Functions of OSI Layers Transport, Presentation session and Network

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

 Next : TCP/IP reference model

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