Basics of real-time measurement, control, and communication using IEEE 1588: Part 6

John C. Eidson, Agilent Technologies

February 12, 2008

John C. Eidson, Agilent Technologies

IEEE 1588 Timing Redundancy
Many applications in both industrial automation and telecommunications have redundancy requirements. Redundancy requirements are found in circumstances where loss of life, unacceptable physical damage, or loss of revenue can result due to system failure. The measures taken by General Electric in the design of their turbine controllers are a classic example of the use of redundancy to mitigate these risks.

In current telecommunications systems, carriers provide alternate paths to ensure connectivity in the face of network outages resulting from broken links or equipment. Similar measures are implemented to protect current telecommunications timing and synchronization services.

If IEEE 1588 is to be used to provide telecommunications timing, then means must be devised to satisfy these redundancy requirements, in principle by using the same techniques currently used in telecommunications.

For example, redundant timing in the wireless applications discussed earlier in this series would be provided by redundant IEEE 1588 time servers, as shown in Figure 9.6 below.

Figure 9.6. IEEE 1588 Redundancy in Telecommunications (courtesy of Zarlink Semiconductor)

Here, primary and secondary time servers would provide timing to separate IEEE 1588 grandmaster clocks. Each IEEE 1588 slave clock would select the appropriate IEEE 1588 grandmaster based on a set of algorithms, which have yet to be defined.

The complete absence of a grandmaster, for example, due to a network failure between the grandmaster and the slave, would no doubt invoke the same holdover techniques currently used in telecommunications.

The difficulty is that in the current version of the IEEE 1588 standard at the time of this writing, there is no provision for implementing this form of redundancy.

The current standard allows selection of the grandmaster based only on the best master clock algorithm. Since the primary and secondary references in a telecommunications system will be categorized by IEEE 1588 as stratum 1 clocks,2 the best master clock algorithm will segment the IEEE 1588 topology into two disjointed regions, which is not what is needed in this application.

The IEEE 1588 standards committee has an agenda item to provide mechanisms for implementing the redundancy requirements for both the industrial and telecommunications applications.

Measurements of IEEE 1588 on Metropolitan Networks
Since the spring of 2005, four companies have collaborated on a field trial to test the viability of IEEE 1588 in distributing time over a metropolitan network. The initial results of these measurements were reported to the ITU by DaveTonks of Sematech. The topology of the trial is illustrated in Figure 9.7 below. The metropolitan network was selected by the carrier as being typical in terms of equipment, operation, and network traffic.

Figure 9.7. Topology of the IEEE 1588 telecommunications field trial

An IEEE 1588 master clock, using an Agilent 5071A cesium clock as a reference, injected IEEE 1588 packets into the network. These packets were carried on a virtual private network (VPN) to a remote office on the network where they were returned on a different VPN.

The path consists of six switch buffers in the Cisco 6509 equipment forming the network, plus two additional buffers in switches local to the experiment. Other than the VPNs, no special handling of the IEEE 1588 packets was used.

The returned packets were received by a slave clock that recovered the IEEE 1588 timing information and generated a T1 signal as an input to the Agilent OmniBER 718 network analyzer. Both the master and slave clocks are Semtech prototype devices. Note that the 718 is measuring the noise of the entire IEEE 1588 system, including the master and slave clocks, as well as the noise introduced by the metropolitan network.

Figure 9.8. MTIE field trial data for 5 June 2005 (courtesy of Semtech)

The 718 collected TIE data at 50 samples per second. The data are transferred via a local control computer to a data store in Agilent's laboratory in Palo Alto, California. TIE data are collected for approximately 24 h before a new measurement sequence is initiated. Additional data collected from the slave clock allow related information, such as the actual round-trip delay as a function of time, to be archived. All collected data could be retrieved from the store by the trial participants.

The test began on 21 April 2005, and has been collecting TIE data more or less continuously ever since. The data were still being collected and analyzed as this book was being prepared. The MTIE and TDEV measurements for 5 June 2005 are shown in Figures 9.8 above and Figure 9.9 below.

Figure 9.9. TDEV field trial data for 5 June 2005 (courtesy of Semtech)

In each figure, the mask shown is the plesiochronous digital hierarchy (PDH) mask based on ITU recommendations. The observed values of both MTIE and TDEV data fall well below the masks. A comparison of the TDEV and MTIE data collected as of 5 June 2005 does not show significant day-to-day variation.

Figure 9.10 below, which is based on data provided by Dave Roe of Semtech, shows the maximum MTIE values for each 24-h measurement period between 21 April and 5 June 2005.

Figure 9.10. MTIE field trial summary data: 21 April to 5 June 2005 (courtesy of Semtech)

These MTIE and TDEV data suggest that, under certain conditions, IEEE 1588 is able to transfer frequency with the necessary phase stability over metropolitan networks. Clearly, additional investigation is needed before this technique can be adopted.

Although one would expect that the 6-week trial period evaluated here encompassed typical network loading conditions, additional testing for performance with loads designed to stress the use of IEEE 1588 in this application must be done. Likewise, testing on additional metropolitan networks must be done to see if either carrier practices or differences in network equipment are important.

Since some protocols require actual time transfer, rather than only frequency, further experiments will be required to determine the effects of asymmetry in telecommunication networks.Dave Tonks of Sematech presented the results of preliminary time transfer experiments using the trial network. These results indicate that time transfer is possible in metropolitan networks to better than 100 ns.