Freqs and volts

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Rock

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Hey people, long time no see, eh (granted I usually hang out in the astronomy section, but I think it still applies)? Sorry I've been away so long, I've had quite a bit to do.

Now to the grist of it:

Assume a hypothetical situation for a moment. Allow that we have in our hands a device which can transceive in the range of ~300 - ~3,400 hz, the frequencies at which telephones operate, approximately.

Now, assuming that one has appropriate software, compatible with the language in which information is encoded therein, is there any reason that one could not, in principle, receive or transmit anything to or from said wires?

Is there a modulation factor to be taken into account? What I'm looking at is a question of EM emanations and signatures vs. imprinting signals. Any ideas?

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Este est percepi - to be is to be perceived.
:) -R.
 
i have absolutly no idea what your talking about but i would love to see the result.

tell us how whatever your trying to do goes.

krusher
 
Rock,

Not quite sure about it, but I believe there will be a bit of a problem receiving from/transmitting to telephone wires. This is because telephone signal is carried on twisted pairs, which are two strands of wire wound around each other in a DNA-like helix. This creates destructive interference in any signal that attempts to leave the wires (hence they are poor transmitters), and the same happens to any signal that attempts to enter the wires (hence they are very poor antennas, which is why they are twisted in the first place -- to reduce noise).

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I am; therefore I think.
 
Krusher - Yes, I'll be glad to keep you posted. Thanks for the encouraging commentary. To translate what I'd posted: I was looking at err... picking up emanations from communications systems and the ability to input signals onto hard lines remotely.

Boris - I'd heard about the wire twist interference problem, but I've got a hunch that provided that the signal is amplified adequately, the translation should pose the only real problem. I figure it this way: granted an individual device or LAN may be shielded, but unless it's an island, it can be accessed, in principle.

This means that one should be able to not only download (access) info from said hardline without a hard wired connection, but that one should logically also be able to upload (imprint signals) onto said hard line given an adequate signature library (this assumes that signatures imprinted upon a line result in the originating signal, if not, then a parent library should be engineerable if required).

If one runs a test signal through a sample line, then one should be able to record and play it back in such a way that the resulting playback signal either positively results in an identical line signal or negatively results in a different line signal. In either case one can then continue with the experiment.

Any idea what modulation is used in telephony? (am, fm, pm, ssb, cw, etc.?)

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Este est percepi - to be is to be perceived.
:) -R.
 
Rock,

The phone system operates on constant voltage, I know that much -- so their signals cannot be am. In fact, given that they operate in a frequency band, I'd say that it's a safe guess to say they use fm.

However, I still think you are going to have huge problems with any scheme of the type you propose. The signals coming off a telephone line are extremely weak and span an entire band of frequencies, so you will have unending fun trying to separate them from the environmental radio background. Not only that, but since only a miniscule portion of an incoming signal actually makes it onto the line, you would have to pound it with a radio signal of insane intensity just to replicate an on-line signal equivalent to a native signal in power.

May I ask why you are interested in this stuff? (Don't worry, I'm not some government spook :))

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I am; therefore I think.
 
Rock,
Are you trying to capture the info from the phone system or merely use it as medium for transfer of your own signals???

As far as I know telephone (anolog lines) use FDM modulation and an ALOHA type access scheme, digital would use a form of CDM/CDMA. You can seperate all the frequencies you like very easily via a FFT device (fast fourier transform)and a DAC. I believe these devices can be found in off the shelf packages, which you then need to bias to operate in the range you need.

The twisted pair scenario is not a problem, infact i had a VERY HIGH bandwidth conection on a twisted pair PAL line. Its normaly used to hard wire a security system but we used it to connect to our ISP, which was accross the road from the office. I got about 10Mb connection. Unfortunatley the council found the line and made us remove it......There is a new digital multiplexed system (can't remember the name) that alows a similar conection with a couple of multiplexers on either end of the line.

BTW bandwidth is a serious consideration. If you want to stay analog then you will only get 28.8Kbits/sec. That is max for a standard analog copper line. It really depends on what you are trying to do.

Could you give us a clearer idea of what you are trying to achieve.
 
Krusher -

Yes, in a sense it is exactly that: a souped up phone tap/phreak design. Somewhat more in depth, but quite a bit more legal (at least, I'm unaware of any laws prohibiting research into this specific line of endeavor).

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Boris -

Yes, well I had originally worried about the signal intensity issue somewhat but then I realised that radar runs many orders of magnitude difference between the emitted signal strength and the strength of the return signal. I imagine that if the sensitivity of rather large devices (sevaral square feet, for example) is sufficient to pick up what I estimate to be around milli-microvolt range pings, then small (laptop or palm) devices have a chance for ground line reception.

As for transmission, if there is a line somewhere, but the exact location is unknown, yes that would pose a power level problem. If on either knows the location of or through whatever means (I don't know, let's say radar?) can locate a line, then the matter is quite different. In the latter case one need only beam an appropriate energy level required for that cross section, at a density great enough to force cross talk. At least, that's the hypothesis I'm proposing to operate under.

And don't worry, I'm not concerned over the possibility of your being a spook; although possible, certainly, it's not a danger since to the best of my knowledge I'm enquiring after legally attainable public information for legitimate research purposes. But thanks for putting any qualms of mine (or anyone else) to rest. :)

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Rambler -

Well, capturing signals from a monitor is illegal (certainly if they're not yours and have no court wire-tap orders), so I imagine that doing something similar with public equipment and signals would be similarly illegal (theft of service, or wiretapping, any number of possibilities), or at very least could set precedent... so no, I'm not looking to do that precisely. If my feedbacks look promising, then I will likely continue on to a physical prototype and experimentation on my own equipment, signals, etc., in order to prove the principle sound.

My main drive here is that I conceive a potential way in which one could log on and off any network (bad day for security unfortunately, but terrific for commuters, vacationers, go-getters, etc.). Familiar with Tempest? This would essentially be the flipside of said technology.

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Este est percepi - to be is to be perceived.
:) -R.


[This message has been edited by Rock (edited May 11, 2000).]

[This message has been edited by Rock (edited May 11, 2000).]
 
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