presentation layer protocols in computer networks

  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

SASE specific application service element

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.

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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:

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 .

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:

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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• Network infrastructure

presentation layer

• Andrew Froehlich, West Gate Networks

What is the presentation layer?

The presentation layer resides at Layer 6 of the Open Systems Interconnection ( OSI ) communications model and ensures that communications that pass through it are in the appropriate form for the recipient application. In other words, the presentation layer presents the data in a readable format from an application layer perspective.

For example, a presentation layer program could format a file transfer request in binary code to ensure a successful file transfer . Because binary is the most rudimentary of computing languages, it ensures that the receiving device can decipher and translate it into a format the application layer understands and expects.

How the presentation layer works

Once the application layer passes data meant for transport to another device in a certain format, the presentation layer then prepares this data in the most appropriate format the receiving application can understand.

Common data formats include the following:

• American Standard Code for Information Interchange and Extended Binary Coded Decimal Interchange Code for text;
• JPEG , GIF and TIFF for images; and
• MPEG, MIDI and QuickTime for video.

Encryption and decryption of data communications are also performed at the presentation layer. Here, encryption methods and keys exchange between the two communicating devices. Only the sender and receiver can properly encode and decode data so it returns to a readable format.

The presentation layer can serialize -- or translate -- more complex application data objects into a storable and transportable format. This helps to rebuild the object once it arrives at the other side of the communications stream. The presentation layer also deserializes the data stream and places it back into an object format that the application can understand by the application.

The tool that manages Hypertext Transfer Protocol ( HTTP ) is an example of a program that loosely adheres to the presentation layer of OSI.

Although it's technically considered an application-layer protocol per the TCP/IP model , HTTP includes presentation layer services within it. HTTP works when the requesting device forwards user requests passed to the web browser onto a web server elsewhere in the network.

HTTP receives a return message from the web server that includes a Multipurpose Internet Mail Extensions ( MIME ) header. The MIME header indicates the type of file -- text, video, or audio -- that has been received so that an appropriate player utility can present the file to the user.

Functions of the presentation layer

• ensures proper formatting and delivery to and from the application layer;
• performs data encryption; and
• manages serialization of data objects.

Editor's note: This article was republished in January 2023 to improve the reader experience.

Continue Reading About presentation layer

• What is the difference between TCP/IP model vs. OSI model?
• Data and file formatting

Related Terms

Dig deeper on network infrastructure.

What are the most important email security protocols?

file extension (file format)

network protocol

MIME (Multipurpose Internet Mail Extensions)

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• Introduction To Computer Networks
• Uses of Computer Networks
• Line Configuration
• Types of Network Topology
• Transmission Modes
• Transmission Mediums
• Bounded/Guided Transmission Media
• UnBounded/UnGuided Transmission Media
• Types of Communication Networks
• Connection Oriented and Connectionless Services
• Quality of Service(QoS)
• Network Layer
• IGMP Protocol
• Reference Models
• Digital Transmission
• Multiplexing
• Circuit-Switched
• Message-Switched Networks
• Packet Switching
• Error Correction
• Data Link Control
• Flow and Error
• Simplest Protocol
• Stop-and-Wait Protocol
• Go-Back-N Automatic Repeat
• Sliding Window Protocol
• HDLC Protocol
• Point-to-Point Protocol
• Multiple Access in DL
• Channelization Protocols
• Gigabit Ethernet
• Random Access Protocol
• Controlled Access Protocols
• Carrier Sense Multiple Access
• Transport Layer
• Telnet vs SSH
• UDP Protocol
• TCP - Protocol
• Introduction to Reference Models
• OSI Model: Physical Layer
• OSI Model: Datalink Layer
• OSI Model: Network Layer
• OSI Model: Transport Layer
• OSI Model: Session Layer
• OSI Model: Presentation Layer
• OSI Model: Application Layer
• The TCP/IP Reference Model
• Difference between OSI and TCP/IP Model
• Key Terms - Computer Network
• Session Layer
• Components of Computer Networks
• Features of Computer Network
• Protocols and Standards
• Connection Oriented and Connectionless
• OSI Vs TCP/IP

Presentation Layer

• HTTP Protocol
• FTP Protocol
• SMTP Protocol
• POP Protocol
• SNMP Protocol
• Electronic Mail
• MIME Protocol
• World Wide Web
• DNS Protocol

In this tutorial, we will be covering the Presentation layer of the OSI reference model in Computer Networks.

The presentation layer is layer-6 of the OSI reference model . This layer mainly responds to the service requests from the application layer(that is layer-7) and issues the service requests to layer-6 that is (the session layer).

This layer mainly acts as the translator of the network. Another name of the presentation layer is the Syntax layer.

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. The presentation layer takes care that the data is sent in such a way that the receiver will understand the information(data) and will be able to use the data. Languages(syntax) can be different between the two communicating systems. Under this condition, the presentation layer plays a role as translator.

In order to make it possible for computers with different data representations to communicate, the data structures to be exchanged can be defined in an abstract way. The presentation layer manages these abstract data structures and allows higher-level data structures(eg: banking records), to be defined and exchanged.

We can say that the presentation layer may represent or encode the data in various ways (like data compression, data encryption). But the receiving device mainly decodes or converts the encoded message into its original form.

For the same data, the sender and receiver must need to agree upon a messaging format that is commonly known as the Presentation format.

Also, the presentation layer is a part of the operating system that mainly converts the data from one presentation format to another presentation format.

