This Global System for Mobile communications Technical Specification ( GTS) gives the stage 2 description of Unstructured Supplementary Servi ce Data (USSD). The unstructured supplementary service data (USSD) mechanism allows th e Mobile Station (MS) user and a PLMN operator defined application to communicate in a way which is transparent to the MS and to intermediat e network entities. The mechanism allows development of PLMN specific supplementary services. This specification defines the requirements for handling USSD at the M S and network entities. It does not include specification of particula r applications, nor does it specify how a particular application is se lected. Where more than one application exists at a network entity, ro uting of messages to the correct application is carried out by the USS D handler. The MMI for USSD is specified in GSM 02.30 and GSM 02.90. T he 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 followi ng ways: - Network initiated USSD (clause 1); - Mobile initiated USSD (clause 2).