GPRS made as simple as ABC

Nick Robins, Technical Director, Alpha Micro Components, is in the know about wireless communication, especially when it comes to embedding GPRS functionality into electronic designs.

With the future of communication between electronic equipment becoming more and more reliant on remote access, GPRS is the technology of the future.

However the challenges that engineers face when working within this environment has held the development of GPRS-enabled products back.

This article outlines the advances made from the traditional fixed PSTN solutions, examines the limitations of the GSM network before considering the benefits that GPRS can offer for mobile applications.

Finally it provides an overview of what Alpha Micro has done to make it easier for engineers to design GPRS into the architecture of their electronic devices.

Everybody is talking about GPRS, what it can already do and what it might do in the future, and yet there are still difficulties in the electronics world in how to upgrade and incorporate GPRS functionality into devices.

Currently design engineers are finding more and more applications for adding remote signalling to field based embedded devices.

In the past, connection to these remote systems was made over the PSTN (public switched telephone network), but this led to fixed solutions that were only suitable for applications that were themselves fixed.

A typical application would be the monitoring of water or sewage-pumping stations, where the station does not move.

However, even this application requires that a PSTN landline be situated near enough for a pair of wires to be brought in, alongside some sort of mains electrical source.

Although offering the obvious benefit of being wireless, GSM data modems continue to behave in a very similar manner to their dial-up counterparts: once a connection has been established the circuit is maintained between the two ends of the link allowing data to be transferred simply by sending asynchronous data serially with no thought for protocols.

It is fair to say that this "circuit switched" approach satisfied the needs for a simple solution, but at the same time could be very wasteful in terms of bandwidth usage: once established, a connection must be kept available even when no data are being transmitted.

This also meant that the telephone link was paid for on a per-second basis (back in the days when GSM billing was paid for by rounding call times up to the next minute integer).

GPRS uses a different approach to GSM for data transfer in that it relies on a "packet switched" architecture, which is very similar to the way data are transferred daily over the worldwide web.

There is no fixed link between two modem end points, rather a "virtual" circuit is present to transfer data.

This "virtual" circuit is free for others to use during periods of inactivity.

The bonus for a user is much lower running costs, as when no data are being sent the bill for the current data call is not incrementing.

The connection may be left open for a significant time period without sending any data, resulting in no extra costs to the end user.

This is perfect for a device such as an electronic point of sale terminal, which is used regularly and for small data transactions.

However everything has a down side, and in the case of GPRS it is the fact that sending data over the network relies on each data byte being formatted into an organised series of packets that must be sent to the radio modem ready for transmission.

There are two main protocol stacks in which packets of information can be sent through when using GPRS: UDP and TCP.

Protocol stacks refer to layers of software that read the packets of information being sent and make sense of it.

User Datagram Protocol (UDP) is based on the "fire and forget" algorithm, where information is sent, but it cannot be guaranteed in what order the packets are sent or if indeed it will get there at all.

It is a faster and cheaper way to send information, at the same time being user friendly from a designer point of view.

The other, more accurate stack in which packets of information can be sent is via Transmission Control Protocol running over Internet Protocol (TCP/IP).

TCP contains error correction algorithms, ensuring that data arrives in full.

It is not ideally suited to GPRS because GPRS stops and starts and is subjected to delay.

TCP will automatically try and correct the delay, slowing down the whole communication process.

The demand and importance for accuracy within many industries, such as financial and retail meaning that TCP/IP is often the only realistic solution TCP running over IP and sitting on a PPP base is the most common set up.

To run these protocols on top of each other successfully is not a trivial matter, and most "stacks" that claim to be "full TCP" do not take into account the multitude of failure mechanisms possible on radio networks, and the rescue algorithms that should be added for smooth system running and recovery.

Although GPRS can offer substantial end-user benefits, the challenges of working with TCP within the GPRS environment has put many OEMs and system builders off the idea of integrating it into their product lines.

To develop a stable and efficient software stack for an embedded system can take many years of development.

To solve this problem, and make GPRS more accessible, Alpha Micro has designed a series of GPRS modem modules that are as easy to use as traditional dial-up devices; yet at the same time allow the embedded system design engineer plenty of flexibility within such a system.

The modem modules are supplied complete with all the protocol stacks required to communicate via GPRS, installed and running royalty free.

Alpha Micro has developed its own TCP stack specifically for GPRS.

We started from the ground up, placing layers of software and embedding the TCP, IP and PPP stacks.

This took around 18 months to develop but now we have a complete device.

Embedding this into GPRS modems is substantially easier and can be done in a matter of weeks.

This is a definite bonus for those wanting to use GPRS but who are concerned about the difficulties of embedding it into their current systems.

With Alpha Micro's TCP stack, a simple tweak of a company's receiving server may be all that is needed to start getting information over the Internet.

Alpha Micro's new GPRS modem modules are based around radio engines from the French company Sagem.

Sagem is already well known for its advanced capabilities in designing GSM/GPRS radio modem engines for a number of major mobile telephone manufacturers.

Combining Alpha Micro's and Sagem's expertise has enabled the new GPRS modem to be set up with a few simple "AT" type commands, resulting in the appearance of a dial-up modem link.

The embedded design engineer hardly needs to possess any knowledge of the GPRS or GSM data network in order to make successful connections into the Alpha Micro test server.

There is even an easy-to-follow setup guide and opportunities for "playing" with a development board to become "expert" in no time.

For designers looking for accurate global position (in addition to a datapath), Alpha Micro has an easy-to-use solution here too.

The same module footprint can also support the GPS (Global Positioning System) function with a receiver engine from the Swiss company, u-Blox.

The GPS fixes produced are accurate to within a few metres, and this may be made even more accurate with the addition of external filtering or statistical analysis.

All Alpha Micro modems have onboard switched mode power supplies that allow for a wide range of input voltages, and there is also a series of aluminium-cased units with RS232 level serial signals suitable for automotive applications.

The encased unit is already being used in a tracking application where the modem collects GPS data, formats these together with an identity and ancillary control information, then delivers them to a nominated server at requested time intervals.

In this application the end user does not have to write additional embedded code in the mobile unit as the whole tracking application runs on the Alpha Micro modem itself.

Switch on your vehicle ignition anywhere and you can be tracked, switch off, and this is also signalled to the server.

GPRS seems to be the way forward, for many applications, including machine-to-machine communication.

It is already being used and is making an impact; the only thing holding design engineers back from using GPRS more is fear of the unknown.

Alpha Micro has used its in-house design capabilities to ease fears and even take them away by presenting engineers with a range of simple "drop-in" modem solutions, allowing GPRS communication functionality to be a straight-forward addition to any device.

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