Protocols used at the Presentation layer

Given below are some of the protocols used at the presentation layer:

AFP(Apple filling protocol)

Secure Socket Layer(SSL)

FTP(file transfer protocol)

Lightweight Presentation Protocol(LPP)

SSH(Secure shell)

Functions of Presentation Layer

Translation: Before being transmitted, the information in the form of characters and numbers should be changed to bitstreams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format of the computer.

Encryption: It carries out encryption at the transmitter and decryption at the receiver.

Compression: It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text, etc.

Design Issues with Presentation Layer

To manage and maintain the Syntax and Semantics of the information transmitted.

Encoding data in a standard agreed-upon way. Eg: String, double, date, etc.

Perform Standard Encoding on the wire.

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The OSI Model

What is the osi model.

How a single bit travels from one computer to the next is a complex concept. In 1984, the open systems interconnection (OSI) model was published as a framework for network communication. The model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. The message is built as it moves down the protocol stack. However, it is not sent to another network until it reaches the physical layer.

The model helps IT, computer science, and cybersecurity professionals understand how a single bit travels from one computer to the next by breaking the system into these layers.

From physical devices to user interfaces (UI), this model explains the communication role of each layer in overall computer networking. This article will start by introducing the Physical Layer (Layer 1).

Layer 1: the physical layer

The physical layer is where data moves across network interfaces as digital signals. Additionally, this is where the transmitting and receiving of network communication occurs. Starting with the Application Layer the message moves down the OSI model, and it eventually reaches the Physical Layer for transmission. When the message is received by the physical layer, the message will then move up the OSI layers until it reaches the final application layer.

Layer 2: data-link layer

Electrical signals received (or transmitted) to the physical layer are linked and translated to digital logic in the data-Link layer . Computer devices may be networked at the Data-Link layer, but only as a Local Area Network (LAN). Connecting a LAN to another LAN occurs at Layer 3.

Within Layer 2, the Protocol Data Unit (PDU) known as a frame consists of a header, footer, and data. Understanding how a frame is structured is important for network traffic analysis.

Additionally, within Layer 2, physical addresses are assigned and are also known as MAC addresses and/or hardware addresses in networking. MAC addresses are unique to each device on a local network. They are 48-bits in length and are assigned in hexadecimal characters.

Some other things to note about Layer 2 is that there are a few protocols that reside in it that we should know about:

• Ethernet : The most common type of LAN, Ethernet is the standard used to connect computing devices, routers, and switches in a wired network.
• IEEE 802.11 : “Wi-Fi” or “Wireless LAN.”
• Fiber Distributed Data Interface (FDDI) : Network standard for fiber optic LAN connections.
• Link Layer Discovery Protocol (LLDP) : A Link Layer protocol used for advertising neighbors, identity, and capabilities on a LAN.
• Address Resolution Protocol (ARP) : Converts and links Internet Protocol (IP) addresses to MAC addresses on a LAN.
• Cisco Discovery Protocol (CDP) : Similar to LLDP, but Cisco proprietary. The protocol collects neighbor information of directly connected LAN devices.

Additionally, Layer 2 is split into two sublayers:

• Logical Link Control (LLC) : Responsible for establishing the logical link between devices on a local network.
• Media Access Control (MAC) : Responsible for the procedures used by devices across a network medium.

Layer 3: network layer

When we think of the internet, we are thinking of interconnected networks. Interconnecting networks refer to a Local Area Network (LAN) connection to neighboring or remote networks. Layer 3 of the OSI model, the network layer , is where internetworking takes place and is where logical addresses are assigned to networked devices. A primary function of this layer is to route network packets from one LAN to another. Routing requires IP addresses and logical mapping of other networks across the internet to properly deliver messages. Another important function of Layer 3 is its ability to fragment and reassemble large communication. When Layer 3 passes a message down to Layer 2 for transmission, message length limits may be encountered in some cases.

Additionally, Layer 3 is the layer where the protocols used to route communication between networks reside. A few common network protocols are:

• Internet Protocol (IP) : IPv4 and IPv6 are two versions of IP, and IPv4 is the most common protocol of the Internet .
• Internet Protocol Secure (IPSec) : A more secure version of IP which leverages cryptography.
• Routing Information Protocol (RIP) : Distance-vector routing protocol that uses hop count as a metric of routing.
• Enhanced Interior Gateway Routing Protocol (EiGRP) : Cisco proprietary. A distance-vectoring protocol used for automating network configurations and routing decisions.
• Internet Control Message Protocol (ICMP) : Network protocol used for error reporting of network issues.
• Border Gateway Protocol (BGP) : A routing protocol designed to exchange routing information automatically on the internet.

Within Layer 3, the Protocol Data Unit (PDU) is the packet . Packets encapsulate data intended for transmission with header and footer data.

The IPv4 protocol encapsulates data with IPv4 header information necessary for delivery. For example, the 32-bit packet format contains the source address, the destination address, protocol, time-to-live (TTL), etc. in the IPv4 header data.

Layer 4: transport layer

The transport layer , Layer 4, is responsible for being the go-between the abstract layers of the OSI model (Layers 7-5) and the concrete communication layers (Layers 3-1).

Depending on the type of application, the transportation of that application’s communication will need to be handled in a specific way. For example, basic web browsing communication uses Hypertext Transfer Protocol (HTTP) . HTTP communicates via a specific connection service type and port. The transport layer is responsible for delivering/receiving the HTTP communication and maintaining the connection throughout the HTTP communication.

