10 Technical Appendix
10.5 The Precision Time Protocol (PTP) / IEEE 1588
Precision Time Protocol (PTP or IEEE 1588) is a time synchronization protocol that offers sub-microsecond
accuracy over a standard Ethernet connection. This accuracy can be achieved by adding a hardware times-
tamping unit to the network ports that are used for PTP time synchronization. The timestamping unit captures
the exact time when a PTP synchronization packet is sent or received. These timestamps are then taken into
account to compensate for transfer delays introduced by the Ethernet network.
In PTP networks there is only one recognized active source of time, referred to as the Grandmaster Clock.
If two or more Grandmaster Clocks exist in a single network, an algorithm defined in the PTP standard is
used to determine which one is the „best“ source of time. This "Best Master Clock" algorithm must be imple-
mented on every PTP/IEEE1588 compliant system to insure that all clients („Slave Clocks“) will select the same
Grandmaster. The remaining deselected Grandmaster Clocks will „step back“ and enter a passive mode, mean-
ing that they do not send synchronization packets as long as that is being done by the designated Grandmaster.
The existing network infrastructure components play a big role in a PTP network and directly influence the
level of accuracy that can be achieved by the clients. Asymmetric network connections degrade the accuracy,
therefore classic layer 2 and 3 Ethernet switches with their “store and forward” technology are not suitable for
PTP networks and should be avoided. With activating the HQ-Filter (see chapter HQ-Filter) the Jitter can be
eliminated. Simple Ethernet hubs with fixed pass-through times are not a problem. In large networks, special
switches with built-in PTP functionality help to maintain high accuracy even over several subnets and longer
distances. These components act as "Boundary Clocks" (BC) or "Transparent Clocks" (TC). They compensate their
internal packet processing times by using timestamping units on each port. When acting as a Boundary Clock,
they synchronize to the Grandmaster clock, and in turn act as a Master to the other subnets they are connected
to. When acting as a Transparent Clock, then the "residence time" of the Masters’ Sync-Packet is measured and
added to the packet as a correction value. Internally the PTP timescale TAI (see chapter Timescale in Global
Parameters).
microSync
Date: 22nd June 2020
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