The technical realization of service accessibility in terms of registration@ handover@ roaming and system selection is defined in the 03 series of GSM specifications. The purpose of this European Telecommunication Standard (ETS) is to describe the service access procedures as presented to the user. Definitions and procedures are provided in this ETS for international roaming@ national roaming and regionally provided service. These are mandatory in relation to the technical realization of the Mobile Station (MS).