et link with less latency and jitter, then we can af- ford higher loss in the radio link and still offer adequate voice quality. Asynchronous Transfer Mode (ATM) deals with the different requirements by having four classes of service, with class D traffic split into two subclasses. In these descrip- tions, a service is time-sensitive if it requires low delay and latency. If the class of ser- vice is sensitive to loss, then it requires a low error rate. Class A, constant bit rate (CBR), serves a low bit rate at a steady stream and is time- sensitive and also sensitive to loss. It is suitable for circuit-switched data service. Class B, variable bit rate (VBR), serves a steady or bursty data stream and is time- sensitive. It is suitable for real-time voice or video applications. Class C, non-real-time (VBR-NRT), serves a steady or bursty data stream and is tolerant of some delay. It is suitable for video or audio streaming. We can think of these losses like airlines losing baggage. Loss of luggage is rare enough that many passengers check their luggage routinely and maintain a backup plan for the few times they arrive and their suitcases do not. Fortunately, packets in subscriber data services typically arrive with a bit error rate (BER) of 10-6, one in a million, considerably better than airlines' rate of lost luggage. Streaming is a technology in which a relatively high quality voice or video file is received and is placed into a buffer. The file can begin to play for the user before it is finished downloading. In- creases in delay or problems with jitter will reduce the buffer size during play. However, if the ser- vice is robust enough, the playback will complete without interruption. Streaming video and au- dio are used commonly on the Internet. 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 TABLE 