Router and WAP use same WiFi channel

Question

I have a WiFi network setup with a router (with WiFi) and a WAP (Wireless Access Point). Specifically I use an ASUS-AC66U as router and as WAP I have an ASUS RT-N56U.

As modern devices do, they choose WiFi channels automatically. They automatically choose different channels for the 2.4GHz band.

However, I have noticed something surprising concerning the 5GHz band. They both choose Channel 36. If I reboot one of them, it chooses 36 again after startup.

Normally using the same channel would cause conflicts, reducing the bandwidth. But these two devices seem to choose the same channel on purpose!

Is it so? Do they perform some sort of negotiation at startup and then cooperate in using the same channel? Or is this a bug?

(The negotiation theory is perhaps strengthened by the fact that they are both fairly modern ASUS devices with similar, up to date firmware, 3.0.0.4.374_726 and 3.0.0.4.374_239, respectively)

I have not detected any reduced bandwidth, but it is hard to tell.

Clarification: The WAP has an ethernet cable connection to the router. So if I understand the term correctly I use "ethernet backhaul".

Answer 1

If you're using wireless as the backhaul between the two, then they're choosing the same channel to make that backhaul work. Each of those units only has one radio interface per band, so if you've got them configured to use a wireless backhaul, and they chose to do it over 5GHz for performance reasons, then they have to both put their 5GHz radios on the same channel so they can talk to each other for the sake of the wireless backhaul. But if you're using wired Ethernet as the backhaul, this wouldn't apply.

Even if you're using Ethernet as the backhaul, there are a lot of reasons why an automatic channel selection algorithm might choose channel 36. Here are some factors off the top of my head:

• It could be that all the other channels have a lot more traffic or noise on them.
• Channel 36 is non-DFS, which means less hassle. In all or almost all regulatory domains, the low end of the 5GHz band (the area occupied by channels 36-48) is never used for radar installations, so devices working on those channels don't need to use radar-avoidance algorithms known as "Dynamic Frequency Selection" (DFS). DFS requires devices to listen passively for up to 60 seconds on a channel before using it, which makes it seem like your AP is taking 60 seconds longer to boot. It also means the AP may have to suddenly switch channels (likely meaning another 60 seconds of downtime) if it detects radar energy pulses on the channel it's on.
• If it was a choice between 36 and 40 for the N AP, and 36, 40, 44, or 48 for the AC AP, then it's best practice for them both to put their primary 20MHz channel (sometimes called the "control" channel) on the same channel. Any time two APs are going to use frequencies that overlap, they'll actually coordinate with each other better and avoid stepping on each others transmissions if they overlap completely rather than just partially.