Spread spectrum radio modems

Hi all, I wanted to find more about the 'Pros and Cons' of using 2.4 GHz Frequency hopping spread spectrum radio modems. One of the established Telemetry
equipment vendors (in Australia) suggested that spread spectrum modems are suitable only for short range applications (e.g., Tank farm) and are ill-suited for a robust Telemetry network spanning up to 10 Kms (radius). In fact the current network we would like to install can be better described as 'Remote IO networks connected by radio modems' rather than the 'TRUE' telemetry network.
I also know that the 2.4 GHz spread spectrum radio modems are relatively a new product (< 2 years on the market) as compared to the licensed 400 MHz (2400 BPS) radios.
I would like to hear from people who have designed, installed and commissioned wireless / telemetry networks about the pitfalls to avoid when using the 2.4 GHz modems. Thanks for sharing your experience in advance. Regards, Raj Sreenevasan
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Hi Raj. I haven't installed/commissioned one of these new systems (yet!) but I *do* know that there are two basic limitations over the 400MHz ones:
1) They are very much line-of-sight, antenna-to-antenna. Usually this just means an external antenna somewhere mounted high enough to pick up *all* the others. Repeaters are available though.
2) They are low power - and have to be to keep to the licensing regs. The frequency-hopping means reasonably high data rates are possible with fairly low signal strength, but once you hit the noise floor you won't get much data back at all (if any).
FWIW, I doubt you could use them in a Tank Farm - there would be too much metal around. In any case, you need to start with a Radio Survey of your site *first* and *then* go ahead and buy/install/commission with confidence.
I hope this helps, Cameron:-)
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Cameron Dorrough wrote:

900MHZ will give you greater distances that 2.4GHz, all things being equal. You certainly can use hi_gain (yaggi) antennae with 2.4 GHz radios to get greater distances. Do not underestimate the hassle/costs of maintaining tall antennae, due to lightning, wind, vandalism, insurance and hackers. Any of the unlicensed bands will have lots of communications hackers perusing your data, especially if you are in a populated areas or an 'industrial' country. Analyze the sensitivity of your data. If it is sensitive, then make sure you include robust encryption (Forget WEP or anything done in hardware) that is not cracked or crackable, if your data sensitivity requirements warranty. When you use encryption, your data payload will bloat in size, often to the point that FHSS does not provide sufficient bandwidth. Then you have to got to a DSSS spread_spectrum scheme.
I have not see a 400MHz that is not trivial to comprimise, if security is required. If it's not required, then any frequency will work. If a vendor is trying to sell you equipment or a system, without robust engineering analysis of your operation and operational needs, then you need to use a *CONTRACT* and have the vendor play integrator with guarantees.
Too often, I've had to 'reimplement' wireless networks, because inexperience persons tried to build something from vendor catalogs. Caveat Emptor!
James
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