5 ms·

I've always wondered, from people more knowledgeable than I -- Any advice on bundling cable runs, re: crosstalk? Does it actually cause issues? For either elect

by ethbr0 4y ago

I've always wondered, from people more knowledgeable than I -- Any advice on bundling cable runs, re: crosstalk? Does it actually cause issues? For either electrical or data? Know it's less on a concern with fully-shielded and -terminated (Cat6a+?), so this may be a somewhat historical question. Interested in theory and/or real-world practice and results. Links welcome too!

bayindirh 4y ago
Gigabit Ethernet doesn't care much if you have a good quality Cat5E+ cable. Doing this a lot in the data center for a long time. However you don't want high power AC cables going alongside with them. Just carry them separately, or keep power source local to target. Similarly, other data cables (Infiniband, SAS, etc.) doesn't care either, but they're both short length when used in copper form and they have ample shielding.
crest 4y ago
Lacing cables together isn't worse than running them parallel using other means e.g. velcro loops unless you're lacing them tight enough to crush the cables.
- ethbr0 4y ago
  From memories of electromag, field coupling from current in parallel cables decays with 2*pi*r, no? So it seems there would be a fair amount of difference at extremely low separation distances. But we never worked through high frequency EE derivations, which I assume are more average-current- or capacitance-dominated in terms of effects?
  - SAI_Peregrinus 4y ago
    The distance between cables doesn't vary much between lacing methods. The balanced nature of the signals also means the common mode rejection ratio is very high, so external EM interference (which almost entirely shows up as common mode) isn't a huge concern.
brk 4y ago
On a decent quality cable you are unlikely to run into any scenarios in a typical residential or small business install where noise and interference causes a degradation in throughput. The balanced signalling used in twisted pair communications is very robust these days, and then you have various protocols on top of that which can handle small errors natively (eg: TCP retransmissions). It is highly recommended, and often required by code, to keep low voltage and high voltage (high voltage being 120V AC power lines, which really aren't that "high" of a voltage) separated physically. This can be air separation, keep them several inches apart, or conduit separation (don't ever run them in the same conduit). This is mostly a safety issue in the case of insulation breakdown to keep any high voltage leakage from entering the low voltage cable and equipment. Similarly, things like minimum bend radius, and untwist amount at termination can often be wildly violated with no ill effects. But, it's still best to not temp fate if not needed. Even max cable length can often be exceeded by 20% or more.
- jaclaz 4y ago
  >It is highly recommended, and often required by code, to keep low voltage and high voltage (high voltage being 120V AC power lines, which really aren't that "high" of a voltage) separated physically. It depends, there are types of data cables certified (insulation above 400 V) to be installed in same conduit as mains wires: https://news.ycombinator.com/item?id=34725851 https://news.ycombinator.com/item?id=34725851
  - brk 4y ago
    Until you wind up with an AHJ who doesn’t care and requires you to separate the cables.
    - jaclaz 4y ago
      Well, that can happen about almost anything, having data and mains cables separated is anyway a good idea, but (with the correct cables) is not (anymore) against code in many situations.
      - 4y ago
bluGill 4y ago
If doing long runs just run fibre. gigabit Ethernet is about as fast as copper can get - yes 10gb Ethernet exists, but it is power hungry (one reason 2.5gigabit starting to appear). You can send a lot more data with fibre, there are no cross talk issues, and it is cheaper than copper. Transceivers are still expensive though, so copper is best for shorter runs (within your house distances), but still everyone who might run some cable in the future should learn about fibre and how to work with it.
- ethbr0 4y ago
  I was looking at it. What's the current best home-hobbyist option for terminating? Last I checked, it looked like it made more sense to just buy pre-terminated, built-to-length cables.
  - bluGill 4y ago
    That is one of the downsides of fiber: you need to figure out your distances before hand and order the right length. One more reason to use copper in the house most of the time. Maybe some ~2ft lengths of fiber for use on your server rack where you need speed between the server and the switch.
- xattt 4y ago
  Is it still worth doing Cat 8 for stringing Ethernet in a new build, or is it okay to cheap out and just go with Cat 6a?
  - crote 4y ago
    Cat6a can do 10GB over 100 meters, cat8 can do 40GB over 30 meters. Current consumer hardware is starting to support 2.5G over 100 meter of cat5, and we miiiight see 5G over 100 meters of cat6 become common in a few years. If you're that worried about upgrade-ability, better stick to fiber.
    - toast0 4y ago
      Note that the specs are 100 meters in a dense conduit. You can probably manage higher than rated speeds in a household environment which likely doesn't have dense conduit nor 100 meter runs. If you're wiring from scratch, sure put the best you can afford, but if you've got cat3 in your walls already, see what runs, your NICs aren't going to look at the label on the insulation. (For better or worse, ethernet speed negotation runs at 1Mbps and the specs don't contemplate testing conditions and reducing speeds until you get to the multi-gig (2.5/5/10g) equipment. Some equipment will drop to 100M in drivers though)
JKCalhoun 4y ago
As a foil, people building point-to-point tube amplifiers are very concerned about "crosstalk" in the circuit itself and try to have signal carrying wires cross at right angles to each other rather than run parallel (for obvious electrical-theory reasons).
- BenjiWiebe 4y ago
  But those signal wires aren't balanced twisted pairs.
- deleted 4y ago
  [deleted]
crote 4y ago
It can definitely causes issues. The risk factors are: - High-current power cables - Analog signal wires - Low-voltage signal wires In practice it is mostly a solved issue. Applications like Ethernet, HDMI, and professional audio have been using balanced signal pairs for decades and they cancel out most interference. We figured out how to do that in the 1880s.
cuSetanta 4y ago
I have spent months of my life trying to diagnose issues with spacecraft hardware that ended up being caused by cross-talk from a noisy clock signal in AWG30 wires. Added some shielding and everything worked perfectly. In the most recent case, the clock was interfering with a series of motors, causing an antenna pointing array from getting lost and moving erratically. We had had some issues with the motors in the past so assumed it was an issue there and performed 100s of tests to try understand the issue. When we realised it was crosstalk I was quite red in the face. In spacecraft we talk a lot of precaution to avoid crosstalk when laying out our harness. Shielding is not always an option.

