exers interface to 2048kbit/s 30 channel (1544kbit/s 24 channel). An economic fit of this digital interface is normally achieved by 20.12 Through connections taking 2 X 2048kbit/s 30 channel bit streams with a total of 60 channels and transmultiplexing to a single FDM supergroup. The Within the network it may be required that a band of channels alternatively fit for a T1 1544kbit/s 24 channel bit stream is trans- (typically group or supergroup) are allocated as a through connec- multiplexed into 2 X FDM groups each carrying 12 channels. A tion path without translation to voice band. This could be at a drop block diagram of a typical 60 channel Transmultiplexer is shown in and insert point for instance as shown in Figure 20.18. Figure 20.20 (Rossiter et. al., 1982). Dropped/Inserted Traffic GP and Channel GP and Channel Translating Translating 4MHz Line 4MHz Line Through Translating Equipment Translating Through connected traffic Figure 20.18 Drop and insertion of traffic Transmultiplexers 20/19 CONVENTIONAL DEMODULATION Demodulator Filter Characteristics THROUGH CONNECTION FILTER Broadband multichannel spectrum Spectrum for demodulation Through Connected Filter Characteristic DEMODULATOR Baseband Baseband for through connection 'Clean' band presented for remodulation Figure 20.19 Through connection filters The incoming supergroup is filtered to remove the adjacent side- A-law companding, PCM coding and time processed for the bands. The gain is adjusted using an AGC from the supergroup pilot 2048kbit/s CEPT structure. extracted within the Discrete Fourier Transform (DFT) and the The opposite procedure is adopted for the PCM to FDM conver- complete band digitised through an analogue to digital converter. sion. Using Digital Signal Processing (DSP) techniques the digitised band is split and then transformed into a TDM representation. 20.13.1 Synchronisation At this point the coefficients representing the pilots and signalling are extracted. The signalling is processed separately and the process The frequency accurac