hat is heard by the callING party. If the callED party picks up the phone, the network senses the off-hook, removes the ringing voltage at the callED party's end, and signals through the network for the office at the callING party's end to remove the artificial ring tone. We call the person at the other end the callED party. We use the uppercase ING to differentiate between the callING party and the callED party, and we emphasize the ING syllable when speaking about telephone calls. Telephone subscribers in rural areas are loathe to pay the monthly charge for Touch Tone ser- vice. Rotary phones are alive and well. 3The uppercase ED is to avoid confusing the callING party and the callED party, and we pro- nounce it in two syllables, "call-ED," with the accent on the second to make the point. 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 The call is then connected and is now said to be a stable call. The call stays in the stable state until one party, either callING or callED, hangs up the receiver or, in tele- phone talk, goes on-hook. Once this happens, the call is complete, and any trunks used to complete the call go away. We are using this traditional call sequence to illustrate the structure of a telephone call. If setting up this call required that the call be routed between offices, then one or more interoffice trunks would have been used. Traditionally, the trunk would have been selected by examining the trunks in the appropriate trunk group until an idle trunk was found, identified by the signaling bits (assuming a digital facility). If the call had to be routed through to another trunk, then the process would be repeated at the next switch in the call path. The method used today by many carriers is to negotiate for available trunks over the entire transmission path using Signali