ing problems is to tem- porarily reduce the encoding rate. The typical encoding rate for DSI is 32 Kbps; in certain situations, the encoding rate may be reduced to 24 Kbps, thus freeing up significant bandwidth for additional channels. Both techniques are widely utilized in DSI systems. Enter the Modern World: Voice-Over IP It would be irresponsible to update this book without giving at least passing mention to voice-over IP (VoIP) as a major telecommunications phenomenon. It is fundamentally changing the way in which people talk 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 Telephony on a daily basis. In this section we'll describe what it is, how it works, and how it is being used by a wide array of both residence and enterprise cus- tomers. Let's examine how a telephone call is carried across an IP network. A customer begins the call using a traditional telephone. The call is carried across the PSTN to an IP telephony gateway, which is nothing more than a special-purpose router designed to interface between the PSTN and a packet-based IP network. As soon as the gateway receives the call, it interrogates an associated gatekeeper device, which provides informa- tion about billing, authorization, authentication, and call routing. As soon as the gatekeeper has delivered the information to the gateway, the gateway transmits the call across the IP network to another gateway, which in turn hands the call off to the local PSTN at the receiving end, completing the call. Let's also address one important misconception. At the risk of sound- ing Aristotelian, "All Internet telephony is VoIP, but all VoIP is not Inter- net telephony." Got it? Let me explain. It is quite possible today to make telephone calls across the Internet, which by definition are IP-based calls simply because IP is the fundamental operational protocol of the Inter- n