le 1 ensures that communication channels can be established and consists of 1,0,1,0,1 reversals for bit timing, and the channel mar- maintained. To achieve this three channels types are needed: ker word (CHM) is used to set up frame synchronisation. Successful synchronisation is marked by the transmission of the SYNC word 1. B-channel for speech or data. from the handset at which point it is able to transmit the same frame 2. D-channel for signalling and control. structure and forward its identification code in the D-channel. 3. SYN-channel for bit and burst synchronisation. CT2/CAI digital specification 49/7 Multiplex 1.2 72 bits (1ms) (that is, one burst) 66 bits B Channel 64 bits 1 bit 1 bit 64 bits 1 bit 1 bit 64 bits 6 bits 6 bits Receive Receive Transmit Multiplex 1.4 72 bits (1ms) (that is, one burst) 68 bits B Channel 64 bits 2 bits 2 bits 64 bits 2 bits 2 bits 64 bits 4 bits 4 bits Receive Receive Transmit Figure 49.5 Multiplexer structures used by CT2/CIA At various stages of a call set up these three are conveyed by the Layer 3 messages are the information elements delivered error above mentioned multiplexes. free by layer 2 to the end of the CT2/CAI radio link. These elements have meaning and can be translated into specific responses. For Layer 2 supports the ability of CT2/CAI to communicate mess- example keyed digits 0-9, star and square, handset display elements ages between end points of the link. Such messages include those and many other messages that a manufacturer may wish to employ associated with the D-channel error detection and correction, mess- in his product. age acknowledgement, call set up and clear down messages etc. A Layer 2 message package consists of up to six words each formed 49.3.4 Transmission plan and digital codec from eight 8-bit bytes. The first word is the address code word and the remainder are data code words. Messages may be formed from To meet the European standard objectives it is necessary to define a number of message packages. the transmission characteristics of b