GSM SMS refers to the process when SMS messages are sent from PC via one or more attached GSM device with the help of an SMS gateway software. GSM SMS is ideal for sending and receiving fewer than 15 000 SMS messages per day, since the capacity of one GSM modem is about 6-10 SMS per minute.

GSM SMS is available by using a suitable GSM device (phone or modem) attached to the computer with a phone-to-PC data cable. For information about suitable phones, check out the GSM devices page. The GSM device has to be equipped with a SIM card that charges (preferably) low rates for SMS messages.

With this setup you can use a computer program (SMS gateway) to send and receive SMS messages. In this case, the SMS gateway software uses the attached device to communicate with the GSM network. If a message is sent out by the gateway running on the computer, it is first sent to the attached GSM device. Then the GSM device transmits it to the SMS Center (SMSC) of the GSM service provider, using a wireless link.

When a message is received, the GSM device stores the message in its memory or on the SIM card, and sends a notification to the SMS gateway. When the program receives this notification, it retrieves (reads) the message from the respective memory cell, and then deletes the message from the device to make room for the next incoming message.

The advantage of using a cellular modem is that you do not need Internet connection for SMS messaging. Sending an SMS message using a cellphone takes about 5-6 seconds. Receiving takes about the same time. Good SMS softwares allow you to attach more than one device to your PC and to use them simultaneously to increase capacity.

As a cellular network, GSM allows mobile phones to connect to it when searching for cells in its proximity. The cell network is created through antennas that transmit radio signals by way of dedicated carrier frequencies (850 MHz, and 1900 MHz in Canada and the United States; 900 MHz, or 1800 MHz in other countries) with a coverage area of 35 km, or 22 miles.

How to connect the GSM modem to the PC

  • The RS232 serial data cable connection

    Connecting a GSM/GPRS modem by way of a serial data cable is one of the simplest methods of connection, which requires an RS232 serial cable to be connected to the serial port of the computer and the modem itself. Serial cables for a mobile phone are called phone-to-PC data cables, which are connected to the PC the same way. After the connection is established, the operating system on the computer recognizes the devices through either COM1 or COM2 serial ports.

    Special industrial GSM modems usually use RS232 serial data cable connection for reliability and sustained performance. Below there are some of the modems that support serial cable connection to PC:

    Figure 1 - Wavecom Fastrack 1206b

    Wavecom produces high-speed and dependable modems for SMS and MMS messaging at an affordable price range. Sending an SMS through these devices require a SIM card, an external GSM antenna and the RS232 serial cable, which is for modem-PC connection.

    Another modem is the Fargo Maestro 20 or Maestro 100. These modems use a similar connection method like the Wavecom Fastrack modems.

    Figure 2 - Fargo Maestro 100

    After connecting a modem such as one of the above to the serial port of the computer, the device must be installed as a standard modem on the Operating System. The image below represents the installation of a GSM modem under Windows XP by simply installing it though the Add Hardware wizard in the XP Control Panel. This installation must be done by manually selecting a Standard 56000 bps modem from the list provided by the Operating System and choosing a dedicated COM port for communication with the modem.

  • The USB data cable connection

    USB connection between the computer and a mobile phone allows for several phones to be connected to the computer simultaneously by using USB hubs, which can be purchased separately. A driver for USB cables, however, must be installed first in order to use them. A disadvantage of using the drivers provided with the USB cables is that they must be periodically checked upon in case a newer driver version becomes available that has bug fixes discovered in the previous version.

    If a USB driver is installed on the computer, it automatically designates a virtual COM port on the system, numbering it anywhere from 3 to 5, or even more if several USB cables are connected. After installing these drivers, the GSM modem protocol for SMS messaging in the SMS Gateway software must be configured to use the COM port that is used by the modem or cell phone.

  • The Bluetooth (BT) connection

    One advantage of connecting a mobile device, or modem to a computer with Bluetooth connection, is its mobility, since BT connection is wireless with a range of 5 meters up to 100 meters, depending on the class of the Bluetooth device. This connection, along with its wireless nature, does not require the device to be in line-of-sight with the receiver on the computer, like the Infrared connection. To establish a Bluetooth connection, a BT antenna needs to be connected to the computer, to recognize the BT device. Nowadays there are computers, or notebooks with built-in Bluetooth feature, therefore a BT connection is often the most convenient method of communication with the device.

    Figure 3 - Connecting a mobile phone to the computer with Bluetooth connection

  • The Infrared (IR) connection

    The Infrared connection is also wireless but, as mentioned above, must remain in line-of-sight with the computer during the connection. If a device is moved, then the connection terminates. Another drawback of the IR connection is that most mobile phone devices turn off their IR sensor after a certain period. Similarly to the USB and Bluetooth connections, the IR connection also creates a virtual COM port on the computer, which then must be set within the SMS gateway program or HyperTerminal of Microsoft Windows.