LMSI, TMSI, P-TMSI, M-TMSI, and S-TMSI | Area and Subscriber Identities

All temporary subscriber identities, Local Mobile Subscriber Identity (LMSI), TMSI, and P-TMSI, will not be seen in E-UTRAN signaling as long as there is no inter-RAT mobility between the E-UTRAN and UTRAN/GERAN. Indeed, for LTE a new NAS protocol was specified (3GPP 24.301) that introduces a new temporary subscriber identity for the E-UTRAN: the GUTI described in Section 1.4.4. However, to fulfill inter-RAT mobility requirements TMSI, P-TMSI, and LMSI will still be found in E-UTRAN NAS messages, or at least it will be indicated if valid values of these parameters are stored on the USIM card.

The LMSI is a four-octet/byte number. It is a pointer to a database record for a particular IMSI in the Visitor Location Register (VLR) database. Although the VLR is no longer found in the EPC network architecture, there is a database with the same function hosted by the MME. The purpose of the LMSI was to speed up the search for particular database records. If this is still required, with the new powerful computer hardware used to build today's network elements it is a design detail to be defined by NEMs. From definitions given in 3GPP 23.003 it can be guessed that the LMSI will not be used by the MME.

The TMSI is also encoded as a four-octet/byte hex number. It is allocated to a particular subscriber (more correctly, to a particular subscriber's (U)SIM card) during initial attach. The TMSI is used to mask the true subscriber's identity, which is the IMSI, in NAS signaling procedures. In the E-UTRAN it is often used together with the GUTI. It can be coded using a full hexadecimal representation. Since the TMSI has only local significance (i.e., within a VLR and the area controlled by a VLR, or within a SGSN and the area controlled by a SGSN, or within a MME and the area controlled by a MME), the structure and coding of it can be chosen by agreement between the operator and manufacturer in order to meet local needs.

The TMSI allocation procedure should always be executed in ciphered mode to prevent unauthorized eavesdropping.

The P-TMSI is the complement of TMSI in the UTRAN/GERAN PS domain. It is allocated by the SGSN and, hence, will be monitored in the EPC and E-UTRAN during inter-RAT handover/relocation preparation and execution. The P-TMSI is encoded in the same way as the TMSI. The difference is in defining value ranges. If the first two leading digits have the value "11" the parameter is identified as a P-TMSI. Thus, in the hexadecimal format, all TMSI values starting with C, D, E, or F as the first hex number are P-TMSIs.

The M-TMSI is a 32-digit binary number that is part of the GUTI and exclusively used in the E-UTRAN.

The S-TMSI consists of the Mobility Management Entity Code (MMEC) and M-TMSI. Indeed, it is just a shorter variant of the GUTI.