20.13 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.

20.13.1 Synchronisation

The frequency accuracy requirement of an FDM supergroup is better than 1 part in 10⁷ whereas the TDM stream is 50 part in 10⁶ (Ribeyre et. al., 1982). To achieve the FDM accuracy the transmultiplexer may have to be locked to an accurate reference and allowance made to frame slip the TDM data.

20.13.2 Pcm alarms

Data and alarms carried on TSO of the PCM frame, data services carried on either the TDM or FDM signals and common channel signalling schemes have to be terminated at the transmultiplexer and treated separately.

20.14 Repeatered cable line equipment

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 terminal end. A constant current d.c. supply is established on the copper transmission wires for supplying power to the dependent repeaters (usually 50 or 110 mA).

Dependent repeaters are installed at regular intervals along the cable, the recommended spacing is as in Table 20.11. Repeaters are normally housed in pressurised boxes, either pole mounted or buried depending on the siting of the cable.

A coaxial line is required to meet an average noise contribution per circuit of better then 1,5pWOp per km. In order to achieve this with a limited DC power budget optimal designs have to be used.