in an RA, a one-to-one relationship exists between the TLLI and the Temporary Mobile Subscriber Identity (TMSI) (an International Mobile Subscriber Identity [IMSI]) that is only known in the mobile station and the SGSN. Whereas a packet TMSI (P_TMSI) is used in the GMM sublayer for identification of a mobile station, a TTLLI is used for addressing the radio resources. TLLI is also derived from the P_TMSI and unambiguously iden- tifies the logical link. TLLI assignment is controlled by the GMM. TLLI is assigned, changed, and unassigned with an assignment request function coming from interaction between the LLC (more precisely in the LLME) and the GMM layer. The mobile station will transmit its IMSI (only when necessary, other- wise the temporary IMSI is used whenever possible for security reasons) or the old P_TMSI to the SGSN when attaching to the SGSN. On attachment, the mobile station uses the P_TMSI for authentication. The SGSN uses the old P_TMSI to reallocate a new one. Therefore, a new P_TMSI will be assigned to the mobile station and passed across the network to the mobile station. The mobile station having received the new P_TMSI from the net- work transmits a TLLI to unambiguously describe the logical link between it and the network. This is the one-to-one relationship previously described. The P_TMSI is coded with a 32-bit sequence to create the TLLI, as shown in Table 8-1. 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 Mobile Station to SGSN Interface Table 8-1 Type of TLLI The Format of the TLLI Mapped in Local TLLI P_TMSI 32 Bits from the Foreign TLLI P_TMSI P_TMSI Random TLLI Auxiliary TLLI Reserved Reserved Reserved How the TLLI Is Used A TLLI is used for addressing on resources used for GPRS. This TLLI is built on the basis of either the local or foreign P_TMSI. This means that a valid __TMSI