SERVICE ACCESS POINTS (SAPs)
Service access points (SAPs) are OSI addresses and identifiers of OSI entities, such as an application that resides in layer 7, or a protocol that resides in layer 3. Figure A-8 illustrates these concepts.
Figure A-8. OSI SErvice Access Points (SAPs)
The identifier idea is straight forward; it simply identifies something in a machine. Unfortunately, SAPs can be confusing because they can be used in more than one way. Just keep in mind that they are used to identify a process (in practice, a software module) in a layer.
We must defer to the OSI Model in its description of the SAP. The Model states that: an (N + 1)-entity ...
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