White Rabbit networks for PTP users

White Rabbit networks for PTP users
Technology News |
Precise time synchronization has become an indispensable tool for supporting the proper operation of distributed real-time systems as required on many industrial, telecommunications and safety critical applications.
By Julien Happich

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For this reason, there is a growing concern about how to avoid uncoordinated actions and the consequent generation of instabilities that impact the reliability of a distributed system operation. As time synchronization and distribution become more critical, new technologies have emerged to enable the management of core industrial operations and decision-making processes in an efficient and resilient way.

One of the most common protocols for industrial time transfer in a network is the well-known IEEE1588 Precision Time Protocol (PTP). IEEE1588 PTP synchronizes multiple clocks over networks such as Ethernet and provides sub-microsecond time synchronization over long distances using just Ethernet links. This requires establishing which device will serve as Master clock (in a Master/Slave scheme) and properly measure the time skew generated by the clock offsets and the network delays.

Fig. 1: Scheme of the message flow in WR-PTP.

The link delay between two nodes (Master and Slave) is evaluated through the exchange of precise time-stamps (typically done using hardware assisted time-stamping mechanisms) in a network segment: An initial message sent by the Master node to the Slave is time-stamped at t1 and received by the Slave at t2. Then, during the way back, a message is sent from the Slave at t3 and received by the master at t4. Assuming that the time it takes for messages to go from Master to Slave is the half of the total time in the two-way path (link symmetry assumption), the offset with respect to the Master can be calculated as 

offset = (t2 + t3 – t1 – t4) /2.

The clock synchronization is achieved by minimizing the offset. Note that the time transfer accuracy is based on the thought of zero delay asymmetry in the round trip which is not always an accurate assumption.

Despite the efforts made to increase IEEE1588 PTP’s accuracy, the PTP links introduce accumulative time inaccuracies over the entire network and, for some applications requiring long distance links and asymmetrical links, PTP’s scalability and synchronization is poor. An improved approach is PTPv2, an industrial evolution of PTP that better define protocol devices cases and result in significant improvements in the accuracy of time synchronization.


By combining PTPv2 with Synchronous Ethernet, also referred as SyncE, the time information and frequency is distributed to all the nodes. Currently, this combination is the most popular among the solutions for industrial time distribution on telecommunications in a similar way to switching networks. SyncE can also be used in power grid and automation applications and it offers a slight improvement in time synchronization regarding PTP. However, problems related to phase propagation and system scalability (long distance and link asymmetry) remain unsolved and the synchronization level is still limited for some applications that stand in need of timing precision better that 1 nanosecond.

The difficulty to achieve the requirements needed on many industrial applications has pushed the development of rather complex network topologies and device types. In a PTP network, equipment can assume different roles (ordinary clock, master clock, slave clock, slave only clock, grandmaster clock, preferred grandmaster, server, client, transparent clock or boundary clock) in order to optimize the timing budget available to each device of the network. In the case of White-Rabbit solutions, the network is much more simplified.

White Rabbit-PTP (WR-PTP) is the name of the enabling technology for sub-nanosecond time precision. The next generation of telecommunication systems, defense applications or high-speed computerized trading in financial markets, among others, expect to benefit from ultra-accurate solutions, that otherwise are not achievable with the current PTPv2 protocol.

The improvements introduced by WR-PTP can be summarized as follows:

Synchronization and time-stamping with sub-nanosecond accuracy and with a jitter lower than 20 picoseconds.

Distribution through thousands of nodes and up to hundred kilometers over standard optical fiber networks.

Dependable (and deterministic) global time reference. Timing is not affected by network traffic, weather conditions or number of hops.


The White Rabbit Network (WRN)

The WRN consist on a set of multi-port White Rabbit Switches and single or dual-port White Rabbit Nodes interconnected though conventional Gibabit Ethernet (GbE) and expandable to thousands of nodes interconnected with optical fibers links up to 10 km. All elements work as boundary clocks with the exception of the grandmaster which is the master clock reference for all the remaining devices. The sub-nanosecond accuracy is provided for all the devices (switches and nodes) and the protocol implementation and network deployment is significantly improved (there is no need for all these equipment types defined on PTPv2 networks). Furthermore, there is no longer need to hire high experts to implement time sensitive applications with outstanding capabilities.

