Re: [PATCH 1/2] cfg80211: Add support to set tx power for a station associated

[Ben Greear <greearb@candelatech.com>]

On 11/07/2016 06:10 AM, Ashok Raj Nagarajan wrote:
> On 2016-08-01 18:57, Ben Greear wrote:
>> On 08/01/2016 02:29 AM, Johannes Berg wrote:
>>>
>>>> Sure.. First use case will be to help with the problem of legacy
>>>> client devices that roam across multiple APs. It is a classic
>>>> enterprise Wi-Fi AP problem,  often managed by a "network controller"
>>>> unit that is connected to all the APs.
>>>> The problem is how to handle seamless handoff of clients between
>>>> multiple  APs while maximizing the client throughput and minimizing
>>>> disruption of IP application services like VoIP calls and video
>>>> streaming. A legacy client will often  hold onto an AP association,
>>>> even down to 1 Mbps as it roams away. Instead,  if the AP can
>>>> recognise that the client RSSI (and therefore throughput) is poor, it
>>>> can "drop" the Tx power significantly (just to that client) such that
>>>> it forcesthe client to look for a better, closer, and therefore
>>>> higher-throughputassociation. It would "give it a kick" without
>>>> blacklisting it. It just needsto hold the power low for the small
>>>> amount of time it takes to convince it to go away.
>>>
>>> Not sure that *works* since implementations may just compare beacon
>>> signal strength and hold on to the AP based on that, but it does seem
>>> like a reasonable use case.
>>
>> How is that better than just kicking the station deliberately and/or
>> refusing to send frames to it at all?
>>
>
> Ben, deliberately kicking out the station can potentially cause the black
> listing behaviour on the client side and results in connection failures. Each
> client handles the kickout logic differently. Reducing the tx power, causes the
> station to trigger its roaming algorithm.

We tested some phones a year or so ago, and used a variable attenuator
to decrease the signal of one AP while ramping up signal of a second AP.
They did not roam until they lost connection, and since we were not using an isolation
chamber, we could not get the AP signal less than around -75 DB, so in our test,
the phones often did not roam at all.

http://www.candelatech.com/cookbook.php?vol=wifire&book=Emulating+Station+Motion+with+Programmable+Attenuator

So, I am not sure you can assume much about scanning behaviour either.  Maybe newer
phones are better...

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com