Rules

There are some basic rules in the OSI model:-

	Each layer can only talk to the one above it and below it e.g. The ‘Network’ layer can only pass information to the ‘Transport’ layer above it and the ‘Data Link’ layer below it.
	Passing information from one layer to another is done in a standard way. This means that if company ‘A’ has written some software for the ‘Transport’ layer – and company ‘B’ has written some software for the ‘Network’ layer – then they are guaranteed to be able talk to each other.
	Any layer does not care or need to know how the layers beneath it work. It will assume that the information it needs, is going to be presented in a standard way. This makes it much simpler to design the layer itself.

The ‘Application Set’ is mainly concerned with controlling how the various applications currently running are making use of the network.

For example, you could be running ‘Internet Explorer’ to view web pages and at the same time be writing a report in ‘Word’ that is stored on the school’s file server. Both are making use of the network in different ways.

On the other hand, the ‘Transport Set’ is only concerned with passing information through the network – it does not care whether the information comes from the Internet Explorer application or the Word application currently running on the machine.

Notice that the top four layers have been grouped together and are called the ‘Application Set’ and the bottom three layers are called the ‘Transport set’.

The next section describes the main task of each layer.