garage remote jamming

I believe Jeff was describing DSSS (Direct Sequence Spread Spectrum) which does use only one carrier frequency but is still subject to jamming. See...

That looks like an excellent reference document.

Yes, GPS uses DSSS. While not immune to jamming, it does tolerate moderate power narrow-band signals within its bandpass. Its resistance to jamming is a function of the data rate. From the summary:

"If there are narrowband interferers of moderate level, then a DSSS system that will completely reject them may be designable. Should there be large interfering signals, then a DSSS link may completely fail while FSSS is likely to continue operating even though the interference is not complete rejected."

The difference is that a jammer may totally block of the discrete frequencies in the frequency hopping link, so that information is lost. The integrator in the DSSS link can reject the jammer until its energy dominates the summation.

Jeff

We're veering off into one of the semantic excursions that often obscure the much simpler, original question: Can a system like RadioRA be jammed by a transmitter of much greater power at the "same frequency?"

The system that Jeff's talking about, DSSS, is described at Wikipedia:

"This noise signal is a pseudorandom sequence of 1 and -1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band."

Band of what? (-:

"As this description suggests, a plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission."

Distribution of what? (-:

Certainly spread spectrum can performed in a variety of ways, and though these technologies obviously provide greater *resistance* to jamming than simple radio designs, nothing I've read so far suggests that any one of them is *immune* to jamming.

They seem best suited for rejecting the low power interference created by other, similar devices. DSSS seems especially adept at recovering very weak signals (like satellite transmissions) from a sea of *normal* radio noise. It also seems quite useful in providing synchronization information between multiple transmitters, a great asset to GPSs. But the military can jam civilian GPSs and they even admit to that capacity openly, along with the ability to keep military GPSs in operation while they do it.

"We would not create a global problem for transport out of spite for Saddam," says a spokesman at the US Department of Defense. However, he admitted that the US military does have the capability to jam civilian GPS signals regionally, and did not rule this option out.

. . .

"A GPS expert at the University of New Brunswick in Canada, Richard Langley says "the civilian GPS signal relies on a so-called "coarse/acquisition code" (c/a), which enables a receiver to determine the distance to the satellite. But the US military relies on the "precise code" (p). The p code is transmitted over a much wider bandwidth than the c/a code explains Langley: "So you can jam the narrower c/a code without jamming the wider signal."

Simple brute force would probably knock out a lot of gear right near the jamming source. A much larger radius could be achieved if the pseudorandom sequence of 1 and -1 values of DSSS or the frequency map of a frequency hopping system were known.

There have been a number of situations I know of where radio traffic shot up so much in such a short time that communications broke down nearly completely. 9/11 and the recent tidal wave come to mind. A lot of radio gear that's never active simultaneously otherwise comes on line and it all fails. I'm not sure what can be done to address those issues but a lot of problems resulted from the breakdown of communications at a time when communications were critical. So there's still room for progress in radio comm.

Of course, this will be more of an issue when HA switches all have embedded GPS receivers to detect sinkholes or landslides or other tectonic activities . . . (-:

-- Bobby G.

It depends on how expensive your kit is.

Assuming you have a suitably powerful* frequency agile transmitter then jamming problem is not easy.

You can either:

Jam every frequency the transmitter is using; If there are up to 20 frequencies you need 20 times as much power as jamming a single frequency for example.

Hop jamming frequencies when you detect that he has changed transmitting freqnecy, which means that he can transmit in the interval required for you to catch up. If the hopping is fast enough you may never catch up.

Attempt to guess which frequency he will use next. If this is essentially unguessable - usin a cryptographic RNG or sufficiently complex (and unrepeated) predetermined pattern for example - then it's very unlikley you can do this.

If you would like to put in an RFQ for an unjammable garage door opening device I'm sure we could come up with something - but it might not fit in your car. (I should point out that neither crypto nor jamming nor transmitters feature in my job and I am in no way connected to the people who work here who do that kind of thing, nor am I talking for my employer, myself, the church, state or any other body with sufficiently deep pockets to cause me to regret any statement I have made regardless of it's accuracy or otherwise.)

you don't have to do any of that, there are other ways, knowledge of a specific frequency is un-necessary, just the band.

While I can in no way vouch for the accuracy of this claim, while looking for something else (Uniden 5.8 phones) I did come across this thread:

audiblesolutions 10-07-06, 03:57 PM

A delurk AND a X-post to sci.crypt where I am out of my league, so I see your X-post, and raise you with a further X-post to a radio newsgroup . . . hmm . . are there really over 500 newsgroups with the word 'radio' in them? . . . hmmm. OK. We'll leave the radio folks out of this because I've answered the primary question in another message in this thread. You win this hand!

Ah yes, but in this case, the whole issue revolves around the asterisk. The question was whether LutronRA, a low power RF home automation switching system with a stated reliable operational radius of 30 feet, could be jammed by a much stronger signal. The miracle of spread spectrum, at least IMHO, is that it allows low-powered devices to communicate with each other as well as receive signals from great distances. The problem of the garage door jamming incidents is that it puts low-powered transceivers up against a transmitter probably powered by a military 500kVA truck-mounted generator set.

