BY AREN GATES IN · EXPERTS · SEPTEMBER 1, 2016

In my previous post, I discussed the two classifications of interference on your wireless network: non-Wi-Fi sources and Wi-Fi sources. In this post, I focus on a source of Wi-Fi interference specific to 802.11ac and how it is mitigated with current technology.

802.11ac brings its own challenge that is not often discussed: Overlapping BSS (OBSS). OBSS is defined as two BSSs that, well, overlap, because their channel configurations occupy some of the same space. 

Consider the following scenario:

Two APs can hear each other. AP1 is set to use channel 36+40+44+48 (primary channel 36+40) while AP2 is set to use channel 44+48. Due to the nature of 802.11 and co-channel interference, AP1 cannot transmit while AP2 is transmitting.

In this scenario, the APs will forever dance the dance of CCI, waiting for the other to finish transmitting before taking over the medium itself. Wouldn’t it be better if instead of waiting, they could both transmit at the same time? 

Figure 1: OBSS Visualized

802.11ac has a built in mechanism to help mitigate the issue of OBSS. AP1 could simply utilize the other 40 MHz that is not occupied by AP2. AP1 has its primary channel set to 36+40, meaning that while AP2 is transmitting on 44+48, AP1 can transmit data simultaneously! 

Be wary, though, in this scenario when both APs are transmitting, the throughput on AP1 is half of the throughput compared to when only AP1 is transmitting. This is because AP1 switches from an 80 MHz channel width down to a 40 MHz channel width. However, while the individual throughput might be lessened, the overall throughput of the system is greater.

Lastly, remember that OBSS doesn’t just occur with 80 and 40 MHz channels. Many high density deployments are choosing to not use channel bonding in order to provide added connectivity instead of added throughput. This means that you might run into a 20 MHz channel overlapping your 40 or 80 MHz deployment.

Don’t worry, though, 802.11ac can shrink the channel width all the way down to 20 MHz in order to “play nice”. That means that if you set your AP to 36+40+44+48 with 36 being the primary channel and you hear another AP on channel 40, your AP will shrink its own channel width down to channel 36 in order to continuously transmit.

Figure 2: 802.11ac primary channel assignments

When designing your wireless network, take into consideration Wi-Fi sources of interference and deploy accordingly. If you are not achieving your expected throughput, Wi-Fi might be the cause. APs on the same channel (CCI) could be severely degrading network performance. Even APs that are broadcasting too loudly on other channels (ACI) can generate issues.

Lastly, beware OBSS. Even though there are mechanisms built in to suppress the effects of OBSS, you might still end up with those pesky support calls that we all dread. Hopefully, at the end of the day, someone will finally compliment you on how stable the wireless is.