Functions of OSI Layers

Transport Layer

The basic function of transport layer is to accept data from the session layer and split it into smaller units, pass these to the network layer.

Other main functions of transport layer are

• Connection control
• Service point addressing
• Segmentation
• Reassembling
• End to end error recovery and flow control

Session Layer

The session layer allows users to establish sessions between different machines. A session allows ordinary data transfer functions with some extended functionality.

Other functions of session layer are

• Token management
• Dialogue control
• Synchronization 

Presentation Layer

 Presentation layer deals with syntax and semantics of the data transmitted.

Functions of presentation layer are

• Encryption
• Translation
• Compression

Application Layer

Application layer contains a rich set of protocols that are commonly used.

Specific services provided by the application layer are

• File transfer
• Mail service
• Directory service

Physical Layer

The physical layer deals with the transmission of raw bits over a communication channel. It specifies the mechanical, procedural and electrical network interface specifications.

The major functions and services performed by the physical layer are

• Decide signal level and data rate
• Whether simultaneous transmissions in both direction
• Deals with physical transmission of data
• Determine number of pins and functions of each pins in the network connector.
• Synchronization of bits
• Determine transmission mode
• Synchronization of bits
• Representation of bits

There exists a variety of physical layer protocols such as RS-232, RS-449 standards developed by Electronic Industries Association (EIA).

Data Link Layer

The main goal of data link layer is to provide reliable, efficient communication between adjacent machines connected y a single communication channel.

Other responsibilities of the data link layer include the following:

• Group the physical layer bit stream into units called frames (Faming)
• Physical addressing
• Error control
• Flow control
• Access control
• Frame synchronization

There exist varieties of methods for determine frame boundaries like length counting, bit stuffing and character stuffing. We will discuss these later.

Network Layer

The network layer is responsible for the source-to-destination delivery of a packet. The data link layer manages the delivery of the packet between two nods on the same network, the network layer guarantee that each packet gets from its source host to destination host.

The major functions and services performed by the network layer are

• Routing
• Congestion control
• Internetworking
• Interface between the host and network
• Logical addressing