[Ieee80211Mac] Beacon frames are not sent at PIFS by AP.

[Tey]

In a correct 802.11 MAC implementation, the AP is supposed to send its beacon at TBTT after detecting the channel as idle for PIFS. In the current implementation (Ieee80211Mac module), the shortest supported IFS is DIFS, so the no frame is ever sent at PIFS. It's even worse: the beacon is put in the Tx queue as a normal management frame, so it will be sent when all the other management frames already present in this queue have been sent, and using a backoff. The beacon must be treated as an urgent frame, so it must not be delayed by other frames or a backoff.

Also, the following quote about buffered multicast/broadcast frames only concerns the power save mode, which this implementation does not support, so it should be removed:

// TODO: 9.3.2.1, If there are buffered multicast or broadcast frames, the PC shall transmit these prior to any unicast frames.