[user5994461]

> the cables don't have a little chip or anything saying "I'm not suitable for high speed" the card will figure out whether this looks plausible and just do it.

You're actually wrong on all of that ^^

The cables actually have a rating to say what they are suitable to. See the markings on the cable: category Cat5/Cat5e/Cat6 + frequency range 100/250 Mhz + insulation UTP/FTP/STP/mix.

Ethernet cards don't negotiate, they typically only check whether the pairs can transmit any signal. You could end up in a situation where they go for gigabit and it doesn't work well.

Fortunately, the main issue for signal transmission is loss over distance. Ethernet is designed to work over 100m every time in a noisy industrial environment. You've got a pretty good chance for it to work on a short run, even with poor cables.

The alternatives being discussed ADSL/VDSL/G.hn actually detect the capability of the medium and adjust the transmission rates and frequency to give the maximum possible speed. IMO they are much more advanced technologically and much more interesting. (Ethernet is doing exactly 250 Mbps on one pair, G.hn can do up to 1700 Mbps on the same pair, automatically adjusted, the article is getting 1300 Mbps which is insane!)

[tialaramex]

It's true that the cable says 5e on it but your device doesn't read the printed reading so it doesn't matter.

That printed category tells you what was tested, not whether the cable works in practice. Which makes sense, but leads to the consequence I described.

[user5994461]

Worthwhile to point out: The Cat5 cable required for gigabit Ethernet is merely twisted pairs with no insulation, which is pretty much a dumb basic cable (with 8 wires). That's why any cable can work in practice.

I don't know how possible it is to find a really bad cable (untwisted) and it might work on a short length anyway. (Your 1980s office cabling must have been 8 wires if you were able to get gigabit later, so it was far beyond basic phone wires or Cat1 from the time).