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IMSI – International Mobile Subscriber Identity

Defined by ITU-T Recommendation E.212, the IMSI is an internationally unique identifier associated with a subscriber account. The IMSI is not associated with a specific piece of equipment. CDMA mobile devices are provisioned with at least one IMSI. The IMSI may be up to 15 digits in length and is formatted as shown:

Diagram IMSI.JPG

There are two classes of IMSI:

  • Class 0 IMSI – Fixed length of 15 digits (i.e., NMSI is 12 digits). This is the class of IMSI typically used in CDMA.
  • Class 1 IMSI – Less than 15 digits in length. Rarely encountered in CDMA.

The following types of IMSI are used:

  • IMSI_M – MIN based IMSI. An IMSI_M contains the MIN in the right-most 10 digits of the NMSI. The MIN part of an IMSI_M can be used to identify a roamer’s HLR.
  • IMSI_T – True IMSI. An IMSI_T is not associated with the MIN and may contain any value for the NMSI. Because the NMSI is not associated with the MIN, it can never be relied upon to identify a roamer’s HLR.
  • IMSI_O – Operational IMSI. IMSI_O is not actually another “type” of IMSI. Rather, it is simply a selection of either IMSI_M or IMSI_T, depending upon which type is being used. IMSI_O is what is actually used by the MS.

The six digit portion of the IMSI that consists of the MCC and MNC is referred to as the Home Network Identifier (HNI). The HNI uniquely identifies a network and may be used to determine the HLR associated with a user. Telcordia is responsible for the assignment and administration of HNI values to network operators.

Click <here> for a technical briefing presentation that thoroughly defines IMSI and relates several identifiers, including MIN, MDN, MSID, ESN, IRM, IMSI, MCC, and MNC.


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