tal 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 Identifiers Managed by the NS Layer The Frame Relay network defines a local identifier for a virtual circuit: the DLCI. The NS layer uses an end-to-end logical identifier to reference the same virtual circuit: the Network Service Virtual Connection Identifier (NSVCI). The purpose of the new identifier is to create software that will manage the NSVCI, not the DLCI. Thus, if the NS Subnetwork evolves (toward ATM, for example), the upper-layer coding will not be impacted. Figure 9-18 represents two PCUSNs connected to an SGSN using two dif- ferent NSVCIs. The Network Service Entity (NSE) at the BSS and the SGSN provides the network management functionality required for the operation of the Gb interface. Each NSE is identified by means of a Network Service Entity Identifier (NSEI). The NSEI together with the BSSGP Vir- tual Connection Identifier (BVCI) uniquely identifies a BVC (for example, a PTP functional entity) within an SGSN. The NSEI is used by the BSS and the SGSN to determine the NSVCs that provide service to a BVCI. An NSE manages a pool of NSVCIs toward a specific node, as shown in Figure 9-19: In a PCU, one NSE is present. In an SGSN, many NSEs are defined (one per PCU). For an NSE, the NS layer performs load sharing between NSVCIs to face traffic, but for a given mobile station, packets will always take the same NSVCI (so that the order is guaranteed). PCU A Figure 9-18 The NS layer identities. BSS A Frame Relay PCU B Network BSS B Example of a SGSN connected to 2 PCUs 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) NSEI 0 Figure 9-19 The