 the congestion is resolved. Frame Relaying Frame Relay networks use DLCIs to route the frames between two adjacent nodes. A DLCI identifies a channel. Frame Relay switches have routing tables that associate an input port and a DLCI to an output port and another DLCI. Each frame that is received is forwarded to the correct port with a new DLCI. The data popu- lating the tables points to the type of circuit being used such as 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) PCUSN-to-SGSN Interface (Gb) Permanent Virtual Circuit (PVC) Used between two users, it consists of data that fills these routing tables; the frames then have the equivalent of a dedicated path through the network. Switched Virtual Circuit (SVC) This may also be established at will by the user on a per-call basis. It is not supported in GPRS in which only PVCs are used. Benefits of Frame Relay Frame Relay saves money on customer premises equipment (CPE), local access, and interexchange network. It also reduces administrative and operations costs. More direct connectivity between locations can be provi- sioned for a minimal incremental cost. Because of this, Frame Relay net- works can be designed to better match the underlying traffic patterns. Locations and connections can be added easily and more cost effectively with Frame Relay compared with private lines. It is easy to redesign and optimize the network because changing port connection and virtual circuit bandwidth is software configurable. Most Frame Relay networks have self- healing or automatic-rerouting capabilities between the Frame Relay switches. Frame Relay is based on statistical multiplexing where bandwidth can be shared between active applications and/or connections only. This lowers the cost of ownership because end users don't have to pay for idle or excess capacity needed to mee