wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Clock Synchronization in Distributed Systems 18-5
Node clock
Ordinary
clock
data sets
Port 1
Port 2
Port 3
Port n
Protocol engine
…
Clock Protocol state
correction engine
Message Configuration
assembling storage
Port
configuration
data and
foreign
master data
Event port
Timestamped
message receipt and
transmission
General
port
Message receipt and
transmission w/o
timestamping
PTP communication path
(Technology 1)
(Technology 2) (Technology 3) (Technology n)
FIGURE 18.4
Model of a PTP boundary clock (PTPBoundaryClock.pdf).
18.7 Precision Time Protocol, IEEE 1588–2008 (PTPv2)
Version 1 of IEEE 1588 was kept as simple as possible, but growing industrial interest in precision clock
synchronization soon made clear that an improved version of the standard was needed. By the time
of writing, the second ballot on IEEE 1588v2 has started and the final publication of the standard is
expected to be in the second half of 2008. The new version of the standard will have several improvements
[IEEE 1588v2]:
• Mappings. As the main clauses of the standard specify a communication protocol, rather then
actual payload positions in packets, mappings to communication protocols such as IP have to be
made. As an improvement to version 1, additional mappings such as to UDP in IPv4 and IPv6,
DeviceNet, raw Ethernet, PROFINET, and ControlNet will be made.
• Type length value fields. Version 1 suffered from the problem that formally, messages could
not be extended with arbitrary information. Such extensions have been added via partly userdefinable
type length value (TLV) fields. Nevertheless, not only proprietary fields are defined
via TLVs. Also, standardized functionality, like security updates, is transported via these fields.
• Transparent clocks. Under the name “on-the-fly timestamping” [Gaderer2004], version 2 supports
transparent clocks, which are basic network elements that timestamp messages when entering
© 2011 by Taylor and Francis Group, LLC