service access point (OSI) (SAP)
The service access point( SAP) is in each case the point in a communication model at which a layer makes its service available to the layer directly above it.
All communication, i.e., all information exchange between two neighboring layers, takes place via this service access point. It is the interface between the lower and higher layers. In the case of the transition between the physical layer and the datalink layer, the service access point is called the Data Link Service Access Point (LSAP); in the case of the network layer, it is the Network Service Access Point ( NSAP); in the case of the transport layer, it is the Transport Service Access Point ( TSAP); and in the case of the communication control layer, it is the Session Service Access Point( SSAP).
The higher layers do not have to comply with the principles of the OSI reference model. Only the functionality required by the network layer at the network service access point, the NSAP, must be fulfilled by the service user.
InIEEE 802.2, the Service Access Point is part of the address specification, which is composed of the Source Service Access Point (SSAP) and the Destination Service Access Point( DSAP). The Destination Service Access Point (DSAP) is the Service Access Point (SAP) of a network node that is specified as the destination address in the header of the data packet. It is a 1-bytedata field in the header of 802.2.