service access point identifier (ISDN) (SAPI)
- Service Access Point Identifier (SAPI) represents an access point to a specific service in the link layer in the D- channel protocol. The SAPI identifier is used to establish a relationship to an instance in the network layer. In the D-channel protocol, four possible instances or services of layer 2 (SAPI) are defined for the terminal equipment, which are expressed in the 64 possible SAPI values. These SAPI values are permanently assigned to the services of the data link layer: SAPI-0 is used for signaling, SAPI-16 for packet-oriented data transfer, SAPI 32 to 47 are reserved for national purposes, and SAPI 63 for management functions such as TEI allocation or the formation of SAPI groups.
- In Link Access Procedure SDH( LAP-S), the Service Access Point Identifier (SAPI) is a 2-bytedata field in which the IP protocol to be transported is entered.