How to configure Wi-Fi channels for top network performance

By Eric Geier (Our Owner & Lead Wi-Fi Consultant)

Originally published on NetworkWorld

Nothing is more important to the performance of your company's Wi-Fi network than the channels used by your wireless access points (APs). You can blanket a building with the latest and greatest APs, placed in the most advantageous spots, yet have a failing network if they aren't set to optimal channels on the Wi-Fi frequency bands.

You need to avoid co-channel interference, which is when APs within range of each other are on the same or overlapping channels. You must also circumvent non-Wi-Fi interference from cordless phones, Bluetooth headsets and other devices that emit wireless signals – even the office microwave. Both types of interference can be reduced by picking better Wi-Fi channels.

That said, it's nearly impossible to fully eliminate all interference, as it can come from inside your building from unknowing staff, such as those who install their own wireless router or enable the Wi-Fi hotspot on their smartphone or tablet, as well as from networks in neighboring offices and buildings that you have no control over. Adding further complication, the interference can change at any moment as users enter and leave the area with their devices, turn their gadgets on and off, and so on. This is why it's crucial to periodically check for interference.

Nearly all APs these days have an auto-channel feature that's supposed to choose the best channel when the AP boots up, and some additionally have a dynamic channel feature that scans the airwaves on an ongoing basis (either continuously or at set intervals or times of day) and switches to the channel with the best signal. But the level of sensing and accuracy differ among APs, so you should always manually verify auto-channel assignments immediately after deployment and periodically afterward. To properly analyze the channels, though, you need to understand the frequency bands and channels.

There are two radio frequency (RF) bands designated for Wi-Fi use: 2.4GHz and 5GHz. Both bands make use of unlicensed radio spectrum, meaning Wi-Fi devices don't get exclusive access to those airwaves but must share with an array of other wireless devices, including cordless phones, wireless security cameras, microwave ovens, Bluetooth and Zigbee devices, radar systems and more. Wi-Fi devices using the older 802.11b and 802.11g standards use just the 2.4GHz band, whereas newer 802.11n and 802.11ac devices can make use of both bands.

You'll find that the 2.4GHz frequency band is quite crowded, and its overlapping channel design, which we'll get to in a moment, limits the number of usable channels. Although the band is really not even big enough for Wi-Fi networks alone, the fact that it shares with several other unlicensed wireless technologies makes it even worse. The 5GHz band is much larger, making it less congested, but there are some stipulations to its usage that can limit the number of usable channels in the band.

Understanding the 2.4GHz band

The 2.4GHz frequency band has a total of 14 channels for Wi-Fi, but in practical terms, there are usually only a maximum of three channels. Why? First, not all regions support each of the 14 channels. In North America, only channels 1 – 11 are fully supported, while most other regions support up to channel 13. In Japan, all channels are available, but 14 is restricted to the old 802.11b standard.

The overlapping of the channels, as depicted in the graph below, causes a dramatic reduction in the number of usable channels. When an AP or other Wi-Fi device transmits on a given channel, it actually spreads the signal over about four channels' worth of space, which is either 20MHz or 22MHz wide, depending upon the wireless standards in use. The channel number of the device corresponds to the center frequency.

courtesy Aerohive Networks

The width of the Wi-Fi channels in the 2.4GHz frequency band causes them to overlap.

As shown in the graph, if you set your AP to channel 1, the signal spreads all the way to channel 3. If the AP is set to channel 6, the signal is spread across channels 4 and 8. When on channel 11, signal goes from channel 9 to 13.

Making use of just channels 1, 6 and 11 (typically called the non-overlapping channels) gives you the most usable bandwidth. Channel 14 would give you another non-overlapping channel, but again, that channel is supported only in Japan.

