ng. A landline subscriber who has both features signals to add another call using three-way calling. At the same time, a new incoming call arrives from someone and is connected through call waiting. The subscriber is waiting for a dial tone, whereas the new caller is waiting for a conversation. In this example, both the components in the glare condition are people, but human beings are part of the telephone system, too, and human beings also can get caught in glare conditions. A common form of glare in the network occurs when two switches attempt to seize the same trunk. The trunk looks idle at both ends, based on the signaling bits, and both will attempt to seize. The signaling protocol is sufficiently clever that both switches are able to detect the glare condition. One of the switches, however, will have been pro- grammed to accept that the other end has seized the trunk successfully and to wait receive the dialed digits. 12.4 Wireless Telephone System Architecture From the perspective of the public switched telephone network (PSTN), the wireless system is an access technology. The role of the wireless network is clear: A mobile switching center (MSC) looks like a private branch exchange (PBX) to the PSTN, and the base stations and air interface are analogous to cross-connects and local loops. The 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. Telephony Engineering Concepts Key Telephony Concepts wireless system looks like a large community of end users with a set of telephone num- bers and a collection of trunks. When wireless systems communicate with the PSTN, they must send signals according to the SS7 standard and calls packaged in pulse code modulation (PCM). 12.5 Wireless Telephony Engineering Issues There are some telephony issues specific to wireless telephone systems. The great big difference is that wireless telephones are typi