Multiplexing techniques and TDM standards

Multiplexing is combining several signals for transmission over one shared medium.

Multiplexing techniques

1. Frequency Division Multiplexing (FDM): each signal is placed in its own frequency band by modulation; guard bands separate the bands; all channels are sent together. Used in analogue carrier telephony, radio and TV broadcasting.
2. Time Division Multiplexing (TDM): each channel is given a time slot in a repeating frame and uses the full bandwidth during its slot. Synchronous TDM (fixed slots, e.g. T1/E1) and statistical TDM (slots on demand, with addresses).
3. Wavelength Division Multiplexing (WDM): in optical fibre, each signal is carried on a different wavelength of light (CWDM, DWDM).
4. Code Division Multiplexing (CDM/CDMA): all users share the same band at the same time, each spread by a unique orthogonal code; used in 3G mobile and GPS.
5. Space Division Multiplexing: separate physical paths (pairs in a cable, fibres, beams).

North American TDM standard

Basic frame: 24 channels × 8 bits + 1 framing bit = 193 bits per 125 µs.

$$\begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frame rate} &= 8000\ \text{frames/s}\ (T = 125\ \mu\text{s}) \\ R_{T1} &= 193 \times 8000 = 1.544\ \text{Mbps} \end{aligned}$$

24 x DS0 --> [MUX] --> T1  1.544 Mbps (24 ch)
 4 x T1  --> [M12] --> T2  6.312 Mbps (96 ch)
 7 x T2  --> [M23] --> T3  44.736 Mbps (672 ch)
 6 x T3  --> [M34] --> T4  274.176 Mbps (4032 ch)

European TDM standard

Basic frame: 32 time slots × 8 bits (TS0 sync, TS16 signalling, 30 voice).

$$\begin{aligned} \text{Bits per frame} &= 32 \times 8 = 256\ \text{bits} \\ R_{E1} &= 256 \times 8000 = 2.048\ \text{Mbps} \end{aligned}$$

30 x 64k --> [MUX] --> E1  2.048 Mbps (30 ch)
 4 x E1  --> [MUX] --> E2  8.448 Mbps (120 ch)
 4 x E2  --> [MUX] --> E3  34.368 Mbps (480 ch)
 4 x E3  --> [MUX] --> E4  139.264 Mbps (1920 ch)
 4 x E4  --> [MUX] --> E5  564.992 Mbps (7680 ch)

The European system uses a regular factor of 4 at every level and keeps signalling in a separate slot; the American system uses robbed-bit signalling and factors 4, 7 and 6.