presentation osi model

Layer 6 Presentation Layer

De/Encryption, Encoding, String representation

The presentation layer (data presentation layer, data provision level) sets the system-dependent representation of the data (for example, ASCII, EBCDIC) into an independent form, enabling the syntactically correct data exchange between different systems. Also, functions such as data compression and encryption are guaranteed that data to be sent by the application layer of a system that can be read by the application layer of another system to the layer 6. The presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable for both systems data format, the ASN.1 (Abstract Syntax Notation One) used.

OSI Layer 6 - Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer. In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format. Wikipedia

Data conversion
Character code translation
Compression
Encryption and Decryption

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

ACSE	Association Control Service Element
ROSE	Remote Operation Service Element
CCR	Commitment Concurrency and Recovery
RTSE	Reliable Transfer Service Element

SASE specific application service element

FTAM	File Transfer, Access and Manager
VT	Virtual Terminal
MOTIS	Message Oriented Text Interchange Standard
CMIP	Common Management Information Protocol
JTM	Job Transfer and Manipulation
MMS	Manufacturing Messaging Service
RDA	Remote Database Access
DTP	Distributed Transaction Processing

Layer 7 Application Layer

Layer 6 presentation layer, layer 5 session layer, layer 4 transport layer, layer 3 network layer, layer 2 data link layer, layer 1 physical layer.

The OSI Model – The 7 Layers of Networking Explained in Plain English

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

Basic familiarity with common networking terms (explained below)
A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

What the OSI model is
The purpose of each of the 7 layers
The problems that can happen at each of the 7 layers
The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

What is Network Topology? Best Guides to Types & Diagrams - DNSstuff

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

Please | Physical Layer
Do | Data Link Layer
Not | Network Layer
Tell (the) | Transport Layer
Secret | Session Layer
Password (to) | Presentation Layer
Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

Defunct cables, for example damaged wires or broken connectors
Broken hardware network devices, for example damaged circuits
Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
Flow control. How much data should be transmitted?
Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

Header: typically includes MAC addresses for the source and destination nodes.
Body: consists of the bits being transmitted.
Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Example of frames, the network layer, and the physical layer

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

All the problems that can occur on Layer 1
Unsuccessful connections (sessions) between two nodes
Sessions that are successfully established but intermittently fail
Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

A routing table showing the destination, subnet mask, and interface

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

All the problems that can crop up on previous layers :)
Faulty or non-functional router or other node
IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

Blocked ports - check your Access Control Lists (ACL) & firewalls
Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

Servers are unavailable
Servers are incorrectly configured, for example Apache or PHP configs
Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

Non-existent or corrupted drivers
Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

All issues on previous layers
Incorrectly configured software applications
User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

What is the OSI model?
What are each of the layers?
How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

The Illustrated Network, 2nd Edition
Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

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Computerworld

What is the OSI model? How to explain and remember its 7 layers

A tutorial on the open systems interconnection (osi) networking reference model plus tips on how to memorize the seven layers..

AI image 7 layers of OSI model telecommunications network

The Open Systems Interconnect (OSI) model is a conceptual framework that describes networking or telecommunications systems as seven layers, each with its own function.

The layers help network pros visualize what is going on within their networks and can help network managers narrow down problems (is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Tech vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”.

The 7 layers of the OSI model

The layers (from bottom to top) are: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

It wasn’t always this way. Conceived in the 1970s when computer networking was taking off, two separate models were merged in 1983 and published in 1984 to create the OSI model that most people are familiar with today. Most descriptions of the OSI model go from top to bottom, with the numbers going from Layer 7 down to Layer 1.

The layers, and what they represent, are as follows:

Layer 7: Application

The Application Layer in the OSI model is the layer that is the “closest to the end user”. It receives information directly from users and displays incoming data to the user. Oddly enough, applications themselves do not reside at the application layer. Instead the layer facilitates communication through lower layers in order to establish connections with applications at the other end. Web browsers (Google Chrome, Firefox, Safari, etc.) TelNet, and FTP, are examples of communications that rely on Layer 7.

Layer 6: Presentation

The Presentation Layer represents the area that is independent of data representation at the application layer. In general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission; this happens at Layer 6.

Layer 5: Session

When two computers or other networked devices need to speak with one another, a session needs to be created, and this is done at the Session Layer . Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

Layer 4: Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

Layer 3: Network

Here at the Network Layer is where you’ll find most of the router functionality that most networking professionals care about and love. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers . You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2: Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sublayers exist here as well–the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2. But it’s not that simple. Some switches also operate at Layer 3 in order to support virtual LANs that may span more than one switch subnet, which requires routing capabilities.

Layer 1: Physical

At the bottom of our OSI model we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in a Wi-Fi network), as well as the layout of pins, voltages, and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example.

Why you need to know the 7 OSI layers

Most people in IT will likely need to know about the different layers when they’re going for their certifications, much like a civics student needs to learn about the three branches of the US government. After that, you hear about the OSI model when vendors are making pitches about which layers their products work with.

In a Quora post asking about the purpose of the OSI model, Vikram Kumar answered this way: “The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.”

While some people may argue that the OSI model is obsolete (due to its conceptual nature) and less important than the four layers of the TCP/IP model, Kumar says that “it is difficult to read about networking technology today without seeing references to the OSI model and its layers, because the model’s structure helps to frame discussions of protocols and contrast various technologies.”

If you can understand the OSI model and its layers, you can also then understand which protocols and devices can interoperate with each other when new technologies are developed and explained.

The OSI model remains relevant

In a post on GeeksforGeeks, contributor Vabhav Bilotia argues several reasons why the OSI model remains relevant, especially when it comes to security and determining where technical risks and vulnerabilities may exist.

For example, by understanding the different layers, enterprise security teams can identify and classify physical access, where the data is sitting, and provide an inventory of the applications that employees use to access data and resources.

“Knowing where the majority of your company’s data is held, whether on-premises or in cloud services, will help define your information security policy,” writes Bilotia. “You can invest in the correct solutions that provide you data visibility within the proper OSI layers once you have this knowledge.”

In addition, the OSI model can be used to understand cloud infrastructure migrations, particularly when it comes to securing data within the cloud.

And because the model has been around for so long and understood by so many, the uniform vocabulary and terms helps networking professionals understand quickly about the components of the networking system “While this paradigm is not directly implemented in today’s TCP/IP networks, it is a useful conceptual model for relating multiple technologies to one another and implementing the appropriate technology in the appropriate way,” Bilotia writes. We couldn’t agree more.

How to remember the OSI Model 7 layers: 8 mnemonic tricks

If you need to memorize the layers for a college or certification test, here are a few sentences to help remember them in order. The first letter of each word is the same as the first letter an OSI layer.

From Application to Physical (Layer 7 to Layer 1):

All People Seem To Need Data Processing
All Pros Search Top Notch Donut Places
A Penguin Said That Nobody Drinks Pepsi
A Priest Saw Two Nuns Doing Pushups

From Physical to Application (Layer 1 to Layer 7):

Please Do Not Throw Sausage Pizza Away
Pew! Dead Ninja Turtles Smell Particularly Awful
People Don’t Need To See Paula Abdul
Pete Doesn’t Need To Sell Pickles Anymore

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What is OSI Model? – Layers of OSI Model

OSI stands for Open Systems Interconnection , where open stands to say non-proprietary. It is a 7-layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe. The OSI reference model was developed by ISO – ‘International Organization for Standardization ‘, in the year 1984.

The OSI model provides a theoretical foundation for understanding network communication . However, it is usually not directly implemented in its entirety in real-world networking hardware or software . Instead, specific protocols and technologies are often designed based on the principles outlined in the OSI model to facilitate efficient data transmission and networking operations

What is OSI Model?

What are the 7 layers of the OSI Model?

Physical Layer – Layer 1

Data link layer (dll) – layer 2, network layer – layer 3, transport layer – layer 4, session layer – layer 5, presentation layer – layer 6, application layer – layer 7.

What is the Flow of Data in OSI Model?

Advantages of OSI Model

OSI Model in a Nutshell

OSI vs TCP/IP Model

The OSI model, created in 1984 by ISO , is a reference framework that explains the process of transmitting data between computers. It is divided into seven layers that work together to carry out specialised network functions , allowing for a more systematic approach to networking.

For those preparing for competitive exams like GATE, a strong understanding of networking concepts, including the OSI model, is crucial. To deepen your knowledge in this area and other key computer science topics, consider enrolling in the GATE CS Self-Paced course . This course offers comprehensive coverage of the syllabus, helping you build a solid foundation for your exam preparation.

Data Flow In OSI Model

When we transfer information from one device to another, it travels through 7 layers of OSI model. First data travels down through 7 layers from the sender’s end and then climbs back 7 layers on the receiver’s end.

Data flows through the OSI model in a step-by-step process:

Application Layer: Applications create the data.
Presentation Layer: Data is formatted and encrypted.
Session Layer: Connections are established and managed.
Transport Layer: Data is broken into segments for reliable delivery.
Network Layer : Segments are packaged into packets and routed.
Data Link Layer: Packets are framed and sent to the next device.
Physical Layer: Frames are converted into bits and transmitted physically.

Each layer adds specific information to ensure the data reaches its destination correctly, and these steps are reversed upon arrival.

Let’s look at it with an Example:

Luffy sends an e-mail to his friend Zoro.

Step 1: Luffy interacts with e-mail application like Gmail , outlook , etc. Writes his email to send. (This happens in Layer 7: Application layer )

Step 2: Mail application prepares for data transmission like encrypting data and formatting it for transmission. (This happens in Layer 6: Presentation Layer )

Step 3: There is a connection established between the sender and receiver on the internet. (This happens in Layer 5: Session Layer )

Step 4: Email data is broken into smaller segments. It adds sequence number and error-checking information to maintain the reliability of the information. (This happens in Layer 4: Transport Layer )

Step 5: Addressing of packets is done in order to find the best route for transfer. (This happens in Layer 3: Network Layer )

Step 6: Data packets are encapsulated into frames, then MAC address is added for local devices and then it checks for error using error detection. (This happens in Layer 2: Data Link Layer )

Step 7: Lastly Frames are transmitted in the form of electrical/ optical signals over a physical network medium like ethernet cable or WiFi.

After the email reaches the receiver i.e. Zoro, the process will reverse and decrypt the e-mail content. At last, the email will be shown on Zoro’s email client.

What Are The 7 Layers of The OSI Model?

The OSI model consists of seven abstraction layers arranged in a top-down order:

Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer

The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.

Functions of the Physical Layer

Bit Synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.
Bit Rate Control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
Physical Topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
Transmission Mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.

Note: Hub, Repeater, Modem, and Cables are Physical Layer devices. Network Layer, Data Link Layer, and Physical Layer are also known as Lower Layers or Hardware Layers .

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address . The Data Link Layer is divided into two sublayers:

Logical Link Control (LLC)
Media Access Control (MAC)

The packet received from the Network layer is further divided into frames depending on the frame size of the NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header.

The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.

Functions of the Data Link Layer

Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
Physical Addressing: After creating frames, the Data link layer adds physical addresses ( MAC addresses ) of the sender and/or receiver in the header of each frame.
Error Control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.
Access Control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

Note: Packet in the Data Link layer is referred to as Frame. Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines. Switch & Bridge are Data Link Layer devices.

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender & receiver’s IP address es are placed in the header by the network layer.

Functions of the Network Layer

Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.
Logical Addressing: To identify each device inter-network uniquely, the network layer defines an addressing scheme. The sender & receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

Note: Segment in the Network layer is referred to as Packet . Network layer is implemented by networking devices such as routers and switches.

The transport layer provides services to the application layer and takes services from the network layer. The data in the transport layer is referred to as Segments . It is responsible for the end-to-end delivery of the complete message. The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.

At the sender’s side: The transport layer receives the formatted data from the upper layers, performs Segmentation , and also implements Flow and error control to ensure proper data transmission. It also adds Source and Destination port number s in its header and forwards the segmented data to the Network Layer.

Note: The sender needs to know the port number associated with the receiver’s application. Generally, this destination port number is configured, either by default or manually. For example, when a web application requests a web server, it typically uses port number 80, because this is the default port assigned to web applications. Many applications have default ports assigned.

At the receiver’s side: Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data.

Functions of the Transport Layer

Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.
Service Point Addressing: To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.

Services Provided by Transport Layer

Connection-Oriented Service
Connectionless Service

1. Connection-Oriented Service: It is a three-phase process that includes:

Connection Establishment
Data Transfer
Termination/disconnection

In this type of transmission, the receiving device sends an acknowledgment, back to the source after a packet or group of packets is received. This type of transmission is reliable and secure.

2. Connectionless service: It is a one-phase process and includes Data Transfer. In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service.

Note: Data in the Transport Layer is called Segments . Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls. The transport layer is called as Heart of the OSI model. Device or Protocol Use : TCP, UDP NetBIOS, PPTP

This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.

Functions of the Session Layer

Session Establishment, Maintenance, and Termination: The layer allows the two processes to establish, use, and terminate a connection.
Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

Note: All the below 3 layers(including Session Layer) are integrated as a single layer in the TCP/IP model as the “Application Layer”. Implementation of these 3 layers is done by the network application itself. These are also known as Upper Layers or Software Layers. Device or Protocol Use : NetBIOS, PPTP.

Let us consider a scenario where a user wants to send a message through some Messenger application running in their browser. The “ Messenger ” here acts as the application layer which provides the user with an interface to create the data. This message or so-called Data is compressed, optionally encrypted (if the data is sensitive), and converted into bits (0’s and 1’s) so that it can be transmitted.

