## 4 Cross phase compatibility

03.903GPPTSUnstructured Supplementary Service Data (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, subclauses 5.6 and 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.