Moxa Managed Ethernet Switch (UI_2.0_FW_5.x) User Manual
IEEE 1588 PTP
The f ollowing inf ormation is taken f rom the NIST website at http://ieee1588.nist.gov/intro.htm:
“Time measurement can be accomplished using the IEEE Standard f or a Precision Clock Synchronization
Protocol f or Networked Measurement and Control Systems (IEEE 1588-2008) to synchronize real-time
clocks incorporated within each component of the electrical power system for power automation
applications.
IEEE 1588, which was published in November 2002, expands the perf ormance capabilities of Ethernet
networks to control systems that operate over a communication network. In recent years an increasing
number of electrical power systems have been using a more distributed architecture with network
technologies that have less stringent timing specifications. IEEE 1588 generates a master-slave relationship
between the clocks, and enforces the specific timing requirements in such power systems. All devices
ultimately get their time f rom a clock known as the grandmaster clock. In its basic f orm, the protocol is
intended to be administration free.”
How Does an Ethernet Switch Affect 1588 Synchronization?
The f ollowing content is taken f rom the NIST website at http://ieee1588.nist.gov/switch.htm:
“An Ethernet switch potentially introduces multi-microsecond f luctuations in the latency between the 1588
grandmaster clock and a 1588 slave clock. Uncorrected these fluctuations will cause synchronization errors.
The magnitude of these f luctuations depends on the design of the Ethernet switch and the details of the
communication traf f ic. 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 recognize signif icant f luctuations and use suitable averaging techniques in the
algorithms controlling the correction of the local 1588 clock will be good design means to achieve the
highest time accuracy.”
Can Ethernet switches be designed to avoid the effects of these
fluctuations?
A switch can be designed to support IEEE 1588 while avoiding the ef f ects of queuing. In this case two
modifications to the usual design of an Ethernet switch are necessary:
1. The Boundary Clock and Transparent Clock f unctionalities def ined by IEEE 1588 must be
implemented in the switch.
2. The switch must be conf igured so that it does not pass IEEE 1588 message traf f ic 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.
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