---
title: "Short note: Grade of service"
subject: "Telecommunications"
chapter: 6
key: "semester_8/telecommunications:short-note-grade-of-service"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-grade-of-service
site: BCT Engineering
---

# Short note: Grade of service

Telecommunications · Chapter 6: Telephone Traffic

- Asked in: 2081 Chaitra, 4 marks
- Video: [Trunking and Grade of Service (GOS) | Traffic Intensity](https://www.youtube.com/watch?v=OGGEmAjMx7I) — ECIT Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-grade-of-service
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-6

## Question

Write a short note on Grade of Service.

## Short answer

Grade of Service (GoS) is the probability of call loss in the busy hour for a loss system, or the probability of excessive delay in a delay system. It is used with Erlang B tables to determine the required number of trunks for a given offered traffic.

## Answer

**Grade of Service (GoS)** is a measure of the quality of service of a telephone network in the busy hour. In a loss system it is the proportion of calls that are lost because all servers are busy:

$$
GoS = \frac{\text{number of calls lost}}{\text{number of calls offered}} = \frac{A_{lost}}{A_{offered}}
$$

In a delay system it is the probability that a call waits longer than a stated time.

#### Key points
- GoS lies between 0 and 1; a **smaller value means better service**. Typical design values: 0.002 for local exchange paths, 0.01 for trunks, 0.02–0.05 for cellular channels.
- It is fixed for the **busy hour**, so service is better at other times.
- For Poisson traffic offered to N trunks, GoS equals the Erlang B blocking probability $B = E_{1,N}(A)$.
- For a connection passing through several stages or links in series, overall GoS ≈ sum of the individual GoS values (when each is small): $B \approx B_1 + B_2 + \dots$

#### Example
In the busy hour 1000 calls are offered and 10 are lost: GoS = 10/1000 = **0.01** (1%).

#### Use
GoS is the design target in traffic engineering: given the offered traffic and GoS, the number of trunks is found from Erlang B (or Engset/Erlang C) tables. A tighter GoS needs more circuits and so costs more.

## Check yourself

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

1. In a loss system, GoS is:
   A. Calls carried / calls offered
   B. Calls lost / calls offered in the busy hour
   C. Average waiting time
   D. Number of trunks / traffic
   Answer: B. GoS is the proportion of calls lost. (Key point: GoS meaning)

2. What does a smaller GoS value mean?
   A. Worse service
   B. Better service
   C. Fewer trunks
   D. Less traffic
   Answer: B. Fewer calls are lost, so service is better, but more circuits are needed. (Key point: Smaller GoS)

3. For links in series with small blocking, overall GoS is approximately:
   A. Product of individual GoS
   B. Sum of individual GoS values
   C. The largest value only
   D. The average
   Answer: B. B ≈ B1 + B2 + ... when each is small. (Key point: Series links)

## Related in Telecommunications

- [Traffic numerical: 1000 calls, 5 lost](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-1000-calls-5-lost) — Telecommunications, chapter 6, asked 6×
- [GOS vs blocking; 600 calls, 2.5 min](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Agos-blocking-600-calls-2-5min) — Telecommunications, chapter 6, asked 6×
- [Traffic numerical: 800 calls, 50 lost](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Agos-800-calls-50-lost) — Telecommunications, chapter 6, asked 6×
- [Traffic numerical: 600 calls, 40 lost, 120 s](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Agos-600-calls-120s) — Telecommunications, chapter 6, asked 5×
- [GOS numerical: 1400 calls, 14 lost](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Agos-busy-hour-1400-calls) — Telecommunications, chapter 6, asked 5×
- [GOS and blocking probability in a loss system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Agos-blocking-loss-system) — Telecommunications, chapter 6

## Related in other subjects

- [Isolated square footing, 650 kN service](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aisolated-square-footing-650-kn-service) — Design of RCC Structure, chapter 11, asked 2×
- [Isolated footing 400x400, 1200 kN service](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aisolated-footing-400x400-1200-kn-service) — Design of RCC Structure, chapter 11
- [Slab base for ISMB 350, 850 kN service](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aslab-base-ismb350-850kn) — Design of Steel and Timber Structure, chapter 6