I’m using openPDC to capture synchrophsor measurements from a PMU and output those measurements. When I checked the outgoing data packets through Wireshark I noticed the fraction of seconds (raw) is not consistent with the received data. I have attached a photo of a Wireshark capture of the outgoing data and as you can see the resolution of the fraction of second (raw) is 167772. Whereas the resolution of the fraction of second (raw) of the incoming data is 100000. I wonder why this is? Also, I noticed there are two data packets for the same timestamp(No. 6453 and No. 6452) in the outgoing data stream as you can see in the outgoing capture. Could someone please help me understand why that is?
The openPDC does not simply forward input data frames along to its output streams. The openPDC needs to work with many different protocols and even mix and match data from different protocols, yet still produce outputs in whatever protocol the user specifies. The process involves decoding the original data frames to produce individual measurement values so they can be processed individually and then concentrated again into a whole new set of data frames to be sent as output. Therefore,
fraction of second of the output is going to be unrelated to
fraction of second of the input, and instead it is derived based on the
timebase field in the openPDC’s output stream configuration. To override
timebase, it must be added to the connection string, and the default is
timebase=16777215 (for IEEE C37.118).
To provide a little more detail,
fraction of second and
timebase work together to define a ratio that will be added to the
second of century to produce the timestamp. Therefore, the timestamp can be derived using the following equation:
timestamp = secondOfCentury + (fractionOfSecond / timebase)
As for the repeated timestamp, I can’t think of anything except maybe a bug in Wireshark. The fraction of second field is clearly different between those two frames, so they should have different timestamps.
Thank you for your reply and the detailed explanation Stephen.