RT-AC68U+Merlin Question on channel designations vs. channel BW

[David R.]

Hi,

I apologize if this has been answered somewhere, but I've been searching quite a bit and haven't found the answer.

I have an Asus RT-AC68U (T-mobile TM-AC1900, actually) with the most current Merlin firmware. I have a question about the relationship between the control channel numbers, the channel bandwidth, and the actual channel ranges being used to form higher bandwidth channels. In many places on the net, I see that for 802.11ac, the 80MHz channel in the high band of 5GHz should be designated as channel 155; however, the Merlin firmware (and perhaps the Asus FW as well) do not allow selection of 155 as the "control channel". Only the base 20MHz channel designations are available to choose.

So, the questions are:

1. what is the relationship between the control channel and the actual 80MHz channel frequency range in the Merlin FW on this router?

2. in the high band, will channel 155 be used as the actual 80MHz channel regardless of which of the 20MHz channels I select as the control channel? I would like to avoid any operation in the DFS area, as I understand this router might be capable of DFS.

For 802.11n, I understand the concept of the control channel plus upper or lower extension, but for 802.11ac, I haven't been able to find much information on how the control channel maps to the full 80MHz channel, or why some sites say to choose 155 yet is is not selectable on this router.

Thanks!

 

[ColinTaylor]

There have been a few discussions about this. Have look here.

The short answer is that the terminology has changed with 802.11ac. Confusingly there a two different ways to refer to the same thing.

So for example, when talking about an 80MHz width; On one hand it should be referred to by centre channel number + channel width. But on the other (more practical) hand, you still need to specify the primary 20MHz channel + channel width.

You will see from that post I linked to (and the one it linked to here) that by just specifying the primary 20MHz channel + width, all the other channels to be used are automatically deduced. Plus, it dictates what channels are used in the event of falling back to 40MHz or 20MHz operation.

 

[sfx2000]

So, the questions are:

1. what is the relationship between the control channel and the actual 80MHz channel frequency range in the Merlin FW on this router?

The "control channel" is akin to an 802.11a 20MHz channel - this is where all the management and supervisory info is on... the other "channels" just carry data frames

2. in the high band, will channel 155 be used as the actual 80MHz channel regardless of which of the 20MHz channels I select as the control channel? I would like to avoid any operation in the DFS area, as I understand this router might be capable of DFS.

For 802.11n, I understand the concept of the control channel plus upper or lower extension, but for 802.11ac, I haven't been able to find much information on how the control channel maps to the full 80MHz channel, or why some sites say to choose 155 yet is is not selectable on this router.

Again - an 80MHz channel is going to have a center - so for example...

The "control channel" might be ch149, which again, is the 11a channel, and also the 802.11n primary - in 11ac context, it can be considered that the center channel is Ch155

155 is not selectable, as it runs into a couple of odd rules going back to 11n... so most consumer routers will offer the following channels as user options...

Notice that the DFS channels aren't available - for good reason - but many routers that support DFS, if placed into Auto, will initialize on non-DFS, do the clear channel process for DFS, and then tune into it, depending on the regulatory region...

 

[David R.]

Thanks for the replies!

Between this and some additional searching, I now understand how the base 20MHz channels aggregate into the larger BW channels in 802.11ac. The fact that the new 802.11ac center-based channel designations don't appear in the router firmware caused some confusion, but it's clear now how it works.

That's interesting information about DFS as well. It makes sense that the router would first map out the DFS "landscape" rather than allowing direct selection of DFS channels. Unfortunately in my case, I'm using WDS, so the channel must be manually selected, which limits me to non-DFS channels only, even if the router is capable of DFS operation, which I'm not certain it is anyway.

 

[sfx2000]

I'm using WDS, so the channel must be manually selected, which limits me to non-DFS channels only, even if the router is capable of DFS operation, which I'm not certain it is anyway.

With classic WDS. this makes total sense, as each WDS node has to probe to find the adjacent node... and in the DFS context, it could be a long time if a node had to reboot and start over...

This is where some of the newer Mesh (and similar) do have a bit of an advantage - as the master can set up the mesh in non-DFS, and then opportunistically scan into DFS space, and see if the channel is clear - and then if it is, it jumps, and sends an 802.11 management action frame for the others to jump as well...

Once the jump to hyperspace is successful, all is good... then the entire Mesh can live in DFS land...

And there's good motive here - as in the US, we have a bit of flexibility in non-DFS space, but in some regions, non-DFS can lead to basically one 80MHz channel only...