ibrary.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 Key Telephony Concepts these customers by providing a twisted pair of wires to connect the telephone to the lo- cal exchange office. 1 A novel feature of the analog loop is that it uses a single twisted pair for both the talk path and the listening path of the call, as well as the signaling of digits, sensing of on- and off-hook, and ringing the telephone. Of particular interest to us is the ability to carry both the talk and listening paths. This is accomplished by us- ing a device called a hybrid circuit at both ends of the loop. The hybrid circuit essen- tially channels the incoming signal to the earpiece while channeling the signal from the microphone or mouthpiece out to the line without excessive energy going to the ear- piece. Ideally, it also ensures that no energy is reflected back to the other end of the line, a phenomenon called echo. Modern local exchange offices may use specialized equipment located close to the telephone to minimize the length of the twisted pair of wires. For the purpose of this brief introduction, the scenario used will be that the twisted pairs extend all the way back to the local exchange office. It is important to note that each telephone number is assigned a physical presence-think of it as a pair of terminals-on the local exchange office. A call to a telephone number will result in the switch sending the call to that physical location on the switch. All telephones connected to the other end of the twisted pair are assumed to have an assigned telephone number. The actual procedures used between the telephone and the telephone switch to com- municate the user's desire to place a call or to alert the user that the telephone num- ber is being called are the topic of Sec. 12.1. A useful concept is that of a line, a transmission path that can be switched only at one end. The twisted pair that connects the telep