r corrupted packets are resent. 10 This class of service is not really of much value for commercial purposes. There really is no specific term for the older send-and-retransmit method because it was the only method available, so it did not need a unique term. Allow us to use the general term error recovery. 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) 17.4.1 Error recovery Error recovery is generally used for data services. True data service requires a much higher guarantee of accuracy than digital voice or video. A single bit error can, for ex- ample, corrupt an entire computer program if it is not detected. However, since pack- ets do not need to arrive in order, the receiving computer system can store all received packets as they are received and request retransmission of missing or failed packets. When the missing packet is received successfully, it is included in the message, which is then assembled into a computer data or program file or other large unit of data. Data transmission methods such as the Transmission Control Protocol/Internet Protocol (TCP/IP) were designed to use data received out of order. As a result, error recovery, which is more accurate but does not guarantee that packets are received quickly, is usually better for transmission of subscriber data. The TCP, as part of TCP/IP, is a data-recovery system. When a packet has an error in it, the receiver sends back a request to retransmit the packet. The detection of error is done using a cyclic redundancy check (CRC). The CRC function performs a binary calculation on a frame and produces a frame check sequence or frame checksum (FCS), which is sent along with the frame. 12 means that the total transmission is the original frame and the CRC-calculated The same calculation is performed at the receiving end, and the packet 