search

aredn / aredn

Amateur Radio Emergency Data Network (AREDN)
https://www.arednmesh.org
Other
175 stars 77 forks source link

[Question]: How is DFS handled in AREDN? #537

Closed T-X closed 1 year ago

T-X commented 2 years ago

Hi, I have a little cross-project question. Sorry for (ab)using the ticket system, but I don't have a HAM radio license, so I can't use your forum, I think. Feel free to redirect me if there is better, other, preferred place to ask this.

tl;dr: How is AREDN currently handling DFS on its WLAN adhoc/mesh interface? Are DFS events an issue for AREDN users, too? Any thoughts on how we could improve this?

For the Freifunk or FunkFeuer projects we are building mesh networks, too. Regarding frequencies we are struggling a bit with an overcrowded 2.4GHz band in cities and not being able to reliably use the 5GHz band outdoors for mesh networking.

The issue we are facing for years now with the 5GHz band outdoors is its requirement for DFS. For one thing we see many false positives. And for another, there is currently no implementation in hostapd or other open source software to reliably migrate and converge a whole mesh network to a new channel. And the 802.11(s) standard provides insufficient / no guidance on how to coordinate a new channel in a mesh network.

So in the firmware framework Gluon, which is based on OpenWrt too and is used by many Freifunk communities these days, we currently disable the 5GHz WLAN interface for meshing if it is an outdoor device. And only use the 5GHz for an AP interface outdoor then, with DFS enabled. For indoor devices we use the 5GHz for both AP and meshing, but on the lower 5GHz channels where no DFS is required then. However in Europe unfortunately these lower, non-DFS 5GHz channels are allowed for indoor use only.

Latest progress I'm aware of on the software side is this patchset that got added to hostapd: https://patchwork.ozlabs.org/project/hostap/list/?series=186663&state=%2A&archive=both. However the patch "mesh: use deterministic channel on channel switch" was unfortunately not merged (though I'm having my doubts that it would be sufficient and reliable enough for a city wide mesh network anyways).

Due to the difficulties in finding and implementing a proper solution in software, for years now, I'm also increasingly wondering if it would make sense to ask regulatory bodies, like ETSI, Bundesnetzagentur and FCC to offer / to help us find a solution, maybe by an alternative or complimentary approach to DFS. But that's where you people on the ham radio side would have more experience and expertise with, the regulatory process, I believe.

I'm curious if people here at the AREDN project have experienced issues with DFS, too. And if so, if you found solutions for them or had ideas on how this could be improved.

Regards, T_X

(PS/Edit: I had searched both this ticket system and your excellent documentation, but could not find anything regarding DFS in AREDN there.)

K6AH commented 2 years ago

The FCC's Part 97 which Amateurs in the US operate under has no specific DFS requirement except that we have secondary rights to the spectrum and are not allowed to interfere with the primary users... in this case Doppler radar.

When a node is configured as an Access Point (when they are operating under Part 15), then the DFS channels are not selectable.

T-X commented 1 year ago

@K6AH thanks for the reply, very interesting!

The FCC's Part 97 which Amateurs in the US operate under has no specific DFS requirement except that we have secondary rights to the spectrum and are not allowed to interfere with the primary users... in this case Doppler radar.

How do you ensure that as a secondary user you are not interfering? Is it more relaxed for a an amateur radio operator, are you allowed to just check this "manually" from time to time? With DFS for normal users they are usually pretty strict on how and how often to scan for radars etc.

When a node is configured as an Access Point (when they are operating under Part 15), then the DFS channels are not selectable.

Is that also including adhoc or 11s mode? Technically these two are more similar to AP than client mode I'd say. When meshing over an adhoc/11s mode do you exclude the DFS channels then, too? Hm, yes, in the US you're quite lucky that you can also use channels 32 to 48 outdoors, without DFS :-).