Gigabit Ethernet Connections

This post was published more than a few years ago (on 2005-06-16) and may contain inaccurate technical information, outmoded thoughts, or cringe takes. Proceed at your own risk.

Yesterday, we had quite a scare. We had run CAT5e cable throughout the ceilings and walls of the office, but when we plugged them into the gigabit Ethernet switch, none of the link lights lit up. We were dumbfounded. All of this cable had tested fine. We tried shorter cable, different cable, none of it worked, except a few old pieces of cable that had been lying around. But we couldn’t figure out why the hell those cables worked and ours didn’t. We even ran out and bought all new CAT6 cable, thinking maybe we had gotten a bad batch, and got better cable to overcompensate for it. We tried that— a 30′ cable, and a 5′ cable, and neither worked. The cable tester said it was good, but it wasn’t working --- Why?

Defeated and frustrated, I frantically called my IT knowledgeable friend, Ben, and he stepped through everything, leaving me with two bits of advice: take a look at the way the cables that do work are crimped and emulate them; and if all else fails, call Linksys, and ask them what the heck is going on. Well, before calling Linksys, I looked on their web site’s support area, and found this gem, which explained the proper pin order of the wires. We cut off the old connectors, re-crimped them with the wires in the new order matching the 568B standard, and lo and behold, it worked.

I guess it has to do with how the twisted pairs shield themselves, and the way we were doing it, there were two pairs that were signal/ground instead of signal/signal or ground/ground like they should be. Good enough for 10/100, but not gigabit. The cable tester only tests the connection order, not which pairs go where, so there’s no way we could have known from that.

Anyway, I’m glad we got this figured out. Of course, now I’m the proud owner of a 1000′ spool of CAT6 that we didn’t really need to buy. I s’pose I’ll keep it handy for the home network— but let this be a lesson to you, if you’re making your own ethernet cables, follow the standard.