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White Rabbit: When Every Nanosecond (and Subnanosecond) Counts

Xilinx Employee
Xilinx Employee
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By Dr. Javier Díaz (Chief Executive Officer), Rafael Rodríguez-Gómez (Chief Technical Officer), and Dr. Eduardo Ros (Chief Operating Officer), Seven Solutions SL



(Excerpted from the latest issue of Xcell Journal)


White Rabbit Logo.jpgAn Ethernet-based technology called White Rabbit, born at CERN, the European Organization for Nuclear Research, promises to meet the precise timing needs of high-speed, widely distributed applications including 100G Ethernet and 5G mobile telecom networks, smart grids, high-frequency trading, and geopositioning systems. Named after the time-obsessed hare in Alice in Wonderland, White Rabbit is based on, and is compatible with, standard mechanisms such as PTPv2 (IEEE-1588v2) and Synchronous Ethernet, but is properly modified to achieve subnanosecond accuracy. White Rabbit inherently performs self-calibration over long-distance links and is capable of distributing time to a very large number of devices with very small degradation.


From the very beginning, Seven Solutions, based in Granada, Spain, has collaborated in the design of White Rabbit products including not only the electronics but also the firmware and gateware. The company also provides customization and turnkey solutions based on this technology. As an extension of Ethernet, White Rabbit technology is being evaluated for possible inclusion in the next Precision Time Protocol standard (IEEE-1588v3) in the framework of a high-accuracy profile. Standardization would facilitate WR’s integration with a wide range of diverse technologies in the future.


White Rabbit incorporates a number of mechanisms designed to optimize its timing accuracy within the framework of an extension of Ethernet (thus keeping the Ethernet communications structure). White Rabbit integrates PTP, Synchronous Ethernet and digital dual-mixer time difference (DMTD) phase tracking. The new WR-ZEN board from Seven Solutions shows how the key elements of White Rabbit come together in a product.



Seven Solutions White Rabbit WR-ZEN Board.jpg



Seven Solutions’ Zynq-based White Rabbit WR-ZEN board




Based on Xilinx’s Zynq-7000 All Programmable SoC, the WR-ZEN board includes the White Rabbit Core (WRC), along with a Gigabit Ethernet MAC implementation that’s capable of providing a high-accuracy clock. Synchronization mechanisms implemented in the WRC include the following elements:


  • Frequency synchronization (synthonization): This is obtained by using SyncE, which encodes the clock signal in the data carrier. To guarantee that all nodes use the same frequency, we employ a mechanism based on a local oscillator disciplined to the external clock that is recovered from the optical link.


  • Phase synchronization: The physical clock of a node is retransmitted to the master element and vice versa so that the master can compare the phase of this signal (coming from the slave) with its own phase. The deviation should be equal to the propagation time of the signal through the fiber (properly measured using PTP). Having this information, the master can determine the phase difference between its own clock and the one from the slave, and request that the slave shift its phase to exactly the same value as the master. This process is done digitally by implementing a digital DMTD in the FPGA gateware.


  • Time synchronization: This is a consequence of using the PTPv2 protocol, which measures the link propagation times and provides the global notion of time. White Rabbit also takes into account the asymmetries in the propagation time due to the utilization of different wavelengths in a bidirectional optical fiber for each communication transfer (back and forward in the loop), thus improving the accuracy of the standard PTP protocol. Since the frequency and phase have been previously synchronized, we can guarantee the global notion of time in all the devices within the White Rabbit network.



Note: This blog is an excerpt. To read the full article in the latest issue of Xcell Journal, click here.



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