High-precision Time Synchronization with PTP and gPTP
The Precision Time Protocol (PTP, IEEE 1588) and the Generalized Precision Time Protocol (gPTP, IEEE 8201.AS) are two related protocols for highly accurate time synchronization over different transmission media. InES has been actively involved in standardization in the IEEE from the beginning. PTP was published in 2002 as IEEE 1588 and was upgraded to version 2, and 2.1 respectively, in 2008 and 2019.
gPTP restricts many options of PTP and defines some specialties. It was standardized by the Audio-Video Bridging (now Time Sensitive Networking) Task Group of IEEE 802.1 in an initial version in 2011, and significantly expanded in a new version as IEEE 802.1AS-2020.
Today, many Ethernet chips and CPUs exist that support hardware timestamping. Both protocols allow accuracies well below one microsecond with HW timestamps and down to the nanosecond range in special implementations.
The diagram on this page shows the measured deviation of a slave from the master clock for a 1588v2 Transparent Clock implemented by InES. The slave is synchronized across 7 heavily loaded switches (each link is loaded with 90% of its theoretical capacity in both directions). During the five-hour test run, the deviation between master and slave varied with σ = 33,9ns.
InES has worked with Hirschmann Automation and Control to implement both IEEE 1588v2 and IEEE 802.1AS-2011, and is currently working on implementing 802.1AS-2020, which will provide enhancements to several TSN profiles currently under development at IEEE, including IEC/IEEE 60802, the «Industrial Automation Profile for Time Sensitive Networking».
The major enhancements include:
- Support for multiple time domains to transmit a free-running, system-wide "working clock" for control and network operation in parallel with a global time base for sequence of event recording.
- Introduction of the "one-step" mechanisms defined in IEEE 1588 for the transmission of synchronization information.
- Performance target: one microsecond synchronization accuracy over 64/100 Ethernet links in series
Not yet standardized, and currently under definition at IEEE, is hot standby redundancy with gPTP.
For PTP in combination with PRP/HSR, a solution for redundant paths in the network already exists with IEC 61439-2 Ed.3.