ars ago. In some very old movies you will see a businessman's desk with over a dozen phones. One rings, and the executive fumbles, picking up phone af- ter phone until he knows who called. Each phone had only one wire leading to one des- tination, with one phone at the other end. A person needed a separate telephone with a direct line to each person he or she might ever want to call. Switches eliminated that limitation, allowing any one telephone to reach any other telephone. The earliest switches were manual patch-bays, where an operator would sit in front of a board and make connections as subscribers requested them. These con- nections may have started with people's names, but the telephone company was soon large enough that individual lines were referred to by number: "Operator, please con- nect me to 7283." These gave rise to another Hollywood image, that of the operator plugging and unplugging cords, one after the other, throughout a busy day. With the advent of automated switches, people were not part of the switch any more, and Hollywood lost interest. But this is where our interest begins. In this chapter we will explore the development and function of the automated telephone switch from the early mechanical days to the latest electronic technology. This chapter's final section will look at the additional functionality added to switches to support mobile telephony services. 11.1 The Public Switched Telephone Network (PSTN) The core of the PSTN is the network of switches that are used to carry a telephone call from the switch that connects to the calling party to the switch that connects the called party of a telephone call. The cabling and equipment that connect telephones to the network of switches are often referred to as the local loop. 11.1.1 The local loop The vast majority of residential subscribers access the network using analog devices such as two-wire analog telephones, fax machines, and modems. The PSTN services Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringl