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Does wireshark show the number of MPDUs in a A-MPDU?

asked 2018-03-02 19:07:25 +0000

lmsbio gravatar image

updated 2018-03-02 19:07:47 +0000

In WiFi, I understand that an A-MPDU is a sequence of MPDUs followed by a block ack. I would like to see if the Block ack or some other frame has a field which shows the number of MPDUs in the A-MPDU.

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answered 2018-03-03 13:22:37 +0000

Bob Jones gravatar image

I find it depends on the driver. For an Atheros 10K chip, such as

wlan104  phy0   ath10k_pci  Qualcomm Atheros QCA986x/988x 802.11ac Wireless Network Adapter

The driver passes up an A-MPDU field with a reference number and flags field indicating the last frame:

Radiotap Header v0, Length 64
    Header revision: 0
    Header pad: 0
    Header length: 64
    Present flags
    Flags: 0x10
    Channel frequency: 5745 [A 149]
    Channel flags: 0x0140, Orthogonal Frequency-Division Multiplexing (OFDM), 5 GHz spectrum
    SSI Signal: -43dBm
    RX flags: 0x0000
    A-MPDU status
        A-MPDU reference number: 23981896                            <-----------A-MPDU field
        A-MPDU flags: 0x0000
    VHT information

.... .... .... 1... = This is the last subframe of this A-MPDU: True

So a screen shot of this in action: C:\fakepath\Atheros.png Notice the same reference number, datarate, signal, etc.

As well, an Intel 8265 passes up a reference number: C:\fakepath\Screen Shot 2018-03-03 at 7.59.39 AM.png Notice here the block ack is not present, so the three frames are sent again. The flag for last frame does not appear to be in use here; it is present, but always 0.

Finally, my Macbook has trouble with this. It does not pass up the reference number in a meaningful way. However, notice the lack of signal strength until the last frame: there are clues as to the collection of frames in use per A-MPDU. C:\fakepath\macbook.png

The Mac has issues with larger size A-MSDUs as well, but that is for another time.

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Asked: 2018-03-02 19:07:25 +0000

Seen: 30 times

Last updated: Mar 03