This International Standard specifies the signalling protocol for the support of the Short Message Service (SMS) at the Q reference point between Private Integrated services Network eXchanges (PINXs) connected together within a Private Integrated Services Network (PISN). This service is based on GSM 03.40. The Service Centre functionality described in this International Standard is equal to the functionality of a Service Centre in GSM 03.40. Thus, for interoperability with a GSM network, it is only necessary to implement a QSIG interface. NOTE 1 - The interworking with other air interfaces is not precluded, but is outside the scope of this International Standard. NOTE 2 - The Short Message Service is a special type of basic service but is described in the present document as a supplementary service. The Short Message Service is a service which permits a served user to send a message of limited size to another user in the same PISN or another network. 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. 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 21989. The signalling protocol for SMS operates on top of the signalling protocol for the connection oriented call independent APDU transport mechanism and uses certain further 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 SMS and supplementary services and ANFs. This International Standard is applicable to PINXs which can be interconnected to form a PISN.