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| GPRS and remote controls | ||||
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GPRS is a magic word much loved by people in marketing and seems to solve any problem. In fact it 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. |
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GPRS basics by Massimo Portolani
We get more and more requests about buying our Telecom Control Systems TCS boards enabled for GPRS. Of course we are working and testing it since a while and we know how to use it and what the advantages are. Unfortunately most of the information our customers have are not correct because it is not easy to get proper information in the mobile wonderland where everything is presented to the media in a simplified and sometimes exaggerated way. Being technical stuff, usually result is disaster. For this reason I have decided to write this simple GPRS primer, that provides the basics to somebody that doesn't know anything about it or only knows what can be found in daily newspapers or weekly magazines. This doesn't mean to be a high level technical explanation and I wish to apologize if some expressions are not perfectly stated. Exact information it is not easy to find and I am just writing what my experience is. Any feedback is welcome. 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. 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 most of the existing terminals (modem and phones) and network settings, you CAN'T send data faster. So a camera in GPRS mode won't send data any faster than in CSD standard data mode. Besides this you have to consider that as a matter of fact nowadays voice is more used than data in GSM networks and operators give a lot of privileges to it. It is unusual to have more than 1 channel available for GPRS in a cell and it must be split among GPRS devices attached. 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. Reattaching can take, and usually takes, a few seconds but can also take more than 30 seconds sometimes. Is GPRS cheaper? It depends on the amount of data you have to send. At the moment there are usually 2 GPRS price schemes (ask your operator for their particular offer): Let's make 2 examples with a small amount of data, 500 bytes (half a Kb), and with an amount of data corresponding to a standard picture, 60 kb, and see how much it would cost in CSD (standard data mode) GPRS with monthly fee and GPRS without monthly fee. We consider one of the most standard billing schemes at a low cost for SIMs belonging to the same contract. We assume that the cost is 5 cents upon answer plus 5 cents per minute. We also assume that the first minute is included in the first 5 cents (this is not always true, usually you pay 5+5 for the first minute). 500 bytes in CSD (standard data connection) would take less than 1 second to send but you have to place a call and establish a connection so you usually end up with 30 seconds plus the fixed cost that most operators charge upon answer, so you would pay 5 cents. 500 bytes in GPRS with monthly fee would be billed the minimum that is a Kb or 0,25 cents. Let's neglet the monthly fee component because we assume that you send enough data to dilute it. 500 bytes in GPRS without a monthly fee would be billed 2,5 cents. If you sent the 500 bytes as 4 SMS it would have costed even more. A standard SMS costs 10 cents, so 4 SMS, that would have carried 140x4=560 bytes would have costed 40 cents !!! Note: an SMS carries 140 bytes that are 160 7 bit characters (140*8/7=160). So if you send a small amount of data, GPRS is in fact convenient and in fact also much faster. 60 Kbytes in CSD would take about a little more than 1 minute, so it would cost 10 cents in the best condition. 60 Kbytes in GPRS with monthly fee would cost 60x0,25=15 cents, more than in CSD. 60 Kbytes in GPRS without monthy fee would cost 60x2,5=150 cents!!! you don't want to do that! Time is certainly a little more than 1 minute in CSD mode, in GPRS mode it will be that in the best possible condition. Usually it will be more. If you send a decent amount of data GPRS is not generally convenient for a remote control system. Of course the story is different for internet navigation. In that case you download a page and start reading it, while you read it you are billed for the connection time if you connect in CSD mode so it means that it if you don't want to disconnect and reconnect every page, it is usually more convenient to navigate in GPRS. Anyway we are not interested in Internet navigation or WAP navigation, I just wanted to make that clear for the sake of clarity and fairness to operators. Phone to Phone Usually you can call a TCS in data mode using a computer with a modem or a Nokia Communicator. This is convenient to change parameters and so on. 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 Server 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? First of all, the number 50 is the minimum that usually justifies setting up a server to handle them. If you have 10 machines to control, it is better to use SMS or CSD. 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. 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. TCS boards in GPRS are more expensive Of course they are a little more expensive because they have a PPP firmware that we have developed and we must repay the investment. GPRS modems are also more expensive nowadays, but this should change because of growing competition. Conclusions What is important to know is that handling a remote control in GPRS is not a trivial task, not from the terminal side and not from the Server side. If you buy a TCS and use it in SMS mode or standard data mode (CSD) you can be up and running in 5 minutes. If you want to set up something in GPRS mode it is a lot more complicated and either you buy or build a complete solution, or forget using it at all. This primer was meant to provide some basic facts and put you in the correct track. The message is: GPRS is very good for some purposes but it is not always necessarily good for remote devices management, we know very well how to handle devices in GPRS mode but it is not simple at all and it is not convenient for small projects because of the infrastructure start up cost. At this point any further question is welcome. |
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