RE: H.261 video problems - fixed

> > - with modern video endpoints at least. Any "good" encoder should
> > generate packets where the media payload starts with a GOB/Slice
> > header anyway and in this case the extra information in the rtp
> > header is mostly redundant
> 
> True, but impossible with plain H.263 (still used by most endpoints
out
> there,
> and all 3G mobile phones) unless you can accept packets of any size,
even
> bigger than the MTU and thus fragmented by TCP/IP.
> 
> H.263+/Slice, MPEG4 and H.264 certainly do provide for what you
described
> above.

For baseline H.263 since you have ~18 GOBs per frame you can get to
around 6Mbps at 30fps without having to generate mode B packets or
exceed the MTU of Ethernet. You are right that when you get to these
very high bitrates you do need to generate mode B packets - but on the
other hand you should not be transmitting at high rate if there is any
significant packet loss, so again the extra information in the RTP
header is not that useful.

As an aside packets bigger than 1400 bytes or so that are sent as
fragmented IP are not received correctly by many endpoints. The only
endpoints that I know of that generate such packets are those made by
VCON (but to be fair to VCON it is rare with the VCONs default
settings). If you call between a vcon and an endpoint that does not
handle big fragmented packets you can get a feedback loop where the
receiving endpoint generates a fast update request which causes the Vcon
to generate a keyframe with big fragmented packets that the receiver
cannot decode which causes more FURs....!

> And jumped over MPEG-4 for probably good reasons. Except that it is
MPEG-4
> being available in 3G mobile phones today (with baseline H.263).

Yeah. We (Codian) use MPEG-4 today for streaming. The (video part of)
Mpeg-4 is not that much of an improvement over H.263+.

Regards,
Marj



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