Hot Off the Press: Arbitration WhitepaperBy CWNP On 09/16/2009 - 26 Comments
One of CWNP’s instructors and CWNEs, GT Hill, has what might be labeled a “fetish” with 802.11 arbitration. Some people like shoes, others like gadgets, but GT likes teaching 802.11 arbitration. He’s a Wi-Fi geek among the best of us. ;) If you get the chance, taking him up on the lesson would be a good use of your time. If you don’t get the chance, the next best thing may be the 802.11 arbitration whitepaper that CWNP just published.
If you’re not already familiar, 802.11 arbitration is the combination of processes and mechanisms used by stations to access the shared wireless medium. Since the wireless medium is half-duplex—shared by everyone in a given area on the same frequency—only one station can usually transmit at a time without adverse effects. If you’ve ever tried to sort through all of these mechanisms by reading the 802.11 spec, you probably walked away scratching your head. I did too. For that reason, I took it upon myself to sort through the complicated details, charts, and processes so that I could boil it all down into 22 pages of mind-numbing whitepaper.
Like myself, I’m sure you’re wondering how it can be “boiled down” into 22 pages. Well, that’s because the topic is complicated, confusing, and comprised of many moving parts. However, even a highly technical whitepaper can serve a few valuable purposes. If you’re not terribly ambitious, it can be a reference for the occasional arbitration question. If you are terribly ambitious, you can read the whole thing and find out the relationship between an AIFSN and an AIFS[AC], you can memorize the default EDCA Parameter Set values for each AC, or find out what in the world an aCWmin or aCWmax even is. Either way, the paper provides high-level concepts, an arbitration flowchart, and lots and lots of good details. I think there is something useful here for everyone with a technical slant.
802.11 arbitration is a foundation upon which functional Wi-Fi is built, so understanding these details can lend considerable credibility to consultants and administrators. Anyways, it is a heavy paper, but well worth the effort to read. You can download the paper at: arbitration.pdf
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