--- title: "FDMA; FDM in telephone hierarchy" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:fdma-fdm-telephony-hierarchy" source: https://www.bctengineer.com/q/semester_6/communication_systems/fdma-fdm-telephony-hierarchy site: BCT Engineering --- # FDMA; FDM in telephone hierarchy Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2072 Magh (CS I), 2+6 marks; 2071 Magh (CS I), 2+6 marks (asked 2 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/fdma-fdm-telephony-hierarchy - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question What do you understand by FDMA? Explain the use of FDM in telephony hierarchy. ## Short answer FDMA assigns each user a frequency sub-band for multiple access; FDM multiplexes signals at one location. Telephony hierarchy: 12 channels → group (60-108 kHz), ×5 → supergroup (60 ch, 312-552 kHz), ×5 → mastergroup (300 ch, 812-2044 kHz), ×3 → supermastergroup (900 ch, 8.5-12.4 MHz). SSB-SC LSB modulation, carriers from master oscillator, pilot tones for control. ## Answer #### FDMA **Frequency Division Multiple Access (FDMA)** is a multiple-access method in which a shared medium (e.g. a satellite transponder or a cellular band) is divided into frequency sub-bands, and **each user/station is assigned its own sub-band** for the duration of its use. All users transmit at the same time on different frequencies. Examples: satellite SCPC/MCPC systems, 1G AMPS cellular (30 kHz channels). FDM is the multiplexing of signals at one location; FDMA is the same idea applied to users at different locations. #### FDM in telephony hierarchy Each voice channel (300–3400 Hz) is given a 4 kHz slot and frequency-translated by **SSB-SC (LSB)** modulation. Channels are built up in standard blocks (ITU-T/CCITT): ```text 12 ch -> Basic group 60-108 kHz x5 -> Supergroup (60) 312-552 kHz x5 -> Mastergroup (300) 812-2044 kHz x3 -> Supermaster (900) 8516-12388 kHz ``` 1. **Basic group:** 12 voice channels modulate carriers 64, 68, ..., 108 kHz; LSBs selected; band 60–108 kHz (48 kHz). 2. **Supergroup:** 5 groups translated by carriers 420, 468, 516, 564, 612 kHz into 312–552 kHz; 60 channels. 3. **Mastergroup:** 5 supergroups, 300 channels, 812–2044 kHz (8 kHz guard bands between supergroups). 4. **Supermastergroup:** 3 mastergroups, 900 channels, 8516–12388 kHz. | Level | Channels | Bandwidth | |---|---|---| | Group | 12 | 48 kHz | | Supergroup | 60 | 240 kHz | | Mastergroup | 300 | 1232 kHz | | Supermastergroup | 900 | 3872 kHz | In the AT&T (Bell) system a mastergroup is 10 supergroups (600 channels, 564–3084 kHz) and a jumbo group is 6 mastergroups (3600 channels). **Advantages of the hierarchy:** standard modular equipment, filters at convenient frequencies, and whole blocks can be switched or routed together. Carriers come from one master oscillator, and pilot tones keep levels and frequencies correct. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In FDMA, how is a shared medium such as a satellite transponder used by many stations? A. Each station transmits in its own time slot on the same frequency B. Each station is assigned its own frequency sub-band and all transmit at the same time C. Each station uses a unique spreading code on the same band D. Stations take turns using the whole band Answer: B. FDMA divides the band into frequency sub-bands, one per user, and users transmit simultaneously on different frequencies. (Key point: FDMA definition) 2. What is the difference between FDM and FDMA? A. FDM is digital and FDMA is analog B. FDM multiplexes signals at one location, FDMA applies the same idea to users at different locations C. FDM uses time slots, FDMA uses frequency slots D. There is no difference in idea, but FDMA needs no filters Answer: B. Both split by frequency; FDM multiplexes at one site while FDMA shares the medium among geographically separate users. (Key point: FDM vs FDMA) 3. How is each voice channel (300-3400 Hz) placed in the FDM telephony hierarchy? A. Given a 4 kHz slot and translated by SSB-SC (lower sideband) B. Given an 8 kHz slot and translated by DSB-FC C. Given a 3.1 kHz slot with no guard band using FM D. Sampled at 8 kHz and given a time slot Answer: A. Each voice channel gets a 4 kHz slot (including guard band) and is shifted by SSB-SC, keeping the lower sideband. (Key point: Channel slot and modulation) 4. Which sequence correctly gives the CCITT FDM building blocks? A. 12 ch group, 3 groups supergroup, 5 supergroups mastergroup B. 12 ch group, 5 groups supergroup (60), 5 supergroups mastergroup (300), 3 mastergroups supermaster (900) C. 12 ch group, 5 groups supergroup, 3 supergroups mastergroup (180) D. 10 ch group, 6 groups supergroup, 5 supergroups mastergroup Answer: B. Group 12 channels, supergroup 5 groups = 60, mastergroup 5 supergroups = 300, supermastergroup 3 mastergroups = 900. (Key point: Hierarchy build-up) ## Related in Communication Systems - [FDM and FDM hierarchy in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-hierarchy-telephony) — Communication Systems, chapter 5, asked 4× - [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 - [FDMA vs TDMA; T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-tdma-t1-e1) — 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 - [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 ## 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 - [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3× - [Switching hierarchy and evolution until the telephone](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-telephone-evolution) — Telecommunications, chapter 1, asked 3× - [E1 telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Ae1-hierarchy) — Computer Networks and Security, chapter 2