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TUCoPS :: Hardware Hacks :: ccurrent.txt

Surveillance with Carrier Current Devices




                              Carrier Current

*Profile

Carrier current transmitters are devices that transmit sound over powered
lines (usually electrical mains) by modulating an independant carrier
frequency (usually AM, though it presents a noisy signal. FM is more
expensive but clearer) with an audio signal and impressing it onto an
electrical line. The signal is sent down the line to be picked up directly
off the line by a specially designed receiver, or by a receiver very close
to the line.

Carrier current surveillance devices can manifest themselves as re-tasked
consumer goods such as telephone jacks and baby monitors. If you leaf
through enough gadget catalogs, you're bound to see a carrier current
transmitter marketed as some magic product that "lets you turn any power
outlet in your home into a phone jack!". Covert carrier current
transmitters will almost exclusively be hidden in plug-in devices, such as
lamps or surge protectors.

The devices also present a dangerous native threat. What the catalog fails
to tell you is that everyone else served by the same pole transformer
(usually everyone else on the block) can turn their power outlet into your
phone jack too. Some carrier current products (usually wireless intercoms)
have several variations on the same model operating at different
frequencies in order to combat the 'crossed wires' effect.

Carrier current devices suffer from a major flaw, in that they are usually
stopped dead by powerline transformers. Bridging the hot leads with a 0.1
microfarad 240VAC capacitor will allow carrier-current transmissions to
bridge the transformer. Alternatively this can sometimes be overcome
through the use of a passive coupler (also called a signal bridge), though
a repeater would be a better option.

Carrier-current devices can be remotely controlled via an X-10 link,
frustrating detection.

*Detection

Carrier current devices can be detected as standard wire line devices with
a TDR, oscilloscope, or with a carrier current receiver (which can be
purchased from <http://www.martykaiser.com>).

Connecting a wet line TDR to the powerline with fused leads will show any
buildouts or undocumented loads on the line. Take care to isolate straight
sections of the circuit, or T-joints will present an overly complex
waveform.

Using an oscilloscope to test for carrier current devices calls for
connecting the scope to the line with fused leads and checking for any RF
on the line other than the 60 hz sine wave thats present on all power
lines.

While there's been some discussion of using commercially available carrier
current receivers (from carrier current telephone extensions or intercoms)
as fill-ins for a good receiver this isn't the best of ideas. Carrier
current devices can transmit on any frequency below 500 kHz (150-300 kHz
being VERY popular) and in one of several modulation formats, while
commercial receivers are generally tuned to only a small fraction of the
available band and using one modulation scheme. An electrical tester
designed to test for EMR noise on power lines would work as a detector, but
could easily be triggered by RF on the line. A tunable carrier-curent
receiver makes the best detection method.

Physical searches for carrier current devices should include removing the
covers of all power outlets, and the disassembly of anything that runs on
AC power.

*Countermeasures

The most obvious way to prevent native carrier current threats is to
prevent friendly threats. Don't use carrier current telephones or
intercoms. Audio jamming will prevent the microphones on CC surveillance
devices from picking up conversations. A good surge protector or power line
conditioner  will incorporate a filter that will block carrier current
emissions. Make sure that the power line conditioner or surge protector you
select blocks RFI down to at least 1000 Hz. Its possible to use blocking
capacitors or low-pass filters (blocking above 60 Hz) on AC power lines in
order to negate carrier current devices, but certainly not recomended for
anyone lacking in electrical knowlege.



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