IMSI (international mobile subscriber identifier)

A unique 15-digit number which designates the subscriber. This number is used for provisioning in network elements. (Cingular)

IMSI[im-zee] is an acronym for International Mobile Subscriber Identity. This is a unique number that is associated with all GSM and UMTS network mobile phone users. The number is stored in the SIM. It is sent by the mobile to the network and is used to look up the other details of the mobile in the HLR or as locally copied in the VLR. In order to avoid the subscriber being identified and tracked by eavesdroppers on the radio interface, the IMSI is sent as rarely as possible and a randomly generated TMSI is sent instead.

An IMSI is usually fifteen digits long. However, they can be shorter (eg. MTN SouthAfrica's are 14 digits). The first three digits are the country code (MCC), and the next digits are the network code (MNC). The MNC can be either two digits long (normal e.g. in Europe) or three digits long (normal in North America), the remaining digits, up to the maximum length are the unique subscriber number (MSIN) within the network's customer base.

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

IMSI Analysis

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

If the subscriber is not from our network, then we must convert the IMSI to a Global Title at this state which can then be used for accessing the subscribers data in the remote HLR. This is mainly important for international mobile roaming.

Outside North America the IMSI is converted, to the hybrid number format, E.214, which is similar to, but not the same as an E.164 number. The E.214 numbering plan is very similar to, but distinct from, the E.164 number format (more or less a telephone number) and is used for international routing of SS7 messages. E.214 provides a method for converting the IMSI into a number that can be used for routing to international SS7 switches. The conversion process described in E.214 can be interpreted as implying that there are two separate stages of conversion; first determine the MCC and convert to E.164 CC then second determine MNC (two or three digits) and convert to national network code for the carrier's network. This process is not used in practise and the GSM numbering authority has clearly stated that a one stage process is used [1].

In North America, the IMSI is just 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 more details please see Global Title Translation.

HNI

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 specific distinction is that in a country with multiple country codes (e.g. the USA has codes 310 up to 316) there may be two different networks, with the same Mobile Network Code, but only one of which is the home network. In order to know which network a mobile belongs to we have to analyse the entire HNI at once.

Global Title problems

E.214's recommendation for Global Title Translation does not take into account countries with more than 1 MCC (for example the U.S., which has 7 MCCs), or shared numbering plans (for example NANPA, or the +1 country code, which applies to the U.S., 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 U.S. So a global title with CC of 1 can indicate any of 7 U.S. MCCs, or Canada, or any Caribbean nation.

This has led to a temporary discontinuation of distributing IMSIs in the U.S. with MCCs other than 310, in an attempt to minimise the ambiguity.

In practise, however, home carriers use a deeper translation process which performs a lookup based on the entire CC+NC in order to better determine the correct country (in the case of NANPA, this would be 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).

Future possibilities for eliminating the global title problem include upgrading international switches to accept IMSIs as global titles, thus eliminating any such ambiguities. This is an especially handy solution, as non-GSM networks begin to transition to IMSIs for subscriber identification. However the expense of such an infrastructure upgrade may not be feasible for all countries anytime soon.

(Wikipedia)