An AAL, specified in ITU Q.2100 "ATM Adaptation Layer for Signalling (SAAL)" that provides reliable transport of signalling messages (Q.2931) between peer Q.2931 entities. These signalling messages are carried over a PVC. Currently, the signalling VC is defined to be a PVC with VPI = 0 and VCI = 5.
The SAAL consists of the following:
+--------------------+ | SSCF | | ------------------ | SSCS | SSCOP | +--------------------+ | CPCS | +--------------------+ | SAR | +--------------------+
SSCOP is a general purpose data transfer layer providing, among other things, assured data transfer.
SSCF is a coordination function that maps SSCOP services into those primitives needed specifically for signalling (by Q.2931). Different SSCFs may be prescribed for different services using the same SSCOP.
The SSCS may be null as well (e.g. IP-over-ATM or LAN Emulation).