The international mobile subscriber identity (IMSI; /ˈɪmziː/) is a number that uniquely identifies every user of a cellular network.^[1] It is stored as a 64-bit field and is sent by the mobile device to the network. It is also used for acquiring other details of the mobile in the home location register (HLR) or as locally copied in the visitor location register. To prevent eavesdroppers from identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly-generated TMSI is sent instead.

The IMSI is used in any mobile network that interconnects with other networks. For GSM, UMTS and LTE networks, this number was provisioned in the SIM card and for cdmaOne and CDMA2000 networks, in the phone directly or in the R-UIM card (the CDMA equivalent of the SIM card). Both cards have been superseded by the UICC.

An IMSI is usually presented as a 15-digit number but can be shorter. For example, MTN South Africa's old IMSIs that are still in use in the market are 14 digits long. The first 3 digits represent the mobile country code (MCC), which is followed by the mobile network code (MNC), either 2-digit (European standard) or 3-digit (North American standard). The length of the MNC depends on the value of the MCC, and it is recommended that the length is uniform within a MCC area.^[2] The remaining digits are the mobile subscription identification number (MSIN) within the network's customer base, usually 9 to 10 digits long, depending on the length of the MNC.

The IMSI conforms to the ITU E.212 numbering standard.

IMSIs can sometimes be mistaken for the ICCID (E.118), which is the identifier for the physical SIM card itself (or now the virtual SIM card if it is an eSIM). The IMSI lives as part of the profile (or one of several profiles if the SIM and operator support multi-IMSI SIMs) on the SIM/ICCID.

Examples of IMSI numeric presentational

IMSI analysis

IMSI analysis is the process of examining a subscriber's IMSI to identify the network the IMSI belongs to, and whether subscribers from that network may use a given network (if they are not local subscribers, this requires a roaming agreement).

If the subscriber is not from the provider's network, the IMSI must be converted to a Global Title, which can then be used for accessing the subscriber's data in the remote HLR. This is mainly important for international mobile roaming. Outside North America, the IMSI is converted to the Mobile Global Title (MGT) format, standard E.214, which is similar to an E.164 number. E.214 provides a method to convert the IMSI into a number that can be used for routing to international SS7 switches. E.214 can be interpreted as implying that there are two separate stages of conversion; first determine the MCC and convert to E.164 country calling code then determine MNC and convert to national network code for the carrier's network. But this process is not used in practice and the GSM numbering authority has clearly stated that a one-stage process is used .

In North America, the IMSI is directly converted to an E.212 number with no modification of its value. This can be routed directly on American SS7 networks.

After this conversion, SCCP is used to send the message to its final destination. For details, see Global Title Translation.

Home Network Identity

The Home Network Identity (HNI) is the combination of the MCC and the MNC. This is the number which fully identifies a subscriber's home network. This combination is also known as the PLMN.

See also

• IMEI
• MSISDN
• MEID
• Electronic Serial Number
• IMSI-catcher

References

External links

• "Cellular Networking Perspectives" article in Wireless Telecom Magazine
• IMSI oversight council responsible for allocating IMSI ranges in the USA Archived 2004-04-23 at the Wayback Machine
• IMSI Lookup Utility
• Explanation of GSM numbering plans