The Protocol Data Unit (PDU) at Layer 4 is known as a data segment . Segmentation is the process of dividing raw data into smaller pieces. Once the raw data is packaged from the higher application layers it is segmented at the transport layer before being passed to the Network Layer.

The transport layer protocols are divided into two categories depending on their connection service type:

Connection-oriented services

This connection type establishes a logical connection between two devices prior to beginning communication across a network. Connection-oriented protocols typically maintain service connection by following a set of rules that initiate, negotiate, manage, and terminate the communication. The Transport Layer protocols will also retransmit any data that is received without acknowledgment. The most common Connection-Oriented protocol is the Transmission Control Protocol (TCP) and its process to manage a connection between two devices is called the Three-Way Handshake . In TCP communication, the communicating devices typically share a client/server relationship where a client initiates communication with a service. The handshake involves the process of sending special TCP messages to synchronize a state of negotiated connection in communication.

Connectionless services

In connectionless communication, the protocol does not establish a connection between client and server. Instead, once a request is made to the server, the server sends all data without initiation, negotiation, or management of connection. Connectionless protocols also do not attempt to correct any interruptions in data transmission. Once the server sends the data, the server is not concerned if the client receives it.

When TCP or UDP are used to establish communication, the communication is assigned a port as the Layer 4 address. A port is a logical assignment given to processes and their respective application protocols on a computing system. A few important facts to memorize about ports are:

• There are 65,535 valid port numbers available to assign to a communication process.
• Ports 0 - 1023 are Well-Known Ports : Assigned to universal TCP/IP application protocols. These protocols are the most common such as HTTPS, SSH, FTP, DNS, and the list goes on. They are registered to these protocols by a global
• Ports 1024 - 49,151 are Registered Ports : Reserved for application protocols that are not specified as universal TCP/IP application protocols.
• Ports 49,152 - 65,535 are Private/Dynamic Ports : These ports may be used for any process without the need to register the port with the global assigning authority.
• When TCP and IP are used together, a Layer 4 port and a Layer 3 IP address are assigned to the connection. This is called a socket. For example, 8.8.8.8:443 is a socket indicating that communication to IP address 8.8.8.8 is to connect to port 443 on the server.

Layer 5: session layer

The session layer starts, manages, and terminates sessions between end-user application processes. Sessions are considered the persistent connection between devices. A session is application-focused; sessions are not concerned with layers 1-4. Instead, the session layer controls dialog between two networked devices. It is considered to facilitate host-to-host communication. Sessions dialog may be controlled through synchronization checkpoints, and through management of communication modes. There are two modes of communication permitted at Layer 5:

• Half-Duplex : Communication travels in both directions between sender and receiver, but only one device may transmit a message at a time.
• Full-Duplex : Communication travels in both directions between sender and receiver, and messages may be sent simultaneously in either direction.

The session layer resembles a phone conversation. For example, when a person picks up a phone and calls someone else a session is created. Once the communication on the call is completed, the session is terminated by hanging up the phone. In computing, software applications are making the phone call and establishing a session.

Two common Layer 5 protocols still used today are:

• Remote Procedure Call (RPC)

Layer 6: presentation layer

The presentation layer is primarily responsible for presenting data so that the recipient will understand the data. Data formatting and encoding protocols apply at Layer 6 to ensure data is legible and presented properly in the application receiving it. Data compression is also a function of Layer 6. If necessary, data may be compressed to improve data throughput over network communication.

Some common Layer 6 protocols are ASCII , JPEG , GIF , MPEG , and PNG .

Another main function of the presentation layer is the encryption and decryption of data sent across a network. Most encryption communication protocols straddle multiple layers of the OSI model, but the actual encryption function is Layer 6.

Two of the most common secure communication protocols are:

• Transport Layer Security (TLS)
• Secure Socket Layer (SSL)

Layer 7: application layer

The topmost layer of the OSI model is the application layer . On computer systems, applications display information to the user via the UI.

Note : Software applications running on a computer are NOT considered to reside in the application layer. Instead, they leverage application layer services and protocols that enable network communication.

For example, the user can craft messages and access the network from the application layer. A web browser application allows a user to access a web page. The user may input information and receive information through the web browser. However, the application layer protocol HTTP performs the network communication function. The web browser and HTTP work closely together, and the distinction between the two may be subtle. Yet, HTTP is the web browsing protocol for all web browser applications. In contrast, no single web browser software exclusively utilizes HTTP.

HTTP is one of many common application layer protocols. Below are a few additional protocols to know. It is also good practice to memorize the associated port assigned to the protocols:

The OSI model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. Understanding the OSI model will help you communicate with other network technologists. Computer networking may seem complex, but, with a bit of study, you can gain this knowledge to become an effective Cybersecurity Analyst.

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Presentation Layer: Protocols, Examples, Services | Functions of Presentation Layer

Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model where all application programmer consider data structure and presentation, beyond of simply sending the data into form of datagram otherwise packets in between the hosts. Now, we will explain about what is presentation layer with its protocols, example, service ; involving with major functions of presentation Layer with ease. At the end of this article, you will completely educate about What is Presentation Layer in OSI Model without any hassle.

What is Presentation Layer?

Presentation layer is capable to handle abstract data structures, and further it helps to defined and exchange of higher-level data structures.

Presentation Layer Tutorial Headlines:

Let’s get started,   functions of presentation layer.

