Outputs Concentration Expansion 4.3.3 The Essential Functions of a Local Switch As we mentioned above, means are provided in a local switch to connect each subscriber line to any other in the same exchange. In addition, any incoming trunk must be able to connect to any subscriber line and any subscriber to any outgoing trunk. 7 These switching functions are remotely controlled by the calling subscriber, whether she/he is a local subscriber or long-distance subscriber. These remote instructions are transmitted to the switch (exchange) by "off-hook," "on-hook,"8 and dial information. There are eight basic functions that must be carried out by a conventional switch or exchange 7 The statement assumes full availability. 8 Off-hook and on-hook are defined in Section 1.3.1. 4.3 INTRODUCTION TO SWITCHING 1. Interconnection 2. Control 3. Alerting 4. Attending 5. Information receiving 6. Information transmitting 7. Busy testing 8. Supervisory Consider a typical manual switching center illustrated in Figure 4.7. Here the eight basic functions are carried on for each call. The important interconnection function is illustrated by the jacks appearing in front of the operator. There are subscriber-line jacks9 and jacks Manual exchange Attending Interconnecting function function Subscriber lines 2-ended Incoming trunks Jacks Orig. Term. Outgoing trunks Supervising function Alerting function Ringing source Operator Information receiving function Information transmitting function Control function Busy testing function Figure 4.7 A manual exchange illustrating switching functions. 9A jack is an electric receptacle. It is a connecting device, ordinarily employed in a fixed location, to which a wire or wires may be attached, and it is arranged for the insertion of a plug. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK for incoming and outgoing trunks. The connection is made by double-ended connecting cords, which can connect subscriber to subscriber or subscriber to trunk. The cords avail- able are always less than half the n