What is IMSI Number? What is the format and IMSI Vs IMEI

The International Mobile Subscriber Identity(IMSI) is an identity of a subscriber in the Telecom Network. IMSI’s full form says that identity is unique across the globe.  For uniqueness, it follows a hierarchical structure.

The length of the IMSI number is 15 digits. The IMSI structure consists of MCC, MNC, and MSIN. The globe has multiple countries, and each has a mobile country code or MCC.  The MCC is not the country code(CC), that we use while dialing an international mobile number. Here is the list of MCC and CC  for all countries. What is IMSI number

Each country gets the MCC from a standard telecommunication body. It is of a fixed length of three digits,

E.g., 404 is the MCC of India, while 91 is the county code.  The first three digits of IMSI are MCC.

Within a country, there can be multiple mobile operators at various locations. Each operator has multiple mobile network codes or MNCs with in the country. Within a nation, according to the area. The MNC specifies a PLMN for a mobile operator. The MNC can be 2 or 3 digits long.

If there is more than one mobile operator at the same location. All will be having different MNCs. After MCC the next 2 or 3 digits are for MNC.

The remaining 10 or 9 digits are mobile subscription identification number (MSIN), A MSIN identifies a subscriber within a PLMN. A mobile operator is free to choose unique MSIN.

IMSI security over the network: 

Being the primary identity of a sim card,  The telecom network uses IMSI for calls and SMS. If someone knows the IMSI and has software to hack the call or SMS, he might succeed on the radio interface.  For security reasons, the radio network uses TMSI, as the temporary identity of the sim card over the radio interface. During the first attach to an MME or SGSN, the core network allocates the TMSI or GUTI for further network requests.

Multi IMSI SIM card:

When a person buys a sim card. It comes with an IMSI or unique identity, which belongs to the network operator of the sim card.  Most of the time, this does not change. But there are sim cards, having multiple IMSIs configured. Depends on the location, the identity changes. This reduces roaming changes. The SIM with multiple identities is also called roaming SIM cards. Many MVNOs use multi IMSI sim cards for low-cost roaming.

IMSI vs. Imei

Like IMSI, IMEI is another unique international identity. But they are different parameters. One is hardware and another is SIM identity. IMEI vs Imei is a comparison in simple terms.

When a mobile subscriber changes the sim card, IMSI changes, but the IMEI remains the same.  IMEI is the 14 Digits long field, while the other is 15 digits long.  Device manufacturers use IMEI while the mobile network operator uses others.

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