Importance and unit of traffic engineering

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:

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.

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