This International Standard specifies service description and control aspects of the multi-rate circuit-mode basic services which may be supported by Private Integrated Services Network (PISN). One of the purposes of the stage 1 and stage 2 specifications is to guide and constrain the work on signalling protocols at stage 3. Therefore, this International Standard is concerned mainly with the control aspects of services. A stage 3 standard shall be in conformance with this International Standard, if the signalling protocols and equipment behavior specified in the stage 3 standard are capable of being used in a PISN which supports any or all of the basic services specified in this International Standard. This International Standard encompasses Circuit-Mode Multi-rate Unrestricted 8 kHz Structured Bearer Service. Negotiation of services at call establishment time, interworking using channel aggregation (N×64kbit/s) and change of service during a call are outside the scope of this International Standard.