ations might be dynamically subdivided into various-sized channels for use by a PBX for voice, a videoconferencing system, and data traffic, as shown in Figure 7-1. Dedicated facilities have the advantage of always being available to the subscriber. They have the disadvantage of being there and accumu- lating charges whether they are being used or not. For the longest time, dedicated circuits represented the only solution that provided guaran- teed bandwidth-switched solutions simply weren't designed for the heavy service requirements of graphical and data-intensive traffic. Over time, however, that has changed. A number of switched solutions have emerged in the last few years that provide guaranteed bandwidth and only accumulate charges when they are being used (although some of 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 Figure 7-1 A high- Voice bandwidth channel, channelized for different traffic Video types Conferencing Transmission Channel them offer very reasonable fixed-rate service). The two most common of these are frame relay and ATM; we begin with frame relay. Before we do, however, let's spend a few minutes discussing the hierarchy of switching. Part of this is review of prior material; most of it, though, is preparation for our discussion of frame relay and ATM, the so-called fast packet switching technologies. The Switching Hierarchy The switching hierarchy, shown in Figure 7-2, has two major subhead- ings - circuit switching and store-and-forward switching. Circuit switch- ing is something of an evolutionary dead end in that it will not become something else: The only evolution for circuit switching is an evolution toward packet switching, at this point. Packet switching evolved as one of the two descendents of store-and- forward technology. Message switching, the alternative and another ev