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.” 2 Some 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 installation.” 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
standard, but
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.” 4 This 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