I have just moved to a new apartment which has cat5e wires running through the walls to each room and to a centralized panel. I got a guy to crimp both ends of the wires and tested the network speed of the wires using a 1gb router. The computer I used to test (a 2017 macbook pro has a gigabit nic).

To my surprise, I only got the speed of 100MiB. I also tested a stock wire I had lying around and connected it directly to the router (got 1GB as expected).

Suspecting something is wrong with the wiring, I used a professional wiring testing tool to check the cable wiring which indicated all 8 wires are connected (see image)

I'm suspecting something is wrong with the wire, but it's brand new and tested well by the wiring check tool. Can you think of a reason for the speed to only get to 100MiB?

wiring tool indication

  • Did you really mean MiB? MiB is MebiBytes. So 100MiB = 838,860,800 bits. That's only a little short of the theoretical max throughput of TCP over IPv4 over standard 1500 Byte frames over gigabit Ethernet, which is about 943,000,000 bits per second. The difference could be the overhead of whatever software/protocol you're using to measure the speed. If you didn't mean MebiBytes, please edit your question to use the right units or unit abbreviation.
    – Spiff
    Apr 21, 2019 at 1:06

2 Answers 2


If you don't follow a color coding inside on the wire, the cable is technically not CAT5e, but only CAT3, which can only reach speeds of 100MiB. You can see the colors they used from inside the plug. Verify that the colors are:


If your cable is not as follows, then the guy who made the cables has no idea what he is doing, and you don't have a CAT5e cable, which is why the speeds are reduced.

  • get a cable tester - will tell you why immediately Apr 20, 2019 at 21:36
  • Thus assumes the 568B standard. The equivalent a standard.woukd also bo acceptable. It assumes eveeything is correctly crimped as well.
    – davidgo
    Apr 21, 2019 at 2:09
  • Actually, thinking about it, isnt it more likely he tried to crimp solid cable? That doesnt work that wwll.
    – davidgo
    Apr 21, 2019 at 2:12
  • @JohnnyVegas. He used a cable tester. It passed. The problem is, the cable testers most DIYers buy at the hardware store or electronics store are just pinout/continuity testers that would never be able to detect a split pair. Most people don't spring for a fancy/expensive Fluke device like a cabling pro would own, so they see that the pinout is right and think the cable's fine. But with a split pair it won't have sufficient noise immunity over a decent-length in-wall run.
    – Spiff
    Apr 21, 2019 at 3:55
  • @spiff good point - Is the cable solid cored or stranded? I once had some awful cable which although had 8 cores it would only get gigabit speeds under two meters Apr 21, 2019 at 11:25

Thanks for the inputs guys.

I believe the wire is standard and says cat5e on the shell. To make sure it's not a connector/wiring problem of the individual pairs - I conducted two additional tests:

  1. Tested another wire going to another room and got the same results
  2. I took a short cable which came with the router and tested it (got 1GB as expected). I then cut both ends of the cable and crimped both ends using the same connectors used on the main wires running in the walls. The cable tested OK and provided 1GB.

I am getting desperate and with no better idea would probably eventually replace the wires :/

Edit: Replaced one of the cables to a brand new CAT6 and now getting 1GB speeds. Cables must be faulty (although it's still weird the cable tester reported them all as functional).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.