 or TMSI) to the SGSN. This message will also contain a Network Service Area Point Identifier (NSAPI), which is specific to a particular network application at the mobile station. The Subnetwork-Dependent Convergence Protocol (SNDCP) layer uses this NSAPI to communicate with the network application. Several NSAPIs may be associated with an individual mobile station. One may be for an Internet browser, whereas another could be an e-mail service. 2. The SGSN verifies whether the user is authorized and authenticated for that particular service by checking with the HLR entry for the mobile station. 3. After authorization, the SGSN sends back a reply to the mobile station with a TLLI. The TLLI is specific to the mobile and is used by the Logical Link Control (LLC) layer in the protocol stack. The purpose of this TTLI is to provide a temporary ID to the mobile station, which can be used for data communication. 4. A database is maintained at the SGSN that maps the mobile identity with the TLLI assigned to it. The NSAPI is associated with and the Figure 5-5 The steps in Radio 1. performing a Frame GPRS attach. Relay 1.2.3.4 NSAPI=2 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. Main GPRS Procedures Main GPRS Procedures Figure 5-6 Table updates TLLI=1, NSAPI=2 correlating the mobile station to the SGSN for attach procedures. More fields to TLLI=2, NSAPI=3 come after activate TLLI=3, NSAPI=2 quality of service (QoS) subscription parameters required by the application. The table of entries for the SGSN correlations of attached mobiles shown in Figure 5-6 is updated after the PDP activation phase. Mobile Station-Initiated GPRS Detach To move from the ready state to the idle state, the mobile station initiates a GPRS detach procedure. The results of the GPRS detach function is that the SGSN may delete the MM and the PDP c