prism2 silence value question

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Maybe somebody out there can help me noodle this one out (you listening
h1kari? ;)

I've been writing some code for a project that looks at the signal/silence
values from prism2 cards, and I've hit one somethign interesting. I was
initially doing my coding against a Netgear MA401 prism2.5 card that
probes out (under OpenBSD 3.2) like this:

wi1 at pcmcia0 function 0 "NETGEAR MA401RA Wireless PC, Card, ISL37300P"
port 0xa400/64
wi1: PRISM 2.5 ISL3873, Firmware 1.0.7 (primary), 1.3.6 (station), address
00:09:5b:24:0b:fd

Seems simple enough, it's an ISL3873-based card. For any given sample of
packets here in my lab, I see silence values ranging from ~3 through ~10,
which seemed to directly contradict a lot of people's driver code that
asserts 27 as a floor value for the silence byte (dstumbler included). In
some testing, I noted that I was seeing packets with my code that were
being discarded by dstumbler because of a silence value < 27. I thought
this was odd/nifty/whatever and hadn't mailed here about it because I
hadn't gotten around to it.

I recently got my hands on a Demarc 200mw Prism2.5 card (which I have to
say is SWEET), and I'm seeing something that's confusing me a bit. The
demarc card probes out like this:

wi0 at pcmcia1 function 0 " , IEEE 802.11 Wireless LAN/PC Card, " port
0xa000/64
wi0: PRISM 2.5 ISL3873, Firmware 1.0.7 (primary), 1.3.6 (station), address
00:60:b3:75:ae:3d

My values for silence however are (again, in my lab) ranging from ~30 to
~38 on average. This roughly seems to bear out the floor value of 27 that
a lot of people seem to use, but I'm confused as to why I'm getting these
different values from 2 cards with the same chipset and firmware versions.

Any thoughts from the peanut gallery?

Alternately, any suggestions on a good way to normalize the data I'm
getting so that either card would be useful?

thanks,


- -d

"You see, wire telegraph is a kind of a very, very long cat.
 You pull his tail in New York and his head is meowing in
 Los Angeles. Do you understand this? And radio operates
 exactly the same way: you send signals here, they receive
 them there. The only difference is that there is no cat. "

                                           --Albert Einstein

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