--- title: "Pulse stuffing; LED vs LASER sources" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:pulse-stuffing-led-laser" source: https://www.bctengineer.com/q/semester_8/telecommunications/pulse-stuffing-led-laser site: BCT Engineering --- # Pulse stuffing; LED vs LASER sources Telecommunications · Chapter 2: Transmission Media - Asked in: 2074 Chaitra, 4+4 marks - Video: [Lec 16: Optical Sources: Laser Diodes over LEDs](https://www.youtube.com/watch?v=G0d34Bhu-Us) — NPTEL-NOC IITM - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/pulse-stuffing-led-laser - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question Why pulse stuffing is needed? Compare the light sources LED and LASER in telecommunications. ## Short answer Pulse stuffing synchronizes plesiochronous tributaries by inserting dummy bits in fixed justification slots and using control bits to identify them, preventing slips. LEDs emit spontaneous, broad-spectrum light with low power, suited for short multimode links. Lasers emit coherent, narrow-spectrum light with high power and speed, used for long-distance high-speed transmission. ## Answer #### Need for pulse stuffing When several digital tributaries (e.g. four 2.048 Mbps E1 streams) are combined into a higher-order stream (8.448 Mbps) in the **plesiochronous digital hierarchy (PDH)**, each tributary has its **own clock**, which is nominally the same but can differ slightly (e.g. ±50 ppm). The multiplexer reads them at one common, slightly higher rate. If nothing is done, a slower tributary would run out of bits (or a faster one would overflow the buffer), causing **slips** and lost data. **Pulse stuffing (justification)** solves this: - The multiplexer output frame has a few **stuffing (justification) bit positions** for each tributary. - When a tributary's buffer is about to empty, a **dummy (stuff) bit** is inserted instead of a data bit; otherwise the position carries data. - **Justification control bits** (sent several times and decided by majority vote) tell the demultiplexer whether the position holds data or a stuff bit, so it can remove the stuff bits and recover the original clock with a phase-locked loop. - Thus tributaries with slightly different rates are brought to exactly the same rate without slips. #### LED vs LASER as optical sources | Feature | LED | LASER (laser diode) | |---|---|---| | Emission | Spontaneous, incoherent | Stimulated, coherent | | Spectral width | Wide (30–60 nm) | Narrow (<1–3 nm; DFB ~0.1 nm) | | Output power | Low (tens of µW into fibre) | High (mW) | | Beam | Wide, divergent | Narrow, directional | | Coupling efficiency | Poor; used with multimode fibre | Good; single-mode fibre | | Modulation speed | Low (up to ~100s of Mbps) | High (Gbps and above) | | Dispersion effect | High chromatic dispersion | Low | | Distance | Short links, LAN | Long-haul, undersea | | Temperature sensitivity | Low | High; needs control | | Drive circuit | Simple | Complex (threshold, feedback) | | Lifetime/cost | Long life, cheap | Shorter life, costly | LEDs suit short, low-cost multimode links; lasers are used for high-speed, long-distance single-mode systems. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is pulse stuffing needed when multiplexing PDH tributaries? A. To encrypt the data B. To add error correction C. To increase the carrier frequency D. Tributary clocks differ slightly, so stuff bits bring them to one common rate and prevent slips Answer: D. Each tributary has its own clock; stuffing justifies the rates so buffers do not empty or overflow. (Key point: Why stuffing) 2. What do justification control bits tell the demultiplexer? A. Whether a position holds data or a stuff bit, so stuff bits can be removed B. The tributary's source address C. The parity of the frame D. The amplitude of the signal Answer: A. Control bits, sent several times and majority voted, mark stuffed positions. (Key point: Justification control) 3. Which is true of an LED compared with a laser diode? A. Narrow spectrum and high power B. Spontaneous incoherent emission, wide spectrum, low power, used with multimode fibre C. Coherent emission and Gbps speeds D. Needs complex temperature control Answer: B. LEDs are simple, cheap, low-power and wide-spectrum; lasers are coherent, narrow and fast. (Key point: LED vs laser) 4. Why are lasers chosen for long-haul single-mode systems? A. They are cheaper and last longer B. They emit wide beams easily coupled to multimode fibre C. Narrow spectrum and high power give low dispersion, good coupling and high speed D. They need no drive circuit Answer: C. Narrow linewidth, high power and directional beam suit high-speed long distance. (Key point: Laser use) ## Related in Telecommunications - [Pulse stuffing and complete TDM PCM system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-pulse-stuffing-tdm-pcm) — 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 - [Pulse stuffing in TDM synchronization](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apulse-stuffing-tdm-sync) — Telecommunications, chapter 3 - [WDM and its light sources](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awdm-light-sources) — Telecommunications, chapter 3 - [DTMF vs pulse dialing and DTMF design](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adtmf-vs-pulse-design) — Telecommunications, chapter 1, asked 2× - [Pulse dialing waveform for number 401](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apulse-dialing-waveform-401) — Telecommunications, chapter 1 ## Related in other subjects - [Light sources and specular reflection principle](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alight-sources-specular-reflection) — Industrial Power Distribution and Illumination, chapter 6 - [Short note: Light sources in optical communication](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alight-sources-short-note) — Propagation and Antenna, chapter 6 - [Error correction vs detection, bit stuffing](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aerror-correct-detect-bitstuff) — Computer Networks and Security, chapter 3 - [Light sources and indoor lighting schemes](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alight-sources-indoor-schemes) — Industrial Power Distribution and Illumination, chapter 6