• Hi there

    I understand that two APs within sight on the same channel cause Co-Channel-Interference, and two APs with overlapping Channels (eg. Channel 1 and 3) cause Adjacent-Channel-Interference.

    I have seen a WLAN-Setup, where some APs have 40MHz wide channels, and some have 20MHz wide channels (in 5GHz). There were for example APs on Channel 48@20MHz and APs on Channel 44@40MHz. From a RF point of view, the spectrum from Channel 48@20MHz is "within" (or part of) the Channel 44@40MHz.

    So i was wondering: what exactly is this? Can the APs handle this situation and recognize it as CCI and avoid collissions with CSMA/CA, or can't they handle this and see this as ACI, ending up in collissions and retransmissions?

    Thank you and Kind Regards


  • By Howard - edited: August 22, 2016

    I don't want to be facetious, but the answer to your question is YES.    What ?!

    It would be easy to argue either way.  The big problem is that our original definitions of Adjacent Channel and ACI were made before both 20 and 40 MHz channels were allowed.

    I think you would have to base your observation on whether you had the same channel allocations on both AP's. e.g. both have a primary of 44, and secodary of 48, and 40MHz only.

    Originally I would have called it ACI. But obviously we now need a more refined definition of ACI.

    Technically, even with 20MHz channels there is some overlap between 5 GHz channels.   Just because the "official" spectrum mask is defined with a certain width, does not mean that weaker signals farther out are not causing some degradation in your received signal. I can see this happen when I perform ACI/sensitivity measurements with my Anrtisu WLAN Test Set.

    From a practical standpoint, if only one of the AP's has its channel defined as 40 MHz only, there's definitely ACI.   Actual duty cycle and the effectiveness of CCA will determine how bad things get.

    With two 20/40 MHz wide AP's using 44 and 48, it will also depend upon which Primary Channel each of them has specified.

    Client devices have a "40 MHz Intolerant" setting that can help tremendously, but from what I have seen not all AP's feel they are bound to obey it.

    It's possible to think up several scenarios based on wider channels.

    IMHO 40 MHz channels in the 2.4 GHz should have been prohibited by the FCC.   802.11ax will probably only make things worse, as will LTE-U.

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