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I know it depends on a lot of things. For example if we are talking about fiber optics, pretty much nothing can interfere with them.

But let's talk about common things like Cat5e/Cat6 cables. Do you need to be concerned if you are running multiple 120 volt lines in near your Ethernet lines?

EDIT

Nevermind for other cables, they are limited in length and number anyways.

Are some cables more sensitive than others? I would guess DVI is far more susceptible to interference. What about USB, firewire, eSATA, DisplayPort, HDMI?

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3 Answers 3

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All digital signals are subject to noise. The noise can be from power cables, florescent lights, wireless telephones, and so on. Most protocols such as TCP/IP have various methods like CRC check sums to find errors and either correct them or have that packet of data resent.

To minimize the problems and keep retransmissions to a minimum you can do several things. First use quality cables. CAT 6 is better then CAT5 which is better then simple twisted pair. Keep cables runs as shorts as possible. Longer cables will capture more interference (that is simplified but basically true), avoid power cables. A short section of cable next to a power line won't cause big problems, but don't run both through a long conduit right next to each other.

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I have heard (but never seen any hard data) that you should not run Ethernet cables parallel to high voltage power for more that a couple feet.

I also would imagine that the other cable types you listed would be much more susceptible to interference, since they can't even travel very long distances (USB cables, for instance, are not supposed to be longer than 6 feet).

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Well there are quite a few factors involved in determining the level of interference that will affect a digital signal. Specifically interference occurs at a particular intensity which is usually measured in decibels, the frequency of the interference often dictates what it will and will not interfere with, the amount of background noise not associated with the interference and lastly there are variances in specific materials and configurations being vulnerable to interference.

AC lines can indeed mess with your cat5 cables and you should avoid trying to run them parallel to to your data cables. Generally you won't have much problems with short cables as there is very little attenuation and the signal is bright and crisp. Cat5 cables are optimized to eliminate/reduce crosstalk between the pairs so this makes it far easier to recover a signal at the other end. The longer an AC power line is close to a cat5 cable will increase the amount of interference that is induced in the wire. This induced interference needs to overpower the number of decibels of signal in the cable. Longer cables are more susceptible to interference as the signal is weaker at the end of a long cable run.

Generally speaking interference is introduced into some circuits through passive inductance which is just a fancy name for crosstalk.

Another thing to consider is that a balanced cable is much less likely to experience disruptive interference as both sides of the cable are more or less grounded to bleed off any extra signals via unused wires.

something to consider, you can interfere with any cable if you have the proper intensity, frequency and/or proximity to induce the interference.

EMP is a good example of universal interference or check out a spark gap generator to see what a nasty multi-spectrum noise source looks like.

I could go on and on but for the sake of brevity I will close with a link to more info...

Wikipedia article on Wave Propagation

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