The ATM Adaptation Layer (AAL) is defined to enhance the services provided by the ATM layer to support the functions required by the next higher layer. Different AALs support various protocols to suit the different needs of a range of AAL service users. One particular type of AAL service is the signalling AAL (SAAL), which comprises AAL functions necessary to support a signalling entity. The structure of the SAAL is defined in Recommendation Q.2100.
The SAAL consists of a Segmentation and Reassembly function (SAR), and a Convergence Sublayer which is divided into two sublayers: a Common Part Convergence Sublayer (CPCS) and a Service Specific Convergence Sublayer (SSCS). The common part protocol is defined in clause 6/I.363 and is used as the underlying protocol for the service specific part for signalling. The SSCS is functionally divided into two parts: The Service Specific Connection Oriented Protocol (SSCOP), which provides an assured data transfer service, and the Service Specific Coordination Function (SSCF). The SSCOP is defined in Recommendation Q.2110 and is equally well suited for use by various SSCFs. This Recommendation specifies the SSCF for signalling at the User-to-Network Interface (UNI).
The SSCF at the UNI performs a coordination function between the service required by the signalling layer 3 (Recommendation Q.2931) user and the service provided by SSCOP.
This Recommendation describes for SSCF at the UNI the mapping of primitives from layer 3 to signals of the SSCOP and vice versa.