--- title: "PCM block diagram; T1 signaling rate and frame" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:pcm-block-t1-rate-frame" source: https://www.bctengineer.com/q/semester_6/communication_systems/pcm-block-t1-rate-frame site: BCT Engineering --- # PCM block diagram; T1 signaling rate and frame Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2070 Asar (CS II), 3+3 marks - Video: [Communication Engineering - A PCM/TDM System: T1 Carrier System](https://www.youtube.com/watch?v=kx5rEYUiKJo) — Dr. Sapna Katiyar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/pcm-block-t1-rate-frame - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question Explain functional block diagram of the PCM system. Find the signaling rate of the T1 system and draw its frame diagram. ## Short answer PCM system includes LPF, sampler, quantizer, encoder at transmitter; regenerative repeaters; decoder and reconstruction LPF at receiver. T1 signaling rate is 1.544 Mb/s. Frame: 1 framing bit plus 8 bits per 24 channels, every 125 µs. ## Answer #### Functional block diagram of PCM **Pulse code modulation (PCM)** turns an analog signal into a stream of binary code words by sampling, quantising and encoding it. ```text TRANSMITTER m(t)->[LPF]->[Sampler]->[Quantizer]->[Encoder]-> PCM bits CHANNEL with REGENERATIVE REPEATERS ->[Equalizer]->[Decision/Timing]->[Regenerate]-> RECEIVER ->[Regenerator]->[Decoder]->[Reconstruction LPF]->m'(t) ``` - **Anti-aliasing LPF:** limits the message to $W$ Hz. - **Sampler:** takes samples at $f_s \ge 2W$ (8 kHz for speech). - **Quantiser:** rounds each sample to one of $L = 2^n$ levels (with companding for speech). - **Encoder:** turns each level into an $n$-bit code word, then a line code is applied. - **Regenerative repeaters:** reshape and retime the pulses along the line, so noise does not build up. - **Receiver:** the regenerator cleans the pulses, the decoder turns code words back into PAM samples, and the LPF rebuilds $m(t)$. #### Signalling rate of T1 T1 sends 24 voice channels, each sampled at 8 kHz with 8 bits per sample, plus 1 framing bit per frame. $$ \begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frames per second} &= f_s = 8000\ \text{frames/s} \\ R_b &= 193 \times 8000 = 1\,544\,000\ \text{b/s} = 1.544\ \text{Mb/s} \end{aligned} $$ **Answer: T1 signalling rate = 1.544 Mb/s (frame of 193 bits every 125 µs).** #### T1 frame diagram ```text |<--------- one frame = 125 us = 193 bits --------->| +---+--------+--------+--------+-------+--------+ | F | Ch 1 | Ch 2 | Ch 3 | ... | Ch 24 | +---+--------+--------+--------+-------+--------+ 1 bit 8 bits 8 bits 8 bits 8 bits F = framing bit (pattern spread over 12 frames) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the correct order of the PCM transmitter blocks? A. Sampler, LPF, encoder, quantiser B. Quantiser, sampler, LPF, encoder C. Encoder, sampler, quantiser, LPF D. Anti-aliasing LPF, sampler, quantiser, encoder Answer: D. The message is band-limited by an LPF, sampled, quantised to L = 2^n levels and then encoded into n-bit code words. (Key point: PCM block order) 2. Why are regenerative repeaters used along a PCM link? A. They amplify noise together with the signal B. They reshape and retime pulses so that noise does not accumulate C. They convert digital pulses into analog form D. They increase the sampling rate Answer: B. Repeaters decide, reshape and retime the pulses, so noise does not build up over many sections. (Key point: Regenerative repeaters) 3. What is the T1 signalling rate? A. 1.536 Mb/s B. 2.048 Mb/s C. 1.544 Mb/s D. 1.92 Mb/s Answer: C. 193 bits per frame x 8000 frames/s = 1.544 Mb/s. (Key point: T1 signalling rate) 4. What does one T1 frame of 125 µs contain? A. 1 framing bit followed by 24 channels of 8 bits each B. 32 slots of 8 bits each C. 24 channels of 8 bits with no framing bit D. 12 channels of 16 bits each Answer: A. Frame = F bit + 24 x 8-bit channels = 193 bits. 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