Presentation layer performs various functions in the OSI model ; below explain each one – 

Protocols of Presentation Layer

Example of presentation layer protocols:.

Here, we will discuss all examples of presentation layer protocols; below explain each one –  

Multipurpose Internet Mail Extensions (MIME) : MIME protocol was introduced by Bell Communications in 1991, and it is an internet standard that provides scalable capable of email for attaching of images, sounds and text in a message.

Network News Transfer Protocol (NNTP) : This protocol is used to make connection with Usenet server and transmit all newsgroup articles in between system over internet.

Apple Filing Protocol (AFP ) : AFP protocol is designed by Apple company for sharing all files over the entire network .

NetWare Core Protocol (NCP) : NCP is a Novell client server model protocol that is designed especially for Local Area Network (LAN). It is capable to perform several functions like as file/print-sharing, clock synchronization, remote processing and messaging.

Network Data Representation (NDR) : NDR is an data encoding standard, and it is implement in the Distributed Computing Environment (DCE).

Tox : The Tox protocol is sometimes regarded as part of both the presentation and application layer , and it is used for sending peer-to-peer instant-messaging as well as video calling.

eXternal Data Representation (XDR) : This protocol provides the description and encoding of entire data, and  it’s main goal is to transfer data in between dissimilar computer architecture.

Presentation Layer Services

Design issues with presentation layer, faqs (frequently asked questions), what is meant by presentation layer in osi model.

Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model that is the lowest layer, where all application programmer consider data structure and presentation, beyond of simply sending the data into form of datagram otherwise packets in between the hosts.

What protocols are used in the presentation layer?

Can you explain some presentation layer examples, what are the main functions of the presentation layer, what are services of presentation layer in osi.

Presentation layer has a responsibility for formatting, translation, and delivery of the information for getting to process otherwise display .

Now, i hope that you have completely learnt about what is presentation layer with its protocols, example, service ; involving with major functions of presentation Layer with ease. If this post is useful for you, then please share it along with your friends, family members or relatives over social media platforms like as Facebook, Instagram, Linked In, Twitter, and more.

Also Read: Data Link Layer: Protocols, Examples | Functions of Data Link Layer

Related posts.

Presentation Layer

Last Edited

What is the Presentation Layer?

Presentation Layer is the Layer 6 of the seven-layer Open Systems Interconnection (OSI) reference model . The presentation layer structures data that is passed down from the application layer into a format suitable for network transmission. This layer is responsible for data encryption, data compression, character set conversion, interpretation of graphics commands, and so on. The network redirector also functions at this layer.

Presentation Layer functions

• Translation:  Before being transmitted, information in the form of characters and numbers should be changed to bit streams. Layer 6 is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
• Encryption:  Encryption at the transmitter and decryption at the receiver
• Compression:  Data compression to reduce the bandwidth of the data to be transmitted. The primary role of  data compression  is to reduce the number of bits to be transmitted. Multimedia files, such as audio and video, are bigger than text files and compression is more important.

Role of Presentation Layer in the OSI Model

This layer is not always used in network communications because its functions are not always necessary. Translation is only needed if different types of machines need to talk with each other. Encryption is optional in communication. If the information is public there is no need to encrypt and decrypt info. Compression is also optional. If files are small there is no need for compression.

Explaining Layer 6 in video

Most real-world protocol suites, such as TCP/IP , do not use separate presentation layer protocols. This layer is mostly an abstraction in real-world networking.

An example of a program that loosely adheres to layer 6 of OSI is the tool that manages the Hypertext Transfer Protocol (HTTP) — although it’s technically considered an application-layer protocol per the TCP/IP model.

However, HTTP includes presentation layer services within it. HTTP works when the requesting device forwards user requests passed to the web browser onto a web server elsewhere in the network.

It receives a return message from the web server that includes a multipurpose internet mail extensions (MIME) header. The MIME header indicates the type of file – text, video, or audio – that has been received so that an appropriate player utility can be used to present the file to the user.

In short, the presentation layer

Makes sure that data which is being transferred or received should be accurate or clear to all the devices which are there, in a closed network.

• ensures proper formatting and delivery to and from the application layer;
• performs data encryption; and
• manages serialization of data objects.

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Presentation layer in osi model.

Last Updated on March 7, 2024 by Abhishek Sharma

The OSI (Open Systems Interconnection) model is a conceptual framework used to understand the functions of a telecommunication or computing system. It consists of seven layers, each responsible for specific tasks. The sixth layer, known as the Presentation Layer, plays a crucial role in ensuring that data exchanged between systems is readable and usable. Let’s explore the functions and importance of the Presentation Layer in the OSI model.

What is Presentation Layer in OSI Model?

The Presentation Layer, the sixth layer of the OSI (Open Systems Interconnection) model, is responsible for ensuring that data exchanged between systems is in a format that can be interpreted and used by the receiving system. It performs various functions, including data translation, encryption, compression, and formatting, to facilitate efficient and secure communication between networked devices.

Functions of the Presentation Layer

Below are some of the functions of the Presentation Layer in OSI Model:

• Data Translation: The Presentation Layer translates data from the format used by the application layer into a format that can be transmitted over the network. This includes encoding, compression, and encryption.
• Data Formatting: It ensures that data is formatted according to the specifications of the application layer. This includes converting between different character sets, such as ASCII and Unicode.
• Data Compression: The Presentation Layer compresses data to reduce the amount of bandwidth required for transmission, improving network efficiency.
• Data Encryption: It encrypts data to ensure that it remains secure during transmission, protecting it from unauthorized access.
• Data Syntax: The Presentation Layer defines the syntax for data representation, ensuring that both the sender and receiver understand the structure of the data being exchanged.

