International Mobile Subscriber Identity

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An International Mobile Subscriber Identity, or IMSI [im-zee], is a unique number associated with all GSM and Universal Mobile Telecommunications System (UMTS) network mobile phone users. It is stored in the Subscriber Identity Module (SIM) inside the phone and is sent by the phone to the network. It is also used to acquire other details of the mobile in the Home Location Register (HLR) or as locally copied in the Visitor Location Register. 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.

The IMSI is used in any mobile network that interconnects with other networks, in particular CDMA and EVDO networks as well as GSM nets. 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, 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 subscriber identification number (MSIN) within the network's customer base.

The IMSI conforms to the International Telecommunication Union (ITU) E.212 numbering standard.

Contents

[edit] Example

IMSI:429011234567890

MCC 429 Nepal
MNC 01 Nepal Telecom
MSIN 1234567890

[edit] Example (American)

IMSI: 310150123456789

MCC 310 USA
MNC 150 Cingular (AT&T)
MSIN 123456789

[edit] IMSI analysis

IMSI analysis is the process of examining a subscriber's IMSI 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 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 [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 details, see Global Title Translation.

[edit] Example of outside World Area 1

This example shows the actual practise which is not clearly as 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)
* add 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

so we get 284011234567890 becomes 359881234567890 numbering plan E.214.

Translation rule:

* match numbers starting 310150 (America first MCC + Cingular MNC)
* remove first six digits (length of MCC+MNC)
* add 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

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.

[edit] Example inside World Area 1 (North America)

Translation rule:

* match numbers 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

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.

[edit] 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. 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. In order to know which network a mobile belongs to we have to analyse 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 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 ambiguities include upgrading international switches to accept IMSIs as global titles. 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 any time soon.

[edit] See also

[edit] External links