Supplementary service SD enables a user to communicate with another user or application by the exchange of keypad and display information transparently over a PISN.
The Q reference point is defined in ISO/IEC 11579-1.
Service specifications are produced in three stages and according to the method specified in ETS 300 387. ISO/IEC 21408:2003 contains the stage 3 specification for the Q reference point and satisfies the requirements identified by the stage 1 and stage 2 specifications in ISO/IEC 21407.
The signalling protocol for SS-SD operates on top of the signalling protocol for basic circuit switched call control, as specified in ISO/IEC 11572, and uses certain aspects of the generic procedures for the control of supplementary services specified in ISO/IEC 11582.
ISO/IEC 21408:2003 also specifies additional signalling protocol requirements for the support of interactions at the Q reference point between SS-SD and other supplementary services and ANFs.
ISO/IEC 21408:2003 is applicable to PINXs which can interconnect to form a PISN.