IMSI stands for International Mobile Subscriber Identity.

  • It is a globally unique permanent subscriber identity associated with the USIM.
  • An IMSI is stored on the USIM and by the UDM(user data management) network function.

Structure of IMSI:

  • MCC (Mobile country code): ITU is responsible for allocating MCC.
  • MNC (Mobile network code): The national administrator is responsible for allocating the MNC. [PLMN = MCC + MNC].
  • MSIN (Mobile Subscriber Identification Number): The MSIN is allocated by the service provider (ISP).

The subscriber is identified within the home PLMN using the Mobile Subscriber Identification Number (MSIN).


IMEI stands for International Mobile Equipment Identity.

  • It is a permanent identity belonging to a device. It is stored in the device hardware and by UDM.

Structure of IMEI:

TAC (Type Allocation Code): This identifies the UE model, as well as a specific version of a UE model. GSM Association (GSMA) is responsible for TAC allocation.

SNR (Serial Number): unique identification of a device with a specific TAC, i.e. all the UE’s having the same TAC will get a different serial number.

CD (Check Digit): It is calculated from a combination of the TAC and Serial Number. CD provides a mechanism for detecting data entry errors, e.g. when the IMEI is manually entered into a system.


SUPI (5G subscription permanent identity): It can be either IMSI or NAI (Network access identifier).

SUCI (Subscription Concealed Identifier): This allows the SUPI to be signaled without exposing the identity of the user.


5G-GUTI stands for 5G-Globally Unique Temporary Identifier. AMF is responsible for its allocation, it is a temporary identity.

Structure of the 5G-GUTI:

5G-GUTI and 4G-GUTI comparison
  • AMF Identifier = AMF Region ID + AMF Set ID + AMF Pointer.
  • GUAMI = AMF Identifier + MCC + MNC.


It is shortened version of the 5G-GUTI i.e. without PLMN(MNC + MCC) and AMF Region Identifier.

  • It is more efficient to use the 5G-S-TMSI rather than the 5G-GUTI.
  • Because of its smaller size help in the reduction of signaling overhead.

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