GSM / GPRS Modem

Figure 15: GSM Modem

A GSM modem is a wireless modem that works with GSM networks. A wireless modem behaves like a Hayes compatible dial-up modem. The main difference between a standard Hayes modem and a GSM modem is that a Hayes modem sends and receives data through a fixed telephone line while a GSM modem sends and receives data through radio waves. A GSM modem can be an external unit or a PCMCIA card (also called PC Card). An external GSM modem is connected to a PC through a serial cable, a USB cable, Bluetooth or Infrared. Like a GSM mobile phone, a GSM modem requires a SIM card from a wireless carrier in order to operate.

GSM/GPRS Modules are similar to modems, but there's one difference: A GSM/GPRS Modem is external equipment, whereas the GSM/GPRS Module is a module that can be integrated within equipment. It is an embedded piece of hardware.

PC's use AT commands to control a modem. GSM modems and normal Hayes modems support a common set of AT commands. A GSM modem can be used just like a Hayes compatible modem. GSM modems support an extended set of AT commands. These extended AT commands are defined in the GSM standards. With the extended AT commands, the things that can be done are like:

1. Read, write and delete SMS messages.
2. Send SMS messages.
3. Monitor the signal strength.
4. Monitor the charging status and charge level of the battery.
5. Read, write and searched phone book entries.

The number of SMS messages that can be processed by a GSM modem is pretty low, approx. six messages per minute.

A GPRS modem is a GSM modem with additional support for GPRS technology for data transmission. GPRS means: 'General Packet Radio Service'. It is based on a packet-switched technology, as an extension to GSM (note that GSM is a circuit-switched). An advantage of GPRS over GSM is that GPRS has a much higher data transmission speed. GPRS can be used as the bearer of SMS. If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages per minute may be achieved. This is much faster than SMS over GSM. A GPRS modem is required to send and receive SMS via GPRS. Some wireless carriers do not support the sending and receiving of SMS via GPRS. A GPRS modem is typically required for MMS.