IMSI Number

An international mobile subscriber identity (IMSI) identifies a mobile station (MS) in the global system for mobile communications (GSM) network uniquely. IMSIs are encoded in E.212 format, stored in the subscriber identity
module (SIM) card, HLR, and VLR, and transmitted over the wireless interface and Mobile Application Part (MAP) interface.

Figure 1 IMSI structure

 

MCC: It is short for mobile country code and is composed of three digits, for example, 460 for China.
MNC: It is short for mobile network code and is composed of two digits, for example, 00 for a carrier. 
MSIN: It is short for mobile subscriber identification number and is a unique identification for an MS in a PLMN. 
NMSI: It is short for national mobile subscriber identification and is a unique identification for an MS in a country. 
A typical IMSI example is 460-00-4777770001.
The principles for IMSI allocation are as follows: 

• An IMSI can contain up to 15 digits.
• MCCs are allocated uniformly worldwide, but NMSIs are allocated by carriers of different countries.
• If more than one GSM PLMN exists in a country, a unique MNC must be allocated to each PLMN.
• For IMSI allocation, it is required that addressing in PLMNs of foreign countries be successful after analysis of only MCC, or only MNC, or MCC+MNC.

The operations UpdateLocation, PurgeMS, and SendAuthenticationInfo must use IMSIs for addressing at the first time. Generally, the operation RestoreData uses IMSIs for addressing. At present, all messages related to supplementary service operations sent to the HLR use IMSIs for addressing.