What set of channels in wifi networks are considered to be non-overlapping channels?

I. Networking Fundamentals

7. Describe wireless principles

7.1 Non overlapping Wi-Fi channels

Wireless communication usually involves a data exchange between two devices. A wireless LAN goes even further, many devices can participate in sharing the medium for data exchanges. Wireless LANs must transmit a signal over radio frequencies (RF) to move data from one device to another. Transmitters and receivers can be fixed in consistent locations, or they can be mobile and free to move around. A WiFi channel is the medium through which our wireless networks can send and receive data. The 2.4 GHz band has 11 channels and the 5 GHz band has 45 channels. Selecting the proper WiFi channel can significantly improve your WiFi coverage and performance. In the 2.4 GHz band, 1, 6, and 11 are the only non-overlapping channels. Selecting one or more of these channels is an important part of setting up your network correctly.

The 802.11 b/g WiFi standard defines a total of 14 frequency channels. The FCC allows channels 1 through 11 in the U.S., and most of Europe allows channels 1 through 13.

WiFi channels actually represent the center frequency that the transceiver uses (e.g., 2.412 GHz for channel 1, 2.417 GHz for channel 2, etc.). There is only 5 MHz separation between the center frequencies, and the signal occupies about 22 MHz of the frequency spectrum. As a result, an 802.11b/g signal overlaps with several adjacent channel frequencies (by 11MHz on each side of the center frequency). This leaves only three non-overlapping channels: 1, 6, and 11 in the U.S. to use without causing some interference between access points.

The best approach when choosing a channel is to do a site survey from your wireless client computer (info), or the router/access point (if available). Note the channels of nearby wireless networks. The default on many routers is channel 6.

Once you have a site survey available, just keep in mind to use non-overlapping channels (1, 6, 11), or minimize overlap of signals by using channels as far apart as possible from other networks in range. You can experiment by checking signal level when you configure your access point on a certain channel as well. This may be important in fine-tuning for the best possible signal, as some channels may experience interference from other sources that are not detected as neighboring networks.

There are many tasks associated with properly designing and deploying a wireless network—one of the most important is developing a channel plan. A well-developed channel scheme will assist with squeezing every bit of precious airtime, which is one of the foundations of high performing Wi-Fi Networks.

Before we go too much further, let’s go over a few of the basics. The IEEE 802.11 standard defines operation for wireless networks in both the 2.4 GHz, 5 GHz and now 6 GHz frequency ranges. Depending on where you are in the world, you will have different amounts of channels that you can have access to with different rules and regulations. 

In the United States, the 2.4 GHz band is broken up into 11 channels (1-11), each 20MHz wide. In the 5 GHz band, we have channels ranging from 36 up to 165, and in the 6 GHz band, we have Wi-Fi channels ranging from 1-233.

2.4 GHz Channel Planning

Even though there are 11 channels available in 2.4 GHz in the US, only 3 of them do not “overlap” or interfere with one other: 1, 6, and 11.

2.4 GHz Channel Overlap & Adjacent Channel Interference

Without getting too deep on how wireless communication happens, when a station (Access Point, client device, etc.) has something to transmit, it must wait for the channel to be clear. Put simply, only one device can successfully transmit at a time. When overlapping channels are used, any stations (STAs) on those channels will transmit independent of what is happening on the other channels, causing a degradation of performance. Think of it like being between radio stations and having a mix of country overlapping on your favorite metal station. This type of interference is called Adjacent Channel Interference (ACI).

2.4GHz Channel Overlap. Source: Ekahau ECSE Design Course 

Adjacent Channel Interference. Source: Ekahau ECSE Design Course

2.4 GHz Co-Channel Interference

Co-Channel Interference (CCI), on the other hand, is when 2 or more AP’s that are in the same area are operating on the same channel. This essentially turns both cells (a cell is the coverage area for an AP) into one big cell. This means that any STA that has anything to transmit now must wait for not only the other STAs associated to the same AP, but also all the STAs associated to the other AP on the same channel. While not as damaging as ACI, CCI will also degrade performance. This is caused by more devices trying to gain access to the wireless medium on the same channel, making STAs wait longer for their chance to transmit.

2.4 GHz CCI (Co-Channel Interference) Source: Ekahau ECSE Design Course

Up to this point, we have only used the 2.4 GHz band for examples. With its limited amount of available spectrum, it is highly recommended that only non-overlapping 20 MHz channels are used.

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!

If you have a small to medium size deployment and have done your homework (with Ekahau Pro, of course!) to ensure you can use 40MHz channels, give it a shot!

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!

Want to Learn More?

For even more tips on Wi-Fi channel planning best practices, register for our upcoming webinar, Demystifying Wi-Fi: Channel Planning Made Simple. In the webinar, we’ll dive deeper into Wi-Fi channel planning by discussing:

• What are Wi-Fi channels?
• How channel overlap occurs and how to plan around it
• The difference between adjacent channel & co-channel interference
• CSMA/CA aka DCF aka “The Game”
• Band comparison of 2.4 GHz and 5 GHz and how they each inform channel planning
• Optimizing for the right channel widths for your network

What is overlapping channels in Wi

What channels are non

Are all 5GHz channels non

How many overlapping channels does 2.4 GHz have?