Short Message Service (SMS) is a text messaging service for telephone or mobile communication systems. It enables the exchange of short text messages between fixed line, mobile devices and computers. The messages are short as they have a maximum size of 160 characters.
SMS was developed to work with the GSM (Global System for Mobile Communications). GSM describes a standard set of protocols for digital cell phone networks and became the standard for mobile communications. SMS was envisaged as a simple messaging system that would work when users' mobile phones were switched off or out of range.
A key feature of GSM is the Subscriber Identity Module (SIM), which is a detachable smart card that fits into communication devices. It has a unique serial number (ICCID), an international mobile subscriber identity (IMSI) (including a telephone number) and a Personal Identification Number (PIN).
ClearSCADA can exchange SMS messages with a remote outstation. To do this, you connect a GSM modem to your computer or to a ClearSCADA server. A GSM modem accepts a SIM card, and enables you to communicate over the GSM network. It can also store SMS messages. The GSM modem can be a dedicated device with a serial connection, or it can be a cell phone with GSM modem functionality.
The remote outstation is equipped with a GSM modem. Communication takes place between the GSM modems using the GSM network.
When ClearSCADA sends an SMS message to a remote outstation, it first routes the message to the local GSM modem, which sends the message to the GSM network. The message then reaches the SMS Center (SMSC), which is a GSM network element. The purpose of an SMSC is to store and deliver SMS messages and to deliver them to the recipient modem. If the recipient modem is unavailable (for example, when the outstation is switched off) an SMSC will store the SMS message. It will deliver the message when the recipient modem is available. Message delivery is 'best effort' - there is no certainty that the GSM network will deliver a message.
In ClearSCADA, and in many modems, you can specify a validity period (or expiry period) after which the SMSC will delete a stored message. Once deleted, the SMS message will no longer be available for delivery to the recipient (even if the recipient becomes available). The validity period is a request - an SMSC can override it.
There are two different modes in which you can send SMS messages: SMS Text mode and PDU (Protocol Data Unit) mode. In SMS Text mode, messages are in simple ASCII text. In PDU mode, messages are in binary strings encoded as hexadecimal characters. The strings contain not only the message, but also meta-data about the sender, the SMS service center, a time stamp and so on.
A common SMS service is Bulk Text Messaging. This is the ability to send (or broadcast) a large volume of SMS text messages to the GSM network for delivery to many cell phones. It is primarily used by organizations for mass communication, marketing, alerts and reminders.