- 2074 Chaitra · 4+4 marks
Pulse stuffing; LED vs LASER sources
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.
Why pulse stuffing is needed? Compare the light sources LED and LASER in telecommunications.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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