This International Standard specifies the Authentication supplementary services, which are applicable to various basic services supported by Private Integrated Services Networks (PISN). Basic services are specified in ISO/IEC 11574. 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 supplementary services are based on the challenge and response method of authentication. Authentication algorithms to be used by these two supplementary services (SS-WTAT and SS-WTAN) are outside the scope of this International Standard. This International Standard provides the information flows to convey the security parameters. Supplementary service specifications are produced in three stages, according to the method described in CCITT Recommendation I.130. This International Standard contains the stage 1 and stage 2 specifications of SS-WTAT and SS-WTAN. The stage 1 specification (clause 6 and 7) specifies the supplementary service as seen by users of PISNs. The stage 2 specification (clause 8 and 9) identifies the functional entities involved in the supplementary service and the information flows between them.