IMO, cost is a big factor here. Adding things like insulation, EM sheilding, or anything else other than the minimum needed water water and electrical lines to the inside of walls raises the cost of construction. So if it isn't required by the function of the room or building code, you're unlikely to see such fanciness, unless it's some high-end luxury place where the shielding is a selling point. Expect higher rent in such a place to cover the additional costs of constructing those fancy EM-shielded units.

IANAP but I would think such a cage would cause WiFi to reflect internally, causing as much if not more trouble than noisy neighbors.

The "real" solution is for people in dense apartment buildings to just turn their damn Tx power down, but that's a prisoner's dilemma if I've ever seen one.

I actually tried to approach this problem in my own apartment complex. The effort died almost immediately. Very few consumer wireless access points even expose Tx power as a configuration parameter at all, even buried in an "Advanced Settings" section somewhere. You're lucky if they even expose channel selection at all.

So with the craptastic firemware of most consumer wifi APs, the AP hops on channels 1, 6 or 11, and stays there, screaming at full power. And the consumer, even if they know enough about radio to realize such is suboptimal, can't do anything about it without changing the firmware or buying a more expensive device.

From my experiences, lots of these consumer APs frequently end up landing somewhere like channel 4 or channel 9. :/

In a really busy area, what's the issue there? If there are tons of people on 1/6/11, you may as well hop into the middle bits - there's already plenty of interference going on in the usual channels.

With 802.11g you're actually taking up four channels out of the available 12? So people hopping in the middle are messing with people on both sides of them. Sorry for the mobile link. https://en.m.wikipedia.org/wiki/IEEE_802.11g-2003

However, if the 'standard' channels are already congested, people are already messing with each other. Look at the semicircle graph on your link - someone on channel 4 is messing a fair bit with someone on channel 5, and a little bit on 6. However, someone on channel 6 is messing a lot with another person on 6.

So, people on the same channel silence each other (CTS/RTS), but on other channels it is simply seen as noise. If there are lots of people on 1/6/11, you're going to spend a lot of time silenced.

It's one of those things that you just have to try out in your own particular circumstances. Will you get better performance if frequently silenced, or better performance dealing with a high level of 'noise'?

Actually WiFi was designed from the very beginning to do well despite reflections. My thesis advisor knew somebody who was applying ideas from "quantum chaos" to make WiFi robust to various environments at Bell Labs, back in the 1990s, before Carly Fiorina bankrupted Lucent.

It's a prisoner's dilemma in the case where everyone controls their own wireless APs, sure.

In the case where the building is served by one major ISP, and they provide combination gateway+APs, I could see the ISP deciding to "solve" the problem by turning the whole building into one AP mesh (with only one AP actually active per unit sphere) where each client gets a VLAN with tagged QoSing over the last-mile "backplane" back to their own gateway.

Though, if you're going that far, you may as well just call yourself a WiMAX provider and tell people their gateways are just extension picocells for the network, rather than end-user equipment. Which is basically what you get already from the ISPs that offer "public wi-fi for customers at our hotspots" (i.e. broadcast by all APs on our business plan), except that said wi-fi would be the only product, and would be served by residential gateways as well.

There is wifi blocking paint and wallpaper. http://www.lessemf.com/paint.html for example. That said, this may not be desirable if you have a cellphone without a local nanocell or voip switching.

There is wifi blocking paint and wallpaper. http://www.lessemf.com/paint.html for example. That said, this may not be desirable if you have a cellphone without a local nanocell or voip switching.