The 802.11n wireless standard, introduced in 2009, added optional channel bonding, which combines two adjacent 20MHz channels to create a single 40MHz-wide channel, as a way to increase the throughput and speed of Wi-Fi connections. However, as you can visualize from the graph, there's only enough space in the 2.4GHz band for one non-overlapping 40MHz channel, with the remaining space offering enough room for just one regular 20MHz non-overlapping channel. These constraints are acceptable in very few environments and networks; thus it's usually best to stick with the legacy channel widths in the 2.4GHz band.

Understanding the 5GHz band

The 5GHz frequency band is quite different from 2.4GHz. As you can see in the illustration below, it offers much more frequency space, providing up to 25 possible channels. As you'll learn, however, there are many caveats to 5GHz usage, and the number of configurable channels on your APs may be significantly fewer than 25.

Immediately apparent is the different numbering scheme. The first Wi-Fi channel is 36 and the last is 165. However, not all channels are available. Instead of allowing you choose from each consecutive channel (36, 37, 38, etc.), Wi-Fi devices are configured for operation only on non-overlapping channels (36, 40, 44, etc.) if the legacy 20MHz channels are used. All of the configurable channels are separated from one another by four channels, but there are gaps (such as the jump from channel 64 to 100) because the frequency space given to Wi-Fi is not fully continuous.

Not all APs support every available channel, and that's largely because of regulations that restrict the usage of different portions of the frequency band. Some APs don't have the necessary technologies to meet the restriction requirements because the vendors may choose not to include these technologies to reduce costs.

The restrictions that most impact wireless networks are for channels 52 through 144. APs that access those channels must support dynamic frequency selection (DFS) and transmit power control (TPC) – detection and avoidance mechanisms that prevent APs from interfering with radar systems that have higher priority in that frequency space.

If an AP detects radar activity, such as from military or weather stations, on the restricted channels over a certain threshold, it must reduce its transmit power via TPC or change channels via DFS. Radar activity may be detected by networks from a few up to 21 miles away from radar stations. The remediation process taken after radar detection can interrupt wireless connectivity for 5GHz users as the AP tries other channels. In addition, the detection accuracy varies, and you might see false positives from other RF sources.

 

As with the 2.4GHz band, the 802.11n standard adds the optional use of channel bonding in the 5GHz band, which gives you up to 12 non-overlapping 40MHz-wide channels. The 2014 802.11ac standard introduced even larger channel widths in the 5GHz band. In the first wave of 802.11ac products, widely available now, up to 80MHz channels are supported, giving up to six possible non-overlapping channels at that size. The second wave of 802.11ac products just beginning to roll out are able to support up to 160MHz wide channels, providing just two possible non-overlapping channels, which isn't acceptable in most environments and networks. As you'll read later, however, there likely will be more space opening up in coming years.

 

Keep in mind that if your APs lack the DFS/TPC channels of 52 through 144, or if you can't use them because of nearby radar activity, the number of channels available to your network is greatly reduced. At the legacy 20MHz channel-widths, you'd have up to nine non-overlapping channels available. Going to 40MHz would give you four non-overlapping channels, and at 80MHz you'd have only two. Thus, for APs that don't support all channels, you'll likely need to stick with 40MHz channels at most.

This illustration shows the 5GHz Wi-Fi channels currently available without the DFS/TPC channels at the different channel widths.

Checking channel usage

For a quick and simple view of the airwaves, you can use a free Wi-Fi stumbler on a laptop or Android device. These list nearby APs and give their basic details, including the channel, signal level and security status. However, almost none of them detects the background noise levels or gives the signal-to-noise ratio (SNR) – two numbers you should consider in addition to signal when determining the best channels. (More on that in a moment.) They also typically lack the ability to reveal the name of hidden SSIDs, which is useful if you ever need to identify a wireless network that has disabled SSID broadcasting.

For more through details, consider professional Wi-Fi analyzers and map-based surveying tools, such as those from AirMagnet, Ekahau or TamoGraph. The analyzers report on noise and SNR levels and also reveal any hidden SSIDs. The map-based surveying tools allow you to visualize channel usage, signal and other parameters on a heat map. This visual look at the network is extremely useful, especially for larger networks.

