 |
         |
 |
| The SMS Story Audience This briefing note is intended to provide both Managers and Engineers with a perspective that will help them more fully appreciate both the technology and wide application areas for SMS functionality. Summary The Short Message Service (SMS) is a facility available on GSM (Global System for Mobile communications) networks that enables text based messages of limited size to be sent either to or from a mobile phone or device. Although most of us are aware of the standard SMS functionality on our mobile phones, few understand the underlying potential for this technology. SMS can be used in numerous applications, from simple text paging through to sophisticated telemetry, location tracking and control applications. The ability to be able to connect directly into an SMS Centre (SMSC) for sending messages to mobiles from a variety of messaging applications, and receiving messages back from those mobiles, creates an enormous range of potential opportunities. Total Remote Control (www.totalremotecontrol.co.uk) has years of experience in implementing mobile technologies into highly sophisticated products and would be pleased to advise potential clients of both the business and technical suitability of such functionality for their products and processes. An Introduction to SMS There are a number of basic scenarios in which SMS can be used, which are both efficient to implement and cost effective. To fully appreciate these, it is important to understand that an SMS does not transmit directly from on mobile device to another: it passes through a "store and forward" mechanism, known as an SMS Centre (SMSC). The SMSC provides a mechanism for both the transmission of SMS-MT (mobile terminated SMS) messages to mobile devices and also for the acceptance of SMS-MO (mobile originated SMS) messages from them. The "devices" do not need to be mobile phones, but they need to have the functionality associated with them, perhaps via a GSM modem. Mobile originated messages are sent either for onward transmission to an ESME (External Short Message Entity) or as SMS-MT messages to other GSM mobiles. An ESME is an "external" system which is typically some sort of computer or 'host' running an SMS application whilst a Mobile is a standard GSM Mobile Handset (which could be further connected to another device such as a Laptop computer or PDA). Through consideration of these systems, it can be seen that 3 different scenarios that can occur:
STORE AND FORWARD SMS has been designed to provide a message delivery system in which messages are not lost, even if a recipient is switched off or moves out of coverage. The service makes use of an SMSC, which acts as a store and forward centre in order to store messages for future delivery attempts. Due to this store and forward nature, the perceived end-to-end delay will be variable if the SMSC is required to make multiple delivery attempts in order to successfully deliver messages. Applications can facilitate a variety of self checking functions such as evaluating coverage, memory capacity and battery level, in order to give the best possible chance of message delivery. Additionally the progress of messages can be evaluated using delivery receipts and querying via SMPP commands. SMS can be used by applications that rely on its speed of response, i.e. its ability to deliver messages relatively quickly, but no "guarantees" are offered by providers, as the SMSC queue cannot be explicitly known at any particular instant. System engineers will therefore need to be aware of such operational constraints at an early stage in the design process. Standards associated with SMS The three standards associated with SMS that will be of significant interest are :-
These open standard protocols support a full-featured set of messaging functions. Some Technical Background SMS messages can be up to 140 octets (8-bit bytes) in length and can carry information coded in a variety of ways, with the most common being the GSM default alphabet. This arrangement allows a simplified text alphabet to be coded into 7 bits per character, which are packed, by the SMSC, into the available "space". A basic calculation therefore shows that 160 (7 bit) characters can be transmitted, giving the normal maximum text message size on a mobile phone. At the other end of the usage spectrum, more advanced applications could use 'freeform' 8-bit data, where the SMSC makes no assumptions on the coding scheme and allows applications to use the 140 octets as they wish. In addition the 140 octet user data, an SMS has a number of standard 'header' elements, which dictate the behaviour of the network and associated applications when handling the message. Some examples are shown below :-
The "information" contained within an SMS message can also control other operational features. Examples of these are :-
|
