340 The backoff procedure of DCF is described in section 9.2.5.2 of [ieee80211]_.

   341 

   342 *  “The backoff procedure shall be invoked for a STA to transfer a frame 

   343    when finding the medium busy as indicated by either the physical or 

   344    virtual CS mechanism.”

   345 *  “A backoff procedure shall be performed immediately after the end of 

   346    every transmission with the More Fragments bit set to 0 of an MPDU of 

   347    type Data, Management, or Control with subtype PS-Poll, even if no 

   348    additional transmissions are currently queued.”

   349 

   350 Thus, if the queue is empty, a newly arrived packet should be transmitted 

   351 immediately after channel is sensed idle for DIFS.  If queue is not empty 

   352 and after a successful MPDU that has no more fragments, a node should 

   353 also start the backoff timer.

   354 

   355 Some users have observed that the 802.11 MAC with an empty queue on an 

   356 idle channel will transmit the first frame arriving to the model 

   357 immediately without waiting for DIFS or backoff, and wonder whether this 

   358 is compliant.  According to the standard, “The backoff procedure shall 

   359 be invoked for a STA to transfer a frame when finding the medium busy 

   360 as indicated by either the physical or virtual CS mechanism.”  So in 

   361 this case, the medium is not found to be busy in recent past and the 

   362 station can transmit immediately. 

   363