What is GUTI in LTE?
The GUTI full form is Globally Unique Temporary ID. In the first initial Attach request, a UE sends the IMSI. On a network attach request, the MME does the authentication and location update with the AUC and HSS, respectively.
Once authentication and location update is successful, MME allocates the GUTI. The GUTI is stored in the SIM card. In any further communication, the sim card uses GUTI, not the IMSI, for security purposes.
The components of GUTI :
- MME ID,
- MNC, T-IMSI
All combination makes the GUIT a globally unique identifier.
GUTI Allocation :
When a UE does the authentication for the first time, the NAS PDU in the initial UE Message is of type attach request. The Attach request has IMSI in the EPS mobile identity parameter. After receiving an ATTACH request, MME does authentication and does an update location procedure with the HSS/AUC.
MME initiates an IntialContextSetup request towards the eNodeB for a UE. The NAS PDU is of type attach accept and activate the default bearer. In the ATTACH Accept NAS Pdu, EPS mobile identity parameter has GUTI allocated by the MME.
The eNode/UE uses GUTI in an LTE network for further ATTACH requests. The MME drives IMSI from GUTI for the authentication and location update with HSS in the home network of LTE.