17.1 Data Service Requirements Service Delay Circuit-switched Steady Voice Steady low Video Steady high Medium Web access Bursty high Medium Variable Data downloads Medium E-mail Class D, available bit rate (ABR), serves a bursty data stream, is tolerant of delays and some cell (packet) loss, and guarantees a level of data throughput. It is suitable for local area network (LAN) traffic. 9 Class D, unspecified bit rate (UBR), is a bursty data stream that is tolerant of delays and more cell (packet) loss. It makes no guarantees other than best effort. We picture an array of data performance requirements as shown in Table 17.1. The circuit-switched data subscriber requires a steady stream of highly accurate bits with low delay, the voice call permits higher error rates but still requires low delay, and a nightly data download requires high accuracy but can wait a few minutes or even hours. 17.4 Error Detection and Correction There are several techniques for dealing with bit errors. To deal with errors, we have to detect them, and this requires enough redundancy so that errors appear wrong. If every combination of letters in English spelled a word, then proofreading would be very difficult, and there would be no spell checkers. In general, we can divide error-correction schemes into two types, forward error cor- rection and error recovery, sometimes called backward error correction. For forward er- ror correction, we transmit enough redundant error-correcting information to allow the receiving system to reassemble lost data (most of the time) without having to request retransmission. In the older error-recovery technology, only enough information is sent to allow the receiving system to determine that an error has occurred. The receiving system must request retransmission of the packet containing the information that was not received correctly. 11 The packet loss is tolerated because in the LAN environment error recovery using the Trans- mission Control Protocol (TCP) or a similar protocol will ensure that the lost o