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PTP Operational Overview
Compensating for Causes of Asymmetric Delay
PRISM MPI2-10-MPX2-10 User Manual
302
Compensating for Causes of Asymmetric Delay
Several factors can cause the PTP message delay to be different for the messages sent
from the master to slave as opposed to the messages the other direction from the slave
to master. Unless corrected, this propagation delay asymmetry causes an offset in the
clock phase equal to ½ the difference in the two path delays. There are several main
causes of asymmetric delay:
•
Rate mismatch in the ports on a switch
•
Traffic mismatch on the two paths
•
Message type mismatch
•
Cable delay variation
This section contains suggestions on how to design the system to minimize the delay
asymmetry. Alternatively, some slaves provide a way to manually enter a correction
value to cancel the delay error.
Rate Mismatch
Rate mismatch in a switch causes a delay asymmetry because the switch performs a
“store and forward” action on messages. This means the entire message must be stored
in the buffer before it starts to be “forwarded” or sent out. For example, for a 100 Mb
input and 1 Gb output, the switch must wait for the entire message to clock in at the
slow rate before it can start outputting the message at the high rate. Conversely for a 1
Gb/s input and 100 Mb/s output, the entire packet is quickly read in at the faster rate, so
then the output can start sooner but at a lower rate.
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 time stamps. 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 insert residence time information to allow
the slave to correct for this delay.