- 2072 Asoj · 4+2 marks
2G vs 3G, prioritized handoff, cell dragging
Short answer
2G (GSM) uses TDMA for digital voice/SMS; 3G (UMTS) uses WCDMA for data and video calls with soft handoff. Prioritized handoff allocates guard channels for handoffs to reduce dropped calls. Cell dragging is when a slow-moving mobile with strong old-cell signal enters a new cell without handing off, causing interference.
Differentiate between 2G and 3G with examples of appropriate technologies used. Explain the terms prioritized handoff and cell dragging.
2G vs 3G
| Feature | 2G | 3G |
|---|---|---|
| Main service | Digital voice, SMS | Voice, video call, mobile internet |
| Example technologies | GSM, IS-136 (TDMA), IS-95 (CDMA), PDC | UMTS/W-CDMA, CDMA2000, TD-SCDMA |
| Multiple access | TDMA/FDMA, narrowband CDMA | Wideband CDMA |
| Channel bandwidth | 200 kHz (GSM), 1.25 MHz (IS-95) | 5 MHz (WCDMA) |
| Data rate | 9.6–14.4 kbps (EDGE ~384 kbps) | 384 kbps–2 Mbps (HSPA higher) |
| Switching | Circuit (packet in 2.5G) | Circuit + packet |
| Handoff | Hard (soft in IS-95) | Soft and softer handoff |
| ITU family | – | IMT-2000 |
Prioritized handoff
Dropping an ongoing call is more annoying than blocking a new call, so systems give priority to handoff requests over new call requests:
- Guard channel concept: a fraction of channels in each cell is reserved only for handoffs.
- Queuing of handoff requests: a handoff request is queued during the time the signal falls from the handoff threshold to the minimum usable level, increasing the chance of getting a free channel.
Cell dragging
Cell dragging happens when a slow-moving user with a strong line-of-sight path (e.g. a pedestrian near the base station) travels far into a neighbouring cell while the received signal from the old base station stays above the handoff threshold. No handoff is triggered, so the user is "dragged" outside the planned cell boundary, causing interference and traffic management problems. It is reduced by careful setting of handoff thresholds and by using umbrella/microcell planning.
✓ Answer checked by Claude Opus 5.5 (high effort)
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