 and SVCs is whether the connec- tions are provisioned or established. Both types of connections need to be defined. The difference is when the connections are defined and resources allocated. The network operator typically provisions PVCs. The network operator can be the carrier (public services) or the Management Information System Support (MIS) manager (private networks). Once the PVC is provisioned, the connection is available for use at all times unless a service outage occurs. On the other hand, the end user, not the network operator, estab- lishes SVCs. Prior to each use, an SVC is established to the destination end user. The connection is cleared after use. SVC UNIs and NNIs The Frame Relay Forum has two implementation agreements specifying the implementation of SVCs over UNIs and Network-to-Network Interfaces (NNIs). The SVC UNI agreement depicts how an SVC can be established and released from an end-user device. On the other hand, the SVC NNI agree- ment depicts how an SVC can be established and released between two or more independent Frame Relay networks. The interconnected Frame Relay networks can both be private, public, or one public and one private. The Network-to-Network Interface (NNI) The initial thrust of the Frame Relay work focused on the UNI. Subsequent work resulted in the publication by the Frame Relay Forum (based on ANSI's T1.617 Annex D) of a NNI. This interface is considered instrumen- 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) tal to the success of Frame Relay because it defines the procedures for dif- ferent networks to interconnect with each other to support the Frame Relay operations. Obviously, the UNI defines the procedures between the user and the Frame Relay network, and the NNI defines the procedures between the Frame Relay networks. A PVC op