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In P802.11-REVma-D9.0-red-line-all-pages:
11.2.1.5 AP operation during the CP

f) Immediately after every DTIM, the AP shall transmit all buffered broadcast/multicast MSDUs. The More Data field of each broadcast/multicast frame shall be set to indicate the presence of further
buffered broadcast/multicast MSDUs. If the AP is unable to transmit all of the buffered broadcast/multicast MSDUs before the TBTT following the DTIM, the AP shall indicate that it will continue to deliver the broadcast/multicast MSDUs by setting the bit for AID 0 (zero) in the bit map control field of the TIM element of every Beacon frame, until all buffered broadcast/multicast frames have been transmitted.

what is the meaning of the sentence in red color? If the AP is unable to transmit all of the buffered broadcast/multicast MSDUs before the TBTT following the DTIM, what will AP do? It delivery spare broadcast/multicast MSDUs after every Beacon after the DTIM? If AP doesn't delivery spare broadcast/multicast MSDUs after every Beacon after the DTIM, what is the intention that it will continue to deliver the broadcast/multicast MSDUs?

Hi zhier81,

Welcome to the forum!

When there is much downlink multicast/broadcast traffic, it is very possible that the AP may not win enough time on the wireless medium to transmit all of it between the DTIM/Beacon and the next TBTT (which is when the next beacon is due to be transmitted). When this happens, the AP is supposed to set the AID-0 bit to 1 in all future beacons (until there is no longer any downlink multicast/broadcast data queued at the AP) to indicate to the BSS that there is more multicast/broadtraffic that needs to be sent from the AP.

Devinator

Thanks for Devinator's answer!

In your words:

When there is much downlink multicast/broadcast traffic, it is very possible that the AP may not win enough time on the wireless medium to transmit all of it between the DTIM/Beacon and the next TBTT (which is when the next beacon is due to be transmitted). When this happens, the AP is supposed to set the AID-0 bit to 1 in all future beacons (until there is no longer any downlink multicast/broadcast data queued at the AP) to indicate to the BSS that there is more multicast/broadtraffic that needs to be sent from the AP.

My question:

[zhier81]: Is it possible that AP delivery the spare BC_MC frames following Beacon(non-DTIM Beacon after the DTIM after which AP begins to delivery the buffered BC_MC frames )?
If AP dosen't delivery the spare BC_MC frames following the non-DTIM Beacon, what is the intention that AP indicate that AP has more BC_MC frames to be sent ?
If AP can delivery the spare BC_MC frames following the non-DTIM Beacon, what is the advantage that AP can delivery the spare BC_MC frames following the non-DTIM Beacon?

Hi Zhier:

TIMs and DTIMs (every so many TIMs is also a DTIM) are information elements transmitted by access points (AP) in beacons for the benefit of associated stations operating in power save mode.

If there are no associated stations operating in power save mode then all TIMs are blank and ignored.

When the first associated station tells its AP that it will be operating in power save mode, the AP begins buffering unicast frames destined for that station and multicast frames destined for the BSS. The AP is bound to transmit the unicast frames only in response to a PS-Poll frame from the station, and transmit the multicast frames only after every TIM that is a DTIM.

With these assumptions in mind a station can choose to lightly doze, waking for every TIM, or deeply doze, waking only for every DTIM.

There is more to this story, but I am unsure what your next question might be.

I hope this helps. Thanks. /criss

Thanks for Criss_Hyde's help.

This sentence I can't understand:
The AP is bound to transmit the unicast frames only in response to a PS-Poll frame from the station, and transmit the multicast frames only after every TIM that is a DTIM.

In P802.11-REVma-D9.0-red-line-all-pages.pdf:
If the AP is unable to transmit all of the buffered broadcast/multicast MSDUs before the TBTT following the DTIM, the AP shall indicate that it will continue to deliver the broadcast/multicast MSDUs by setting the bit for AID 0 (zero) in the bit map control field of the TIM element of every Beacon frame, until all buffered broadcast/multicast frames have been transmitted.

There is one thing I want to know:
If the AP is unable to transmit all of the buffered broadcast/multicast MSDUs before the TBTT following the DTIM, shall the AP continue to deliver the broadcast/multicast MSDUs following every Beacon frame, until all buffered broadcast/multicast frames have been transmitted?

yes

Thanks!

But any reason? I think STAs staying awake for BC_MC frames for so long time should be considered about power management!

Another Question:

If the AP is unable to transmit all of the buffered broadcast/multicast MSDUs before the TBTT following the DTIM, shall the AP deliver the spare broadcast/multicast MSDUs prior the buffered unicast frames following every Beacon frame?

Anyone know the previous reason? Thanks......