The Common Air Interface (CAI) is the radio or air interface for radio technical equipment. The radio interface is standardized by the European Telecommunications Standards Institute (ETSI) and defines the entirety of the physical and protocol specifications for data exchange. The interface conventions include modulation procedures, 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. Signalling 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 call.Traffic Channel (TCH), is used for the transmission of user data from the Bm data and the Lm data respectively. 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.