Communication in Session Layer

The presentation layer is also called the Translation layer . The data from the application layer is extracted here and manipulated as per the required format to transmit over the network.

Functions of the Presentation Layer

Translation: For example, ASCII to EBCDIC .
Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
Compression: Reduces the number of bits that need to be transmitted on the network.

Note: Device or Protocol Use: JPEG, MPEG, GIF.

At the very top of the OSI Reference Model stack of layers, we find the Application layer which is implemented by the network applications. These applications produce the data to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user.

Example : Application – Browsers, Skype Messenger, etc.

Note: The application Layer is also called Desktop Layer. Device or Protocol Use : SMTP .

Functions of the Application Layer

The main functions of the application layer are given below.

Network Virtual Terminal(NVT): It allows a user to log on to a remote host.
File Transfer Access and Management(FTAM): This application allows a user to access files in a remote host, retrieve files in a remote host, and manage or control files from a remote computer.
Mail Services: Provide email service.
Directory Services: This application provides distributed database sources and access for global information about various objects and services.

Note: The OSI model acts as a reference model and is not implemented on the Internet because of its late invention. The current model being used is the TCP/IP model.

OSI Model – Layer Architecture


7	Helps in identifying the client and synchronizing communication.	Message
6	Data from the application layer is extracted and manipulated in the required format for transmission.	Message	, ,
5	Establishes Connection, Maintenance, Ensures Authentication and Ensures security.	Message (or encrypted message)
4	Take Service from Network Layer and provide it to the Application Layer.	Segment
3	Transmission of data from one host to another, located in different networks.	Packet
2	Node to Node Delivery of Message.	Frame	,
1	Establishing Physical Connections between Devices.	Bits	, , , Cables

TCP/IP protocol ( Transfer Control Protocol/Internet Protocol ) was created by U.S. Department of Defense’s Advanced Research Projects Agency (ARPA) in 1970s.

Some key differences between the OSI model and the TCP/IP Model are:

TCP/IP model consists of 4 layers but OSI model has 7 layers. Layers 5,6,7 of the OSI model are combined into the Application Layer of TCP/IP model and OSI layers 1 and 2 are combined into Network Access Layers of TCP/IP protocol.
The TCP/IP model is older than the OSI model, hence it is a foundational protocol that defines how should data be transferred online.
Compared to the OSI model, the TCP/IP model has less strict layer boundaries.
All layers of the TCP/IP model are needed for data transmission but in the OSI model, some applications can skip certain layers. Only layers 1,2 and 3 of the OSI model are necessary for data transmission.

OSI vs TCP/IP

Why Does The OSI Model Matter?

Even though the modern Internet doesn’t strictly use the OSI Model (it uses a simpler Internet protocol suite), the OSI Model is still very helpful for solving network problems. Whether it’s one person having trouble getting their laptop online, or a website being down for thousands of users, the OSI Model helps to identify the problem. If you can narrow down the issue to one specific layer of the model, you can avoid a lot of unnecessary work.

Imperva Application Security

Imperva security solutions protect your applications at different levels of the OSI model. They use DDoS mitigation to secure the network layer and provide web application firewall (WAF), bot management, and API security to protect the application layer.

To secure applications and networks across the OSI stack, Imperva offers multi-layered protection to ensure websites and applications are always available, accessible, and safe. The Imperva application security solution includes:

DDoS Mitigation: Protects the network layer from Distributed Denial of Service attacks.
Web Application Firewall (WAF) : Shields the application layer from threats.
Bot Management: Prevents malicious bots from affecting the application.
API Security: Secures APIs from various vulnerabilities and attacks.

The OSI Model defines the communication of a computing system into 7 different layers. Its advantages include:

It divides network communication into 7 layers which makes it easier to understand and troubleshoot.
It standardizes network communications, as each layer has fixed functions and protocols.
Diagnosing network problems is easier with the OSI model .
It is easier to improve with advancements as each layer can get updates separately.

Disadvantages of OSI Model

Complexity: The OSI Model has seven layers, which can be complicated and hard to understand for beginners.
Not Practical: In real-life networking, most systems use a simpler model called the Internet protocol suite (TCP/IP), so the OSI Model isn’t always directly applicable.
Slow Adoption: When it was introduced, the OSI Model was not quickly adopted by the industry, which preferred the simpler and already-established TCP/IP model.
Overhead: Each layer in the OSI Model adds its own set of rules and operations, which can make the process more time-consuming and less efficient.
Theoretical: The OSI Model is more of a theoretical framework, meaning it’s great for understanding concepts but not always practical for implementation.

In conclusion, the OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven distinct layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer has specific responsibilities and interacts with the layers directly above and below it, ensuring seamless communication and data exchange across diverse network environments. Understanding the OSI model helps in troubleshooting network issues, designing robust network architectures, and facilitating interoperability between different networking products and technologies.

Frequently Asked Questions on OSI Model – FAQs

Is osi layer still used.

Yes, the OSI model is still used by networking professionals to understand data abstraction paths and processes better.

What is the highest layer of the OSI model?

Layer 7 or Application layer is highest layer of OSI model.

What is layer 8?

Layer 8 doesn’t actually exist in the OSI model but is often jokingly used to refer to the end user. For example: a layer 8 error would be a user error.

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The OSI Model’s 7 Layers, Explained

The seven layers in the Open Systems Interconnection (OSI) model each serve a specific function and work together to create an efficient network communication system.

The Open Systems Interconnection (OSI) model is a framework in network communication that simplifies complex network interactions into a structured format.

What Is the OSI Model?

The Open Systems Interconnection model is a framework in network communication designed to simplify complex network interactions into a structured format. This architecture has seven layers, each of which serves a specific function. All seven layers work together to create a robust and efficient network communication system.

Each of its seven layers has a distinct role, ensuring efficient data transfer from one device to another . The OSI model is essential for understanding how data is transmitted in a network and is also a practical guide for network protocol design and problem solving.

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The OSI model, developed by the International Organization for Standardization , outlines the essential functions of networking and telecommunications systems for practical application. It plays a crucial role in telecommunications, where vendors use it to define the features and capabilities of their products and services.

This approach allows for a detailed explanation of different aspects of network communication, including transport protocols, addressing schemes and data packaging methods. As a result, the OSI model resolves the complexities of network communication and fosters a more integrated and coherent digital world .

The 7 Layers of the OSI Model

Each layer of the OSI model serves a specific function, yet they work in harmony to create a robust and efficient network communication system. Understanding these layers provides valuable insights into the complexities of network design and operation, showcasing the intricate nature of modern digital communication.

Layer 7: Application Layer

Functionality: The Application Layer is the closest to the end user. It facilitates user interaction with networked systems, providing interfaces and protocols for web browsers, email clients and other applications.

Key protocols: Protocols like HTTP, FTP and SMTP operate at this layer, enabling services such as web browsing, file transfers and email communications.

Layer 6: Presentation Layer

Role: The Presentation Layer acts as a translator, converting data formats from the application layer into a network-compatible format and vice versa. It ensures that data sent from one system is readable by another.

Data formatting: This layer is responsible for data encryption and compression, playing a significant role in maintaining data privacy and efficient transmission.

Layer 5: Session Layer

Managing sessions: It establishes, manages and terminates sessions between applications. This layer ensures that sessions are maintained for the duration of the communication.

Coordination: The Session Layer coordinates communication between systems, managing dialogues and synchronizing data exchange.

Layer 4: Transport Layer

Data segmentation and control: The Transport Layer is crucial for segmenting data into smaller packets. It ensures end-to-end data integrity and delivery, managing flow control, error correction and sequencing.

Protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are key protocols in this layer, differing in their approach to data transmission.

Layer 3: Network Layer

Routing and addressing: This layer is responsible for logical addressing and routing data packets across different networks. It determines the best path for data to travel from source to destination.

Internet protocol: The Internet Protocol (IP), fundamental for internet data exchange, operates at this layer.

Layer 2: Data Link Layer

Framing and MAC addressing: The Data Link Layer frames data into packets. It handles physical addressing through MAC addresses, ensuring that data is directed to the correct hardware.

Error detection: This layer is also involved in error detection and handling, improving overall data transmission reliability.

Layer 1: Physical Layer

Physical transmission: The Physical Layer deals with the physical aspects of data transmission, including cable types, electrical signals and data rates.

Hardware components: It involves hardware components like cables, switches and network interface cards, forming the foundation of network communication.

How Data Flows in the OSI Model

Understanding this data flow process is crucial for professionals, as it aids in diagnosing and troubleshooting network issues, designing efficient network solutions and ensuring robust data security and management.

Encapsulation Process

When data is sent, it begins at the Application Layer and moves down through the layers. At each stage, it is encapsulated with the necessary headers, trailers, and other control information relevant to that layer. For instance, at the Transport Layer, data is segmented and encapsulated with port numbers, while at the Network Layer, IP addresses are added.

Each layer plays a role in preparing the data for transmission. The Presentation Layer may encrypt the data for security, while the Data Link Layer ensures it is formatted into frames suitable for physical transmission.

Data Transmission Across the Network

The Physical Layer transmits the raw bits over a physical medium, such as a cable or wireless network. This transmission is the actual movement of data across the network. In cases where data must move across different networks, the Network Layer’s routing functionalities become crucial. It ensures that data packets find the most efficient path to their destination.

Decapsulation Process

Upon reaching the destination, the data moves up the OSI model, with each layer removing its respective encapsulation. The Data Link Layer, for instance, removes framing, and the Transport Layer checks for transmission errors and reassembles the data segments. Once the data reaches the Application Layer, it is in its original format and ready to be used by the receiving application, whether it’s an email client, a web browser or any other networked software.

Seamless Data Flow

The OSI model ensures that each layer only communicates with its immediate upper and lower layers, creating a seamless flow. This layered approach means changes in one layer’s protocols or functionalities can occur without disrupting the entire network.

OSI Model Advantages

The OSI model is a cornerstone in network architecture for several reasons:

Simplification of network design

The OSI model’s layered approach breaks down complex network processes, making design and operation more manageable. Each layer focuses on a specific aspect of communication, allowing for independent development and easier troubleshooting.

Standardization and interoperability

It establishes universal standards for network communication, enabling different technologies to interact seamlessly. This interoperability is crucial for the efficient functioning of diverse network devices and applications.

Flexibility and Scalability

Adaptable to technological advancements, the OSI model allows individual layers to evolve without overhauling the entire system. This scalability makes it suitable for various network sizes and types.

Enhanced Security

Security measures are integrated at multiple layers, providing a robust defense against threats. Each layer can address specific security concerns, leading to comprehensive network protection.

Real-World Applications of the OSI Model

The OSI model’s influence extends well beyond theoretical concepts, playing a crucial role in various practical aspects of networking:

Network Design and Protocol Development

Network professionals use the OSI model as a blueprint for structuring and developing robust networks. It guides the creation of new protocols, ensuring seamless integration and functionality across different network layers.

Efficient Troubleshooting and Management

In troubleshooting, the OSI model provides a systematic approach for identifying issues, from physical connectivity to application-level errors. It also aids in network maintenance and performance optimization, addressing each layer to enhance overall efficiency.

Cybersecurity Strategy

The model is foundational in crafting layered security strategies . By implementing security measures at different layers, it offers comprehensive protection against various cyber threats. Understanding the OSI layers is key in detecting and mitigating attacks targeting specific network segments.

Educational and Training Tool

It serves as an essential framework in networking education, helping students and professionals alike understand complex network operations. The OSI model is a cornerstone in training programs , emphasizing the intricacies of network architecture and security.

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OSI Model vs. TCP/IP Model

While the OSI model offers a detailed conceptual framework, the TCP/IP model is recognized for its practical application in today’s internet-driven world.

Structural Differences

OSI model : Introduced as a comprehensive, protocol-independent framework, the OSI model details seven distinct layers, offering a more granular approach to network communication.

TCP/IP model : Developed earlier by the U.S. Department of Defense, the TCP/IP model consists of four layers (Application, Transport, Internet and Network Access), combining certain OSI layers.

Theoretical vs. Practical Approach

OSI model : Developed as a theoretical and universal networking model, it’s used more for educational purposes to explain how networks operate.

TCP/IP model : This model is designed around specific standard protocols, focusing on solving practical communication issues. It leaves sequencing and acknowledgment functions to the transport layer, differing from the OSI approach.

Adoption and Use

OSI model: While not widely implemented in its entirety, the OSI model’s clear layer separation is influential in protocol design and network education; simpler applications in the OSI framework may not utilize all seven layers, with only the first three layers (Physical, Data Link, and Network) being mandatory for basic data communication.

TCP/IP model : The dominant model used in most network architectures today, especially in internet-related communications. In TCP/IP, most applications engage all layers for communication.

Frequently Asked Questions

Why is the osi model important.

The OSI model is crucial for standardizing network communication and ensuring interoperability between various devices and systems. It simplifies network design and troubleshooting and serves as a fundamental educational tool in networking.

What are the 7 layers of the OSI model?

Layer 1: Physical Layer — Transmits raw data.

Layer 2: Data Link Layer — Manages direct links and framing.

Layer 3: Network Layer — Handles addressing and routing.

Layer 4: Transport Layer — Ensures reliable data transfer.

Layer 5: Session Layer — Manages connections.

Layer 6: Presentation Layer — Translates data formats.

