Question: On my local network there are (among others) 5 machines (running
Debian Jessie or Arch) wirelessly connected to a Netgear WNDR4000
router. Below is a graph of the ping times to the router
from each of the machines, collected over a period of around half an hour.

![Ping times](https://i.stack.imgur.com/U5SFs.jpg)

Observations:

* When things are going well, the ping times are all below 3ms (under
1ms for two of the machines, including the problem machine *purple*)
* At irregular intervals (of the order of 100s), three of these
machines (*red*, *green*, *purple*) suffer degradation of ping times, while the other two appear unaffected.
* The degradation periods coincide for all 3 machines.
* The degradation for *purple* is two orders of magnitude more severe than
for *green* and *red*, with ping times typically reaching over 20000ms
for *purple* and 200ms for *red* and *green*.
* If *purple* is physically moved nearer the router, the degradation
completely disappears for *purple* while continuing as before for both
*red* and *green*.
* *Red* is 3m away and in direct line of sight from the base station;
*purple*'s usual location is about 10m away without direct line of
sight.

This makes network access on *purple* intolarably slow (when it is in
its normal location).

Can you suggest how to go about diagnosing and fixing the problem?

Answer: 
Clearly you have an interference problem. Interference can come from passive elements like aluminum wall studs or thick floors, but those are not likely to show the periodic pattern you see. So something electric or electronic is periodically emitting. Finding it may be expensive or tough, but you have a few options.

1. **Graph more.** It would be nice to make sure that this isn't a bandwidth limitation of your own creation. Looking at the bandwidth consumed on your uplink and each of the clients might lead to a similar looking graph and the source of your problem.
2. **Upgrade firmware.** Being on the latest firmware could get you past a bug in their code causing this. Plus it helps reduce the chances of your router being remotely compromised.
3. **Mitigate it.** Get a wifi analyzer for your phone. Look at who is using what bands when purple is pokey particularly. You may find that switching from 7 to 1 or 14 takes care of your problem. The analyzer should show you how the channels spread out into each other, so if you are in a really busy area going for the in-betweens of 4 or 11 would let you reduce the congestion.
4. **Migrate.** Can you move the WAP to a different location? Placing it in a place just a foot away can significantly effect propagation within a building.
5. **Ground it.** Make sure all of your AV equipment (TV's, stereos, etc.) is properly grounded.
6. **Find it.** You could use a spectrum analyzer and a directional antenna to find the emitter, but the spectrum analyzer is big bucks. If you know a ham (an amateur radio operator), they may have this gear laying around already; offer them food and don't be surprised if more than one shows up. The ham will also know the FCC regs inside and out which will be great if you actually find the source. Without bribing anyone you could try cutting things off and see if gets better, but it might not be your stuff causing the problem. When dealing with VCR's and TV's you may need to completely unplug them.

Good luck.
I had it with an idle Wi-Fi network (Windows 7, various TP-LINK USB adapters). The ping fluctuates, making any remote terminal work a nightmare.

Solved by putting a constant load on the Wi-Fi network (e.g. fetching a small file from the router's web server in a loop).