A GSM modem is an external modem device, connecting to a computer through a serial cable or a USB cable. It is a wireless modem that works with a GSM wireless network. Like a GSM mobile phone, a GSM modem requires a SIM card from a wireless carrier in order to operate. GSM modem is recommended for use with a computer to send and receive messages. This is because some mobile phones have certain limitations comparing to GSM modems.