Layer 7: Application Layer — Interfaces with applications.

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1. Overview

The OSI model stands for Open Systems Interconnection model and is a standardized framework that describes a networking system’s communication functions. It consists of 7 layers, and each layer has specific functionality to perform.

In this tutorial, we’ll discuss the OSI model and its layers in detail . We’ll present the description and functionality of each layer.

2. Introduction to the OSI Model

In the early days of networking systems, the networking devices were limited to communicate to the devices manufactured by the same vendors. For example, a networking device manufactured by a company X could only communicate with other devices manufactured by the same company, not with any other companies’ devices.

To ensure that the networking devices manufactured nationally and internationally can communicate with each other, the devices should be developed through a standard procedure. To bring standardization in the computing and communication system, the International Organization for Standardization (ISO) published the OSI model in 1984.

The OSI model consists of 7 layers: Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Datalink Layer, Physical Layer. Each of these layers has a different role to play, and they work collaboratively to transmit the data from one networking device to another.

Let’s see how the OSI model:

Now, we’ll discuss the primary function of each of the layer:

3. Physical Layer

The lowest layer in the OSI model, the physical layer, is responsible for the transmission and reception of raw data between the network devices and the transmission medium . It stores the information or data in the form of bits and transit bits from one node to another:

The physical layer plays a vital role in controlling the transmission rate . Whenever some device sends data to the physical layer, it receives the data, converts the data to bits, and sends the data link layer. While transferring the bits, it defines the number of bits transmitted per second.

It facilitates the synchronization at the bit level by setting a clock that controls both the receivers and senders.

It defines the transmission mode in which the data flows between two networking devices.

The physical layer also defines the physical topology . A physical topology is an actual layout of the computer cables and other network devices.

The network protocols which are used in the physical layer are RS-232 , UTP cables , DSL .

4. Data Link Layer

The data link layer is responsible for the data transfer between two directly connected nodes. When the physical layer receives data, it converts it to bits and sends it to the data link layer. The data link layer ensures the data or packet received from the physical layer is error-free and then transfers the data from one node to another. It sends the packet to another node using its MAC address.

The data link layer is further divided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC) . The LLC layer is responsible for checking the errors in the received packet, and it synchronizes the frames. The MAC layer controls the access of networking devices to the transmission medium. It also gives permission to transfer the data from one node to another.

As soon as the raw data from the physical layer reaches the data link layer, it converts the raw data into packets, called frames. With a frame, it also adds a header and trailer. A header and trailer include information for hardware destination and source address. In this way, it also helps the packet to reach the destination:

The primary responsibility of the data link layer is to control the flow of the data. Suppose some data is transferred from one high processing server to another low processing server. The data link layer makes sure the data rate matches between two servers and no data get corrupted.

The data link layer is also responsible for error control and access control .

The most popular computer networking technology used in the data link layer is Ethernet .

5. Network Layer

The network layer transmits data from one transmitting station to another. It also helps select the shortest path from one host to another to deliver the data at the best possible time. This process is called routing. It also puts the IP addresses of the sender and receiver into the header.

The network layer provides a logical connection between different network devices. It also plays an important role in addressing. When the incoming data arrives at the network layer, it adds the destination and source address with the header:

The network layers use various protocols like IPv4/IPv6 , ARP , ICMP to carry out its functionalities.

6. Transport Layer

The transport layer works between the network layer and the application layer. It ensures the end-to-end and complete transfer of the data. It also provides acknowledgment to the sender node when the data transmission is complete. If there is an error occurred during transmission, the transport layer re-transmit the data.

The transport layer receives the packets from the network layer and divides the packets into smaller packets, known as segments. This process is called segmentation . It also controls the flow and error of the data and helps data to reach the receiver end successfully:

At the receiver’s end, the transport layer receives the data in a segment form. It reassembles the segment and gives acknowledgment for the successful transformation of data.

As the transportation layer deals with packet transfer between sender and receiver station, it mainly uses TCP , UDP protocols to transfer packets.

7. Session Layer

The session layer is used to establish a connection, maintain and synchronizes the sessions between communicating devices. It also provides authentication and ensures security.

When transmitting data in the form of segments, the session layer adds some synchronization points. If some error occurs, the transmission restarts from the last synchronization point. This process is called synchronization and recovery:

The session layer also plays an important role as a dialog controller. A dialog controller identifies the mode of communication for a session. It allows communication between two systems in half-duplex or full-duplex .

In order to allow applications to communicate with different computers, the session layer uses network protocols like NetBIOS or PPTP .

8. Presentation Layer

The presentation layer gives syntax and semantics of the data exchanged between the two networking systems. It provides translations and is also known as a translation layer.

It also ensures data privacy by providing encryption and decryption to the data. It helps in the reduction of the number of bits needed to represent data. This process is formally known as data compression:

In order to ensure the privacy and security of the data being transferred, the presentation layer uses SSL or TSL protocol.

9. Application Layer

The application layer is the last layer in the OSI model, and it is very close to the software application. It acts as a window between a software application and an end-user. It helps to access the data from the network and display the information to the user.

The application layer performs several functions like it allows a user to access a file remotely, manage the files, retrieve specific data from a computer:

Popular protocols that are used in the application layer are e.g. HTTP , FTP , DNS , SNMP .

10. Conclusion

In this tutorial, we’ve discussed the OSI model in detail. We presented a concise discussion on each layer of the OSI model with its functionalities.

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The presentation layer is the 6 layer from the bottom in the OSI model. This layer presents the incoming data from the application layer of the sender machine to the receiver machine. It converts one format of data to another format of data if both sender and receiver understand different formats; hence this layer is also called the translation layer. It deals with the semantics and syntax of the data, so this layer is also called the syntax layer. It uses operations such as data compression, data encryption & decryption, data conversion, etc.

Data is sent from sender to receiver, but what if the sender device and receiver device understand different formats of code? For example, suppose one device understands ASCII code and another device understands EBCDIC code. In that case, the data must be translated into a code that the recipient understands to determine what data has been sent. The presentation layer is responsible for translating ASCII codes to EBCDIC or vice versa. With the help of the presentation layer, the receiver understands the data effectively and uses it efficiently. Whatever data is being transmitted between the sender and the receiver, that data must be secure because an intruder can hack the data passing between the sender and the receiver. Hackers can modify the data and send the modified data to the receiver to create false communication. The presentation layer is responsible for encrypting and decrypting data to avoid data leakage and data modification.
The plaintext data at the source is encrypted into ciphertext (unreadable format), then it is sent to the receiver, where the ciphertext is decrypted into plaintext. Now, if the hacker tries to hack the data, the hacker receives an encrypted, unreadable form, and if the hacker tries to send modified data, the receiver can detect the modification during decryption; thereby, the data remains safe. If the file size is large, it becomes difficult to transmit the large file over the network. File size can be decreased by compressing the file for easy transmission of data. Compression is the method of diminishing the size of a file to transmit data easily in less time. When the compressed data reaches the receiver, the data is reconstructed back to the original size, and this process is called decompression.

The presentation layer in the OSI model is classified into two sublayers:

This sublayer offers services to layer-7, i.e., the application layer, and requests services from layer-5, i.e., the session layer. It supports various application services, such as Reliable Transfer Service Element (RTSE), Remote Operation Service Element (ROSE), Association Control Service Element (ACSE), and Commitment Concurrency and Recovery (CCR). This sublayer offers application-specific protocols, such as Message Oriented Text Interchange Standard (MOTIS), Remote Database Access (RDA), File Transfer Access and Manager (FTAM), Common Management Information Protocol (CMIP), Virtual Terminal (VT), Distributed Transaction Processing (DTP), Job Transfer and Manipulation (JTM), and others. It is a presentation layer protocol in the OSI model, which was formed by Citrix Systems. It is used for transferring data from server to client. It is a very thin protocol as it does not require much overhead in order to transmit data from the server over to the client. It is well-optimized for the WAN. It is the protocol that is used to implement the presentation layer of the OSI model. It provides different kinds of data representation, such as images, video, audio, numbers, etc. It is used for Microsoft Remote Procedure Call (Microsoft RPC) and Distributed Computing Environment (DCE) / Remote Procedure Calls (RPC). It is a communication protocol that was specifically designed for macOS by Apple, Inc. It provides file services for Classic Mac OS and macOS. This protocol is used to share files over the network. It is a protocol that is associated with the client-server operating system. The user can access the directory, print, message, file, clock synchronization, etc., with the help of this protocol. It supports many platforms, such as Linux, Classic Mac OS, Windows NT, Mac OS X, and Microsoft Windows. It is a telecommunications equipment that splits a stream of data into separate packets and formats packet headers for asynchronous communication on X.25 networks. It receives packets from the network and converts them into a stream of data. The PAD provides many asynchronous terminal connectivities to a host computer. It is a computer network protocol that is used to transfer data between two systems. It was first published in 1987. XDR is used by various systems such as NDMP, Network File System, NetCDF, ZFS, Open Network Computer Remote Procedure Call, and others. It is a protocol that offers ISO presentation services over TCP/IP based networks. This protocol explains an approach to provide stream-line support for OSI over TCP/IP based networks.

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What is the OSI Model?

The OSI Model Defined, Explained, and Explored

The OSI Model Defined

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

Created at a time when network computing was in its infancy, the OSI was published in 1984 by the International Organization for Standardization (ISO). Though it does not always map directly to specific systems, the OSI Model is still used today as a means to describe Network Architecture.

The 7 Layers of the OSI Model

Physical layer.

The lowest layer of the OSI Model is concerned with electrically or optically transmitting raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find “physical” resources such as network hubs, cabling, repeaters, network adapters or modems.

Data Link Layer

At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer.

The data link layer encompasses two sub-layers of its own. The first, media access control (MAC), provides flow control and multiplexing for device transmissions over a network. The second, the logical link control (LLC), provides flow and error control over the physical medium as well as identifies line protocols.

Network Layer

The network layer is responsible for receiving frames from the data link layer, and delivering them to their intended destinations among based on the addresses contained inside the frame. The network layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to quite literally route information where it needs to go between networks.

Transport Layer

The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. One of the most common examples of the transport layer is TCP or the Transmission Control Protocol.

Session Layer

The session layer controls the conversations between different computers. A session or connection between machines is set up, managed, and termined at layer 5. Session layer services also include authentication and reconnections.

Presentation Layer

The presentation layer formats or translates data for the application layer based on the syntax or semantics that the application accepts. Because of this, it at times also called the syntax layer. This layer can also handle the encryption and decryption required by the application layer.

Application Layer

At this layer, both the end user and the application layer interact directly with the software application. This layer sees network services provided to end-user applications such as a web browser or Office 365. The application layer identifies communication partners, resource availability, and synchronizes communication.

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Published: 11 June 2024 Contributors: Chrystal R. China, Michael Goodwin

The Open Systems Interconnection (OSI) model—also called the OSI reference model—is a conceptual model that divides network communication and interoperability into seven abstract layers. It provides a standardized model that enables different applications, computer systems and networks to communicate.

The OSI model emerged as a solution to communication incompatibilities between the diverse array of networking protocols in use around the turn of the century. The layers of OSI gave developers and engineers a framework for building interoperable hardware and software across networks by providing a categorical approach to networking .

At each layer of the stack—typically shown in reverse order to illustrate how data moves through a network—the OSI model provides guidelines and criteria for network components and their unique computing functions.

The layers are:

Layer 7: The application layer initiates communication with the network, including the protocols and data manipulation processes that convert computer-readable network data into user-readable responses.
Layer 6: The presentation layer prepares data for the application layer, including data translation, compression and encryption .
Layer 5: The session layer initiates and terminates connections between two devices interacting on the network, making sure that resources are neither overused nor underutilized.
Layer 4: The transport layer transmits end-to-end data between two devices interacting on the network, making sure that data isn’t lost, misconfigured or corrupted.
Layer 3: The network layer handles data addressing, routing and forwarding processes for devices interacting across different networks. If the devices are on the same network, they don’t need the network layer to interact.
Layer 2: Unlike the network layer, the data link layer manages data routing between two interacting devices on the same network.
Layer 1: The physical layer comprises the physical assets, like routers and USB cables, that convert data into strings of 1s and 0s for transmission to higher layers.

The OSI model focuses on providing a list of tasks for engineers to complete in building each layer of a network architecture, instead of specifying protocols for communication between layers. Its theoretical approach enables developers to visualize and build highly complex computing networks, even without prior knowledge of the networking system itself. It also helps teams better understand how data traverses a network and tailor network functions with layer-specific coding.

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Although the OSI model isn’t the direct basis for modern computer networking technologies, it’s had a profound impact on computing standards development, helping shape contemporary understandings of network architecture.

In the late 1970s and early 1980s, computer systems were becoming increasingly interconnected, but manufacturers often developed their own networking solutions, creating a patchwork of proprietary and non-interoperable systems.

Several early networking efforts attempted to address compatibility issues with the ARPANET (which laid the groundwork for the modern internet) and the TCP/IP protocol suite (commissioned by the Department of Defense). Both represented significant advancements, but they also highlighted the need for a more comprehensive and universally accepted approach.

Recognizing the growing importance of networking and the need for a universal framework, the International Organization for Standardization (ISO) and the International Telegraph and Telephone Consultative Committee (CCITT) initiated the development of a standardized networking model.

