ating current. This is connected from a 31.2.4 Exchange structure common supply at typically 75V r.m.s. and 25 Hz connected through a 500 ohm resistance at the exchange. The most elementary form of switching network consists of a single square switch, where connection can be made between any one inlet and any one outlet by connecting the associated horizontal and 31.2.2 Types of customer (subscriber) vertical circuits at the point where they cross. However, to provide installation an exchange capable of serving say 10,000 customers, it would not be economic either to construct a single large switch or simply to Telephones are installed at customers' (subscribers') own premises gang a large number of smaller switches to form a square matrix of or made available to the general public at call offices (paystations) 10,000 X 10,000 crosspoints. A more practical arrangement is to with coin collecting boxes (CCB) or card reading devices. Each of connect the switches in two or more stages. To illustrate this Figure these installations is normally connected to a local exchange by a 31.8 shows first a single square matrix of 81 crosspoints providing pair of wires but in isolated locations radio links may be used and for nine paths between nine inlets and nine outlets and then how to optical fibres are now being used increasingly for at least the part of meet the same requirement with only 54 crosspoints. The advantage the connection nearest to the exchange. In some cases more than one of this approach is even greater with larger numbers of circuits and customer's instrument may be connected to the same line which is practical switches having typically 10 X 20 crosspoints or more, then described as a party line. In some countries a party line, rather than only 3 X 3 as shown in the example. especially in remote rural areas, may serve as many as 20 customers This simple example provides only for the distribution of calls with a system of coded ringing signals. In the UK the only party from the inlets to an equal number of ou