Radio Frequency Engineering with WiFi, or how to stream in comfort and politeness

2013 October 28
by Daniel Lakeland

The vast majority of middle class residences have at least one WiFi router these days. Usually connected to cable or DSL internet services. These internet services work a lot better and cheaper than doing everything over your phone's data connection, but they are inconvenient without wireless distribution throughout your house. Many people get a WiFi router with their cable/DSL set modem up, and a few buy an after-market one and set it up themselves. Most of these devices come with some default settings, enough people just keep those defaults that you will frequently see network IDs (ESSID) that look like "linksys" or "NETGEAR" or some other default name. (In the following I assume mostly US based rules and regulations)

Unfortunately, WiFi doesn't have much in the way of non-interference standards. So for example routers out of the box don't periodically search for the "best" channel and switch to it. Doing so would cause a brief interruption of service, but it wouldn't be that hard to take samples throughout the day of the channel usage, and whenever a very low traffic moment arrives, switching to the "best" channel. Few people would notice a 1-3 second downtime at 2am for example. There's the possibility of some interesting and strange iterative dynamics with such a scheme I suppose. As it is, I see routers tuned to all sorts of channels. For 2.4Ghz there are channels 1-11, and I can see 1, 3,4,6,7,9,11 nearby my house. Few everyday non-tech-geeks know that there are actually only 3 independent channels. These are (if you're a geek, say it with me) ... 1, 6, and 11 (this is different for the newer 5Ghz band where channels range from 36 up to 165 and some are forbidden). Channel 1 interferes with 1,2,3,4,5 but not 6-11, channel 6 interferes with 2,3,4,5,6,7,8,9,10 but not 11, and channel 11 interferes with 7,8,9,10,11. In Europe and some other countries, there may be an extra set of channels 12-14 so that you can get 4 non-interfering ones, 1, 6, 11, 14. In some circumstances where there's sufficient separation between the stations, 1,5,9,11 can be used with only minimal interference but added channel capacity that makes up for it. That kind of deployment requires that you have control over all the stations, like inside a hotel or shopping center, since other people are not likely to choose the right channels.

Another thing access points don't usually do is automatically scale back their power output. In fact, manufacturers tout their high power outputs, and they typically come off the shelf configured to max out the power. High power is useful in a point-to-point link where two wireless devices have directional antennas pointed at each other and are trying to send data over long distances, from a few hundred meters to kilometers in some cases. But in a typical single family home or apartment complex, high power output is a nightmare that doesn't improve the range, but does dramatically increase interference. In one example, a wireless networking conference found that their hotel WiFi was terrible, and the biggest fix was to go around turning off access points and turning down power output until the interference was eliminated.

As an analogy, imagine the following scenario, you are in a parking lot with a megaphone. You are trying to have a conversation with your wife (husband) about what items to buy in the store. Your partner has no megaphone. When you're sitting across from each other in your car, the fact that you are pointing a megaphone at your wife and blasting her in the face with 100dB of sound is probably not going to improve your chances of communicating clearly (try it some time!). And, when she's all the way across the parking lot at the entrance to the store, blasting in your megaphone "Did you get the eggs?" doesn't really help, because without a megaphone on her end, you just can't hear her reply. In the mean time, everyone else in the neighboring cars suffers through hearing every detail of your shopping list when you could be quietly talking inside your car without the megaphone.

The same thing goes for WiFi. I was recently trying to improve my WiFi performance and found that I could turn my access point down to 13dBm and still get just as good performance throughout my house as when I had it cranked to the default of 18 dBm. That's about 1/3 of max power output. Sure, it meant that my signal wasn't FULL BARS at the street in front of my house, but that's actually a huge advantage for everyone. I don't need full bar streaming at my curb, and across the street from me, they don't need my WiFi router to be interfering with their laptop. Whereas in fact my across the street neighbors have their power cranked up in such a way that on my front porch their signal is roughly 83 dBm which is a respectable signal strength. They might be able to sit on my porch and stream their netflix if they had a high enough power laptop wifi.

So what is the upshot of it all? Please go to your wireless router and select a channel from 1,6,11 and turn down your power output to about 12 to 17 dBm (for most people this will work). You can check your wireless signal strength using Wifi Analyzer, the Android phone app. Walk around and make sure your coverage works well inside your living area, and in important outside areas (on your deck or whatever) but not across the street or in the apartment 4 doors down the hall. Finally, if you need to extend your range, consider rather than turning up your power output, to add another station connected via wired ethernet to your main router, on a different channel, with low power, and placed in the middle of the "worst" signal area. This is the same as bringing your wife closer to you in the parking lot instead of turning on the megaphone, your neighbors will thank you. Of course sometimes running the wired line is a pain, in which case, try repositioning your main router, and if that doesn't work, turn the signal up slowly until you get coverage.

Perhaps consider discussing this article politely with your closest neighbors if they aren't using channels 1,6,11 or are using a default high power output and leaking a lot of signal into your dwelling.

2 Responses leave one →
  1. November 6, 2013

    Suppose I want to convince my neighbors that this is a worthwhile investment of time and effort. What concrete steps can I take to show that turning down the power and selecting non-interfering channels will affect something they care about, like, say, download speed?

  2. Daniel Lakeland
    November 6, 2013

    It's a good question. I haven't entirely come up with the answer. In part, most people are not very much power users of networking. Surprisingly few people even back up their laptops. Since the bottleneck in almost all conditions is the cable/dsl speed, most people will never really experience significant problems with their wifi inside their home in a suburban setting. Also, the typical symptom might be something like a 3 second pause in their Netflix stream while their tablet disassociates and then reassociates to their AP and they just get used to that.

    Sure, if they're trying to backup their laptop to an NAS box or something they'll notice it, instead of getting 50 or 100 Mbps they'll be getting 20 or something like that so their backup will take an hour instead of 15-20 minutes, maybe really long processes like that will get disconnected and have to start over. It can be really annoying. But it's only a fraction of people who even know what an NAS box is.

    If you live in a denser housing situation than I do, like an apartment complex, then the problem can be a LOT worse. There it can be difficult to even connect if you're not on the correct half of your living room etc. I think those are the situations where it's relatively easy to demonstrate the improvement, but only AFTER getting large numbers of people to agree to make changes. One single change is unlikely to help much.

    In an apartment complex situation the 5Ghz band is probably your good friend. It will be heavily attenuated by even 2 layers of gypsum wallboard and there are more channels, so that neighboring apartments can simply avoid a channel their neighbors are on and get good coverage within their dwelling. If you have dual band on all your devices (ie. tablets, phones, laptops etc) then you could actually just TURN OFF your 2.4Ghz radio and force all your devices to connect to the lower interference 5Ghz band.

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