aled digits of the called subscriber, 447-8765. To route the call, the exchange will only work upon the first three digits. It accesses its local look-up table for the routing to the 447 exchange and takes action upon that information. An appropriate vacant trunk is selected for this route, and the signaling for the call advances to the 447 exchange. Here this exchange identifies the dialed number as its own and connects it to the correct subscriber loop, namely the one matching the 8765 The term office or central office is commonly used in North America for a switch or exchange. The terms switch, office, and exchange are synonymous. 1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS Exchange Exchange Called subscriber Calling subscriber Figure 1.5 Example connectivity subscriber-to-subscriber through two adjacent exchanges. number. Ringing current is applied to the loop to alert the called subscriber. The called subscriber takes her/his telephone off hook, and conversation can begin. Phases 2 and 3 of this telephone call are similar to our previous description. 1.3.3 The Use of Tandem Switches in a Local Area Connectivity Routing through a tandem switch is an important economic expedient for a telephone company or administration. We could call a tandem switch a traffic concentrator. Up to now we have discussed direct trunk circuits. To employ a direct trunk circuit, there must be sufficient traffic to justify such a circuit. One reference (Ref. 3) suggests a break point of 20 erlangs. 6 For a connectivity with traffic intensity under 20 erlangs for the busy hour (BH), the traffic should be routed through a tandem (exchange). For traffic intensities over that value, establish a direct route. Direct route and tandem connectivities are illustrated in Figure 1.6. 1.3.4 Introduction to the Busy Hour and Grade of Service The PSTN is very inefficient. This inefficiency stems from the number of circuits and the revenue received per circuit. The PSTN would approach 100% efficiency if all the circuits were used all the time. T