20.2 History

The first UK FDM cable systems started service with a 12 channel system between Bristol and Plymouth in 1936. (Young, 1983) Previously all telephony transmission was at audio frequency and the move to FDM was inspired for economic reasons and the pressure on cable utilisation. From then onwards the drive was for lower loss and higher bandwidth cables to support ever higher bandwidth systems.

The period during the 1960's to the mid 1970's saw the peak of FDM systems in service and the development of transistorised systems to 60MHz. However during this period developments of digital systems started to fully occupy the R&D budgets and pene­tration of digital into the trunk transmission network rapidly over­took the FDM analogue network.

20.3 Fdm hierarchy

20.3.1 General considerations

A number of different transmission media are available, such as open wire cable, coaxial cable, radio or satellite systems, all with different bandwidth capability. The multiplexing schemes use a hierarchy of building blocks to construct systems to the required bandwidth. Many stages of translation may be required with the final stages of modulation only being specific to the particular transmission medium.

The building blocks are optimised for cable transmission. The dimensions of the various blocks are largely historic and based on the economics of filter design.

20.3.2 Channel bandwidth

Channel spacing is standardised on 4kHz. This provides enough space between the voice frequencies (0.3 to 3.4kHz) to economi­cally filter the carrier, pilots, outband signalling tones, etc. that are positioned in the gaps between the speech signals.

Where transmission bandwidth is at a premium then the channel spacing may alternatively be based on 3kHz (voice bandwidth 0.2 to 3.050kHz). This provides a 4/3 increase in channel capacity but is only achieved with increased cost of the channel translation stage.

20.3.3 Group and supergroup

With reference to Figure 20.2 the channel translating section con­verts 12 voice frequency channels (or 16 if 3kHz channelling) and assembles them into a basic 'group' in the range 60 to 108kHz. Five groups are translated using carriers spaced 48kHz apart to form a 'supergroup' in the range 312kHz to 552kHz. A supergroup contains 60 channels.

Where a suffix has been added i.e. group 5 or supergroup 12, the suffix refers to the carrier that will be used to translate that particular set of channels and identifies it in the higher order band of frequen­cies.

Groups and supergroups are used to construct systems on open wire and symmetrical cable. The line frequency spectrum of the typical systems are shown in Figure 20.3.

20.3.4 Higher order translation

Above supergroup level different administrations have adopted different hierarchies to build large systems. Three schemes have been identified and are described below:

1. The 15 supergroup assemblies (UK).

2. The mastergroup and supermastergroup assemblies (Europe).

3. The Bell mastergroup plan (USA).