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GSM stands for Global System for Mobile Communications and is one of two major global technologies developed for digital cellular networks. The other major such standard is CDMA (Code Division Multiple Access). GSM phones are recognized by the presence of a SIM or Subscriber Identification Module, which contains the user’s subscriber information.

GSM is a cellular network, which means that mobile phones connect to the network by searching for cell towers in the immediate vicinity. These cell towers broadcast the GSM signal. GSM communicates across radio spectrum bands (think of them as “channels”) with different spectrum bands licensed to various carriers.

SIM Card Explanation
The SIM, or Subscriber Identification Module, is an electronic chip embedded into a plastic card, which contains the subscriber’s phone number and network identifying information. An activated SIM, inserted in the subscriber’s device, is required for GSM devices to function on the carrier network. The beauty of having a SIM-based device is the convenience of being able to move between devices and retain that subscriber’s information.

SIM Locks On some networks, devices are "locked" to the SIM card of the carrier that offers the phone so that the device will only work with that carrier's SIM. This is usually done because phones are typically subsidized by carriers which then recoup their subsidy cost by requiring the customer to sign a two-year agreement. In some countries, this practice is now prohibited. In most others, carriers are required to provide some form of unlocking support at the customer's request. This allows the customer to take the device to another compatible network as desired.

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