Should PTP replace NTP altogether? If
not, what do system designers need to
know in order to choose the appropriate protocol? Perhaps the answers to
these questions can be discerned from
the circumstances that led up to the
definition of PTP and a more in-depth
look at this newer standard.
measurement and Control Devices:
time for Something new?
Measurement and control devices
have always been a vanguard for high-precision event synchronization. To
achieve the degree of synchronization
that devices of this nature require, signals sent over specialized cables can
be used to synchronize events between
devices. These cables, which are used
exclusively for event synchronization,
are often matched in length to ensure
propagation delay is consistent. Synchronization using this dedicated cabling results in extremely high precision, where events can be coordinated
to within picoseconds of each other
across multiple devices in proximity.
This type of synchronization is commonly referred to as signal-based.
While nearly unbeatable for applications requiring the most accurate
synchronization possible, signal-based synchronization can be highly
impractical and sometimes not possible. The dedicated cabling needed
to synchronize separate devices can
be cost prohibitive, and signal-based
synchronization requires specialized
hardware and software to generate
and receive the signals on the cable.
The signaling protocol can be proprietary, resulting in potential vendor
tie-in, risk of discontinuation, or other legal or technical restrictions. The
cables themselves are often subject to
varying propagation delay over time
and temperature, and as more devices
are added to a system, the complexity
of cabling multiple devices increases
the maintenance burden and effort in
troubleshooting failures. Signal-based
synchronization also requires the devices be relatively close to each other
and does not scale over long distances
when compared with other synchronization mechanisms.
Meanwhile, with Ethernet becoming more and more ubiquitous in the
laboratories and on factory floors
where measurement and control de-
measurement and
control devices
have always been
a vanguard for
high-precision event
synchronization.
vices are deployed, a need arose for
these devices to be able to use a LAN
or even a wide-area network (WAN)
for control and data communication.
NTP was even leveraged to set system
time for these devices, but the need for
dedicated event synchronization cables still existed. Despite the presence
of all the basic ingredients for event
synchronization using coordinated,
distributed timekeepers (sometimes
referred to as time-based
synchronization), an acceptable technology that
could use this infrastructure to replace
signal-based synchronization had yet
to be created.
Because signal-based implementations impose the constraints previously mentioned, time-based solutions using Ethernet were investigated further
as a synchronization solution. At first
glance, NTP seems like a good candidate for a low-cost time-based synchronization solution—and it is for many
applications. Signal-based synchronization, however, provides an extremely
high level of precision, and NTP version 3 (until recently the officially
supported NTP release) provides only
millisecond precision, which is not
even close to being sufficient for applications using signal-based solutions.
PTP was designed to meet the needs of
the measurement and control industry and is capable of near-nanosecond
precision while taking advantage of infrastructure that is similar to what NTP
uses. A closer look at PTP reveals why
it is successful for measurement and
control applications—and, as it developed, many other applications as well.
introducing PtP
PTP’s primary design goals have been
listed in numerous presentations and
documents, including the IEEE 1588
standard: 1
˲ To provide sub-microsecond synchronization of real-time clocks in
components of a networked distributed measurement and control system;
˲ To perform best with relatively localized systems typical of industrial automation and test and measurement
environments;
˲ To be applicable to LANs supporting multicast communications (
including but not limited to Ethernet);
˲ To provide a simple, administration-free installation;