Multicast over WIFI

Good old multicast over WIFI, it’s a problem due to the simple fact that 802.11 ACKs for multicast do not exist. Unicast ACKs is built into the protocol. Each time a STA and AP communicate the packets are acknowledge via ack or block ack .

Multicast frames are sent at the lowest mandatory data rates which will be problematic for clients especially those at the cell edge. In order for multicast streams to efficient there is a need for acknowledgements as the STA will not know if the frame/packet has been received. As a baseline a few items are typically required as good wireless multicast starting point.

In order to efficiently receive multicast streams the frames/packets will be sent as unicast and at higher data rates. This feature is called multicast to unicast, which will allow the multicast transmissions to be acknowledge therefore allowing a STA to retransmit frames as needed. Using 802.11n/ac higher RSSI and SNR provide a more reliable and stable RF environment. Another important factor is end to end QoS and and end to end wired multicast network. IGMP and PIM will also play an important factor on the wired network.

When the above features are implemented only STAs that require multicast traffic will receive the stream, therefore eliminating congestion on the medium. Below is the baseline configuration required when configuring multicast.

Wired network

  1. Most networks today are already using Gigabit Ethernet, this will allow higher wired data throughput.
  2. Enable multicast routing and IGMP snooping on the wired side.

ip multicast-routing distributed

3560-CORE-1#show ip igmp snooping

Global IGMP Snooping configuration:

IGMP snooping : Enabled
IGMPv3 snooping (minimal) : Enabled
Report suppression : Enabled
TCN solicit query : Disabled
TCN flood query count : 2
Robustness variable : 2
Last member query count : 2
Last member query interval : 1000

Enable IGMP Snooping/Proxy.

  • IGMP Snooping uses the WLAN controller to monitor the clients that are subscribed to multicast Video groups, and only sends multicast traffic to AP`s when required and also, only on a per-group basis
  • Helps to identify which AP`s have clients that have subscribed to multicast streams, and then block replication of Mcast stream to other AP`s
  • IGMP proxy implements multicast routing by re-originating IGMP joins and leaves from the source of the controller.
  • IGMP proxy is enabled on the user VLAN IP interface, also stating the interface where the multicast frames are being transmitted into the controller and this will make sure that the IGMP client membership table is filled out properly.
  • Without the IGMP client membership table properly filled out, dynamic multicast optimization will not take place, leading to video quality degradation

reference –

Configure Protocol-Independent Multicast (PIM) – preferably sparse-mode

QoS on the wired network

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