The present document gives the stage 2 description of Unstructured Supplementary Service Data (USSD). The unstructured supplementary service data (USSD) mechanism allows the Mobile Station (MS) user and a PLMN operator defined application to communicate in a way which is transparent to the MS and to intermediate network entities. The mechanism allows development of PLMN specific supplementary services. The following diagram shows how handling of USSD is carried out, independently of the applications.

The present document defines the requirements for handling USSD at the MS and network entities. It does not include specification of particular applications, nor does it specify how a particular application is selected. Where more than one application exists at a network entity, routing of messages to the correct application is carried out by the USSD handler. The MMI for USSD is specified in TS 22.030 and TS 22.090. The alphabet indicator and the data coding scheme are defined in TS 23.038. USSD may be initiated by the MS user, or by the network in the following ways:

• Network initiated USSD  (clause 1);
• Mobile initiated USSD  (clause 2).

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

In addition to those below, abbreviations used in the present document are listed in TR 21.905. AI MI USSD

The Phase 1 series of GSM specifications defined the signalling protocol which may be used, but they did not specify the operation of USSD as a service. The main body of the present document assumes that the MS and all network entities comply with this phase of USSD. In order to minimize any possible problems between a Phase 1 implementation of USSD and this phase, clause 5.6 and clause 6.6 define the additional requirements for when one or more entity complies with the Phase 1 USSD specification for network initiated and mobile initiated USSD respectively.