What is the GUTI?
In LTE, the GUTI is the Globally Unique Temporary ID, and identifies the mobile device to the LTE network. The GUTI is assigned to the device by the Mobility Management Entity (MME), the access network’s primary control node. By allocating a temporary ID to the device, the MME maintains the security of the IMSI (International Mobile Subscriber Identification) when transmitting over radio interface.
Components of the GUTI
The GUTI has two parts. These are the globally unique Globally Unique Mobility Management Entity Identifier (GUMMEI), which identifies the network, and the M-TMSI, which identifies the device.  The GUMMEI is 42 bits in length and the M-TMSi is 32 bits long.
The GUMMEI portion of the GUTI is comprised of several parts. These are the Mobile Network Code (MNC), the Mobile Country Code (MCC), and the MME ID (Mobility Management Entity ID). The Mobility Management Entity is the primary control node for the LTE access network. MMEs are usually clustered in pools, and the MME ID identifies both the MME pool and the node within that pool.
LTE vs. 3G
The GUTI takes the place of the P-TMSI, which is the temporary id used in 2G and 3G communications. The P-TMSI is still needed since LTE networks must “hand off” users to 3G or 2G networks for voice communication. When a mobile device operating on a 3G network tries to contact an LTE cell, the network will send the P-TMSI nunber. The MME uses this number to get the subscriber’s IP address and other information from the serving GPRS Support Node (SGSN). The SGSN supports both radio access and IP access.
Once the user is on the LTE network, his/her device will be allocated a GUTI. Then when the user wants to use voice or has another reason to move back to the 3G network, the device sends a GUTI instead of a P-TMSI.This procedure is called a Routeing Area Update (RAU). The MME is responsible for mapping the GUTI to P-TMSI, and vice versa.
If a user moves from one MME to another, the second MME will contact the first one for user information. Both MMEs will use the GUTI to identify the user.
Avoiding duplication
Each GUTI must be unique, and networks use different protocols to avoid duplication of numbers. Since mobile country codes, mobile network codes and MME IDs can all be the same for different devices, the M-TMSI portion of the GUTI is the part that must be unique. The risk of duplication occurs when an allocating MME node restarts. Â Sometimes the potential problem is avoided by using the time of allocation as part of the M-TMSI.
The M-TMSI is stored in the SIM card, and therefore it cannot have all 32 digits equal to one. A string of 32 one’s signifies that no valid ID is available, so a device with 32 one’s for its M-TMSI could not operate.
Abbreviated GUTIs
For radio communication, the GUTI can be shortened. The shortened form is called the S-TMSI and is comprised of the full M-TMSI and a portion of the GUMMEI. The part of the GUMMEI that identifies the MME node is called the MMEC, and this is included in the S-TMSI. The part of the GUMMEI that identifies the MME pool is called the MMEGI. This part is NOT included in the abbreviated GUTI. Therefore it is necessary to avoid using the same MMEC number to identify two Mobility Management Entities that are in adjacent or overlapping pools. However, in all cases, the M-TMSI portion of the GUTI should be unique for each device.
