Over the past few months, and especially in our update on Class B instruments in the September 2008 issue “The Killer Bs Are Coming,” we have been reporting on the advantages of the IEEE 1588 precision timing protocol (PTP) and the application areas it is opening up for LXI instruments. There are, however, considerable 1588 interest and emerging activity in other industrial branches. In some cases, it even could initiate a switch away from some of today’s dominant timing methods.
Application areas already involved are power generation/grids, industrial automation, and electronic test while telecom, consumer/professional AV, military test, and even financial applications are showing very high interest and conducting trials. According to Bill Seitz of IXXAT, a company that provides protocol stacks, “A better question is this: What applications won’t be converting to 1588?”
He also reported that in the last year his company received an average of one inquiry per day from companies wishing to put together a 1588-based system. And even in this business environment, the frequency of inquiries continues to pick up. “There’s no recession in 1588 that we can see,” he added.
Endorsing this feeling was Doug Arnold, a product architect at Symmetricom, a company that manufactures timing equipment. “As timing mechanisms go, 1588 is going to be bigger than anything we’ve ever seen before due to its broad appeal. It’s easy to add 1588 as an extra feature to any number of products, and it will become very pervasive,” he explained.
What’s the Attraction?
What, exactly, are these advantages? IEEE 1588 allows any number of devices connected with low-cost commercial Ethernet cable, hubs, and switches to synchronize their operations within roughly 500 µs using software alone. Devices equipped with hardware-assisted PTP clocks increase accuracy to the nanosecond range. A number of companies now are selling microcontrollers and Ethernet controllers with the hardware assist, making it easy to incorporate this functionality into slave devices.
So how does this PTP work? Every 1588-based system must have a grandmaster clock. Through the multiple exchange of timing and verification messages, each slave clock can determine the amount of delay that message-passing takes and then synchronize its own local time to that of the grandmaster (Figure 1).
To account for variations in network loads, synchronization messages are resent every second. At this rate in most circumstances when using a hardware-assist, boundary clocks, and other assists, it is quite easy to get synchronization of 50 ns or better.
Synchronized time does not necessarily have a relation to the absolute time of day; rather, here a 1588 network consists of an island of highly synchronized devices working on their own agreed-upon time scheme. You can, however, coordinate this system time to absolute time by adding a GPS receiver to the grandmaster.
It’s instructive to compare the performance of 1588 to precision timing schemes that have dominated until now (Figure 2). One that also is based on standard Ethernet is network time protocol (NTP), which has been in use for more than 20 years. Its big drawback is accuracy: It can deliver 1 to 2 ms performance on a LAN or 1 to 20 ms on a WAN. But this is not guaranteed because of the delays inherent in switches and routers and because many NTP clients run on non-real-time operating systems. Windows, for instance, often can add clock corrections of 10 to 50 ms because the system is performing tasks deemed more important than timekeeping.
Another popular scheme is the Inter-Range Instrumentation Group (IRIG), developed in the 1960s and very popular in mission-critical applications such as military, aerospace, and power utilities. It increases accuracy to 1 to 10 µs. The big drawback is that a dedicated timing cable, typically coax or optical fiber, must go to each slave device being synchronized.
And so the appeal of IEEE 1588: It improves precision beyond that of IRIG while using commercial Ethernet equipment and sharing timing data with regular network traffic as does NTP.
Devices such as Ethernet switches can add nondeterministic latency and jitter to packet transit times from a 1588 master to a slave. With enough switches and routers in a system, 1588’s performance can drop to that of NTP. In these cases, system designers often set up subnets controlled by a boundary clock, a switch with multiple 1588 ports. It is a slave to the grandmaster or another boundary clock on one port, and on all other ports, it serves as a master for further downstream subnets.
Because so many industries were involved in the IEEE 1588 committee, the standard ended up with almost a superset of features. As a result, many industry groups are defining profiles of the specific feature sets they need. There still is some question as to what degree these profiles will be interoperable.
IEEE 1588 in Action
As you’ve read previously in this magazine, test engineers are starting to take advantage of these benefits. Some of the most common applications for 1588 so far involve distances where dedicated triggering buses won’t reach. With 1588, for instance, it’s possible to synchronize the acquisition of data from widely distributed sensors or perform a stimulus/response test where the two pieces of equipment are many meters or perhaps even a kilometer or more apart.
