[streaming] Re: Reliable streaming format for wireless links

[Marshall Eubanks]

Hello

On Mar 13, 2006, at 10:51 AM, Jean-Paul Saman wrote:

> Niall Donegan wrote:
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>> Romuald CARI wrote:
>>> UDP + Wireless create creepy things.
> Not true by definition. UDP + Wireless works well for unicast, it  
> is only multicast that is a problem for most WLAN AP.

I do multicasts over my (otherwise pretty unloaded) wireless LAN  
without any real problems.

Unicast will not help you if it's UDP and there are packet drops.

AND, TCP streaming will also get into problems if there are very many  
packet drops.

Some base stations limit multicasts in terms of bps or pps. You should
dig into the WLAN configuration and see if there isn't some  
artificial limit
you can increase. If the limit is 1 Mbps (as it is on Apple's  
Airports), and you are sending 1.5 Mbps MPEG,
you will not be very happy.

The other thing is the size of your GOP. Can you shorten it ? Suppose  
you are sending out 100 pps
(packets per second) and the GOP is 30 seconds. Then, 1 packet lost  
out of 3000 will mess things up,
on average for 15 seconds. If you can lower the GOP to 30 frames (1  
second), then the same packet loss rate
will case errors for 1 second out of 30, lasting on average for 0.5  
seconds, which is a lot
more tolerable.

I always try and lower the GOP for streaming.

Regards
Marshall


>> That I discovered!
>> <snip>
>>> You could try to reduce the stream bandwidth and create unicast TCP
>>> connections to your clients using HTTP, good luck !
>> The main problem I see with that is that if I have a few clients  
>> at the
>> other end of the wireless link as is going to happen, then I run into
>> congestion which I was trying to avoid with multicast.
>> Is there any more resiliant codecs than MPEG2 that I can transcode to
>> which will handle the missing packets more gracefully?
>
> The problem is here that WLAN is not designed for multicast video.  
> It is hard to find a good WLAN AP that handles UDP and multicast well.
>
> The other option is using unicast over the WLAN link and lower the  
> bitrate to a value that is appropriate for the quality of the  
> signal. If the client and the AP are far apart, then you must use a  
> lower bitrate video.
>
> What you want is a transport layer that is more resilient or a  
> network setup that can deal with the transport protocol to use.
>
> Grtz,
> Jean-Paul Saman.
>
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