engineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Hybrid Voice-Data Networks Key Data Concepts On cellular networks, such codecs are already in place for use on the air interface. Keep- ing the signal encoded all the way to the mobile switching center (MSC) makes sense. VoIP can be used by large corporations on their own networks. The functionality similar to the case of an internal ATM network carrying both data and voice: If the ca- pacity is there for data, why not put voice on it when we can? As of 2001, one major company answered this question: Because we cannot ensure voice quality well enough with any product now on the market. But this is changing, if not this year, then very soon. Companies are beginning to adopt VoIP for internal long-distance services to re- duce long-distance telephony charges. In addition, VoIP is available for general use on the Internet, and it will be coming into use on the PSTN. 19.4.1 VolP on the Internet VoIP on the Internet first arrived in the late 1990s as a form of free long-distance tele- phone service. Using a microphone and speaker attached to a personal computer, spe- cial software, and an Internet connection, people could talk to other people over the Web and not pay any long-distance charges. This gave it a small niche, especially for international telephone calls. However, early VoIP simply relied on the fact that the Internet had available bandwidth and would deliver acceptable call quality for the price. Some efforts have been made since then to create a viable commercial product. These efforts must overcome several technical hurdles: Service must be available to telephones, not just computers. The callING party must be able to place a call from a telephone to a telephone switch that then moves the call to In- ternet transport. The service must reach all telephones. It must become possible for any callED party with a computer, landline phone, or cellular phone to 