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;