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7 posts

Wannabe Geek

  #3030016 31-Jan-2023 17:57
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@Antoniosk - I was going to test the living room line speeds as a comparison to the iMac in the office.
But came across an odd situation....

Yesterday we checked the living room ethernet using an older Macbook with an ethernet port (my newer Air doesn't). Detected the network fine. 

Tonight I went to check the speeds in the living room and found:
- Macbook = no network.
- Moved the iMac to the living room and used a different ethernet cable = no network. 
- Turned on the TV, plugged in the same cable = network detected. 

Anyone care to guess what's happened here? Even if one of the pairs isn't working that would not explain the issue as the TV uses the same pairs as a computer yes? 




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  #3030100 31-Jan-2023 21:36
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Gigabit ethernet needs all four pairs. 100Mb/s only needs two pairs (orange & green). Some ethernet controllers will degrade to 100Mb/s if one of the other pairs is faulty, some won't.



Ethernet is not a very fault-tolerant protocol. The endpoints negotiate based on what they support, then they run with that. If the cable isn't good enough, you drop packets and potentially drop the connection. Unlike ADSL or old-school dial-up, it doesn't try to reduce speed to improve stability.



As for pair twisting, I'm not sure it's even standardised as to which pairs get twisted more & less. Theoretically all should be 100 ohms and with the same velocity regardless of twistedness. Skew (difference in speed between pairs) is one of the parameters that is specified in the cable standards.

26418 posts

Uber Geek


  #3030108 31-Jan-2023 22:19
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Or you have a dodgey crimp and the movement made it connect better.



7 posts

Wannabe Geek

  #3031554 3-Feb-2023 22:18
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@Antoniosk - found the problem and ran a line test; the download speed in both rooms is identical. There is no loss in speed despite three joiners. 325.49 Mbps / 108.55 Mbps
I found one dodgy crimp as @richms suggested. 

  #3031980 4-Feb-2023 21:14
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Yeah, you would usually get packet loss, not speed issues, from excessive joints and/or cable length. Highly unlikely to be an issue in domestic situations though; you're nowhere near the 100m limits.



Failed crimps like that are why the industry is really trying to pull away from DIY-crimped plugs and say that DIY terminations should only be punch-down, then use a patch lead. Patch leads can be made of stranded cable, machine-terminated and tested, and potted to prevent movement.

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