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I know (because of this question) that mixing CAT5e and CAT6 is not supposed to matter, but when installing a wired network I did run into the following problem - persistenty:

  • When adding a CAT6 extension cable to a CAT5e cable running from my modem to a switch elsewhere in the house, the wired speed was only 10 Mb;

  • When omitting that extension cable there and instead adding the very same cable to another CAT5e cable running from the switch to yet another room in the house, the speed was 100 Mb as it's supposed to be (for my modem).

Riddle me this! The extension cable is not at fault, obviously. The only theory I have is that the modem detected something funny on its outgoing cable and decided to drop the speed - but is that even possible?

(The speeds of 10 resp. 100 Mb were as reported by the modem, but confirmed by a manual speed test.)

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    "The extension cable is not at fault, obviously." Why not? You describe that your issue only occurs when you are using this cable.
    – MMM
    Feb 10, 2020 at 8:10
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    How does the extension cable connect to the existing cable? Does it have a socket on one end, or are you using one of those connector/gender-changer widgets? Is it possible to use an active switch instead? Feb 10, 2020 at 8:23
  • @MMM Yes, but when using the cable in a different position, the issue is not there. Ergo, the cable can carry the 100Mb
    – Arend
    Feb 11, 2020 at 21:02
  • @user1686 It's a cable with a socket on one end. It's possible to use a switch instead, and that's what I ended up doing (so the problem is solved, actually - I'm just still curious what the problem was in the first place). The reason to initially try the extension cable is that the switch is now very close to the ground; I would have liked to have it higher up on the wall so as to be more easily reachable.
    – Arend
    Feb 11, 2020 at 21:04
  • Well, using transmission line theory possible explanation would be some impedance mismatch at your modem <-> switch input path (e.g. extension connector used if I understood your description correctly), which caused high reflections / return loss etc. That is happening when mixing cables with different electrical properties at high speed (although impedance of UTP is defined as 100 +/- 15 ohms). So, the logic reduced the speed to increase S/N ratio. It's just my best guess.
    – dev101
    Apr 6, 2021 at 14:44

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