GUTI | Area and Subscriber Identities

The GUTI is assigned only by the MME during initial attach of a UE to the E-UTRAN.

The purpose of the GUTI is to provide an unambiguous identification of the UE that does not reveal the UE or the user's permanent identity in the E-UTRAN. It also allows identification of the MME and network to which the UE attaches. The GUTI can be used by the network to identify each UE unambiguously during signaling connections.

The GUTI has two main components: the Globally Unique Mobility Management Entity Identifier (GUMMEI) that uniquely identifies the MME which allocated the GUTI; and the M-TMSI that uniquely identifies the UE within the MME that allocated the GUTI. The GUMMEI is constructed from the MCC, MNC, and Mobility Management Entity Identifier (MMEI).

The MMEI should be constructed from a Mobility Management Entity Group ID (MMEGI) and a MMEC. The GUTI should be constructed from the GUMMEI and the M-TMSI as shown in Figure 1.

 

Figure 1: Format of GUTI and S-TMSI

For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI. It is correct to say that the S-TMSI is a shorter format of GUTI that can be used because, after successful registration of a UE, the serving network as well as the serving MME group are known and stored in the core network databases.

The operator needs to ensure that the MMEC is unique within the MME pool area and, if overlapping pool areas are in use, unique within the area of overlapping MME pools.

The GUTI should be used to support subscriber identity confidentiality and, in the shortened S-TMSI form, to enable more efficient radio signaling procedures (e.g., paging and service request).

MCC and MNC should have the same field size as described for the IMSI.

The M-TMSI has a length of 32 bits, MMEGI is 16 bits in length, and MMEC 8 bits in length.

It is important to understand that on the S1 interface the IMSI is typically not seen, just like the GUTI. Exceptions are initial attach to the network when no old GUTI is stored on the USIM card or the true subscriber's identity is checked using NAS signaling, which regularly happens when roaming subscribers attach. Also, in the case of the paging procedure the IMSI might be seen.

For monitoring and network performance measurement the IMSI on S1 can only be revealed if the changing temporary identities are tracked with a quite sophisticated architecture. Full IMSI tracking can only be ensured by monitoring all S1 interfaces of an operator's E-UTRAN and ideally all S6a interfaces and storing the current GUTI/IMSI relations in a central point as stored in the HSS.