SUBJECT G3)

Most people agree that the ATM standards cover 3 distinct layers -- Physical Layer, ATM Layer, and ATM Adaptation Layer (AAL).

The Physical Layer (corresponding to OSI Physical) is usually taken to be SONET/SDH (which itself has 4 layers...) but can be other things as well. The PHY deals with medium-related issues.

The ATM Layer is responsible for creating cells and formatting the cell header (5 octets). Some argue that it also corresponds to OSI Physical (it deals with bit transport) and others say its OSI Data Link (formatting, addressing, flow control, etc.).

The AAL is responsible for adapting ATM's cell switching capabilities to the needs of specific higher layer protocols. The AAL is responsible for formatting the cell payload (48 bytes). Some argue that this layer corresponds to OSI data link (data error control, above Physical), others OSI transport (it's end-to-end).

I think that this all proves that the OSI model is an excellent model as a basis for discusion and comparison but becoming hopelessly inadequate to discuss many new services.

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Last Changed 25 August 1996