Why bother?

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Layer Overlap!

Exam Summary





“Coding and conversion functions. Ensure information sent from the application layer of one system will be readable by the application layer of the other system.”

Internetworking involves the interaction of different machines. A user might have to transfer a text file from an Apple Mac to a Unix workstation and then on to an NT-based PC.

A problem arises from the fact that different machines represent data in different ways. A good example explaining this concept is the two different ways in which text (characters and numbers) can be represented in code, ASCII or EBCDIC.

ASCII (pronounced askee) is the acronym for the American Standard Code for Information Interchange. Fortunately, most computers use ASCII codes to represent text, which makes it possible for a computer to understand data sent from another computer.

Pronounced eb-suh-dik, EBCDIC is an abbreviation of Extended Binary-Coded Decimal Interchange Code. EBCDIC is widely used on large IBM computers (mainframe).

Imagine trying to send text from an IBM mainframe to a PC. For example, the ASCII code for uppercase M is 77. Code 77 has no corresponding meaning in EBCDIC, which uses code 212 to represent uppercase M. Without proper presentation layer coding and conversion functions, text from these two computers would be incomprehensible to each other. It is therefore the job of the presentation layer to make sure that data sent from the application layer of one system is understandable by the application layer of another system.

Another way to summarize what the presentation layer does is “it converts data from the application layer into a standard format.”

The presentation layer can also encrypt and compress data before passing it on the next layer.


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