[streaming] Multicast errors: discontinuity at reception

[Javier Gálvez Guerrero]

Hi again,

I know some multicast wifi behaviour issues that make multicast streaming
less reliable than unicast but what I can not understand is why, if I have
made sure that every network device is working at 54 Mbps in a mandatory
way, with no power save mode but even with the DTIM wifi parameter set at
the minimum (1) and loading the wifi channel with less than 2 Mbps of data
(monitored with Wireshark), I have heavy packet losses (1/10 packets are
lost in the wifi channel). I know how huge is the wifi overhead, but even
having an effective rate of 36 or 24 Mbps and having the half for upstream
and the other half for downstream, 12 Mbps (let's say 10 Mbps) should be
more than enough to cope with ONE multicast streaming video (less than 400
kbps) with not such errors.

I have checked it with the client device at less of one meter from the AP
and there are many lost packets, tested in different environments, networks
and with different client devices. If I transcode the streamed files to mp4
256 kbps video and 128 kbps audio it works pretty well but the whole bitrate
( 256 + 128 ) is bigger than the original one (333 kbps), what still gets me
more confused. It has something to do that the original video can be encoded
with MPEG-1 or 2? Anyway, how can be possible that I have this amount of
lost packets if I have the radio channel almost "dedicated" with such a
higher available rate than needed?


Thank you all,
Javi

2008/3/3, Sebastien Chaumontet <sebastien at chaumontet.net>:
>
> Hi,
>
> Problem with multicast (as broadcast) over wireless is that it is not
> reliable:
> In the unicast world, wireless is retransmitting lost packets.
> With broadcast or multicast on wireless (downstream way: from AP to
> clients) if packets are lost they are not retransmitted.
>
> An other behavior is that multicast is used as the rate of the lowest
> client associated with the AP. ie: if all your clients on the AP are
> using 11Mb/s except one at 1Mb/s, multicast and broadcast will be sent
> at 1Mb/s.
>
> Regards
> Seb
>
>
> On Mon, Mar 3, 2008 at 2:53 PM, Javier Gálvez Guerrero
> <dulceangustia at gmail.com> wrote:
> > Hi,
> >
> > I am trying to stream multicast videos through a hybrid wireless-wired
> > network and I am having some problems at wireless clients. I have
> checked
> > with differents devices and even with different networks (laboratory and
> > home), but the results are the same: "ts warning: discontinuity received
> 0xX
> > instead of 0xX (pid = 68 or 69)" messages, so having some video and
> audio
> > interruptions and poor frame decoding results.
> >
> > I stream from a VLC server (through VLM, VLC 0.8.6d), Ubuntu Linux 32
> bits
> > based, running in a laptop. It serves RTSP VOD videos and UDP multicast,
> > both configured with VLM telnet interface. It streams via 802.11g at 54
> Mbps
> > to a Cisco Aironet 1200 series access point, connected to a switch with
> many
> > other computers. Then I have two clients: a desktop and a laptop. The
> > desktop is wired and it receives both VOD and the multicast streams
> > perfectly, but the laptop, connected to the network with 802.11greceives
> > VOD with no problems but the multicast stream with many discontinuity
> > errors. If I connect the laptop to the wired network the multicast
> streams
> > are received with no problems.
> >
> > I configure the scheduled multicast streaming:
> >
> > > new test broadcast enabled
> > > setup test input /media/sda2/diptv/server/contents/test.mpg
> > > setup test output #standard{mux=ts,access=udp,dst=239.255.1.5:60101}
> >  > new test_tv schedule enabled
> > > setup test_tv date 2008/02/28-14:50:00
> > > setup test_tv append control test play
> >
> > I have tried with different videos, different wireless receivers,
> different
> > 802.11g channels and transmission rates (and many other access point
> > features), streaming from a wired and wireless server, with a WIFI
> > Conceptronic router or a D-Link DWL700 access point instead the Cisco
> > Aironet 1200 and receiving in a Linux or WinXP client with the same
> results,
> > the raw of discontinuity errors at receiver. I have also tried
> increasing
> > the buffer size of VLC for UDP muxer with no effect.
> >
> > I have launched Wireshark protocol analyzer to track the udp packets on
> the
> > wired desktop and on the wireless laptop and the difference are some
> random
> > missing packets on the wireless laptop network interface, obvious
> losses.
> >
> > So what I would like to know is if somebody has faced this problem with
> > multicast streaming in a wireless environment and, if it has been
> finally
> > solved, how to do it. If the solution is a different access point I
> would
> > like to know which ones have worked properly with this issue.
> >
> >
> > Thanks a lot.
>
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> >
>
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