determine that an error has occurred, but to fix it. No resend is required. The third and perhaps most common form of error detection and cor- rection is called detect and retransmit. Detect and retransmit systems use the CRC field to detect errors when they occur. The errored frames are then discarded, and the previous switch is ordered to resend the errored frame. This implies a number of things: The frames must be numbered, 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 Protocols there must be some form of positive and negative acknowledgment sys- tem in place, the transmitter must keep a copy of the frame until its receipt has been acknowledged, and there must be some facility in place to allow the receiver to communicate upstream to the transmitter. Two recovery techniques are commonly used in synchronous sys- tems. To understand them, we must first introduce a couple of trans- mission protocols used to meter the transmission of frames between switches. In early communications systems (1970s), the network was known to be relatively hostile to data transmission. After all, if noise occurred dur- ing a voice conversation, no problem-it was a simple matter to ignore it, provided it wasn't too bad. In data, however, a small amount of noise could be catastrophic, easily capable of destroying a long series of frames in a few milliseconds. As a result, early data systems such as IBM's Binary Synchronous Communications (BISYNC) used a protocol called Stop-and-Wait that would permit only a single frame at a time to be out- standing without acknowledgment from the receiver. This was obviously terribly inefficient, but in those early days was as good as it got. Thus, if a major problem occurred, the maximum number of frames that would ever have to be resent was one. As time passed and network quality improved, designers got brave and be