ers, routers, and Frame Relay switches about congestion, and (b) take corrective action. The BECN bit and the FECN bit achieve both capabilities. Figure 9-12 shows the FECN and BECN flow. Assume that a Frame Relay switch is starting to experience congestion problems due to its buffers (queues becoming full and/or experiencing a problem with memory management). It may inform both the upstream nodes and the downstream nodes of the problem by the use of the FECN and BECN bits, respectively. The BECN bit is turned on in the frame and is sent downstream to notify (potentially) the source of the traffic that con- gestion exists at a switch. This would permit the source to flow control its traffic until the congestion problem is solved. In addition, the FECN bit could be set and placed in a frame and sent to the upstream nodes to inform them that congestion is occurring downstream. One might question why the FECN is used to notify upstream devices that congestion is occurring Figure 9-12 The FECN and BECN. Router Router Destination Switches Source Router Hosts 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. PCUSN-to-SGSN Interface (Gb) Chapter 9 downstream. After all, the downstream device is the one creating the traf- fic problem. The answer is that it varies, depending on remedial action at the upstream (destination) machines might want to take. For example, this FECN bit could be passed to an upper-layer protocol (such as the transport layer) that enables it to (a) slow down its acknowledgments (which in some protocols would close the transmit window at the destination device) or (b) establish its own more restrictive flow control agreement with its commu- nications source machine (which also is permitted in some protocols). An obvious solution to the problem is for the source machine(s) to flow control itself to ameliorate the network congestio