[streaming] Multicast errors: discontinuity at reception

Sat Mar 8 10:33:52 CET 2008

Wireless is not a wire: on wireless it is normal to get errors/loss.

Have you really understood the difference between upstream and
downstream behavior in multicast over wireless?

http://www.sydneywireless.com/?p=736
------------------
The 802.11 (Wi-Fi) standards specify support for multicasting as part
of asynchronous services. An 802.11 client station, such as a wireless
laptop or PDA (not an access point), begins a multicast delivery by
sending multicast packets in 802.11 unicast data frames directed to
only the access point. The access point responds with an 802.11
acknowledgement frame sent to the source station if no errors are
found in the data frame.

If the client sending the frame doesnt receive an acknowledgement,
then the client will retransmit the frame. With multicasting, the leg
of the data path from the wireless client to the access point includes
transmission error recovery. The 802.11 protocols ensure reliability
between stations in both infrastructure and ad hoc configurations when
using unicast data frame transmissions.

After receiving the unicast data frame from the client, the access
point transmits the data (that the originating client wants to
multicast) as a multicast frame, which contains a group address as the
destination for the intended recipients. Each of the destination
stations can receive the frame; however, they do not respond with
acknowledgements. As a result, multicasting doesnt ensure a complete,
reliable flow of data.

The lack of acknowledgments with multicasting means that some of the
data your application is sending may not make it to all of the
destinations, and theres no indication of a successful reception.
This may be okay, though, for some applications, especially ones where
its okay to have gaps in data. For instance, the continual streaming
of telemetry from a control valve monitor can likely miss status
updates from time-to-time.
------------------

Some solutions are already in TODO list for VLC :
https://trac.videolan.org/vlc/ticket/820

---
Seb

On 3/8/08, Javier Gálvez Guerrero <dulceangustia at gmail.com> wrote:
> 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 802.11g
> receives
> > > 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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