This chapter evaluates and concludes the test results. It also summarizes the methodolo-gies and characteristics that are useful when measuring time synchronization accuracy.
The reasons that are causing the inaccuracies to time synchronization are also summa-rized in this chapter.
For measuring time synchronization the traditional way is the 1PPS method which is sim-ple to imsim-plement especially in laboratory environments. However, this is not convenient in large scale networks. Ingres, egress and reverse sync methods are however better op-tion for larger networks because they use in-band reporting method and the calculaop-tions are done by slave or master.
When measuring or monitoring the time synchronization accuracy, the most important characteristics that should be measured are Maximum Time Interval Error (MTIE), Time deviation (TDEV) and Mean Path Delay. MTIE clarifies the highest offset peak between synchronized devices. TDEV defines the stability between a measured phase to a refer-ence phase. Mean Path Delay defines the average time of the link between master and slave.
There are various factors that have and impact to the time synchronization accuracy. The common reasons that are causing the time errors in time synchronization process are listed below:
• Clock frequency
• Quality and stability of an oscillator
• Propagation delays at PHY
From these, the performed test cases verified the enviromental changes, stability of an oscillator, propagation delays and traffic. These are the factors that should be taken in consideration then developing an Ethernet device for synchronous network.
This will ease the costs and the effort of the development of a PTP capable industrial network device. With the right architectural choices, the development of such devices would be more cost efficient and the device itself becomes more durable.
Overall the DUT used in this thesis achieved its goals to maintain the time synchroniza-tion accuracy below 1 µs. These test gives the rough estimate how the device behaves in different situations. Summary of the test results are presented in Figure 5.1 and Fig-ure 5.2. The FigFig-ure 5.1 presents the average offsets and the FigFig-ure 5.2 presents the average mean path delays from every test case.
Figure 5.1. MTIE and Average Offset results
Figure 5.1 shows that there are no remarkable differencies between average offsets ex-ecpt with the high load traffic test. That is the one that is approximately 5nshigher than other test cases. The high load traffic case is also above others when comparing the MTIE results. Long data path increases also the time error noticeably.
Figure 5.2.Average Mean Path Delay results
Based on these results it is important to take the traffic controlling in consideration if the device will handle heavy traffic. This will cause some inaccuracies in the time synchro-nization. Also, when creating an industrial network it is important to take in consideration the length and topology of the data path. Long data paths will cause minor inaccuracies in time and it will also increase significantly the path delay as shown in Figure 5.2.
The overall average path delays were not impacted almost at all excluding the long dat-apath test. In conclusion, only longer data paths have an effect on the mean path delay based on these tests.
Another useful test case for measuring the time synchronization is testing with different oscillators. With different oscillators, the enviromental changes are playing a bigger role.
Especially if the oscillator is a low-end model and it has no thermal controlling, it might be affected easily by the temperature or humidity changes. In this case the DUT had a good quality oscillator which was not affected that much from the enviromental changes.
The test setup which included the Calnex Paragon device (Figure 3.7) is a useful setup for creating various test cases. Unfortunately, the test setup did have some functional diffuculties thus some important test cases were not feasible to execute. Test case such as creating asymmetric data path is a great test for accuracy testing because the PTP relies on a symmetric data path. Another helpful test case is a test where the resolution of the timestamp is decreased which will reduce the precision of the time calculations.
Third useful test case with the Calnex testing device is a test where jitter, wander and broken packets are generated to the test network for creating disturbance to the time synchronization calculations.
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