GSM technology is simply the technology that you (most probably) and 80% of mobile users use for making calls on their mobile phones. In a way, it is the standard and default wireless protocol used for mobile communication.
GSM started back in 1982 and was then named after the group that devised it, Groupe Spécial Mobile, whence the GSM acronym. The official protocol was launched itself in Finland in 1991. It is now called Global Systems for Mobile communications.
GSM is considered a 2G (second generation) protocol. It works with cells, which is why a GSM network is also called a cellular network, and phones working on GSM are called cell phones. Now what is a cell? A GSM network is split into cells, each one of which covers a small area. Devices (phones) are then located and communicated with through these cells.
A GSM network consists mainly of connection devices (gateways etc.), repeaters or relays, which people commonly call antennas – these massive metal structures that stand as high towers -, and the mobile phones of users.
The SIM Card
Each mobile phone is connected to a GSM network and identified in it through a SIM (Subscriber Identity Module) card, which is a small card that is inserted inside the mobile phone. Each SIM card is assigned a phone number, hard-coded into it, that is used as unique identification element for the device on the network. This is how your phone rings (and not anyone else’s) when someone dials your mobile phone number.
GSM people have developed a communication system that is a cheap alternative to the somewhat expensive voice communication; it is the Short Messaging System (SMS). This consists of transmitting short text messages between mobile phones using phone numbers for addressing.
GSM and Voice over IP
GSM is not very cheap, but it is widely accepted because it offers mobility in communication. It is even more expensive when it comes to making international calls. But by coupling it with VoIP, that cost can be considerably cut down. With VoIP, you can make an international call and pay only the price of a local call plus some cents only for each minute of communication.
Here is a list of some of the VoIP service providers that allow you to make cheap international calls anywhere there is GSM coverage, without requiring Wi-Fi or 3G: