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Introduction to IEEE 1588
Measurement and control systems are widely used in traditional test and measurement, industrial automation, communication systems, electrical power systems and many other areas of modern technology. The timing requirements placed on these measurement and control systems are becoming increasingly stringent. Traditionally these measurement and control systems have been implemented in a centralized architecture in which the timing constraints are met by careful attention to programming combined with communication technologies with deterministic latency. In recent years an increasing number of such systems utilize a more distributed architecture and increasingly networking technologies having less stringent timing specifications than the older more specialized technologies. In particular Ethernet communications are becoming more common in measurement and control applications. This has led to alternate means for enforcing the timing requirements in such systems. One such technique is the use of system components that contain real-time clocks, all of which are synchronized to each other within the system. This is very common in the general computing industry. For example essentially all general purpose computers contain a clock. These clocks are used to manage distributed file systems, backup and recovery systems and many other similar activities. These computers typically interact via LANs and the Internet. In this environment the most widely used technique for synchronizing the clocks is the Network Time Protocol, NTP, or the related SNTP.
Measurement and control systems have a number of requirements that must be met by a clock synchronization technology. In particular:
- Timing accuracies are often in the sub-microsecond range,
- These technologies must be available on a range of networking technologies, including Ethernet, but also other technologies found in, for example, aerospace, industrial automation, power and utility, semiconductor manufacturing, telecommunication, test and measurement,
- A minimum of administration is highly desirable,
- The technology must be capable of implementation on low cost and low-end devices,
- The required network and computing resources should be minimal.
In contrast to the general computing environment of intranets or the Internet, measurement and control systems typically are more spatially localized.
IEEE 1588 addresses the clock synchronization requirements of measurement and control systems.
Objective of IEEE 1588
The objective of IEEE 1588 is defined in the 'Scope' section of the Project Authorization Request approved by the Standard Board of the IEEE as follows.
“This standard defines a network protocol enabling accurate and precise synchronization of the real-time clocks of devices in networked distributed systems. The protocol is applicable to systems where devices communicate via networks, including Ethernet. The standard allows multicast communication, unicast communication or both. The standard specifies requirements for mapping the protocol to specific network implementations and defines such mappings, including User Datagram Protocol (UDP)/Internet Protocol (IPversions 4 and 6), and layer-2 IEEE 802.3 Ethernet.
The protocol enables heterogeneous systems that include clocks of various inherent precision, resolution, and stability to synchronize to a grandmaster clock. The protocol supports synchronization in the sub-microsecond range with minimal network bandwidth and local clock computing resources. The protocol enhances support for synchronization to better than 1 nanosecond. The protocol specifies how corrections for path asymmetry are made, if the asymmetry values are known. The grandmaster can be synchronized to a source of time external to the system, if time traceable to international standards or other source of time is required. The protocol provides information for devices to compute Coordinated Universal Time (UTC) from the protocol distributed time, if the grandmaster is traceable to international standards and is able to access pending leap second changes. Options are also provided to allow end devices to compute other time scales from the protocol distributed time scale.
The protocol defines timing domains in which system timing is consistent and independent of timing in other such domains. The protocol establishes the timing topology. The default behavior of the protocol allows simple systems to be installed and operated without requiring the administrative attention of users to determine the system timing topology.
The standard defines all needed data types, message formats, required computations, internal states, the behavior of devices with respect to transmitting, receiving, and processing protocol communications. The standard provides for the management of protocol artifacts in devices, including a Simple Network Management Protocol (SNMP)-compliant management information base (MIB). The standard defines formal mechanisms for message extensions and the requirements for profiles that allow customization for specific application domains.
The standard defines conformance requirements. Optional specifications are provided for protocol security. This standard documents conditions under which this standard is backward compatible with IEEE 1588-2008.”
History of IEEE 1588
The last ten years have seen a number of academic and commercial organizations develop techniques for synchronizing clocks in devices typically used in measurement and control applications. Public discussion of the standardization of such a technology occurred between engineers developing technologies and standards applicable to distributed systems in industrial automation, the IEEE 1451 family of standards.
By November of 2000 there was sufficient interest in starting a standardization activity on clock synchronization to warrant forming a committee and seeking sponsorship. The initial committee met for the first time in April of 2001 and decided to seek sponsorship from the Institute of Electrical and Electronics Engineers (IEEE) Technical Committee on Sensor Technology of the Instrumentation and Measurement Society which had also sponsored, along with NIST, the IEEE 1451 activity. The committee membership included engineers from the automation, robotics, test and measurement, and time keeping industry as well as representatives from NIST and the military. The committee submitted a formal application to the IEEE which was approved on June 18, 2001.
The committee produced a draft of the standard which was submitted for ballot under the usual IEEE rules in April of 2002. This first ballot passed but there were a number of helpful comments submitted by the reviewing balloters. The committee incorporated these suggestions and resubmitted the standard for a second ballot which passed in May of 2002. The committee has submitted this final balloted version to the IEEE Standards Board Review Committee for final approval. The draft was approved as an IEEE standard by the review committee on September 12, 2002. The standard was published in November of 2002.
The IEEE 1588-2002 standard was later revised and became the IEEE 1588-2008 standard, many in industry refers to it as the IEEE 1588 version 2 standard.
Five years later, on June 14, 2013, a Project Authorization Request (PAR) was approved to revise the IEEE 1588-2008 standard to reflect the common needs of various industries for a new version of the IEEE 1588 standard that will meet the requirements of a more secure precision clock synchronization protocol that can work with both IPv4 and IPv6, and also suitable for higher speed applications in the sub-nanosecond range. See the Scope above for more detail.