Recently I was asked to look at a local college that is experiencing issues in the evening with wireless users. In this example the complaints from students who live in the dorm are obscure and lacking details. They range from “Wifi goes out” to “Video’s buffer”, and “Its just so slooooow”. So, with this little information known we decided to go over and check out the spectrum to see what was going on.
First I talked with the admins to find out about the building, construction, Bandwidth/networking and how students use the dorm wireless. From our conversation I found that the dorm has dedicated internet of around 400 MBits and monitoring is show it busy but not maxed out. Students are working throughout the day in their dorms, and complaints pretty much only come in at night. Construction is new, but lots of brick – signal should not propagate to far (this is important for later). The building which is 5 floors, stands by itself with little sources of wifi interference outside of internal APs.
We then went on site and did a passive survey with spectrum analysis to check coverage, interference, and if the spectrum was busy. Wifi coverage with was great, lots of APs to cover needed space, maybe too many APs (we will get to that later). When conducting the spectrum analysis —- WHOA! its not busy, but look at the signals that are seen. Below shows an screen shot of the spectrum using Ekahau.
My first thought was that’s a lot of APs! I looked at our layout and we only had about 6 APs in this area, but I see many more than that. So the idea that the building materials (brick) would attenuate the enough that we would not see APs on other floors was incorrect. This gives a lot of credit to on site surveys before installing APs – building materials might be different and react different then we think sometimes. I was seeing APs 2 floors above with usable signal. Another thing to notice that there is plenty of open spectrum in the 5 gig range, why are all the channels being used close to each other and at the beginning of the spectrum? At this client Ruckus wireless is being used and after checking documentation and asking vendor reps, seems like it might be a firmware thing. Also notice that we have a lot of APs on the same channel that can hear each other, this would cause contention on the channel.
This shows the importance of always doing a site survey and checking spectrum to see what AP coverage is.
To help resolve the issues we created a channel plan to statically set channels and spread them throughout the spectrum, also lowering channel width to 20 MHZ since we have so much AP coverage between floors. By doing these two things we were able to decrease channel contention and give our users a better wireless experience.
One additional setting we modified was to block broadcast/multicast from clients. When doing a packet capture on the spectrum we also noticed that around 30 percent of the traffic was just MDNS. We created ACLs to block this traffic.
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