Awakening Power of Physical Layer: High Precision Time Synchronization for Industrial Ethernet

High-precision time synchronization is critical for nowadays industrial Ethernet systems. Most existing time synchronization mechanisms are implemented based on packet communication. This interaction pattern, however, greatly limits their synchronizing frequency. In order to achieve microsecond-level synchronization precision, expensive high-quality oscillator is necessary for maintaining low clock skew under this long synchronization period (usually several seconds). Furthermore, packet processing introduces many nondeterministic variances (e.g. network stack overhead), which needs to be carefully eliminated. In this paper, we propose the brand-new Industrial Time Protocol (ITP). We deploy the entire ITP in the physical layer, so it eliminates most time uncertainties caused by network stack processing. Furthermore, ITP leverages the InterFrame Gap (IFG), which is the inherent interval between any two Ethernet frames, to carry the synchronization message. With this novel design, ITP can synchronize peer devices at very high frequency without degrading the goodput. The accuracy of ITP is bounded by 16ns for two adjacent devices with only intrinsic cheap oscillator. Furthermore, our theoretical analysis deduces that ITP guarantees 16N-nanosecond accuracy for N-hop network. We implement ITP design with NetFPGA. Experiments show that ITP can provide about 76-nanosecond accuracy for #hops=16 network under severe congestions. In addition, the design of ITP is scalable. It only consumes about 0.67% of logic cells in the low-end FPGA for supporting every ITP-aware port increase.

[1]  Thomas Kunz,et al.  A recursive method for bias estimation in asymmetric packet-based networks , 2014, 2014 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control, and Communication (ISPCS).

[2]  James Aweya,et al.  Clock Synchronization Over Communication Paths With Queue-Induced Delay Asymmetries , 2014, IEEE Communications Letters.

[3]  Maciej Lipinski,et al.  White rabbit: a PTP application for robust sub-nanosecond synchronization , 2011, 2011 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication.

[4]  Augusto Ciuffoletti.  Preventing the Collision of Requests From Slave Clocks in the Precision Time Protocol (PTP) , 2011, IEEE Transactions on Instrumentation and Measurement.

[5]  J. Jasperneite,et al.  Enhancements to the time synchronization standard IEEE-1588 for a system of cascaded bridges , 2004, IEEE International Workshop on Factory Communication Systems, 2004. Proceedings..

[6]  Radim Bartos,et al.  The impact of network latency on the synchronization of real-world IEEE 1588-2008 devices , 2010, 2010 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication.

[7]  Cristian Marinescu,et al.  Experimental verification of the egress and ingress latency correction in PTP clocks , 2014, 2014 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control, and Communication (ISPCS).

[8]  David Tipper,et al.  A Survey of Clock Synchronization Over Packet-Switched Networks , 2016, IEEE Communications Surveys & Tutorials.

[9]  Nikolaus Kero,et al.  Synchronizing an IEEE 1588 slave clock over both paths of a redundant Ethernet system , 2016, 2016 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control, and Communication (ISPCS).

[10]  J. Elson,et al.  Fine-grained network time synchronization using reference broadcasts , 2002, OSDI '02.

[11]  Vern Paxson,et al.  On calibrating measurements of packet transit times , 1998, SIGMETRICS '98/PERFORMANCE '98.

[12]  Hermann Kopetz,et al.  Real-time systems , 2018, CSC '73.

[13]  David L. Mills,et al.  Internet time synchronization: the network time protocol , 1991, IEEE Trans. Commun..

[14]  Donald F. Towsley,et al.  Estimation and removal of clock skew from network delay measurements , 1999, IEEE INFOCOM '99. Conference on Computer Communications. Proceedings. Eighteenth Annual Joint Conference of the IEEE Computer and Communications Societies. The Future is Now (Cat. No.99CH36320).

[15]  Darryl Veitch,et al.  Robust synchronization of software clocks across the internet , 2004, IMC '04.

[16]  Radim Bartos,et al.  Transparent Clocks vs. Enterprise Ethernet switches , 2011, 2011 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication.

[17]  J. Ridoux,et al.  An IEEE-1588 compatible RADclock , 2012, 2012 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication Proceedings.

[18]  Darryl Veitch,et al.  PC based precision timing without GPS , 2002, SIGMETRICS '02.