Saturday, July 30, 2011

FreeBSD: DFS update

I've spent the last few weeks sorting out the DFS support in FreeBSD. I've made some decent progress with it however there's still a long way to go. This has been paid work and I'm glad to see increasing interest in 802.11 support under FreeBSD.

I've debugged DFS channel switching behaviour in station and access point mode. This now functions correctly in both instances.

The ath driver now behaves "better" when doing a DFS channel change. The driver would change channels and wait for the first beacon to program the beacon timers - which are then used to signal when there's a lack of beacons (and thus net80211 marks the state as "SCAN" and begins hunting for another AP.) If a channel change occured to a channel that required a CAC (clear access check) and a radar event occured, a second channel switch would occur - and there'd never be any beacons on that channel. The NIC would sit there forever waiting to hear a first beacon that never came.

I've updated the FCC3 regulatory domain to include frequency bands that require DFS.

I've also introduced some radar device setup and event parsing code into the ath HAL layer which would allow an interested party to tinker with radar detection/classification code on the AR5212 series NICs. I don't have permission to commit the radar frame decoding code for the 11N NICs - I'm working on this.

Finally, I've introduced a radar detection layer - called "ath_dfs" - which currently does nothing (hence the only module is "dfs_null".) This provides all the hooks needed to do radar detection, classification and signaling.

Now, what I haven't done:
  • The project also includes working porting the radar classification code and getting it to work. This unfortunately won't be open sourced. I'm going to try and get as much of the radar hardware setup and radar event frame parsing code committed to FreeBSD so an interested party can begin tinkering with this.
  • I haven't yet ported a lot of the "channel interference" support. It's there, but I haven't really fleshed it out to be useful and then used it.
  • There's no DFS support for 11N yet - in particular, 11N HT/40 channels aren't correctly marked as "interference" or "radar"; 11N channels also aren't scanned for during DFS channel selection. That's a later project.
  • I need to extend things a bit to handle the weather radar stuff when doing DFS - the ETSI requirements include different channel quiet/scan times.
  • There's no support in net80211 (yet) for the DFS, TPC and Quiet elements. This is going to be required for a fully compliant DFS implementation.
  • I've not investigated implementing/fixing up the DFS support for IBSS and mesh modes.
However, even though there are shortcomings and some missing support in the DFS support, FreeBSD-9.0 will support DFS STA mode well enough to correctly obey instructions from a DFS-compliant access point.

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