GPRS Communication Using Embedded Device

GPRS Communication Using Embedded Device

Abstract


The task of creating a cheap and flexible device for Supervisory Control And Data Acquisition (SCADA) is the goal of this project. SCADA systems generally cover larger geographic areas, and rely on a variety of communications systems that are normally less reliable than a LAN.

The device is intended to operate as part of a previously defined architecture. By using a very simple and cheap microprocessor, and relying on an existing communications infrastructure – the mobile phone network, the task was accomplished to the point that a functional system now exists. GPRS provides us with a relatively cheap, continuous connection to the Internet, and a web interface hides complexity from the user.

Introduction

An architecture has been defined [1] for building complex Supervisory Control And Data Acquisition (SCADA) applications utilising shared network infrastructure which communicates with simple distributed devices that monitor and act on the world. Cost is minimised by using devices with only minimal processing power and supporting these devices with network based services. This allows complex control and data acquisition applications to be implemented at lower cost than is possible using current techniques. Existing infrastructure supports communication between devices and supporting services using several methods provided by the GSM mobile phone network. It is envisaged other communication mediums particularly those that support TCP/IP will also be suitable for connecting devices to supporting services. Having designed an architecture and provided some network services a cheap, generic and reprogrammable device is required for interacting with the physical world. The device should conform to the following specification:

• Utilise General Packet Radio Service (GPRS) for communications. The existing infrastructure used other communication methods available through mobile phones. Using the caller ID of a phone ringing a bank of phone numbers is the most basic method of communication and currently the cheapest, as there is no communication cost if the called number is not answered. SMS text messages and circuit switched GSM calls where the mobile phone acts like a modem have also been employed. GPRS provides an ‘always on’ communications channel without incurring time based charges. Data is packet switched and charges are volume based. GPRS, costing about $25 per megabyte of data transferred , is expensive where large volumes of data are to be transferred but can cost no more than a few cents per day for the volumes of data that are required to be transmitted for many applications. GPRS enables a constant TCP/IP connection with the device (for monitoring, controlling or acquiring data) whilst avoiding the prohibitive costs associated with a permanent circuit switched connection.

• General purpose. The range of applications the infrastructure supports is broad, and rather than creating a purpose built device for each application, we aimed to create a generic device which could be reconfigured for any particular application as required. The work required to apply the device to a given application is limited to attaching analogue, digital or serial I/O to the spare I/O pins on the device, and programming the required logic into the device.

• Reprogrammable over the air. Rather than physically connecting the device to programming hardware (via a serial cable or any other method), it was decided that the device should be ‘over the air’ programmable in situ. This allows code to be easily updated without special equipment when physically separated from the device and where there are tens or even hundreds of devices that all need a similar code upgrade.

• Simple to use. Interacting with devices using a web browser provides a familiar interface and avoids the requirement for installing specialist software. Messages should be sent to the device through a web browser interface. It should be possible to write code, compile it, and upload it to a device utilising only a web browser. A networked database is used to archive all data arriving from the device and this too is accessible via a web browser or as an XML web service. Users are therefore able to control and monitor their devices programmatically using HTTP requests from within their own programs.

• Low Cost. The devices should be as low cost as possible ideally costing no more than a few dollars to produce in volume so as they can be deployed without adding significantly to the cost of appliances in which they are deployed. It is envisaged that these devices will be deployed in great numbers and the data acquisition and control architecture is aimed at providing as much of the functionality as possible with network based services. Device costs will become more important in future when cheaper communications technologies, for example blue tooth are employed.