Object The object of this document is to define the minimum requirements in terms of services@ operating scenario and service attributes of a common air interface (CAI) standard for a nominally Geostationary Earth Orbit (GEO) Mobile Satellite Service (MSS) enabling singlemode or multimode user terminal operation with the terrestrial GSM network. Additional features@ over and above those mandated by the minimum requirements are optional and shown italicized and enclosed in brackets. The subject of standardization is the satellite-mobile link in both uplink and downlink directions. A GEO mobile satellite system has several elements which differentiate it from cellular systems@ low earth orbit (LEO) systems and mid-earth orbit (MEO) systems. In particular@ the propagation path is significantly longer resulting in larger path losses and longer round trip delays which must be accommodated by the protocol. On the other hand@ there are opportunities to simplify the protocol or to introduce new features and services of which the other systems might not be capable. Multimode operation with terrestrial GSM introduces the opportunity of minimizing the complexity of the ground station and the user terminal by reusing existing GSM hardware. As such@ the CAI shall be developed based on concepts@ services and Technical Specifications of Phase 2 GSM and allow reuse of existing GSM network infrastructure [1@2@3@4]. This requirements document is limited in scope and does not address@ nor is it intended to preclude other standards which might describe mobile satellite communication interoperating with GSM or other types of existing or future wireless networks.