iency and packaged in small frames to minimize the processing delay of the frames. Several hundred voice channels can be encoded over a single PVC, although the number is usually smaller when actually implemented. Frame relay is a virtual circuit service. When a customer wishes to connect two locations using frame relay, he or she contacts his or her ser- vice provider and tells the service representative the locations of the end- points and the bandwidth they require. The service provider issues a service order to create the circuit. If at some point in the future the cus- tomer decides to change the circuit endpoints or upgrade the bandwidth, another service order must be issued. This service is a PVC and is the most commonly deployed frame-relay solution. Frame relay is also capable of supporting switched virtual circuit (SVC) service, but SVCs, for the most part, are not available from service providers. With SVC service, customers can make their own modifica- tions to the circuit by accessing the frame-relay switch in the central office and requesting changes. However, service providers do not cur- rently offer SVC service because of billing and tracking concerns (cus- tomer activities are difficult to monitor). Instead, they allow customers to create a fully meshed network between all locations for a very reasonable price. Instead of making routing changes in the switch, the customer has a circuit between every possible combination of desired endpoints. As a result, customers get the functionality of a switched network, while the service provider avoids the difficulty of administering a network within which the customer is actively making changes. 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. Transport Technologies Transport Technologies Figure 7-3 Frame relay service delivery 512 Kbps 1.544 Mbps 384 Kbps 768 Kbps In frame relay, PVC