The ISO formally published the OSI model, a seminal framework for developing interoperable network solutions, in 1984. Unlike previous standardization attempts, the layered configuration of OSI enables disparate systems to communicate despite differences in their underlying architectures and protocols.

The OSI model remains integral to understanding network architecture even as technologies evolve and new models emerge. Whether a team is designing a simple local area network (LAN) or managing a complex global network , the principles of the OSI model provide a clear, structured approach to networking.

The OSI model includes seven distinct layers. The application layer (layer 7), the presentation layer (layer 6) and the session layer (layer 5) comprise the software layers of an OSI, where all transmissions to and from software apps (including operating systems and utilities, such as web browsers and email clients) occur.

The transport layer (layer 4) is the “heart of OSI,” handling all data communication between networks and systems. Finally, the network layer (layer 3), the data layer (layer 2) and the physical layer (layer 1) comprise the hardware layers of OSI, where data moves through the physical components of the network as it’s processed.

Data moves bi-directionally through the OSI model; each layer communicates with the layers below and above it in the stack. Furthermore, both the sending and receiving devices transmit data through the data layers; and senders and receivers often switch roles in the process.

For example, if a user wants to send an email to another person, the user would first write the email and send it. When the user presses “send,” their email goes to the application layer, which will choose the correct protocol (typically SMTP) and send the email to the presentation layer. The presentation layer then compresses the message data and sends it to the session layer, which initiates a communication session and sends the data to the transport layer for segmentation.

Since the email is going to another network, the email data must go to the network layer, where it’s divided into packets and then to the data link layer where it’s further broken down into frames. Those frames are subsequently transmitted through the physical layer (the recipient’s wifi), at which point the recipient’s device receives the bit stream and the email data traverses the same layers in reverse. At the end of the process, the email data lands in the application layer of the recipient’s device where it’s delivered, in human-readable form, to the recipient’s inbox.

The OSI model is foundational to protocol development, with each layer of the framework managing specific network processes.

The application layer is the OSI layer closest to the end user. It provides network services directly to user applications and facilitates communication between API endpoints and lower layers of the OSI model. In other words, software applications use the application layer to initiate communication with the network and send data to the presentation layer.

Applications themselves are not part of this layer. Rather, the application layer provides the protocols (HTTP, FTP, DNS and SMTP, for instance) that enable software to send and receive data. It’s responsible for processes such as:

File transfer. The application layer takes human-readable data files from the user’s device and transmits them to the presentation layer.
Communication and authentication. The application layer makes sure that the receiving device can accept the data and that the communications interfaces needed for the transfer exist. It can also be used to authenticate the devices involved in the transfer.
Remote access. The application layer enables users to access web browsers, email clients and other services from various geographical locations. It also enables users to access and manage files on a remote computer.

Directory services. The application layer provides directory services—a shared database of information about network devices and users—to facilitate network resource management.

The presentation layer transforms data into a format that the application layer can accept for transmission across the network (from an EBCDIC-coded text file to an ASCII-coded file, for instance). Due to its role in converting data and graphics into a displayable format for the application layer, it is sometimes referred to as the syntax layer.

It supports secure sockets layer/transport layer security (SSL/TLS) protocols, JPEG protocols (for image compression) and MPEG protocols (for video The presentation layer transforms data into a format that the application layer can accept for transmission across the network (from an EBCDIC-coded text file to an ASCII-coded file, for instance). Due to its role in converting data and graphics into a displayable format for the application layer, it is sometimes referred to as the syntax layer.

It supports secure sockets layer/transport layer security (SSL/TLS) protocols, JPEG protocols (for image compression) and MPEG protocols (for video compression). The presentation layer is responsible for:

Data translation. The presentation layer converts data into the correct format (specified by the application layer) during the encapsulation process, as outgoing messages move down the protocol stack from sender to receiver.

Data encryption and decryption. The presentation layer encrypts data for secure transmission and decrypts it upon delivery.

Data compression. The presentation layer reduces the size of a data stream for transmissions and decompresses it for use.

Sometimes formatting and translation are reversed during the de-encapsulation process, as incoming messages move up the protocol stack. In those instances, outgoing messages are converted into the specified format during encapsulation, while incoming messages undergo a reverse conversion during de-encapsulation.

The session layer is responsible for session management, the process of establishing, managing and terminating connections—called "sessions"—between two or more computers. It initiates the connections between local and remote applications, keeping the session open long enough to transmit the necessary data and closing them when complete to preserve network resources.

Key functions of the session layer include:

Session interactions. The session layer manages user logon (establishment) and user logoff (termination), including any authentication protocols integrated into client software.

Synchronization. The session layer helps ensure that data streams are properly synchronized and handles recovery points (checkpoints that allow devices to resume a session from a specific point, if interrupted).

Session recovery. The session layer manages session failures and re-establishes connections if there are network problems.

It also establishes protocols for connecting and disconnecting sessions between related data streams, such as audio and video in web conferencing. Therefore, the session layer is often explicitly implemented in network environments that utilize remote procedure calls.

The transport layer uses protocols like transmission control protocol (TCP) and the user datagram protocol (UDP) to manage the end-to-end delivery of complete messages. It takes messages from the session layer and breaks them into smaller units (called “segments”), each with an associated header. At the destination, the transport layer reassembles the segments in the correct order to reconstruct the original message.

The transport layer also handles:

Service point addressing. The transport layer helps ensure that messages are delivered to the correct process by attaching a transport layer header (including a service point or port address).

Flow control. The transport layer prevents data overflow and manages the rate of data transmission between two devices interacting on the network, making sure that sending devices transmit data to receiving devices (and vice versa) at the appropriate speed.

Multiplexing. The transport layer allows multiple network applications to use the same connection simultaneously.

At the sender's end, the transport layer receives formatted data from the upper layers, performs segmentation and implements flow and error control to ensure accurate data transmission. It adds source and destination port numbers to the header and then forwards the segmented data to the network layer.

At the receiver's end, the transport layer reads the port number from the header and forwards the received data to the appropriate application. It also handles the sequencing and reassembly of the segmented data and retransmits data if errors are detected.

The transport layer provides two types of service.

With connection-oriented service , a three-part process including connection establishment, data transfer and termination (or disconnection), the data receiver sends an acknowledgment of receipt back to the sender when the data packet is delivered. Connectionless service , however, only involves data transfer. The receiver does not confirm receipt, which accelerates communication but can be less reliable than connection-oriented service.

The network layer of the OSI model is responsible for facilitating data transfer from one node to another across different networks. The network layer determines the best path (routing) for data to travel between nodes. If segments are too large, the network layer breaks them up into smaller “packets” for transport and reassembles them on the receiving end.

A network serves as a medium where multiple nodes (each with a unique address) can connect. The network layer allows nodes to send messages to nodes on other networks by providing the message content and the destination address, leaving the network to determine the optimal delivery path (which may involve routing through intermediate nodes).

The network layer primarily uses the Internet Protocol v4 (IPv4) and IPv6 and is responsible for:

Packet fragmentation and reassembly. The network layer splits large packets (those that exceed the size limits of the data link layer) into smaller ones for transmission and reassembles them at the destination.

Traffic control. The network layer manages network traffic to prevent congestion and safeguard efficient data flow.

Reliability isn’t guaranteed in the network layer; while many network layer protocols offer reliable message delivery, some do not. Furthermore, error reporting isn’t mandatory at this layer of OSI, so data senders may or may not receive confirmation of delivery.

The data link layer’s primary function is to manage error-free data transfer between multiple devices interacting on the same network.

The DLL is divided into two sublayers.

The logical link control (LLC) layer —which serves as an interface between the media access control (MAC) layer and the network layer—handles flow control, synchronization and multiplexing (where two or more data streams share a single connection to the host). The MAC layer controls how devices access network mediums and transmit data.

When the DLL receives a packet from the network layer, it divides the packet into data “frames”—according to the frame size of the network interface card (NIC)— and transmits it to the host using its MAC address.

DLL functions include:

Framing. The DLL allows the sender to transmit a set of bits (data) that are meaningful to the receiver by attaching special bit patterns to the beginning and end of the frame.

Physical addressing. The DLL uses the Address Resolution Protocol (ARP) to convert IP addresses to MAC addresses and then adds the MAC addresses of the sender and receiver to the header of each frame after framing is complete.

Error control. The DLL detects damaged or lost frames and manages retransmission (if necessary) to ensure data integrity.

Flow control. To prevent corruption, the DLL dictates how much data a sender can send before receiving an acknowledgment of delivery, keeping the data rate consistent on both sides.

Access control. When multiple devices share a single communication channel, the MAC sublayer determines which device has control over the channel at a given moment.

The physical layer comprises the physical network components responsible for transmitting raw data—in the form of “bits,” or strings of 1s and 0s—between devices (connectors, routers, repeaters and fiber optic cables, for instance) and a physical medium (like wi-fi).

The physical layer is responsible for:

Bit rate control. The physical layer defines the data transmission rates, often in bits per second.

Bit synchronization. The physical layer imposes a clock on bit streams, ensuring that the sender and receiver are synchronized at the bit level.

Transmission mode. The physical layer defines how data will flow between connected devices (as simplex, half duplex or full duplex transmission).

Physical topologies. The physical level specifies how network devices and nodes are situated (in bus, star or mesh topologies , for example). Standards like USB, Bluetooth and Ethernet include physical layer specifications.

The physical layer also defines how encoding occurs over a physical signal (using electrical voltage, radio or light pulses, for example).

The OSI reference model provides a theoretical underpinning that helps engineers and developers understand the intricacies of network communication. However, it’s sometimes compared with another networking model: the transmission control protocol/internet protocol (TCP/IP) model.

Unlike the OSI model, the TCP/IP model is based on standardized protocols that are widely and directly implemented and in real-world networks. It consists of four layers—instead of seven—but each layer corresponds to one or more layers of the OSI model.

Network access layer. Also called the data link layer or the physical layer, the network access layer of a TCP/IP network includes both the hardware and software components necessary for interfacing with the network medium, combining the OSI model’s physical and data link layers. It handles physical data transmission—by using Ethernet (for LANs) and ARP protocols—between devices on the same network.

Internet layer. Similar to the OSI model's network layer, the internet layer is responsible for logical addressing, routing and packet forwarding. It primarily relies on the IP protocol and the Internet Control Message Protocol, which manages addressing and routing of packets across different networks.

Transport layer. The TCP/IP transport layer serves the same function as the OSI model's transport layer; it enables reliable data transfer between upper and lower layers. Using TCP and UDP protocols, it also provides mechanisms for error checking and flow control.

Application layer. TCP/IP’s application layer encompasses the OSI model’s session, presentation and application layers. It uses HTTP, FTP, Post Office Protocol 3 (POP3), SMTP, DNS and SSH protocols to provide network services directly to applications and manages all the protocols that support user applications.

The OSI model's primary value lies in its educational utility and its role as a conceptual framework for designing new protocols, making sure that they can interoperate with existing systems and technologies.

However, the TCP/IP model's practical focus and real-world applicability have made it the backbone of modern networking. Its robust, scalable design and horizontal layering approach has driven the explosive growth of the internet, accommodating billions of devices and massive amounts of data traffic.

Its abstracted, vertically layered approach to networking enables modular protocol design, where each layer can be developed and updated independently.

The modularity of the OSI model encourages faster innovation in protocol development, since software engineers can integrate new technologies without overhauling the entire network stack.

It also enables developers to abstract away the lower layers of the model to simplify development.

Software engineers can separate the operating layers of each network component and organize them according to their primary roles in the network. This decomposability makes it easier for developers to conceptualize a network and share simplified models between development teams.

When a device on the network fails or an app loses connection, the OSI model allows teams to pinpoint and isolate the problematic layer to address any security issues or networking vulnerabilities without disrupting the entire framework.

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Modern network infrastructures built for digital transformation require solutions that can be just as dynamic, flexible, and scalable as the new environments. IBM SevOne provides application-centric, network observability to help NetOps spot, address and prevent network performance issues in hybrid environments.

With IBM Cloud load balancers, you can load balance traffic among your servers to help improve uptime. You can also easily scale your applications by adding or removing servers, with minimal disruption to your traffic flows.

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Three-tier architecture is a well-established software application architecture that organizes applications into three logical and physical computing tiers.

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Cloud computing is the on-demand access of computing resources—physical servers or virtual servers, data storage, networking capabilities, application development tools, software, AI-powered analytic tools and more—over the internet with pay-per-use pricing.

IBM NS1 Connect provides fast, secure connections to users anywhere in the world with premium DNS and advanced, customizable traffic steering. NS1 Connect’s always-on, API-first architecture enables your IT teams to more efficiently monitor networks, deploy changes and conduct routine maintenance.

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The Layers of the OSI Model Illustrated

Each layer explained

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The Open Systems Interconnection (OSI) model defines a networking framework to implement protocols in layers, with control passed from one layer to the next. It is primarily used today as a teaching tool. It conceptually divides computer network architecture into 7 layers in a logical progression.

The lower layers deal with electrical signals, chunks of binary data , and routing of these data across networks. Higher levels cover network requests and responses, representation of data, and network protocols, as seen from a user's point of view.

The OSI model was originally conceived as a standard architecture for building network systems, and many popular network technologies today reflect the layered design of OSI.

Physical Layer

At Layer 1, the Physical layer of the OSI model is responsible for the ultimate transmission of digital data bits from the Physical layer of the sending (source) device over network communications media to the Physical layer of the receiving (destination) device.

