Role of traffic engineering

Traffic engineering applies probability theory (queueing and teletraffic theory) to measured traffic so that a telecommunication network is built with just enough equipment to give a required quality of service at the lowest cost. Subscribers do not all call at once, so an exchange is built with far fewer paths than subscribers; traffic engineering decides how many.

Why it is needed

• A network with one trunk per subscriber would be very expensive and mostly idle.
• Too little equipment causes blocking, long dial-tone delays and lost revenue.
• Traffic is random and varies with time of day, day of week and season, so planning must use statistics.

Roles of traffic engineering

1. Traffic measurement and characterisation. Measure the number of calls, holding times and busy-hour traffic (in erlang) on each route, and find the busy hour (CCITT E.600).

2. Forecasting. Predict future traffic from subscriber growth, new services (data, mobile) and tariff changes, so equipment is ready in time.

3. Choice of grade of service. Fix the acceptable blocking probability, e.g. 1% on local routes, 0.5% on final routes, and delay targets such as dial tone within 3 s for 95% of calls.

4. Dimensioning. Using Erlang B (loss systems), Erlang C (delay systems), Engset or binomial (finite sources), calculate the number of:

   • trunks on each route,
   • switching paths and network stages,
   • common-control equipment: registers, markers, processors, signalling links,
   • radio channels in a mobile cell.

5. Switching network design. Calculate blocking of multistage (e.g. Clos) networks using the Lee graph or Jacobaeus methods.

6. Routing plans. Design high-usage and final routes with alternate (overflow) routing so trunks are used efficiently.

7. Processor and queue performance. Find delays in stored-program control processors, message queues and data networks with queueing models (M/M/1, M/M/n).

8. Performance monitoring and overload control. Compare measured blocking with target and add capacity or apply call gapping when traffic exceeds design values.

9. Cost optimisation. Balance equipment cost against lost-call revenue and customer satisfaction.

Example

A route carries 10 E in the busy hour. For GoS 0.01, Erlang B gives about 18 trunks. Without traffic engineering, one might install 30 (wasteful) or 12 (blocking about 12%). The table shows the main tools used:

Thus traffic engineering links traffic demand, quality of service and cost, and is the basis of planning every exchange, trunk route and mobile cell.