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I'm asking here after finding and reading Why is my LAN not gigabit?

Similarly, I have a CAT5e patch cable which will only connect at 100Mbps. All my other cabling is CAT6, but this one patch cable isn't. It is 30M long. From everything I read, that should be capable of gigabit, right? Swap it with a CAT6 and I get gig... no other changes.

Is it a bad cable, or is 5e really not so giga-capable as I thought? I have confirmed with a multimeter that all 8 wires are good from end-to-end and they have the same color sequence from left-to-right.

No errors. No dropped packets. Just won't negotiate at gig.

Thanks.

Edit: Almost immediately after posting, I found another 5e cable to test with although it is shorter (50ft vs 30M... about half). It successfully makes a 1Gbps link. So the question now is: Is something wrong with my 30M cable? Or is 30M just too long for Giga speed?

Community
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bcsteeve
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2 Answers2

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It's because the cable isn't properly wired with all four pairs. 100BASE-TX (100 Mbps) requires two pairs (1-2, 3-6), but 1000BASE-T (1 Gbps) requires all four pairs (1-2, 3-6, 4-5, 7-8).

The 1000BASE-T standard requires autonegotiation. If you don't have all four pairs wire correctly, it will negotiate to 100BASE-TX.

Ron Maupin
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It sounds like the pins line up alright, but somewhere along the cable it may have been kinked or bent too tightly. Even if you straighten them out, some cables will never work at their highest rated speed again. CAT6 cables have a built-in separator to keep the individual pairs spaced properly and are more resilient to abuse while still staying in tolerance for 1Gbps.

There's some really interesting high frequency dynamics created in the cable the faster we pump data through them. Once a cable degrades outside of a certain tolerance, signals start to bounce back or lose cohesiveness, and the cards auto-negotiate down to a lower level. It's the kind of stuff that keeps electrical engineers up at night.

atmarx
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