fra- structure is already optimized for voice-quality calls with low noise, low latency, and low jitter. Designing IP switches is a major job, and retrofitting the network by replac- ing traditional circuit switches with IP packet switches is an even bigger job. However, the job is internal to the local exchange company or long-distance provider who takes on the challenge. The other components of the network and their quality are already determined. It is much easier to ensure quality when switching out only one component of a network than when trying to make an inherently unreliable network reliable enough for real-time service. In addition, the business case is much simpler. There is no need to attract new cus- tomers. In fact, the company might be better off if the customers do not even know that VoIP has arrived. The company, whether it offers local or long-distance service or both, simply can continue to provide service to the same customers at the same rate while saving money through reduced demand for transport capacity. Over time, the reduced "We say this with some confidence when we consider the number of subscribers who change cel- lular providers if their calls are dropped by their current provider. 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. Hybrid Voice-Data Networks Key Data Concepts demand for capacity will generate enough savings to justify the investment in the purchase and installation of new switches. Added value can be realized in that these switches support IP, which means, in addition to supporting VoIP, that they are also a reliable environment for data services. If a customer is given a duplex IP packet-based virtual circuit over the PSTN, the customer can add Transmission Control Protocol (TCP) error correction at each end and receive reliable high-speed data service. This configuration is good for the provider be