Measuring Propagation Delays and Assessing Performance of Power Utility Communication Networks

Author: Fred Steinhauser, OMICRON electronics, Austria

Applications in WAN scenarios

Applications in WAN scenarios

The same measurement principles as applied above work with wide area networks as well. But in such scenarios, the measurement of the propagation delay between the different ends is a particular challenge. Time synchronized test equipment has to be used. The equipment available until recently was special gear for telecommunications engineers, which is normally not at hand in electrical utilities.

Therefore, the assessment of such communication links has often been performed by pinging a remote device. But this allows only a very raw and incomplete assessment. The ping utility measures a round-trip time, which is the total of several times. The dominant factors will be the propagation times in both directions (tAB, tBA) and the response delay of the responding device (trd), which is usually not specified at all. Thus, the total round trip time (trt) measured in site A when pinging a remote device in site B can be written as:

                                      (3)

The statistics provided refer to the round trip times (trt,A), the influence of the individual summands cannot be taken apart. It is not possible to conclude if a jitter comes mainly from the response delay or from the propagation delay in the network. Essential parameters like channel asymmetry, which is crucial for the applicability of time synchronization, cannot be assessed this way.

Also, the ping utility is typically executed on a PC and this limits the accuracy of the measurements because the timing uncertainty on PCs can easily be in the range of several milliseconds.

Figure 7 shows (very much simplified) a setup for such a propagation delay measurement in a WAN scenario. The controlling PC and several details about tapping the traffic (e.g. mirror ports) are not depicted in detail.

The measurement devices on both ends are precisely time synchronized with an error not larger than 1µs. This is indicated by the GPS receivers, but time synchronization can as well be established if precision time protocol is provided in the local networks. To control the whole measurement system from one site, an IP route through the WAN is required to control the measurement device located at the remote site.

The packets to be captured for the propagation delay measurement must not be altered when passing through the network, so they can be recognized on both ends. The measurements can be performed with operational traffic (e.g. GOOSE messages) or with traffic injected especially for the measurements, like ICMP packets for pinging, but now the request and response packets are measured individually on their one-way trips and not just the round-trip as a whole. This delivers the propagation delay values individually for each direction and allows the evaluation of the channel asymmetry as well.

Conclusion

The performance of a power utility communication network is not really challenged when only client/server and GOOSE traffic is present. But when Sampled Values come into play, it is worthwhile to look into this in more detail, especially when other traffic may interfere with the Sampled Values. The explained interferences can lead to considerable jitter. When such interferences occur repeatedly in a network, the jitter may become so big that following Sampled Values packets may catch up to their predecessors.

The effects of interference of Ethernet packets in communication networks can be well understood. By careful examination of the network, expected values for the packet delays can be derived. These expected values are confirmed by precise measurements. This leads to the converse argument that malfunctions of network components can be revealed when measurements are performed and the results deviate from the expectations.

The assessment of the performance of wide area networks is crucial for protection applications that require timely delivery of their mission critical data between the different sites. A distributed, time synchronized measurement system delivers the performance data for each link and individual direction.

With today's modern equipment, such measurements are feasible not only for genuine communication experts, but also for power utility engineers who have to deal with communication networks that are mission critical components of their protection, automation, and control systems.

Relion advanced protection & control.
BeijingSifang June 2016