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Thread: 802.11n capture - data missing?

  1. #1

    Default 802.11n capture - data missing?

    Greetings,

    I am trying to do 802.11g and 802.11n pcaps between an ASUS N16 and an iPad 4th generation on 2.4GHz. I am using IXChariot to send traffic for 30 seconds (high performance throughput in either the downstream or upstream direction, never simultaneous).

    When doing 802.11g, I get captures between 80-120MB. When doing 802.11n the captures are much smaller (20MB) even though the throughput is much higher.

    I am using an airpcap device, I have it close to the AP/client (within 20'), and I am using a Windows XP laptop.

    Any suggestions on what I could do to improve the 802.11n captures?

  2. Default

    Hi Ben,

    Are you using channel bonding on your 802.11n AP? Maybe the AirPcap NX is missing the bonded channel.

  3. #3

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    Quote Originally Posted by Joel View Post
    Hi Ben,

    Are you using channel bonding on your 802.11n AP? Maybe the AirPcap NX is missing the bonded channel.
    Hi Joel,

    Thanks for the reply. I was using 20MHz channels.

    However, I think I found the answer - it seems that the AirPCAP device does not support SGI20, which is what my AP advertises. A colleague of mine is suspecting that our clients "sometimes" use SGI20, and therefore the capture has some data, but not all.

    Do you have any input?

  4. Default

    Quote Originally Posted by Ben_S View Post
    Hi Joel,

    Thanks for the reply. I was using 20MHz channels.

    However, I think I found the answer - it seems that the AirPCAP device does not support SGI20, which is what my AP advertises. A colleague of mine is suspecting that our clients "sometimes" use SGI20, and therefore the capture has some data, but not all.

    Do you have any input?
    Hi Ben,

    The best input that I've got is that I can find no mention of SGI20 on on the AirPcap NX datasheet, so I think that is a very likely theory. Let me ping Trent to see if he has any ideas. He's on travel right now so it might take me a week or so to get back to you.

  5. #5

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    Hi Joel,

    Any word from Trent?

  6. Default

    Quote Originally Posted by Ben_S View Post
    Hi Joel,

    Any word from Trent?
    I spaced it and forgot to ask him about it. Lyndon called me today, and I shot Trent an email about it with Lyndon looped in on the email thread.

  7. #7

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    Roger. Thank you, sir.

  8. Default

    I think you guys ended up finding the answer before we did. Looks like the AirPcap NX doesn't support SGI20/Greenfield. I'd recommend getting captures with a MacBook running Mac OS 10.8.

    Short Guard Interval 20 (SGI20) or High Throughput (aka Greenfield) modes are not supported.

    The AirPcap Nx adapter does not currently support the Short Guard Interval 20 or Greenfield transmission modes. If your access point and stations are configured to use SGI20 or Greenfield modes, you will likely see garbled information or no packets. The AirPcap Nx adapter will support the following 802.11n modes:

    Legacy Mode
    HT20 Mixed mode
    HT40 Mixed mode

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