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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?

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  • I've heard that CAT5e can vary in quality, and CAT5e is barely good enough for Gigabit, usually (but sometimes it isn't quite good enough), and CAT6 is recommended because CAT6 really ought to be good enough for Gigabit. Still, since that might just be hearsay, I'm +1'ing Ron Maupin's answer as a more likely answer. – TOOGAM Jun 7 '16 at 2:00
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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.

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  • And that's what that linked-to question/answer suggested... but, as I meant to say in the question (and I'm updating now), I've confirmed with a meter that all 8 are end-to-end good. – bcsteeve Jun 7 '16 at 2:04
  • They are simply not wired correctly, or a pair could have a broken conductor. This is very common with cheap patch cables. A meter is not a good cable test. There are many, many required things it simply cannot measure. You can have electrical conductivity, but the cable is not be able to be used. A proper cable tester costs several thousand dollars. – Ron Maupin Jun 7 '16 at 2:06
  • I did just read a page that says CAT5 cabling colors are "in order": blue, orange, green, brown. Looking at this patch cable, the orange paid and blue pair are reversed from that. However, that can't possibly make a difference, can it? It is reversed on both ends, so the physical copper is still 1:1. But thought I'd mention it in case I'm missing something on that. Also, I can't tell 100% sure if they haven't mixed up the green/white and blue/white wires. In other words, it may go like this: Orange/white, Orange, Green/white, Blue, Blue/white, Green, Brown/white, Brown. But again... – bcsteeve Jun 7 '16 at 2:11
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    DC continuity doesn't mean anything at the frequencies used for gigabit ethernet. All it takes is a little RLC anomaly somewhere to foul up one pair and the cable's just flat junk. We're dealing with inductance, capacitance and impedance which do not reflect in simple DC. – Fiasco Labs Jun 7 '16 at 2:16
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    The primary cable tests are Wire Map, Length,.Insertion Loss, Near End Cross Talk (NEXT), Power Sum Near End Cross Talk (PSNEXT), Equal Level Far End Cross Talk (ELFEXT), Power Sum Equal Level Far End Crosstalk (PSELFEXT), Return Loss, Propagation Delay, and Delay Skew The color coding has nothing to do with the cable category. That has to do with TIA568-A or TIA568-B. – Ron Maupin Jun 7 '16 at 2:20
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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.

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