Let's say I have a long (> 30m) LAN cable, that connect two (very close) devices.
Could some of the unused wire, disposed as a huge coil, cause any issue ? I'm thinking about the special wire layout that would create a magnetic field (like in an inductance) and cause problems for transmitting the signal.
I have done extensive testing at work with various cable and the answer is... possibly, but unlikely.
It really depends how tight the coil is - with Cat 6 - at the bend radius that it comes sold as in a box - no problems... but if you do any sort of tight bend, the chances are high that you could cause problems, but, with cat5e, you could tie it in knots and I doubt there would be any difference.
... Tested Cat 5e, 100 meters, 85 ish meters wrapped in a box, 15 meters up to this punishment:
and I saw no difference when compared to a shorter point to point cable.
Theoretically, yes. However I have never seen excessive coiled wire cause a problem. I dont think ethernet really puts out enough power to create a strong enough magnetic field to cause issues. Unshielded power cables have been known to cause problems.
Rolling up excess wire in a coil can greatly impact its electrical properties and cause interference by literally turning them into inductors and antennas. but you will not effect any significant loss in signal quality by putting away an ethernet cable like this., unless you are subjecting it to enough abuse for mechanical stress to become an issue. A common rule of thumb is to never bend it with a radius under four times the cable's diameter. The coil in your image does not look like it is anywhere near violating that.
When a single wire is rolled up, magnetic fields from whatever source induce currents, adding noise to the signal. This phenomenon also occurs in ethernet cables, but the latter has multiple wires inside, carrying currents in opposite directions. Pairs of wires are generally twisted so that 'on average', both are occupying the same physical space and are exposed to the same magnetic effects. The resulting forces cancel, very similar to someone pulling equally hard on both ends of a rope on a pulley.
Rolling up LAN cables like this is perfectly safe.
While in theory there can be a (small) problem, in practice I've never seen it, and others haven't reported a problem either. The fact that the cable is balanced twisted pair eliminates most inductive effects in the coil.
There is a limit, though, to the length of an individual run from hub to device -- 100 meters. Over that and timings get screwed up, even if the electrical signal is solid. So if someone were to attempt this with a much longer spool of wire there could be a problem.
Ok, I have tested this in the worst case scenario. I work for a company that tests and refurbs cable modems, gateways and routers. Our testers sre designed to test 12 units at a time. The cable were coiled up tightly out of the way. This caused units to show a false failure on lost packets. We used shorter ethernet cables to avoid coiling and the problem has been resolved. The longer cables were re-used elsewhere without issues.
A coil of wire, usually around a ferrite (iron) core is an electrical choke or high pass filter. I had exposure to these building crossovers for speakers. I also experienced a coil of 300 ohm antenna wire with no core effectively filtering out UHF TV channels. When the coil was removed, the channels returned.
This simple choke coil principal backs up Janes real world tests, a tight coil of wire can filter out high frequencies and drop packets. That is exactly what we send along Ethernet cables.
If you need to take up slack, don't coil Ethernet cables, layer back and forth.
Environment is the critical factor wherever noise/contamination/interference is suspect. Are there halogen lights or unshielded motors/heating elements etc in the vicinity?
Are you using a DSL connection that is susceptible to external interference?
Packet loss and errors are most often caused by the environment, not just by faulty hardware. A coiled wire is basically a magnet for problems (pardon the pun).
Also, Is the UPS close by? not good, Is there an unshielded radio or speaker? Not good. Older 2.4ghz cordless phones? Not good. Flickering street light? Not Good.
To find if your environment is a potential influence for your coiled wire or DSL setup. Take an old AM Radio, tune it to the high end around (1600) and move around your area. If you hear feedback, that is RF and a cause for concern.
