PT-7828 User’s Manual
Featured Functions
3-11
How Does an Ethernet Switch Affect 1588
Synchronization?
The following content is taken from the NIST website at http://ieee1588.nist.gov/switch.htm:
An Ethernet switch potentially introduces multi-microsecond fluctuations in the latency between
the 1588 grandmaster clock and a 1588 slave clock. Uncorrected these fluctuations will cause
synchronization errors. The magnitude of these fluctuations depend on the design of the Ethernet
switch and the details of the communication traffic. Experiments with prototype implementations
of IEEE 1588 indicate that with suitable care the effect of these fluctuations can be successfully
managed. For example, use of appropriate statistics in the 1588 devices to recognized significant
fluctuations and use suitable averaging techniques in the algorithms controlling the correction of
the local 1588 clock will be the good design means to achieve the highest time accuracy.
Can Ethernet switches be designed to avoid the effects of these fluctuations?
A switch may be designed to support IEEE 1588 to avoide the effects of queuing. In this case two
modifications to the usual design of an Ethernet switch are necessary:
z
The Boundary Clock functionality defined by IEEE 1588 must be implemented in the switch
z
The switch must be configured such that it does not pass IEEE 1588 message traffic using the
normal communication mechanisms of the switch.
Such an Ethernet switch will synchronize clocks directly connected to one of its ports to the
highest possible accuracy.
S
S
S
M
S
M
M
M
Grandmaster
Clock:
Determines the
time base for the
system
Boundary Clock:
Slave to the
grandmaster clock
and master to its
slave
Ordinary Clock:
Slave to its master