What happens to the signal if you jam 10 of those channels? Does the message still get through? What about 5 channels? It all depends, I assume, on the redundancy present in the transmitted message. What if you set up your jammer to randomly transmit on the 20 frequencies? Will that be as effective in jamming the signal as flip-flopping between two of 20 known frequencies?

Wouldn't you only need to jam as many channels as it would take to destroy the signal integrity of the tranmission? Bad static in a voice transmission might not be as a serious as data packets that are corrupted. Wouldn't a lot depend on the redundancy (if any) built into the distribution of the signal into different frequencies?

Of course, you may never have to if you manage to step on enough of the transmission often enough to prevent its reconstruction at the receiving end.

Agreed. But this sort of technology is not likely to be found in garage door openers. Those devices do use some pretty sophisticated rolling code techniques but it's to prevent bad guys from recording and playing back the opening signal and not to precent jamming.

(-: Already covered. Pan and tilt mount automatically-aligning laser sender with line of sight to the garage receiver. When your GPS senses you're close to home, the laser sender deploys and begins searching for the receiver. Nothing short of a supernova or a direct atomic blast should be able to interrupt the transmission.

Shucks, no one would *ever* sue anybody for something they posted on the net, would they?

I wonder exactly how strong the transmissions were that knocked out 100's of garage door openers and whether we're likely to be seeing more or less interference on these bands in the future.

-- Bobby G.

That's about equal parts puffery and technical and seems to be more their response to ZigBee than anything else. Their critique of "RF standards" and mesh networks would apply equally to Z-Wave.

They use 418MHz ASK/OOK which is also used by almost all of the IR extenders (e.g. Powermid) and most of the RF-capable high-end touchscreen remotes (e.g. Pronto). In Europe, all of the above as well as several RF based lights/appliance systems use 433.92MHz (i.e. their "B" frequency) and, if RadioRA sells in Europe they are also required to use 433.92MHz.

Any nearby ~418MHz transmitter will jam their "A" frequency" and any nearby ~434MHz transmitter will jam their "B" frequency.

In this specific case the military frequency was far enough away from 418MHz that it was not likely to affect any RadioRA but, in general, their "immunity" from interference is more a matter of miniscule market share than technical prowess.

For those who reacted by blaming the garage door makers, a glance at FCC frequency allocations shows that nearly the entire spectrum allowed for low-power unlicensed use under FCC Part 15 is also allocated for some type of licensed use. I doubt there's any easy way to determine what particular sub-band might avoid some future problem like this - it goes with the territory.

Interesting. Zigbee probably would be their main competitive concern. Proprietary protocol vendors always fear open system designs the most and with good reason. Let me count the proprietary schemes that have fallen over dead. Microchannel PC cards. Microsoft's HELP file format.

From what I could decipher they based their design decisions on that band being restricted to some very narrow "on time" broadcasting parameters, i.e. no continuous or automated traffic on that band as opposed to 2.4GHz's wireless phones, LANs and cameras. Lutron says they based their design on their analysis of the RF environment but that environment is already out of date if I recall their timeline correctly. There's really no telling if they're really future proof. There's more and more wireless equipment coming on line each day. Many millions of Homeland Security $'s have gone into the purchase of emergency radios and other goodies. Who knows what band they'll be transmitting on and for what? I think the only thing one can say about the RF spectrum 20 years from now is that it will be a lot noisier, all over the world.

Some poor radio astronomer out there probably knows precisely how much earth-generated RF is floating around and how much it increases each year. This site:

says:

"These tiny communication devices will in many cases be disposable items and are likely to give rise to a general increase in overall background r.f. signals"

So it seems to me the RF spectrum is going to get as nasty as home powerlines have become in the thirty years or so that X-10's been around.

RA's reliability may simply come from the conservative 30 foot radial distance rating of the RadioRA units. As RR pointed out, moving transmitter and receiver closer even slightly results in significantly more transmitter power reaching the receiver. Couple that power to the transmission speeds RA uses, and it's a vast improvement over X-10's PLC abilities. I'm assuming RA could try/retry a complex 100+ digit command string at least 100 times before either of us could blink. (-:

Probably. But I haven't seen many failure reports at all and they claim to have 1 million somethings sold (installations, clients, switches, something - sorry but I can't remember what the million things were but I was surprised it was that many!) Still, I wouldn't mind seeing a "shootout" at a trade show to see exactly how it stacks up against other transceivers in that band.

The experience with the door openers also indicates that if the military or the government owns the frequency (or perhaps just wants to borrow it for a while), low power users might not be on the "need to inform" list. (-: I'd expect to see more jamming incidents as the military configures itself to fight against IEDs for the next war, which, if military history is any indicator, will have nothing to actually *do* with IEDs. But we will be ready for it. Just like we were prepped to fight either a jungle war or a Baltic war in the 80's but ended up fighting in the desert. I just wish Congress could fast-track some troop and vehicle armor the way they do trade agreements. )-:

Lots and lots of gummint bands in dem dere lists!

What I'd really like to know is how Lutron's customer service dealt with the situation. What did it cost their NYC customers to switch to channel B? Were the units field upgradeable? Even better, could they be upgraded without being pulled? With radio modules as cheap as they are these days, what would it have cost them to design a switch with complete different fall back frequency, in other words, both A and B in the same switch with either a DIP or some other way to switch them? Perhaps even auto-hopping if the switch can't get through on the primary channel.

-- Bobby G.