# Telephone Traffic & Trunking Principle

## TELEPHONE TRAFFIC

Telephone traffic means, number of subscribers .trying to contact other subscribers. It has an interesting similarity with vehicular traffic on a road in a city. In practice, it is very difficult to predict. that-at a particular time someone wants to make a call. It may happen any time. So it is a probability that someone needs to make a call.

For a given area, the distribution of calls is not uniform but it varies from time to time. There is seasonal variation of calls also. An average over a year for day to day variation gives an idea, that how many people are making calls at a particular time. This variation is shown by a curve in figure

A thorough study of the telephone traffic gives an idea to decide as how many selectors are to provide at each stage, and how should they be interconnected, so that the ideal objective of maximum efficiency with minimum cost can be approached as far as possible. It is now necessary to define certain basic terms so that a quantitative meaning can be attributed to the telephone traffic.

## BUSY HOUR

As we know that the telephone traffic varies from time to time, so any calculations based on the average will be misleading for the requirements most of the time. To get a reasonable standard of service, the calculations for the numbers of switching equipment should be based on some internationally agreed criteria. This internationally agreed criteria, on which all the calculations are based is the busy hour. The busy hour is defined as a period of sixty consecutive minutes during which the telephone traffic is the highest. The busy hour varies from exchange to exchange depending on its location, and community interest of its subscribers.

## BUSY HOUR CALLING RATE

It is defined as the average number of calls per subscriber during the busy hour.

## HOLDING TIME

This is called holding time because during this time, the switches in the exchange are held busy. The holding time per call consists of two parts: Operating time, is the time during which the call is set up and after conversation the switches are released and Conversation time, is the actual time used by the subscriber in talking. From the subscriber point of view the conversation time is important while for the telephone administration the total time is of importance because the switches remain busy until both the subscriberput down their receivers.

## TRAFFIC UNIT - THE ERLANG

The telephone traffic is measured in the unit called "erlang", which is defined as the product of the number of calls perbusy hour and the average holding time per call, measured in hours. The abbreviation used for erlang is E. Thus the telephone traffic A in erlang is given by:

A = N x T       erlang
Where             A = telephone traffic in erlang
N = number of calls in the busy hour
T = average holding time of calls expressed in Hours.
An erlang can also be defined as one call of one hour duration.