User`s manual

TN-5516/5518 Series Featured Functions
3-11
specific timing requirements. All devices ultimately derive their time from a clock known as the grandmaster
clock. In its basic form, the protocol is intended to be administration free.
How does an Ethernet switch affect 1588 synchronization?
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 recognize significant fluctuations and use suitable averaging techniques in the algorithms
controlling the correction of the local 1588 clock will 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 while avoiding the effects of queuing. In this case two
modifications to the usual design of an Ethernet switch are necessary:
• The Boundary Clock functionality defined by IEEE 1588 must be implemented in the switch, and
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.
Basic Ethernet Communication with IEEE 1588 PTP Topology.