• Just want of confirm If an 802.11 STA (AP or client ) can decode the preamble and PLCP header of an 802.11 frame on adjacent channels, then extract the value of length of transmitting time of this frame to avoid collision.

    For example, if AP1 working on Channel 1, AP2 is working on Channel 2. They are very close. When AP2 is transmitting a frame, can AP1's CCA functionality decode the preamble and PLCP header of this frame?


  • Technically they should not since they are on different channels. But since this RF there is always a percentage of RF seen on the adjacent channels @ high power.

  • So, you think the STA always thinks of the energy caused by the wi-fi transmitting on adjacent channels as "Noise"?

    However, I got a white paper from cisco, ""

    It states: "This proximity is important because 802.11 specifies the protocol using carrier sense multiple access (CSMA), meaning listen before transmitting. In this case, channel 4 clients refrain from transmitting until the client that is transmitting on channel 1 is finished."

    The contention does works there, Channel1 over Channel 4...

    Any thoughts?


  • HI

    Most clients also are possible to use energy measurements in their CCA calculation and will keep quiet even if they can not decode the preamble.

    This is what Cisco means.

    Does not matter if this was real Noise or an adjacent WLAN packet on the next channel. As far as I know WLAN radios do not use all the OFDM sub carriers to pick up the Preamble only the ones in the middle and will thus have problems picking up the length field in the preamble of an adjacent channel.

    BR Martin

  • Hi Martin,

    Good point!

    As far as I know, the STA would not transmit frames only when the energy detected is higher than -65dBm. If the strength is lower than -65dBm, it will try to decode the frame if it can recognize the preamble.

    In addition server days ago, I also did a quick test ---- setup an air capture station which was monitoring Channel 10 by using ( LINUX + Wireshark + arimon + Atheros Wireless card ). I found it can decode frames sent by an AP and a client which were on Channel 11. So, I think 802.11 STA should be able to detect and encode the frames on Adjacent Channels as long as the signal strength is strong enough.


    Lin Tao

  • I have definitely seen OmniPeek seeing and decoding transmissions on nearby channels.

    But I can also see over 100 SSID's at my disk - unfortunately I don't control that aspect of the business.

  • Guys. My guess is below.

    If you look at the spectral mask of 802.11b you will see the strong side bands, which contains pretty much a copy of the main band, and stretches easily over to the next channel. Remember how 802.11b uses the spectrum. Shows quite clear in Metageek. Must beacons runs at 802,1b speeds if you have not turned them off.

    That is what you are recording.

    in OFDM the effect is much lower due to the way it is designed. I think one have to get information from nearly all of the sub-carriers to be able to decode the frame.

    i do not know if OFDM creates harmonics that can be picked up.

    Can anyone comment on this!

  • By Howard - edited: October 10, 2014

    If by "picked up" you mean, "Do OFDM transmissions create harmonics that can be decoded, by a Wi-Fi radio ?"  Then the answer is NO.

    If you mean, can an OFDM transmission create harmonics that you can you see on a spectrum analyzer ?"  The answer is YES.    However, if it passed its compliance tests, the signal levels will be so low as to be insignificant and cause no problems.  

    Problems arise however, if the power amplifier is driven into compression, and the harmonics that are generated are too strong.   These can easily exceed the regulatory limits too.

    I haven't seen it personally, but I've heard this is seen more often with Bluetooth than with Wi-Fi. 

    Internally generated Inter Modulation Distortion can also cause small amounts of noise within a band, and this can severely reduce a radios Dynamic Range, and be very difficult to diagnose.

    A common problem, especially with older radios, is when a signal levels extends beyond the upper or lower limits of the band-end (2.4 or 5 Ghz).   This is one reason some radios put out less power on channels at one end of the band than the other, and why power levels are rarely flat across the 2.4 GHz ISM band. 

    I have also seen more un-eveness in UNII-2e than in the other UNII bands.

    As radio technology advances, there are fewer problems in this area, and there are now some radios that are remarkably flat across the band(s).

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