using Frame Relay G1 Between SGSN and HLR, extension of the Mobile Application Part (MAP) Gn Between SGSN and GGSN, using the GTP (tunneling) protocol Gi Between GGSN and PDNs (X.25 and Internet Protocol [IP]) Between SGSN and MSC/VLR, for some simultaneous GPRS and GSM operations (same as Base Station Mobile Application Part [BSSMAP] but optional) Gd Delivers SMS messages via GPRS (same as MAP from GSM) G Between GGSN and HLR (same as MAP but optional) Figure 3-7 The new interfaces in GPRS. PSTN/ PSPDN Agprs PCUSN SGSN of other PLMN 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. System Architecture System Architecture Frames going out from the BTS will be transparently conveyed by the BSC to the PCUSN, which handles GPRS-specific Packet Processing (the normal process uses Frame Relay, but, this may change to Asynchronous Transfer Mode [ATM] or other protocols in the future). The Different Backbones Used Each SGSN is linked to Packet Control Unit Switching Nodes (PCUSN) with a Frame Relay network, which is The only protocol possible (today) in the actual state of the ETSI specifications Simpler than X.25 Capable of supporting data rates up to 2 Mbps The SGSN and GGSN are linked together within the GPRS backbone based on IP routing. GPRS tunnels the protocol data unit (PDU) using the GPRS Tunneling Protocol (GTP). GTP IPv4 is used as a GPRS backbone network layer protocol. Figure 3-8 provides a summary of the various back- bone networks used. The GTP header contains a tunnel endpoint identifier for point-to-point and multicast packets as well as a group identity for point-to-multipoint GGSNs Figure 3-8 The different backbones used. Internet IP Backbone Intranets Agprs Agprs Agprs Frame Relay Network Agprs Agprs PC USNs Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hi