 planners to decide which carriers to allocate to DO services. 1In data communications on systems with very low error rates, the data system may be designed not to use error recovery. In this case, the data transfer relies on FEC alone, just as voice calls do. Application designers can choose depending on the criticality of the information being delivered and the consequences of an error. For example, a Web browser reading HyperTe: Markup Lan- guage (HTML) will, at worst be unable to read the page if it contains an error. The user will be notified of the problem and press or click on Refresh to solve it. However, if a file is being down- loaded for later use, using the File Transfer Protocol (FTP) rather than HTTP, error recovery is essential to ensure that the file, which may not be used until much later, is received correctly. As we discussed in Chap. 9, we use the term detail standard to refer to a standard that is part of a larger standard. 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. Quality of Service (QoS) Key Data Concepts 17.2.4 Mixed voice and data carriers Existing carriers already support low data rates, and improvements to the channel will increase these rates. However, mixing voice and data on a single CDMA channel is likely to be less efficient than using separate carriers most of the time. When a DO carrier is in place, it can offer a very high data rate to a single subscriber by temporarily reducing service to other subscribers on the same cell or even by lock- ing them out for brief periods. This is acceptable to most users in data communications. We are all familiar with clicking to receive new e-mail and seeing that sometimes we get an immediate response, but sometimes we have to wait several seconds before any- thing comes through. Current DO channels rely on this willingness to wait SO that they can provide high-speed 