White Rabbit networks for PTP users

June 24, 2016 // By Trinidad Garcia
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.

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.

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