While 1588 is just starting to gain traction in the test world, other industries have been on-board for several years. The industry currently using the most 1588 clocks is power generation.
Because it found what it needed in Version 1 of 1588, released in 2002, GE was an early adopter. The company has since built in excess of 50,000 IO packs with 1588 clocks that work in its Mark VIe Control Platform, reported Mark Shepard, a consulting engineer with GE Energy’s Controls & Power Engineering Center of Excellence.
This platform is used widely in GE Energy products from wind turbines to large gas and steam turbines as well as on distributed control system applications. For instance, when a utility suddenly sees large loads being added or shed, such as when a big factory suddenly starts multiple large machines, it must adjust the routing of its power to that location, adding turbines within microseconds, if necessary. A high level of synchronization is a common requirement for motion control, robotics, and sequential CNC operations as well as timestamping events for debugging.
IEEE 1588 is used on GE’s IONet to maintain synchronization of the controllers and I/O packs in a system for both sampling and diagnostics. Using 1588, all input I/O packs in a system autonomously sample inputs at exactly the same time and simultaneously put their data packets on an Ethernet network where the switches take care of queuing everything up for transmission to the controllers. Then the control software makes its calculations, and the output packets are sent and applied simultaneously.
Frame synchronization is achieved with the 1588 protocol. Every device knows what time it is, what the precise frame period is, and the epoch for the start of Frame 0. From there, each device can calculate the start-of-frame time for every subsequent frame, and no other communications are needed to maintain complete system synchronization.
IONet is a closed network and not used as a general-purpose LAN. Interoperability with other 1588 devices is not supported by GE’s products. And although GE uses the protocol, it is careful not to claim full compliance with the standard. This is the model that other fieldbus manufacturers follow with schemes using 1588 including ProfiNet and CIPSync, a time-synchronization enhancement to the CIP factory network.
Before 1588, such Ethernet-based schemes relied on NTP, which is suitable for applications that don’t need the highest level of accuracy. But in motion-control applications, such as with multicolor printing presses moving paper at incredible speeds, resolutions in the microsecond range are necessary.
ProfiNet doesn’t use every part of 1588. Instead, it has its own profile, explained Franz-Josef Goetz, a communications system architect at Siemens. He added that 1588 is simpler to implement with higher performance, and it places fewer requirements on the PLLs used to maintain accuracy for time synchronization.
We might have to wait a while before there is a universal standard for PTP in the heavily regulated energy/power sector. However, a standard called IEC 61850 defines, among other things, Ethernet-based communications in that sector, noted Heiko Gerstung, director of technical sales and marketing at Meinberg. The company develops timing equipment for a variety of protocols.
The standard currently mentions only simple network time protocol (SNTP) as a network time-sync protocol, but people are working to add PTP to it. This work is not yet completed, but Mr. Gerstung believes we will see something like this as soon as standardization bodies integrate it and that steps will lead to a significant increase in the number of 1588-capable devices. And while Meinberg sells many NTP devices into this market, its 1588 business in power generation, transmission, and distribution has been limited to a few grandmaster devices used mainly in R&D and test labs.
When Mobile Phones Are in Motion
Many other industries are embracing 1588 with great enthusiasm. One of them is telecom which is the most active for new developments, according to John Eidson of the Agilent Technologies Measurement Research Lab and chairman of the 1588 standards committee. Representatives from the industry, he added, make up at least 30% of the 1588 committee that produced IEEE 1588-2008. And with the added features in that recent update, the doors are open for telecom and other new applications.
For the last few decades, telecom networks have used digital sampling to provide high-quality voice services over any distance. The regular sampling rate of voice services has been matched by the use of time-division multiplexing (TDM) in both switching and transmission equipment. Nonvoice services, such as data and video, also have been carried, quite often on dedicated links known as leased lines where the service is maintained to a very high degree of availability—and for a high fee.
Until just a few years ago, voice service was predominant, but nonvoice services now consume a greater capacity. Leased lines have been a lucrative offering, but guaranteed availability carries its own costs, and leased lines are difficult to adapt to load changes.
