GSM feature phones require a small microchip called a
Subscriber Identity Module or SIM Card, to function. The SIM card is
approximately the size of a small postage stamp and is usually placed
underneath the battery in the rear of the unit. The SIM securely stores the
service-subscriber key (IMSI) and the Ki used to identify and authenticate the
user of the mobile phone. The SIM card allows users to change phones by simply
removing the SIM card from one mobile phone and inserting it into another
mobile phone or broadband telephony device.
The first SIM card was made in 1991 by Munich smart card
maker Giesecke & Devrient for the Finnish wireless network operator
Radiolinja.
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