es face the same challenge: providing voice quality for voice while providing data quality for data. The voice channel is time-sensitive, requiring low de- lay and low jitter, but it can tolerate a higher error rate. The data channel can tolerate more delay and jitter but requires a much lower error rate. Due to the different methods of achieving these different classes of service, the net- work must function differently for voice and for data. For voice, the signal includes for- ward error correction (FEC). As a result, it is best to reassemble the entire data stream, even if it contains errors. It is better to have a corrupted bit than to discard an entire frame or packet. However, for data services, a bad packet should be discarded as soon 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. Hybrid Voice-Data Networks Key Data Concepts as possible, allowing a request for retransmission. Very low-level protocols, at the In- ternet Protocol (IP) packet level and even at the data-link layer, need to be redesigned to accommodate this flexibility. Switches and routers operating at these levels must be able to identify the type of service needed for the data in the packet and act differently depending on that need, that class-of-service request. As networks become more sophisticated, the class-of-service request has other func- tions as well. On networks with multiple paths, it is possible for routers and switches to pass information back and forth, which informs each piece of equipment of the state of the network. Routers can then identify paths with low latency and jitter that should be used for real-time service. Using these paths maintains a low-delay, low-jitter cir- cuit, or virtual circuit, for telephone signals. Data packets are sent on slower routes to keep the high-speed route available for voice. We see an analogy for this in the opera- t