The IMSI (International Mobile Subscriber Identity) is a number stored on a  GSM/HSPA/LTE based SIM and is used to uniquely identify a subscriber on a wireless network.

In North America the IMSI is based on the ITU E.212 based numbering plan and is a 15 digit number assigned to subscribers (SIM).  The number is broken down as follows:

  • MCC – Mobile Country Code (first 3 digits)
  • MNC – Mobile Network Code (next 2 or 3 digits)
  • MSIN- Mobile Subscription Identification number (remaining digits)

You can find a list of Canada’s wireless network operators and their associated E.212 number plan assignments here.

When you remove your SIM card from your cell phone and insert it in another cell phone, you are moving your IMSI and all your wireless account information (identity) to the new phone.

How is IMSI Used?

IMSI Analysis is the process of examining a subscriber’s IMSI to identify the network the IMSI belongs to, and whether subscribers from that network may use a given network (if they are not local subscribers, this requires 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 SS7switches. 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 practice and the GSM numbering authority has clearly stated that a one-stage process is used [1].

In North America, the IMSI is 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.  (from Wikipedia)

Depending on the country, mobile subscribers and wireless network nodes are assigned E.212, E.214 E.164 numbering plan based numbers to enable the proper translation and routing of MAP based messages in telecom networks.  These messages are transported between telecom infrastructure globally such that a user can be identified no matter where they are located in the world, and calls can be routed to the subscriber with no loss of calls.

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