For a even more accurate look at the Wi-Fi channels, consider using an RF spectrum analyzer such as AirMagnet Spectrum XT or Wi-Spy, which report on signal and noise, even from non-Wi-Fi devices. Typically, Wi-Fi stumblers and even professional analyzers alone can't read non-Wi-Fi signals. However, most professional analyzers and surveying tools offer some type of integration with RF spectrum analyzers.

Configuring bands and channels

Now that you have a good understanding of the Wi-Fi bands and channels, you can better manage the airwaves. When deploying a network, it's usually best to design for full 5GHz coverage, which has shorter transmission ranges than 2.4GHz. That means you'll need to place APs closer together than you would with a 2.4GHz-only network. This could be anywhere from 20 to 40 feet closer but depends upon the environment; placement should be determined by surveying. Consider using any band steering functionality on the APs as well, to get as many users as possible onto the bigger, less congested band.

 

When manually assigning channels to APs, remember that it's best to stick with legacy 20MHz-wide channels on 1, 6 or 11 for 2.4GHz, in order to avoid the overlapping channel issue. For 5GHz, try staying with 40MHz channel widths, and avoid the DFS/TPC channels of 52 to 144 even if your APs support them. If you need to increase throughput further for faster speeds or denser networks, and you don't think radar interference will be an issue, you can certainly try using all channels with larger channel widths.

Your APs' wireless coverage areas should overlap each other somewhat so there are no gaps in the coverage (a.k.a. dead zones), but make sure you alternate the channel that each AP uses so there's no same- or overlapping-channel interference. When assigning channels, perhaps start with the APs on the outer edges of the desired coverage area, as you'll have to consider channel usage of any neighboring networks as well. If there are multiple floors, this all gets even trickier.

This AP setup alternates non-overlapping channels on the 2.4GHz band.

As mentioned above, it's best to look at noise and SNR levels in addition to signal when evaluating channels and verifying coverage. Noise is basically the amount of interference the APs and wireless devices have to contend with. The SNR is the difference between the noise and the signal – it gives you a quick value to judge the overall quality of the signal. Typically, the higher the SNR, the better the quality of the signal and potentially the better the connection between the APs and wireless devices.

When optimizing your wireless network, you should define some minimum values, such as a signal of -60 decibel-milliwatts (dBm), noise of -90 dBm and SNR of 30 dB, and make sure you see these values or better when taking measurements in the Wi-Fi coverage areas. You could raise or lower these baselines depending upon the performance level you want for your network.

 

In addition to looking at signal, noise and SNR when choosing channels, you may need to consider the average network usage or the typical amount of traffic for individual APs, which some Wi-Fi analyzers can tell you. For instance, if the 2.4GHz band is too crowded and there's no completely free channel, you'd want to compare the activity of the APs. There's a chance it might be better to use a channel where a neighboring AP shows a higher signal (and thus more potential for interference with the AP you're configuring) with much less wireless traffic than choosing a channel where a neighboring AP shows a lower signal (less potential interference) with much more wireless activity.

You can read more on channel usage and other ways of boosting Wi-Fi performance in our past article, "9 tips for speeding up your business Wi-Fi."

More 5GHz channels coming

In 2014, the FCC allocated more space in the 5GHz band for Wi-Fi, but we won't see new APs that support these for a few years. The new space will be enough for 11 more 20MHz channels, six more 40MHz channels, three more 80MHz channels or two more 160MHz channels. This will make using the larger channel-widths much more practical.

The 5GHz Wi-Fi channels marked Currently Available and New Channels in the graph are all available now; those marked Next Channels will be available in the next few years.

In the meantime, however, the new Multi-User MIMO (MU-MIMO) technology coming with second wave 802.11ac can help increase speeds without wider channels. We're just now seeing it appear on consumer routers, and we'll see it on business and enterprise APs within the next year.

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