Is SIFS, DIFS, AIFS or nothing used before beacon frame is sent?
I got the frame duration in us by adding 192 us preample and data_bits/1Mbps together but I don't know if I should add DIFS SIFS etc to get total airtime that is required to send one beacon.
I am not totally clear about what you are trying to ask.There are so many parameters regarding airtime for any 802.11 frame.
You mentioned 192us that [Long preamble] is time taken to send PHY layer header of 802.11 Beacon frame.
And Beacon frame is kept in Highest Priority queue in AP and uses the same CSMA/CA method to gain access to wireless medium.I am not sure about which IFS time AP uses before sending beacon.I will get back to you on this.
Update: --Please ignore my comment.I was not allowed to edit :P --
After discussing with Experienced WLAN Developer,this is the best information i got. There is no special facilities are given for Beacon.This means, to send Beacon AP has to content the Wireless medium as usual.But beacon is given higher priority than other TX packets ,present in Queue.Thats why you will notice there will be delay [ from actual TBTT ] in Beacon Timestamps [As medium was busy so AP could not able to send Beacon in TBTT]. But STAs will be synchronized according to TimeStamp in Beacon.