The present document defines the service offered by the physical layer to the upper layers of the MES-GS interface in the GMR-1 Mobile Satellite System. Its main objective is to provide guidance for the interface between the GMR-1 Technical Specifications in the 45-series and the 44-series. It also specifies the format of signalling channels, traffic channels (Iu mode) and the order of bit transmission.

As far as possible, the present document makes use of the layering principles of the Reference Model for Open System Interconnection (OSI) as contained in ITU-T Recommendations X.200 [i.1] and X.210 [i.2].