• I'm preparing for the CWAP exam and I have a question. The study guide clearly states that non-802.11 interference will not cause CCA to go into busy state. Yet, in the spectrum analysis chapter it states that a jammer could cause CCA to fail. Could someone clear this up for me? Thanks-Justin

  • Non-802.11 interference can cause CCA to fail. CCA is comprised of physical carrier sense (decoding PLCP headers to determine how long the medium will stay busy) and energy detection. The latter detects raw RF energy (including non-802.11 energy) to determine whether the medium is available. The energy level that indicates available medium varies depending on frequency, chipset, and configured parameters like receive threshold. Non-802.11 energy will not cause the CCA carrier sense to indicate that the medium is busy, but can cause CCA energy detection to do so. If it didn't, the other end would likely fail to decode the transmission, forcing retries and increasing overhead.

  • Hey @@ron, Thanks for the reply and sorry for not getting back as quickly as I would have like to. So....while reading CWAP the first time through, one thing that I thought was clear to me was that NICs cannot detect raw energy. On page 244/245 it states that "CCA is the physical carrier sense mechanism for 802.11 stations and is only set to busy if there is a high enough level of energy detected coming from valid, modulated bits". Then later states "interference from non-80211 devices does not cause the CCA to go into a busy state". In your opinion, are these statements false? I know that NICs cannot detect the level of the noise floor so in this case do you think the raw energy is detected but not measurable? Sorry but I'm still a bit confused. Could you recommend any other reading material on this subject? Thanks again.

  • I found a great description of CCA on revolution wi-fi. As @@ron mentioned CCA is broken up into two parts; carrier sense and energy detection. Energy detection will detect non-Wi-Fi interference or corrupted Wi-Fi signals. Although an enormous source of information, I do feel that the CWAP study guide is a little misleading on this topic and that this book alone will not get me through the CWAP exam. Do any CWAPs out there have any suggestions on additional study material? BTW I think it's time to buy myself a jammer 8-)

  • By Howard - edited: June 29, 2012

    The secret to passing the CWAP, is the same as all of the other CWNP Certs " Study to the Official Objectives", not to what's in the book or in the practice exams. Specifically, get lots of practice with a wireless packet analyser, memorize the patterns in every spectrum analyzer picture you can find and be able to identify them. Sniff packets for every kind of wireless security that you can too. As far as the practice tests go, be able to get 95% on every practice test or chapter question without memorizing the questions themselves. As far as the actual CCA mechanism works, its almost impossible to experiment with unless you can get to the actual CCA pin output on the radio. Good luck.

  • I can't speak definitively for all 802.11 chips, as implementations do vary quite a bit. I would think that all must do some level of ED, even if they don't expose configuration or visibility of this process. I've worked with some chips that expose "802.11 busy" and "non-802.11 busy" registers, and others that only expose "802.11 busy". To calculate the noise floor, 802.11 chips do need to demodulate several 802.11 frames. I'm not a hardware guru, so perhaps somebody with more RF/hardware experience can give more detail. I used the following resources for the CWAP exam: - exam objectives - 802.11 spec to be very helpful (you can get a digital copy of 802.11-2012 for $5.00) - official study guide - real-world, hands-on experience (troubleshooting, spec analysis, packet analysis, network design) - practice exams - whitepapers from CWNP CWAP was the hardest exam, in my opinion, and well worth the effort to pursue it. I spent more time in preparation for it than the other CWNP exams. Be sure to check the study guide errata and do serious research on any exam objective that you don't fully understand. And hit up the forum -- there are active members here who are anxious to help!

  • Thanks for the advice.

  • Definitely the 802.11 specification. Know for all PHYs.

Page 1 of 1
  • 1