y requirement of an FDM supergroup is algorithms depend on the type of signalling adopted (i.e. CCITT R1 better than 1 part in 107 whereas the TDM stream is 50 part in 106 or R2, E&M scheme etc.). The TDM band is then formatted for (Ribeyre et. al., 1982). To achieve the FDM accuracy the transmulti- GROUP Format 60 channel Signalling/ Pilots SUPERGROUP 2 x 30 CHANNEL 312 552 kHz 2Mbit/s G703 Interface Pulse Code Modulated Format Signalling/ Pilots Figure 20.20 Supergroup transmultiplexer 20/20 Frequency division multiplexing plexer may have to be locked to an accurate reference and allowance made to frame slip the TDM data. Table 20.11 Repeatered cable spans 20.13.2 PCM alarms System Cable Repeatered Data and alarms carried on TSO of the PCM frame, data services 1.3MHz (300 channels) 1.2/2.4mm carried on either the TDM or FDM signals and common channel signalling schemes have to be terminated at the transmultiplexer 4MHz (960 channels) 1.2/2.4mm and treated separately. 12MHz (2700 channels) 1.2/2.4mm 18MHz (3600 channels) 2.6/9.5mm 20.14 Repeatered cable line equipment 60MHz (10800 channels) 2.6/9.5mm 1.5km The main function of the line terminal equipment is to service the cable transmission equipment. A supervisory overlay system is usually provided whereby a faulty repeater can be located from the 20.14.1 Pre-Emphasis terminal end. A constant current d.c. supply is established on the copper transmission wires for supplying power to the dependent The noise contributions from a single repeater are shown in Figure repeaters (usually 50 or 110 mA). 20.21(a). The greatest contribution is thermal noise at high frequen- Dependent repeaters are installed at regular intervals along the cies as the highest gain is required at this end to overcome to cable cable, the recommended spacing is as in Table 20.11. Repeaters are loss. normally housed in pressurised boxes, either pole mounted or buried By applying deliberate shaping to the transmission path at the depending on the siting of the cable. transmitting terminal end (p