ence point 34.5.1 Analogue line circuits standard at 144kbit/s with 2B+D channels. In 2-wire systems the line interface is more complex and two line Analogue PABX line circuits are very similar to those of public systems are common, burst mode or echo cancellation. Echo can- exchanges providing the same functionality for connection of the cellation is much more complex to implement in silicon and PABX plain ordinary telephone. line circuits have tended to use the simpler burst mode technique. The PABX line circuit needs to provide the BORSCHT functions Echo cancelling is a full duplex solution in that it transmits data but the specification can be less severe in some areas, as the line continuously in both directions at the same time. In burst mode length of the average PABX line is under 1km. operation the line circuit transmits a burst of data to the terminal, the line is switched off for a guard period and then a burst of data is transmitted from the terminal to the line circuit. The line trans- 34.5.2 Key system line circuits mission rate in burst mode systems is therefore much higher than in echo cancellation systems for the same rate of information transfer In key telephone systems the line circuits differ from that of a PABX between the terminal and the line circuit. The technique is based on in that additional signalling information needs to be transmitted to the fact that duplex transmission can be achieved on a single pair of and from the telephone to provide the functionality of a key system. wires by increasing the line transmission rate by some factor greater No standards exist for these signalling systems and they are all than 2 such that the transmit and receive signals at each end are proprietary to the individual key system manufacturer. However the compressed in time and never overlap, as in Figure 34.10. principle is the same in all the systems even if the physical and Transmission rates of 384kbit/s on cable up to 1000 ohm loops are logical implementations may differ. possible. A typical trans