Section: 2.8.2.1.3 Mapping in the old MME
Content: A new SGSN attempts to retrieve information regarding the UE, e.g. the IMSI, from the old MME. In order to find the UE context, the MME needs to map the RAI, P-TMSI (or TLLI) and the P-TMSI Signature (sent by the SGSN) to create the GUTI and compare it with the stored GUTI. The MME shall perform a reverse mapping to the mapping procedure specified in clause 2.8.2.1.2 "Mapping in the UE" (see 3GPP TS 29.060[ General Packet Radio Service (GPRS); GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface ]  [6] and 3GPP TS 29.274[ 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 ]  [88] for specifics of the messaging). For the reverse mapping, the E-UTRAN <MME Code> within the GUTI shall be set either to bits 23 to 16 of the GERAN/UTRAN <P-TMSI> (i.e., the NRI field) or to the GERAN/UTRAN <RAC>. For GERAN TLLI, the old MME replaces the two topmost bits of TLLI, received from new SGSN via GTPv1, with '11' before mapping the TLLI to the GUTI used for looking up the "UE Context".