[ns] backoff timer 802.11

[sasan zhalehpour]

hi,
i read in ns2.33 in MAC802_11.cc/....MAC802_11:: send(...) class
...
/*
if the medium is not IDLE, then we start the backoff timer  */


 but as i know backoff timer starts when the channel is idle after
expiration of defer timer.
please tell if i made a mistake!!!
best regards
sasan.