Open System Interconnection – OSI Model

OSI model

An ISO standard that covers all the aspects of network communication is the open system interconnection model.

Open system

A model that allows any two different systems to communicate without depending on their software and hardware.

Purpose of OSI model

Open communication between different system without requiring changes to their software and hardware.

Design of OSI MODEL

The OSI model based upon seven layers.

  1. Physical layer
  2. Data link layer
  3. Network layer
  4. Transport layer
  5. Session layer
  6. Presentation layer
  7. Application layer


In communication every layer has assigned a specific task different protocols are used in different layer.

At Sender End

For communication OSI layer must be on available on both system when sender send data to the receiver the data moves in descending order at sender end. Data moves from layer 7 and towards layer 1. When the data passed from every layer its adds its header or trailer (header and trailer contain information) and send it’s to other layer where the same process is repeated by adding header or trailer.

At the Receiver End

When the data reaches to receiver end it moves in ascending order first its comes form application layer than to other layer during visiting each layer at receiver end the layer remove the header or trailer which is assigned by the same layer at receiver end doing this process the data is accurately receive by the receiver but its not an easy task to transfer data from one computer to another accurately without any error every application have some function or responsibilities which we will discussed now.

OSI layer Functions

Physical Layer

The physical layer is responsible for sending raw bits via transmission medium between the source and destination nodes. It deals with the mechanical and electrical specifications of the cables, connectors, and interfaces such as RS 232-c etc.


The physical layer is concerned with the following.

· Signal encoding. How are the bits 0 and 1 to be represented in the form of signals the sender and receiver must be agreed upon the same pattern.

e.g. 0 represents 0V and the 1 bit represents 5V.

· Medium. What is the medium used and what its properties are.

E.g. the medium used may be twisted pair, coaxial and their properties are different.

· Data Rate. The number of bits per second also defined by physical layer.

· Transmission Type. The physical layer checks the transmission type whether

It is serial or parallel.

· Line Configuration:- The physical Layer is concerned with the connection of devices to the medium

E.g. Point-to-Point (dedicated line), Multipoint configuration (shared).

· Topology. The topology defines how devices are connected to make network.

E.g. Mesh, star, Bus etc.

· Transmission Mode. The physical layer also defines the direction of transmission.

e.g. Simplex, Half duplex and Full Duplex.

· Multiplexing. The physical Layer also defines by which method the media is divided, what is the type?

E.g. FDM (frequency division multiplexing), TDM (Time division multiplexing).

· Signal Type. The Physical layer defines that it is an analog signal or digital signal.

Data Link Layer

The data link layer is the second layer in OSI layer stack. The data Link layer is responsible for transmitting a group of bits between adjacent nodes. The group of bits are generally called frame. The data Link layer is responsible for node-to node delivery. The data link layer assigns physical address to each frame which is the address of source and destination. In case of more than one network the physical address will be the address of the next node or connecting device (router).

If the connection is a multipoint type or shared then the problem that should send how much data and at what time has to be solved. The problem commonly arises in LANs and is solved by Media Access Control (MAC) protocol.


Therefore the Data Link layer is divided into two layers.

1. LLC(Logic Link Control)

2. MAC(Media Access Control)

Media Access Control (MAC). The MAC sub layer controls the means by which multiple devices share the same media channel. This includes contention methods and other media access details. The MAC layer also provides addressing information for communication between network devices.

Logical Link Control (LLC). The LLC sub layer establishes and maintains links between communicating devices.


Following are the responsibilities of data link layer:-

· Framing. The Data Link layer divides the stream of bits received from the network layer into small chunks or data units called frames.

· Addressing. Headers and trailers are added, containing the physical addresses of the adjacent nodes, and removed upon a successful delivery.

· Flow Control. If the data rate of send is faster and the data absorbed by the receiver is less then to the Data Link Layer control the flow control so the receiver can easily accurately receives the data.

· Error Control. The Data Link Layer checks each frame CRC if there is any error in the frame or any frame is missing it send the request to retransmit the damage or lost frame

· Node-to-Node delivery:-It is responsible for error free delivery of the entire frame to the next adjacent nodes.

Network Layer

This network layer route the packet from the source to the destination node across multiple nodes in the same network, or across multiple networks. Network layer is responsible for source-to destination delivery of the packet across multiple networks. If the two computers attached with the same link than there is no need of network layer. To deliver the Packets among different networks logical addresses is assigned by network layer to each packet which remains same throughout the journey of the packet while the physical addresses changes form node-to-node.


