TDM principle and T1 carrier

Working principle of TDM

In Time Division Multiplexing, several signals share one line by taking turns. Time is divided into frames, and each frame into time slots; each channel gets one slot per frame and uses the full line bandwidth during it.

 ch1 -[LPF]-o                     o-[LPF]- ch1
 ch2 -[LPF]-o \                 / o-[LPF]- ch2
              (rotating)  link (rotating)
 chN -[LPF]-o / switch ------> \  o-[LPF]- chN
            commutator     decommutator
              ^                    ^
              +-- clock / sync ----+

1. Each signal is band-limited by an LPF.
2. A commutator samples the channels in turn at $f_s \geq 2 f_m$ (8 kHz for voice), so one frame lasts $1/f_s$ = 125 µs.
3. The samples are coded (PCM) and sent with framing bits.
4. A decommutator at the receiver, kept in step by the framing pattern, routes each slot to its own output, where an LPF restores the signal.

T1 carrier system

Frame structure: 24 voice channels × 8 bits + 1 framing bit.

 1 bit  8 bits  8 bits        8 bits
+-----+-------+-------+-----+--------+
|  F  |  Ch1  |  Ch2  | ... |  Ch24  |
+-----+-------+-------+-----+--------+
<--------- 193 bits in 125 us -------->
F = framing bit

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

• Framing bits over 12 frames (superframe) give the alignment pattern; robbed-bit signalling uses the LSB of each channel in frames 6 and 12.
• µ-law coding, AMI/B8ZS line code, repeaters about every 1.8 km.

Multiplexing levels:

$T2 = 4 \times T1 + 136$ kbps overhead; $T3 = 7 \times T2 + 552$ kbps; $T4 = 6 \times T3 + 5.76$ Mbps (framing and stuffing bits).