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GPRS and remote controls GPRS can provide very powerful solutions not obtainable in any other way. The drawback is that it can't always be used for small projects because of the need of a central server, speed is not necessarily faster than a standard data call and sometimes it is not even cheaper. Read details below. GPRS basics by Massimo Portolani (2002) GPRS stands for General Packet Radio Service and it means exactly that. Data sent through a GPRS connected terminal (a telephone or a Modem) are sent as packets, like in an IP network, like the Internet. The GPRS capability is an addition to the traditional GSM services, like voice and SMS. Voice is sent in the usual mode and not in GPRS mode, it has nothing to do with it. GPRS is just a different way of transmitting data. The traditional way of transmitting data is CSD (Circuit Switched Data) and some terminals and some providers also provide HSCSD (High Speed Circuit Switched Data), that is a multi channel connection, unfortunately not available in most places. In GPRS enabled terminals (a phone or a modem) the standard data mode (CSD) is still available as an alternative to GPRS data transmission. What is the difference between standard CSD and Packet data? In circuit switched mode the terminal opens a connection and as long as it is connected there is a circuit (a line) available for it and only for it. If you send data or not, you are billed for the time you use the line, exactly same thing that happens for the voice. In packet mode, your data are sent as packets, and you are billed according to quantity of data sent. Somebody refers to GPRS as faster, constantly ON and cheaper, most people think this is true in all conditions. In fact it is not always true. Is GPRS faster? The usual speed in CSD (standard data mode) is 9600 baud or bits per seconds that means more or less 900 Bytes per seconds. This is theoretical, because of network checking protocols in fact the speed can be lower, expecially if quality of signal is not good. In GPRS mode, operators usually reserve at least 1 channel in a cell for GPRS data and this means that if you are the only one connected you will have a full channel that have more or less the same capability as a CSD channel. If the operator has reserved only one channel and there are 2 customers attached to that cell in GPRS mode, you actually end up having half CSD bandwith, that means you are sending data at half the speed of a standard data connection. If there are 10 people attached you are sending data at 1/10th of the traditional data speed. Of course, as the networks improve, more and more channels will be reserved for GPRS. Besides this there is an asymmetrical speed for most of the existing terminals and usually they allow 1 channel in upload (sending data from the terminal) and 4 in download (receiving data to the terminal). This is because the idea they had in mind when writing the specifications was that you need to receive data faster than you send them, in a fashion similar to a http internet connection. You type an address and make a short request and receive a full page with images and text. This was what they thought (and how it works now) but in fact it could not be ideal in most applications. So in theory it is possible to receive data faster (up to 4 times) but even in theory, with many of the existing terminals (modem and phones) and network settings, you CAN'T send data faster. Is GPRS constantly ON? This means that you don't have to place a data call, you attach once and then you can send data. This saves time when sending small amounts of data. As a matter of fact anyway, even in GPRS sometimes the network disconnects and the terminal must re-attach, and this takes a few seconds. Is GPRS cheaper? This depends on the type of contract you subscribe. If you can get a flat rate and stay within it, of course it is usually cheap. If you pay an amount each Kbyte, it can be more expensive. If yo uhave to transmit 1 Kbyte and they bill you minim 100Kbytes every time you attach, it is easy to exceed the allowed flat quota. Every operator has different offers. We recommend that you consider them well because, as usual, not all things that shine are gold. Phone to Phone Usually you can control a GSM remote controller calling or sending an SMS to a number. In GPRS mode this is not possible. It is not possible to call a phone or modem in GPRS mode. The phone or modem must start the connection in the first place and must ask for data that you will send. Besides this, the phone/modem cannot call another phone or modem directly. In GPRS mode it will always attach to a IP network and you must have a computer that answers to its requests. A GPRS terminal, either phone or modem, must connect to a IP network and send and receive packets of data through that network. Remote monitoring scenario in GPRS mode Let's say that you have 50 machines equipped with our boards and you want to control them in GPRS mode. How does it works? You will install the units in the machines and then you will set up a server that will communicate with them through packets of data, usually UDP packets. There is no ready software to do it so you have to be capable of writing it, ask us to do it or find somebody that does it for you. Not a trivial task. We will be glad to provide an offer for the complete package including the server software, once we are supplied the necessary specifications. Machines will attach in GPRS mode and send packets now and then and the Server will react to them answering or sending alarms to you or to any of your people, using SMS or e-mail in most cases. A well designed system must take in consideration that GPRS could be down or not working in some areas, so the system should be designed to use also SMS in that case, as an alternative to GPRS connection for small amounts of data (or CSD for big amounts). If the server wants to query a machine usually it has to wait for the machine to send a packet of data first or it has to wake up the machine sending an SMS or placing a call. There are more technical details that are involved, as for example the IP address of the terminal that is assigned in dynamic mode and that will change and other stuff, but we would go to deeper in technical details. There are lower cost solutions, that use the Internet to send data from the GSM operator GPRS designated web gateway to the control server, that can be in your factory but also at an ASP (application service provider), that could be us for example. There are more sophisticated aand more expensive solutions, that don't use the public net but in fact are a VPN (Virtual Private Network), in which server is attached to a special firewall of the operators network through a physical link like a CDN. IN this way mobile GPRS terminals are seen as a part of the customer private network and it is even possible to decide which SIMs can access the server, for a higher security. Conclusions GPRS provides a very good solution for handling remote monitoring of devices or for acquiring parameters from a weather station. It is only important to consider the basic facts and make sure they fit in your requirement and in your budget.

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