This International Standard specifies the Short Message Service (SMS). SMS enables a user to send and receive Short Messages (SMs) to and from another user. 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 kind of basic service but is described in this document in the style of a supplementary service. Supplementary service specifications are produced in three stages, according to the method described in ETS 300 387. This International Standard contains the stage 1 and stage 2 specifications of SMS. The stage 1 specification (clause 6) specifies the service as seen by users of PISNs. The stage 2 specification (clause 7) identifies the functional entities involved in the service and the information flows between them.