Re: Roaming questions

[Denis Kenzior <denkenz@gmail.com>]

Hi Alexander,

On 2/13/25 9:01 AM, Alexander Ganslandt wrote:
> Hello,
> 
> I'm looking into different roaming solutions and currently evaluating iwd, which 
> seems promising so far! My use-case is a device live streaming video while 
> moving around large areas, and the goal is to do so without hiccups in the live 
> stream. I have a few questions:
> 
> Looking at station_roam_failed(), it will schedule a full scan if the previous 
> limited scan failed to find a better BSS to roam to. A full scan on our chip 
> takes about 7 seconds, which is often enough time for the signal to get low 
> enough to deauthenticate the station when you're moving. It seems to me that a 
> better approach would be to schedule a scan for either the "known frequencies", 

The idea is to use neighbor reports when possible.  If neighbor reports are 
supported, iwd scans the frequencies in the neighbor report.  Otherwise the 
limited scan uses the frequencies that iwd knows the network operates on.  If 
the limited scan fails, then iwd falls back to the catch-all full scan.  The 
hope is that falling back to the full scan should never happen in practice.

> or all non-DFS frequencies (since passive scanning takes extra time) or some 
> other popular group of frequencies that won't take long to scan. If we're lucky 
> there is a good BSS in that scan, otherwise we schedule a scan for the rest of 
> the frequencies. In the worst case, this should be identical to a full scan with 
> minor extra overhead. Do you have any thoughts about this from your side? Is it 
> something that could be accepted into iwd or could it cause issues for other 
> use-cases?

If you find that full scans do indeed happen when roaming, then introducing 
intermediate limited / non-DFS scan(s) prior to the catch-all scan would be just 
fine.

> 
> Another question is about the "CriticalRoamThreshold". There seems to be a 
> config for this and some functions for lowering and raising the roam threshold, 
> but I can't see that they're called from anywhere? It seems to me that only 
> "RoamThreshold" is used, or am I missing something? There's also a hardcoded 
> delay of 5 seconds from the point where the roam threshold is passed before a 
> scan is started, is that just a "hysteresis" to not immediately start a scan if 
> the signal temporarily drops, or does it have some other function?

This is currently used with the Affinities property on the Station interface. 
For now it is only useful to lock the station to the currently connected BSS to 
prevent roaming.  For example, for the duration of firmware upgrade or similar 
scenarios.

> 
> Thanks in advance!
> 
> Best regards,
> Alexander
> 

Regards,
-Denis