Carriers have been searching for a way to converge all types of traffic onto a single network for cost and flexibility reasons, and for this they’ve chosen packet networks. They don’t want to lose revenue from leased lines, and customers don’t want to replace their interfaces so techniques have been developed to carry the leased line services over packet networks.
There are other implications of moving to a packet network. TDM services are tied to a very stable reference clock. This clock makes possible the use of relatively small storage buffers in switches, allowing the voice service to have very low levels of latency.
A complete synchronization network then is implemented to distribute this stable reference around the carrier’s network. However, packet switches usually have considerable amounts of buffer storage so a packet network does not inherently need synchronization.
When a service is provided via a pure packet link, there is no connection to the carrier’s reference source. This means the customer has to get synchronization from somewhere else. This is where 1588 Version 2 comes into play, explained Dave Tonks, a principle engineer at Semtech, an analog and digital semiconductor company that has added the 1588-based timing over packet synchronization (ToPSync) products to address the needs of telecom companies.
Taking a cellular wireless network as an example, there are two key requirements:
•?Cell sites must reliably distribute their traffic to customers, whether on other cell sites or on a landline.
•?On-air frequencies used by the cell site must be stable and reliably held within a tight frequency margin to aid hand-off and avoid interference with neighboring channels.
The first requirement is satisfied by serving the cell site with a small number of T1/E1 leased lines. In many cases, the second requirement also can be met using the traceability of the leased lines.
Some cellular equipment, though, needs to tie the transmissions to timeslots. These cell sites use GPS receivers to provide this timing, although GPS is notoriously easy to jam. Leased lines are something of a straightjacket; they are expensive and very inflexible.
With the growth in wireless data traffic, cell sites need more leased lines, yet these take a long time to procure, and their high cost impacts the profitability of the wireless operator. A better way is to connect the cell sites using a packet network. This offers inherent flexibility and can be paid for on a packet-by-packet basis, matching cost to demand. However, the use of a packet network divorces the cell site from the carrier’s timing reference point and spoils the stability and accuracy of the air frequencies.
Telecom providers have been looking for a way to deliver synchronization within the data packets themselves. NTP doesn’t have the necessary accuracy. IEEE 1588 Version 2 now allows systems to carry timing information from one end of a packet network to the other end. In addition, it can provide the UTC traceability needed by some cell sites.
IEEE 1588 Version 2 meets other telecom needs since it allows for shorter message formats and higher update rates and defines the concept of a transparent clock, a means of compensating for the message delay through network elements. Further, a 1588 profile for telecom is being developed in ITU-T SG15, Q13 which is responsible for timing and synchronization in telecom networks, and this ITU-T recommendation may include other material that is not part of the profile.
Lips Sync With Audio
It’s nothing new that all sorts of digital media are streaming into our homes today, primarily into our computers or digital settop boxes. Consumer electronics manufacturers, however, have the dream that we will be able to route high-quality audio and video throughout our homes with a guaranteed quality of service using inexpensive Ethernet links, routers, and switches. Originally dubbed residential Ethernet, the concept now is referred to as audio/video bridging (AVB). These techniques also will be applied to professional audio and in-car AV systems.
Today, you could transport digital content over conventional Ethernet as long as the line is dedicated to that purpose. As soon as you start transporting another best-effort stream such as file transfer or Internet access, it likely will significantly degrade the quality of the AV stream. There must be some means to synchronize the packets so audio matches the lip movements of actors or the sound coming out of a pair of loudspeakers is in phase, especially in Wi-Fi speakers.
It’s also necessary with satellite receiver streaming that the clock rates in the receivers be identical because you can’t tell the satellite to change its speed. In addition, manufacturers want to develop a plug-and-play scheme that will ensure equipment from all vendors is compatible with an absolute minimum of customer adjustments.
In response to this requirement, consumer-electronics manufacturers are working with IEEE on the standardization of the required timing with IEEE 802.1AS. The group also is focused on parallel standards, 802.1Qat, 802.1Qav, and 802.1BA, to address other needs including streaming, queuing, and profiling.
