mission system would have the following The key telephone and the line circuit have a bi-directional data key parameters: 4kHz burst repetition rate; 2 PCM characters (16 circuit that connects the processor in the telephone with the proces- bits); 1 data word (16 bits); signal/aux channel (4 bits); total sor in the key system. A message is sent from the the key telephone bits/burst (36 bits); 384kbit/s line transmission; scrambled AMI to the system requesting service and this is acknowledged by the pulse transmission. system returning a message to light a lamp or change the indication Figure 34.11 shows how the voice and data are combined on the on the telephone display. Messages are sent between the telephone one pair of wires, then separated at the line circuit end to be and the system until the call is completed. The system may require switched through the system. to send messages to idle or busy telephones on the system, depend- ing on the system configuration, informing each telephone of the status of all the other telephones, the status of tie lines, or trunk lines. As the system size increases SO does the need for more messages on Control and administration the line. In this form of system the telephone is normally analogue and In the modern PABX the system is under software control, running similar to an ordinary telephone but the signalling can be digital or on either a single processor for the smaller systems or with the analogue on a separate pair of wires from the system to the tele- functions divided between processors in the larger systems. In any phone, depending upon the individual designs. case the particular architecture is governed by the system designer's Burst repetition period card Tx Terminal Rx Terminal Tx Transmission Line card Rx delay Figure 34.10 Burst mode transmission 34/12 PABX and key systems Codec voice Digital Transmission Digital or data interface filters interface Control Signalling Signalling & message & message interface interface Control Displays Line card Terminal Figure 34