[vlc] Re: Wireless Multicast Stream

[Benjamin PRACHT]

On Sat, Nov 20, 2004, Eftychios Eftychiou wrote :
> Can someone recommend a specific Wireless router that will multicast a 
> video stream effectively? I know the Dlink 624 will not allow any mutlicast 
> traffic to the wireless AP.
> 
> Thanks
> 
> Eftychios 
> 

Well, I'v successfully streamed about 6Mbit/s of udp multicast traffic
on a Symbol Spectrum 24 AP 4131 11 Mbit/s Access Point with an
acceptable packet loss (I haven't tried with other hardware). However,
don't expect it to work out of the box (default settings give about
a 10 kbit/s bitrate). Indeed, the layer 2 part or the 802.11 norm
wasn't really designed for broadcast of high rate streams. Broadcast
/ Multicast frames are sent using the lowest available transfer rate
available on the AP (1 Mbit/s by default), so that every endpoint, even
those who are at the limit of the range can receive them. Moreover,
for energy saving reasons, broadcast / multicast frames are only sent
every so called DTIM intervals (every 0.3-1s by default on most AP).
Indeed, every node of a wireless network is supposed to wake up at such
intervals, to receive broadcast frames. As a consequence, broadcast
frames have to be buffered in the AP between 2 DTIM.

So, to make short, to improve the multicast bandwidth on an AP:

- Increase the rate the minimum available bandwidth of your AP (at the
  cost of range of the AP
- Increase the size of the buffer allowed for broadcast / multicast
  frames (not all AP offer this feature, the Symbol one did)
- Decrease the interval between 2 DTIM (send one every beacon, for
  instance) at the cost of the power consumption of the nodes on the
  network...

-- 
BigBen

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