The  UE identity is very important for authenticating the subscriber to access the network. authentication of subscriber is happened in various procedure  via RRC signalings. But to keep the subscriber safe, network allocated some unique identification, which  are used to identify the UE at various nodes. 


        The international Mobile Subscriber Identity (IMSI) is a globally unique permanent subscriber identity associated with in the USIM. An IMSI can be moved between UE by moving the USIM.

An IMSI is stored on the USIM and by the User Data Management (UDM) Network Function within the 5G Core Network.

The structure of an IMST is illustrated in below Figure.
  • The 'home' Public Land Mobile Network (PLMN) is identified using a combination of the Mobile Country Code (MCC) and Mobile Network Code (MNC). The ITU is responsible for allocating the MCC, whereas the national administrator is responsible for allocating the MNC. A UE uses the 'home' PLMN Identity when searching for a network, e.g. when completing a band scan, a UE will search for a cell which is broadcasting the 'home' PLMN Identity within SIB 1.
  • The subscriber is identified within the home PLMN using the Mobile Subscriber Identification Number (MSIN). 
  • MSIN is allocated by the service provider.

The IMSI can be used as the '5GS Mobile Identity' within NAS signalling procedures. 

For security reasons, the IMSI is included within NAS messages using a 'concealed' format which can hide the actual value. 

The 'Protection Scheme' indicated as part of the '5GS Mobile Identity' field can be set to 'null' in which case the IMSI is visible within the message.  

Some protection schemes are used to encrypt the IMSI prior to including within the message.

In the case of 5G, the IMSI is not used for paging procedures.


The International Mobile Equipment Identity (IMEI) is a permanent identity belonging to a device. 
It is stored within the device hardware and by the User Data Management (UDM) Network Function within the 5G Core Network.

The structure o1ran IMEI is illustrated in below figure.
  • The Type Allocation Code (TAC) is an 8 digit number which identifies the UE model. It can also identify a specific version of a UE model, i.e. different versions of the same UE model can be allocated different TAC. The TAC is allocated by the GSM Association (GSMA).
  • The Serial Number (SNR) uniquely identifies a device with a specific TAC. All UE which have the same TAC should be allocated different Serial Numbers. The Serial Number is allocated by the device manufacturer.
  • The Check Digit (CD) is calculated from a combination of the TAC and Serial Number. It provides a mechanism for detecting data entry errors, e.g. when the IMEI is manually entered into a system. 

The IMEI can be used within NAS signalling procedures as the '5GS Mobile Identity'. 

In contrast to the IMSI, the IMEI does not use a 'Protection Scheme' to provide encryption. Instead, the IMEI can be included directly within NAS signalling messages.

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