---
title: "2G vs 3G, prioritized handoff, cell dragging"
subject: "Wireless Communication"
chapter: 1
key: "semester_7/wireless_communication:2g-vs-3g-prioritized-handoff"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/2g-vs-3g-prioritized-handoff
site: BCT Engineering
---

# 2G vs 3G, prioritized handoff, cell dragging

Wireless Communication · Chapter 1: Introduction

- Asked in: 2072 Asoj, 4+2 marks
- Video: [What is handoff? Handoff Strategies](https://www.youtube.com/watch?v=ktnBPBOZY4E) — ECIT Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/2g-vs-3g-prioritized-handoff
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-1

## Question

Differentiate between 2G and 3G with examples of appropriate technologies used. Explain the terms prioritized handoff and 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.

## Answer

#### 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What channel bandwidth does WCDMA 3G use?
   A. 200 kHz
   B. 5 MHz
   C. 1.25 MHz
   D. 30 kHz
   Answer: B. 3G WCDMA uses 5 MHz carriers; GSM is 200 kHz and IS-95 1.25 MHz. (Key point: Wideband)

2. What is the guard channel concept?
   A. Reserving a fraction of channels in a cell exclusively for handoff requests
   B. Reserving channels for new calls only
   C. Adding guard time in TDD
   D. Queuing new calls
   Answer: A. It gives handoffs priority because dropping a call is worse than blocking a new one. (Key point: Guard channel)

3. What causes cell dragging?
   A. A fast user crossing cells quickly
   B. A slow user with strong line-of-sight staying above the handoff threshold deep inside a neighbouring cell
   C. Too many guard channels
   D. Using umbrella cells
   Answer: B. No handoff is triggered, so the user is dragged beyond the cell boundary causing interference. (Key point: Cell dragging)

4. Why can queuing of handoff requests work?
   A. There is a finite time between the handoff threshold and the minimum usable signal level
   B. It guarantees zero dropped calls
   C. It reserves channels permanently
   D. It removes the need for thresholds
   Answer: A. The request waits in that interval for a free channel, but waiting is limited so drops cannot be eliminated. (Key point: Queuing)

## Related in Wireless Communication

- [Handoff prioritization and trunking terms](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahandoff-prioritizing-gos-terms) — Wireless Communication, chapter 2
- [Improvements in 2G, 3G and beyond 3G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aimprovements-2g-3g-beyond) — Wireless Communication, chapter 1
- [Proper vs improper handoff; cellular concept N = 7](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahandoff-situation-cellular-n7) — Wireless Communication, chapter 2
- [Handoff margin](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahandoff-margin) — Wireless Communication, chapter 2
- [3G versus 4G mobile communication](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3A3g-vs-4g) — Wireless Communication, chapter 1, asked 6×

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