---
title: "Handoff prioritization and trunking terms"
subject: "Wireless Communication"
chapter: 2
key: "semester_7/wireless_communication:handoff-prioritizing-gos-terms"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/handoff-prioritizing-gos-terms
site: BCT Engineering
---

# Handoff prioritization and trunking terms

Wireless Communication · Chapter 2: Cellular Mobile Communication Concepts

- Asked in: 2082 Bhadra, 3+4 marks
- Video: [Prioritizing Handoff, Umbrella Cell Concept, Cell Dragging, Microcell Zone Concept](https://www.youtube.com/watch?v=-ArTzZf_Xmk) — 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/handoff-prioritizing-gos-terms
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-2

## Question

Explain two schemes for prioritizing handoff. Define terms grade of service, traffic intensity, average holding time and block call.

## Short answer

Guard channels reserve capacity exclusively for handoffs; queuing handoff requests uses the time before signal drops. GOS is the probability a call is blocked, traffic intensity is channel occupancy in Erlangs, average holding time is mean call duration, and a blocked call is one that cannot be completed due to congestion.

## Answer

#### Two schemes for prioritising handoff
Dropping an ongoing call is worse than blocking a new call, so handoffs are given priority:

1. **Guard channel concept**
   - A fraction of the channels in each cell is reserved exclusively for handoff requests; new calls can use only the remaining channels.
   - Reduces dropped calls, but lowers total carried traffic because guard channels may be idle. Dynamic channel assignment makes it more efficient.

2. **Queuing of handoff requests**
   - There is a finite time interval between the signal reaching the handoff threshold and falling to the minimum usable level. During this time the handoff request is placed in a queue and served as soon as a channel becomes free.
   - Reduces forced termination, but cannot guarantee zero drops because the waiting time is limited.

```text
 Signal level
   │ \
   │  \ ← handoff threshold: request queued
   │   \
   │    \ ← minimum usable level: call drops
   └──────────── time
       |<-->| queuing time available
```

#### Trunking terms
- **Grade of service (GOS):** a measure of the ability of a user to access the trunked system during the busiest hour; usually given as the **probability that a call is blocked** (Erlang B) or delayed beyond a given time (Erlang C). Example: GOS = 2 %.
- **Traffic intensity:** the measure of channel time utilisation, i.e. the average channel occupancy, measured in **Erlangs**. For one user, Au = λH; total offered traffic A = U·λ·H. One Erlang = one channel occupied continuously.
- **Average holding time (H):** the average duration of a typical call. Example: H = 3 minutes.
- **Blocked call (lost call):** a call that cannot be completed at the time of request because all channels are busy (congestion). In Erlang B systems blocked calls are cleared; in Erlang C they are queued.

## Check yourself

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

1. Drawback of guard channels?
   A. They raise dropped calls
   B. They lower total carried traffic when idle
   C. They need queues
   D. They block all handoffs
   Answer: B. Reserved channels may sit idle. (Key point: Guard channel drawback)

2. Traffic intensity is measured in:
   A. Erlangs
   B. Hertz
   C. Bits per second
   D. Decibels
   Answer: A. One Erlang is one channel occupied continuously. (Key point: Traffic intensity)

3. Grade of service usually means:
   A. Probability that a call is blocked (or delayed)
   B. Data rate
   C. Coverage radius
   D. Number of cells
   Answer: A. E.g. 2 % blocking. (Key point: GOS)

4. In Erlang B systems blocked calls are:
   A. Queued
   B. Cleared (lost)
   C. Handed off
   D. Delayed until free
   Answer: B. Erlang C queues them. (Key point: Erlang B)

## Related in Wireless Communication

- [Numerical: trunking capacity loss with sectoring](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atrunking-loss-sectoring) — Wireless Communication, chapter 2
- [2G vs 3G, prioritized handoff, cell dragging](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3A2g-vs-3g-prioritized-handoff) — Wireless Communication, chapter 1
- [GSM frame structure and channels during a call](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-call-channels) — Wireless Communication, chapter 8
- [GoS and optimal N for 15 dB S/I (n = 3)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agos-optimal-cluster-size-n3) — Wireless Communication, chapter 2
- [MLSE and time diversity](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amlse-time-diversity) — Wireless Communication, chapter 5
- [Significance of coherence bandwidth and time](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acoherence-bandwidth-time-significance) — Wireless Communication, chapter 3, asked 3×

## Related in other subjects

- [Traffic intensity, GOS and call duration probability](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-intensity-call-duration-probability) — Telecommunications, chapter 6
- [Two methods of calculating traffic intensity](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-intensity-two-methods) — Telecommunications, chapter 6
- [Traffic intensity, GOS and national numbering](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-intensity-gos-numbering) — Telecommunications, chapter 6
- [Device holding time and calls per hour](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adevice-holding-time-calls) — Telecommunications, chapter 6