• By risshuu - edited: March 22, 2016

    I just read through chapter 19 of the CWNA study guide, and I saw the question in the title in the summary section.

    I have reread the chapter, and I have no idea why 802.11ac would or would not allow this.

    I see the part about the VHT-MCS Map fields--and that the first eight MCSs are mandatory, but would that alone be a sufficient explanation?

    Any help is appreciated.

  • By Howard - edited: February 9, 2018

    On pages 42 and 43 of Matthew Gasts 802.11ac book it is explained as:

           "Because the MCS map field [of the VHT Capabilities Information Element] only allows three options (MCS 0 through 7, MCS 0 through 8, and MCS 0 through 9), it is not possible to disallow low data rates in an 11ac network."

    Only two bits are used to define the 3 possibilities ( 0, 1, or 2), and the value of 3 is reserved.

    My original confusion over this was that the Supported MCS Set map in an OmniPeek trace seems to imply something else, and that on some Cisco AP's, the GUI interface implies that you can make other choices - but doesn't allow them.  

    In a Motorola  AP CLI manual, the available options only give you the three choices, but does not explain why.  In fact, because it's not explained, it can mislead you to believe that the manual is missing parameters or keywords.

  • Thank you for the reply and the extra information.

  • You're welcome.

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