Examples of layer 1 technologies include Ethernet cables and hubs . Also, hubs and other repeaters are standard network devices that function at the Physical layer, as are cable connectors.

At the Physical layer, data is transmitted using the type of signaling supported by the physical medium: electric voltages, radio frequencies, or pulses of infrared or ordinary light.

Data Link Layer

When obtaining data from the Physical layer, the Data Link layer checks for physical transmission errors and packages bits into data frames. The Data Link layer also manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of network devices to the physical medium.

Because the Data Link layer is the most complex layer in the OSI model, it is often divided into two parts: the Media Access Control sub-layer and the Logical Link Control sub-layer.

Network Layer

The Network layer adds the concept of routing above the Data Link layer. When data arrives at the Network layer, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination. If the data has reached the final destination, layer 3 formats the data into packets delivered to the Transport layer. Otherwise, the Network layer updates the destination address and pushes the frame down to the lower layers.

To support routing, the Network layer maintains logical addresses such as IP addresses for devices on the network. The Network layer also manages the mapping between these logical addresses and physical addresses. In IPv4 networking, this mapping is accomplished through the Address Resolution Protocol (ARP); IPv6 uses Neighbor Discovery Protocol (NDP).

Transport Layer

The Transport Layer delivers data across network connections. TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are the most common examples of Transport Layer 4 network protocols. Different transport protocols may support a range of optional capabilities, including error recovery, flow control, and support for re-transmission.

Session Layer

The Session Layer manages the sequence and flow of events that initiate and tear down network connections. At layer 5, it is built to support multiple types of connections that can be created dynamically and run over individual networks.

Presentation Layer

The Presentation layer has the simplest function of any piece of the OSI model. At layer 6, it handles syntax processing of message data such as format conversions and encryption/decryption needed to support the Application layer above it.

Application Layer

The Application layer supplies network services to end-user applications. Network services are protocols that work with the user's data. For example, in a web browser application, the Application layer protocol HTTP packages the data needed to send and receive web page content. This layer 7 provides data to (and obtains data from) the Presentation layer.

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OSI Seven Layers Model Explained with Examples

This tutorial explains the OSI reference model. Learn the seven layers of the OSI model and the functions of each layer in detail through examples.

The OSI (Open System Interconnection) reference model is a comprehensive set of standards and rules for hardware manufacturers and software developers. By following these standards, they can build networking components and software applications that work in any environment. It was published in 1984 by ISO (International Organization for Standardization).

It provides a framework for creating and implementing networking standards, devices, and internetworking schemes. It explains the networking from a modular perspective, making it easier to understand and troubleshoot.

Seven layers of the OSI Model

The OSI model has seven different layers, which are divided into two groups. The following table lists all the layers with their names and numbers.

Group	Layer Number	Layer Name	Description
Top Layers	7	Application	Provide a user interface for sending and receiving data
	6	Presentation	Encrypt, format, and compress data for transmission
	5	Session	Initiate and terminate a session with the remote system
Bottom Layers	4	Transport	Break the data stream into smaller segments and provide reliable and unreliable data delivery
	3	Network	Provide logical addressing
	2	Data Link	Prepare data for transmission
	1	Physical	Move data between devices

Let’s understand each layer in detail.

This tutorial is the second part of the article " Networking reference models explained in detail with examples. ". Other parts of this article are the following.

This tutorial is the first part of the article. It summarizes why the OSI model was created and what advantages it has.

This tutorial is the third part of the article. It compares the OSI reference model with the TCP/IP model and lists the similarities and differences between both.

This tutorial is the fourth part of the article. It explains the five layers of the TCP/IP model in detail.

This tutorial is the fifth part of the article. It explains how data is encapsulated and de-encapsulated when it passes through the layers.

The Application Layer

This is the last and topmost layer of the OSI model. This layer provides an interface between the local system and the application program running on the network. If an application wants to use the resources available on the remote system, it interacts with this layer. Then, this layer provides the protocols and services that the application needs to access those resources.

There are two types of application programs: Network-aware and Network-unaware . An application program is considered a Network-aware application if it can make any type of network request. If an application program cannot make any type of network request, it is considered a Network-unaware program.

Network-aware programs are further divided into two types.

Programs that are mainly built to work on a local system. This type of program occasionally accesses the network for particular reasons such as updates, documentation, and troubleshooting. MS-Word, Adobe-Photoshop, and VLC Player are examples of this type of program.

Programs that are mainly built to work with a remote system. This type of program provides a platform to access resources available on a remote system. This type of program only works if the system is connected to the network. SSH, FTP, and TFTP are examples of this type of program.

The Application layer describes only the programs which fall in the second type. But it doesn’t mean that the first type of programs can’t take the advantage of the Application layer. It simply means that they are not documented in the Application layer. But if required, they can also connect to the network through the Application layer.

The Top layer of the OSI model is the application layer. It provides the protocols and services that are required by the network-aware applications to connect to the network. FTP, TFTP, POP3, SMTP, and HTTP are examples of standards and protocols used in this layer.

The Presentation Layer

The sixth layer of the OSI model is the Presentation layer. Applications running on the local system may or may not understand the format that is used to transmit the data over the network. The presentation layer works as a translator. When receiving data from the Application layer, it converts that data in such a format that can be sent over the network. When receiving data from the Session layer, it reconverts the data in such a format that the application, which will use it, can understand.

Conversion, compression, and encryption are the main functions that the Presentation layer performs on the sending computer while on the receiving computer these functions are reconversion, decompression, and decryption. ASCII, BMP, GIF, JPEG, WAV, AVI, and MPEG are examples of standards and protocols that work in this layer.

The Session Layer

The session layer is the fifth layer of the OSI model. It is responsible for setting up, managing, and dismantling sessions between presentation layer entities and providing dialogs between computers.

When an application makes a network request, this layer checks whether the requested resource is available on the local system or on a remote system. If the requested resource is available on a remote system, it tests whether a network connection to access that resource is available or not. If a network connection is not available, it sends an error message back to the application informing that the connection is not available.

If a network connection is available, it establishes a session with the remote system. For each request, it uses a separate session. This allows multiple applications to send or receive data simultaneously. When data transmission is completed, it terminates the session.

The session layer is responsible for establishing, managing, and terminating communications between two computers. RPCs and NFS are examples of the session layer.

The Transport Layer

The transport layer is the fourth layer of the OSI model. It provides the following functionalities: -

Segmentation

On the sending computer, it breaks the data stream into smaller pieces. Each piece is known as a segment and the process of breaking the data stream into smaller pieces is known as the segmentation . On the receiving computer, it joins all segments to recreate the original data stream.

Data transportation

This layer establishes a logical connection between the sending system and receiving system and uses that connection to provide end-to-end data transportation. This process uses two protocols: TCP and UDP.

The TCP protocol is used for reliable data transportation. TCP is a connection-oriented protocol. UDP protocol is used for unreliable data transportation. UDP is a connection-less protocol.

The main difference between a connection-less and connection-oriented protocol is that a connection-oriented protocol provides reliable data delivery. For reliable data delivery, it uses several mechanisms such as the three-way handshake process, acknowledgments, sequencing, and flow control.

Multiplexing

Through the use of port numbers, this layer also provides connection multiplexing. Connection multiplexing allows multiple applications to send and receive data simultaneously.

The main functionalities of the Transport layer are segmentation, data transportation, and connection multiplexing. For data transportation, it uses TCP and UDP protocols. TCP is a connection-oriented protocol. It provides reliable data delivery.

The Network Layer

The third layer of the OSI model is the Network Layer. This layer takes the data segment from the Transport layer and adds a logical address to it. A logical address has two components; network partition and host partition. The Network partition is used to group networking components while the host partition is used to uniquely identify a system on the network. A logical address is known as the IP address. Once the logical address and other related information are added to the segment , it becomes the packet .

This layer decides whether the packet is intended for the local system or a remote system. It also specifies the standards and protocols which are used to move data packets over networks.

To move data packets between two different networks, a device known as the router is used. Routers use the logical address to make the routing decision. Routing is the process of forwarding data packets to their destination.

Defining logical addresses and finding the best path to reach the destination address are the main functions of this layer. Routers work in this layer. Routing also takes place in this layer. IP, IPX, and AppleTalk are examples of this layer.

The Data Link Layer

The Data Link Layer is the second layer of the OSI model. This layer defines how networking components access the media and what transmission methods they use. This layer has two sub-layers: MAC and LLC.

MAC (Media Access Control)

This sub-layer defines how the data packets are placed in media. It also provides physical addressing. The physical address is known as the MAC address. Unlike logical addresses that need to be configured, physical addresses are pre-configured in NIC. The MAC address is used to uniquely identify a host in the local network.

LLC (Logical Link Control)

This sub-layer identifies the network layer protocol. On the sending computer, it encapsulates the information of the Network Layer protocol in the LLC header from which the Data Link layer receives the data packet. On the receiving computer, it checks the LLC header to get the information about the network layer protocol. This way, a data packet is always delivered to the same network layer protocol from which it was sent.

Defining physical addresses, finding hosts in the local network, specifying standards and methods to access the media are the primary functions of this layer. Switching takes place in this layer. Switches and Bridges work in this layer. HDLC, PPP, and Frame Relay are examples of this layer.

The Physical Layer

The Physical Layer is the first layer of the OSI model. This layer specifies the standards for devices, media, and technologies that are used in moving the data across the network such as:-

Type of cable used in connecting the devices
Patterns of pins used in both sides of the cable
Type of interface-card used in the networking device
Type of connector used to connect the cable with the network interface
Encoding of digital signals received from the Data Link layer based on the attached media type such as electrical for copper, light for fiber, or a radio wave for wireless.

On the sending computer, it converts digital signals received from the Data Link layer, into analog signals and loads them on the physical media. On the receiving computer, it picks analog signals from the media and converts them into digital signals, and transfers them to the Data Link layer for further processing.

The Physical Layer mainly defines standards for media and devices that are used to move data across the network. 10BaseT, 10Base100, CSU/DSU, DCE, and DTE are examples of the standards used in this layer.

That’s all for this tutorial. In the next part of this article, we will compare the OSI model with the TCP/IP model and explains the similarities and differences between both models. If you like this tutorial, please don’t forget to share it with friends.

By ComputerNetworkingNotes Updated on 2024-06-09

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Network Basics: The 7 Layers of the OSI Model

In the 1980s, networking was in its infancy. Engineers needed a way to visualize the different elements of a networking system. The computing world urgently required a standard language to communicate across companies, business sectors and even cultures. The OSI model filled the gap, providing a functional way to describe and analyze network structures.

The 7-layer OSI model is now common knowledge across the world. Despite decades of IT development and the emergence of the internet, it remains relevant. This article will explain how the model works, why it is still useful -- when used carefully.

What is the OSI model (Open Systems Interconnection)?

The OSI (Open Systems Interconnection) model was first published in 1984 by the International Organization for Standardization (IOS). IOS sought to create a standardized language for network analysis and communication. The OSI model provided this language, enabling different devices and networks to transmit data smoothly.

The OSI model divides networking into 7 separate "layers". Each OSI model layer is part of a seven-stage stack. Information descends and ascends the stack as data flows through networks. In theory, the stacks represent critical processes in data transmission. These stages could include encryption, packet creation, flow management, and presentation.

How does the OSI model work?

The OSI reference model generally flows downwards from Level 7 (the Application Layer) to Level 1 (the Physical Layer).

Every stack in the model describes a stage in the journey of an idealized data packet through a communication system. In a typical transmission, data flows from Layer 7 downwards to Layer 1, and then back upwards to Layer 7 where it can be used by recipients.

The model layers communicate with each other. Each layer deals with levels directly above and below, creating a neat chain of activity.

This stacked construction makes sense from a troubleshooting perspective. Engineers can isolate problems at the network or application layer. Or they might look at physical medium issues such as cabling.

By focusing on specific network issues, engineers can reduce workloads and diagnose problems more effectively.

Importance of OSI Model

The OSI model was important because it represented the first systematic attempt to standardize networking language. The fact that the model became used worldwide shows that the creators succeeded. And forty years later the OSI concept still has many uses.

Troubleshooting -- The OSI hierarchy is a good shorthand for detecting network flaws. Technicians can use the model to detect network-wide problems, application issues, or faults in physical equipment. OSI provides a clear way to break problems down into manageable chunks.
Marketing – The OSI layer model allows software and hardware vendors to describe the functions of products. Marketers can clearly explain to buyers where their products fit into the OSI hierarchy. Buyers can understand how those products will fit into network architecture.
Software Development – The OSI model helps developers during planning and coding phases. Developers can model how applications will function at specific layers. The layer model provides a guide to how apps will interact with other network components.
Security awareness – The OSI system allows security teams to identify security vulnerabilities. Security teams can classify risks according to OSI layers. They can identify where data rests in the network hierarchy, and assign protective controls to ensure data security. OSI layers also help to stage secure cloud migrations.

The OSI hierarchy is just a model. But it is a very useful way to conceptualize network structures and connections between communication partners. The model makes it easier to compare applications, protocols, hardware profiles, and much more.

The OSI model provides a language for experts to use when discussing IT architecture. So while newer models have appeared, we still rely on the OSI template to understand networking.

Advantages of the OSI model

Despite being published in 1984, the OSI concept has many advantages. Put simply, the OSI model:

Helps with sourcing hardware and software to build network architecture
Assists IT teams in understanding how network components communicate
Allows experts to troubleshoot network problems
Makes it easier to develop tools that can communicate with those from other vendors
Provides a way to communicate how software and hardware operate in networks. Allows technicians to talk to outsiders in a simple manner, with a reasonable degree of accuracy.

