Re: [dnssd] multicast over wireless links

[James Andrewartha <[e-mail]>]

Hi Tom,

On 25/07/14 07:47, Tom Pusateri wrote:
> Having spent a good portion of my career on IP multicast (from Token Ring to IP Multicast VPNs), it baffles me how a radio can have such a difficult time doing IP multicast. The 802.11 link layer clearly needs improvement in this area. Access points regularly fall over dead when bridging an ethernet wired connection to wireless when there is a high data rate multicast stream (actually 4Mbps isn't that high). This is just unacceptable.
> 
> With most radios, multicast is the most efficient form of transmission. But in 802.11, every receiver in range currently receives N copies of the multicast to their antenna before throwing all of them but one away. Surely, this can be improved. Multi-unicast over radio broadcast has to be the least efficient mechanism to achieve this.

I don't know how familiar you are with 802.11, but the core problem is
the methods used to increase the data rates to clients mean each client
can speak to the AP at a different rate. In short, multicast is
inefficient for 802.11 because it must speak at the slowest rate the AP
allows clients to connect to it at. This can be as low as 1Mbps if
802.11b clients are allowed to connect.

The multicast to unicast conversion you speak of is an optimisation
implemented by some vendors, and works because if there are say 5
clients connected at say 300Mbps, and one client at 12Mbps, they can all
receive the transmission in unicast faster than if it were sent out as
multicast at 1Mbps for all clients to receive. Another optimisation some
vendors implement is to broadcast/multicast at the minimum rate of the
connected clients, which in this case would allow transmission at 12Mbps.

Also note that multicast/broadcast traffic will only ever use one
spatial stream as that is the minimum required by 802.11n/ac, while some
clients suport two or three spatial streams for higher data rates. So
again multicast to unicast translation will result in more efficient
transmission.

802.11ac MU-MIMO will further complicate things, as it will allow higher
unicast data rates and simultaneous transmission of possibly different
data streams to different clients at the same time.

Thanks,

-- 
James Andrewartha
Network & Projects Engineer
Christ Church Grammar School
Claremont, Western Australia
Ph. [phone]
Mob. [phone]