as stated in the PTP standard, should
consult the Precise Networked Clock
Synchronization Working Group of
the IM/ST (Instrumentation and Mea-surements/Sensor Technology) Committee for technical review.
PTP profiles not only change several
aspects of the PTP standard, but also
extend it. A profile may define its own
BMC algorithm; configuration and
monitoring (“management”) mechanism; path-delay mechanism (
end-to-end or peer-to-peer); use of multicast
or unicast; transport mechanism;
node types; and any options that are required, permitted, or prohibited. Profiles may also define completely new
transport mechanisms and data types.
The flexibility that profiles have in morphing PTP to the needs of almost any
particular application has proven useful to telecommunication and energy
industries, among others.
Unicast. PTP was designed assuming a multicast communication mode,
but support for unicast operation was
eventually added as an optional feature.
The PTP standard does not describe a
unicast PTP implementation in detail,
but instead describes several optional
unicast features that can be used for an
implementation “as long as the behavior of the protocol is preserved.”
implementations may require that
slave clocks use a configuration that
specifies a list of known master clocks
by protocol address (for example, a list
of IP addresses when used over Ethernet) to discover the potential masters.
This unicast discovery mechanism
is optional, meaning a unicast implementation could choose to use multicast for discovery of master clocks
and unicast for all other messaging.
Furthermore, this discovery mechanism may also require some amount
of configuration to define the list of
masters, since that is most likely specific to a given system and stretches the
interpretation of the PTP objective to
“provide a simple, administration-free
1 Another optional implementation detail defined by PTP is the
use of the unicast-negotiation mechanism, which involves sending specific
signaling messages to master devices
indicating that they respond with a unicast Announce, Sync, Delay Response,
or Peer Delay Response to the signaling
slave device. This flexibility in allow-
PtP profiles not
only change several
aspects of the PtP
also extend it.
ing unicast operation and providing
several optional features to implement
it allows profiles to define the specific
unicast implementation details best
suited for their applications.
Timescale. The timescale for a PTP
network is defined by the grandmaster
and can be one of two types: the default
PTP timescale or an ARB (arbitrary) timescale.
5 With the ARB timescale, the epoch is set by some predetermined procedure and can be set again using that
procedure during normal operation.
The PTP timescale uses the PTP epoch,
and its unit of time is the SI second. The
PTP epoch is 1 January 1970 00:00:00
TAI (International Atomic Time), which
is 31 December 1969 23:59: 51.999918
UTC (Coordinated Universal Time).
using the Right tool for
the Job: ntP or PtP?
The requirements of devices for the
measurement and control industry are
similar to those of many other industries—and many innovative outcomes
have resulted from applying technology in ways that its designers had not
originally considered—but the intended applications for any technology still
should be considered before adopting
it, regardless of the similarities it may
have to the incumbent technology.
Objectives. As described earlier and
stated in the standard, PTP was designed to be used over a LAN, or more
specifically, “spatially localized systems
with options for larger systems.”
is one of the more significant differentiators between PTP and NTP. The use
of a LAN allows other PTP objectives to
be fulfilled using techniques such as
multicast for discovery and automatic selection of PTP masters, network
equipment such as boundary clocks
and transparent switches, and very high
message exchange rates that may not be
feasible over a WAN. A LAN also gives
PTP some liberties that NTP does not
usually have, such as assuming—with a
reasonable degree of confidence—that
unrelated network traffic and security
risks are both low, given that LAN usage
is usually confined and controlled.
In contrast, NTP is typically used
over the Internet and is therefore subject to a large amount of nondeterministic delays from intermediate network
elements (such as routers) and exposed
to a far greater number of security