5GHz Extension Channels - Explain in plain english please?

[namurt]

I have two R7000's. Both are running dd-wrt (Kong 24345). One is my main router and the other is an AP. I utilize both the 2.4GHz and 5GHz radios in each.

For 2.4GHz, I'm using 40MHz channel widths and I understand the way extension channels work with the N spec. I found this table: http://en.wikipedia.org/wiki/IEEE_802.11n-2009#40.C2.A0MHz_in_2.4.C2.A0GHz to be very helpful in visualizing which blocks I'm taking up based on what I pick for a primary channel and whether I choose upper or lower in the webif of dd-wrt for the extension channel.

5GHz, on the other hand, I can't seem to understand. I'm using VHT (80MHz + 80MHz) for channel widths. This is the terminology used by the dd-wrt webif. I do get that there are far more non-overlapping channels with 5GHz as opposed to 2.4GHz. What I am baffled by is how the extension channels work? I have options for "lower lower" "lower upper" "upper lower" and "upper upper" and the available primary channels the dd-wrt webif offers change when I change the extension channel.

Short of going through every single combination of primary and extension channel on both R7000's and running some tests, I'm hoping there's some general advice that can be offered as to how I should configure 5GHz. And if not that, if someone can explain 5GHz extension channels (more specifically, dd-wrt's implementation) I would be very grateful!

 

[stevech]

2.4GHz. Use 20MHz mode rather than 40MHz mode. Unless you live in a very rural area.

5GHz band is divided into "sub-bands" if you will. Depending on what country you live in, there are various regulations on limited transmitter power, automatic/dynamic channel reselection, and others. Products ares shipped with configuration constraints based on country (regulatory domain). The regulations are lengthy.

You'll likely find the 40MHz mode in 5GHz band works better than 80MHz, due to interference and other factors.

 

[4Leaf]

20 mhz = 1 wireless channel overall

40 mhz = 2 wireless channels overall

80 mhz = 3 wireless channels overall

I doesn't matter which ghz band you use. For Example: Both the 2.4 ghz and 5 ghz bands would both take up 2 wireless channels if you are using 40 mhz on both. Each additional channel adds more bandwidth you are taking up.

 

[thiggins]

Unless you specifically need to avoid certain channels, let the router decide which extension channel to use.

 

[azazel1024]

20 mhz = 1 wireless channel overall

40 mhz = 2 wireless channels overall

80 mhz = 3 wireless channels overall

I doesn't matter which ghz band you use. For Example: Both the 2.4 ghz and 5 ghz bands would both take up 2 wireless channels if you are using 40 mhz on both. Each additional channel adds more bandwidth you are taking up.

80MHz = 4 channels, not 3.

Also I am with THiggins. Just let the router decide unless it presents problems. In general you are probably going to have to choose upper or lower 5GHz band as DFS channels are normally not available (not sure if DD-WRT will allow you to choose them, or if the radio firmware on the R7000 will allow them to be used at all).

The normal selection is channel 36-48 and 142-161 (I think it is 161. sometimes 167 is also available).

Basically you get an 80MHz lower slice and an 80MHz upper slice, sometimes with an extra 20MHz added on top of the 5GHz upper slice. With a pair of 11ac routers, basically you set one to the lower slice and one to the upper slice and dust your hands off.

The issue with 11ac is that we'll actually have LESS choice once 160MHz bonded channels come out with wave 2, as there is effectively ONE choice that'll eat up effectively all 5GHz bandwidth. At least on 2.4GHz, you can have a 40MHz and a 20MHz and not overlap, or 3 20MHz. With 11ac and 5GHz you can have two non-overlapping 80MHz or a single 160MHz.

Biggest reason I am constantly saying that manufacturers/FCC need to get their stuff in gear and free up DFS channels. Either ship their gear with the capability to do DFS and utilize the channels, or some new rule making to make DFS easier to use/more readily available (like allowing it for indoor use without DFS or something, or free up some of the channels for non-DFS use).

DFS represents something like another ~190MHz or so of spectrum that is currently almost never used on anything. If it could be, in most locations you could have a pair of non-overlapping 160MHz channels, or 4 80MHz channels!

Anyway, just set the channel and forget about upper/lower stuff with 5GHz, especially when using 80MHz channels.

 

[stevech]

As I recall, the push for DFS regulations in the US was driven by the issue of FAA/DoD airport radars near those frequencies.
Nothing much changed on that.

 

[azazel1024]

Correct, but there really isn't anything that stops gear from utilizing the DFS channels. They just need to implement DFS radar search. It really should be simple firmware to do the search and block channels that are currently in use. In general in an area where a channel it utilized, you are looking at only having one channel blocked (I think it is possible adjascent channels are also supposed to be blocked), not the whole range.

I don't see FCC necessarily throwing the DFS channels to non-DFS use, but there is nothing stopping gear from implementing DFS.

 

[stevech]

Correct, but there really isn't anything that stops gear from utilizing the DFS channels. They just need to implement DFS radar search. It really should be simple firmware to do the search and block channels that are currently in use. In general in an area where a channel it utilized, you are looking at only having one channel blocked (I think it is possible adjascent channels are also supposed to be blocked), not the whole range.

I don't see FCC necessarily throwing the DFS channels to non-DFS use, but there is nothing stopping gear from implementing DFS.

To get US FCC type acceptance, needed to legally sell in the US, a vendor's product/firmware must implement the FCC restrictions in that band.
Same for many other countries (regulatory domain). Several countries have restrictions much more onerous than those of the FCC.

 

[sfx2000]

To get US FCC type acceptance, needed to legally sell in the US, a vendor's product/firmware must implement the FCC restrictions in that band.
Same for many other countries (regulatory domain). Several countries have restrictions much more onerous than those of the FCC.

To back up stevech's comment...

US has certain requirements - as does the EU and other areas (e.g. LATAM, EMEA, APAC) - in 5GHz, the channels are not as harmonized as they are in the ISM band - even within markets, there are regulatory restrictions in the 5Ghz band to prevent radar interference (Weather Radar is one, Defense Radar is another)...