IEEE 1588 Keeps Time In Sync
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/5/2005 9:01:00 AM
An interview with John Hirsekorn, Paul Skoog, and Doug Arnold of Symmetricom.
IEEE 1588 Resources The IEEE 1588 Website at the National Institute if Standards and Technology contains links to tutorials, news, and conference information. Eidson, John C., "IEEE 1588: Applications in Measurement, Control, and Communication," January 18, 2005. Eidson, John C., "The Application of IEEE 1588 to Test and Measurement Systems." Agilent and IEEE 1588 When we asked Agilent Technologies to define its position on IEEE 1588 and LXI, a company spokesperson replied, "It is currently under development for use in LXI. Although it is not used today with traditional test and measurement LAN-based instruments, IEEE 1588 will likely prove quite valuable to a set of users with needs for strict timing. It will be a key in the construction of synthetic instruments, where it can coordinate actions across a collection of devices to perform a single orchestrated measurement. Agilent considers 1588 an important specification to finalize and could very likely change how timing and synchronization are carried out over LAN." |
IEEE 1588, Standard for A Precision Clock Synchronization Protocol for Networked Measurement and Control Systems is poised to move into test and measurement equipment. Published in November 2002, and based on the work of John Eidson at Agilent Labs, IEEE 1588 specifies hardware and software that lets networked equipment (clients) synchronize their clocks to a network's master clock. The standard has seen implementations in industrial automation where precise control wasn't previously possible over a LAN.
To learn how IEEE 1588 can improve timing of measurements, I spoke with three engineers at Symmetricom: John Hirsekorn, VP of marketing for Symmetricom's timing, test, and measurement division; Paul Skoog, product marketing manager for NPT and bus card timing products; and Doug Arnold, chief scientist. Symmetricom manufactures network time servers and time and frequency sources.
IEEE 1588 is capable of synchronizing the internal clocks of networked equipment to within 10 µs because it requires hardware in every network node that can adjust its internal clock. Previous protocols such as Network Time Protocol (NTP, www.ntp.org) use a software-only protocol to synchronize clocks on networked equipment, but it can't provide the accuracy needed for industrial control and test-and-measurement applications. To learn more about IEEE 1588, see "IEEE 1588 Resources" in the sidebar.
T&MW: Please explain why synchronization is needed in networked equipment.
Hirsekorn: We noticed that engineers were using coaxial cables to distribute clocks from clock generators to all kinds of equipment, some of which was test equipment. With an increasing number of products communicating by Ethernet, we saw a need to eliminate the coax cable and use Ethernet to synchronize equipment clocks. We started developing our own protocol, but soon realized that an IEEE standard was in the works. So, we abandoned development of our protocol in favor of IEEE 1588 because we saw the market begin to adopt it.
Arnold: Moving to a network-based protocol for clock synchronization eliminates the second cable in many applications. You can synchronize equipment using your data cable. That's a huge troubleshooting and cost advantage.
Skoog: The master clock and the client clocks work with regular Ethernet cables, hubs, and switches, which hold costs down. IEEE 1588 is a time-distribution mechanism. It can replace IRIG-B, which sends one pulse per second. As long as the client receives the pulse, it stays synchronized to the master clock. IEEE 1588 introduces a notion called scheduling, where you can use an inexpensive oscillator in a client and schedule it through software to stay synchronized to the network's master clock.
We know of a turbine control system that uses IEEE 1588 to synchronize the sensors to within 1 µs. A series of measurements starts at the top of each second and continues at 5-ms intervals. Each sensor relies on its own schedule to make a measurement. Measurements occur 200 times per second, within 1 µs of each other. The manufacturer saves a large sum of money because network drops are inexpensive compared to running cables to each sensor.
T&MW: Are the sensors connected directly to the network?
Skoog: The sensors are connected to a data-acquisition board that contains its own accurate clock. That clock is synchronized to the network's master clock through IEEE 1588.
Hirsekorn: With IEEE 1588, you can have multiple instruments making measurements on cue. You can then capture the data, which will be aligned in time, and analyze it. It should open new doors for test and measurement applications.
Skoog: You can collect data over a network. In the future, test equipment will not only measure amplitude, but will do so based on a precise time scale. Engineers will be able to time correlate their data. IEEE 1588 also eliminates trigger-delay latencies in measurements because measurements will use local clocks.
T&MW: Do you know of any test and measurement products that currently incorporate IEEE 1588?
Hirsekorn: IEEE 1588 is in its infancy. Over the next year or two, you'll begin to see test equipment that incorporates IEEE 1588.
Skoog: The industrial automation folks have been the early adopters of IEEE 1588. If you scroll through the papers presented at the IEEE 1588 conference, you can see that people are clearly looking into implementing it. But it's still early. IC support for 1588 has, until recently, been limited to FPGAs. Intel has announced an IEEE 1588 support for some of its processors. With Intel behind IEEE 1588, you'll see it become more common over the next few years in test instruments.
Skoog: IEEE 1588 has it limitation, though. If you use it through a network router, you may as well use NTP. The router will introduce nondeterministic effects. Ethernet switches can also the protocol's effectiveness. You're best off to use an Ethernet hub.
Arnold: In the factory automation world, companies are designing switches that preserve the precision that IEEE 1588 offers. They're called "transparent" switches or boundary-clock switches.
Hirsekorn: These switches measure the delays they introduce into the network and compensate for those delays.
T&MW: Where can readers go to learn more about IEEE 1588?
Arnold: John Eidson of Agilent Technologies produced a slide presentation that's a very good tutorial.
T&MW: What is Symmetricom's participation in IEEE 1588?
Hirsekorn: While we're not a test and measurement company, we do produce equipment that generates clock signals. We can produce a time standard for networks.
T&MW: What are the biggest hurdles to overcome for IEEE 1588 to gain widespread use?
Arnold: Silicon is needed so that equipment manufacturers can build inexpensive slaves in devices by the thousands.
Skoog: To make 1588 work, you need hardware on both ends. IEEE 1588 is just a protocol. You need a hardware clock and a hardware client to gain the advantages over NTP, a software-only timing protocol. You need to be able to plug network devices into a network and get the timing accuracy. If someone is looking to set up a network of sensors, they're probably willing to buy a master clock, but they want the sensors and data-acquisition equipment to be relatively inexpensive.
T&MW: Do you envision IEEE 1588 hardware going into test equipment? That is, do you see it going into stand-alone equipment with LAN ports or do you see it on plug-in data-acquisition boards as well?
Skoog: I see it going soup to nuts. Some people will want to add precise timing to existing instrumentation systems while others will want to build systems from the ground up.
Hirsekorn: Soon, you will see implementations that replace an expensive cable that transports precision time with equipment that uses IEEE 1588. There's an economic benefit today.
Skoog: We didn't mention that IEEE 1588 will increase time-of-day accuracy over IRIG-B, which currently delivers accuracy to within 1 µs. IEEE 1588 will improve time-of-day accuracy to perhaps 100 ns. To get timing accuracy better than 1 µs today, you have to move a 1 pulse per second clock to each piece of equipment in your system. And, you have to tell the client what time of day to associate with each pulse. IEEE 1588 eliminates that because the protocol can pass time-of-day information from master clock to each client over the network.
Hirsekorn: I expect test-equipment makers to adopt IEEE 1588 earlier that the broader IT community because of the relative high cost of sophisticated test equipment. Adding IEEE 1588 hardware and software will have little impact on the cost of test equipment.
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