ibaClock
Manual
Issue 1.4
53
The offset that is used to correct the slave clock is calculated as follows.
𝑂𝑓𝑓𝑠𝑒𝑡 = 𝑀𝑎𝑠𝑡𝑒𝑟𝑆𝑙𝑎𝑣𝑒 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 − 𝑟𝑢𝑛 − 𝑡𝑖𝑚𝑒
or
𝑂𝑓𝑓𝑠𝑒𝑡 = (𝑡
2
− 𝑡
1
) −
(𝑡
2
− 𝑡
1
+ 𝑡
4
− 𝑡
3
)
2
However, this calculation of the offset is only correct on the assumption that a constant
propagation speed prevails in the network. No accurate synchronization can occur if de-
vices are installed in the infrastructure that cause different run-times between the master
and slave.
Different run-times can be caused, for example, by switches that are not designed to
operate PTP applications.
15.2
Network requirements
With an optimally developed network architecture, PTP allows for a synchronization of
under 100 nanoseconds. To be able to achieve this, however, the network architecture
must be fully IEEE 1588 compliant.
The three main components of grandmaster clock (best with GPS receiver), network
switch (transparent or boundary clock) and PTP slave must support the time stamping
on the hardware side.
Network switch
Switches can be categorized as “standard” and “IEEE1588-supporting.” A standard
switch temporarily stores the network packets before sending them on again. The time
of the caching may vary depending on the utilization of the switch. This in turn affects the
calculation of the delay and the offset.
High-Speed, Low-Latency Switch
Like the standard switch, this switch caches the packets. In the event of a slightly higher
network load, it also allows for an accurate and stable synchronization. However, since
the cache time also varies with this switch, it has a negative impact on the calculation of
the delay and the subsequent synchronization.
Transparent Clock
A transparent clock (TC) is a network switch that handles the IEEE1588 packets differ-
ently than the standard switch. The transparent clock measures the cache time of the
packets and enters this in a field of the packet. The slave then uses the original time
stamp and the time in the additional field to correctly calculate the delay.
Boundary Clock
The boundary clock (BC) is another switch where the handling of the IEEE1588 packets
is again different from that of a transparent clock or a standard switch.
The boundary clock can transport the PTP time information across network boundaries.
For this purpose, it receives the time information as a slave and passes this on as a
master.
Slaves connected to the boundary clock see this as a grandmaster clock and synchro-
nize themselves, even if they are in a different subnetwork.
