International mobile subscriber identity

The international mobile subscriber identity or IMSI /ˈɪmziː/ is used to identify the user of a cellular network and is a unique identification associated with all cellular networks. It is stored as a 64 bit field and is sent by the phone 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 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 Both cards have been superseded by the UICC.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 being used in the market are shown as 14 digits. The first 3 digits are the mobile country code (MCC), which are followed by the mobile network code (MNC), either 2 digits (European standard) or 3 digits (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.[1] The remaining digits are the mobile subscription identification number (MSIN) within the network's customer base (mostly 10 or 9 digits depending on the MNC length).

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

Examples of IMSI numeric presentation

IMSI:410072821393853
MCC 410 PAKISTAN
MNC 07 Warid Pakistan
MSIN 2821393853
IMSI:310150123456789
MCC 310 USA
MNC 150 AT&T Mobility
MSIN 123456789
IMSI:502130123456789
MCC 502 MY
MNC 13 Celcom
MSIN 123456789
IMSI:460001357924680
MCC 460 CHINA
MNC 00 CMCC
MSIN 1357924680
IMSI:520031234567890
MCC 520 THAILAND
MNC 03 AIS
MSIN 1234567890
IMSI:470010171566423
MCC 470 Bangladesh
MNC 01 Grameenphone
MSIN 0171566423

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.

Example of outside World Area 1

This example shows the actual practice which is not clearly described in the standards.

Translation rule:

  • match numbers starting 28401 (Bulgaria mobile country code + MobilTel MNC)
  • identify this as belonging to MobilTel-Bulgaria network
  • remove first five digits (length of MCC+MNC)
  • prepend 35988 (Bulgaria E.164 country code + a Bulgarian local prefix reaching MobilTel's network)
  • mark the number as having E.214 numbering plan.
  • route message on Global Title across SCCP network

Therefore, 284011234567890 becomes 359881234567890 under the E.214 numbering plan.

Translation rule:

  • match numbers starting 310150 (America first MCC + Cingular MNC)
  • remove first six digits (length of MCC+MNC)
  • prepend 14054 (North America E.164 country code + Network Code for Cingular)
  • mark the number as having E.214 numbering plan.
  • route message on Global Title across SCCP network

Therefore, 310150123456789 becomes 14054123456789 under the E.214 numbering plan.

The result is an E.214 compliant Global Title, (Numbering Plan Indicator is set to 7 in the SCCP message). This number can now be sent to Global Title Analysis.

Example inside World Area 1 (North America)

Translation rule:

  • match number starting 28401 (Bulgaria MCC + MobilTel MNC)
  • identify this as belonging to MobilTel-Bulgaria network
  • do not alter the digits of the number
  • mark the number as having E.212 numbering plan.
  • route message on Global Title across SCCP network

Therefore, 284011234567890 becomes 284011234567890 under the E.212 numbering plan.

This number has to be converted on the ANSI to ITU boundary. For more details please 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

References

  1. ↑ 3GPP specification 23.003 Release 14.3.0 (2017-03), 2.2 "Composition of IMSI"
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