by ethbr0 4y ago

bayindirh 4y ago

Gigabit Ethernet doesn't care much if you have a good quality Cat5E+ cable. Doing this a lot in the data center for a long time. However you don't want high power AC cables going alongside with them. Just carry them separately, or keep power source local to target. Similarly, other data cables (Infiniband, SAS, etc.) doesn't care either, but they're both short length when used in copper form and they have ample shielding.

crest 4y ago

Lacing cables together isn't worse than running them parallel using other means e.g. velcro loops unless you're lacing them tight enough to crush the cables.

ethbr0 4y ago
From memories of electromag, field coupling from current in parallel cables decays with 2*pi*r, no? So it seems there would be a fair amount of difference at extremely low separation distances. But we never worked through high frequency EE derivations, which I assume are more average-current- or capacitance-dominated in terms of effects?
- SAI_Peregrinus 4y ago
  The distance between cables doesn't vary much between lacing methods. The balanced nature of the signals also means the common mode rejection ratio is very high, so external EM interference (which almost entirely shows up as common mode) isn't a huge concern.

brk 4y ago

On a decent quality cable you are unlikely to run into any scenarios in a typical residential or small business install where noise and interference causes a degradation in throughput. The balanced signalling used in twisted pair communications is very robust these days, and then you have various protocols on top of that which can handle small errors natively (eg: TCP retransmissions). It is highly recommended, and often required by code, to keep low voltage and high voltage (high voltage being 120V AC power lines, which really aren't that "high" of a voltage) separated physically. This can be air separation, keep them several inches apart, or conduit separation (don't ever run them in the same conduit). This is mostly a safety issue in the case of insulation breakdown to keep any high voltage leakage from entering the low voltage cable and equipment. Similarly, things like minimum bend radius, and untwist amount at termination can often be wildly violated with no ill effects. But, it's still best to not temp fate if not needed. Even max cable length can often be exceeded by 20% or more.

