Section: 2.8.2.1.2 Mapping in the UE
Content: When a UE moves from an E-UTRAN to a GERAN/UTRAN, the UE needs to map the GUTI to an RAI, a P-TMSI and a P-TMSI Signature, for them to be sent to the SGSN. For GERAN, the TLLI is derived from the P-TMSI by the UE and is a foreign TLLI (see clause 2.6). The mapping of the GUTI shall be done to the combination of RAI of GERAN / UTRAN and the P-TMSI: E-UTRAN <MCC> maps to GERAN/UTRAN <MCC> E-UTRAN <MNC> maps to GERAN/UTRAN <MNC> E-UTRAN <MME Group ID> maps to GERAN/UTRAN <LAC> E-UTRAN <MME Code> maps to GERAN/UTRAN <RAC> and is also copied into the 8 Most Significant Bits of the NRI field within the P-TMSI; E-UTRAN <M-TMSI> maps as follows: - 6 bits of the E-UTRAN <M-TMSI> starting at bit 29 and down to bit 24 are mapped into bit 29 and down to bit 24 of the GERAN/UTRAN <P-TMSI>; - 16 bits of the E-UTRAN <M-TMSI> starting at bit 15 and down to bit 0 are mapped into bit 15 and down to bit 0 of the GERAN/UTRAN <P-TMSI>; - and the remaining 8 bits of the E-UTRAN <M-TMSI> are mapped into the 8 Most Significant Bits of the <P-TMSI signature> field. The UE shall fill the remaining 2 octets of the <P-TMSI signature> according to clauses 9.1.1, 9.4.1, 10.2.1, or 10.5.1 of 3GPP TS.33.401 [89] , as appropriate, for RAU/Attach procedures. For UTRAN, the 10-bit long NRI bits are masked out from the P-TMSI and are also supplied by the UE to the RAN node as IDNNS (Intra Domain NAS Node Selector) (see 3GPP TS 23.236[ Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes ]  [23]). However, the RAN configured NRI length should not exceed 8 bits.