circuit to which it is to be applied. 31.2.6 Types of transmission 31.3 Teletraffic Transmission may be either analogue, in which the electrical signal 31.3.1 Grade of service represents directly the speech signal, or digital, in which speech or data are coded in pulses. At present, telephone instruments and customers' lines are generally analogue, but the trunk and junction It would be uneconomic to provide switching equipment in quan- network is increasingly digital. tities sufficient for all customers to be simultaneously engaged on The essential feature of digital transmission, as opposed to anal- calls. In practice, systems are engineered to provide an acceptable ogue, is regeneration. The digital signals consist of a sequence of service under normal peak (i.e. busy hour) conditions. For this digits (or symbols) each of which have a few recognised levels purpose, a measure called the grade of service (g.o.s.) is defined as (typically four or less) and occupy equal periods of time known as the proportion of call attempts made in the busy hour which fail to digit (or symbol) periods. Transmission systems are designed so that mature due to the equipment concerned being already engaged on corruption of the digits, due to noise, interference or distortion, does other calls. A typical value would be 0.005 (one lost call in 200) for not cause the signal level in each digit period to become indetermi- a single switching stage. To avoid the possibility of serious deterior- nate. A regenerator measures the level in each digit period and ation of service under sudden abnormal traffic it is also usual to generates a new digit of the original amplitude. This means that specify grades of service to be met under selected overload condi- noise and corruption of the signal do not accumulate through a series tions. of regenerators. Only two types of impairment are left. Digital errors occur when excessive noise and corruption cause a regener- ator to make wrong decisions about the level and jitter, which is 31.3.2 Traffic uni