--- title: "FDM vs TDM; FDM telephone hierarchy" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:fdm-vs-tdm-fdm-hierarchy" source: https://www.bctengineer.com/q/semester_6/communication_systems/fdm-vs-tdm-fdm-hierarchy site: BCT Engineering --- # FDM vs TDM; FDM telephone hierarchy Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2068 Jestha (old course), 4+6 marks - Video: [Frequency Division Multiplexing (FDM) Explained](https://www.youtube.com/watch?v=UwWmDwbsDBs) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/fdm-vs-tdm-fdm-hierarchy - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question Differentiate between FDM and TDM. Draw spectral details and explain various standard groups of FDM telephone hierarchy. ## Short answer FDM allocates frequency bands to simultaneous analog signals; TDM assigns time slots sequentially to digital signals. The FDM hierarchy: 12 voice channels form a basic group (60–108 kHz), five groups form a supergroup (312–552 kHz), and five supergroups form a mastergroup (812–2044 kHz). ## Answer #### Difference between FDM and TDM | Point | FDM | TDM | |---|---|---| | Sharing basis | Each signal gets its own frequency band | Each signal gets its own time slot | | Time use | All signals sent at the same time | Signals sent one after another | | Signal type | Mainly analog | Mainly digital (PAM/PCM samples) | | Hardware | Modulators, sharp band-pass filters, oscillators per channel | Sampler, commutator, mostly digital circuits | | Guard | Guard bands between channels | Guard times between slots | | Crosstalk cause | Non-linearity, poor filters | Pulse spreading (ISI), poor sync | | Synchronisation | Not critical | Frame sync is essential | | Fading effect | Narrow-band fading hits only some channels | Fading during a slot hits one channel | | Example | Radio/TV broadcast, old analog telephone trunks | T1/E1 digital telephony, GSM slots | #### FDM telephone hierarchy and spectral details Telephone FDM uses SSB-SC modulation. Each voice channel (300–3400 Hz) gets a 4 kHz slot that includes its guard band. The standard (CCITT) groups are: | Level (CCITT) | Made of | Voice channels | Frequency band | Bandwidth | |---|---|---|---|---| | Voice channel | — | 1 | 0–4 kHz (0.3–3.4 kHz used) | 4 kHz | | Basic group | 12 voice channels | 12 | 60–108 kHz | 48 kHz | | Supergroup | 5 groups | 60 | 312–552 kHz | 240 kHz | | Mastergroup | 5 supergroups | 300 | 812–2044 kHz | 1232 kHz | | Supermastergroup | 3 mastergroups | 900 | 8516–12388 kHz | 3872 kHz | **1. Basic group (12 channels, 60–108 kHz).** Channel $k$ modulates a carrier $f_k = 112 - 4k$ kHz ($k = 1 \dots 12$, i.e. 108, 104, …, 64 kHz). The lower sideband is kept, so channel $k$ occupies $f_k - 4$ to $f_k$ kHz. **2. Supergroup (60 channels, 312–552 kHz).** Five groups modulate carriers at 420, 468, 516, 564 and 612 kHz. Taking the lower sideband turns each 60–108 kHz group into a 48 kHz block. For example, group 1 on the 420 kHz carrier occupies $420 - 108 = 312$ to $420 - 60 = 360$ kHz. **3. Mastergroup (300 channels, 812–2044 kHz).** Five supergroups are placed with 8 kHz guard bands between them. (The Bell system mastergroup instead uses 10 supergroups = 600 channels in 564–3084 kHz.) **4. Supermastergroup (900 channels, 8516–12388 kHz).** Three mastergroups. ```text Group: 12 channels, carriers 64, 68, ..., 108 kHz (LSB of each channel kept) |ch12|ch11|ch10| ch9| ch8| ... | ch2| ch1| 60 64 68 72 76 100 104 108 kHz |<----------------- 48 kHz ----------------->| Supergroup: 5 groups, carriers 420, 468, 516, 564, 612 kHz (LSB kept) | G1 | G2 | G3 | G4 | G5 | 312 360 408 456 504 552 kHz |<-------- 240 kHz -------->| ``` Each step needs only one set of modulators and filters for a whole block of channels, which keeps the equipment simple. The wide guard bands at the higher levels make the filters easier to build. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which row correctly contrasts FDM and TDM? A. FDM: signals sent at the same time; TDM: one after another B. FDM: one after another; TDM: same time C. Both same time D. FDM needs frame sync; TDM does not Answer: A. FDM sends all simultaneously on different frequencies; TDM sends in successive slots. (Key point: FDM vs TDM) 2. Channel k in the basic group uses carrier f_k = ? A. 112 - 4k kHz, lower sideband kept B. 60 + 4k kHz, upper sideband C. 420 + 48k kHz D. 64 kHz for all Answer: A. k = 1..12 gives 108, 104, ..., 64 kHz with LSB, filling 60-108 kHz. (Key point: Basic group carriers) 3. With carrier 420 kHz and lower sideband, where does the 60-108 kHz group go? A. 312-360 kHz B. 420-468 kHz C. 60-108 kHz D. 360-408 kHz Answer: A. 420 - 108 = 312 to 420 - 60 = 360 kHz. (Key point: Supergroup example) 4. Why is the hierarchy used? A. One set of modulators and filters per block keeps equipment simple, and wide guard bands at higher levels ease filter design B. It reduces the total bandwidth below 4 kHz per channel C. It removes the need for SSB D. It converts speech to digital Answer: A. Standard blocks give modular equipment and easier filters. (Key point: Why a hierarchy) ## Related in Communication Systems - [FDMA; FDM in telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-fdm-telephony-hierarchy) — Communication Systems, chapter 5, asked 2× - [Short note: FDM vs TDM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm-vs-tdm) — Communication Systems, chapter 5 - [TDM vs FDM for voice; E1 and T1 hierarchies](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Atdm-fdm-voice-e1-t1) — Communication Systems, chapter 5 - [T1 and E1 TDM-PCM hierarchy with frame diagrams](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-e1-frame-diagrams) — Communication Systems, chapter 5 - [E1 frame, TDM hierarchy and signaling rate](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-frame-hierarchy) — Communication Systems, chapter 5 - [Companding; T1 hierarchy of TDM-PCM telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acompanding-t1) — Communication Systems, chapter 5 ## Related in other subjects - [FDM hierarchy in telephone system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-hierarchy-telephone) — Data Communication, chapter 6 - [Comparison of FDM, TDM and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-wdm) — Telecommunications, chapter 3, asked 2× - [Compare FDM, WDM and TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acompare-fdm-wdm-tdm) — Data Communication, chapter 6 - [FDM and TDM with block diagrams](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-block) — Telecommunications, chapter 3