scope:

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 GSM 02.30 and GSM 02.90. The alphabet indicator and the data coding scheme are defined in GSM 03.38.

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).