The present document specifies the functional architecture of the Short Message Service (SM) service for the pan-European Integrated Services Digital Network (ISDN) as provided by European public telecommunications operators at the T reference point or coincident S and T reference point (as defined in ITU-T Recommendation 1.41 1 [6]) by means of the Digital Subscriber Signalling SystemNo. one (DSS1) protocol. The Functional Architecture as defiied in Stage two needed to support a telecommunication service (see ITU-T Recommendation I. 130 [4]). The present document does not speciSl the additional functional entities or architecture where the service is provided to the user via a telecommunication network that is not an ISDN but it does include intenvorking requirements of other networks with the public ISDN. The Short Message Service is provided independently of a call. Charging principles are outside the scope of the present document. The Short Message Service enables the originating SMS user to send Short Messages (SMs) to the receiving SMS user via a Short Message Service Centre@ belonging to the network of the SMS originating user (served user) or separated from the network of the SMS originating user. NOTE: The Short Message Service is typically used between a Short Message service provider and a user (receiving user) of the Short Message service provided. Further parts of the present document speciSl the method of testing required to identiSl conformance to the present document. The present document is applicable to equipment supporting the Short Message Service@ to be attached at either side of a T reference point or coincident S and T reference point when used as an access to the public ISDN.