OSI Model - Session Layer (Layer 5)

OSI Model – Session Layer (Layer 5)

In a Data Transfer process, after the Presentation Layer has done its done job of providing the presentation service like data encryption, compression, etc. the data is then handed to the next layer which is known as the Session Layer.

What does it do?? Well, the name “Session” suggests that it might have something to do with creating sessions. Yes, correct, it helps in creating sessions of different application that a user might be using at a time for data transfer.

It maintains dialog control, which means, if a user is sending an email through Windows mail, it should not go to any other application like Yahoo Messenger or Gmail Talk…

Before the actual data transfer takes place, session layers creates a virtual session with the destination host, and then maintains dialog control so that the data is handed to correct application, and after data transfer is done, it ends up the session, termed as tearing down session.

It provides communication by offering three different modes: Simple, Half-Duplex and Full Duplex.

Simplex: It is a one sided communication.

Half Duplex: It is a two sided communication, but one at a time can communicate.

Full Duplex: It is a two sided communication, and both the side can communicate with each other    simultaneously.

From the receiving side, session layer notifies the presentation layer about an incoming data stream, and it will only notify if a series of messages are complete, in some cases.

For example: An ATM will never debit your account and then fail, without handling the cash.

The session layer creates ways to imply which flows are part of the same session and which flows must be complete before terminating the virtual session.

User	Service
Application Layer	Web Browser, Email Clients
Presentation Layer	Formats data, compression, encryption, etc
Session Layer	Maintains dialog control, creating session and controlling session and ending session.

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