Importance of the Presentation Layer

Importance of Presentation Layer are:

• Data Integrity: By ensuring that data is formatted correctly and encrypted, the Presentation Layer helps maintain the integrity of data during transmission.
• Interoperability: The Presentation Layer enables different systems to communicate with each other by ensuring that data is translated into a common format that both systems understand.
• Efficiency: Data compression reduces the amount of data that needs to be transmitted, improving network efficiency and reducing bandwidth requirements.
• Security: Encryption provided by the Presentation Layer ensures that data remains secure and protected from unauthorized access.

Conclusion The Presentation Layer is a crucial component of the OSI model, responsible for ensuring that data exchanged between systems is in a format that can be understood and used. By performing functions such as data translation, formatting, compression, and encryption, the Presentation Layer plays a vital role in maintaining data integrity, facilitating interoperability, and ensuring the security of data during transmission.

FAQs related to Presentation Layer in OSI Model

Here are some of the FAQs related to Presentation Layer in OSI Model:

Q1: What is the role of the Presentation Layer in the OSI model? The Presentation Layer ensures that data exchanged between systems is in a usable format, performing functions such as data translation, encryption, compression, and formatting.

Q2: How does the Presentation Layer ensure data security? The Presentation Layer encrypts data before transmission, making it unreadable to unauthorized parties, thus ensuring data security.

Q3: Why is data compression important in the Presentation Layer? Data compression reduces the size of data packets, leading to faster transmission speeds and optimized bandwidth usage, which is crucial in high-traffic networks.

Q4: How does the Presentation Layer facilitate interoperability between systems? By translating data into a common format that both sender and receiver understand, the Presentation Layer enables different systems to communicate with each other seamlessly.

Q5: Can the Presentation Layer be bypassed in data transmission? While it is possible to bypass the Presentation Layer in some cases, doing so can lead to compatibility issues between systems and is not recommended.

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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.

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

ComputerNetworkingNotes CCNA Study Guide OSI Seven Layers Model Explained with Examples

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

The Open Systems Interconnection (OSI) networking model defines a conceptual framework for communications between computer systems. The model is an ISO standard which identifies seven fundamental networking layers, from the physical hardware up to high-level software applications.

Each layer in the model handles a specific networking function. The standard helps administrators to visualize networks, isolate problems, and understand the use cases for new technologies. Many network equipment vendors advertise the OSI layer that their products are designed to slot into.

OSI was adopted as an international standard in 1984. It remains relevant today despite the changes to network implementation that have occurred since first publication. Cloud, edge, and IoT can all be accommodated within the model.

In this article, we'll explain each of the seven OSI layers in turn. We'll start from the lowest level, labelled as Layer 1.

1. Physical Layer

All networking begins with physical equipment. This layer encapsulates the hardware involved in the communications, such as switches and cables. Data is transferred as a stream of binary digits - 0 or 1 - that the hardware prepares from input it's been fed. The physical layer specifies the electrical signals that are used to encode the data over the wire, such as a 5-volt pulse to indicate a binary "1."

Errors in the physical layer tend to result in data not being transferred at all. There could be a break in the connection due to a missing plug or incorrect power supply. Problems can also arise when two components disagree on the physical encoding of data values. In the case of wireless connections, a weak signal can lead to bit loss during transmission.

2. Data Link Layer

The model's second layer concerns communication between two devices that are directly connected to each other in the same network. It's responsible for establishing a link that allows data to be exchanged using an agreed protocol. Many network switches operate at Layer 2.

The data link layer will eventually pass bits to the physical layer. As it sits above the hardware, the data link layer can perform basic error detection and correction in response to physical transfer issues. There are two sub-layers that define these responsibilities: Logical Link Control (LLC) that handles frame synchronization and error detection, and Media Access Control (MAC) which uses MAC addresses to constrain how devices acquire permission to transfer data.

3. Network Layer

The network layer is the first level to support data transfer between two separately maintained networks. It's redundant in situations where all your devices exist on the same network.

Data that comes to the network layer from higher levels is first broken up into packets suitable for transmission. Packets received from the remote network in response are reassembled into usable data.

The network layer is where several important protocols are first encountered. These include IP (for determining the path to a destination), ICMP, routing, and virtual LAN. Together these mechanisms facilitate inter-network communications with a familiar degree of usability. However operations at this level aren't necessarily reliable: messages aren't required to succeed and may not necessarily be retried.

4. Transport Layer

The transport layer provides higher-level abstractions for coordinating data transfers between devices. Transport controllers determine where data will be sent and the rate it should be transferred at.

Layer 4 is where TCP and UDP are implemented, providing the port numbers that allow devices to expose multiple communication channels. Load balancing is often situated at Layer 4 as a result, allowing traffic to be routed between ports on a target device.

Transport mechanisms are expected to guarantee successful communication. Stringent error controls are applied to recover from packet loss and retry failed transfers. Flow control is enforced so the sender doesn't overwhelm the remote device by sending data more quickly than the available bandwidth permits.

5. Session Layer

Layer 5 creates ongoing communication sessions between two devices. Sessions are used to negotiate new connections, agree on their duration, and gracefully close down the connection once the data exchange is complete. This layer ensures that sessions remain open long enough to transfer all the data that's being sent.

