 with the ETSI specifications, a new thrust in the international internetworking comes one step closer. The ANSI model for GPRS, although similar and dif- ferent at the same time, is shown in Figure 3-4. Data Routing One of the main issues in the GPRS network is the routing of data packets to/from a mobile user. The issue can be divided into two areas: data packet routing and mobility management. Data Packet Routing The main functions of the GGSN involve interaction with the external data network. The GGSN updates the location directory using routing informa- Figure 3-4 = Signaling Interface ANSI-41 The GPRS reference = Signaling and Data HLR/AC Transfer Interface model for North America (ANSI). ANSI-41 ANSI-41 ANSI-41 Serving Gateway- MC/OTAF MSC/VLR MSC/VLR 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 tion supplied by the SGSNs about the location of a mobile station (MS) and routes the external data network protocol packet encapsulated over the GPRS backbone to the SGSN currently serving the MS. It also decapsulates and forwards external data network packets to the appropriate data net- work and collects charging data that is forwarded to a charging gateway. Figure 3-5 shows the use of the various tools in a GPRS network. Three different routing schemes are possible: mobile-originated message (possibility 1), network-initiated messages when the MS is in its home net- work (possibility 2), and network-initiated messages when the MS has roamed to another GPRS operator's network (possibility 3). In these exam- ples, the operator's GPRS network consists of multiple GSNs (with a gate- way and serving functionality) and an intraoperator backbone network. GPRS operators will allow roaming through an interoperator backbone network. The GPRS operators connect to the interoperator network via a B