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Strategy for MediaWindow Analysis

This section provides further insight into MediaWindow analysis and suggests how the

Ethernet threshold settings can be configured to maximise 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 neighbouring 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.

PROBE USERS' MANUAL VERSION 5.0

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