There are even GSM break-out boards specially designed for the Raspberry Pi 40-pin header, like the 'RPI GSM Add-on V2.0' found on eBay. Since it needs a 4.3V power supply, check that the board includes a voltage regulator, so it can be powered with an external 5V USB power supply. This chapter is based on the SIM800 chip that is very versatile and available on several modules. Most modems uses a serial (RS-232) communication link and the good old (extended) Hayes command set, with the 'AT' prefix. On the hardware side you need a GSM module, also called GSM modem, that can be interfaced with the Raspberry Pi. In most applications only SMS or data transmission with little data volume and low transfer speed is needed, so you may purchase a cheap 'data only' SIM card. Like with mobile phones you need a SIM card and a subscription from any provider. If your Raspberry Pi needs to communicate with the outside world when it is not in the range of a WiFi access point, a digital cellular network may be used.
The source code of all examples can be downloaded from here.