Disadvantages of the OSI model

The advantages above are significant but need to be qualified with a few important drawbacks:

Most networking experts argue that the OSI system is outdated. The division of network structures into 7 different layers no longer makes sense in the age of the internet and cloud computing. The internet generally suits the TCP/IP model more closely than OSI.
The 7-layer model may also feature redundant elements. For instance, the Session Layer and Presentation Layer may not have practical relevance in real-world networks.
Some network functions reach across OSI layers, creating unnecessary confusion.

7 layers of the OSI model and their functions

The 7 layers of the OSI model are usually viewed from 7 downwards. So it makes sense to explain each one as data descends the hierarchy.

Layer 7 – The Application Layer

The application layer is where users interact with data. This does not include all applications at the edge of the network. For instance, email clients or video conferencing apps would not be included. Instead, the application layer includes the software used to allow network-facing apps to function.

Application layer functions include the operation of protocols and data formatting tools. Common layer 7 protocols include SMTP and HTTP. The function of the application layer is to accept data for software to use, or to carry out preparations before sending data down the OSI chain.

Layer 6 – The Presentation Layer

The presentation layer manipulates data before the application layer can use it. This layer "presents" raw data. The presentation layer turns it from a bitstream into something that applications can decode and use.

The presentation layer is important in a security context. This is the stage where data is encrypted and compressed (or decrypted and decompressed). Data encryption allows secure transmission. Compression allows networks to transmit more data at higher speeds.

Layer 5 – The Session Layer

When data is transferred in computer networking, two devices agree to create a "session." The session layer applies agreed rules about how data will be transmitted and authenticated. It expires when the transmission is complete.

The Session Layer is responsible for commencing communication between devices. It determines how long sessions last and checks that data is transmitted accurately. This generally involves the use of data checkpoints. Checkpoints break down data into smaller segments. Each segment is checked for fidelity before the session closes.

The Session Layer has a security function. Sessions must close quickly and include authentication systems to identify data sources and recipients. But the main function of the session layer is ensuring efficient data transfer with minimal resource use.

Layer 4 – The Transport Layer

The Transport Layer involves setting up direct communication between connected devices. This layer may also break down data, an operation that reaches across OSI layers. But the overall function of the Transport Layer is ensuring that data leaves and arrives in the same condition.

The Transport Layer controls the flow of data in end-to-end communication. Tools decide the correct speed for data transmission. This may vary depending on the connection speeds involved. Devices with faster connections can flood those with slower speeds, creating performance issues.

The transport layer also carries out error control. Error control tools assess data packets at the receiving device. If data arrives in poor quality, Transport Layer tools will request a repeat transmission.

Well-known Transport Layer protocols include the Transmission Control Protocol (TCP). This protocol functions alongside Internet Protocol (IP) information, forming the TCP/IP standard.

Layer 3 – The Network Layer

The Network Layer is where data is actually sent between connected devices. This makes the network layer a common area of focus for network engineers, and one of the most important nodes in the OSI chain.

The role of the Network Layer is to create and maintain stable network connections. Data is divided into packets that are ready for network transmission. These packets are then put back together at the receiving end of the transmission, reconstituting the original data.

Hardware and software tools at the network layer are also responsible for routing data. Routers decide an optimal route for a data transfer. At Layer 3, routing generally involves communication between different networks. Layer 2 tends to deal with local data routing.

Layer 2 – The Data Link Layer

The Data Link layer is closely related to the Network Layer but usually refers to communication between locally-connected devices. For instance, the data link layer might model connections between on-premises workstations and routers.

At the data link layer, data is accepted and broken down into frames. Frames are suited to local transmission, and interact with two sub-layers of the data link layer:

Media Access Control (MAC) layer – The media access control layer connects related local devices and manages flow rates across the network.
Logical Link Control (LLC) layer – Sets up the logical basis for local data transmission.

The data link layer regulates flows between local devices in a similar way to the network layer. The two layers are therefore often analyzed together when assessing network problems.

Layer 1 – The Physical Layer

The Physical Layer covers all of the physical infrastructure and equipment needed to transfer data. The physical layer includes network cables and switches, as well as radio frequency links, voltage regulators, and routing devices.

Data is converted into a digital bitstream formed from 1s and 0s at the physical layer. The form of this bitstream is agreed by two devices before transmission. This makes it possible to reconstruct data at the receiving end.

The Physical Layer is often the first place to look when troubleshooting networks. Cable connections and faulty power supplies are common problems with relatively simple solutions.

Cross-layer functions

Many applications or services bridge different layers in the OSI hierarchy. These services are called cross-layer functions. Cross-layer functions include critical services that affect multiple parts of the data transmission process. Examples could include:

Security management tools to configure and monitor communications between network devices.
Multi-protocol label switching (MPLS) services to carry data frames between networks.
Protocols that translate IP addresses into MAC addresses and work across the data link layer and the network layer.
Domain Name System (DNS) lookup services.
General security architecture recommended by ITU's x.800 standard.

Cross-layer functions tend to deliver security, availability or reliability. They work across network layers to regulate and monitor traffic, ensuring data security and resolving problems as they arise. Because of this, cross-layer services are a core part of network security planning.

OSI Model vs TCP IP Model

The Transfer Control Protocol/Internet Protocol (TCP/IP) model is the major alternative to the OSI reference model.

TCP/IP actually pre-dates OSI, and was created by the US Department of Defense in the 1970s. Many argue that the emergence of the internet as the dominant form of telecommunication has made TCP/IP more useful as a way of describing network environments.

The main difference between the TCP/IP and OSI models is the number of layers they include. OSI includes 7 layers. TCP/IP removes OSI layers 5-7 and blends them into a single application layer. OSI layers 1 and 2 are also combined in a Network Interface Layer.

The TCP/IP model tends to be a good fit for networks extended across the public internet. It also accurately models the operation of internet communication protocols. OSI is a much more general model. It does not refer to any specific protocols. Instead, the OSI reference model describes network communication as a whole.

TCP/IP is more focused on practical operations. All of the layers are used by relevant applications. In the OSI model, applications may only use a few of the layers. Layers 1-3 are the only essential elements in transmitting data.

In practice, security architects can learn from both models. OSI remains valuable in comparing products and troubleshooting networks. Both the TCP/IP model and the OSI model have roles to play in the way we visualize network security.

OSI Model PPT: Definition, Layers and Purpose

OSI Model PPT: Definition, Layers and Purpose Free Download: The Open Systems Interconnection version (OSI version) is a conceptual version that ‘gives a common foundation for the coordination of [ISO] requirements improvement for the reason of structures interconnection’.

In the OSI reference version, the communications among a computing machine are break up into seven specific abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

It is a 7 layer structure with every layer having unique capability to perform. All those 7 layers work collaboratively to transmit the statistics from one person to some other throughout the globe.

Table of Content

Introduction
Layers of OSI Model
Purpose of OSI Model

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What is the OSI Model? 7 Layers of OSI Model Explained

What is the osi model, 7 layers of the osi model, osi vs tcp model, frequently asked questions.

As a tech expert, I have worked closely with the OSI model and layers to make it easier for individuals to transfer data and information from one computer to another. This is a seamless way of data transfer that also ensures data security. The OSI model is made up of seven layers and each of the layers has certain distinct features that help the system to perform various network functions.

I have curated this detailed tutorial for you to better understand the different layers in OSI model as well as the function of OSI model in this ever-evolving technological era.

The Open System Interconnection model, as the name suggests, is an abstract structure that makes it possible for different systems of communication to interact using common standard protocols. In simple words, the OSI paradigm offers a norm that allows various computer systems to link and share information.

Here, I am attaching a descriptive OSI model diagram that will help you to better understand what the structure is about and how it actually works. The OSI model was developed by the International Organization for Standardization in 1984 with an aim to allow a variety of communication systems to interact with each other by employing certain standard protocols, also known as OSI model layers protocols. This model consists of seven descriptive layers that together make up the means to interact across a network.

This architectural model splits the assignment into seven simpler and manageable sub-tasks. Different layers in the OSI model have a specific task entrusted to it. Since every layer is standalone, tasks allocated to each layer can be handled and accomplished on their own.

Now that you are well-versed in the concept and significance of the OSI model, let me walk you through the different OSI model layers with examples.

Application Layer

The application layer in OSI model allows users and software operations to access network services. In my years of experience, this layer has dealt with concerns like allocation and distribution of resources, network transparency, seamless transmission of data, and so on.

However, the software applications are not inclusive of this layer. The application layer only assists you in carrying out application layer activities. It is the final users who acquire the network amenities through this layer. Application layer protocols encompass both HTTP and SMTP signals that permit communication via email.

In my early days in the tech field, the most important function of the application layer that I experienced was its ability to allow users to access, manage, and transfer files on a remote computer. This layer also allows users to forward and store email.

Presentation Layer

The presentation layer is popularly known as the syntax layer. This layer mostly deals with the Sintic and semantic components of the data communicated between two computer systems. The OSI model security architecture greatly depends upon the presentation layer as it operates as the data interpreter of a network.

The OSI model uses the data prepared by the presentation layer. You can make the presentation layer as secure as possible by encrypting the data that you want to transmit to another device.

Session Layer

This is the third layer in the OSI model, which is primarily used to build, sustain, and coordinate the interaction between two or more devices. This layer is in charge of establishing and terminating communication among the devices. The session layer, as the name suggests, denotes the period of time between the start and completion of a communication to avoid the wastage of resources.

This particular layer will allow you to remain in session for a communication and it will end the communication after the transmission of data.

Transport Layer

The OSI model, transport layer is one of the most crucial layers in the entire structure. Earlier I used to find it very difficult to understand the mechanism of this layer. Hence, I am explaining the functionalities of the transport layer in the easiest manner.

This layer guarantees that there is no redundancy of data and that the messages are dispatched and delivered in the particular sequence as they were sent. The primary responsibility of the transport layer is to ensure that the transmission of information is done properly and completely. Whenever you try to send a message, this layer breaks it into smaller segments and sends it in a sequence.

This layer is popularly known as end to end layer because it builds a dependable point-to-point connection between the place of origin, origin, and destination in order to transmit the data.

Network Layer

The network layer in OSI model keeps track of the location of various devices on a particular network. This is done so that your system can find the best path through which it can move the data packets from one system to another.

It helps decide the path via which the data will travel from the source to the destination. This layer also converts the information into small data packets which makes it easier to travel through the route channels.

Data Link Layer

The data link layer in OSI model has the duty to monitor and ensure error-free transmission of data. This layer builds trust and reliability in the communication channel between two or more computer systems.

This data layer in OSI model helps uniquely identify and locate every device existing on the given local network. This layer defines the hardware destination and the address of the originator.

Physical Layer

The physical layer, also layer 1 of the OSI model transmits small bits from one node to another. This is regarded as the lowest layer in the OSI model that sets up, promotes, and terminates the physical connection.

It specifies the necessities associated with the procedural and mechanical network connections. Throughout these years, the physical layer has mostly assisted me in deciding the kind of signal that I should use while transferring data.

We know the TCP/IP model as a new-age model which handles communication over the Internet. However, the OSI model is quite different from the TCI model.

Let me illustrate the differences between the OSI model and TCP/IP model:

OSI model	TCP/IP model
It consists of seven layers	The TCP/IP model has a four-layer structure
It follows a horizontal strategy	It implements a vertical methodology
The OSI model is considered a reference model, which implements protocol-independent standards	The TCP/IP model is regarded as a foundation upon which the Internet has been established
Protocols in the OSI model can get easily affected because of changes and technology as they remain hidden	On the other hand, changing or replacing the protocols is not so easy in the TCP/IP model
The OSI model was developed even before the protocols were established	In the TCP/IP model, the protocols were developed first and then the model came into existence
The OSI model is an ideal paradigm and is considered to be an abstract concept	The TCP/IP model has a true existence and is used in real-time.
This model is not compatible with internet technology.	The TCP/IP model is designed in a way that it aligns with the Internet standard.

Having worked so many years in the tech industry, I can say that the OSI model has genuinely helped me to work around network communication. Although it is incompatible with the Internet, it still has helped me a lot in understanding how communication within a network should be conducted.

Considering this context, you may check out the authentic and reputable certification programs offered by upGrad if you want to further develop your expertise. These courses will not only give you additional expertise but also a greater chance at a higher employment position because they are affiliated with prestigious universities and have curriculums designed by industry professionals.

Is the OSI Model still relevant today?

The TCP/IP model serves as the foundation of Internet programs as opposed to the OSI model. However, the OSI model is still used for facilitating communication and visualization of network processes.

Are there any limitations to the OSI Model?

The OSI structure has several drawbacks such as it is incompatible with the Internet, it does not back Internet-based protocols and applications such as HTTP, TCP, IP, UDP, etc.

What is the purpose of the OSI Model?

The OSI model serves as a handbook and a universal language to define computer networking. It offers a foundation for various computer systems to be able to interact with each other.

What are some examples of protocols that operate at the Network Layer?

The major used protocols at the network layer are IP and ICMP. Apart from these two, there are certain other protocols such as address resolution protocol, domain name system, border Gateway protocol, etc.

How does the OSI Model handle data transmission errors?

