Monitoring the bit rate
The bit rate of the streamed image may be monitored at the Streams information area of
the Bandwidth Management page.
The Streams list shows network statistics for current RTP streams. For each RTP stream,
three values are presented: the Transmit Bandwidth (in Megabits-per-second), the
packet Drop Percentage and the Round Trip delay Time (RTT — in microseconds). The
Tramsmit Bandwidth is the true bandwidth of the source measured over the last second.
All of this data may not have arrived at the destination if the link shows packets are
being dropped. Most networks show a small amount of dropped traffic, but when this
rises above 5% it indicates the capacity of the link has probably been exceeded. A lightly
loaded network will show a fairly constant RTT. When this value starts to rise or fluctuates
excessively it indicates the network is congested. Usually when network capacity has just
been exceeded, the RTT will rise to a large value just before packets start being dropped.
The link latency will rise as RTT increases.
Bandwidth Management Settings The VNC 200 can apply various control modes that allow the bit rate to be managed.
These control modes are selected in the Mode drop-down box as described below. The
Target bandwidth (Mbps) field and the Frame Drop Percentage fields are also
described.
• None – No bandwidth management policy will be followed apart from the underlying
compression settings.
• Manual Frame Drop – Allows the user to specify the precise fraction of frames
to drop. This doesn’t manage the bandwidth at a fixed level, but does result in a
smoother update given rapidly changing video content types. The percentage of
frames to discard is entered into the Frame Drop Percentage field. For example, a
value of 95 (95%) will discard 19 out of every 20 frames and will therefore reduce a
60 fps video signal to 3 fps.
NOTE: Slowing the frame rate to around 1 fps may cause the decoder to behave
as if the source stream has been interrupted and it may start flashing up
the no source splash screen.
• Shared Flowrate – the total network video traffic for all streams of this source is
limited to the flowrate (in Mbps) specified in the Target Bandwidth field. Frames are
dropped if the instantaneous data rate is higher than the flowrate.
• Peak Flowrate – the network video-traffic for each stream of this source is limited
to the flowrate (in Mbps) specified in the Target Bandwidth field. Frames are
dropped if the instantaneous data rate is higher than the flowrate.
VN-Matrix 200 Series • Advanced Setup Procedures 70