IMSI stands for International Mobile Subscriber Identity. It is a unique identifier of mobile subscriber globally. An IMSI consist MCC, MNC and MSIN. It is of 15 or shorter digits unique identifier stored as a 64 bit field. The 15 digits or less than 15 digits depends on MNC(assigned by national authority) and MSIN(assigned by operator in different countries).
IMSI = MCC(3 digits) + MNC(2-3 digits) + MSIN(9-10 digits)
MCC – Mobile country code
MNC – Mobile network code
MSIN – Mobile subscriber identification number
GUTI stands for Globally Unique Temporary UE Identity. It is used to identify a UE between the UEs and MME on behalf of IMSI for security reasons, because an IMSI is an unique identity of a mobile subscriber so there might be security issues for subscriber if it frequently exposed over the radio link. GUTI is allocated by an MME when UE first time attached to the network and subsequently used instead of IMSI. The length of GUTI is less than 64 bits.
GUTI = GUMMEI + M-TMSI