 

WR equipment

Currently, the WR features are provided by a range of industrial products developed by Seven Solutions. The White Rabbit Switch (WRS) is the main element of the WR technology. It distributes Time and Frequency within sub-nanosecond accuracy to thousands of nodes through standard optical fiber for distances above 80 km. The WRS is fully compatible with Ethernet and provides deterministic delivery and a reliable communication using redundant network topology. In addition, the WRS self-calibrates timing links.

From left to right: WR-ZEN TP, WR-LEN and WRS

Another interesting product is the White Rabbit Zynq Embedded Node (WR-ZEN). It is the versatile and full-programmable standalone node that provides the White Rabbit features to a wide range of applications exploiting its redundant connections. Moreover, the WR-ZEN Time Provider (WR-ZEN-TP) easily distributes time and frequency to other equipment by implementing standard timing protocols such as PTP, NTP, NMEA, IRIG-B, ToD, etc. Furthermore, the FMC expansion port allows to add additional card to the system to develop market-specific products.

During the last years Seven Solutions has also developed cost effective products to distribute PPS/10MHz signals or IRIG-B protocol. This is the case of the WR Lite Embedded Node, (WR-LEN) that is the competitive WR alternative capable of supporting daisy chain configurations and is also available in its OEM version for being integrated in other systems.

Moreover, several kits have been designed to ease the user’s first contact with the WR technology, like the KIT WR-LEN, consisting on a set of WR-LEN nodes. Each node includes two ports and offers several possibilities, including synchronization and time distribution.


Experiments

Experiment 1. WR synchronization over 15 km

The hardware and timing group of CERN carried out an experiment to determine the precision and accuracy of a WR network in 2013. 4 WRS were connected with 5 km fiber rolls in a daisy chain along 15 km. Within this configuration the first WRS was used as Master (in a Master/Slave scheme) and the environmental conditions were simulated by heating the fiber with a hot air gun.

A reference frequency signal of 10 MHz was provided to the Master through an external oscillator. The integrated jitter from 10 MHz to 40 MHz was determined from the Power Spectral Density of the master and each slave phase noise and the obtained value was about 2 ps. The accuracy, given by the deviation between the clocks of a Master node and the Slaves, was determined with the aid of an oscilloscope measuring during 1 hour. The resulting value was several tens of picoseconds between the master and the second slave. In the case of the other slaves the accuracy was not greater than 200 ps.

Experiment 2. WR synchronization at Tunka-HISCORE

Another experiment was carried out by a team from the Paul Scherrer Institute (Switzerland) and the Germany’s largest accelerator center DESY (Germany) in the framework of the Cherenkov Telescope Array (CTA) project. The WR protocol was incorporated to the modern timing system of the prototype Tunkaa-HiSCORe (Hundred Square-km Cosmic Origin Explorer) array. Currently, this detector is under construction at Tunka Valley, Siberia and will cover an area of more than 100 km2. To embody the real behavior, the light arrival had to be measured with sub-nanosecond accuracy.

Before proceeding with the experiment in situ, a laboratory test has been carried out including climate chamber temperature tests from -20 to 40 ºC (in the fiber) and from 0 to 30 ºC in the nodes. The timing precision was better than 0.2 ns and the results showed an excellent trigger time-stamping (in the order of the ns).


Conclusions

Current industrial and scientific facilities incorporate modern timing systems to generate and distribute timing to entire infrastructures. This includes the generation of programmable triggers as well as proper timestamping mechanism to identify machine state. To do this, a set of nodes interconnected in a network performs well-defined functions to synchronize their activities and properly provide a picture of machine operation. Given the need of globally synchronized time, each node no longer need to contain a high-precision clock because the master clock capabilities are transmitted thought the telecommunication network. It enables the node to establish the time the events occur and their duration and because the same timing link can be used to distribute data, it is possible to reduce the wiring necessities.

WR-PTP is the enabling technology to provide sub-nanosecond synchronization to thousands of nodes over tens of kilometers. This technology follows a hierarchical structure its accuracy is provided by means of inserting new calibration procedures and correcting the link delays and asymmetries. It represents a great advance in time and frequency distribution since the Ethernet-based WRN had low-latency, it is highly reliable and distributes deterministic data in a transparent way.

Over the years, significant efforts have been made to include WR into the PTP as an option for ultra-accurate time customers. Standardization will take place in 2018 and is expected to facilitate WR’s integration with a wide range of diverse technologies.

 

About the author:

Trinidad Garcia is Postdoctoral Researcher at Seven Solutions – www.sevensols.com

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