ed. An RA update is required. A combined RA and location area update is required. In all three scenarios, the mobile station stores the cell identity in its MM context. If the mobile station enters a new PLMN, the mobile station either performs an RA update or enters the idle state. A cell update takes place when the mobile station enters a new cell inside the current RA and the mobile station is in the ready state. If, how- ever, the RA has changed, an RA update is executed instead of a cell update. GPRS Attach Procedure In the attach procedure, the mobile station provides its identity. The iden- tity provided to the network is the mobile station's P_TMSI or IMSI: P_TMSI and the RAI associated with the P_TMSI is provided if the mobile station has a valid P_TMSI. If the mobile station does not have a valid P_TMSI, then the mobile station provides its IMSI. 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. Mobile Station to SGSN Interface Chapter 8 At the RLC/MAC layer (through the air interface), the mobile station identifies itself with A foreign TLLI if a valid P_TMSI is available A random TLLI if a valid P_TMSI is not available The foreign or random TLLI is used as an identifier during the attach procedure until a new P_TMSI is allocated. Note, though, that the mobile station identifies itself with a local or foreign TLLI if the mobile station is already GPRS-attached and is performing an IMSI attach. At BSSGP layer (through the Gb interface), the mobile station identifies itself with the information field TLLI in its messages. Upon receipt of the Attach Request message, SGSN allocates a new P_TMSI in this RAI, and transmits it to the mobile station in an Attach Accept (new P_TMSI) message. If P_TMSI was changed, the mobile station acknowledges the received P_TMSI with the Attach Complete (P_TMSI) message. Cell Update in Packet Idle Mode No