Regarding ofdm_demod

I’m relatively new to both gnuradio and python it self.
Current I am using the --log option in ftw_80211_ofdm_tx to create a
‘final.dat’ file which is modulated signal that would be sent to USRP2.

I am trying to learn how will I be able to decode the ‘final.dat’ file
and was looking around at a lot of block and example that would be able
to do that.

One question I have right now is there is this function ofdm_demod ( )
that I was playing with earlier.

The usage is
blks2.ofdm_demod (options, callback=None)

The description says that the Demodulated packets are sent to the
handler which I suspect would be the callback.
However, I do not understand how it work since it seems like that
ofdm_demod still send out output after I tried connecting it to a

What output exactly does this function give and how would I be able to
utilize it?
My current goal is to be able to get the bit stream or the payload back
from the signal.

Thank you for any help, comments, advises