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I have a Linksys WRT1900ACS V1 with DD-WRT installed and want to choose the optimal WiFi channels since we just moved. I'm using both the 2.4 GHz and the 5 GHz networks (the 2.4 GHz is mostly for our visitors, we use the 5 GHz one when supported) and I have Net Analyzer installed on my Android phone.

After looking at the charts, I picked up channels 4 and 140 as shown below:

2.4 GHz5 GHz

Now, I'd like to know if I chose wisely or not. The general rule of thumb according to what I've read on choosing a good WiFi channel would be to use channels 1, 6 or 11 since they don't overlap. However, looking at everyone else, they're ONLY on these channels already. Is it better for me to be on the same channel as them (in this case channel 1 seems to be the best) to avoid overlapping with those on channels 6, or is it better for me to stay on channel 4 because there is nobody there? As for the 5 GHz one, it seems easier to avoid any interference, however I'm not sure if I should stay as close as possible to the others or go the furthest away possible (something like channel 108). Any help would be appreciated.

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    Channel 4 is not unused though. It overlaps with channel 6, as indicated in the chart. – Daniel B Sep 5 '18 at 12:51
  • It does, but barely. Less than if I used channel 6 directly for instance. – dnLL Sep 5 '18 at 13:32
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    What using channel 4 means is that you are going to be getting interference from both channels 1 & 6 rather than just one of them. If anyone is transmitting on either of those channels then your signal will be worse than if you just accepted noise from one of those sources. – Mokubai Sep 5 '18 at 13:58
  • @Mokubai I see. In this case seeing there are 2 networks on channel 1 and only 1 network on channel 6, which one am I best to use? The 2 networks on channel 1 aren't as strong as the signal on channel 6. Also, what about the 5 GHz network, there is no difference between any of the 24 non-overlapping channels? – dnLL Sep 5 '18 at 14:01
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Multiple factors, in theory, affect your choices.

Because Channel 4 in the 2.4GHz band is overlapped by active channels, and Channel 140 in the 5GHz band is not, one solo user will find better throughput on Channel 140.

The specs for 802.11 actually call for 22MHz bandwidth, not 20MHz, so the interference between Channel 4 and other channels will be greater than expected.

However, if there are multiple active users on Channel 140, and no active users on channels 1-9 in the 2.4GHz band, there will be fewer collisions (we're using Carrier Sense Multiple Access/Collision Avoidance in WiFi so avoiding collisions is useful) and therefore throughput will be closer to the maximum which 2.4GHz can provide.

A 2.4GHz signal can travel further and has better penetration capabilities than 5GHz; this can be useful as it restricts the number of users which are able to connect and thereby may reduce collisions. However, if there are walls, windows, and/or other objects in the way, the higher frequency user suffers more.

There's also the issue of interference from other users; hams have oft called the ISM bands, including 2.4GHz and 5GHz, 'garbage' bands because of the number of devices which use them but do't coordinate.

Since you have some elbow room in 5GHz, check to see if all your equipment and client devices are 802.11n compatible (which includes 802.11ac) and not using the antique 802.11a spec. If all of them are, you can use multiple channels and bond them together for more throughput.

Enough about theory: Why not measure throughput, and see? The speed test of Sourceforge is your friend.

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For the 5GHz band, it looks like 140 should be perfectly fine. If you really want to get into micro-optimization, you might get a few fractions of a percent better latency and throughput on a different channel depending on the exact arrangement of items in your house, but the moment you move, the exact optimal channel is likely to change. Do make sure though that if you have multiple transceivers for the 5GHz band that you set each 'network' to it's own channel.

For the 2.4GHz band, it's hard to say without actually testing. Based on what is there, your best bet if you want to share a channel is 1, and if you don't want to is either 3 or 4. Testing is your friend here though, since switching channels is generally easy. I would also suggest running multiple tests and averaging the results, especially if you have other devices in the area using the 2.4GHz band (especially Bluetooth devices, as they hop pseudo-randomly between the channels defined by Bluetooth).

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