Overlapping Basic Service Sets

In 802.11n & 802.11ac it is possible to bound channels together in 5GHz band. Up to 8 channels can theoretically be bounded together in 802.11ac wave2. The main goal for this solution was to acquire higher data rates.
In enterprise environments, this has already been found to be unfit because of the scalability issue relating to airtime and the inefficiency of these wider channels. OBSS occurs due to two or more unrelated or even related BSSs, used in the same physical space, at the same time and with the same frequency channel.

Example:

In this example SSID:

vw-group

is the corporate WLAN of which 2 AP’s are seen in this area: 1 on CH36 & 1 on CH44

SSID:

Diouf & Partners

is a single neighbouring Access Point.

vw-group

is using 20MHz wide channels

Diouf & Partners

is using 80MHz wide channels.

What happens here is that

Diouf & Partners AP

will send all of its Management Frames on the Primary channel 36, but will use CH40,44&48 for sending/receiving DATA frames. So, whenever this AP is sending/receiving DATA to/from an 802.11ac 4 stream capable client, like a MacBookPro for example, it will interfere with all Corporate AP’s on channel 40,44&48 in the same area. In this case CH44 is used by a corporate AP in the same area. CH36 will be less impacted because the operation on the primary channel is the same as on a 20MHz channel and CCA will allow for airtime fairness. Although throughput will go down because of contention will go up when 2 AP’s and their respective clients need to compete for the same airtime.

On top of this there’s a known issue with the way the protocol is set up which negatively impacts the clear channel assessment. On the primary signal CCA dictates that the medium will report as busy when signals are heard stronger than -82dBm. On the secondaries however it will need to hear signals stronger than -72dBm to report as busy. This means it is a lot less likely to hear all clients in its coverage area and start transmitting at the same time. Ultimately this will lead to more collisions, retries, more overhead on the WLAN and poorer performance of the corporate WLAN. And of course, when more 802.11ac clients become prevalent, the amount of data frames will rise and the number of collisions will rise.

Results:

2 instances of OBSS are detected on floor 2.

In both cases, a neighbouring AP is deployed using 802.11ac 80MHz wide channels.

AP 1:
TP Link SSID: Diouf & partners, is using CHA42 as center Frequency (CH36 as primary and CH40, 44, 48 as secondary).

AP2:
Unknown, SSID:Devolo-eed, is using CH 106 as center Frequency (CH100 as primary and CH104, 108 &112 as secondary)

Recommendations:

On this floor, we recommend to only use UNII-2 channels 52,56,60,64 in order to avoid those 2 AP’s that are effectively interfering with UNII-1 & UNII-2e channels.

Useful: