Session Layer

The session layer shares information with the PRESENTATION layer above it and the TRANSPORT layer beneath it.

A ‘session’ is a bit like a music group setting up a gig for the evening at a great venue. A session has a beginning, a middle and an end. The session manager ensures that everything works smoothly and in a timely manner.

Going on with the music analogy, it would be pretty distracting to the audience if in mid-song, a sound engineer steps up to the microphone and starts doing “One, Two, Three” sound checks alongside the singer! Some kind of sensible scheduling and separation of data is needed – and this is the main task of the session layer.

In the world of networking, a session begins when an application wants to make connection to a remote server - the session layer opens a temporary ‘channel’ between the two.

You can have more than one session running at the same time.

So they are going to need some management – for example, you may have a Word session open and wish to save your work, at the same time you may have a web browser open and are flicking across the web. Each application wants the commands and data it is using to go to the right place i.e. your Word file ends up on the file server and the Web pages are displayed in your browser.

The session layer carries out the following tasks.

	Starts and ends a session across a network (on request)
	Allows applications to share information.
	Ensures that information is flowing to the right place
	Talks to the PRESENTATION layer above
	Talks to the TRANSPORT layer below.

Recovering from failure.

Another handy task the SESSION layer can undertake is managing the transfer of large files (such as video) across an unreliable or time-limited network. Imagine you have a 3 Gigabyte file to move and on a perfect day the network you are using will take, for example, 3 hours to complete the task – but its not a perfect day and the network breaks down after an hour! Wouldn’t it be useful to continue from the point of failure rather than wasting another hour to get to the same point as before? The session layer can ‘time stamp’ your data so that it knows where to re-start the transfer.