• I'm currently studying for my CWNA and I'm trying to find out how an 802.11a access point causes all-band interference in the 2.4 GHz ISM band.

  • Hi Alfredo

    Under normal circumstances, an 802.11a access point would not cause all-band interference in the 2.4 GHz ISM band. 802.11a access points operate in the 5 GHz frequency bands.

    802.11a access points never really "caught on" in the commercial arena. However,  when the "principles" of 802.11a were "transferred" to the 2.4 GHz band, to become 802.11g, those systems were highly popular and commercially successful. Even today, with 802.11n and 802.11ac, there are still plenty of 802.11g client devices out there.

    It is possible for some radio transmitters to produce spurious signals which can cause interference in certain frequency regions. However. most modern transmitters are fitted with good filters which allow ( for example ) only 5 GHz signals to be emitted from a 5 GHz device. The filter roll-off characteristics are normally so good that it would be impossible for any 2.4 GHz erroneous signals to be transmitted, far less have any type of all-band interference.

    Where you reading a book/tech paper  that mentioned this scenario ?


  • Dave is certainly right, it can't really cause interference in the 2.4 band. It is either a typo or a trick question you are seeing perhaps?

  • Thank you Neal and David for your reply I was thinking it was probably a typo but I wanted to make sure that I wasn't misinterpreting the question in the "CWNA 105- Practice Exam 1 (Question 16 of 60)."  Which asks:

    Which of these devices cause all-band interference in the 2.4 GHz ISM band? (Choose all that apply.)

    A. Bluetooth,

    B.FHSS cordless phones

    C. DSSS cordless phones

    D. 802.11a access points

    E.All of the above

    Then the answer is: A, B, D

    All-band interference is typically associated with frequency hopping spread spectrum (FHSS) communications that can disrupt 2.4 GHz Wi-Fi radio communications. While hopping and dwelling, an FHSS device will transmit in sections of the frequency space occupied by an 802.11a/b/g/n radio. The frame transmissions by the 802.11a/b/g/n radios can be corrupted from the all-band transmissions of the FHSS interfering devices. Bluetooth uses frequency hopping, and some cordless phones also use frequency hopping.

    The above question is the one that led me to my original question.  Appreciate your help.

  • Ditto to all above.  There is no way an 802.11a radio (5 GHz) can cause interference in the 2.4 GHz band.  I too must think it is a typo (CWNP--please note!)


    And hello to Dave1234--have not seen any comments from you recently, Dave.  Welcome again!


  • The nice thing about a few errors in the practice questions is you have to know for sure what is right and wrong. Don't fully trust what it says, that way you learn the material and can pick out the wrong answers from the right ones.

  • Alfredo, can you tell me if this is the practice exam with the CWNA book or here at the website?


  • DItto to gcate:

    Welcome back DAVE1234    !!!!!!!     You have been missed.

  • Howard,

    I really did not use enough exclamation points to welcome Dave back!  :-)   His forum comments have been sorely missed!

    I usually think I have just read a chapter in a tech book after reading one of his posts!!

    A super resource for us all here at CWNP forums.

    So, welcome back again, Dave!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  • Hi Guys

    Thanks very much for your kind words. Glad to be back.


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