Safely Using 80 MHz Channels with 802.11ac

The big appeal of 802.11ac is higher bandwidth, which will be accomplished in first generation 11ac products primarily through the use of wider 80 MHz channels. Channel planning in 802.11ac also involves assigning primary channels to allow for dynamic per-frame channel width adjustments to reduce co-channel interference (CCI). In my last post, I recommended that you select only one primary 20 MHz channel at the channel width you can likely "guarantee" is free from CCI. This means that most enterprises should design around 20 MHz or 40 MHz channels since they provide more non-overlapping channels to reduce CCI.

Why shouldn't you plan around 80 MHz channels? There likely aren't enough non-overlapping channels at 80 MHz to reduce co-channel interference, so you are better off planning around non-overlapping 40 MHz channels. 

But many organizations will still want to benefit from the higher peak performance gains that 802.11ac can provide. That's the big appeal, right?! The answer is to take advantage of the per-frame channel width capabilities of 802.11ac to dynamically allow wider channel use when the entire 80 MHz channel is clear (not busy).

Let's demonstrate by using two examples...

Example 1 - Planning around 40 MHz channels
You design your enterprise WLAN around non-overlapping 40 MHz channels because there are a sufficient number of channels for you to safely re-use channels across your environment without creating co-channel interference. You also enable 80 MHz channel width on your WLAN, which will be used on a "best-effort" basis if the entire 80 MHz channel is clear.

Note - One big assumption with this example is that DFS channels are supported by your clients. If not, then you're still best off planning around non-overlapping 20 MHz channels and using both 40 MHz and 80 MHz on a best-effort basis.

You designate primary 20 MHz channels so that it results in non-overlapping 40 MHz channels. If you're in the U.S. you can't use 40 MHz channels 118 and 126 (due to TDWR restrictions), so this results in 10 non-overlapping channels. If you're in the UK/EU you can't use 40 MHz channels 151 and 159 (due to Band C licensing), so this also results in 10 non-overlapping channels.

802.11ac Non-Overlapping 40 MHz Channels
Remember that administrators only configure the primary 20 MHz channel, and the primary 40 MHz and 80 MHz channels are dynamically assigned by the AP. I provide a deeper explanation in my post on 802.11ac Channel Planning. In this graphic, primary channels at various channel widths are denoted with gray and dotted-gray shading.

You'll also need to consider AP channel assignment based on physical AP locations in order to maximize the likelihood that 80 MHz channel widths can be used without co-channel interference. You can accomplish this by skipping one primary 20 MHz channel when assigning channels to neighboring APs. For example, if AP1 and AP2 are neighbors, assign AP1 primary channel 36 and AP2 primary channel 52, skipping channel 44. In this manner, neighboring APs result with different 80 MHz channels which are less likely to interfere with one another.

Here you can see that the greater number of 40 MHz channels reduces CCI when compared to 80 MHz channels. 80 MHz channel width can still be on a best-effort basis, if enabled, but remember that two adjacent 40 MHz channels will still use the same 80 MHz channel width. In this example, channels 38 and 46 would share the same 80 MHz channel 42. We have staggered them in our RF design to decrease the signal strength between the two and maximize the possibility of 80 MHz use, even though we can't guarantee it.

40 MHz Co-Channel Interference is Less Likely
In this manner, we have enabled 80 MHz channel use, but have assured ourselves that we can safely fallback to 40 MHz channel width on a per-frame basis if the larger channel width is busy. This allows us to take advantage of the higher performance that 802.11ac wide channels offer without creating large collision domains and high levels of co-channel interference.

Example 2 - Planning around 80 MHz channels is a recipe for disaster!
You design your enterprise WLAN around non-overlapping 80 MHz channels, even though there is greater AP density than non-overlapping channels. You've decided to take a gamble and see if you can get the higher performance that wider channels bring all the time, at the risk of creating more co-channel interference.

You designate primary 20 MHz channels that result in non-overlapping 80 MHz channels (36, 52, 100, 116, 132, 149). If you're in the U.S. you won't be able to use channel 122 (due to TDWR restrictions), so you're left with 5 non-overlapping 80 MHz channels. If you're in the UK/EU you won't be able to use channel 155 (due to Band C licensing), so you're left with 5 non-overlapping 80 MHz channels as well.

802.11ac Non-Overlapping 80 MHz Channels

However, you have a fairly dense AP deployment, resulting in some co-channel interference between APs. Let's say two APs, both using 80 MHz channel 42 can hear one another and sense that the air is busy. Using the per-frame channel width capabilities of 802.11ac, they attempt to back-down to smaller channel widths. However, there is a problem... since you've designed your primary channels based on an 80 MHz channel width, both APs attempt to back-down to the same primary 40 MHz channel (ch38) and primary 20 MHz channel (ch36). They can't avoid the co-channel interference! This results in both APs sharing airtime and reducing network performance and capacity.

80 MHz Co-Channel Interference is Likely

This happens because when you plan around the larger 80 MHz channel width, the primary channels that are assigned at the smaller channel widths are more likely to result in co-channel interference as well. Therefore, if co-channel interference does occur, the neighboring APs will be unable to back-down to smaller channel widths to avoid the interference.

It would be better to allow them to back-down to non-overlapping 40 MHz channels, breaking apart their collision domains so they can both transmit at the same time and avoid co-channel interference. This is exactly what happens when you plan around smaller channel widths instead!

Final Thoughts
With 802.11ac it may be tempting to use 80 MHz channel widths for peak performance. However, in order to reduce co-channel interference it is recommended that you derive your channel plan using non-overlapping 20 MHz or 40 MHz channels instead, allowing 80 MHz channel width on a best-effort basis. This allows APs to back-down to smaller channel widths that are non-overlapping when 80 MHz CCI is present using per-frame channel width capabilities available with 802.11ac. This allows APs to use the higher peak performance when possible, while maintaining separate collision domains at smaller channel widths when 80 MHz transmissions are not possible.

Cheers,
Andrew


802.11ac Gigabit Wi-Fi Series: