20 November 2017 | Article
Figure 1: Schematic structure of a PTP network 
An “Ordinary Clock” is a simple clock (a port, usually a client) that is connected to a master as a slave and adjusts its time to it.
A “Boundary Clock” is a clock which contains several “Ordinary Clocks” (several ports) and which can synchronize several slaves with their time as master. The “Boundary Clock” can also be connected as a slave to a master and its time can be adjusted.
A “Transparent Clock” is a clock that does not actively intervene in the time synchronization; it is more of a hardware that provides the time synchronization data packets, typically a network switch. “Transparent Clocks” correct the time stamps within the data packets by the amount of time spent within the hardware.
The Grandmaster Clock is an “Ordinary Clock” that normally has access to GPS or other very accurate time, providing this very accurate time for all subordinate nodes.
Within the PTP network, each clock (port) can act as master or slave. Within a PTP domain, however, there can always be only one master. The master actively distributes its time to the slaves. It is thus responsible for the synchronization of the slaves in its PTP domain. A slave is a passive element within PTP that responds to the master’s master synchronization requests. In principle, each clock (port) can function as master or slave within PTP. The decision as to whether the clock has to work as master or slave is determined when entering the PTP network. The “Best Master Clock” algorithm was defined in IEEE 1588 standard. This algorithm regulates the clock with the best accuracy, the role of the master.
Synchronization of the time between master and slave is carried out via so-called “event messages”. These messages are distributed by the master via the UDP network protocol in the network and answered by slaves. There are two different methods for synchronization, the one-step synchronization, and the two-step method. The difference in the two methods lies in the number of messages which must be exchanged and can be seen in the following diagram. The two-step method sends four messages to perform a synchronization step. The one-step method requires only three messages, but this is only possible with very good hardware support and cannot be found in any PTP domain.
In order for a slave to perform the synchronization, only four time stamps (t1, t2, t3, t4) are necessary. The time stamps are generated when sending or receiving certain messages. The slave receives these four time stamps by exchanging the following “Messages”:
The following diagram shows the exact flow of the PTP communication and the associated exchange of the required time stamps. The time stamps t1 and t4 are recorded on the master side and are sent to the slaves with the messages. For this node to have its four time stamps, t2 and t3 are created on the slave side upon
receipt of the master messages.
Figure 2: Schematic sequence of the messages 
Figure 3: Formula for calculating the meanPathDelay 
Figure 4: Measurement of the slave offset over 100 seconds 
Figure 5: Measurement of the slave offset over 100 seconds 
In order to be able to recognize and evaluate such disturbance variables, a Kalman filter can be used, for example. In the T & M area, with the help of the exact time synchronization via IEEE 1588, many new and interesting approaches are now emerging. If one considers that a trigger signal requires approx. 5ns running time per meter, one can certainly imagine some measurement tasks which can be solved better and more precisely with the help of the IEEE 1588 time synchronization. For measuring tasks, which have to do with satellite signals, for example, it can happen that measuring devices are 100 and more meters apart, the approach with IEEE 1588 would certainly be an alternative.
Also in the course of the increasing distribution of the IoT sensors and the offline measurement data acquisition, an accurate and stable time synchronization of the individual measuring sensors would be of great advantage. Thus, the individual IoT
sensors can continuously supply their data and store it in a cloud, for example. This data can then be simply correlated with an offline measurement data evaluation via the time stamp, which was created during the measurements.
Since IEEE 1588 also defines time-controlled events, for example, several devices can process sequences of measurement tasks without having to be connected with trigger lines or the like. For example, a signal generator could process its frequency list in a timed manner and the analyzer can perform its measurements. Both tasks would then be controlled by the time-bound.
With the use of IEEE 1588, a wide range of measurement tasks can certainly be implemented much more easily in the field of mobile communications, since the temporal coupling between base station and mobile radio is of relevance for various
These application examples are all very promising in the first approach, but are based on the assumption that the manufacturers of the measuring devices have also implemented this standard in their devices. Even if all parties would use the IEEE 1588 standard, a further standardization is still necessary to coordinate the individual actions and tasks.
In order to make this easier for instrument manufacturers, the LXI standard (Lan eXentsion for Instruments) was launched in 2004 by the leading measuring instrument manufacturer. In 2008, the IEEE 1588 standard was included in the LXI standard and extended by some extensions, such as the Lan Event Messages. With the help of this standard it is now possible for measuring instrument manufacturers to efficiently use the advantages of IEEE 1588 in measuring instruments. Until the introduction of the Intel I21x network chips, cost-intensive or proprietary hardware was always required to integrate the IEEE 1588 standard in the measuring instruments. With the Intel network chips, it is now possible for the first time to realize this standard with consumer electronics.
As early as 2014, the LXI consortium decided to provide a reference design for your members to further spread the use of the standard. After the core functionality of the standard was available in the LXI reference design 2016, the LXI consortium, together with the German company TSEP, began evaluating the possibility of a reference implementation of the LXI Time Synchronization (corresponding to IEEE 1588) and to create a prototype. In February 2017, the first prototype was presented by TSEP at one of the regular meetings of the LXI consortium in the USA. Both on Linux and under Windows, the prototype was running on a commercially available computer board with Intel I211 chip.
The LinuxPacket “LinuxPTP” was used for the Linux prototype. However, the implementation of the actual clock functionality was not sufficient to meet the required framework conditions of the LXI and IEEE 1588 standards. TSEP has therefore reimplemented this. The control algorithm for frequency tracking of the IEEE 1588 Clock has been completely redesigned and optimized.
As mentioned above, Windows does not provide any support for IEEE 1588, so the complete clock functionality as well as the driver support of TSEP was created. TSEP has anchored the necessary drivers for the control of the Intel I21x network chips for the timestamping and the recognition of IEEEE 1588 packets in the Windows Network Stack. Both Windows 7 and Windows 10 tested this functionality.
The first discussions with the LXI Consortium on the provision of LXI Time Synchronization (equivalent to IEEE 1588) were consistently positive and are likely to lead to a free provision for LXI members this year. TSEP also plans to provide its implementation for other non-LXI members.
In summary, it can be said that with the commercial use of Intel I21x network chips it is now possible to use IEEE 1588 in T & M devices. The new solutions developed with IEEE 1588 extend the possibility to solve measurement tasks in the T & M area by a new dimension. If additional extensions like the LXI standard are offered in T & M devices, complex measurement tasks can be structured more simply and efficiently. Self-configuring systems are now possible with these approaches, since device data and actions can be configured, exchanged and triggered network-wide. It remains to be seen how the measuring instrument manufacturers react to this new technology,
but there is a great potential in it.
 IEEE standard for a precision clock synchronization protocol for networked measurement and control systems, 2008.
 Bachelor Arbeit von Dominik Richstein, „Usability of the Intel Ethernet Controllers I210, I211 and I350 for the LXI Extended Function Clock Synchronization“
 CERN. White rabbit specification: Draft for comments, 06.07.2011.
 White Rabbit Project. White rabbit wiki, 2016.
Peter Plazotta, Dipl. Ing (FH)
CEO of TSEP, and has been involved in the design and development of system software for the past 30 years in the fields of T & M, telecommunications and automotive.
Dominik Richstein, B. Sc,
Studied at the Technical University of Ingolstadt and has written his Bachelor thesis on the topic of IEEE 1588 and Intel I21x network chips at TSEP.