Ethernet is switched and under 20Mb/s is more than usable for everything unless you transfer movie files and wait, in which it would take like up a few minutes to watch a video. If you browse it's more than enough.
20 Mb/s is terrible. Sure, it'll work in some sense, but I'd go mad if that's the best I could get. A good wifi setup is way better than a cabled 20 Mb/s unless you absolutely need consist and better latency for some reason.
He said hard to reach places. If an old house has CAT4 in the basement, I'll take the lower pings, reliability, and at worst I'll WebDAV. Unless your home server is in the basement I can't see a normal realistic scenario where the data transfer isn't sufficient.
If you get old equipment to go with your old cabling, ethernet is not necessarily switched. 10 and 100 can be run as a shared medium.
Re: the sibling's suggestion of wifi: If your cabling works at 100, I think the case is pretty clear for wired 100 vs wifi; consistent 100m is better than variable whatever you get. At 10m, not so clear, especially since cabling that's that old is also likely to be daisy chained, so you're looking at maybe a daisy chain of switches running at 10m. That said, cabling is often better than the spec it was tested to, and ethernet cable requirements are for long runs in dense conduit; it's worth trying 100M on old telephone wiring if it's already in the wall to see what you can get.
1. It's not enough when you're on an active connection doing other things at the same time as the video. Live streams at full quality will use a third to half of 20Mbps and demand very little interruption. Loading a single page with a few images can interfere. And even dedicated 20Mbps, with tightly encoded content, can be too little for 4k.
2. The idea in the comment is streaming at original bluray quality. Not transferring.
20Mb/s is enough? Is this a joke? A single youtube full HD stream can reach 7Mb/s. Literallywatching a youtube movie and doing something else will saturate that link. In fact I think it's so low it will negatively impact basic website loading time. Just going to reddit.com loads 18.2 MB of resources. This will take about 8 seconds on your "useable for everything".
My internet connection has 15Mb/s down Thank you very much. So while I'm not your parent I can totally see how that 20 is totally fine. So if you are in a corner of the house you can't get Wifi to that's on par why not use that cable if it exists?
Would I want to use it to transfer large files around internally nowadays? No.
Works for everything else assuming you are the only user of that cable in that corner basement room? Absolutely.
Also insert appropriate "kids these days" joke. I guess it's like HD. Once you have it, you are not going back. Do I need a triple A game I just bought and want to play to download in 2 minutes vs 2 hours? No. But kids these days expect it I guess. No patience.
