This International Standard specifies the signalling protocol for the support of the Wireless Terminal Authentication supplementary services (SS-WTAT and SS-WTAN) at the Q reference point between Private Integrated services Network eXchanges (PINXs) connected together within a Private Integrated Services Network (PISN). Authentication of a WTM user (SS-WTAT) is a supplementary service that enables a PISN, as a security measure, to validate the identity provided by the WTM user. Authentication of the PISN (SS-WTAN) is a supplementary service that enables a served WTM user, as a security measure, to validate the identity of the PISN. The mechanisms used in these services are based on the challenge and response method of authentication. Authentication algorithms to be used by these supplementary services are outside the scope of this International Standard. 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 ITU-T Rec. 1.130. This International Standard 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 15432. The signalling protocol for SS-WTAT and SS-WTAN uses certain aspects of the generic procedures for the control of supplementary services specified in ISO/IEC 11582. This International Standard also specifies additional signalling protocol requirements for the support of interactions at the Q reference point between SS-WTAT / SS-WTAN and other supplementary services and ANFs. This International Standard is applicable to PINXs which can interconnect to form a PISN.