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 service supported by the AAL is the Connection Oriented Transport Service (COTS). The AAL for COTS comprises functions necessary to support COTS data communications between peer entities in the user plane.
The AAL for COTS 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 and is used as the underlying protocol for the service specific part for COTS. 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 [4] and is suitable for use by various SSCFs. This Recommendation specifies the SSCF for COTS.
This Recommendation describes for the SSCF-COTS the mapping of primitives from the Synchronization and Coordination Function (SCF) (defined in Recommendation Q.923 [3]) to signals of the SSCOP and vice versa. It also specifies the exchange of signals between Layer Management and the SSCF-COTS.