loaded 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 It is important to realize that these checks only work on the links where they are be- ing used. IP systems often have host computers and routers at the ends of these TCP links, and there is no error recovery or error correction in those machines. In any data system, it is a good idea to have some kind of end-to-end error detection and correction. 17.4.2 Forward error correction (FEC) FEC is generally used for real-time voice (and video) transmission. High-quality voice communication must occur in real time with the entire signal received in order, but the occasional dropping of small bits of sound is not terrible. FEC is better for these sys- tems because the receiving system can assemble almost all the message accurately al- most all the time. This simple example illustrates FEC: We are going to send 8 bytes of data, 64 bits. Let us lay those bits out on an eight-by-eight checkerboard and for each row and for each column we will add a parity bit, a 0 if the sum of the row or column is even and a 1 if the sum is odd. If we have a single error in our original 64-bit square, then there will be one row and one column that do not add up, as shown in Fig. 17.1. A single er- ror in the parity row will leave all the row sums intact, and the same is true for the parity column. If there are two errors in the 80 bits, then there is no way of knowing the original 64 bits. 00o16 Parity Column 10001 Parity Row Figure 17.1 Simple forward error correction (FEC). 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) Quality of Service (QoS) Thus our error-resolution algorithm at t