Checkpoint control is another responsibility that's held by Layer 5. Sessions can define checkpoints to facilitate progress updates and resumable transmissions. A new checkpoint could be set every few megabytes for a file upload, allowing the sender to continue from a particular point if the transfer gets interrupted.

Many significant protocols operate at Layer 5 including authentication and logon technologies such as LDAP and NetBIOS. These establish semi-permanent communication channels for managing an end user session on a specific device.

6. Presentation Layer

The presentation layer handles preparation of data for the application layer that comes next in the model. After data has made it up from the hardware, through the data link, and across the transport, it's almost ready to be consumed by high-level components. The presentation layer completes the process by performing any formatting tasks that may be required.

Decryption, decoding, and decompression are three common operations found at this level. The presentation layer processes received data into formats that can be eventually utilized by a client application. Similarly, outward-bound data is reformatted into compressed and encrypted structures that are suitable for network transmission.

TLS is one major technology that's part of the presentation layer. Certificate verification and data decryption is handled before requests reach the network client, allowing information to be consumed with confidence that it's authentic.

7. Application Layer

The application layer is the top of the stack. It represents the functionality that's perceived by network end users. Applications in the OSI model provide a convenient end-to-end interface to facilitate complete data transfers, without making you think about hardware, data links, sessions, and compression.

Despite its name, this layer doesn't relate to client-side software such as your web browser or email client. An application in OSI terms is a protocol that caters for the complete communication of complex data through layers 1-6.

HTTP, FTP, DHCP, DNS, and SSH all exist at the application layer. These are high-level mechanisms which permit direct transfers of user data between an origin device and a remote server. You only need minimal knowledge of the workings of the other layers.

The seven OSI layers describe the transfer of data through computer networks. Understanding the functions and responsibilities of each layer can help you identify the source of problems and assess the intended use case for new components.

OSI is an abstract model that doesn't directly map to the specific networking implementations commonly used today. As an example, the TCP/IP protocol works on its own simpler system of four layers: Network Access, Internet, Transport, and Application. These abstract and absorb the equivalent OSI layers: the application layer spans OSI L5 to L7, while L1 and L2 are combined in TCP/IP's concept of Network Access.

OSI remains applicable despite its lack of direct real-world application. It's been around so long that it's widely understood among administrators from all backgrounds. Its relatively high level of abstraction has also ensured it's remained relevant in the face of new networking paradigms, many of which have targeted Layer 3 and above. An awareness of the seven layers and their responsibilities can still help you appreciate the flow of data through a network while uncovering integration opportunities for new components.

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Presentation Layer: What It Is, Design Issues, Functionalities

Description and Functions of Presentation Layer in the OSI model: In this tutorial, we are going to learn what the Presentation layer is and the Functions of the Presentation Layer in the OSI model in Computer Networking. We will also discuss the Design issues with the Presentation Layer and the working of the Presentation Layer with the help of its diagram. By Monika Jha Last updated : May 05, 2023

What is Presentation Layer?

The Presentation Layer is concerned with the syntax and semantics of the information exchanged between two communicating devices.

• The presentation layer takes care that the data is sent in that way the receiver of the data will understand the information (data) and will be able to use the data.
• Languages that are syntax can be different from the two communicating machines. In this condition, the presentation layer plays the role of translator between them.
• It is possible for two machines to communicate with different data representations, data structures to be exchanged can be defined in an abstract way.
• These abstract data structures will be managed by the presentation layer and this layer allows higher-level data structures (For example banking records), to be defined and exchanged.

This figure shows the relationship of the presentation layer to the session layer and application layer.

Design Issues with Presentation Layer

The following are the design issues with presentation layer:

• To manage and maintain the Syntax and Semantics of the information transmitted.
• Encoding data in a standard agreed-upon way just like a string, double, date, etc.
• It Performs Standard Encoding scheme on the wire.

Functionalities of the Presentation Layer

Specific functionalities of the presentation layer are as follows:

1. Translation

• The processes or running programs in two machines are usually exchanging the information in the form of numbers, character strings and so on before being transmitted. The information should be changed to bitstreams because different computers use different encoding schemes.
• The Presentation layer is responsible for compatibility between these encoding methods.
• The Presentation layer at the sender's side changes the information from its sender dependent format.
• The Presentation layer at the receiving machine changes the common format into its receivers dependent format.

Example: Convert ASCII code to EBCDIC code.

2. Encryption

• The system must be able to assure privacy regarding the message or information as it also carries sensitive information.
• Encryption means that the sender transforms the original information or message to another form, this data after encryption is known as the ciphertext and this ciphertext sends the resulting message out over the network.
• Decryption concerned with the transform of the message back to its original form. This decrypted data is known as plain text.

3. Compression

• Data Compression means reduces the number of bits to be transmitted by this reduce the bandwidth of the data.
• Data Compression becomes particularly important in the transmission of multimedia such as audio, video, text, etc.

