Pos Service Disruption - Agilent Technologies OmniBER 718 SDH User Manual

Packet over SDH (POS)

POS Service Disruption

POS Service Disruption
Description A common feature of modern secure networks is the ability to switch the user traffic
to a backup path when the main path fails, or under administrative control. This can
happen at the SDH layer, or at higher protocol layers (for example MPLS). The
switching will usually only disrupt the traffic briefly, and the OmniBER can
measure this disruption using the method described below.
Service disruption can be measured on a POS signal on the disrupted path or virtual
path.
Measurement method
The measurement is made by sending and receiving a PRBS pattern at the IP layer.
The detection of a service disruption is based on the detection of bit errors on the
received PRBS. The disruption period is the time from the end of a received error-
free packet to the occurrence of the next error-free word (32-bits) after any bit errors
have been detected. The measurement is recorded only if 200ms of no bit errors
occur after the disruption period.
POS Service Disruption measurement is only available when PRBS is chosen as the
payload.
Measurement accuracy
Measurement accuracy is proportional to the packet rate. The packet rate in turn is
dependent on 4 factors: packet size, inter-packet gap, stuffing ratio and bandwidth
(see "What determines the packet rate?" page 209).
To maximize the accuracy of this measurement, the inter-packet gap should be kept
to a minimum and the packets should be kept small. However, the resulting packet
rate should not exceed the specification of the system under test. The measurement
accuracy is typically ± 2 x (1÷ packet rate). For example a packet rate of 10,000
packets/s gives an accuracy of typically ± 0.2 milliseconds, which is accurate
enough to measure typical disruptions of many milliseconds
Hence it is only practical to approximate the accuracy. For best results, the smallest
possible inter-packet gap should be chosen along with the smallest possible packet
size.
Hence it is only practical to approximate the accuracy. For best results, the smallest
possible inter-packet gap should be chosen along with the smallest possible packet
size.
The figure on the next page illustrates the measurement technique and also shows
how the packet size and rate affects the measurement accuracy.
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