Digital carrier system and multiplexing techniques

A digital carrier system is a transmission system that carries many telephone (or data) channels in digital form over one medium using PCM and time division multiplexing, with regenerative repeaters along the line. Each voice channel is sampled 8000 times per second and coded in 8 bits (64 kbps); channels are interleaved into frames of 125 µs.

Main parts

 voice -> [codec: sample, quantise, code] -> [TDM mux]
       -> [line coder] -> ~[regen. repeater]~
       -> [line decoder] -> [demux] -> [codec] -> voice

• Primary carriers: T1 (24 ch, 1.544 Mbps, North America/Japan) and E1 (30 ch, 2.048 Mbps, Europe/Nepal).
• Higher orders: T2/T3 (6.312, 44.736 Mbps), E2/E3/E4 (8.448, 34.368, 139.264 Mbps), then SDH (STM-1 155.52 Mbps and above) over fibre and microwave.
• Uses line codes (AMI, B8ZS, HDB3) and frame/signalling bits.

Advantages: noise does not accumulate (regeneration), easy integration with digital switching, cheap ICs, easy encryption and error monitoring. Disadvantages: needs more bandwidth than analogue, and needs strict synchronisation.

Advantages and disadvantages of multiplexing techniques

1. Frequency Division Multiplexing (FDM)

• Advantages: simple for analogue signals; no synchronisation; continuous transmission for all users; well suited to radio/TV broadcast.
• Disadvantages: bulky, costly filters and oscillators; guard bands waste bandwidth; crosstalk and intermodulation from non-linear amplifiers; noise accumulates in analogue repeaters.

2. Synchronous Time Division Multiplexing (TDM)

• Advantages: digital, IC based and cheap; little crosstalk; regenerative repeaters; easily mixed voice and data; fits digital switching (T1/E1).
• Disadvantages: needs accurate bit and frame synchronisation; slots wasted when sources are idle; higher bandwidth than analogue FDM for voice.

3. Statistical TDM

• Advantages: slots only for active sources, so better link utilisation; more terminals per line.
• Disadvantages: address/length overhead; buffering, variable delay and possible overflow; complex.

4. Wavelength Division Multiplexing (WDM)

• Advantages: huge capacity on existing fibre; protocol and bit-rate transparent; easy upgrade by adding wavelengths.
• Disadvantages: costly stable lasers and optical filters; channel crosstalk, dispersion and non-linear effects; only for fibre.

5. Code Division Multiplexing (CDM/CDMA)

• Advantages: all users share the whole band at all times; resistant to interference and jamming; soft capacity; secure.
• Disadvantages: needs power control (near–far problem); complex receivers; capacity limited by mutual interference.