If two sets of Sync and Follow up messages are sent, then the drift between
the two clocks (the phase change rate) can be found by comparing the Δtime
between the successive sync messages.
Drift =
HINT/TIP
Grand masters might have a setting to control the number of synchronization
events sent per second time interval. For GEN series system to synchronize to
a PTP grandmaster within its published specifications, a minimum of one PTP
synchronization per second is required. Two updates per second improves the
short
-
term stability. Higher update rates have not proven to be more stable.
14.3.2
Switch delays
The majority of Ethernet switches on the market use a store
-
and
-
forward
method to decide where to send individual packets. Incoming packets are
stored in local memory. The packet is checked for errors before being sent out
from the appropriate port/ports. This process introduces variations in the
forward and return latency time of the packet. The variations in these delays
mean that the assumption that packet delay is the same in each direction is
invalid, thus rendering the path delay calculations of PTP inoperable. This issue
has been compensated for with the use of two special switches,
boundary
clocks
and
transparent clocks
. For more information, please refer to "PTP
GEN3t
I3995-3.1 en HBM: public
367