PRISM MPI2-25 and MPX2-25 Media Analysis Platform User Manual
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While the total time is the same for both directions, the delay on the fast-in, slow-
out direction is shorter on the packets which contain the PTP timestamps.
This effect is significant on a 100 Mb/1 Gb rate mismatch, but much less
significant on a 1 G/10 G switch since the message time at 1 Gb/s is quite short. If
the switch supports Transparent Clock (TC) mode, then this rate mismatch delay
effect is corrected by the TC delay correction.
Traffic mismatch
Another cause of asymmetric delay is traffic mismatch on the two paths. If there is
a high percentage of traffic utilization on the master to slave path, then most of
the PTP messages may be delayed significantly. If the traffic on the other
direction is significantly different, then the messages on the path may not suffer
the same delay.
This effect can be quite large depending on the traffic, the quality of the switch,
and the number of switches in the network. To reduce this effect, a user should
avoid large loading on the PTP network, use high-quality switches that avoid this
delay variance, or use PTP Transparent mode switches which will insert residence
time information to allow the slave to correct for this delay.
Setting the Diff Serve QoS on the switch and configuring the PTP messages with
an appropriate DSCP can also mitigate the effects of traffic delay.
Message type mismatch
Another cause of delay asymmetry is message type mismatch. For example, if the
sync message is Multicast and the delay request is Unicast, then some switches
may process the messages differently and cause a differential delay. Since the
SMPTE profile allows a mixed mode operation, switches should be evaluated to
ensure they do not have this effect. If transparent mode switches are used, then
this effect should be corrected by the slave.
Cable delay variation
The actual propagation delay through Cat 5 cable can be significantly different in
one direction versus the other direction. This can be due to the twist rate on the
pairs, routing inside the sheath, material differences, etc. The PTP system is
incapable of removing this effect, so the best way to prevent this is to use high-
quality cable with a minimum variation in the physical propagation delay.
