erating across more than one network is called a multinetwork PVC. Each piece of the PVC provided by each net- work is a PVC segment. Therefore, the multinetwork PVC is the combina- tion of the relevant PVC segments. In addition, the NNI uses the bidirectional network procedures, published in ANSI T1.617 Annex D, and further requires that all networks involved in the PVC must support NNI procedures as well as UNI procedures. Full internetworking operations between Frame Relay networks require that the procedures stipulated in ANSIT1.617 Annex D be used at the UNI and the NNI. This concept means that a user sends a status enquiry (SE) message to the network, and the network responds with a status (S) mes- sage. In addition, bidirectional procedures at the NNI require that either network be able to send SE and S messages. Frame Relay/ATM Interworking Two types of Frame Relay-to-ATM interworking are available: network and service. Service providers use network interworking to reduce congestion and achieve economies of scale in the backbone. End users are not impacted by such a deployment, as the protocol in and out of the cloud is still Frame Relay. Figures 9-14 and 9-15 show the use of service and network interworking. Demand continues to increase for service interworking as some end users are beginning to see a need for ATM for some corporate locations. The Figure 9-14 Service interworking. Frame Relay Router Router Frame Relay 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 Frame Relay Frame Relay Figure 9-15 ATM interworking. Router Router @@@@@@@@@@@@@@ most typical example is the end user with a large number of remote loca- tions running Frame Relay-all needing connectivity with the headquar- ters location. The headquarters supporting a large quantity of remote-site traffic may require higher bandwid