To explain the OSI model, it is important to know its capability of handling data transmission errors. The transport layer uses an error detection approach and correction methodology to make sure that the data is safe and secure during transmission.

How many layers are in the OSI model?

There are a total of seven layers in the OSI structure that provide a detailed approach to networking and communication.

Why is the OSI model used?

The OSI model combines computing operations into an integrated set of rules and standards in order to encourage portability and connectivity throughout different software and hardware components.

What are the 7 network protocols?

The are numerous network protocols, but the seven most common ones include, Hypertext Transfer Protocol (HTTP), Internet Protocol (IP), File Transfer Protocol (FTP), User Datagram Protocol (UDP), Simple Network Management Protocol (SNMP), Internet Control Message Protocol (ICMP) and Post Office Protocol (POP).

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OSI vs TCP/IP: Differences and Similarities

By: hostwinds team / august 29, 2024.

The Open Systems Interconnection (OSI) model and the TCP/IP (Transmission Control Protocol/Internet Protocol) model are two network communication frameworks that explain how data is transmitted between devices like phones, computers, and servers. Both models use a layered approach to help conceptualize the processes involved in data transmission and reception, though they differ in their levels of detail, number of layers, and practicality of real-world implementation.

7 Layers of the OSI Model

The OSI model is a conceptual framework that outlines seven distinct layers to help explain how networks interact and how data moves through them. While very useful for developing a broad understanding of network communications, it's more of a theoretical tool rather than a direct reflection of real-world network architectures. The model provides a structured way to think about the different functions involved in networking, but it doesn't enforce a strict set of protocols used in actual implementations.

Layer 1: Physical

The Physical Layer is the first level of the OSI model, and it's all about the actual transmission of raw data over a physical medium.

Here's what it handles:

Hardware and Technologies: It manages the physical components and technologies, like cables and wireless signals, that move raw binary data (bits) from one place to another.

Communication Properties: It defines the electrical, optical, and mechanical properties needed for successful communication.

Data Encoding: This layer takes care of how data is encoded into signals for transmission.

Synchronization: It ensures that data transmission is perfectly synchronized between devices.

In short, the Physical Layer deals with the nuts and bolts of sending data from one device to another.

Layer 2: Data Link

The Data Link Layer is the second level of the OSI model and is responsible for the transfer of data packets between devices on the same network.

It handles:

Framing: It packages raw data into frames, making it ready for transmission over the physical layer.

Error Detection and Correction: This layer detects errors in transmitted data and corrects them, ensuring data integrity.

MAC Addressing: It uses MAC (Media Access Control) addresses to identify devices on the same network segment, facilitating communication between them.

Flow Control: It regulates the data flow to prevent overwhelming the receiving device.

This layer essentially makes sure that data sent from one device arrives intact and in the correct sequence to the next device on the network.

Layer 3: Network

The Network Layer is responsible for routing data between devices across different networks. Its key functions include:

Routing: It determines the best path for data to travel from the source to the destination across multiple networks.

Logical Addressing: It assigns and manages IP addresses, allowing devices to be uniquely identified on the network.

Packet Forwarding: This layer breaks down data into packets and forwards them to their destination.

Handling Congestion: It manages network congestion to ensure data flows smoothly.

Think of the Network Layer as the GPS of the network, guiding data to where it needs to go.

Layer 4: Transport

The Transport Layer focuses on reliable data transfer between devices, regardless of the underlying network. It manages:

Segmentation and Reassembly: It breaks down large messages into smaller segments for transmission and reassembles them at the destination.

Error Detection and Recovery: This layer detects any errors during transmission and retransmits data if necessary.

Flow Control: It controls the rate of data transmission to prevent overwhelming the receiver.

Connection Management: It establishes, maintains, and terminates connections between devices.

In short, the Transport Layer is responsible making sure that data arrives accurately and in the correct order (e.g. TCP, UDP).

Layer 5: Session

The Session Layer is responsible for establishing, managing, and terminating connections between applications on different devices.

Session Connection: It sets up and coordinates communication between devices.

Session Maintenance: It keeps the session active while data is being exchanged and synchronizes data flow.

Session Termination: This layer gracefully closes the session once communication is complete.

Synchronization: Ensures data is synchronized by managing checkpoints and recovery.

In essence, the Session Layer is like the conversation manager, keeping communication organized and on track.

Layer 6: Presentation

The Presentation Layer is responsible for translating, encrypting, and compressing data to ensure it is properly formatted for application use.

It takes care of:

Data Translation: It converts data between the format used by the application layer and the format used by the network.
Data Encryption/Decryption: It ensures data security by handling encryption before transmission and decryption upon reception.
Data Compression: This layer compresses data to reduce the amount of data that needs to be transmitted.

In short, the Presentation Layer makes sure that data is in the right format and secure before it's sent or received (e.g., SSL/TLS).

Layer 7: Application

The Application Layer is the interface through which end-user applications interact with the network.

Network Services: It provides services like email, file transfer, and web browsing, directly to end users.

Data Representation: It ensures that data is presented in a way that applications and users can understand.

User Interface: This layer interacts with the software applications that users use to access the network.

Simply put, the Application Layer is the point where users and software applications access the network and its services (e.g., HTTP, FTP).

TCP/IP Model

Unlike the OSI model, the TCP/IP model is a real-world model used to design and implement based on protocols that are actually used in the Internet and other networks. It consists of four layers and provides a more direct approach to data transmission, encompassing real protocols and standards used in today's networking.

Layer 1: Network Interface

The Network Interface Layer, also known as the Link Layer, combines aspects of the OSI Physical and Data Link layers, dealing with hardware and data framing (e.g., Ethernet, ARP). It's also responsible for addressing and error detection at the local network level.

The Network Interface Layer deals with:

Physical Transmission: Oversees the actual transmission of data over the network medium (e.g., cables, wireless signals).

Frame Handling: Packages data into frames for transmission and unpacks it at the receiving end.

MAC Addressing: Uses MAC addresses to identify devices on the same network for accurate delivery.

Error Detection: Ensures that data is transmitted accurately, detecting and correcting errors at the local network level.

In essence, the Link Layer handles the nuts and bolts of getting data from one device to another within the same network.

Layer 2: Internet

Corresponding to the OSI Network layer, the TCP/IP's Internet Layer is responsible for routing data packets across networks. The IP (Internet Protocol) operates at this layer to direct data from the source to the destination across different networks.

Internet Layer's key roles include:

Routing: Determines the best path for data to travel across multiple networks.

IP Addressing: Manages IP addresses, allowing devices to be uniquely identified on the network.

Packet Handling: Breaks data into packets for transmission and handles their delivery across different networks.

In short, the Internet Layer is like the traffic controller, directing data across various networks.

Layer 3: Transport

Similar to the OSI Transport layer, TCP/IP's Transport Layer handles data transfer between devices, managing data flow and reliability.

The Transport layer handles:

Data Transfer: Uses protocols like TCP and UDP for reliable, ordered delivery and faster, connectionless communication, respectively.

Segmentation and Reassembly: Breaks data into segments for transmission and reassembles them at the destination.

Error Detection and Correction: Identifies and corrects errors in data transmission.

Flow Control: Regulates data flow to prevent congestion and ensure smooth communication.

In essence, the Transport Layer makes sure data gets where it needs to go accurately and reliably.

Layer 4: Application

Encompassing the OSI Session, Presentation, and Application layers, The Application Layer in the TCP/IP model is where network applications and user services operate. (e.g., HTTP, FTP, SMTP).

User Interaction: Provides the interface for users to interact with network services, like web browsing, email, and file transfers.

High-Level Protocols: Supports protocols like HTTP, FTP, SMTP, and DNS that facilitate different network services.

Data Representation: Ensures data is formatted properly for both communication and user understanding.

In short, the Application Layer is where users and software applications connect with the network.

OSI Model vs TCP/IP Model

Now that we know how each model works, let's go over a few of the key differences between them.

Layer Functionality

Structured Layered Approach: Clearly defines each layer's functionality and interactions.
Detailed Layers: Includes more layers with specific functions, providing a more granular approach.
Pragmatic Approach: Focuses on practical aspects and real-world implementations.
Simplified Layers: Fewer layers that combine multiple functions, making it more straightforward and adaptable.

Development and Use

Theoretical Framework: Developed by the International Organization for Standardization (ISO) as a theoretical model for understanding network communication.
Educational Use: Often used as a reference model for teaching and understanding network protocols.

TCP/IP Model:

Practical Implementation: Developed by the U.S. Department of Defense for practical implementation in the ARPANET, the precursor to the modern internet.
Widely Used: Forms the basis of the internet and most modern network architectures.

Protocol Specificity

Protocol-Agnostic: Designed to be independent of specific protocols, providing a general framework for understanding how different protocols interact.
Protocol-Specific: Directly associated with the TCP/IP protocol suite, reflecting the protocols used in real-world network communication.

Flexibility and Adaptability

More Rigid: Provides a structured and detailed approach, which can be less flexible in accommodating new protocols.
More Flexible: Adapted to real-world use and can accommodate new protocols and technologies as needed.

Written by Hostwinds Team / August 29, 2024

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What is Layer 7?

Layer 7 refers to the Application Layer in the OSI networking model. It is the top layer of this network model and deals with standard protocols that users interact with directly, such as HTTP traffic for web browsing.

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Il Modello OSI

The Open Systems Interconnection (OSI) model is a conceptual model for how network traffic is structured. The seven layers of the OSI model include:

Physical Layer: Moves data over a physical medium via electrons, light, etc.
Data Link Layer: Transfers data between nodes, managing the physical medium and error correction. Ethernet is a Layer 2 protocol.
Network Layer: Manages network addressing and routing to move data between networks. IP is a Layer 3 protocol.
Transport Layer: Uses protocols such as TCP and UDP to transmit data between systems and may offer error correction.
Session Layer: Manages connections and sessions between two computers.
Presentation Layer: Performs data encryption, compression, and formatting to ensure data is translated properly between network and application.
Application Layer: Enables end-user software applications to send and receive data over the network.

Importance of Layer 7

Layer 7 is the highest layer of the OSI model and deals with applications that interact with the user directly.

Lower application levels of the OSI model are concerned with ensuring that data gets where it needs to go and is formatted appropriately. Layer 7 is where applications that interact with the user operate. For instance, when browsing the web, a user will be using the HTTPS web protocol to communicate with the remote web server.

HTTPS is a Layer 7 protocol whose traffic is encapsulated within lower-layer protocols, such as:

These protocols are responsible for ensuring that data gets from a particular application on the client computer to a particular application on the server, while HTTPS carries the actual data that makes the web browsing session work.

Load Balancing at Layer 7

An organization may choose to implement load balancing at Layer 7 of the OSI model. This means that legitimate traffic for a single application is distributed across multiple different servers, ensuring that they’re not overloaded.

Therefore, load balancing improves overall application performance. From a user’s perspective, all of the servers behind a Layer 7 load balancer are indistinguishable since they’d have the same public-facing IP address and port numbers. But, the load balancer can route the traffic to servers based on utilization.

Additionally, the load balancer may use cookies or other information included in requests to ensure that traffic from the same session goes to the same server, enabling caching and optimizing the service.

Load balancing can also happen at Layer 4, the Transport Layer of the OSI model. In this case, different upstream servers would use different TCP/ UDP ports, enabling a load balancer to quickly send traffic from the same session to the same server without inspecting its actual contents. However, this approach offers less granular control over the sessions sent to each backend server.

Protecting Against DDoS attacks

Layer 7 is also relevant in the context of distributed denial-of-service (DDoS) attacks . In DDoS application layer attacks, an attacker-controlled botnet attempts to render a target service unavailable to users and customers. DDoS attacks can occur at multiple different layers of the OSI model. One approach is to attempt to overwhelm a system with the sheer volume of requests.

These attacks operate at Layers 3 (Network) and 4 (Transport) of the OSI model. For instance, a SYN flood attack exhausts the number of TCP sessions that a server keeps open at one time.

A SYN Flood is a type of DDoS attack that overwhelms a server with connection requests, making the server unavailable to legitimate customers.

Normally, a client sends a SYN (Synchronize) message to a server, to request a connection to a server.
The server acknowledges this request by sending a SYN-ACK message back to the client.
Then, the client normally responds with an ACK (acknowledgment), and the connection is established.

However, in the case of SYN Flood attacks, the DDoS attacker sends a barrage of SYN requests to the server but purposefully does not reply with a final ACK to any of the SYN-ACK messages sent by the server. As a result, the server is stuck waiting for a large volume of ACK responses that never arrive from the client.

This process overwhelms the servers’ limited compute resources as they are tied up trying to manage a huge volume of half-open connections. This is why SYN Flood attacks are also known as ‘half-open attacks’.

Layer 7 DDoS Attack

Layer 7 DDoS attacks are designed to exploit vulnerabilities and bottlenecks in particular applications or services. For example, HTTP flood attacks try to send a web server more HTTP requests than it can handle. This may be substantially less than the number of simultaneous TCP sessions it can handle, making this a more efficient attack.

Different types of DDoS attacks have to be handled at different OSI layers. While many application firewalls can handle Layer 3/4 attacks, protecting against Layer 7 attacks requires a Layer 7 firewall that inspects and understands application-layer data.

Le soluzioni Check Point e il Modello OSI

Companies can suffer cyberattacks that operate at multiple different layers of the OSI model. For example, DDOS attacks can be performed at Layers 3, 4, or 7. Each of these types of attacks operates differently, and a network security solution providing protection only at Layers 3 and 4 will be blind to attacks occurring at Layer 7.

