The school networks I look after have a common problem: the access points are all in the hallway, leading to massive co-channel interference (CCI). The only way to really fix this is a complete redesign, so my title might be a little misleading. For now, budget and time restraints dictate that I must make them work as well as possible as-is. Have lemons, make lemonade, right? Add some gin and call it Party-Fi.
So, for my first trick, I shall make an incredible amount of co-channel interference disappear. With the help of my lovely assistant Ekahau Site Survey (Vanna White) and the not-so-lovely master of ceremonies Juniper RingMaster (think James Franco hosting the Oscars), I’ll demonstrate just what putting AP’s in your hallway does to your network. Here is what your coverage might look like.
That is a lot of green right? And green is good! Green means go! Go dog go! Or is it? Lets have a look at another important visualization: channel overlap. You can read about channel overlap in this excellent blog post by Andrew Campbell (@WiFiAndrew) of Ekahau.
Not so green anymore, is it? Not quite 50 shades, but I would call that about 95% grey. You may be asking, what does the grey mean? Well, the grey indicates an area in which a station can hear 3 or more access points on the same channel at a signal strength of -80 dBm or greater; there are at least three APs trying to share the same airtime on the same channel, as well as their associated clients. Some areas had 5 or 6 radios overlapping. The problem with hallway-fi is that there is no attenuation between the access points, so RF energy travels a long way, and co-channel interference is very hard to avoid. Here is an example from my initial survey, showing how the RF energy from the selected AP in orange travels down those hallways and reaches 9 other APs. If any of those 9 APs are on the same channel as the selected one, they are sharing airtime and may as well be one access point. Besides, how many of your clients sit in the hallways?
If you haven’t counted the access points yet, there are 18, some of which are mounted vertically. The vertical APs really help keep the signal out of the classrooms as well, because of the antenna characteristics. I hope to post on that soon as well. Eighteen access points works out to just less than 1 AP per 2 rooms; there are about 30 rooms (shops, offices, classrooms) that house clients. It’s not quite the salesman’s bullseye of 1 per room, but it is still too many. How many should there be? I’ll get to that.
Another issue is the transmit power. All 2.4 GHz radios were set to 18 dBm (63 milliwatts). The 5 GHz radios were set to 15 dBm (31.6 mW). Both of those levels are too high, in fact they are the maximum for that model of access point (Juniper WLA532).
Most of my clients aren’t going to transmit at that power level at a desirable date rate (ie MCS5 or higher for 20 MHz/1SS), and the resulting cell sizes are too big for the number of access points. Those power levels will also lead clients to connect to the 2.4 GHz radio, as its signal will be stronger. Since it’s summer and no students are in school, a graph of client connections isn’t helpful to compare results.
One last thing: all legacy (802.11a/b/g) data rates were enabled. Those beacons at 1 Mbps are going an awfully long way. My LG G3 would not roam until it was across the building from where it first associated (although I believe Android clients are notoriously sticky); it just rate shifted lower and lower as I moved away from the associated AP. This is what the RF neighbor table looks like from one AP.
So, in this school, we had too much co-channel interference for four reasons:
- too many access points,
- access points in hallways,
- 802.11b and lower 802.11a/g data rates enabled, and
- transmit power too high.
Depending on the network, some CCI in some places in acceptable. Andrew von Nagy (@revolutionwifi) presented at the last WLPC and talked about not being able to avoid some CCI in designs. CCI can be a pretty tough itch to scratch in certain deployments such as very high density, but I haven’t had that pleasure yet.
I started off with a survey to see exactly what was going on, the results of which are in the two heat maps above. Side note: I quit using continuous surveys in classrooms as there is just too much stuff to walk around. I still use continuous for hallways, but stop-and-go for classrooms. Attenuation values didn’t matter to me at this point, so I took four measurements in each room, around where students would sit.
My first configuration change, regardless of survey results, was to eliminate legacy rates. We have a few 802.11g clients left, but certainly nothing 802.11b. I also set the minimum basic (and therefore the beacon) rate to 24 Mbps for 802.11g and 802.11a. After taking a good look at the coverage of each access point, I ended up disabling half of them. Transmit power levels were set to 4 dBm on 2.4 GHz radios, and 12 dBm on 5 GHz. I have to manually steer clients to 5 GHz because Juniper’s band steering is part of their load balancing feature. I’ve found things work better with that disabled, and my sites don’t have a high enough density of clients to require it. After those changes, I went back out and resurveyed. Here is the signal strength map with 9 access points.
You can see there is not nearly as much green, a little yellow, and even some (GASP!) orange. Above is the 5 GHz coverage. The 2.4 GHz coverage is a little less good, with grey in the bottom left corner and middle portion just above the AP on channels 1 and 157. I plan to address the grey space in the center (gymnasium) with an AP and directional antenna. I ended up making a change in the upper right hallway, which I’ll go over shortly. Next is the 2.4 GHz channel overlap of the new configuration.
Lots of green on that channel overlap map, which in this case is a good thing. The channels that do overlap are not the same as the strongest AP in the area, so the effects should be minimal. Here is the 5 GHz overlap.
Those are 40 MHz channels in 5 GHz. So for the changes in the upper right hallway, I disabled the two APs that were on and enabled the other two that were in that hallway. I kept the channel plan the same and moved them, for lack of a better phrase, to the left. The rooms at the top end of the hall on both sides are computer labs full of desktops, so some poor coverage there is not a problem. Here are the survey results from after those changes.
I only resurveyed the hallway in question and a bit beyond, and I was happy with the results. The new channel overlap was basically the same.
Now, did I have enough access points? I pulled up the Capacity Planner from Revolution Wi-Fi to find out. This is a great tool. If you haven’t checked it out, I highly recommend you do so, as well as the blog. It really takes the guessing out of the capacity portion of a WLAN design. Here are my input and results.
I know, I know… Wire up those Apple TVs; working on it. Either way, you can see with 9 access points I should be doing OK until we upgrade and redesign.
The final thing I wanted to check was how an iPad would connect in the rooms that had signal levels in the -70’s (the orange areas, specifically the bottom left of the map). I grabbed an iPad 2 running iOS 8.3, found this post about wi-fi scanning in iOS by George Stefanick (@MY80211), and downloaded the nPerf app for testing. Here is the setup in the room. Pretty simple.
The iPad scanning results looked like this.
The Ekahau signal levels looked like this. I’ll call those close enough to make no difference. A former coworker of mine used to say, “What’s a couple dB between friends?”.
The nPerf results on the iPad were OK. In this particular room, there will only ever be 3 or 4 iPads connected using core apps; no Apple TVs or 802.11g laptops.
For comparison, here is a test from directly under an AP with a received signal level of about -40 dBm.
Granted, tests to the internet, and throughput tests in general, may not be the best indicator of network performance, especially with the WAN we have.
So, to optimize this hallway “design” and minimize co-channel interference, I did the following:
- turn off legacy rates
- turn off half of the access points
- turn 2.4 GHz transmit power WAY down
- slightly reduce 5 GHz transmit power
- turn off Juniper’s auto-channel setting and do a manual channel plan
All in all, lots of turning things off. It’s a shame when you consider doing a proper design in the first place would have showed only 8 dual-band APs were needed, instead of 18! I am pretty happy with the results of the changes I made, and we’ll see how things work when September rolls around and kids are back in school.
Thanks for reading. Please comment if you have anything to add or share, or have any questions!