Siemens SCALANCE X-300 Configuration Manual page 239

Synchronization regardless of the topology of the network
The calculations shown in the sections above apply only on condition that the message
exchange is via a direct connecting cable between the two communications partners.
Normally, however, networks consist of several switches that have to transport the time of
day messages between the time master and slave. How the synchronization is acheived via
several switches depends on the device category to which a switches are assigned
(boundary clock or transparent clock) and which method is used to calculate the message
delay time (delay request response mechanism or peer delay mechanism).
The mechanism used to handle PTP messages must be configured for each device. Both
delay mechanisms cannot be used at the same time in one network section. All the devices
within a section must be configured for either the delay request response mechanism or the
peer delay mechanism. All the switches involved should support PTP to achieve precise
time-of-day synchronization. A switch that does not support PTP cannot guarantee constant
message delay times between the master and slave due to queuing.
Boundary clock
This switch adopts the role of slave at one port and synchronizes itself with the time master.
For the other connected devices, it adopts the function of master and sends synchronization
frames cyclically to these nodes. In a network with several switches and end devices, the
BMC algorithm handles the task of selecting the most precise clock in the network
automatically. A master-slave hierarchy results in which each switch synchronizes itself with
the neighboring switch in the direction of the BMC.
Synchronization mechanisms with boundary clocks
If a boundary clock is configured for the delay request response mechanism, it sends delay
request messages to the time master and sync and follow-up messages to the slaves.
With the peer delay mechanism, the boundary clock calculates the message delay time to
the neighboring device for each port. It synchronizes itself by evaluating the sync and follow-
up messages of the master. The boundary clock allows the synchronization of the slaves by
sending sync and follow-up messages.
Transparent clock
A transparent clock does not synchronize itself with a time master but forwards PTP
messages between the time master and the slaves to be synchronized. Compared with the
boundary clock, the transplant clock allows more precise synchronization because the error
in the synchronization of the boundary clock is omitted. With several switches in a row in a
linear bus or ring topology, it is therefore preferable to configure these as transparent clocks.
Even when there are topology changes in the network, the transparent clock still provides a
more precise synchronization than the boundary clock. Regardless of its position in the
topology, the function of the transparent clock is to forward synchronization frames. With a
boundary clock, the assignments of master and slave to the individual ports and therefore to
the entire synchronization hierarchy change. It can take several seconds before all the
devices have resynchronized with the time master.
SCALANCE X-300 / X-400
Configuration Manual, 10/2014, C79000-G8976-C187-22
Configuration using Web Based Management and Command Line Interface
4.5 The Switch menu
239