Re: Roaming questions

[James Prestwood <prestwoj@gmail.com>]

On 2/13/25 7:24 AM, Denis Kenzior wrote:
> 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.

This is an area I want to improve since we do see full scans routinely. 
Not because we didn't see any acceptable BSS's via neighbor reports, but 
because the current one was simply better. Likely another limited scan 
using the very same neighbor reports, just done later, would result in a 
successful roam. But its tough because you can't always the APs are 
sending accurate neighbor reports.
>
>>
>> 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
>