Question

Give function of each layer of OSI reference model.

Answer 1

OSI model (open system interconnection) model was developed by ISO (international standard organization) 

Function of OSI model: i. It provides way to understand how internetwork operates. ii. It gives guideline for creating network standard. 

OSI model has 7 layers as shown in the figure.

OSI model has following 7 layers as Physical layer, data link layer, Network layer, Transport layer, session layer, presentation layer, application layer. 

1. Physical layer: It co-ordinates the functions required to transmit bit stream over physical medium. It deals with mechanical and electrical specifications of interface and transmission medium. For transmission it defines procedures and functions that devices and transmission medium has to perform 

• Physical characteristics of interfaces and media.
• Representation of bits: Data rate (transmission rate).
• Synchronization of bits.
• Line configuration: Point to point or multipoint configuration should be used. 

2. Data link layer: It is responsible for transmitting group of bits between the adjacent nodes. The group of bits is called as frame. The network layer passes a data unit to the data link layer. Header and trailer is added to the data unit by data link layer. This data unit is passed to the physical layer. Data link layer is responsible for moving frames from one node to the next. 

Functions of data link layer are:

1) Framing

2) Physical addressing

3) Flow control

4) Error control

5) Media access control

6) Node to node delivery 

3. Network layer: It is responsible for routing the packets within the subnet i.e. from source to destination. It is responsible for source e to destination delivery of individual packets across multiple networks. It ensures that packet is delivered from point of origin to destination. 

Functions of network layer:

1) Logical addressing

2) Routing.

3) Congestion control

4) Accounting and billing

5) Address transformation

6) Source host to destination host error free delivery of packet.

4. Transport layer: Responsibility of process to process delivery of message Ensure that whole message arrives in order.

Functions of Transport layer:

1) Service point addressing

2) Segmentation and reassembly

3) Connection control

4) Flow control: Flow control is performed end to end 5) Error control 

5. Session layer: Establishes, maintains, and synchronizes the interaction among communication systems It is responsible for dialog control and synchronization.

Functions of Session layer:

1) Dialog control

2) Synchronization, session and sub session

3) Session closure

6. Presentation layer: It is concerned with syntax, semantics of information exchanged between the two systems.

Functions of Presentation layer:

Translation: presentation layer is responsible for converting various formats into required format of the recipient

Encryption: Data encryption and decryption is done by presentation layer for security.

Compression and Decompression: data to be transform compressed while sending and decompress while receiving for reducing time of transmission.

7. Application layer: It enables user to access the network. It provides user interfaces and support for services like email, remote file access.

Functions of Application layer:

Network virtual terminal

file transfer access and management

mail services and directory services