Networking – cat6 cable only getting 100mbps over larger distance

cablecat6ethernetnetworking

I know this question has already been asked a few times but I feel that my case is a little different since I've tried all the solutions provided in the other threads.

I created a small diagram to avoid any potential confusion.

My attempt to solve the kink in the cable, also a diagram how things are laid out currently

So I have a cat6 cable of 80 meters. This cable has worked in the past. However, 2 years later I want to use that cable again and now it only gives me 95 mbps(tested with iperf3), it also gets recognized as a 100mbps link instead of a 1000mbps link. Image 1 provides a visual representation of how everything is laid out at this point.

Parts:

My troubleshooting steps:

  • Redo the connector, router side (changed nothing)
  • Redo the lsa wallplug, pc side (changed nothing)
  • Test the cable with a cable tester. (all wires are working, but still only 100mbps)
  • Check the entire cable for kinks (Found two kinks and cut that piece out. Connected the now 2 cables by adding an rj45 to the ends and connecting them with an ethernet extender, see the after part in the image) (solved nothing)
  • Test the cable again with cable tester (all wires are working, still 100 mbps)
  • Test the cable with iperf from the router to the extender (950 mbps, recognized as 1000mbps link)
  • Test the cable with iperf from the extender to the PC (95 mbps, recognized as 100mbps link)
  • (The previous 2 test results do not change if I do them with or without the extender included.)
  • Redo the connector for the cable from the extender to the pc (nothing changes)
  • Redo the lsa plug for the cable from the extender to the PC (nothing changes).
  • Test again with the cable tester (all wires are working)
  • Nothing changes no matter what I try.

Things I know for sure:

  • The rj45 connectors support solid core cable.
  • The lsa plug supports cat6.
  • I use the T568B standard everywhere.
  • The type of cable I'm using is S/FTP cat6.
  • The cable does not run anywhere near any other cables(except where it connects to the router).
  • Short distance of about 15 meters works just as it's supposed to.
  • Longer distance of about 65 meters gets recognized as a 100 mbps link
  • The entire cable runs inside a conduit.

What else could it be?

The only thing I can think of right now is that the signal isn't strong enough by the time It reaches the pc. So I am still planning putting a switch in the middle of the 65 meter part of the cable. However I would rather not do this since this is outdoors and I don't have a good way of protecting the switch or the not interrupted cable from the elements.

If this also does not help, would upgrading to a cat6a help? I read somewhere that cat6 Is not that reliable after about 50 meters.

PS: If you find any spelling/grammar mistakes, please let me know. I am still learning English 😉

UPDATE
I fixed the issue by adding a gigabit switch in the middle. Now I have a 1000mbps link with a PC to PC transfer speed of +-870 mbps.
Update

However, there are still a few very interesting answers which you should definitely read if you stumble upon this post in search of a solution to your ethernet problems. Especially the answers from @Zac67 and @The Unix Janitor

Best Answer

Ethernet auto-negotiates using (low-frequency) link pulses. It does not check or train the cable. Link pulses are exchanged on the two 10/100 pairs and for 1 Gbit/s, all four pairs do an additional link check each.

Some Ethernet devices may fall back to 100 Mbit/s when 1000 Mbit/s has been negotiated but fails to link due to one or two missing pairs - 1 Gbit/s requires all four pairs, 10/100 Mbit/s only use two pairs. This is not by standard though and usually limited to Broadcom chips.

So possibly, one of the pairs in the cable has failed. Another possibility is that you've mispaired some contacts when terminating the cable. Cheap cable testers cannot detected split pairs as they only check for continuity and shorts. Double check the color codes inside the connectors - you can (but shouldn't) swap entire pairs or the wires within a pair but you must not swap wires across pairs.

You should note that while Ethernet over twisted pair is generally limited to 100 m - 90 m of those are required to use solid wire cable and only 10 m are allowed to run over stranded (patch) cable. Anything beyond that may throw (more or less) errors or fail at any time.

Also, DIY cables for Gigabit Ethernet can very easily exceed the specification margins and cause transmission errors. You need to make sure that you obey all twisting and pairing rules and generally do a neat job. Connectors must fit the cable (stranded or solid-core) and crimping tools must match the connectors.

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