Re: garage remote jamming

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

"Richard Revis" <richard.revis@xxxxxxxxxxxxxxxxxxx> wrote in message
news:45f13815$1_1@xxxxxxxxxxxxxxxxxxxxxxxxx
> "Robert Green" <ROBERT_GREEN1963@xxxxxxxxx> wrote in message
> news:WoSdnfla97CuGXXYnZ2dnUVZ_tLinZ2d@xxxxxxxxxx
>
> > Would it work if more than one of those frequencies were deliberately
> > jammed?
>
> <delurk>

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!

> It depends on how expensive your kit is.
>
> Assuming you have a suitably powerful* frequency agile transmitter then
> jamming problem is not easy.

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.

> 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.

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?

> 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.

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.

> 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.

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.

> 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.

(-:  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.

> (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.)

Shucks, no one would *ever* sue anybody for something they posted on the
net, would they?  <deep sarcasm!>

> *In relationship to the reciever, bear in mind the power square law, you
> don't need megawatts to get a high recieved power at 5 feet compared to a
> transmitter at 5 miles.

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.