SIM card, IMSI, IMEI, ICCID and ISDN background on Mobiles


IMSI = International Mobile Subscriber Identity. This is a unique identifier that defines a subscriber in the wireless world, including the country and mobile network to which the subscriber belongs. It has the format MCC-MNC-MSIN.

MCC = Mobile Country Code (e.g. 310 for USA, 505 for Australia)
MNC = Mobile Network Code (e.g. 410 for AT&T in USA, 02 for Optus in Australia)
MSIN = Sequential serial number.

Click here for these International Mobile Country and Network Codes


The IMSI is one of the pieces of information stored on a SIM card. It can be used in any mobile network that can be connected with other networks. It is typically a 15-digit ID, but it may be shorter in some cases. All signaling and messaging in GSM (2G) and UMTS (3G) and LTE (4G) networks uses the IMSI as the primary identifier of a subscriber.
Once registered however, sent as rarely as possible because this protects users from eavesdropping and tracking on the radio interface.

So, instead of using an IMSI, a TMSI Temporary Mobile Subscriber Identity may be used. The TMSI is randomly generated. In LTE, this P-TMSI (Packet-TMSI) is now called the Globally Unique Temporary ID, or the GUTI.

Note, in Australia, where "roaming" between networks does not occur, if your phone is out of range of a cell tower belonging to your network, no calls can be made. Internet access is WiFi only.


ICCID = Integrated Circuit Card ID. This is the identifier of the actual SIM card itself.
i.e. an identifier for the SIM chip. While the information contained on a SIM (e.g. the IMSI) can change, the identity of the SIM itself remains the same.

IMEI is short for International Mobile Equipment Identity and is a unique number given to every single mobile phone, typically found behind the battery.
In Australia, IMEI numbers of cellular phones connected to a GSM network are stored in a database (EIR - Equipment Identity Register) containing all valid mobile phone equipment.
When a phone is reported stolen or is not type approved, the number is marked invalid.
 
MS ISDN = Mobile Subscriber ISDN number (Integrated services digital network).
This is the full phone number of a subscriber, including the national country code (e.g. 1 for US, 44 for UK, 61 for Australia etc.). The purpose of the MS ISDN is to allow a device to be called. A subscriber can have multiple MS ISDNs (e.g. one phone number for business, one for personal calls, one for fax, etc.), but generally only one IMSI. The MS ISDN does not need to be stored on the SIM card. In cases where it is stored on the SIM, the main reason is so that the user can check to see what their MS ISDN is (in case they forget). The MS ISDN is never signaled to or from the device.

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