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14.2.2 Switches using transparent clocks
Switches using transparent clocks forward the master clock synchronization
message to every port of the switch. The time required to transfer the message
from the incoming port to the outgoing port must be measured and transferred
together with the original synchronization message. There are two methods
used to communicate the internal delay information to the original
synchronization messages:
l
End
-
to
-
End
l
Peer
-
to
-
Peer
Each of these two methods has its advantages and disadvantages.
End
-
to
-
End transparent clocks
End
-
to
-
End transparent clocks create a higher load on the master clock, as the
master “sees” all the slaves. End
-
to
-
End transparent clocks support a 1:N
topology with one master communicating with a large number of slaves. They
are, however, good for linear systems with a number of daisy
-
chained clocks.
Peer
-
to
-
Peer transparent clocks
Peer
-
to
-
Peer clocks avoid the higher master load, but introduce the need to be
aware of how the synchronization messages are routed through the network
topology. They cannot resolve 1:N topologies, as they cannot determine which
line delay is being calculated and they must also maintain path delay
measurements.
14.2.3
One
-
Step and Two
-
Step clock synchronization
PTP allows for two different types of time stamping methods:
l
One
-
Step clock synchronization
One
-
Step clocks update time information by adjusting the time information
within the original synchronization messages (sync and delay request) on
-
the
-
fly.
l
Two
-
Step clock synchronization
Two
-
Step clocks transmit the precise timestamps of packets using
additional general messages (follow
-
up and delay response).
A One
-
Step End
-
to
-
End transparent clock updates for switch delay in sync and
delay request messages as they pass through the switch while a Two
-
Step
transparent clock updates a field in the non
-
time
-
critical general message.
GEN3i
322 I3763-3.1 en HBM: public