IMSI, format, definition with examples

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IMSI is acronym from International Mobile Subscriber Identity, and those number is one of the most important technical terms in network telecom layer. Hire will be given explanation of purpose and importance of those number. But before all, we will try to explain how SIM works.

SIM - is acronym from subscriber identity module or subscriber identification module. So, SIM is an integrated circuit which stores the service-subscriber key (IMSI) used to identify a subscriber on mobile telephony devices (such as mobile phones and computers).
One SIM is held on a removable SIM card, which can be transferred between different mobile devices. Old SIM cards were first made for the same size as a credit card (85.60 mm x 53.98 mm x 0.76 mm). The development of electronics and physically smaller mobile devices prompted the development of a smaller SIM card, the mini-SIM card. Mini-SIM cards have the same thickness as full-size cards, but their length and width are reduced to 25 mm x 15 mm.
A SIM card contains its unique serial number (ICCID), internationally unique number of the mobile user (IMSI), security authentication and ciphering information, temporary information related to the local network, a list of the services the user has access to and two passwords (PIN for usual use and PUK for unlocking).



It is good place to define 3 every day used numbers: PIN, PUK and EMEI.

PIN - personal identification number, as name says this is a secret numeric password shared between a user and a system that can be used to authenticate the user to the system.
PUK - PIN Unlock Key (PUK) also known as PIN Unlock Code. This password is used in GSM mobile phones and some smartcards to unblock a blocked card.
Mobile phones usually (if is not disable) on startup looking for PIN. If the wrong PIN is typed in more than three times, SIM card, the device or both become locked. This is also everyday situation, so good suggestion is, write PUK code on some paper and keep it in your wallet. And onse more interesting term, for advenced telecom testers, developers and engineers. Also useful for people who frequently lose their mobile phones.
IMEI - International Mobile Equipment Identity, is a 15 digit unqiue code that is used to identify the GSM phone to a GSM network. When a phone is switched on, this unique IMEI number is transmitted and checked against a database of blacklisted or greylisted phones in the network's Equipment database register. EIR determines whether the phone can log onto the network to send and receive calls, sms, mms etc. And in such a way, this is the type of secondary containment. For example, if a mobile phone is stolen, the owner can call network provider and inform them to "blacklist" the phone using stolen phone with according IMEI number. Also one usefull trick. If you plan to buy second hand mobile phone, type *#06# on the keypad and check with provider regularity of phone. This code works on most phones. Wiretapping can be specified by its IMEI number as well as IMSI and MSISDN.

Definition:
IMSI is a unique identification associated with all GSM and UMTS network mobile phone users. It is stored as a 64 bit field 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 (VLR). 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. HLR and VLR are home and visitor subscribers databases.

Format:
IMSI is usually presented as a 15 digit long number. 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 Subscription Identification Number (MSIN) within the network's customer base.
The IMSI conforms to the ITU E.212 numbering standard.

Examples:

IMSI: 381 01 1234567890
MCC: 381 - Serbia
MNC: 01 - Telecom Serbia
MSIN: 1234567890

IMSI: 310 150 123456789
MCC: 310 - USA
MNC: 150 - America Telecom
MSIN: 123456789

Therefore it is necessary to have excellent knowledge of all basic parameters and interconnection between them for successful design of completely new or quality non kernel development on some of actual billing systems.