minating) jacks. The terminating and originating jacks can be interchangeable. The called subscriber at one moment in time can become the calling subscriber at another moment in time. On the other hand, incoming and outgoing trunks may be separated. In this case they would be one-way circuits. If not separated, they would be both-way circuits, accepting both incoming and outgoing traffic. 4.3.4 Introductory Switching Concepts All local telephone switches have, as a minimum, three functional elements: concentration, distribution, and expansion. Concentration and expansion were discussed in Section 4.3.3. Viewing a switch another way, we can say that it has originating line appearances and terminating line appearances. These are illustrated in a simplified conceptual drawing in Figure 4.8, which shows three different call possibilities of a typical local exchange: A call originated by a subscriber who is served by the exchange and bound for a subscriber who is served by the same exchange (route A-B-C-D-E). 2. A call originated by a subscriber who is served by the exchange and bound for a subscriber who is served by another exchange (route A-B-F). 3. A call originated by a subscriber who is served by another exchange and bound for a subscriber served by the exchange in question (route G-D-E). 10Ring-down is a method of signaling to alert an operator or a distant subscriber. In old-time telephone systems, a magneto was manually turned, thereby generating an alternating current that would ring a bell at the other end. Today, special ringing generators are used. 4.3 INTRODUCTION TO SWITCHING Trunks to/from other exchanges Outgoing Incoming Originating Terminating appearances Originating Terminating appearances connections connections Interconnecting trunks Figure 4.8 Originating and terminating line appearances. Concentration Expansion Distribution Distribution Subscriber Subscriber lines lines Originating Terminating stages stages Figure 4.9 The concept of distribution. Call concentration takes place in B and call exp