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I am trying to achieve periodic multicast with WAP (Wireless Access Point) [basically trying to stream real time audio data to multiple devices]. The regularity in the interval between successive UDP audio data packets matter in my application. Currently that interval is 16 ms and need to be kept lower to have lower latency.

From some resources I came to know that multicast does not employ random back-off mechanism (as no ACK is involved). But still, if channel is busy then WAP should logically postpone the transmission of multicast packets. My questions are :

  1. How fast(i.e. reducing interval between successive multicast packets) multi-casting can be done with present 802.11 b/g/n systems.
  2. Multicast packets have more priority than unicast packets, how is this priority implemented in WAP functioning ? ( By 'priority' I mean, as ACK not involved and reliable multicast packet delievery is still desired, does WAP tries to reserve the channel prior to sending multicast packets by some means or what? )
  3. What's the size of the contention window in this case? What will happen if channel is busy for sufficiently long time and AP has multicast packets to send ?
  4. Is there anything like post back-off period (the period in which WAP should not send any multicast packets)?
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    Your questions are a bit confusing. Multicast has no higher priorty than other packets unless you make it that way with QoS or some other mechanism. Some switches have built in storm control that can stop this traffic if it's too much at the port the AP is plugged into. It can be turned off or adjusted in most switches. I think if you clearly state the problem your having and the model of devices your using you'd get better feed back. Maybe even include some configurations of your devices. – Fixitrod Aug 19 '17 at 21:52
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Feb 21 '18 at 17:43
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Unfortunately, multicast or broadcast on Wi-Fi is a problem. Your WAP will send multicast and broadcast at the lowest possible rate. This is part of the Wi-Fi standard. There is a proposed standard that is supposed to fix some of the problems with multicast on Wi-Fi, but nothing currently supports it, as far as I can tell.

For example, if you have a WAP that supports 802.11b (as you seem to indicate), it will send multicast traffic at 1 Mbps.

Multicast packets have more priority than unicast packets.

I'm not sure where you get that idea. From the perspective of layer-2 (what the WAP works with), you don't have many options for prioritizing traffic. The WAP, like a switch, does not look at the packets, only the frames. It doesn't matter what marking you have on the packets if the devices doesn't look at the packets in order to treat the packets differently.

Many people try to set up something like you describe, but it almost always ends in disappointment. What seems to look good on paper, and would work well on a wired network, will simply not work as envisioned on Wi-Fi.

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