---
title: "Traffic intensity, GOS and national numbering"
subject: "Telecommunications"
chapter: 6
key: "semester_8/telecommunications:traffic-intensity-gos-numbering"
source: https://www.bctengineer.com/q/semester_8/telecommunications/traffic-intensity-gos-numbering
site: BCT Engineering
---

# Traffic intensity, GOS and national numbering

Telecommunications · Chapter 6: Telephone Traffic

- Asked in: 2080 Baisakh, 4+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/traffic-intensity-gos-numbering
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-6

## Question

What do you mean by traffic intensity and Grade of service (GOS)? Explain the national numbering planning with standard format.

## Short answer

Traffic intensity is the average number of simultaneous calls in erlangs, A = C h / T. Grade of Service (GoS) is the fraction of offered calls lost in the busy hour. The national numbering plan follows ITU-T E.164: CC (1–3 digits), NDC, SN, max 15 digits. Example for Nepal: +977 1 4xxxxxx or +977 98X XXXXXXX.

## Answer

#### Traffic intensity
**Traffic intensity** is the average number of calls (servers) simultaneously in progress during a period, measured in **Erlangs (E)**:
$$
A = \frac{C\,h}{T}
$$
where C = number of calls in time T and h = mean holding time. 1 E means one circuit continuously busy. Example: 120 calls of 3 min in one hour give $A = 120 \times 3/60 = 6$ E. Other unit: 1 E = 36 CCS (hundred call-seconds).

#### Grade of Service (GoS)
**GoS** is the proportion of calls that are lost (or delayed beyond a limit) in the busy hour because all servers are busy:
$$
GoS = \frac{\text{calls lost}}{\text{calls offered}} = \frac{A_{lost}}{A_{offered}}
$$
A smaller value means better service; e.g. 0.01 = 1 call lost in 100. It is used with Erlang B tables to find the number of trunks needed.

#### National numbering plan (ITU-T E.164 format)
A **numbering plan** gives every subscriber a unique number so that calls can be routed and charged. ITU-T **E.164** sets the international format, with at most **15 digits** (excluding prefixes):

```text
 |<------------ max 15 digits ------------->|
 +------+----------+--------------------------+
 |  CC  |   NDC    |           SN             |
 +------+----------+--------------------------+
 1-3 dig  |<----- national (significant) no. ----->|
```

- **CC – Country Code:** 1 to 3 digits, given by ITU (Nepal 977, India 91, USA 1). First digit is the world zone.
- **NDC – National Destination Code:** area/trunk code or mobile network code (Kathmandu valley 1; Nepal Telecom mobile 984/985/986, Ncell 980/981/982).
- **SN – Subscriber Number:** identifies the line within the NDC area.
- **NSN (National Significant Number) = NDC + SN**; its length is fixed by the national plan (maximum 15 − length of CC).

**Prefixes (not part of the number):**
- **Trunk (national) prefix:** dialled before the NSN for calls inside the country, usually 0 (e.g. 01-4xxxxxx in Nepal).
- **International prefix:** dialled before CC for outgoing international calls, ITU recommends 00.
- Short codes for emergency and services (100 police, 101 fire, 102 ambulance in Nepal).

**Example:** a Kathmandu fixed line: international format +977 1 4xxxxxx; national format 01-4xxxxxx; local format 4xxxxxx. A Nepali mobile: +977 98X XXXXXXX.

**Requirements of a good national plan:** unique numbers, enough spare capacity for growth (planned for 30–50 years), easy routing and charging from the leading digits, uniform length where possible, compatibility with E.164 and with number portability.

## Check yourself

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

1. 120 calls of 3 min in one hour give traffic of:
   A. 3 E
   B. 6 E
   C. 12 E
   D. 360 E
   Answer: B. A = 120 × 3/60 = 6 E. (Key point: Erlang)

2. ITU-T E.164 limits the number to at most:
   A. 10 digits
   B. 12 digits
   C. 15 digits
   D. 20 digits
   Answer: C. Max 15 digits excluding prefixes. (Key point: E.164 length)

3. The National Significant Number is:
   A. CC + NDC
   B. NDC + SN
   C. CC + SN
   D. Trunk prefix + CC
   Answer: B. NSN = NDC + SN. (Key point: NSN)

4. Which statement about prefixes is right?
   A. Trunk prefix (usually 0) is for national calls and is not part of the number
   B. International prefix is 977
   C. Trunk prefix is part of E.164
   D. Prefix is dialled after SN
   Answer: A. Prefixes are dialled before the number and are not part of it. (Key point: Prefixes)

## Related in Telecommunications

- [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
- [National and international numbering plans](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anumbering-plan-national-international) — Telecommunications, chapter 7, asked 3×
- [GOS vs blocking probability; E.164 plan](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Agos-vs-blocking-e164) — Telecommunications, chapter 6
- [Short note: Grade of service](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-grade-of-service) — Telecommunications, chapter 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 intensity in CCS, CM and CS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-intensity-ccs-cm) — Telecommunications, chapter 6

## Related in other subjects

- [Numerical: 12-cell system capacity at 2% GOS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acellular-capacity-12-cell-system) — Wireless Communication, chapter 2
- [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