Meinberg M900, Руководство по эксплуатации

Страница: 25 / 170

Precision Time Protocol (PTP) / IEEE1588v2
PTP/IEEE1588 is a time synchronization protocol that offers sub-microsecond
accuracy over a standard ethernet connection. This level of accuracy can be reached
by adding a so-called hardware time stamping unit to the network ports that are used
for PTP time synchronization. The time stamping unit captures the exact time when a
PTP synchronization packet is sent or received. These time stamps are taken into
account in order to compensate transfer delays introduced by the ethernet network.
In PTP networks there is only one active source of time, the so-called Grandmaster
Clock. If two or more Grandmaster Clocks exist in one network, a algorithm defined
in the standard is used to find out which one is the „best“ source of time. This „Best
Master Clock“ algorithm has to be implemented on every PTP/IEEE1588 compliant
system and therefore all clients („Slave Clocks“) will select the same Grandmaster.
The other, not selected Grandmaster Clocks will „step back“ and enter passive mode,
meaning that they do not send out synchronization packets as long as that is done by
the selected master.
The installed network infrastructure components play a big role in a PTP network and
directly influence the level of accuracy that can be reached on 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/IEEE1588 networks and should be avoided. Simple ethernet hubs, at least the
ones with fixed pass-through times, are no problem. In large networks special
switches with built-in PTP/IEEE1588 functionality help to maintain a good level of
accuracy even over several subnets and longer distances. These components act as so-
called „Boundary Clocks“, they compensate their internal packet processing times by
using time stamping units on each port. They synchronize to the Grandmaster Clock
and in turn act as a Grandmaster to the other subnets they are connected to.
25