The Data Link Layer Performs The following functions.

· Routing. When independent networks or links are connected together to create an internetwork, the connecting devices (router and gateways) route the packet to their final destination.

· Addressing. When the packets delivers among several networks so the network layer adds mechanism to ensure that the packets.

· Congestion Control. This layer is also responsible for handling the congestion problem at the node, when there are too many packets stored at the node to be forwarded to the next node.

· Address Transformations. Interpreting logical addresses to get their physical equivalent.

· Accounting and Billing. This layer has to carry out the accounting function to facilitate based on how many packets are routed, when etc.

Transport Layer.

The transport layer is the first end-to-end layer. This layer is responsible for end-to-end delivery of the entire message, although the message broken into packets may travel through a number o intermediate nodes. The transport layer ensures that the complete message arrives at the destination and in the proper order and is passed on to the proper application.


For adding security, the transport layer may create a connection between the two end ports. A connection is a single logical path between source and destination that is associated with all the packets.

Creating a connection involves three steps.

1. Connection establishment

2. Data transfer

3. Connection release

Transport layers activities include the following:

  • Segment sequencing. Segment sequencing is one connection-oriented service provided by the Transport layer. When large messages are divided into segments for transport, the Transport layer must resequence the segments when they are received before reassembling the original message.
  • Error control. When segments are lost during transmission or when segments have duplicate segment IDs, the Transport layer must initiate error recovery. The Transport layer also detects corrupted segments by managing end-to-end error control using techniques such as checksums.
  • End-to-end flow control. The Transport layer uses acknowledgments to manage end-to-end flow control between two connected devices. Besides negative acknowledgments, some Transport-layer protocols can request the retransmission of the most recent segments.
  • Connection control. The transport layer might create a logical connection between the source and the destination for the duration of the complete message transfer for better control over the message transfer.

Session layer

The main functions of the session layer are to establish, maintain and synchronize the interaction between two hosts.

The session layer checks and establishes connections between two hosts of different users. For this the user might need to enter identification information such as login and password. The session layer also decides whether both users can send as well s receive data at the same time, or only one host can send and other can receive , and so on communication is simplex, half duplex, or full duplex.


In case of the large files there is more chance of error if the error occurs in file we have to retransmit the whole file and the congestion problem occurs in the network. The session layer at the two hosts creates sub-session. After each sub-session is over, a checkpoint can be taken. For instance, the session layers at the two hosts could decide that after a successful transmission of a set of every 10 pages, they would take a check point. This means that if the connection breaks after the first 105 pages have been transmitted, after the connection is restored, the transmission would start at the 101st page. This is because the last checkpoint would have been taken after the 100th page was transmitted.

The responsibilities of the session layer are as follows.

  • Session and sub-session. The session layer divides a session into sub-sessions for avoiding retransmission of the entire messages by adding checkpoint features.
  • Synchronization. The session layer decides the order in which data need to be passed to the transport layer.
  • Dialog control. The session layer also decides the order in which data need to be passed to the transport layer.
  • Session closure. The session layer ensures that the session between the hosts is closed gracefully.

Presentation layer

When two hosts are communicating they may be using same or different coding standards, character sets, string for representing data internally. The sender session layer converts the sender-dependent information into a common format. The presentation layer at the receiving, machine changes common format into its receiver-dependent format. Data encryption, decryption and data compression, decompression takes place in presentation layer.


The are responsibilities of the presentation layer are as follows.

Translation. The translation between the sender and the receiver’s message formats done by the presentation layer if the two formats are different.

Encryption. Data encryption and decryption is performing in presentation layer for security purpose.

Compression. Data compression is one of important task of presentation layer due to the data compression the number of bits reduced and it take less time to reach the destination at receiver end the de-compression take place. The compression is important specially when sending audio, video and pictures.

Application Layer. The topmost and last layer of the OSI model it allows the user to access the network. The program which using the network also reside to application layer. This layer provides the interface to use the network application such as WWW,TELNET,FTP,email,remote database etc. The files downloading and uploading from the web also the part of application layer.

A common misunderstanding is that the Application layer is responsible for running user applications such as word processors. This is not the case. The Application layer, however, does provide an interface whereby applications can communicate with the network.


Following are the responsibilities of application layer.

Network abstraction. It allows the user to access the network by hiding the complexities.

File access and transfer. It allows the user to download or upload files from/to a remote host.

Mail service. It allows the user to use mail service.

World Wide Web. Accessing the web pages is the part of this layer.

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