Videoquality is bad with H.320 Videoconferencing-Systems (kb2252)

The information in this article applies to:

  • SwyxWare v4.00

[ Question | Answer ]


What's the reason for the bad videoquality with my H.320 Videoconferencing-System, if it's connected to SwyxWare ?


H.320 Videoconferencing-Systems use their own framing for transmission of video- and audiosignals inside the B-channels. These signals are transmitted in the data-mode, which is recognized by the CIP-mask of the call (UDI=Unrestriced Digital Information). SwyxWare does use very small buffers on datatransmissions to minimize the delay, but this does increase the probability of gaps in the streams (underrun of CAPI buffers). This is no problem with protocols like X.75, as these protocols recognize data loss/manipulation by themself and will repeat broken packets. But H.320 is very sensitive to data loss.
To avoid data loss/manipulation you have the chance to enlarge the used buffers. For this you have to add a new key into the Windows Registry:

Location: HKEY_USERS\<SID of the user account the service is running with>\Software\Swyx\IpPbxGate\CurrentVersion\Options
Name: NumOfPreBuffersDATA
Value: Number of empty data blocks which will be sent in advance to avoid buffer underruns for datatransmissions. The default is 0 as most protocols will handle retransmissions themself (e.g. X.75). In case of critical protocols as H.320 you can configure a bigger buffer, e.g. 3 (3 blocks of 30ms each).

Additionally it should be mentioned, that all controllers of the ISDN boards should use the same clocking. At best you should use the clocking of the PSTN. It may be necessary to set some jumpers on the ISDN boards, if you use the Digi ISDN boards. Explanations of how this is done can be found inside the SwyxWare manual.



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