Basics of real-time measurement, control, and communication using IEEE 1588: Part 6
Specific Concerns and Likely
Outcomes
In the author's opinion, IEEE 1588 is at a critical point in its
development. As noted, the IEEE has authorized a standards effort to
extend the existing standard. The responsible IEEE working group,
P1588, was in the early phase of discussions as this book was being
prepared.
The group that produced the original standard numbered no more than a dozen members. By contrast, the current P1588 committee regularly draws between 30 and 45 members to each meeting.
Furthermore, the current committee represents a far more diverse set of interests and potential application requirements than did the original group. This is an indication that IEEE 1588 is perceived as useful technology, but carries with it the potential for fragmentation of the standard.
IEEE 1588 is an infrastructure standard. End users must be able to obtain a variety of affordable IEEE 1588-enabled devices for their applications. All IEEE 1588 based systems will require not only end devices particular to the specific application, but also common components such as boundary clocks, clocks linked to a GPS system, bridges between network protocols, software tools for monitoring performance, and, in the future, transparent clocks and other devices yet to be conceived.
The P1588 committee must ensure that the revised standard not only provides the needed enhancements, but does this in a way that keeps the standard simple enough to enable manufacturers to make cost-effective common components useful for all.
None of the application areas is large enough by itself to create the usage numbers needed to drive down the cost of these common components to the point where IEEE 1588 is not only technically attractive, but also financially compelling.
This will require compromise within the P1588 group, which is always tricky to achieve with any group. Standards that are the union of everyone's design preferences are rarely successful, and certainly do not serve the end customers very well.
The outcome of the P1588 work is likely to be successful. The major enhancements under consideration offer considerable improvements for all application areas. This should provide sufficient incentive to find common solutions.
While predicting the outcome of any standards committee effort, much less the response of balloters, is an art, not a science, the early discussions indicate that the P1588 committee recognizes the need for compromise and is willing to devote the necessary time to achieve it.
There are also technology and economic concerns. Fundamentally, IEEE 1588 is a standard that accurately synchronizes clocks. The accuracy currently required by most applications can easily be met by IEEE 1588. However, accuracy requirements are likely to become more, rather than less stringent.
It is therefore imperative that efforts to achieve low or even sub-nanosecond accuracy succeed. At this level, there are all sorts of technology problems, such as the PHYs, asymmetry in network links, and oscillator stability and noise, that must be addressed.
The solutions will probably require silicon, rather than FPGA or software techniques. Again, cheap silicon requires volume, which is further reason to make sure the standard is no more complex than needed. Data published at the 2004 IEEE 1588 conference and more recent data provide reason for optimism that the needed accuracies can be achieved.
Concern has been voiced that the MII or GMII interfaces between the PHY and MAC layers in Ethernet technology will be integrated into silicon, and not be available for use by IEEE 1588. This is definitely an issue that will be resolved based on the demand that can be generated by the IEEE 1588 user community.
Improvements to IEEE 1588
In the short term, there are a number of improvements and extensions to
IEEE 1588 that will emerge from the P1588 group. These include:
1) The transparent clock. The standard will definitely have some form of transparent clock that will make it feasible to construct reasonably long linear IEEE 1588 topologies. Transparent clocks may also prove useful in tree topologies involving large numbers of end devices as a way to reduce the number of cascaded servos at the top layers of the hierarchy.
2) Short frames and short sync interval. The combination of these will prove useful in achieving higher accuracy, in providing advantageous cost optimization of oscillator quality, and in enabling several proposed telecommunications applications.
3) Layer 2 mapping. A layer 2 implementation of IEEE 1588 has been requested by almost all application areas. It holds the potential of enabling easier silicon-based solutions, and more efficient switch technology.
4) Gigabit implementations. Several of these have been reported to date, and more are sure to emerge. Implementations on fiber optic media will also appear, particularly if IEEE 1588 finds adoption in the power industry.
5) Wireless implementations. Although not on the current agenda for the P1588 group, it is quite likely that serious efforts will be made over the next few years to implement IEEE 1588 on one or more of the wireless protocols.
6) The emergence of silicon incorporating IEEE 1588. This will be driven by increasing adoption of IEEE 1588, and broadening of the experience base to guide the design of chips that incorporate appropriate support for applications using IEEE 1588. Intel was one of the first to announce such a chip, and more are likely to follow.
Regarding applications, the next few years should see significant numbers of products and installations in the industrial automation area. These will occur initially in motion control, but will subsequently spread to monitoring and general control situations as well. In test and measurement, the adoption in the data acquisition market is very likely. Adoption in high-end test depends on the success of the LXI effort.
In telecommunications, there will be continued field trials. If these prove successful, then the adoption depends on the willingness of the major equipment suppliers to provide the necessary devices, and the identification of service areas where there is a clear reason to switch technologies. The early results reviewed earlier are encouraging, but not yet definitive.
Final Thoughts
IEEE 1588 provides another tool to the designer of hard real-time
systems. It will be used in conjunction with the existing practices, as
appropriate.
What IEEE 1588 has introduced is the ability to actually specify and execute operations based on time to an accuracy that is appropriate for the kinds of hard real-time problems discussed here.
It has been commented that hard real-time programming is difficult in part because computer science has abstracted away the notion of time. Perhaps the introduction of IEEE 1588 will prove to be the needed incentive to address this issue. If so, the next few years should prove exciting.
To read Part 1, go to "The varieties of system temporal specifications."To read Part 2, go to "Overview of the 1588 clock synchronization standard."
To read Part 3, go to "Master-slave Synchronization Hierarchy"
To read Part 4, go to "Achieving submicrosecond synchronization accuracy"
To read Part 5, go to "Applying 1588 to wideband nets, wireless, cable and telecom."
Used with permission of its publisher, Springer Science and
Business Media, this series of articles is based on material from "Measurement,
Control and Communication Using IEEE 1588," by John C. Eidson and
can be purchased on line.
John C. Eidson, Ph.D., received a B.S. and an M.S. from Michigan State University and a Ph.D. from Stanford University, all in electrical engineering. He held a postdoctoral position at Stanford for two years, spent six years with the Central Research Laboratory of Varian Associates, and joined the Central Research Laboratories of Hewlett-Packard in 1972. When HP split in 1999, he transferred to the Central Research Laboratory of Agilent Technologies. Dr. Eidson was heavily involved in IEEE 1451.2 and IEEE 1451.1 and is the chairperson of the IEEE 1588 standards committee and a life fellow of the IEEE.
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