--- title: "Need for multiplexing and WDM" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:multiplexing-need-wdm" source: https://www.bctengineer.com/q/semester_8/telecommunications/multiplexing-need-wdm site: BCT Engineering --- # Need for multiplexing and WDM Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2071 Shrawan, 2+5 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/multiplexing-need-wdm - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question Why multiplexing is needed in telecommunication? Describe wavelength division multiplexing in brief. ## Short answer Multiplexing is needed to share the high capacity of transmission media, reducing per-channel cost and spectrum demands. WDM combines multiple optical carriers on a single fibre using passive multiplexers and demultiplexers, each wavelength carrying an independent data stream, achieving very high aggregate capacity. ## Answer #### Why multiplexing is needed **Multiplexing** lets many signals share one transmission medium. It is needed because: - A transmission medium (coax, fibre, microwave) has far more capacity than one voice channel needs; multiplexing uses that capacity fully. - It greatly reduces the **cost per channel**: one cable, fibre or radio link replaces hundreds of separate lines. - Fewer physical lines mean less right-of-way, duct space and maintenance. - Radio spectrum is limited; multiplexing lets many users share it. - It makes it possible to build trunk networks and hierarchies (T1/E1, SDH) between exchanges. #### Wavelength Division Multiplexing **WDM** carries several optical signals over a single fibre, each on a different wavelength of light. It is the optical equivalent of FDM. ```text l1 -->|\ /|--> l1 l2 -->| MUX ==== fibre ==== DEMUX|--> l2 l3 -->|/ (l1+l2+...+lN) \|--> l3 lN --> --> lN l = wavelength (lambda) ``` **Working:** 1. Each input signal modulates its own laser at a different wavelength $\lambda_1, \lambda_2, \ldots, \lambda_N$ (in the 1310/1550 nm windows). 2. A passive optical **multiplexer** (diffraction grating, prism, arrayed waveguide grating or thin-film filter) combines all wavelengths into one fibre. 3. Optical amplifiers (EDFA) can boost all wavelengths together on long routes. 4. At the far end a **demultiplexer** separates the wavelengths and photodetectors convert each to an electrical signal. **Types:** - **CWDM** (coarse): 20 nm spacing, up to 18 channels, cheap uncooled lasers, metro distances. - **DWDM** (dense): 0.8 nm (100 GHz) or less, 40–160 channels, long-haul backbones, terabits per second per fibre. **Advantages:** huge increase in capacity of existing fibre; each wavelength is independent and can carry any format and bit rate; easy upgrades; bidirectional use of one fibre is possible. **Limitations:** costly, stable lasers and filters; crosstalk between adjacent wavelengths; dispersion and fibre non-linearities at high power. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main economic reason for multiplexing? A. It increases the capacity of each medium B. It lowers cost per channel by sharing one cable, fibre or radio link among many signals C. It removes the need for exchanges D. It improves voice quality Answer: B. A medium has far more capacity than one voice channel needs, so sharing it replaces many lines and cuts cost per channel. (Key point: Why multiplex) 2. What does the WDM demultiplexer at the far end do? A. Amplifies all wavelengths B. Combines wavelengths into one fibre C. Separates the wavelengths, each going to a photodetector D. Modulates each laser Answer: C. The demultiplexer separates the wavelengths and photodetectors convert each into an electrical signal. (Key point: WDM working) 3. How do CWDM and DWDM differ? A. CWDM has 0.8 nm spacing and 160 channels; DWDM has 20 nm spacing B. CWDM has 20 nm spacing, up to 18 channels, uncooled lasers; DWDM has 0.8 nm or less, 40-160 channels, long haul C. Both use 20 nm spacing D. CWDM is for long haul backbones; DWDM for metro Answer: B. CWDM is coarse and cheap for metro distances; DWDM is dense, used for long-haul backbones. (Key point: CWDM vs DWDM) ## Related in Telecommunications - [WDM and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awdm-t-carrier-t1) — Telecommunications, chapter 3, asked 2× - [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× - [WDM and its light sources](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awdm-light-sources) — 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 - [TDM types and pulse stuffing](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-types-pulse-stuffing) — Telecommunications, chapter 3 ## Related in other subjects - [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3× - [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3× - [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2× - [Multiplexing: definition and applications](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-define-why) — Data Communication, chapter 6, asked 5×