On 4/16/08, alan smith wrote: > Well...I have this nice RF links folder that I tried to save off all my RF link and related stuff but nothing about GSM modules. > > Client now wants to replace his RF link with a GSM module, and do it cheap (who da thunk that?). So, has anyone done a rather inexpensive design around one? This is a US based design, so only need to do CDMA (I think thats the US standard). So the data being sent is pretty small, essentially a control link sending and getting very small amounts of data. Presently its a 9600baud serial link, so like to stick with that as well. In the US we have both CDMA and GSM networks. Verizon and Sprint use CDMA. AT&T (was Cingular) and TMobile use GSM. One major advantage this sort of project has with GSM is ease of access to the network - you can walk into a TMobile store, buy a data plan with a phone, and then take the SIM card from that phone and use it with most GSM cellular modems that are US compatible. Be aware that GSM frequencies differ from country to country, though, so make sure your modem is compatible. CDMA is more difficult to deal with, but there are companies that provide the modems and the service. A bit more expensive, though. Most cellular modems will allow you to use them as modem to modem digital links, but they'll consume talk time. It's cheaper to use the packet interface with a data plan, but you may have your traffic going over the internet. Most modems also have a built-in TCP/IP stack, so you can open a TCP link between the two modems fairly easily and then chat as you need to. The cost of the modem, however, is eclipsed by the cost of the service. Unlimited data is usually $50/mo, with cheaper plans for a fixed amount of data per month (although you can work out a deal with a carrier to get better bulk rates). Tmobile's pre-pay system costs $1 per day for unlimited data, but I don't know how difficult it is to set that up without using their phones. -Adam -- EARTH DAY 2008 Tuesday April 22 Save Money * Save Oil * Save Lives * Save the Planet http://www.driveslowly.org -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist