OSI Model

The International Standards Organization (ISO) developed a network model in 1970's to standardize the network design. This model was called the ISO-OSI (Open Systems Interconnection) reference model.

OSI Reference Model

An open system is a set of protocols which allows communication between two different systems irrespective of their underlying architecture.

The OSI model handles the connecting systems open for communication with other divcerse systems. The model simply defines which tasks need to be done and which layer handles those tasks.

The OSI Model breaks different aspects of computer networking into seven layers. Following figure shows the depiction.

Layers in OSI Model

1. Physical Layer

The physical layer of transmits raw data bits over a communication channel from one node to the next node. This layer handles the physical devices and components required for data communication.

Functions of physical layer

a) Mechanical

This involes the physical properties of a medium and interfaces between devices like connectors and cables.

b) Signal representation

These means figuring out how the data should be represented on transmission medium. It consists of deciding the encoding mechanisms.

c) Timing

Transmission rate is a number which represents number of raw data bits sent per second. Controling transmission rate is one of the responsibilities of physical layer.

d) Synchronization

Making the sender and receiver in synchronized state, establishing a connection, releasing a connection, handling simplex and duplex communication are the repsonsibilities of phycial layer.

e) Line Configuration

The physical layer handles connection links such as point to point and multipoint under line configuration responsibility.

f) Physical Topology

This involves the connection topologies, how to connect devices, which topology to use out of mesh topology, star topology, ring topology etc.

The devices used at Physical Layer: Hub, Repeater.

2. Data Link Layer

Data Link Layer is responsible for error free transmission of frames from one node to the next node. The primary task of data link layer is to transform raw data bits into a frame that is free of transmission errors.

This error free transmission is achieved by dividing the data into frames and handling the identification of frames.

The IEEE had split data link layer into two sub layers.

i) Logical Link Control
This sublayer establishes and maintain the the communication links between devices.
ii) Media Access Control (MAC)
This sublayer controls access to the communication channel. It controls the way multiple devices share the same channel for communication.

Functions of Data Link Layer

i) Framing

Dividing the stream of data bits into logical units is called as framing.

ii) Error Control

A mechanism is used to detect and correct errors.

iii) Flow Control

Flow control involves controling the flow of data so that the fast sender does not overwhelm the slow receiver.

iv) Physical Addressing

Identifying a machine in the network using its physical address is called as physical addressing.

v) Multiple Access Control (MAC)

In case of multipoint links, MAC provides a mechanism to access shared communication channel between multiple machines.

Data Link Layer Devices: Bridge, Switch

3. Network Layer

The network layer transforms the data packets from source to destination device across multiple networks.

Functions of Network Layer

i) Addressing

Identifying a machine in the network on the basis of logical address is known as addressing.

ii) Routing

Network layer uses routing algorithms to route data from source to destination.

iii) Congestion Control

Network layer also performs congestion control since the data has to travel across communication subnet.

Devices Operating at Network Layer: Router

4. Transport Layer

Transport layer performs process to process delivery of data from source application to destination application. Transport layer is an end to end layer.

This layer ensure that the data message arrives intact and in order at the destination machine. Transport layer isolates the upper layer an lower layer so that the complex characteristics of subnet are hidden from end users.

Functions of Transport Layer

i) Post Addressing

Transport layer uses port address to identify application in the host machine for which the data is intended.

ii) Segmentation and Reassembly

If a message is too large it can be split up. Transport layer performs sequencing and reassembly at destination machine.

iii) Services

This layers decide whether to offer connection oriented or connection less service to upper layer.

iv) Flow and Error Control

This layer ensures end to end flow control and error free delivery of data.

Devices Operating at Transport Layer: Gateway

5. Session Layer

This layer helps to establish a communication session between two different machines.

Sessions offer various services like controling the type of communication (half duplex or full duplex), token management, synchronization, authorization. It also allows check point mechanism to allow transmission to continue from last point where it got interrupted (in case of crash).

Function of Session Layer

i) Dialog Control

This layer helps to establish a dialog between two communicating entities which can either be half duplex or full duplex.

ii) Synchronization

This allows adding checkpoints at certain intervals so that the communication can continue from that point if in case of diconnection.

6. Presentation Layer

This layer is concerned with the syntax and semantics of data to be transmitted. In other words, it defines the format of the data to be exchanged between applications and offers services like format transformation, encryption, compression.

Functions of Presentation Layer

i) Translation

Presentation Layer performs translation. Translation is the process of converting data from application dependant format to a standard format so that it can be transmitted over network.

ii) Encryption

Privacy of data is ensured by encrypting it. Presentation Layer performs encryption for both communicating parties.

iii) Compression

Large amount of data can be compressed to make it small in size. This compressed data can then be sent through the network so that bandwith utilization is less. Compression is important and useful when video, audio or images are to be transmitted throught the network.

7. Application Layer

Application Layer provide services which are commonly required by the user such as file transfer, e-mail, remote terminal access, access to World Wide Web.

Functions of Application Layer

i) File Transfer

This function allows the user to send, receive or retrive files on a remove machine (cloud).

ii) e-mail

This function allows the user to create, send, receive and retrive electonica mails.

iii) Remote Access

This function allows the user to sign in to the remote machine and access any file.