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Analog vs digital multiplexing; 24-channel system

Short answer

Analog multiplexing (FDM) separates channels in frequency, while digital multiplexing (TDM) separates them in time using sampled and coded signals. The American T1 system band‑limits 24 voice channels, samples each at 8 kHz, compresses with µ‑law to 8‑bit PCM, adds one framing bit per 125 µs frame, and transmits at 1.544 Mbps using bipolar AMI line coding.

Difference between Analog and Digital Multiplexing. Explain the mechanism involved in American 24 voice channel multiplex system.

Analog versus digital multiplexing

PointAnalog multiplexing (FDM)Digital multiplexing (TDM/PCM)
PrincipleChannels separated in frequencyChannels separated in time slots
SignalContinuous analogueSampled, quantised, coded bits
EquipmentModulators, band-pass filters, oscillatorsSampler, coder (codec), clock, digital logic
NoiseAccumulates over every repeaterRegenerated at each repeater; no accumulation
CrosstalkFrom filter overlap, non-linearityVery low
SynchronisationNot neededNeeded (frame and bit sync)
Cost/sizeBulky filtersCheap, IC-based
Example12-channel group, 60-channel supergroupT1 (24 ch), E1 (30 ch)

American 24-voice-channel PCM multiplex (T1 / DS1)

Mechanism:

  1. Filtering: each voice signal is band-limited to 300–3400 Hz by a low-pass filter.
  2. Sampling: each channel is sampled at 8000 samples/s (> 2 × 3.4 kHz) by a PAM gate controlled by the commutator; samples of the 24 channels are interleaved in time.
  3. Compression and coding: samples are compressed by µ-law (μ=255\mu = 255) and coded into 8-bit words by a shared codec (256 levels).
  4. Framing: the 24 words (192 bits) plus one framing bit form a frame of 193 bits, repeated every 125 µs.
 1 bit  8 bits  8 bits        8 bits
+-----+-------+-------+-----+--------+
|  F  |  Ch1  |  Ch2  | ... |  Ch24  |
+-----+-------+-------+-----+--------+
<--------- 193 bits in 125 us -------->
F = framing bit
Bits per frame=24×8+1=193 bitsFrame rate=8000 frames/s (T=125 μs)RT1=193×8000=1.544 Mbps\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}
  1. Superframe and signalling: 12 frames form a superframe (D4); the F-bits form the pattern 100011011100 for frame and superframe alignment. In frames 6 and 12 the LSB of every channel is "robbed" to carry signalling bits A and B (on-hook/off-hook, dialling).
  2. Line coding: the stream is sent in bipolar AMI (later B8ZS) on twisted pair, with regenerative repeaters about every 6000 ft (1.8 km).
  3. Receiver: the decoder finds frame alignment from the F-bits, distributes each 8-bit word to its channel, decodes and expands it, and a low-pass filter recovers the voice.
ch1 -[LPF]-\                         /-[LPF]- ch1
ch2 -[LPF]-- sampler -> encoder ->   - decoder ...
ch24-[LPF]-/  (8 kHz)  (8 bit, u-law) \-[LPF]- ch24
                  + F bit -> AMI line 1.544 Mbps

✓ Answer checked by Claude Opus 5.5 (high effort)

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