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  • Hello's.

    Studying for my CWNA and came across some reading that I wanted some clarification.

    Assuming I have an SSID on channel 6 and my neighbor also has an SSID on channel 6. When my laptop is performing it's CCA before transmitting, it is listening for any RF energy on channel 6 regardless of SSID correct? Meaning, that my client would also have to make sure that it does not hear my neighbors devices using the medium correct?

    thanks for your time,

    sky

  • Your station is looking for no Wi-Fi traffic, *and* no RF.  SSID doesn't matter.  We are all sharing the airwaves.

    The PW0-106 book does gloss over this.  And the index says that the RF detection is mentioned on 564, when it is on 565.  Mathew Gast's 802.11n book has a nice description, as does page nine of Marcus Burton's white paper.  The white paper is at http://www.cwnp.com/uploads/802-11_arbitration.pdf  You should read it.

    Briefly, your radio doesn't want to talk over anyone else.  It listens for frames (802.11 traffic from anyone, to anyone) on the airwaves.  If it detects any, it won't transmit.  There is also a chance that a device your station doesn't understand (2.4GHz wireless cameras used to be a huge offender)  is talking.  Your station doesn't want to talk over them either, so it is also listening for *anything* on the airwaves.  Transmissions are energy, so this is referred to as Energy Detection (ED.)  If your station detects a energy (a transmission) above a specific level, it won't transmit either.  Generally if it detects energy 20dB greater (100 times stronger) than the weakest signal it can decode, your station will not transmit.

    I hope that made sense.  The important thing to remember is that we all share the air.

  • Thanks very much for your clear explanations and the doc link. This answered my question

    thanks,

    Sky

  • By Howard - edited: November 3, 2014

    Depending on your RF environment, 2.4GHz wireless cameras are still a huge offender.    It can make b/g almost useless.

  • I just haven't seen one in a long time.  Back when X10.com was advertisingly heavily it seemed like everyone had one.  These days it seems like most cameras are WiFi.  I don't even see microwave ovens like I used to.  Just lucky, I guess.

  • By Howard - edited: November 4, 2014

    Definitely true about Microwave ovens.   I think the better shielding and "cooking" efficiency in the ovens is what helped.

    BTW, Freescale recently announced a solid state Magnetron which will run with as little as 30 volts.     Previous versions could use over 1,500 volts.

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