ts displacement of the digit periods from their ideal positions. Time is essentially analogue in digital transmission and so is subject to Traffic is measured in terms of a unit of traffic intensity called the analogue types of distortion. Erlang (formerly known as the traffic unit or TU) which may be defined as the number of call hours per hour (usually, but not necessarily, the busy hour), or call seconds per second, etc. This is 31.2.7 Signalling a dimensionless unit which expresses the rate of flow of calls and, for a group of circuits, it is numerically equal to the average number To connect calls through a network of exchanges, it is necessary to of simultaneous calls. It also equates on a single circuit to the send data referring to the call between the exchanges concerned. In proportion of time for which that circuit is engaged and conse- the simplest case, this consists of loop/disconnect signalling similar quently to the probability of finding that circuit engaged (i.e. the to that used on customers' lines. Each speech circuit consists of a grade of service on that circuit). The traffic on one circuit can, of pair of copper wires which have direct current flowing when in use course, never be greater than one Erlang. Typically a single selector on a call and this current is interrupted to form a train of break pulses or junction circuit would carry about 0.6 Erlang and customers' for each digit corresponding to those produced by the telephone lines vary from about 0.05 or less for residential lines to 0.5 Erlang dial. When the called party answers, the called exchange sends or more for some business lines. back an 'answer' signal to the calling exchange by reversing the It follows from the definition that C calls of average duration t direction of the current flow. seconds occurring in a period of T seconds constitute a traffic of A Longer circuits are unsuitable for direct current signalling e.g. due Erlangs given by Equation 31.1. to the use of amplifiers, multiplexing equipment or radio links. Voice f