What is a SIM CARD

SIM is an acronym for “Subscriber Identity Module”. A SIM CARD is a small integrated chip with a globally unique number that was first introduced in the second generation mobile networks such as GSM in Europe and IS-95 in U.S.A. The SIM number along with the Mobile identification number is used to locate a mobile station in the network for routing of calls between the mobile station and the Mobile Switching Centre.

The SIM number is also used for purposes such as:-

To find whether a mobile unit trying to access a network is a valid one. This is done by the Authorization Centre in the MSC checking the SIM number of the mobile unit against all the registered SIM numbers in the Home Location Register and the Visitor Location Register. To find whether a mobile unit is in the home network or roaming in a foreign network. If the mobile unit is in the home network then its SIM number is stored in the HLR of the home MSC. If it is in a foreign network then its SIM number in the HLR of the home MSC is combined with the address of the foreign MSC in which the mobile unit is roaming. In the foreign MSC, the SIM number will be stored in the Visitor Location Register. To blacklist a user in the Network by simply banning the SIM number in the Authorization Centre of the MSC. For charging and billing of calls and other services accessed by the user.

One of the best features of a SIM card is that it makes the mobile hardware very much irrelevant in terms of Network access i.e. by simply switching the SIM Card from one mobile set to another; the user can access the network without any change in his Network identity.

Besides the SIM number that is used to identify the user on the Network along with being used by the Mobile Network Operators for a variety of other functions mentioned above, the SIM card also contains a memory space. This memory space is used for:-

Storage of other Mobile numbers by the user so that the user can carry his address book from one unit to another if he is switching mobile sets. There is also a separate space provided for storing SMS messages. Storage of data for accessing various features of the Network and for authorization of the Subscriber in the Network.

Storage of information about various control channels used in the Network.

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