The Global System for Mobile communication (GSM) is the most widespread mobile telephony system in the world. Over 3 billion consumers in the global market use this system. GSM’s popularity enabled the practice of international roaming, a standard process provided by many mobile phone service providers. This allows subscribers to use their cellular phones around the world.

Both signal and speech channels are digital under GSM – considered a second-generation (2G) platform. It enables data communication upgrades in the system. Among Time Division Multiple Access (TDMA), GSM, and Code Division Multiple Access (CDMA), GSM (a variation on TDMA) is most prevalent.

GSM operates by digitizing and compressing data, then sending it through a channel with a pair of user data streams, with each stream in a specific time slot. GSM’s frequency bands are 900 or 1800 MHz.

The Subscriber Identity Module (SIM) card is one of the main features of GSM. The SIM card is a detachable device containing the user’s data, such as a phone book. The SIM allows information to be stored even when switching handsets. Conversely, the user can change the SIM, thus changing service providers while keeping the handset.

GSM-based mobile phone services were launched in 1991 in Finland. GSM now represents 82.4% of all mobile connections, with over 2 billion GSM users around the globe. China, the largest GSM market, has over 370 million subscribers, with Russia following at 145 million, India at 83 million, and the United States at 78 million subscribers.

GSM is a facet of the wireless mobile telecommunications evolution that includes High-speed Circuit-Switched Data (HCSD), Enhanced Data GSM Environment (EDGE), General Packet Radio System (GPRS), and Universal Mobile Telecommunications Service (UMTS).

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