requency (VF) signalling is then used in which pulses of (31.1) alternating current are used at frequencies and levels compatible with circuits designed for handling speech currents (typically 600, 750, 2280 and 2400 Hz). This use of frequencies within the speech An alternative unit of traffic sometimes used is the C.C.S. (call cent band (300-3400 Hz) is termed -band signalling. second) defined as hundreds of call seconds per hour (36 C.C.S. = 1 An alternative is the use of out-of-band (outband) signalling, in Erlang). which a signalling frequency (e.g. outside the speech band is separately modulated onto the carrier in a frequency division transmission system. In digital transmission systems, the signals are digitally encoded and certain time slot(s) are reserved for this 31.3.3 Erlang's full availability formula purpose separate from the speech time slot(s). These are all channel associated signalling systems, in which the Full availability means that all inlets to a switching stage have signalling is physically and permanently associated with the indi- access to all the outlets of that stage. Trunking arrangements, in Analogue switching 31/9 which some of the outlets are accessible from only some of the queue at any one time must be limited and, therefore, dimensioning inlets, are said to have limited availability. must take account of the loss probability as well as delay conditions. For a full availability group of N circuits offered a traffic of A Many practical systems involve multiple queues dependent on the Erlangs, the grade of service B is given by Equation 31.2. service required and this results effectively in a queuing network which can be quite complex to analyse. (31.2) 31.4 Analogue switching 31.4.1 The manual exchange This formula assumes pure chance traffic and that all calls origin- ating when all trunks are busy are lost and have zero duration. The first telephone exchanges were operated by hand. Instructions were passed verbally between the subscriber and the operator (tele- 31.3.4 Busy hour 