• I have a question I have been asked about what device controls the media. In reading my CWNA study guide, I have gotten slightly confused and hope to get unconfused, with your help.

    I understand that in an infrastructure mode BSS, all frames must go through the AP.

    Next when one is researching PCF and DCF, as per the CWNA study guide. ?¡é?€???¡é?€??PCF is using controlled access policy and DCF is using random access policy. With this in mind in PCF mode, during the contention-free period the AP is in complete control of all functions on the wireless network; outside of the contention-free period, stations arbitrate and randomly gain control of the medium. ?¡é?€???¡é?€??

    My question is, does that quote mean the stations are arbitrating which one gets to talk to the AP? Also I am not clear on whether the AP has to enter into this arbitration as well?

    I attempted to find this in the search method but without any luck. Your help with this would be greatly appreciated.

  • Hi M/Q:

    During PCF contention free period (CFP) the access point (AP) controls access to the medium.

    During PCF contention period (CP), and all the time during DCF, all stations, AP stations and non-AP stations, contend on an equal footing for access to the medium.

    Although I am told that PCF was built in the initial days of 802.11 (before 1999), I know of but one person who has seen such a device, and I have never found one offered on eBay.

    I hope this helps. Can you add your location to your forum profile? Thanks. /criss

    P.S. I corrected this post on 09Sep2006.

  • Thank you Chris,

    I should have been clearer. I was just using PCF as an example, as I realize it is extremely out of date protocol. I was mainly concerned with trying to determine if STA?¡é?€??s and AP?¡é?€??s were on the same footing if you will.

    I guess the aspect that I did not think about was that the AP?¡é?€??s will still receive the frames and that they only have to contend for the slot to return the appropriate response.

    Sorry, I forgot to add the location. It is taken care of. I am one of the older CWNA?¡é?€??s, kind of returning to the flock. I have bothered Devin enough that I suspect he may remember me. This is truly an amazing forum. I am a mod at Wi-Fi Planet, but this is a learning forum and I am humbled and proud to be a member of it.

  • Hi M/Q:

    Your 4000 Wi-Fi Planet posts in three years is pretty impressive.

    How did you get that job?

    Thanks. /criss

  • Hello Chris,

    It is not a job, it is an adventure as I am a volunteer. I have been involved with RF technology (ie Amateur Radio) since 1964. I also am fortunate to be involved with computers professionally for almost as long (ie punched cards and IBM ?¡é?€??Big Iron?¡é?€?? kind of dates me). This is why I also volunteer as a trusted source on Security Forums Dot Com.

    The move to 802.11 was a simple on because of my interests. Now days I actually spend more time working in the microwave bands than all of the other frequency bands that are available to me. It also is a special gift to be able to meld professional endeavors with one?¡é?€??s lifestyle.

    One interesting side note is that for a portion of the 2.4GHz band (frequencies around channels 1-5) my license allows me to use up to 100 watts. We mainly use this for satellite comms and emergency nets, but it gets the attention of people familiar with 802.11 and the EIRP requirements .

    I have lurked here for several years now and realize that the skill sets of the members here are quite amazing, I am sure most of you have forgotten more than I will ever hope to know.

  • M/Q,

    Welcome to the forum. I hate suspense....(drum roll please....)... What's your name? :-)


  • Hello Devin,

    Well, I did not want to create any kind of stir or anything. I just really love anything that deals with RF. I am one of those types of people, rare I suspect, that think radio towers are very cool and I strive to determine what sort of antennas are on everyone I see.

  • Hello,
    It's nice to have an RF guy here in the forums.

    To answer your question:

    I was mainly concerned with trying to determine if STA?¡é?€??s and AP?¡é?€??s were on the same footing if you will.

    If you are speaking of DCF mode, then the answer is yes, the AP's and the STA's are on equal footing. The AP has no priority over the STA's. They both use the same arbitration methods. To avoid collisions, each device must randomize a backoff timer and both the AP's and STA's use the same random number seeds.

    If you are talking about PCF mode, it is a lot like token ring, except the AP is giving out the tokens. Each STA will get equal access.

    Nice to have you here!

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