Sencore VB330, User Manual, Page: 191 / 203

B.3 Strategy for MediaWindow Analysis
This section provides further insight into MediaWindow analysis and suggests how the Ethernet threshold
settings can be configured to maximize the usefulness of the MediaWindow graphs and alarms.
The MLR value is always calculated using the continuity counter inside the transport stream packets.
Since the continuity counter is expected to increase by one for each packet of the same PID it is possible
to detect missing TS packets by noting gaps in the continuity counters. Knowing that there are usually 7
transport stream packets inside one UDP packet you expect a continuity counter error of 7 if one UDP
packet goes missing. This corresponds to an MLR value of 7. The range of the continuity counter is 4
bits meaning that if you are unlucky and lose exactly 16 packets for the same PID you will not be able to
detect the packet loss at all. Losing 16 or more packets of the same PID is very rare and will only happen
in networks with plenty of obvious problems.
Not all PIDs carry continuity counters. The null packets (PID 8191) and PIDs carrying PCR (program
clock reference) do not carry continuity counters. This is the reason why losing one UDP packet does not
necessarily result in an MLR of 7 but maybe 6 or even 5 (assuming the mapping is 7TS/UDP).
Systems typically do not mix the mappings among their streams so there is seldom a need to remember
the mapping for streams in order to interpret the exact impact of MLR values.
The range of the MediaWindow graphs can be configured by the user. Even when the graph is updated
in “real-time” each bar in the graph will represent a large number of elementary measurements. For
a 5Mbit/s stream there will be approximately 500 elementary measurements per second, assuming
a mapping of 7 TS packets into each UDP-frame (i.e. there are approximately 500 UDP packets per
second). An elementary measurement is generated for each interval between two neighboring UDP
frames.
Within each update-interval only the extreme IAT and MLR values are displayed in the graph. For IAT
the peak inter-arrival time over the measurement period represents the IAT for that period. For MLR the
highest loss ratio within any second represents the MLR for that period.
When the range of the graph is set to larger intervals, even more elementary measurements are merged
for each bar-interval.
The rest of this discussion assumes the MediaWindow graph range is set to “running” since that lowers
the probability that more packet losses occurred inside the same bar-interval.
The following figure shows how a large number of elementary measurements are represented by one bar
in the graph.
VB3xx 10G Probe User’s Manual version 5.3 191