The Common Air Interface(CAI) is the radio or air interface for radio technology equipment. The radio interface is standardized by the European Telecommunications Standards Institute( ETSI) and defines all the physical and protocol specifications for data exchange. Interface conventions include modulation schemes, bandwidth, frequency deviation, access procedures, security procedures and switching techniques.
In a GSM network, the radio interface is characterized by the user data( Bm, Lm) and signaling( Dm) channels. Signaling data and general traffic data are transmitted via three different logical control channels, the Broadcast Control Channel( BCCH), Common Control Channel( CCCH) and the Dedicated Control Channel( DCCH), for call setup and during an existing connection.
All four channels are integrated and transmitted in TDMA data packets in a multiplexer. The GSM frame or DCS frame structure is designed in such a way that eight connections are transmitted in a staggered manner. Additional necessary information such as synchronization data and header and trailer bits are added to the TDMA frame. The data stream is modulated in a modulator with Gaussian Minimum Shift Keying( GMSK) and superimposed on the carrier frequency.