tch. The receiving switch performs the same calculation and then compares the two remainders. As long as they are the same, the switch knows that the bits arrived unaltered. If they are different, the received frame is dis- carded and the transmitting switch is ordered to resend the frame, a process that is repeated until the frame is received correctly. This process can result in transmission delay, because the Data Link Layer will not 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. Protocols Chapter 2 Figure 2-37 CRC-based bit- Quotient + Remainder level error control 0010010101000010 Address Control PKT 5 400 allow a bad frame to be carried through the network. Thus, the Data Link Layer converts errors into delay. Error Recovery Options There are a number of techniques in common use that allow receiving devices to recover from bit errors. The simplest of these is frame discard, the technique used by frame relay and ATM networks. In frame discard environments, an errored frame is simply discarded-period. No other form of recovery takes place within the network. Instead, the end devices (the originator and receiver) have the end-to-end responsibility to detect that a frame is missing and take whatever steps are necessary to gener- ate a second copy. There are reasons for this strategy that will be dis- cussed in the section on fast-packet services. A second common technique is called forward error correction (FEC). FEC is used when (1) there is no backward channel available over which to request the resend of an errored packet, or (2) the transit delay is SO great that a resend would take longer than the application would allow, such as in a satellite hop over which an application is transmitting delay- sensitive traffic. Instead, FEC systems transmit the application data with additional information that allows a receive device to not only 