GSM Modems

A GSM modem, 3G modem or SIM box is basically a GSM phone without keyboard, speaker, battery and display. The terms SIM box, GSM gateway or GSM router are mostly used for devices that route phone calls through the GSM network and the term GSM modem or 3G modem for devices specialized in wireless data transmission. However for GSM modem software they often are interchangeable.

A GSM modem provides the SMS software with a secure gateway to the GSM network, with only two limitations. The amount of SMS message that can be sent and/or received is limited to 12-30 messages per minute, depending on the GSM modem and network, and it is not possible to send SMS messages with alternative numeric or alphanumeric sender id’s.

GSM modems with a serial port

Wavecom GSM modem

Wavecom Fastrack Supreme GSM modem


  • Very reliable
  • No modem drivers necessary
  • Connector can be fixed securely to the serial port
  • Industrial design, usually very robust
  • External power supply
  • External antenna, suitable for server rooms
  • Designed for 24/7 operation


  • Serial ports are often not available on new computers

Brands and models

  • Sierra Wireless AirLink Fastrack Xtend
  • Wavecom Fastrack Supreme/M1306B/M1206, MOD2
  • Cinterion MC55iT, MC52iT, TC65T
  • Siemens MC55iT, TC65T, TC45T, TC35i
  • Multitech MTCBA-G-F4

Siemens tc35i GSM modem

Siemens tc35i GSM modem

Serial GSM modem over TCP/IP

A very reliable way to use a serial GSM modem on a server that has no serial port is with a TCP/IP to RS232 convertor. An extra advantage is that the actual GSM modems can be placed anywhere in the network, even on remote locations. Diafaan SMS Server can use GSM modems over a TCP/IP connection directly, without the need for any additional COM port drivers.
Review of the ATC-1000 TCP/IP to RS232 converter

USB to RS232 converter

Another, slightly less reliable, way to use a serial GSM modem on a server that has no serial port is with a USB to RS232 converter. The USB to RS232 converter installs a virtual serial port on the PC that is used by Diafaan SMS Server.

GSM modems with a USB port

Huawei E220 USB GSM modem

Huawei E220 USB GSM modem

  • Easy to install and use
  • Readily available and often quite cheap with contract
  • Older models are often available cheap and SIM lock free
  • Most turn out to be sufficiently reliable


  • USB drivers not always reliable
  • Multiple USB modems on one computer can cause problems
  • Power comes from the USB port and is not always sufficient
  • Drivers not always support latest Windows version
  • Most have no support for external antenna
  • Not designed for 24/7 operation

Brands and models

  • Huawei E1750, E220, E173, E160, E153
  • Option iCON 505M, 225
  • ZTE MF627, MF112
  • And many more

GSM phones

GSM phones supporting the GSM 07.05 standard can be used as a GSM modem. For non-supervised systems, a dedicated GSM modem is a more secure option but for less critical applications a GSM phone can be a cost-effective way to send and receive SMS messages. A GSM phone can be connected to the server in a number of ways. On older phones a serial port connection is often available, newer phones usually support a USB connection or bluetooth.


Some smartphones can be used with short message software to send messages but most not to receive messages. The reason is that older phones store the received SMS messages on your phone’s SIM card, the SMS software reads the message list from the SIM card and then removes them to make room for new messages. Many smartphones store the messages in the internal memory of the phone that cannot be read out by the SMS software.