binary; it has two possible values, which means that a customer has two choices: The information being sent is either important, or it isn't-not particularly useful for establishing a variety of QoS levels. Consequently, a number of vendors have implemented proprietary solutions for QoS management. Within their routers (sometimes called frame-relay access devices, or FRADs), they have established queuing mechanisms that allow cus- tomers to create multiple priority levels for differing traffic flows. For example, voice and video-which don't tolerate delay well-could be 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 assigned to a higher priority queue than the one to which asynchronous data traffic would be assigned. This allows frame relay to provide highly granular service. The downside is that this approach is proprietary, which means that the same vendor's equipment must be used on both ends of the circuit. Given the strong move toward interoperability, this is not an ideal solution because it locks the customer into a single-vendor situation. Congestion Control in Frame Relay Frame relay has two congestion control mechanisms. Embedded in the header of each frame-relay frame are two additional bits called the for- ward explicit congestion notification bit (FECN) and the backward explicit congestion notification bit (BECN). Both are used to notify devices in the network of congestion situations that could affect throughput. Consider the following scenario. A frame-relay frame arrives at the second of three switches along the path to its intended destination, where it encounters severe local congestion (see Figure 7-4). The con- gested switch sets the FECN bit to indicate the presence of congestion and transmits the frame to the next switch in the chain. When the frame arrives, the receiving sw