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jumpy video over wireless lan - can't force bitrate on card
http://forums.linhes.org/viewtopic.php?f=5&t=8302		 Page 1 of 1
Author:  mschlott [ Sat Feb 04, 2006 11:04 pm ]
Post subject:  jumpy video over wireless lan - can't force bitrate on card
I did a auto install of knoppmyth on a laptop then reconfigured it to point the my backend server. So it is now just a front end that boots from the disk.

I managed to get wireless working today and all seems well, exept the video plays for 5 seconds or so, then begins pausing every second or 2 after that.

This occurs with out any difference when the wireless is connected any where from 24MB to 54MB, so I don't belive it is a bandwith issue. This happens with 2 different wireless adapters. One is a linksys WUSB54G v1, the other is a SMC28534. The linksys was set up using ndiswrapper and the other using prism54 drivers.

If this same laptop is connected via 100MB wired adapter to the same router. The video plays fine.

Is there something I'm missing? Is there something I can look at to see what is going on? I don't see any logs being updated during playback.



Thanks to everyone here for helping me get this far.
Author:  mschlott [ Sun Feb 05, 2006 9:23 am ]
Post subject: 
I have a theory. The video is being interupted as the wireless adapter changes it't bitrate on the fly. I am trying to force the bitrate to 24M becuase I know I can sustain that without a problem.

When I run iwconfig wlan0 rate 24, the command is accepted.

iwconfig wlan0 reports the bit rate has been changed, but it says...
Bit Rate:24 Mb/s which means it can change.

I am expecting it to report Bit Rate=24 MB/s as described in the man page. Wich would indicate that it will not change.

As a result of the above, the interface does not stay at 24M it increases to 54M and then varies between 36M and 54M.

How do I force this interface to 24M and keep it there?

Another question, the Tx excessive retries:2120 Invalid misc:13434 shown by iwconfig increase continuously while trying to watch live tv or a recording. Is this something that needs to be corected or is this normal?

Thanks
mike

Code:

root@mythlaptop:~# iwconfig wlan0 rate 24M
root@mythlaptop:~# iwconfig wlan0
wlan0     IEEE 802.11g  ESSID:"myssid"
          Mode:Auto  Frequency:2.427 GHz  Access Point: 00:0F:66:2A:AA:EC
          Bit Rate:24 Mb/s
          RTS thr:2432 B   Fragment thr:2346 B
          Encryption key:????-????-?? Security mode:restricted
          Power Management:off
          Link Quality:96/100  Signal level:-60 dBm  Noise level:-256 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:2105  Invalid misc:12883   Missed beacon:0

root@mythlaptop:~# cat /etc/network/interfaces
# /etc/network/interfaces -- configuration file for ifup(8), ifdown(8)

auto lo
iface lo inet loopback



auto wlan0

iface wlan0 inet dhcp
   #wireless_mode managed
   wireless_mode auto
   wireless_rate 24M
   wireless_essid myssid
   wireless_key ????????
Author:  tjc [ Sun Feb 05, 2006 12:18 pm ]
Post subject: 
Last time I did the calculations a normal MPEG-2 video stream, from a PVR card using the default profile, ran about 5Mbits/sec, just for the data before we even talk about protocol overhead, and that is already pushing it with most wireless adaptors. Add the fact that the stated throughput for a wireless adapter is an "average" and that video playback is a a "realtime" application that isn't tolerant of "lumpy" data delivery (which that average hides) and a little light bub should be going off...

Scan through the settings pages and look for the settings related to buffering. I don't remember the exact wording something about "extra buffering". Enabling this should help smooth things out.

Someday (in my "copious free time" ;-)) I need to write a wiki page about the implications of the realtime nature of video/audio recording and playback that I can point folks at...
Author:  mschlott [ Sun Feb 05, 2006 2:54 pm ]
Post subject: 
Thanks for the info. I'll try that when I get home.

I can't help but think a bigger buffer is a bandaid for the underlying network problem. I have great signal strength but those excesive retries are bothering me.

I also have problems if I mount the /myth file system from my backend onto my frontend and access a large file. If I go into /myth/tv and cat a large file to /dev/null I get NFS not responding errors. This should not be happening.

I may start over with a standard linux dist and get the wireless working, then install mythtv and get that working. It was just so easy to do it with the auto install. I'd rather do it using the knoppmyth distro, so I can write a detailed howto for the 2 adapters I have.

Thanks again to tjc
mike
Author:  mschlott [ Sun Feb 05, 2006 3:53 pm ]
Post subject: 
If I'm looking at the right thing, extra buffering was already turned on.

The setting I saw was "Utilities/Setup/TV Settings/Playback/General playback/Extra audio buffering".

This setting sounds like it is for audio, I would think I would want to be buffering more video as well.


I also turned on aggressive buffering for the audio and tried adding cache=1024 in /home/mythtv/.mplayer/config as suggested in another thread.
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