The intent of this Recommendation is to provide a new protocol specification that can be used in the B-ISDN ATM Adaptation Layer (AAL). This protocol, called the Service Specific Connection Oriented Protocol (SSCOP), provides assured data delivery between AAL connection endpoints.
The ATM Adaptation layer (AAL) is defined as enhancing 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 user is the signalling entity wishing to communicate with a peer entity. The signalling AAL (SAAL) comprises AAL functions necessary to support such a signalling entity. The structure of the SAAL is defined in Recommendation Q.2100.
The SSCOP has been defined to provide functions required in the SAAL. The SAAL is a combination of two sublayers: a common part and a service specific part. The common part protocol is defined in Recommendation I.363. The service specific part is also known as the Service Specific Convergence Sublayer (SSCS). In the SAAL, the SSCS itself is functionally divided into the SSCOP and an Service Specific Coordination Function (SSCF) which maps the services provided by the SSCOP to the needs of the user of the SAAL. This structure allows a common connection oriented protocol with error recovery (the SSCOP) to provide a generic reliable data transfer service for different AAL interfaces defined by the SSCF. Two such SSCFs, one for signalling at the User Network Interface (UNI) and one for signalling at the Network to Network Interface (NNI), have been defined in Recommendations Q.2130 and Q.2140, respectively. It is also possible to define additional SSCFs over the common SSCOP to provide different AAL services.
The SSCOP is a peer-to-peer protocol which provides the following functions:
– transfer of user data with sequence integrity;
– error correction by selective retransmission;
– flow control;
– connection control;
– error reporting to layer management;
– connection maintenance in the prolonged absence of data transfer;
– local data retrieval by the user;
– error detection of protocol control information; and
– status reporting.
This Recommendation describes for SSCOP the necessary elements for layer to layer communication, the elements for peer-to-peer communication, a detailed protocol specification, and examples of SSCOP operation.