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Presentation Layer of OSI Model (Layer-6)

Presentation Layer is responsible for representation & formatting of data for session Layer in encapsulation process. It is the 6th Layer in the seven layer OSI Model after Session Layer. Presentation layer serves like a translator & takes care that the data is sent in such a way that the receiver will understand the information or data and will be able to use the data. OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it. This seven Layer model starts from Physical till Application Layer & Presentation Layer is on 2nd place in this model as in below figure:

Functions/Duties of Presentation Layer

Each Layer in OSI Model Performs some important duties. Important functions performed by Presentation Layer are listed here:

• The first & most important is, of course Data Formatting & Representation . When the presentation layer receives data from the application layer, to be sent over the network, it makes sure that the data is in the proper format. If it is not, the presentation layer converts the data to the proper format. On the other side of communication, when the presentation layer receives network data from the Presentation layer, it makes sure that the data is in the proper format and once again converts it if it is not.
• It is also responsible for Data Encryption/Decryption: Presentation Layer carries out encryption at the transmitter end and decryption at the receiver end to keep data secure during transmission.
• Data Compression/De-compression also falls under the responsibility matrix of Presentation Layer . Presentation Layer compresses data to a small size to reduce resource usage such as data storage space or transmission capacity.

*Encryption is typically done at this layer as well, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages

Presentation Layer Protocols

The OSI Model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. Each layer on the OSI Model has some protocols associated with it. Some important protocols on Presentation layer are listed in below:

• JPEG/GIF/PNG/TIFF

Network Equipment/Components at Presentation Layer

• Load Balancers
• End Devices e.g. Computers, Smart Phones, Servers, …

Presentation Layer is the 6th Layer in seven Layer OSI Model. It performs important functions like Data Formatting, Data Representation, Data Encryption/Decryption, Data Compression and De-compression. Important Protocols at Presentation Layer include ASCII, EBCDIC, JPEG, MPEG, GIF, PNG, TIFF, SSL & TLS. Equipment operating at Presentation Layer include Firewalls, Gateways, Load Balancers & Computers.

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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..

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 are:

Layer 1: Physical

Layer 2: data link, layer 3: network, layer 4: transport, layer 5: session, layer 6: presentation, layer 7: 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:

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.

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.

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.

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.

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.

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.

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

Keith Shaw was a Network World editor and the writer of the Cool Tools column. He is now a freelance writer and editor from Worcester, Mass.

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The first gadget Keith Shaw ever wanted was the Merlin, a red plastic toy that beeped and played Tic-Tac-Toe and various other games. A child of the '70s and teenager of the '80s, Shaw has been a fan of computers, technology and video games right from the start. He won an award in 8th grade for programming a game on the school's only computer, and saved his allowance to buy an Atari 2600.

Shaw has a bachelor's degree in newspaper journalism from Syracuse University and has worked at a variety of newspapers in New York, Florida and Massachusetts, as well as Computerworld and Network World. He won an award from the American Society of Business Publication Editors for a 2003 article on anti-spam testing, and a Gold Award in their 2010 Digital Awards Competition for the "ABCs of IT" video series.

Shaw is also the co-creator of taquitos.net , the crunchiest site on the InterWeb, which has taste-tested and reviewed more than 4,000 varieties of snack foods.

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OSI Model Layers and Protocols in Computer Network

What is OSI Model?

The OSI Model is a logical and conceptual model that defines network communication used by systems open to interconnection and communication with other systems. The Open System Interconnection (OSI Model) also defines a logical network and effectively describes computer packet transfer by using various layers of protocols.

Characteristics of OSI Model

Here are some important characteristics of the OSI model:

• A layer should only be created where the definite levels of abstraction are needed.
• The function of each layer should be selected as per the internationally standardized protocols.
• The number of layers should be large so that separate functions should not be put in the same layer. At the same time, it should be small enough so that architecture doesn’t become very complicated.
• In the OSI model, each layer relies on the next lower layer to perform primitive functions. Every level should able to provide services to the next higher layer
• Changes made in one layer should not need changes in other lavers.

Why of OSI Model?

• Helps you to understand communication over a network
• Troubleshooting is easier by separating functions into different network layers.
• Helps you to understand new technologies as they are developed.
• Allows you to compare primary functional relationships on various network layers.

History of OSI Model

Here are essential landmarks from the history of OSI model:

• In the late 1970s, the ISO conducted a program to develop general standards and methods of networking.
• In 1973, an Experimental Packet Switched System in the UK identified the requirement for defining the higher-level protocols.
• In the year 1983, OSI model was initially intended to be a detailed specification of actual interfaces.
• In 1984, the OSI architecture was formally adopted by ISO as an international standard

7 Layers of the OSI Model

OSI model is a layered server architecture system in which each layer is defined according to a specific function to perform. All these seven layers work collaboratively to transmit the data from one layer to another.

• The Upper Layers : It deals with application issues and mostly implemented only in software. The highest is closest to the end system user. In this layer, communication from one end-user to another begins by using the interaction between the application layer. It will process all the way to end-user.
• The Lower Layers : These layers handle activities related to data transport. The physical layer and datalink layers also implemented in software and hardware.

Upper and Lower layers further divide network architecture into seven different layers as below

• Application
• Presentation
• Network, Data-link
• Physical layers

Let’s Study each layer in detail:

Physical Layer

The physical layer helps you to define the electrical and physical specifications of the data connection. This level establishes the relationship between a device and a physical transmission medium. The physical layer is not concerned with protocols or other such higher-layer items. One example of a technology that operates at the physical layer in telecommunications is PRI (Primary Rate Interface). To learn more about PRI and how it works , you can visit this informative article.

Examples of hardware in the physical layer are network adapters, ethernet, repeaters, networking hubs, etc.

Data Link Layer

Data link layer corrects errors which can occur at the physical layer. The layer allows you to define the protocol to establish and terminates a connection between two connected network devices.

