contacts and the wipers are moved one step A method sometimes used in spc systems is known as Trunk from each contact to the next for each pulse, as in Figure 31.11, by Reservation, whereby the overflowing traffic is only given access to means of a ratchet and pawl mechanism. the secondary route if more than a set level of circuits are free, while In early Strowger exchanges, each customer was provided with the traffic normally using this route always has access to it. his own 100-outlet two-motion selector. The expense of providing one large switch or selector is now avoided by using a stage of 31.3.7 Queuing smaller cheaper uniselectors to connect the callers to the selectors as and when they require them. Expansion of the exchange beyond A queuing system provides facilities for storing calls in order until 100 lines is by adding further stages of selectors as shown in Figure the server is available. The number of calls which can be held in a 31.12. 31/10 Principles of switching systems Concentration is provided at the uniselectors. In this example 10 level there are then three stages of group selectors to provide the "dis- contact tribution" function. The final two digits of the number are used to position the final selector (connector) wipers to the contacts con- nected to the required line, thus providing the expansion function. outlets Two-motion The final selectors also contain the circuit element known as the selector 'transmission bridge' which provides current to the calling and called telephones from the central supply (battery) at the exchange. It is here that 'answer' and 'clear' conditions are detected to control Wiper charging (metering) and release of the connection. A large city often has a large number of exchanges with overlap- ping service areas where it would be unacceptable for callers to have Uniselector to consult a different list of dialling codes according to which exchange the telephone they happen to be using is connected. To outlets Feeder spring Director equipment Figure 31.11 Strowg