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A Beginner’s Guide to IMSI

The acronym IMSI has most of us scouring Google for answers, however, once digested, the benefits of the 15-digit code come to light. IMSI translates as International Mobile Subscriber Identity and is used to recognise the user of a mobile phone network. The unique identification number is associated with all mobile networks and stored as a 64-bit field and sent to the phone via the specific network.

In more superfluous terms, the code is associated with the Global System for Mobile Communications (GSM) and the Universal Mobile Telecommunications System (UMTS) mobile phone network users and the IMSI code is unique to the GSM subscriber – not dissimilar to farmers (the network providers) number tagging livestock (the users).

The number itself is split into two parts. The initial five digits (European standard) identifies the GSM operator in a specific country from which the mobile phone account holder hails. The remaining digits are allocated by the network provider, such as EE, Vodafone or O2, to uniquely identify the subscriber.

The IMSI is stored on the SIM card inside the mobile phone handset and is delivered by the phone to the appropriate network.

The IMSI is used to acquire the details of the mobile in the Home Location Register (HLR) or the Visitor Location Register (VLR) which are both databases of permanent subscriber information for a mobile network.

When a mobile is initially affiliated it is given a temporary ISMI that is used to identify the subscriber in future exchanges and is provided anytime the network is accessed. This temporary code is transmitted during initialisation.

To prevent phones from being tracked or hacked, the ISMI is not transmitted. Instead, a random temporary mobile subscriber identity (TMSI) is generated and provided to the network which ensures confidentiality for the user.

Mobile stations comprise of all the user equipment and software needed for communication with a mobile network. When these stations are powered on, they perform a location update which works by propelling their IMSI to the network. This initial update is referred to as the IMSI attach procedure.

These powered on stations can also perform a location update to indicate the current location when a device moves to a new location and these updates are regularly performed to keep as true a picture as to where the devices are operating.

This knowledge to essentially track mobile phone users via their IMSI code has been adopted by some companies to produce devices called IMSI catchers. IMSI catchers pose as mobile base stations and operate by providing the strongest signal, forcing all phones in the vicinity to automatically connect to what they think is a standard network tower. Once this connection has been made, IMSI catchers hoover up all the information, including the subscriber’s identity, via the phone and some can even intercept calls and text messages.

Although catchers are used by the government and police forces to catch criminals, there is a growing public concern as to lack of privacy for general citizens. With the feeling that Big Brother is constantly watching you, members of the public are sceptical of the use of IMSI catchers – especially when many officials are tight-lipped about even their existence.

This is no fault of the IMSI code, it is an essential piece of data that cannot be avoided. It is an issue surrounding the production of the IMSI catchers from which people can’t escape unless they abandon their mobile telephones.

Despite these ethically questionable catchers, IMSI in itself, is a seamless system for identification between networks and users. Luminet offers a ubiquitous and unified IMSI mobile data connectivity solution. Operating 3G and 4G connectivity via network providers EE, Vodafone and O2, the use of their intelligent cloud platform provides operational insight and control over any application and lowers IT costs while improving user experience. The ability to view, control, protect and monitor the connectivity simply means no IT expertise or IT footprint is essential.

Offering a high bandwidth, ranging from 1 to 5Gbps, and access to 3G and 4G data, high speeds and seamless connections are fixed and allow for connection globally to over 370 international operators in over 120 countries worldwide.





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