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I know I can and should check the standard but after looking at it for all the phy stuff I am a little lazy.

I know this may be a bit(no pun) much but it may help clarify it for me and others. Page 50, second paragraph, about the CFP and its first beacon. I just need to clarify this for it is a little confusing.

Does the beacon just before the pifs and the cfp beacon contains a normal duration of 0(since it is a management frame beacon) but in its CF paramater set it has the CFP max duration set value set so all stations can adjust their navs. THEN in the very fist CFP beacon the beacon's Duration value is the one set to 32,768 to instruct the stations during the cfp to look at the cfpdurremaining for their navs. Is this correct?

Simply put I just need to know what the very last CP beacon's duration is set to just before the first CFP beacon. Is it 0 or 32,768????

Sadly I cannot get any traces of CFP modes for the three APs I have just don't support it so I cannot get any traces to confirm.

Any assistance would be helpful.

Hi JS:

I am away from my annotated documentation, books, and notes so please forgive me for not using all of the proper terms below. Also, I was not involved in creating or implementing the standards nor equipment based on the standards (especially not with PCF since it is used virtually nowhere), but I have read the standards more often and more recently than many and here is what I think:

When an access point (AP) supports point coordination function (PCF) its every beacon contains a count of how many beacons before the next contention free period (CFP), the maximum time duration of any one CFP, and the time remaining in the current CFP, if any.

Stations of that basic service set (BSS) begin their CFP when the time comes for that beacon to arrive with a CFP beacon count of zero, even when that beacon is delayed in arriving! Immediately they set their NAV's to the maximum duration and begin counting down time from there. Perhaps an instant later the expected beacon arrives. Because the CFP is in operation these stations ignore duration values in received frames, but reset their NAVs based on the value of CFPdurationRemaining (spelling?) in beacons of their BSS.

Stations of another overlapping BSS ignore the CFP values in beacons from outside their BSS but honor duration values in all properly received frames. These stations find themselves repeatedly resetting their NAV's to 32768 time units while the CFP of the overlapping BSS is in operation.

It is a common misapprehension that a station begins its CFP operation upon receiving a beacon with a duration value of 32768. The standard requires that all frames transmitted by the BSS "during" the CFP, except the one announcing the end of the CFP, carry the duration value of 32768 in order to force alien stations to defer a goodly amount of time.

I several times pondered whether the beacon announcing the CFP is itself a part of the CFP or the CP. The question itself is confused -- there is no such announcement! Rather it is the previous beacon that determines a future point in time when a station will shift itself into PCF mode and await instructions from its point coordinator.

Thanks. /criss

I forgot. Thanks Criss. Very helpful I printed it out and kept your post in the book. /JS

Whats the concept of dual beacon ? Need to know more on dual beacon .

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