and efficient network. The European PSTN is functionally and architecturally similar to the American PSTN. It is a highly reliable network using SS7 switching. However, the digital facili- ties for voice and signaling are E lines rather than DS lines, as described in Sec. 13.1. The European network's system of telephone numbers is quite different because of its different history. The European system is developing through the increasing unifica- tion of what were separate national phone systems, whereas the North American sys- tem is developing largely through the continuing diversification of what was once a cor- porate monopoly. Other nations around the world have a variety of PSTN networks of greater or lesser reliability. Most are based on either European or North American models. The same is true with the allocation of frequency for wireless telephony: Most nations follow either the European model or the North American model of radio bandwidth allocation. 5A tandem office is used to switch a call received over a trunk to another trunk. 6Sometimes SS7 is referred to as Common Channel Signaling 7 (CCS7). Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The PSTN and Telephone Switching The PSTN and Telephone Switching 11.2 Telephone Switch Technology Once switches were automated, the process of providing a number to the switch needed to be automated as well. The pulse dialing method was adopted, and the telephone com- pany developed the rotary dial that could send electrical pulses in the form of momen- tary line disconnections. 7 The telephone central office switches detected these pulses and completed calls using the dialed digits. These were step-by-step switches in which each pulse would move a motor one step at a time and, later, crossbar switches in which com- binations of dialed digits made connections. The crossbar was named for its 