A.2. SMS Overview

The Short Message Service (SMS) allows text-based messages to be sent to and from mobile telephones on a GSM network [2]. Each message has a maximum length of 160 characters. SMS messages are divided into two categories: Mobile Terminate (MT — where the SMS message originates from the network provider) and Mobile Originate (MO — where the consumer can send messages to other consumers) [3]. In the context of MO and MT messages, the consumer refers to the end-user, the person with a cell phone. This is as opposed to the network provider, who provides the consumer with such services. In this section, we are only concerned with MO SMS messages.

Typically, SMS messages are sent and received by cellular consumers using cellular telephone handsets. Cell phones are not the only devices that have this capability though. Anything that is capable of talking to a GSM network, in theory, has the ability to send and receive SMS messages. Since we are trying to interface a computer with the GSM network, it makes sense to use a device designed to do so, in other words a GSM modem.

Almost all GSM modems (and this includes many cell phone hand sets) use RS-232 as a transport protocol. On top of this, they use a protocol called the AT+ command set to communicate with their controlling devices. AT+ was defined by the European Telecommunication Standards Institute [4], and was designed to be a backward compatible set of extensions to the Hayes AT command set [5]. Device manufacturers are free to add their own extensions to this command set, and such extensions usually have an identifying prefix.

The communications hardware used to implement the service described in this paper was just such a GSM modem, specifically a modem based on the Wavecom chipset. It was pleasing to find such modems manufactured by a South African based company called TeliMatrix.