ng circuits. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK 2. It has the ability not only to establish and maintain (or hold) a physical connection between a caller and the called party for the duration of the call, but also to be able to disconnect (i.e., "clear") it after call termination. 3. It also has the ability to prevent new calls from intruding into circuits that are already in use. To avoid this, a new call must be diverted to another circuit that is free or it must be temporarily denied access where the caller will hear a "busy back" (i.e., a tone cadence indicating that the line is busy) or an "all trunks busy" tone cadence signal or voice announcement (i.e., indicating congestion or blockage). Let's differentiate local and tandem/transit exchanges. A local exchange connects lines (subscriber loops) to other lines or to trunks. A tandem/transit exchange switches trunks. Local exchanges concentrate and expand. Tandem and transit exchanges do not. 4.3.2 Concentration and Expansion Trunks are expensive assets. Ideally, there should be one trunk available for every sub- scriber line (loop). Then there never would be a chance of blockage. Thus, whenever a subscriber wished to connect to a distant subscriber, there would be a trunk facility avail- able for that call. Our knowledge of telephone calling habits of subscribers tells us that during the busy hour, on the order of 30% of subscriber lines will be required to connect to trunks for business customers and some 10% for residential customers. Of course, these values are rough estimates. We'd have to apply the appropriate traffic formula based on a grade of service, as described in Section 4.2.1, for refined estimates. Based on these arguments, a local exchange serving residential customers might have 10,000 lines, and only 1000 trunks would be required. This is concentration. Consider that those 1000 incoming trunks to that exchange must expand out to 10,000 subscribers. This is expansion. It provides all subscribers served by the switch with ac