• All,

    802.11-1999 says this about TIMs: TIM

    The Bitmap Control field is a single octet. Bit 0 of the field contains the Traffic Indicator bit associated with Association ID 0. This bit is set to 1 in TIM elements with a value of 0 in the DTIM Count field when one or more broadcast or multicast frames are buffered at the AP. The remaining 7 bits of the field form the Bitmap Offset. AP TIM transmissions

    The TIM shall identify the STAs for which traffic is pending and buffered in the AP. This information is coded in a partial virtual bitmap, as described in In addition, the TIM contains an indication whether broadcast/multicast traffic is pending. Every STA is assigned an Association ID code (AID) by the AP as part of the association process. AID 0 (zero) is reserved to indicate the presence of buffered broadcast/multicast MSDUs The AP shall identify those STAs for which it is prepared to deliver buffered MSDUs by setting bits in the TIM?¡é?€??s partial virtual bitmap that correspond to the appropriate SIDs.

    My questions are:

    Do we need to use both the Traffic Indicator Bit and bit 0 of the Virtual Bit Map to indicate BC/MC traffic? Why or why not?

    In the quote from above is the last acronym "SID" synonymous with "AID"?



  • Hi Moe:

    The traffic indicator bit for multicast is always in the bitmap control field and never in the partial virtual bitmap field. I speculate that this makes for a more efficient (smaller) virtual bitmap field when the access point is buffering both multicast and unicast frames.

    The reference to "SID" is changed to "AID" in the 802.11m-draft5.2 proposed base standard.

    I hope this helps. Thanks. /criss

  • Chris,

    Thanks for the clarification. I have been holding off studying 802.11n until I finish CWAP- don't want to make it murkier than it is at the moment :)



  • TGm is working on a new base document, not an amendment.

  • Thanks for the info on 802.11m- I wasn't aware of it. For anyone else who may be wondering:,,sid40_gci1000597,00.html


    - 802.11m is an initiative to perform editorial maintenance, corrections, improvements, clarifications, and interpretations relevant to documentation for 802.11 family specifications. The term 802.11m also refers to the set of maintenance releases itself.
    The 802.11m initiative, sometimes called "802.11 housekeeping" or "802.11 cleanup," was begun in 1999 by IEEE Task Group M, a part of the IEEE 802.11 Working Group. The 802.11 family is an evolving set of standards for wireless local area networks (WLANs

    I add it to my reference docs.

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