Chinese Version

Introduction to IEEE 1588 PTP

The IEEE 1588 standard is known as "Precision Clock Synchronisation Protocol for Networked Measurement and Control Systems" or "PTP" for short. PTP defines a procedure allowing many spatially distributed real-time clocks to be synchronized through a "package-compatible" network (normally Ethernet). The technology behind the standard was originally developed by Agilent and was used for distributed measuring and control tasks. The challenge was to synchronize networked measureing devices with each other in terms of time, so that they are able to record measured values and provide them with a precise system time stamp. Based on this time stamp, the measured values can then be correlated with each other.

Synchronization principle

In Ethernet systems, unpredictable collisions due to the CSMA/CD procedure may lead to time packages being delayed or disappearing completely. For this reason, IEEE 1588 defines a special "clock synchronization" procedure.

First, one node (master clock) transmits a "Sync" telegram, which contains the estimated transmission time. The exact transmission time is captured by a clock and transmitted in a second "Follow Up" message. Based on the first and second telegram and by means of its own clock, the receiver can now calculate the time difference between its clock and the master clock. To achieve the best possible results, the timestamps should be generated in hardware, or as close as possible to the hardware.

The telegram propagation time is determined cyclically in a second transmission process between the slave and the master ("delay" telegrams). The slave can then correct its clock and adapt it to the current bus propagation time.

Configuration

A PTP network configures and segments itself automatically. For this, each node uses the "best master clock" algorithm (BMC), in order to determine the best clock in the segment. Every PTP clock stores its features within a specified sataset. These features are transmitted to other nodes are able to synchronize their datasets with the features of the actual master and can adjust their clocks. Due to the cyclic running of the BMC, nodes can also be connected or removed during propagation time (hot plugging).

No distinction is made in the protocol between a software clock and a hardware clock. To be able to work with synchronism in the microsecond range, however, hardware support is required. Generally the synchronization errors - caused by softwarfe jitter - can not be eliminated. With a pure software solution (e.g. Windows system), the error may actually be in the millisecond range.

Fax: +86-10-82357574