This Recommendation defines a Service Specific Convergence Sublayer that operates above the Common Part Sublayer of an AAL type 2 connection. The purpose of the SSCS is to convey narrow-band calls consisting of voice, voiceband data, or circuit mode data. The SSCS specifies packet formats and procedures to encode the different information streams for bandwidth-efficient transport by AAL type 2. It accommodates known techniques of low rate audio encoding, silence compression, and facsimile demodulation/remodulation. It makes provision for in-band signalling of narrow-band calls and for control of the SSCS operating state. The SSCS anticipates that multiplexing in the Common Part Sublayer will be used to carry many calls over a single ATM connection, which will therefore act as a trunk group for transmission between two points of access. Additional details needed to configure and manage a trunk group, however, are beyond the scope of this Recommendation. The focus is on normative aspects of SSCS coding and behaviour within a single AAL type 2 connection.