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Help with calculated transmission time vs real-world result

2 posts by 2 authors in: Forums > CWDP & CWAP - Enterprise Wi-Fi Design & Analysis
Last Post: February 16, 2018:
  • By ericlin
    Reply

    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?

    Thank you,

    Eric

  • By Howard
    Reply

    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?

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