---
title: "Short note: Telephone traffic engineering"
subject: "Telecommunications"
chapter: 6
key: "semester_8/telecommunications:short-note-traffic-engineering"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-traffic-engineering
site: BCT Engineering
---

# Short note: Telephone traffic engineering

Telecommunications · Chapter 6: Telephone Traffic

- Asked in: 2069 Bhadra (old course), 10 marks; 2068 Bhadra (old course), 10 marks (asked 2 times)
- 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/short-note-traffic-engineering
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-6

## Question

Write a short note on telephone traffic engineering.

## Short answer

Telephone traffic engineering uses probability and queuing theory to dimension trunks and switches for a target grade of service at minimum cost. Key concepts include traffic intensity in Erlangs, busy hour, and GoS; loss systems use Erlang B, delay systems use Erlang C.

## Answer

**Telephone traffic engineering** is the study and design of switching and transmission resources (trunks, switch paths, registers, processors) so that a network gives an acceptable **grade of service** at minimum cost. It uses probability and queuing theory to relate traffic load, number of servers and service quality.

#### Basic quantities
- **Traffic intensity (Erlang):** average number of simultaneously busy servers, $A = C\,h/T$ (C calls of mean holding time h in period T). 1 E = one circuit busy continuously. 1 E = 36 CCS (hundred call-seconds per hour).
- **Holding time (h):** mean duration a server is occupied per call; usually taken as exponential.
- **Busy hour:** the continuous 60-min period of the day with the highest traffic; networks are dimensioned for it. **Busy hour calling rate** and **day-to-busy-hour ratio** are used to forecast it.
- **Grade of Service (GoS):** probability that a call is lost (loss system) or delayed beyond a time (delay system), e.g. GoS = 0.01 means 1 call in 100 lost in the busy hour.

#### Types of systems
| Type | Blocked calls | Model / formula |
|---|---|---|
| Loss system | Cleared | Erlang B (infinite sources), Engset (finite) |
| Delay system | Wait in queue | Erlang C, M/M/1 |
| Lost-calls-held | Retry/held | Poisson formula |

#### Traffic assumptions
1. Pure chance traffic: Poisson call arrivals (large number of independent sources).
2. Exponential holding times.
3. Statistical equilibrium: traffic does not change during the period.
4. Full availability: any free server can be used.

#### Main formulas
- **Erlang B:** $B = \dfrac{A^N/N!}{\sum_{k=0}^{N} A^k/k!}$ – probability of loss with N trunks.
- **Erlang C:** probability of delay in an N-server delay system; mean delay $= P_D\,h/(N-A)$.
- **Engset:** loss with a finite number of sources (PBX).

#### Uses
- Dimensioning number of trunks between exchanges and of switching stages.
- Choosing between direct and tandem routes (overflow and alternate routing).
- Planning call centres, cellular channels per cell, and buffers in data networks.
- Measuring and forecasting load for network growth.

**Example:** with A = 5 E offered and GoS 0.01, the Erlang B table gives N = 11 trunks.

Traffic engineering therefore balances **cost** (fewer circuits) against **service** (low blocking or delay).

## Check yourself

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

1. What is the aim of telephone traffic engineering?
   A. Maximise number of circuits
   B. Give acceptable grade of service at minimum cost
   C. Remove all blocking
   D. Measure only holding time
   Answer: B. It balances cost against service quality using probability and queuing theory. (Key point: Purpose)

2. Why are networks dimensioned for the busy hour?
   A. It is the lowest traffic period
   B. It is the continuous 60-min period of highest traffic, so GoS is met at peak
   C. It is the hour with the most revenue
   D. It is the first hour of the day
   Answer: B. Designing for the peak ensures service is met at other times too. (Key point: Busy hour)

3. Which pairing of system and model is correct?
   A. Loss system, infinite sources: Engset
   B. Loss system, infinite sources: Erlang B
   C. Delay system: Erlang B
   D. Finite sources loss: Erlang C
   Answer: B. Erlang B for loss with infinite sources, Engset for finite, Erlang C for delay. (Key point: Models)

4. Which is NOT an assumption of pure chance traffic?
   A. Poisson arrivals
   B. Exponential holding times
   C. Statistical equilibrium
   D. Traffic changes constantly during the period
   Answer: D. Equilibrium requires the traffic to stay stable during the period. (Key point: Assumptions)

## 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
- [Role of traffic engineering](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arole-of-traffic-engineering) — Telecommunications, chapter 6
- [Importance and unit of traffic engineering](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atraffic-engineering-importance-unit) — 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×
- [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×

## Related in other subjects

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- [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
- [Equivalence partitioning: telephone numbers](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Aequivalence-partitioning) — Software Engineering, chapter 8
- [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×