ts to honor the request for ser- vice. Several factors must first be considered: Is the mobile a subscriber to GPRS services? The act of verifying the mobile station's subscription information is called authorization. Is the mobile who it says it is? The process of verifying the identity of the mobile station is called authentication. Radio Resource IP/X.25 GPRS PLMN Intra- PLMN IP Backbone 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. GPRS Introduction Chapter 2 What level of quality of service (QoS) is the station requesting? Has the mobile subscribed to the level of QoS being requested (is the owner willing to pay for it?) and can the network provide this level of service while still providing the levels of service already promised to the other attached users? Once the SGSN decides to accept an attachment, it keeps track of the mobile station as the mobile moves around in the coverage area. The SGSN needs to know where the mobile is in case data packets arrive and need to be routed to the mobile. Attaching to the SGSN is similar to creating a log- ical connection (or pipe) between the mobile and the SGSN. The logical con- nection is maintained as the mobile moves within the coverage area controlled by the SGSN. The attachment to an SGSN is not sufficient to enable the mobile to begin transferring packet data. To do that, the mobile needs to activate (and possibly acquire) a PDP address (such as an IP address). Packet Data Protocol (PDP) Contexts The PDP addresses are network layer addresses (Open Standards Inter- connect [OSI] model Layer 3). GPRS systems support both X.25 and IP net- work layer protocols. Therefore, PDP addresses can be X.25, IP, or both. Each PDP address is anchored at a Gateway GPRS Support Node (GGSN), as shown in Figure 2-16. All packet data traffic sent from the public packet data network for the PDP address goes 