Transmultiplexers

A transmultiplexer provides a link between FDM and TDM hierar­chies without having to translate to baseband voice frequencies.

The drive for transmultiplexer development came from analogue network operators updating to digital switches. With a digital switch the interface to the trunk network is at a 2.048kbit/s CEPT defined digital bit stream (or 1.554Mbit/s Tl for the USA digital hierarchy) i.e. there is no point that is suitable for direct connection into an FDM hierarchy without the cost and attendant distortions of trans­lation to speech band.

As a prime function therefore transmultiplexer interface to 2048kbit/s 30 channel (1544kbit/s 24 channel).

An economic fit of this digital interface is normally achieved by taking 2 X 2048kbit/s 30 channel bit streams with a total of 60 channels and transmultiplexing to a single FDM supergroup. The alternatively fit for a Tl 1544kbit/s 24 channel bit stream is trans-multiplexed into 2 X FDM groups each carrying 12 channels. A block diagram of a typical 60 channel Transmultiplexer is shown in Figure 20.20 (Rossiter et. al.,1982).

The incoming supergroup is filtered to remove the adjacent side­bands. The gain is adjusted using an AGC from the supergroup pilot extracted within the Discrete Fourier Transform (DFT) and the complete band digitised through an analogue to digital converter. Using Digital Signal Processing (DSP) techniques the digitised band is split and then transformed into a TDM representation.

At this point the coefficients representing the pilots and signalling are extracted. The signalling is processed separately and the process algorithms depend on the type of signalling adopted (i.e. CC1TT Rl or R2, E&M scheme etc.). The TDM band is then formatted for A-law companding, PCM coding and time processed for the 2O48kbit/s CEPT structure.

The opposite procedure is adopted for the PCM to FDM conver­sion.