An International Mobile Subscriber Identity or IMSI (pronounced /ˈɪmzi/) is a unique number associated with all GSM and UMTS network mobile phone users. It is stored in the SIM inside the phone 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, in particular CDMA and EVDO networks as well as GSM networks. This number is provisioned in the phone directly or in the R-UIM card (a CDMA analogue equivalent to a SIM card in GSM).
An IMSI is usually 15 digits long, but can be shorter (for example MTN South Africa's old IMSIs that are still being used in the market are 14 digits). The first 3 digits are the Mobile Country Code (MCC), and is followed by the Mobile Network Code (MNC), either 2 digits (European standard) or 3 digits (North American standard). The remaining digits are the mobile station identification number (MSIN) within the network's customer base.
The IMSI conforms to the ITU E.212 numbering standard.
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 but different from E.164 number (more or less a telephone 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 practise 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.
This example shows the actual practice which is not clearly as described in the standards.
so we get 284011234567890 becomes 359881234567890 numbering plan E.214.
so we get 310150123456789 becomes 14054123456789 numbering plan E.214.
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. by onmobile
so we get 284011234567890 becomes 284011234567890 numbering plan E.212.
This number has to be converted on the ANSI to ITU boundary. For more details please see Global Title Translation.
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. The reason to make this distinction is that in a country with multiple country codes (e.g. USA has codes 310 to 316) there may be two different networks, with the same Mobile Network Code, but only one of which is the home network. To know which network a mobile belongs to requires analyzing the entire HNI at once.
Because of the unlikeness in the Global Title Translation, it is extremely hard to pin point the exact location of the American international mobile prefix number.
E.214's recommendation for Global Title Translation does not take into account countries with more than one mobile country code (MCC) (for example the US, which has 7 MCCs), or shared numbering plans (for example North American Numbering Plan, or the +1 country code, which applies to the US, Canada, and all the countries in the Caribbean).
The problem lies in de-translation of the global title back into a mobile network E.212 IMSI. Since E.214 recommends that the country part of the translation be done first, it presumes that a given E.164 country code only relates to a single E.212 mobile country code. Unfortunately this is untrue in NANPA member nations, and doubly untrue in the US. So, a global title with CC of 1 can indicate any of 7 US MCCs, or Canada, or any Caribbean nation.
This has led to a temporary practice of distributing IMSIs in the US with only MCCs of 310, in an attempt to minimise the ambiguity.
In practice, however, home carriers use a deeper translation process, which performs a lookup based on the entire CC+NC, to better determine the correct country. In the case of NANPA, this is 1+area code, which can uniquely identify a country, but there are hundreds of area codes. More of the number then has to be used to determine the carrier network (in some cases up to 4 digits).