5 GHz Channel Planning
Now that we have that covered, let’s move the discussion over to 5 GHz. There is significantly more spectrum available in this band, with each channel occupying its own 20 MHz non-overlapping slice. This is where the topic of channel width gets interesting.
Choosing the Right Channel Width for Your Wi-Fi Network
Whether you are using a static channel plan or a vendor’s dynamic channel assessment/assignment algorithm (pretty much all of them offer some version of this functionality), there are a few things to consider besides just picking Wi-Fi channels. One of the most important is deciding on the proper channel width to use.
Standard 20 MHz channels can be combined to increase the size of the channel with the goal of achieving a higher data rate. The wider the channel, the more data can be pushed through it. You know those impressive throughput numbers vendor’s love to tout in the AP datasheets? Those are achieved by using these wide channels. Some vendors’ equipment these days is even set to these wide channels by default right out of the box.
These wide Wi-Fi channels are created by bonding multiple adjacent 20MHz channels together, using the center frequency to denote the channel. For example, channels 36 and 40 (each 20MHz) are bound together to make 40MHz channel 38, etc.
Source: Wireless LAN Professionals
Sounds great, right? So why not just set your APs to the widest channel available and call it a day? Let’s refer back to the beginning of this post, particularly where we discussed Co-Channel Interference (CCI). The 5 GHz band allows for 9 20MHz channels in UNII-1 and UNII-3 (non-DFS). There are another 16 20MHz channels in UNII-2 (DFS), but these come with their own set of complications, which we will discuss later in the blog.
Let’s say we have decided to use 80MHz channels for our deployment. We just went from 25 non-overlapping channels down to 6. Now, for APs that are at opposite ends of the facility that cannot hear each other too loudly, this is not really a problem. Where problems begin is APs that are in close proximity to each other (hearing each other with at least 4dB above noise floor, typically around -85 dBm or higher). These APs, and any STAs associated to them, now all become part of the same cell, slowing everything down due to increased contention. All STAs need to wait their turn to access the medium.
The other item to consider here is that every time you widen the channel, (20MHz – 40MHz & 40MHz – 80MHz, etc.) you introduce an extra 3dB of noise to the channel. That is effectively doubling the noise. Simplifying this, you now have more noise and no gain in signal. This equates to a lower SNR (Signal-to-Noise ratio), which will in turn force a lower MCS rate, shrinking your data rate and throughput, and possibly negating the benefits of using channel aggregation entirely or even resulting in lower capacity vs 20 MHz channels.
Using Mixed Channel Widths in Wi-Fi
Adding to the topic of bonding channels together in 5GHz, if you are in an environment where you have a mixture of wide and non-wide channels on your own infrastructure, or there are neighboring wireless network infrastructures around you that are using wide channels on 5GHz, this could be a big potential cause of degradation on yours and their Wi-Fi’s performance due to increased amount of collisions (and therefore retransmissions) and potentially using protection mechanisms in the form of Request to Send / Clear to Send (RTS / CTS) control frames that add to the protocol overhead.
One of the hallmarks of a high-performing Wi-Fi network is channel reuse. This is the practice of deploying channels in such a manner that they limit the amount of CCI introduced into the environment. The best way to achieve this is by having as many channels to deploy as possible. While a 20MHz channel will not achieve the higher data rates that are advertised with 80MHz, clients can still reach acceptable speeds, allowing you to optimally use every bit of available airtime.
All of this said, every situation is different. What if you have one AP at your small or home office, decent SNR everywhere and no neighbors/outside sources of contention? Set it to 80MHz or 160MHz and let it rip!
Use wide channels until you can’t.
The bottom line is that for most enterprise-type deployments with many APs, sticking with narrow Wi-Fi channels will give you the spatial reuse you need for your WLAN to perform optimally and leave users satisfied.
Other Considerations for your 5 GHz Channel Planning
Some of the 5 GHz band may be affected by radar activity, called DFS (Dynamic Frequency Selection). Out of 25 available 5 GHz channels in the US and EU, only 9 single 20MHz wide channels (UNII-1 and UNII-3) are unaffected by it. As part of the 802.11h DFS compliance standard, when DFS activity is detected, APs must close channel transmission within 200 ms of DFS detection, clients have 10 seconds to move to a different channel, the AP will not transmit for 60 seconds and will change to a channel that is not DFS affected before it starts transmitting again. The channel that DFS activity was observed on also goes into ‘nonoccupancy’ mode for 30 minutes. But it’s not just the access points that respond to DFS channels. Wi-Fi client devices also behave differently depending on if they are using DFS channels or not.
Passive Scanning Clients
If our Wi-Fi client devices are using passive scanning to discover an SSID, this will mean that they are going on to a Wi-Fi channel — let’s say channel 36 on the 5GHz band as an example — to wait a period of time (around 105ms) for a beacon. Once the device has finished waiting on channel 36 it will then move on to the next channel (40 in this example), wait 105ms for a beacon and, if it hasn’t heard the SSID that it wants to connect to, it will continue to move through the channels until it finally does and then will being the association process.
I know that 105ms doesn’t sound like a long time, but when you multiply that by the 25 channels available in 5GHz, it quickly adds up!
Active Scanning Clients
Moving on to active scanning, rather than the device waiting on a channel to listen for beacons, they go on to the channels and send a frame that is called a ‘Probe Request’ which upon hearing, the APs will respond to with a frame that is called a “Probe Response’ containing the list of SSIDs that the AP’s radios support. The main difference here is that active scanning can be up to 5X faster than passive scanning, as sending a probe request and getting a probe response typically takes around 20ms.
Sounds good, right? So why do our devices not always use active scanning instead of passive scanning? Well, there is a slight catch here. Wi-Fi client devices can only send Probe Request frames on non DFS channels. That means they can only do active scanning on the UNII-1 & UNII-3 channels, whereas on the UNII-2 & UNII-2c channels, they can only do passive scanning.
If you have an environment where you are using the DFS channels in 5 GHz, roaming for your client devices may be noticeably slower. If your devices are using any time-sensitive applications over Wi-Fi, like a voice call for example, the experience may be poor.
Remember, not all Wi-Fi client devices support all of the DFS channels, and some devices may not support them at all! If this is the case and you have the DFS channels enabled in your environment and a Wi-Fi client device comes along that does not have support for the DFS channels you have enabled – well guess what, they will not even be able to hear or discover any Wi-Fi on 5 GHz in that area. To these devices, it will seem like there is no Wi-Fi there at all or that you have a massive coverage hole in your design.
Please make sure that you check your device’s manufacturer data sheet to get a clear idea of which 5GHz channels they support!