It is IP address understandable layer, which helps you to define logical addressing so that any endpoint should be identified.

The layer also helps you implement routing of packets through a network. It helps you to define the best path, which allows you to take data from the source to the destination.

The data link layer is subdivided into two types of sublayers:

• Media Access Control (MAC) layer- It is responsible for controlling how device in a network gain access to medium and permits to transmit data.
• Logical link control layer- This layer is responsible for identity and encapsulating network-layer protocols and allows you to find the error.

Important Functions of Datalink Layer

• Framing which divides the data from Network layer into frames.
• Allows you to add header to the frame to define the physical address of the source and the destination machine
• Adds Logical addresses of the sender and receivers
• It is also responsible for the sourcing process to the destination process delivery of the entire message.
• It also offers a system for error control in which it detects retransmits damage or lost frames.
• Datalink layer also provides a mechanism to transmit data over independent networks which are linked together.

Transport Layer

The transport layer builds on the network layer to provide data transport from a process on a source machine to a process on a destination machine. It is hosted using single or multiple networks, and also maintains the quality of service functions.

It determines how much data should be sent where and at what rate. This layer builds on the message which are received from the application layer. It helps ensure that data units are delivered error-free and in sequence.

Transport layer helps you to control the reliability of a link through flow control, error control, and segmentation or desegmentation.

The transport layer also offers an acknowledgment of the successful data transmission and sends the next data in case no errors occurred. TCP is the best-known example of the transport layer.

Important functions of Transport Layers

• It divides the message received from the session layer into segments and numbers them to make a sequence.
• Transport layer makes sure that the message is delivered to the correct process on the destination machine.
• It also makes sure that the entire message arrives without any error else it should be retransmitted.

Network Layer

The network layer provides the functional and procedural means of transferring variable length data sequences from one node to another connected in “different networks”.

Message delivery at the network layer does not give any guaranteed to be reliable network layer protocol.

Layer-management protocols that belong to the network layer are:

• routing protocols
• multicast group management
• network-layer address assignment.

Session Layer

Session Layer controls the dialogues between computers. It helps you to establish starting and terminating the connections between the local and remote application.

This layer request for a logical connection which should be established on end user’s requirement. This layer handles all the important log-on or password validation.

Session layer offers services like dialog discipline, which can be duplex or half-duplex. It is mostly implemented in application environments that use remote procedure calls.

Important function of Session Layer

• It establishes, maintains, and ends a session.
• Session layer enables two systems to enter into a dialog
• It also allows a process to add a checkpoint to steam of data.

Presentation Layer

Presentation layer allows you to define the form in which the data is to exchange between the two communicating entities. It also helps you to handles data compression and data encryption.

This layer transforms data into the form which is accepted by the application. It also formats and encrypts data which should be sent across all the networks. This layer is also known as a syntax layer .

The function of Presentation Layers

• Character code translation from ASCII to EBCDIC.
• Data compression: Allows to reduce the number of bits that needs to be transmitted on the network.
• Data encryption: Helps you to encrypt data for security purposes — for example, password encryption.
• It provides a user interface and support for services like email and file transfer.

Application Layer

Application layer interacts with an application program, which is the highest level of OSI model. The application layer is the OSI layer, which is closest to the end-user. It means OSI application layer allows users to interact with other software application.

Application layer interacts with software applications to implement a communicating component. The interpretation of data by the application program is always outside the scope of the OSI model.

Example of the application layer is an application such as file transfer, email, remote login, etc.

The function of the Application Layers are

• Application-layer helps you to identify communication partners, determining resource availability, and synchronizing communication.
• It allows users to log on to a remote host
• This layer provides various e-mail services
• This application offers distributed database sources and access for global information about various objects and services.

Interaction Between OSI Model Layers

Information sent from a one computer application to another needs to pass through each of the OSI layers.

This is explained in the below-given example:

• Every layer within an OSI model communicates with the other two layers which are below it and its peer layer in some another networked computing system.
• In the below-given diagram, you can see that the data link layer of the first system communicates with two layers, the network layer and the physical layer of the system. It also helps you to communicate with the data link layer of, the second system.

Protocols supported at various levels

Differences between OSI & TCP/IP

Here, are some important differences between the OSI & TCP/IP model:

Advantages of the OSI Model

Here, are major benefits/pros of using the OSI model :

• It helps you to standardize router, switch, motherboard, and other hardware
• Reduces complexity and standardizes interfaces
• Facilitates modular engineering
• Helps you to ensure interoperable technology
• Helps you to accelerate the evolution
• Protocols can be replaced by new protocols when technology changes.
• Provide support for connection-oriented services as well as connectionless service.
• It is a standard model in computer networking.
• Supports connectionless and connection-oriented services.
• Offers flexibility to adapt to various types of protocols

Disadvantages of the OSI Model

Here are some cons/ drawbacks of using OSI Model:

• Fitting of protocols is a tedious task.
• You can only use it as a reference model.
• Doesn’t define any specific protocol.
• In the OSI network layer model, some services are duplicated in many layers such as the transport and data link layers
• Layers can’t work in parallel as each layer need to wait to obtain data from the previous layer.
• The OSI Model is a logical and conceptual model that defines network communication which is used by systems open to interconnection and communication with other systems
• In OSI model, layer should only be created where the definite levels of abstraction are needed.
• OSI layer helps you to understand communication over a network

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• Goals of Networks
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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.

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
• Session Layer
• Presentation 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

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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