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. One particular type of AAL service is the signalling AAL (SAAL) which comprises AAL functions necessary to support a signalling. The structure of the SAAL is defined in Recommenda-tion Q.2100 [8].
The SAAL consists of a Segmentation and Reassembly (SAR) function, and a Convergence Sublayer which is specified as two sublayers: a Common Part Convergence Sublayer (CPCS) and a Service Specific Convergence Sublayer (SSCS). The CPCS is defined in clause 6/I.363 [5] 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 [9] and is suitable for use by various SSCFs. This Recommendation specifies the SSCF for signalling at the Network Node Interface (NNI).
The SSCF at the NNI performs a coordination function between the service required by the signalling layer 3 user and the service provided by SSCOP. This Recommendation describes for the SSCF at the NNI the mapping of primitives from layer 3 to signals of the SSCOP and vice versa. Similarly, it specifies the exchange of signals between layer management and the SSCF at the NNI.