Ethernet cable impedance mismatch

[yello]

I appreciate that this a bit techie for a cycling forum but it seemed daft to me to create a login on a tech forum only to ask one question... besides, there are some clever techie types on here

I've got a Netgear managed switch (a fancy hub in effect) that has a 'cable tester' function. I've never really paid much attention to it in honesty. Anyways, it reports a 'cable mismatch' on one of the cables; the main cat5e cable from the router - so potentially affecting all the devices plugged into the switch. The help tells me it's an "impedance mismatch" (first I've ever heard of such a thing in honesty) and I replaced the cable as suggested but I get the same error having tried a few cables.

Web searching tells me what the error is and whilst I can understand the issue, I'm a little lost to know whether it's actually a problem in practice, as there doesn't seem to be anything untoward happening on any of the devices connected to the switch.

The cable from the router to the switch is maybe 20m long, from inside the house to my work space in a detached building, mainly underground and through a couple of stone walls (so not possible to replace!) with 2 rj45 sockets either end. Is it possible there is a problem with one or other of the sockets? A simple cable tester reports no problem.

I'm inclined to ignore the error. Maybe the Netgear diags are a little simplistic/not to be trusted, I don't know.

 

[Ming the Merciless]

Do an online speed test on a device connected through the switch. Then take that device and connect directly to router with a shorter cable. Repeat speed test. See if there’s any significant difference in the results. If they are ballpark the same. Then ignore it.

If the impedance is lower than expected then switch may be seeing higher currents than expected. Higher currents, mean data moves faster down the cable between router and switch. If impedance is higher than expected then it will be seeing lower currents than expected. The fact it’s working means it’s not damaged the switch or router at least. So it can’t be a huge mismatch.

 

[slowmotion]

I would get another Cat 5 cable and also a Cat 6 one and temporarily use them to replace the existing one. They don't cost much. If your system still doesn't work, at least you'll know where the problem isn't.

 

[flake99please]

Is there anything printed on the cable? Cat5e or cat 6 perhaps?

 

[Tenkaykev]

I appreciate that this a bit techie for a cycling forum but it seemed daft to me to create a login on a tech forum only to ask one question... besides, there are some clever techie types on here

I've got a Netgear managed switch (a fancy hub in effect) that has a 'cable tester' function. I've never really paid much attention to it in honesty. Anyways, it reports a 'cable mismatch' on one of the cables; the main cat5e cable from the router - so potentially affecting all the devices plugged into the switch. The help tells me it's an "impedance mismatch" (first I've ever heard of such a thing in honesty) and I replaced the cable as suggested but I get the same error having tried a few cables.

Web searching tells me what the error is and whilst I can understand the issue, I'm a little lost to know whether it's actually a problem in practice, as there doesn't seem to be anything untoward happening on any of the devices connected to the switch.

The cable from the router to the switch is maybe 20m long, from inside the house to my work space in a detached building, mainly underground and through a couple of stone walls (so not possible to replace!) with 2 rj45 sockets either end. Is it possible there is a problem with one or other of the sockets? A simple cable tester reports no problem.

I'm inclined to ignore the error. Maybe the Netgear diags are a little simplistic/not to be trusted, I don't know.

I'd go with ignoring the error. The cable tester rings out ok, the cable is to all intents and purposes inaccessible. As Mr Ming says, do an online speed test and if there's negligible difference in speeds then ignore it.
If it was possible to determine which core was faulty ( might need a more sophisticated cable tester ) then I think you could swap the dodgy core for an unused one ( the blue cores are unused AFAIK )
( Stands back and waits for a networking guru to show me the error of my ways 🤔)

 

[T.M.H.N.E.T]

is it wired correctly on both ends across the 8 pins, it sounds like the cable may have been made at some point rather than bought at length? Broken or frayed somewhere along it?

Should be Orange white - orange - green white - blue - blue white - green - brown white - brown

 

[DaveReading]

"Impedance mismatch" sounds like something from the bad old days of Ethernet over coax cable - 10Base2, I seem to recall.

 

[mistyoptic]

Is it running near a mains cable?

 

[bruce1530]

Make sure there's nothing plugged into the remote end when doing the test. That'll skew it.
But I wouldn't be overly worried - these test functions built into the switch aren't particularly accurate. A proper "fluke" tester will cost you a 4 figure sum and a decent no-name one will still set you back several hundred.

The real test is to log into the switch when it's in use and look at the port statistics. CRC errors are bad.

 

[si_c]

An impedance mismatch usually means the wiring grade in the cable isn't quite the same - it'll cause small errors requiring re-transmission or error correction which will degrade the performance of the network over that link. As @bruce1530 says you'll see the errors on the port information on the switch.

Swapping the cable out will fix the problem and to be honest it's not the sort of thing you'd expect to see within a single cable, more likely over multiple cable spans, but it could just be a cheap cable. That being said the sort of performance you see over a domestic internet connection is unlikely to see any difficulty with a poor quality cable on the home side as it's not fast enough or have the sustained throughput.

 

[newfhouse]

If the Netgear diagnostic is to be believed my guess is that the way the twisted pairs are connected is wrong. It will check out OK using a DC tester but you probably have Tx or Rx connected using one leg from a different balanced pair.

If a speed comparison shows no problem you don’t need to worry, otherwise it’s worth checking the colour order as @T.M.H.N.E.T has said above.

 

[yello]

Do an online speed test on a device connected through the switch.

Why didn't I think of that! Here's me saying "doesn't seem anything untoward" when I could have put real numbers to it! Anyway, speed tests duly done, first group of three direct from router....

download / upload / ping
4.18 3.27 29
2.17 3.12 35
2.11 4.02 31

and from the switch...
3.19 3.08 28
3.49 3.00 30
3.58 2.20 30

It's a 4G connection so there's going to be fluctuations due to that but I'd say there's nothing there that points to a problem switch side.

In answer to other questions (in no particular order), it's a cat 5e shielded cable run in its own conduit from house to outbuilding. It will pass electrical cable in places (and that might account for some noise maybe) but it is otherwise isolated. It's a cable I ran myself and I wired the sockets myself too - consistent colour coding both ends and tested ok with a simple cable tester.

Most importantly, and thanks @bruce1530, looking at the port stats on the switch shows no CRC errors on any port. I think that the switch might well be correctly reading the impedance mismatch, due to whatever, but it's of no real concern. Indeed, one the devices on the switch is a media server on which I watched streamed films & TV - and if I'm going to see any issues, it might well be there I suspect, as it is the most data hungry and throughput intensive.

Thanks folks, I think I can put this one to bed.

 

[T.M.H.N.E.T]

Yeah 4g will definitely fluctuate! Mine was playing around yesterday and ironically, where I had the greatest signal offered the weakest dl/ul speeds.

*I have a cheap & cheerful mobile wifi/huawei router contract with 3 at home.

 

[biggs682]

Price of apples is horrendous

 

[yello]

My 4G modem/router routinely protests 'no signal' yet continues to work just fine. Oh, and if I reboot it then I will sometimes lose the WiFi entirely - requiring a hw reset and reconfigure. It's going to give up entirely at some point I suspect.