We have a departmental wifi network that exists in six buildings on campus (I think, perhaps more) and uses access points that are operated by other people (the university wifi people use enterprise access points that support multiple SSIDs). This leaves us with a little bit of a wifi monitoring problem, since all we see is basically the very top level of our wifi network activity (as it's all brought to into our wifi network gateway). If there's some problem with our SSID on an access point in some building, or the uplink to us from the building isn't working for some reason, we've traditionally had to wait for people to notice and report it to us. A while back, we decided that we would like to do better, which meant actively monitoring the state of our wifi network in various locations.

There are at least two broad ways to do this; you can put devices on the wifi network and then verify that you can reach them over it, or you can have dual-network devices accessible through another network but also connecting to the wifi. The former requires one less network connection but means you risk false positives (when what's down is your monitoring device, not the wifi network).

If you're a certain sort of person, you're now enthused about the idea of building your own little Linux-based wifi monitoring computers, using one of a number of basic single-board computers with built in wifi, no doubt put in a 3D printed case, and probably powered over USB (from a cheap USB wall charger). We looked at this briefly and concluded that we were not enthused. We didn't have anyone who actively wanted to build such units and we suspected that they'd be surprisingly costly and wouldn't necessarily be as reliable and trustworthy as we wanted them to be. So we picked another option, namely wifi-controlled power plugs that were designed for home automation.

These ticked off more or less all of the boxes we were looking for. Wifi controlled power plugs are obviously on your wifi network, as that's how you're nominally supposed to control them, and they have a simple story of how they're powered, since they just plug into your wall socket (so you don't have to worry about a separate power unit or batteries). They're not very big, they're relatively unobtrusive, they're likely to reliably stay on the wifi, they're probably not going to burst into flames if you buy a decent brand (an important attribute for something we would be leaving plugged into 120V power 24/7), and if you don't have enough outlets, in a pinch you can plug something into them. And basic units are available relatively inexpensively, for far less money than it would cost us to build our own.

There are a variety of other wifi-controlled or wifi-accessible things that you could also use for this and we looked at some of them. But most of them have more complicated stories about (long term) power, such as using batteries that would need changing every six months or so, and many of them seemed likely to want to send potentially alarming and intrusive information off to cloud servers (such as the temperature of their surroundings). If you never actually control power with your wifi-controlled power plugs, they can't tell a cloud service anything very interesting.

The specific model we wound up using is the TP-Link HS103 'Kasa Smart Wifi-Plug' (I believe in the 'Lite' version). These work about as well for us as we could ask for something in their price range, which is to say that almost all of the time they stop responding to ICMP pings, it's because something has happened to the wifi network in their location. And for buildings that are a long way away from our offices, they're cheap enough that you can get two and only be alarmed if both drop off the network at once.

(Some of the time, what happened is that someone unplugged one. We try to put enough labels on them (and locate them in secure spots) so that people will neither pull them out or try to use them to power their devices, but it doesn't always work.)

Update: it turns out that I wrote about the early stages of this back in late 2022 in Monitoring if our wireless network is actually working in locations. The experimentation from then has graduated to production status with a half dozen of these deployed so far.