---
title: "Importance and unit of traffic engineering"
subject: "Telecommunications"
chapter: 6
key: "semester_8/telecommunications:traffic-engineering-importance-unit"
source: https://www.bctengineer.com/q/semester_8/telecommunications/traffic-engineering-importance-unit
site: BCT Engineering
---

# Importance and unit of traffic engineering

Telecommunications · Chapter 6: Telephone Traffic

- Asked in: 2065 Magh (old course), 12 marks
- Video: [Lec-18_Traffic Engineering | Telecommunication Engineering | ICT Engineering](https://www.youtube.com/watch?v=9Rc_XrgN9kA) — Computer_IT_ICT Engineering Department : LJIET
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/traffic-engineering-importance-unit
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-6

## Question

Explain the importance of traffic engineering and its unit in telecommunication.

## Short answer

Traffic engineering ensures sufficient equipment to meet grade of service at minimum cost, avoiding waste and overload. It dimensions trunks, switches, and control units using busy-hour traffic models. The unit of traffic intensity is the erlang (E), defined as one circuit occupied continuously; 1 E = 36 CCS = 60 CM = 3600 CS.

## Answer

**Traffic engineering** is the branch of telecommunication engineering that uses traffic measurements and probability theory to find the amount of equipment (trunks, switch paths, control units, channels) needed to carry the offered traffic at a specified grade of service with minimum cost.

#### Importance of traffic engineering

1. **Economic design.** Subscribers use their phones only a small part of the day (typically 0.05–0.1 E each). An exchange of 10,000 lines may need only a few hundred simultaneous paths. Traffic engineering finds this number so expensive equipment is not over-provided.

2. **Quality of service.** Too little equipment causes blocked calls, busy tones and long waits. Traffic engineering fixes the **grade of service (GoS)**, e.g. not more than 1 call in 100 lost in the busy hour, and designs to meet it.

3. **Dimensioning of trunks and switches.** Using Erlang B, Erlang C, Engset and binomial models it calculates trunk group sizes, number of switching stages, registers, processors and signalling links.

4. **Planning for growth.** Forecasts of traffic allow timely extension of exchanges and routes.

5. **Routing.** It helps design high-usage and alternate routes and overflow arrangements to use trunks efficiently.

6. **Performance evaluation.** It measures actual blocking and delay and shows where congestion occurs.

7. **Overload protection.** It sets limits and controls (call gapping, priority) so the network does not collapse under heavy traffic, e.g. festivals or disasters.

8. **Revenue.** Lost calls mean lost revenue; correct dimensioning maximises carried traffic and income.

#### Grade of service and busy hour

- **GoS** = calls lost / calls offered = traffic lost / traffic offered.
- Equipment is designed for the **busy hour**, the continuous 60-minute period with the highest traffic (CCITT E.600).

#### Unit of traffic

Traffic intensity is the average number of simultaneous calls; its unit is the **erlang (E)**, named after A. K. Erlang.

$$
A = \frac{C\,h}{T}
$$

where $C$ = calls in period $T$ and $h$ = mean holding time. One erlang is one circuit occupied continuously for one hour.

Other units:

| Unit | Definition | Relation |
|---|---|---|
| Erlang (E) | average simultaneous occupancy | 1 E |
| CCS | hundred call-seconds per hour | 1 E = 36 CCS |
| CM | call-minutes per hour | 1 E = 60 CM |
| CS | call-seconds per hour | 1 E = 3600 CS |
| EBHC | equated busy hour call (2 min) | 1 E = 30 EBHC |
| TU | traffic unit | 1 TU = 1 E |

**Traffic volume** = $C h$ (erlang-hours or call-hours) is the total occupancy time.

#### Example

During the busy hour 600 calls are made with mean holding time 3 min:

$$
\begin{aligned}
A &= \frac{600 \times 3}{60} = 30\ \text{E} \\
&= 30 \times 36 = 1080\ \text{CCS}
\end{aligned}
$$

If the GoS required is 0.01, Erlang B tables give about 42 trunks for 30 E. Without traffic engineering, one might install 600 trunks (one per call), which is highly wasteful.

```text
 Subscribers       Concentration      Trunks
 (many lines) ---> [ switch  ] ---> (few paths)
   10,000            traffic           ~ 500
                  engineering sets
                   this number
```

## Check yourself

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

1. Why can an exchange with 10,000 lines use only a few hundred paths?
   A. Each subscriber uses the phone only a small part of the day (about 0.05-0.1 E)
   B. Every call uses several lines at once
   C. Subscribers are never active in the busy hour
   D. Paths can be shared without any blocking
   Answer: A. Because each line is busy only a small fraction of the time, concentration is possible. (Key point: Economic design)

2. Grade of service is defined as:
   A. calls carried / calls offered
   B. calls lost / calls offered
   C. mean holding time / busy hour
   D. trunks / subscribers
   Answer: B. GoS = calls lost / calls offered, equal to traffic lost / traffic offered. (Key point: Grade of service)

3. Equipment is designed for the busy hour, which is:
   A. the average hour of the day
   B. the first hour of the working day
   C. the continuous 60-minute period with highest traffic
   D. any hour with 1 E traffic
   Answer: C. The busy hour is the continuous 60 minutes of highest traffic (CCITT E.600). (Key point: Busy hour)

4. 600 calls in the busy hour, mean holding time 3 min. The traffic is:
   A. 1800 E
   B. 10 E
   C. 60 E
   D. 30 E
   Answer: D. A = 600 x 3 / 60 = 30 E (= 1080 CCS). (Key point: Unit of traffic)

## Related in Telecommunications

- [Traffic engineering objectives; Engset formula](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-objectives-engset) — Telecommunications, chapter 6
- [Traffic engineering goals; E.600 busy hours](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-engineering-busy-hour-e600) — Telecommunications, chapter 6
- [Short note: Telephone traffic engineering](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-traffic-engineering) — Telecommunications, chapter 6, asked 2×
- [Role of traffic engineering](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arole-of-traffic-engineering) — Telecommunications, chapter 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
- [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×

## Related in other subjects

- [Traffic engineering; road user characteristics and HVE](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Ate-definition-road-user-hve) — Transportation Engineering II, chapter 1
- [Traffic engineering; road users and vehicular characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Ate-definition-road-users-vehicular) — Transportation Engineering II, chapter 1
- [Traffic control devices and traffic signs](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atraffic-control-device-requirements-signs) — Transportation Engineering II, chapter 1, asked 2×
- [Driver vision characteristics affecting traffic](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Adriver-vision-characteristics) — Transportation Engineering II, chapter 1