IEEE 1588 allows profiles, that is, collections of 1588 options and defaults, to be defined for various applications, such as telecom and AVB. 802.1AS includes the 1588 profile defined for AVB. It takes much of what it needs from 1588 but not all of it. It then adds aspects not included in 1588 such as support for 802.11 wireless networking.
The 802.1AS jitter and wander specs are defined by existing requirements in the Audio Engineering Society (AES) AES3 digital audio standard for professional audio equipment and the European Broadcasting Union (EBU) and Sony/Philips digital interface format (S/PDIF). S/PDIF is a specification for carrying digital audio signals that is part of IEC 60958 and represents a minor modification of the original AES/EBU standard for consumer use, requiring less expensive hardware. These specifications for jitter and wander are quite tight, in the range of 10 ns for jitter and long-term frequency offsets of ±50 ppm for consumer interfaces and ±1 ppm for professional interfaces for wander.
Manufacturers indicate that the synchronization of multiple speakers in different locations should be less than ±500 ns for an acceptable consumer experience. None of these values are close to the accuracy afforded by 1588.
There will be AVB-capable devices available in the near future, starting with professional audio/video systems and working down toward consumer electronics over time, explained Geoff Garner, a consultant in AVB timing and synchronization for Samsung Electronics and the editor of the 802.1AS standard. He added that the standard is stable but still being worked on, and it is expected to go out for sponsor ballot at the end of this year.
All other industries are watching 802.1AS with great interest. If the consumer industry adopts 1588 in this way, it will make enormous investments to drive prices down.
IXXAT’s Mr. Seitz agrees that 802.1AS is an area of tremendous interest. In addition to home electronics, he mentioned possible uses in large systems such as concert stages and stadiums where unsynchronized loudspeakers can lead to barely intelligible speech or distorted music. In fact, his company is working on a project for a large baseball park that will run 1588 to synchronize the entire stadium, with a trial planned for spring of this year.
The Military Reevaluates IRIG-B
IEEE 1588 offers an excellent alternative to IRIG-B, which is used heavily in military test installations. Engineers are looking for ways to get rid of the dedicated coax or fiber-optic cables needed for timing in their sensor beds, especially projects such as large-scale weapons tests across large distances or airborne tests. Eliminating the weight of extra cables will be especially attractive in things that move like ships, subs, and planes.
Further, added Meinberg’s Mr. Gerstung, these test beds are a living infrastructure continually being changed and expanded. With 1588, engineers find it much simpler and less expensive than replacing all their dedicated connections.
And if you’re going to invest in infrastructure, he noted, why not add nanosecond accuracy? Besides, companies such as Meinberg sell 1588 synchronization units that can generate IRIG timing codes to allow the use of legacy equipment. This will be a huge market for 1588 because there are literally hundreds of millions of dollars of IRIG test gear that can easily be upgraded.
So far, we’ve mentioned some obvious candidates for the use of 1588, but there are many others you wouldn’t normally suspect. For instance, IXXAT has received inquiries from financial institutions that want to detect when a message has been received with greater than the millisecond resolution of NTP. A large trader might receive a thousand or more trading orders in a microsecond and must be able to resolve the order of reception and make trades on that basis. If the resolution of the messages with NTP is only 1 ms, all these messages have the identical timestamp. “These companies have told us that this capability can save them hundreds of millions of dollars per day,” said Mr. Seitz from IXXAT.
Clearly, 1588 has started something big. It will be fascinating to see which other industries find ways to use this exciting technology.
Acknowledgement
A special acknowledgement goes to John Eidson for providing considerable information and contacts that were invaluable in preparing this story.
Reference
Eidson, J. C., Measurement, Control, and Communication Using IEEE 1588, 2006.
Paul G. Schreier is a technical journalist and marketing consultant working in Zurich, Switzerland. He was the founding editor of Personal Engineering & Instrumentation News, served as chief editor of EDN Magazine, and has written articles for countless technical magazines. Currently, he is the editor for LXI ConneXion at EE-Evaluation Engineering. Mr. Schreier earned a B.S.E.E. and a B.A. in humanities from the University of Notre Dame and an M.S. in engineering management from Northeastern University. e-mail: [email protected]
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