SIM, subscriber identity module, is a smart chip for mobile phones which is used in all GSM and 3G networks. In 3G it is called USIM. It’s also used in some version of CDMA. The SIM is identified on their network by their unique IMSI (International Mobile Subscriber Identity) number.
SIM is behaving like a mini computer with its own operating system and storage. It can add delete and control information in its memory and both send and receive data. SIM is available in different sizes, First one named as “Full size SIM card” which was intrdouced in 1991. The lenght of this Full Size SIM card is 85.60 mm while width is 53.98 mm. Thinckness of this full size sim is 0.76 mm. Second one is “Mini SIM” which was introduced in 1996. The lenght of this Mini SIM card is 25.00 mm while width is 15.00 mm. Thinckness of this mini sim is 0.76 mm. Third one is “Micro SIM” which was introduced in 2003. The lenght of this Micro SIM card is 15.00 mm while width is 12.00 mm. Thinckness of this micro sim is 0.76 mm. Last but not the least is “Nano SIM” which was intrduced in early 2012. This Sim is currenlty used in most part of the world. Althhough there is new “Embedded SIM” has been introduced but it is still inn early stages of development. The lenght of this Nano SIM card is 12.30 mm while width is 8.80 mm. Thinckness of this micro sim is 0.67 mm. The basic functionality of SIM is to give access of network to the mobile phone. It authenticates the user to the network and check that they billed correctly with respect to the airtime they used.
SIM allows the storage of messages, personal phone directory, and give the roaming facility. Through SIM the user can access the news, games, and lots more. Through SIM use can connect internet via GPRS. Through SIM one can send SMS, MMS to their friends. But these all things depends upon the type of service company is providing and the mobile phone customer is using.
SIM also stores information of its current local area identity (LAI). When user changes his area then the SIM updates the location and send the information to the network about the new location. When the phone turn off and back on again it will check the LAI and send update to the network with respect to it.
SIM is available in different storage sizes the biggest being using is 256 Kb while the mostly widely used version has storage space of 64k.