[PATCH] roaming: start a scan if the SNR is below customized threshold

Holger Schurig hs4233 at mail.mn-solutions.de
Fri Sep 18 03:16:58 EDT 2009


> What is roam_snr supposed to be in?  dBm?  What about cards
> that have no idea what their dBm is?

When it comes to user-space roaming *WITH A KNOB*, then I don't 
actually care what is it. As long as it has the following 
properties:

* the farther away from the AP the lower the number
* we have one number reserved for "invalid", "don't know"
  or "not yet measured", e.g. -1.
* the value should react promptly to changed environment,
  this allows roaming of vehicle-mounted WLAN devices


Say one card gives me the quality between 0..70. The I'd set 
the "knob" to scan when it's below 30 or 25.

Another card gives qual between 0..100. I'd set the knob to 30.

Another card gives me a signal in dBm. I could set the knob 
to -65.


You see, the actual range doesn't really care. When I produce a 
dvice (and I'm a producer of hand-held and fork-lift terminals) 
I know what card I have, so I can pre-set a sane value. The 
customer can then modify that value, e.g. to roam early, late or 
normal. But he has a number to start with, as orientation.


Now about the necessity of the knob.

Suppose for a second that every card under the linux earth gives 
me mBm. And that, by an astonishing miracale, all cards are even 
calibrated. So they give exactly -53 dBm if the card is in the 
same position/condition relative to the AP. Could I then life 
without a justification knob?

No way. As soon as I put another antenna on my device, or change 
the housing of the fork-lift terminal, I get back different 
values, because I changed the electromagnetic environment.

So I need a knob anyway. And because of the knob, I don't care if 
I get Qual, Signal, SNR, RSSI or whatever. As long as a lower 
number means "worse signal" or "farther away from AP".

We can solve (or try to solve) the dBm/SNR/RSSI/qual mess, but 
it's not a pre-requisite for good roaming.



However, I'd prefer "Signal in dBm", so that applications like 
can come up with a sane default.


-- 
http://www.holgerschurig.de


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