of the popularity of digital switches, implementations of analog switches quickly declined. The 4ESS is a digital switch using the 1A processor originally used in the analog 1AESS. The 4ESS is a tandem switch designed to connect to trunks rather than lines. Trunks are shared facilities that can be used by any call received by the switch rather than dedicated, as in the case of lines. Therefore, trunks can be engineered for higher utilization. The result is that the number of trunks terminated on a tandem switch is a fraction of the number of lines terminated at a local exchange office. "In many places one can still dial a call by pressing the switchhook rapidly up and down on a regular telephone, even a Touch Tone telephone. Diehard fans of analog switches miss the sound of a telephone office. Other than for the whirring of cooling fans, a telephone office with a digital switch is quiet. Step-by-step, crossbar, and reed switches have their own distinctive audible character, and a telephone switching office used to be a noisy place. 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 Key Telephony Concepts 11.3 Telephone Switch Functions Telephone switches are specialized according to the functions they are to perform. Some of this specialization is achieved by optimizing the architecture of the switch, and for some functions, specialization is achieved by the activation of software features. 11.3.1 Connections Local exchange offices are optimized for the functions that are needed to terminate in- dividual lines (telephones). They are highly modular, capable of terminating a large number of individual lines. Each line is physically associated with a telephone number. The average utilization of a line is very low, so local exchange offices are designed to terminate a large number of lines but typically have a somew