--- title: "FDM and FDM hierarchy in telephony" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:fdm-hierarchy-telephony" source: https://www.bctengineer.com/q/semester_6/communication_systems/fdm-hierarchy-telephony site: BCT Engineering --- # FDM and FDM hierarchy in telephony Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2079 Chaitra (CS I), 4+6 marks; 2075 Baisakh (CS I), 2+5 marks; 2069 Bhadra (CS I), 4+6 marks; 2067 Shrawan (CS I), 4+6 marks (asked 4 times) - 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-hierarchy-telephony - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question Define FDM. Explain FDM hierarchy used in telephony. ## Short answer FDM places signals in non-overlapping frequency bands. Telephony hierarchy: 12 voice channels (4 kHz each) form group (60-108 kHz) via SSB-SC LSB modulation. 5 groups → supergroup (60 ch, 312-552 kHz), 5 supergroups → mastergroup (300 ch, 812-2044 kHz), 3 mastergroups → supermastergroup (900 ch, 8.5-12.4 MHz). Modular equipment, filters at practical frequencies. ## Answer #### FDM **Frequency Division Multiplexing (FDM)** is a technique in which several signals share one channel by being placed in **different, non-overlapping frequency bands**, each modulating its own carrier. All signals are transmitted simultaneously; guard bands between them prevent interference, and band-pass filters separate them at the receiver. ```text |ch1|g|ch2|g|ch3|g| ... |chN| g = guard band f1 f2 f3 fN --> frequency ``` #### FDM hierarchy in telephony In analog telephony, each voice signal (300–3400 Hz) is allotted **4 kHz** and translated by **SSB-SC (LSB)** modulation. Channels are combined step by step into larger blocks. The ITU-T (CCITT) hierarchy: ```text 12 voice ch --> Group (60-108 kHz) 5 Groups --> Supergroup (312-552 kHz) 5 Supergrps --> Mastergroup (812-2044 kHz) 3 Mastergrps--> Supermastergroup (8516-12388 kHz) ``` **1. Basic group** - 12 voice channels. - Carriers at 64, 68, 72, ..., 108 kHz; lower sidebands selected. - Occupies **60–108 kHz** (48 kHz bandwidth). **2. Supergroup** - 5 groups = **60 channels**. - Each group modulates carriers 420, 468, 516, 564, 612 kHz (LSB). - Occupies **312–552 kHz** (240 kHz). **3. Mastergroup** - 5 supergroups = **300 channels** (CCITT). - Occupies **812–2044 kHz**, with 8 kHz guard bands between supergroups. **4. Supermastergroup** - 3 mastergroups = **900 channels**, occupying **8516–12388 kHz**. | Level | Composition | Channels | Band | |---|---|---|---| | Group | 12 voice | 12 | 60–108 kHz | | Supergroup | 5 groups | 60 | 312–552 kHz | | Mastergroup | 5 supergroups | 300 | 812–2044 kHz | | Supermastergroup | 3 mastergroups | 900 | 8.516–12.388 MHz | **AT&T (North American) version:** mastergroup = 10 supergroups = 600 channels (564–3084 kHz); jumbo group = 6 mastergroups = 3600 channels (0.564–17.548 MHz). **Why a hierarchy:** the same standard equipment (channel banks, group and supergroup translators) is reused; filters at each stage work at practical frequencies; blocks of channels can be routed as a unit. **Requirements:** sharp SSB filters, carriers derived from one master oscillator, pilot tones for gain/frequency control, linear wideband amplifiers to limit intermodulation. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. FDM shares a channel by: A. Assigning separate time slots B. Placing signals in different non-overlapping frequency bands C. Using different codes D. Alternating polarity Answer: B. Each signal modulates its own carrier and is separated by band-pass filters. (Key point: FDM definition) 2. Basic group carriers are: A. 64, 68, ..., 108 kHz with LSB B. 420-612 kHz C. 4 kHz only D. 60-108 kHz with USB Answer: A. Twelve carriers 4 kHz apart, LSB selected. (Key point: Basic group carriers) 3. A CCITT mastergroup contains: A. 300 channels (5 supergroups), 812-2044 kHz B. 60 channels C. 600 channels D. 900 channels Answer: A. 5 supergroups x 60 = 300. (Key point: Mastergroup) 4. A supermastergroup has: A. 3 mastergroups, 900 channels B. 5 mastergroups, 1500 channels C. 10 supergroups D. 2 mastergroups Answer: A. 3 x 300 = 900, occupying 8516-12388 kHz. (Key point: Supermastergroup) ## 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 in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm-telephony) — Communication Systems, chapter 5, asked 5× - [FDM in telephony and uses of filters](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-filters) — Communication Systems, chapter 5 - [FDM and its method in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-method) — 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 - [FDM vs TDM; FDM telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-vs-tdm-fdm-hierarchy) — Communication Systems, chapter 5 ## Related in other subjects - [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× - [FDM hierarchy in telephone system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-hierarchy-telephone) — Data Communication, chapter 6 - [Software used for FEM and FDM](https://www.bctengineer.com/question.md?key=semester_8%2Fcomputational_techniques_in_civil_engineering%3Asoftware-fem-fdm) — Computational Techniques in Civil Engineering, chapter 1 - [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×