This is part two of a multi-part post describing the tuning of Single Channel Architecture/Virtual Cell and taming ChromeOS roaming behaviors. Part one can be found here.
It is no secret at this point that I maintain a very large Single Channel Architecture/Virtual Cell wireless network. From time to time there are certain nuances that force our hand into making adjustments to the wireless network, which really is no different than any other network, SCA or MCA. Since we do primarily use a single channel throughout our instructional spaces, a single, identical, BSSID is seen by all clients within a Virtual Cell. Recently I was troubleshooting network performance issues on Chromebooks and found that they were constantly performing four way handshakes, several times per minute. Other devices in the area, including my Mac, did not experience the same problem. I decided to pull some pcaps to see what I could find. I ran Wireshark for a period of time then filtered out beacons (wlan.fc.type_subtype == 0x0008) and sorted them by lowest to highest RSSI. What I found next started to pull things together from several other recent troubleshooting sessions. I found that my beacons, all from the same BSSID, had valid FCS and were being received with RSSI into the high -80s (dBm). This only left room for an SNR in the single digits.
Because the SNR was so low on these distant beacons, unnecessary roams were being triggered on the Chromebooks even though there was ample signal available from much closer access points. From one second to another, a device could hear several beacons with the same BSSID, some with very poor SNR and others with very favorable SNR. For the last couple of years we have found a sweet spot with TX power and minimum supported rates which has worked nicely with the vast majority of the 80,000 devices we see at peak on a daily basis. Until the recent influx of Chromebooks (tens of thousands) in the past school year there has been no reason to make any adjustments.
Stay tuned for an upcoming post on changes that were made to the infrastructure to make these sensitive devices behave better.