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COMMUNICATIONS 225
LAYER 5: SESSION LAYER
This layer handles the life of the current connection and keeps the data traffic moving.
LAYER 6: PRESENTATION LAYER
This layer handles the data from applications. It performs packing, encryption, decryp-
tion, compression, and so on.

LAYER 7: APPLICATION LAYER

This layer is where the application software resides. More information about the seven-
layer model can be found at the following PDF and web sites:

www.itp-journals.com/nasample/t04124.pdf
www.itp-journals.com/OSI_7_layer_model_page1.htm
www.scit.wlv.ac.uk/ jphb/comms/std.7layer.html
www.cs.cf.ac.uk/Dave/Internet/node51.html

Not everyone is happy with the seven-layer OSI model. Check out www.randywanker
.com/OSI/ (rated R) and www.scit.wlv.ac.uk/ jphb/comms/osirm.crit.html
A couple of underlying ideas are behind the layering of this stack that applies across
most communications:

Hidden functions The stack layers interact with a fixed interface. Portions of
the stack can be redesigned internally and still function properly.
Common interfaces Because the stack layers interact with a fixed interface,
two different machines can communicate with each other without a problem. They
simply communicate from the same level to the same level. For example, TCP
information at level 4 in one machine travels down the stack to the physical level
and is sent to the other machine. At the receiving machine, it enters the physical
level and travels up to level 4 where it appears as TCP information again.

Many communication techniques lead to standards that can be observed by all
designers at various stack levels. Most communication standards are limited to just a
few levels of complexity. They all have physical and link layers. Many have network
and transport levels, but not many go to higher levels.