USRP2 80211_mac Carrier Sensing



I had a couple of questions about the carrier sensing functionality of
USRP2 and the BBN 802.11 code.
When we call the carrier_sensing function and it returns a value in dB,
does the USRP2 sample its data?

I know that it takes an average of the samples of the channel by using
IIR filter with the parameter alpha = 0.001.
I calculated the sample period to be approximately 10 usec.
Does the USRP2 actually sample faster (i.e. 1 usec), filter/decimate the
data before returning a value every 10 usec?
How fast can the USRP2 actually sample data?

Does anyone suggest a value for alpha? With the given alpha, the carrier
sensing block is storing values of the channel on the order of 10s of
ago. Packets should last about 1-2 msec, so storing information for that
long does not make a lot of sense.

Miklos Christine