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0 pointsssl-35mo ago0 comments

Gigabit ethernet "requires" 4 pairs of no-less-than cat5. The 100mbps standard that won the race -- 100BASE-TX -- also "requires" no-less-than cat5, but only 2 pairs of it.

Either may "work" with cat3, but that's by no means a certainty. The twists are simply not very twisty with cat3 compared to any of its successors...and this does make a difference.

But at least: If gigabit is flaky over a given span of whatever wire, then the connection can be forced to be not-gigabit by eliminating the brown and blue pairs. Neither end will get stuck trying to make a 1000BASE-T connection with only the orange and green pairs being contiguous.

I think I even still have a couple of factory-made cat5-ish patch cords kicking around that feature only 2 pairs; the grey patch cord that came with the OG Xbox is one such contrivance. Putting one of these in at either end brings the link down to no more than 100BASE-TX without any additional work.

(Scare quotes intentional, but it may be worth trying if the wire is already there.

Disclaimers: I've made many thousands of terminations of cat3 -- it's nice and fast to work with using things like 66 blocks. I've also spent waaaaay too much time trying to troubleshoot Ethernet networks that had been made with in-situ wiring that wasn't quite cutting the mustard.)

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toast05mo ago

> Neither end will get stuck trying to make a 1000BASE-T connection with only the orange and green pairs being contiguous.

They can get stuck, because negotiation happens on the two original pairs (at 1Mbps), and to-spec negotiation advertises the NIC capabilities and selects the best mutually supported option. Advertising fewer capabilities for retries is not within the spec, but obviously helps a lot with wiring problems.

The key thing with the ethernet wiring requirements is that most of the specs are for 100m of cabling with the bulk of that in a dense conduit with all the other cables running ethernet or similar. Most houses don't have 100m of cabling, and if you're reusing phone cabling, it's almost certainly low density, so you get a lot of margin from that. I wouldn't pull new cat3 for anything (and largely, nobody has since the 90s; my current house was built in 2001, it has cat5e for ethernet and cat5e in blue sheaths for phone), but wire in the wall is worth trying.

ssl-3OP5mo ago

TIL that they can get stuck in no-man's-mand with 2 pairs. That seems stupidly-incompatible, and it isn't something I've witnessed myself, but it makes sense that it can happen.

My intent wasn't to dissuade anyone from trying to make existing cat 3 wire work (which I've never encountered in any home, but I've not been everywhere), but to try to set reasonable expectations and offer some workarounds.

If a person has a house that is still full of old 2- or 4-pair wire, and that wire is actually cat3, and is actually home-run (or at least, features aspects that can usefully-intercepted), then they should absolutely give it a fair shot.

I agree that the as a practical matter, the specifications are more guidelines than anything else.

I've also gone beyond 100 meters with fast ethernet (when that was still the most commonly-encountered) and achieved proven-good results: The customer understood the problem very well and wanted to try it, so we did try it, and it was reliable for years and years (until that building got destroyed in a flood).

If the wiring is already present and convenient, then there's no downside other than some time and some small materials cost to giving it a go. Decent-enough termination tools are cheap these days. :)

(Most of the cat3 I've ran has been for controls and voice, not data. Think stuff like jails, with passive, analog intercom stations in every cell, and doors from Southern Steel that operate on relay logic...because that was the style at the time when it was constructed. Cat3, punch blocks, and a sea of cross-connect wire still provides a flexible way to deal with that kind of thing in an existing and rather-impervious building -- especially when that building's infrastructure already terminates on 25-pair Amphenols. I'll do it again if I have to, but IP has been the way forward even in that stodgy slow-moving space for a good bit now.)

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