out the replies. After the download, the data session can move into a dormant state, reducing the demand on the capacity of the cell. When the user finishes creating replies and sends them, the system can re- activate the data transmission link. This response does not have to happen within a fraction of a second. If it takes a few seconds for the e-mail to start moving, then that is no problem. And a few seconds is a few thousand milliseconds. This flexibility in time creates opportunities for optimizing capacity for multiple users. For example, a system could be designed so that if capacity is near peak at a particular moment, a new request for data services encounters a brief delay. This allows one or more of the other trans- missions to complete and go dormant. Rather than getting excessive interference, each transmission takes its turn, and all subscribers are satisfied. 17.2.2 Error recovery In voice transmission, forward error correction (FEC) is used. As a result, the trans- mission of each person's voice relies on only one-half of the duplex channel. This is fa- 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) miliar to us in the case of static on only one channel. One party is saying, "I can't hear you," while the other party is saying, "I can hear you just fine." With data transmission, the situation is quite different. Data transmission relies on error recovery, where the receiving system checks the packets it receives and requests retransmission of any corrupted or missing packets. As a result, even if a data file is being sent in only one direction, it relies on an operating two-way communications channel. This can be observed by watching the number of bytes moving in each direc- tion while downloading a file from the Internet. Although most of the bytes are coming on the downlink