IEEE 1588 standard for a precision clock synchronization protocol for networked measurement and control systems

This paper discusses the major features and design objectives of the IEEE-1588 standard. Recent performance results of prototype implementations of this standard in an Ethernet environment are presented. Potential areas of application of this standard are outlined. INTRODUCTION Temporal relationships have always been an important element in the measurement and control of industrial physical systems. In small closed systems time is usually implicit in the operation of electronic circuits or in the execution patterns of computer programs. As these industrial systems become more complex with sensors, actuators, and computers distributed in space and communicating via networks, the explicit representation of time is often necessary for robust implementations. The temporal and other implementation requirements on industrial systems differ considerably from those found in typical office distributed computing environments. IEEE-1588-2002, “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” was designed to serve the clock synchronization needs of industrial systems. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE DEC 2002 2. REPORT TYPE 3. DATES COVERED 00-00-2002 to 00-00-2002 4. TITLE AND SUBTITLE IEEE-1588 (tm) Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Agilent Laboratories,3500 Deer Creek Rd,Palo Alto,CA,94304 8. PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES See also ADM001507. 34th Annual Precise Time and Time Interval (PTTI) Planning Meeting, 3-5 December 2002, Reston, VA 14. ABSTRACT This paper discusses the major features and design objectives of the IEEE-1588 standard. Recent performance results of prototype implementations of this standard in an Ethernet environment are presented. Potential areas of application of this standard are outlined. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT Same as Report (SAR) 18. NUMBER OF PAGES 12 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 34 Annual Precise Time and Time Interval (PTTI) Meeting 244 TEMPORAL REQUIREMENTS FOR INDUSTRIAL APPLICATIONS Application targets for IEEE-1588 are systems typically found in laboratories or in product test, industrial automation, motion control, power, or telecommunications system installations and similar industrial settings involving multiple sensors, actuators, instruments, and computer/controllers. Temporal, or synchronization, requirements in these applications are typically met in one of three ways: 1. Message-based. In message-based timing, the sensing of a datum, the setting of an actuator, or the initiation of a control procedure is synchronized based on the event of receiving a command or message. IEEE-488 instrument systems and many industrial control systems based on proprietary networks are good examples. In these systems, precise control of communication latency and execution timing is required for accurate system wide synchronization. 2. Cyclic-systems. In cyclic-systems, timing is periodic and is usually defined by the characteristics of a cyclic network or bus, for example the SERCOS bus widely used in the motion control industry. Synchronization accuracy depends of the accuracy of the cycle period and on the latency and fluctuations introduced by participating devices in converting the cycle timing into the required actions. 3. Time-based. In time-based systems, the execution of events is based on a common sense of time. For each event an execution time is specified, with the event execution occurring when the specified time matches the measure of real-time. This is the most flexible of the three schemes in that different, and if needed incommensurate, timing schedules for each device are easily implemented. In addition, synchronization accuracy depends on the accuracy of the common sense of time and on the implementation of the participating devices, rather than on precise control of messaging latency. For the most accurate synchronization in a time-based system, the common sense of time will be implemented by having a local clock in each participating node synchronized to its peers via a protocol such as IEEE-1588. The required accuracy in synchronizing these clocks depends on the application. Typical applications and their required accuracies are listed in Table 1. Table 1. Typical application synchronization requirements. Application area Required synchronization accuracy Low speed sensors (e.g. pressure, temperature) Milliseconds Common electro-mechanical devices (e.g. relays, breakers, solenoids, valves) Milliseconds General automation (e.g. materials handling, chemical processing) Milliseconds Precise motion control (e.g. high-speed packaging, printing, robotics) A few microseconds High speed electrical devices (e.g. synchrophasor measurements) Microseconds Electronic ranging (e.g. fault detection, triangulation) Sub-microsecond In addition to synchronization requirements, the targeted application areas typically include one or more of the following additional requirements: 1. Networking. Distributed systems increasingly use networks such as Ethernet for communication. However, there is still a need to provide a common sense of time on other networks. 2. Heterogeneous systems. Most systems have a range of synchronization accuracies. Therefore, the synchronization protocol must accommodate devices with varying accuracy capabilities. 34 Annual Precise Time and Time Interval (PTTI) Meeting 245 3. Cost. Systems will include both highand low-cost devices. Low-cost devices typically will have limited computational capability and memory available to execute a synchronization protocol. 4. Spatial scale. Most systems are compact enough both physically and logically to be implemented in a few local subnets of the communication system. Larger scale application can usually be treated as islands of locally precise time in which a separate protocol, for example GPS, is used for inter-island synchronization. 5. Low administrative overhead. In the simplest case, the protocol should be self-configuring. IEEE-1588 is designed to satisfy all of these requirements. It is not designed to work over the Internet or the general computing environment typically served by the popular Network Time Protocol [1]. REQUIREMENTS FOR MILITARY APPLICATIONS While military applications were not explicitly considered during the development of IEEE-1588, it appears that many of the characteristics discussed above for industrial systems are applicable to the newer generations of military systems. In particular, military systems are evolving from stand-alone systems into architecture of interoperable systems with strong synchronization or coordination requirements. A common sense of time is to be the foundation for this architecture, with the worldwide GPS system as the key component. However, operation must be insensitive to the loss of GPS signals [2]. At least in local environments such as a ship or command center, a system based on IEEE-1588 may allow more robust local operation by maintaining local time consistency, even when isolated from the global system. As system requirements extend to smaller, less capable, and cheaper military devices, the low implementation footprints for IEEE-1588 may be advantageous as well. Further discussion of applications of IEEE 1588 may be found other papers [3,4]. The remainder of this paper discusses performance experience and how IEEE-1588 is structured to meet timing requirements. IEEE-1588 DESIGN FOR MEETING TEMPORAL REQUIREMENTS Within a subnet IEEE-1588 automatically establishes a master-slave relationship among the participating clocks communicating via the subnet. The master is selected as the best clock based on defined descriptors for inherent accuracy, traceability to UTC, etc. Provision is made to designate a set of clocks to be preferred in this selection for applications where this is important. The slaves synchronize their local clocks to that of their master by an exchange of messages illustrated in Figure 1. Periodically the master clock sends a distinguished message, a Sync message, as a multicast to all its slaves. This message contains an estimate of when the Sync message will be placed on the network. In the most accurate implementations, the master will contain a mechanism for detecting and time stamping based on the master’s local clock, the time that the Sync message is actually placed on the network. In Ethernet, an ideal place to attach this detector is at the MII interface of the PHY c