jaclaz 4y ago
>It is highly recommended, and often required by code, to keep low voltage and high voltage (high voltage being 120V AC power lines, which really aren't that "high" of a voltage) separated physically. It depends, there are types of data cables certified (insulation above 400 V) to be installed in same conduit as mains wires: https://news.ycombinator.com/item?id=34725851 https://news.ycombinator.com/item?id=34725851
- brk 4y ago
  Until you wind up with an AHJ who doesn’t care and requires you to separate the cables.
  - jaclaz 4y ago
    Well, that can happen about almost anything, having data and mains cables separated is anyway a good idea, but (with the correct cables) is not (anymore) against code in many situations.
    - 4y ago

bluGill 4y ago

If doing long runs just run fibre. gigabit Ethernet is about as fast as copper can get - yes 10gb Ethernet exists, but it is power hungry (one reason 2.5gigabit starting to appear). You can send a lot more data with fibre, there are no cross talk issues, and it is cheaper than copper. Transceivers are still expensive though, so copper is best for shorter runs (within your house distances), but still everyone who might run some cable in the future should learn about fibre and how to work with it.

ethbr0 4y ago
I was looking at it. What's the current best home-hobbyist option for terminating? Last I checked, it looked like it made more sense to just buy pre-terminated, built-to-length cables.
- bluGill 4y ago
  That is one of the downsides of fiber: you need to figure out your distances before hand and order the right length. One more reason to use copper in the house most of the time. Maybe some ~2ft lengths of fiber for use on your server rack where you need speed between the server and the switch.
xattt 4y ago
Is it still worth doing Cat 8 for stringing Ethernet in a new build, or is it okay to cheap out and just go with Cat 6a?
- crote 4y ago
  Cat6a can do 10GB over 100 meters, cat8 can do 40GB over 30 meters. Current consumer hardware is starting to support 2.5G over 100 meter of cat5, and we miiiight see 5G over 100 meters of cat6 become common in a few years. If you're that worried about upgrade-ability, better stick to fiber.
  - toast0 4y ago
    Note that the specs are 100 meters in a dense conduit. You can probably manage higher than rated speeds in a household environment which likely doesn't have dense conduit nor 100 meter runs. If you're wiring from scratch, sure put the best you can afford, but if you've got cat3 in your walls already, see what runs, your NICs aren't going to look at the label on the insulation. (For better or worse, ethernet speed negotation runs at 1Mbps and the specs don't contemplate testing conditions and reducing speeds until you get to the multi-gig (2.5/5/10g) equipment. Some equipment will drop to 100M in drivers though)

JKCalhoun 4y ago

As a foil, people building point-to-point tube amplifiers are very concerned about "crosstalk" in the circuit itself and try to have signal carrying wires cross at right angles to each other rather than run parallel (for obvious electrical-theory reasons).

BenjiWiebe 4y ago
But those signal wires aren't balanced twisted pairs.
deleted 4y ago
[deleted]

crote 4y ago

It can definitely causes issues. The risk factors are: - High-current power cables - Analog signal wires - Low-voltage signal wires In practice it is mostly a solved issue. Applications like Ethernet, HDMI, and professional audio have been using balanced signal pairs for decades and they cancel out most interference. We figured out how to do that in the 1880s.

cuSetanta 4y ago

I have spent months of my life trying to diagnose issues with spacecraft hardware that ended up being caused by cross-talk from a noisy clock signal in AWG30 wires. Added some shielding and everything worked perfectly. In the most recent case, the clock was interfering with a series of motors, causing an antenna pointing array from getting lost and moving erratically. We had had some issues with the motors in the past so assumed it was an issue there and performed 100s of tests to try understand the issue. When we realised it was crosstalk I was quite red in the face. In spacecraft we talk a lot of precaution to avoid crosstalk when laying out our harness. Shielding is not always an option.