Check Point next-generation firewalls (NGFWs) provide protection at multiple layers of the OSI model, including the ability to inspect and understand network packet payloads to offer application-layer protection. Learn more about the Layer 7 protection that Check Point Quantum Force NGFWs provides by signing up for a free demo .

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What Is the OSI Model?

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Sicurezza della rete vs sicurezza dell'applicazione

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Was ist das OSI-Modell?

Das osi-modell bildet die grundlage für technische kommunikation. das müssen sie über das open systems interconnection model wissen..

Die sieben Säulen der Weisheit sehen (manche) IT-Experten auch im OSI-Modell.

Foto: WitR – shutterstock.com

Ab 1977 wurde das Open Systems Interconnection Model – zu deutsch OSI-Modell – entwickelt. Seit dem Jahr 1984 ist das OSI-Modell von der International Organization for Standardization (ISO) offiziell als Standard anerkannt.

OSI-Modell – Definition

Dabei bildet das OSI-Modell einen konzeptionellen Rahmen für Netzwerk- oder Telekommunikationssysteme, der sich über sieben verschiedene Schichten mit jeweils eigener Funktion erstreckt. Die unterschiedlichen OSI-Schichten helfen dabei, die Vorgänge innerhalb eines Netzwerks zu visualisieren und können auch dabei unterstützen, Probleme einzugrenzen.

Die Anbieter neuer, technischer Produkte beziehen sich oft auf das Open Systems Interconnection Model, um den Kunden zu vermitteln, mit welcher Schicht ihre Produkte arbeiten oder ob sie “über den OSI Stack” funktionieren. Zwar wird das OSI-Modell aufgrund seines konzeptionellen Charakters teilweise als veraltet und weniger bedeutsam als das vierschichtige TCP/IP-Modell erachtet. Nichtsdestotrotz ist das Open Systems Interconnection Model für das Verständnis von Netzwerktechnologien bedeutsam. Wenn Sie das Modell und seine sieben Schichten verstehen, können Sie auch nachvollziehen, welche Protokolle und Geräte miteinander interagieren können.

OSI-Schichten – Layer 7 bis 1

Dass das OSI-Modell sieben Schichten aufweist, darüber war man sich nicht immer einig: 1983 wurden zwei getrennte Modelle zum Open Systems Interconnection Model vereinigt. Die meisten Beschreibungen des OSI-Modells erfolgen von “oben” nach “unten”:

7. Anwendungsschicht / Application Layer

Die Anwendungsschicht ist dem Endbenutzer im OSI-Modell am nächsten. Sie empfängt Informationen direkt von den Benutzern und stellt die eingehenden Daten dar. Die Applikationen selbst sind wider Erwarten nicht auf der Anwendungsschicht angesiedelt. Stattdessen erleichtert der Application Layer die Kommunikation durch niedrigere Schichten, um Verbindungen mit Anwendungen am anderen Ende herzustellen. Beispiele für Kommunikationsprozesse, die auf dieser OSI-Schicht ablaufen, sind etwa Webbrowser oder FTP-Clients.

6. Darstellungsschicht / Presentation Layer

Der Presentation Layer ist von der Datendarstellung auf der Anwendungsschicht unabhängig. Allgemein gesprochen, wird hier die Übersetzung vom Anwendungsformat in das Netzwerkformat vorbereitet. Anders ausgedrückt “präsentiert” die Darstellungsschicht Daten für Applikationen oder Netzwerke. Ein gutes Beispiel hierfür ist die Ver- beziehungsweise Entschlüsselung von Daten zur sicheren Übertragung.

5. Sitzungsschicht / Session Layer

Wenn zwei Rechner oder andere, vernetzte Geräte miteinander kommunizieren müssen, muss dazu eine Sitzung eingerichtet werden. Das geschieht auf dem Session Layer. Die Funktionen auf dieser OSI-Schicht umfassen dabei Setup, Koordinierung und die Beendigung der Session.

4. Transportschicht / Transport Layer

Auf dem Transport Layer werden Datentransfers zwischen Systemen und Hosts koordiniert. Dabei geht es beispielsweise darum, wie viele Daten mit welcher Geschwindigkeit wohin gesendet werden sollen. Das prominenteste Beispiel für die Transportschicht ist das Transmission Control Protocol (TCP), das auf dem Internet Protocol (IP) aufbaut und allgemein als TCP/IP bezeichnet wird. TCP- und UDP-Portnummern arbeiten auf Schicht 4, während IP-Adressen auf Schicht 3 arbeiten.

3. Vermittlungsschicht / Network Layer

Im Grunde ist die Vermittlungsschicht für die Weiterleitung von Datenpaketen zuständig – einschließlich der Weiterleitung durch verschiedene Router . Wenn Ihr Computer in München eine Verbindung zu einem Server in den USA herstellen möchte, stehen dazu Millionen verschiedener Pfade zur Verfügung. Auf dem Network Layer wird sichergestellt, dass der effizienteste Pfad gewählt wird.

2. Sicherungsschicht / Data Link Layer

Der Data Link Layer sorgt für die Datenübertragung zwischen (miteinander direkt verbundenen) Knoten und übernimmt außerdem die Fehlerkorrektur des Physical Layer. Dabei existieren zwei “Unterschichten”: die Media Access Control (MAC)-Schicht und die Logical Link Control (LLC)-Schicht. In der Netzwerkwelt arbeiten die meisten Switches auf Schicht 2, einige aber auch auf Layer 3, um Support für virtuelle LANs bieten zu können, die sich wiederum über mehr als ein Switch-Subnetz erstrecken können und deshalb Routing-Funktionen erfordern.

1. Bitübertragungsschicht / Physical Layer

Der Physical Layer repräsentiert die elektrische und physikalische Darstellung eines Systems. Das kann vom Kabeltyp über die Funkverbindung (wie in einem Wi-Fi-Netzwerk) bis hin zur Anordnung der Stifte, Spannungen und anderen physikalischen Anforderungen so gut wie alles umfassen. Tritt ein Netzwerkproblem auf, wenden sich viele Netzwerkexperten direkt an die Bitübertragungsschicht, um zu prüfen, ob alle Kabel richtig angeschlossen sind.

OSI-Modell – Merksätze

Wenn Sie die Schichten des OSI-Modells – aus welchem Grund auch immer – verinnerlichen wollen oder müssen, können Sie auf einen dieser praktischen Merksätze zurückgreifen (von Layer 1 nach Layer 7):

P lease d o N ot t hrow S alami P izza A way

P ew! d ead N inja t urtles S mell P articularly A wful

P eople d on’t N eed t o S ee P aula A bdul

P ete d oesn’t N eed t o S ell P ickles A nymore

Falls Sie lieber die deutschen Begriffe präsent halten wollen, empfiehlt sich diese Eselsbrücke (hier von Layer 7 nach Layer 1):

A lle d eutschen S tudenten t rinken v erschiedene S orten B ier

Dieser Beitrag basiert auf einem Artikel unserer US-Schwesterpublikation Network World. (fm)

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COMMENTS

Presentation Layer in OSI model
Prerequisite : OSI Model. Introduction : Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model. This layer is also known as Translation layer, as this layer serves as a data translator for the network. The data which this layer receives from the Application Layer is extracted and manipulated here as per the required ...
Presentation Layer
In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the ...
The OSI Model
The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each. ... OSI Layer 6. Layer 6 is the presentation layer. This layer is responsible for data formatting, such as character encoding and conversions, and data ...
Presentation layer
2. Data link layer. 1. Physical layer. v. t. e. In the seven-layer OSI model of computer networking, the presentation layer is layer 6 and serves as the data translator for the network. [ 2][ 3][ 4] It is sometimes called the syntax layer. [ 5]
What is the OSI model? How to explain and remember its 7 layers
The 7 layers of the OSI model. The layers (from bottom to top) are: Physical, Data Link, Network, Transport, Session, Presentation, and Application. It wasn't always this way. Conceived in the ...
What is OSI Model
Prerequisite : OSI Model Introduction : Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model. This layer is also known as Translation layer, as this layer serves as a data translator for the network. The data which this layer receives from the Application Layer is extracted and manipulated here as per the required form
Presentation Layer of the OSI Model
The presentation layer in the OSI model has five main functions within the frame of presentation layer protocols. Character Code Translation A character code is a representation of text using a ...
The OSI Model's 7 Layers Explained
This article explains the seven layers in the OSI model, a network communication framework that simplifies complex network interactions into a structured format. ... Layer 6: Presentation Layer. Role: The Presentation Layer acts as a translator, converting data formats from the application layer into a network-compatible format and vice versa ...
OSI Model
The OSI model consists of 7 layers: Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Datalink Layer, Physical Layer. Each of these layers has a different role to play, and they work collaboratively to transmit the data from one networking device to another.
OSI model
The Open Systems Interconnection (OSI) model is a reference model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection." [2] In the OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network ...
Presentation Layer in OSI Model
The presentation layer is the 6 th layer from the bottom in the OSI model. This layer presents the incoming data from the application layer of the sender machine to the receiver machine. It converts one format of data to another format of data if both sender and receiver understand different formats; hence this layer is also called the ...
The OSI Model & The 7 Layers Explained
In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Created at a time when network computing was in its infancy, the OSI was published in 1984 by the International Organization for ...
What Is the OSI Model?
The application layer is the OSI layer closest to the end user. It provides network services directly to user applications and facilitates communication between API endpoints and lower layers of the OSI model. In other words, software applications use the application layer to initiate communication with the network and send data to the presentation layer.
The OSI Model Layers from Physical to Application
Jerrick Leger. The Open Systems Interconnection (OSI) model defines a networking framework to implement protocols in layers, with control passed from one layer to the next. It is primarily used today as a teaching tool. It conceptually divides computer network architecture into 7 layers in a logical progression.
OSI Seven Layers Model Explained with Examples
The sixth layer of the OSI model is the Presentation layer. Applications running on the local system may or may not understand the format that is used to transmit the data over the network. The presentation layer works as a translator. When receiving data from the Application layer, it converts that data in such a format that can be sent over ...
OSI Model Layers and Protocols in Computer Network
OSI model, the transport layer is only connection-oriented. A layer of the TCP/IP model is both connection-oriented and connectionless. In OSI model, data link layer and physical are separate layers. In TCP data link layer and physical layer are combined as a single host-to-network layer. The minimum size of the OSI header is 5 bytes.
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The Open Systems Interconnection (OSI) model is a conceptual framework that divides network communications functions into seven layers. Sending data over a network is complex because various hardware and software technologies must work cohesively across geographical and political boundaries. The OSI data model provides a universal language for ...
7 Layers Explained: OSI Model Guide
The OSI model divides networking into 7 separate "layers". Each OSI model layer is part of a seven-stage stack. Information descends and ascends the stack as data flows through networks. In theory, the stacks represent critical processes in data transmission. These stages could include encryption, packet creation, flow management, and presentation.
OSI Model PPT: Definition, Layers and Purpose
OSI Model PPT: Definition, Layers and Purpose. In the OSI reference version, the communications among a computing machine are break up into seven specific abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. It is a 7 layer structure with every layer having unique capability to perform.
What is the OSI Model
The OSI model security architecture greatly depends upon the presentation layer as it operates as the data interpreter of a network. The OSI model uses the data prepared by the presentation layer. You can make the presentation layer as secure as possible by encrypting the data that you want to transmit to another device.
OSI vs TCP/IP: Differences and Similarities
The Open Systems Interconnection (OSI) model and the TCP/IP (Transmission Control Protocol/Internet Protocol) model are two network communication frameworks that explain how data is transmitted between devices like phones, computers, and servers. Both models use a layered approach to help conceptualize the processes involved in data transmission and reception, though they differ in their ...
What is Layer 7?
Presentation Layer: Performs data encryption, compression, and formatting to ensure data is translated properly between network and application. ... Lower application levels of the OSI model are concerned with ensuring that data gets where it needs to go and is formatted appropriately. Layer 7 is where applications that interact with the user ...
Was ist das OSI-Modell?
Ab 1977 wurde das Open Systems Interconnection Model - zu deutsch OSI-Modell - entwickelt. Seit dem Jahr 1984 ist das OSI-Modell von der International Organization for Standardization (ISO) offiziell als Standard anerkannt. ... Der Presentation Layer ist von der Datendarstellung auf der Anwendungsschicht unabhängig. Allgemein gesprochen ...

Layer 6 Presentation Layer

OSI Layer 6 - Presentation Layer

CASE common application service element

SASE specific application service element

Layer 7 Application Layer

The OSI Model – The 7 Layers of Networking Explained in Plain English

Prerequisites

Learning Objectives

Common Networking Terms

What is the OSI Model?

OSI Layer 1

How to Troubleshoot OSI Layer 1 Problems

OSI Layer 2

How to Troubleshoot OSI Layer 2 Problems

OSI Layer 3

How to Troubleshoot OSI Layer 3 Problems

OSI Layer 4

How to Troubleshoot OSI Layer 4 Problems

OSI Layer 5

How to Troubleshoot OSI Layer 5 Problems

OSI Layer 6

How to Troubleshoot OSI Layer 6 Problems

OSI Layer 7

How to Troubleshoot OSI Layer 7 Problems

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The 7 layers of the OSI model

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Layer 4: Transport

Layer 3: Network

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The OSI Model’s 7 Layers, Explained

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The 7 Layers of the OSI Model

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