l delay, and relatively low cost. The switches accomplish this by making several assumptions about the network. First, they assume (correctly) that the network is digital and based largely on a fiber infrastructure, the result of which is an inordinately low error rate. Second, they assume that, unlike their predecessors, the end-user devices are intelligent and there- fore have the ability to detect and correct errors on an end-to-end basis (at a higher protocol layer), rather than stopping every packet at every node to check it for errors and correct them. These switches still check for errors, but if they find errored packets, they simply discard them, know- ing that the end devices will realize that there is a problem and take cor- rective measures on an end-to-end basis. Think back to the protocol chapter for a moment: This is the difference between layer-two error detection and layer-four error detection. 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 Chapter 7 Frame Relay Frame relay came about as a private-line replacement technology and was originally intended as a data-only service. Today, it carries not only data but voice and video as well, and while it was originally crafted with a top speed of T1/E1, it now provides connectivity at much higher band- width levels. In frame-relay networks, the incoming data stream is packaged as a series of variable length frames that can transport any kind of data- LAN traffic, IP packets, SNA frames, even voice and video. In fact, it has been recognized as a highly effective transport mechanism for voice, allowing frame-relay-capable PBXs to be connected to a frame-relay per- manent virtual circuit (PVC), which can cost effectively replace private line circuits used for the same purpose. When voice is carried over frame relay, it is usually compressed for transport effic