How does a sim card function in a cellphone?
It is a plastic card, which is called a subscriber identity module (SIM). It consists of an integrated microprocessor chip, which keeps data relating to a subscriber, and provides communication with GSM network.

Territory covered with GSM network is divided into hexagonal cells. The diameter of each hexagonal cell may be from 400 m up to 50 km, which consists of base station that provides communication — receive and transmission, and antennae.

All GSM cellular communication telephone cells are preformed via these antennae and stations, which are regulated by switching centre.

Switching Centre provides communication between city telephone network, base stations and other cellular communication operators.

Every time you switch on your cell phone, the communication is performed with the nearest base station. At the same time permanent receive-transmission of signals from your terminal is performed.

When a terminal (phone) is switched on, the system automatically searches and defines subscriber's location by using the subscriber identity module SIM. The sim card along with your cell phone can enable the secure transfer of messages.

SIM card serial number is printed on its backside. This number contains the information necessary to obtain a new card, in case SIM card or GSM phone is lost or stolen.

SIM-card does not limit talks' duration. Unauthorised access is prevented by the password. Functional opportunities of SIM-card are an opportunity to keep cost per minute and talk duration data; to block all outgoing calls except specified numbers; to keep up to 150 telephone numbers in card's memory; to keep last 10 outgoing telephone numbers in card's memory