People often want a definition of International Mobile Subscriber Identity (IMSI) after hearing the term. Though the name reveals a lot about the meaning, there are so many other details interested users would have to know. This is not pivotal, though, since mobile network technology is designed to be user-friendly. That way, customers don’t have to be tech-savvy or remember various numbers and terms. Nonetheless, knowing where to find subscriber information can be crucial. Common reasons include needing to repair the device, having the device stolen, or deciding to switch carriers and/or devices. With that said, let’s explain what International Mobile Subscriber Identity (IMSI) is.
What does IMSI (International Mobile Subscriber Identity) represent, and how long is it?
IMSI (International Mobile Subscriber Identity) is usually a 15-digit long unique number that identifies a subscriber on a cellular network. Contrary to popular belief, though most frequently used for GSM (Global System for Mobile Communications) mobile networks, it is not limited to it. In reality, it can be applied to any cellular network interconnected with others.
For instance, like with GSM, IMSI can also be provisioned to SIM cards that use Universal Mobile Telecommunications System (UMTS), Evolution data only (EVDO), and LTE (Long-Term Evolution) networks. Moreover, IMSI can be embedded into devices using cdmaOne and CDMA2000 networks. However, because those are deprecated, it has regular utilization in the modern CDMA version of a SIM card named R-UIM, especially starting in the 2010s.
What does International Mobile Subscriber Identity (IMSI) consist of?
International Mobile Subscriber Identity (IMSI) can have 14 to 15 digits, depending on the three components it has:
- MCC (Mobile Country Code) — This is a 3-digit long code that represents the country in the international cellular network system. Similar to LAI (Location Area Identifier) number in meaning and format.
- MNC (Mobile Network Code) — A 2-digit or 3-digit number that identifies a mobile network provider, i.e., a carrier within the country. The former, two digits, means that the carrier follows a European standard of the MCC, while the latter, three digits, represents a North American standard of the MCC. Same meaning and format above, but assigned by the government.
- MSIN (Mobile Subscription Identification Number) — MSIN regularly has 9 or 10 digits, based on the length of the MNC, and provides information about the identity of a customer of a specific MNC.
Because of similarities when it comes to customer identification, users frequently confuse the ICCID number of a SIM card. Without getting into too many specifics, a key difference is a length, since ICCID (Integrated Circuit Card Identifier) has 18-22 numeric characters. Also, there’s a variety in what they represent, since ICCID is a unique identification number of a SIM while IMSI only takes up a portion of a profile of a SIM (more on that in the section below). Additionally, MNC is usually a part of both ICCID and IMSI and may be in the same place (4th to 6th number character).
Examples of IMSI
Now that you understand what they represent, you may want to see how IMSI looks in practice. For IMSI 310150123456789, we can conclude that:
- 310 represents the home country, in this case, the United States
- 150 is the carrier, in this case, AT&T Wireless
- 123456789 is a random customer identification number
If you were to encounter IMSI that is 734011234567890, it would mean:
- 734 is the home country, now Venezuela
- 01 is the mobile network (not 3 digits like above), now DigiTel C.A.
- 1234567890 represents the customer, but this time with 10 digits
Finally, because IMSI is a tad outdated in countries with a less-developed infrastructure, you may see a 14-digit number such as 65507123456789:
- 655 — South Africa
- 07 – Cell C
- 123456789 – a 9-digit customer identification number
Who allocates IMSI, and where is it stored?
IMSI is allocated by the international cellular country-based system, then individual mobile network providers and with their MNC and MSIN. Its development, application, and standard-upholding are overseen by the Study Group 2 subsidiary of the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) since its inception in 1988. With that said, IMSI is stored in a 64-bit field of a SIM card. It is then forwarded to the network when authentication is required. This means that if a SIM card only has one IMSI, it only occupies a section of a SIM profile.
In contrast, if the SIM card was created by an operator that permits multi-IMSI cards, it is stored in portions of each profile. The aforementioned broadcasting for identification has issues we’ll address next. Although, this functionality makes it capable of sending mobile-related data to the Home location register (HLR). HLR is a database that stores information about any mobile device and its subscribers that have permission to use a GSM network. Moreover, that information is then copied locally to Visitor Location Register (VLR). This is a database that tracks any mobile stations that roam inside the area with the authority of a specific Mobile Switching Center (MSC).
When the device or SIM comes online, it reveals its IMSI to the nearest base station and thus the network. This tells the network where the SIM or device with a specific identity currently is, known as the “attach procedure”. When they go offline, permanently or temporarily, an IMSI detach procedure occurs. Both processes can occur when a subscriber is “handed over” from one mobile station to another.
Protection against IMSI spying
With everything we stated up until this point, it’s clear that IMSI has to be transmitted to the network upon request. This makes it exceptionally vulnerable to eavesdroppers that monitor radio waves and can put customers at peril. Therefore, mobile networks worldwide have adopted TMSI (Temporary Mobile Subscriber Identity). They seldom permit customers’ SIM cards or devices from sending their real IMSI. Instead, IMSI is forwarded to the aforementioned VLR, which generates a TMSI.
VLR can and does change IMSI at key points. For example, when the device or SIM turns on or becomes inactive or invalid before switching on. That involves the procedure called “paging” where the nearest base contacts a mobile device to request information broadcasting. Each type of cellular system has its ways of transmitting data between the two.