I am trying to calculate the transmission time (TXTIME) and see if it agrees with a real-world frame capture. I am using the TXTIME equation from the 802.11ac spec in 22.4.3 (also found here on page 10: https://www.ekahau.com/wp-content/uploads/2017/01/Wi-Fi_Capacity_Analysis_WP.pdf). However, my calculations do not agree with real-world observations.
I have a frame capture of A-MSDU frames, each with 2 subframes. They are ~41us apart. However, my calculations for TXTIME yields 73us (44us preamble + 29us data).
The capture frames are 802.11ac, MCS 9x2, SGI, 80MHz, 867Mbps data rate. They were captured in an RF-shielded room with a 3x3 Macbook Pro. PSDU/MPDU is 3076 bytes, no STBC, NDBPS=3120.
What is wrong with my calculation?
First guess - the Macbook software. IIs it possible that the MCS/rate/#streams that you are using to make your predictions does not match what the data is really being sent at ? Do you have another sniffer that you can use to verify the results you are getting ?
Have you verified that the MACbook is actually receiving/seeing all of the data?