--- title: "Digital carrier system and multiplexing techniques" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:digital-carrier-multiplexing" source: https://www.bctengineer.com/q/semester_8/telecommunications/digital-carrier-multiplexing site: BCT Engineering --- # Digital carrier system and multiplexing techniques Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2072 Kartik, 6+4 marks - Video: [Types of Multiplexing | FDM TDM WDM | Analog Digital | Computer Networks](https://www.youtube.com/watch?v=WXof7bg_Zys) — Dr. Ankit Verma - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/digital-carrier-multiplexing - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question What is digital carrier system? Discuss the advantages and disadvantages of various multiplexing techniques. ## Short answer A digital carrier system uses PCM and TDM to carry many voice channels, using regenerative repeaters. Main advantages: no noise accumulation and easy digital switching. Synchronous TDM is simple but wastes idle slots; statistical TDM improves efficiency for bursty data; FDM needs complex filters; WDM offers huge fibre capacity; CDM shares spectrum flexibly. ## Answer 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 ```text 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. | Technique | Main gain | Main cost | |---|---|---| | FDM | No sync, analogue | Filters, guard bands | | TDM | Digital, cheap ICs | Sync, idle slots | | STDM | Efficiency for bursty data | Overhead, delay | | WDM | Very high capacity | Optical components | | CDM | Robust, flexible | Power control, complexity | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a digital carrier system? A. Analogue FDM carrying many channels with SSB B. A system carrying many channels in digital form over one medium using PCM and TDM with regenerative repeaters C. A system sending each channel on a separate fibre D. A radio system using spreading codes Answer: B. It uses PCM (64 kbps per voice channel) and TDM frames of 125 µs, with regenerative repeaters along the line. (Key point: Digital carrier definition) 2. Which is a disadvantage of FDM stated in the answer? A. It needs accurate frame synchronisation B. Guard bands waste bandwidth and bulky filters are costly C. It needs power control for the near-far problem D. It works only on fibre Answer: B. FDM needs filters and oscillators, wastes bandwidth in guard bands and suffers crosstalk and noise accumulation. (Key point: Disadvantages of FDM) 3. What is the main gain and cost of statistical TDM? A. Gain: no synchronisation; cost: guard bands B. Gain: very high fibre capacity; cost: optical filters C. Gain: better link utilisation for bursty sources; cost: address overhead, buffering and delay D. Gain: interference resistance; cost: power control Answer: C. Slots go only to active sources, but addresses, buffers and variable delay add overhead and complexity. (Key point: Statistical TDM trade-off) ## Related in Telecommunications - [Analog vs digital multiplexing; 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aanalog-digital-mux-24-channel) — Telecommunications, chapter 3 - [Multiplexing techniques and TDM standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-techniques-tdm-standards) — Telecommunications, chapter 3 - [Compare multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acompare-multiplexing-techniques) — Telecommunications, chapter 3 - [WDM and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awdm-t-carrier-t1) — Telecommunications, chapter 3, asked 2× - [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2× - [TDM and T1/E1 carrier standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-t1-e1-carriers) — Telecommunications, chapter 3 ## Related in other subjects - [T1 digital carrier system; adaptive delta modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-hierarchy-adm) — Communication Systems, chapter 5 - [Multiplexing techniques; analog one explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Alist-mux-techniques) — Data Communication, chapter 6 - [List digital-to-digital line coding techniques](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aline-coding-techniques-list) — Data Communication, chapter 5 - [Digital transmission system; analog vs digital](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aanalog-digital-transmission-compare) — Instrumentation II, chapter 5