I could use some advice about verifying the performance of a home-built stratum-1 NTP server.
The server is based on an HP55300A reference clock. The PPS signal and time-of-day serial port are connected to an Arduino Mega with an Ethernet shield and some miscellaneous interface circuitry. The Mega tracks time from the PPS signal generally within +-500ns by using internal timers, and a software-based PLL controlling a software-based fractional-N divider. There are hardware timestamps on incoming NTP requests. Transmit timestamps are not hardware generated but as best I can figure, are accurate to 10us worst case and probably just a couple of us in reality.
At least I ***think*** that's how accurate this thing is. In an attempt at independent verification, I've been running ntpd 4.8.2 on a Win7 PC. It is configured to use the Arduino NTP server (which is on the same network hub as the PC). Also configured are three internet stratum-1 servers over a DSL connection (one is ACTS, the other two are GPS ref clocks). Polling between 16 and 32 seconds on each. The PC's loop stats look fine -- it tracks within 500us typically and jitter is around 300us or so.
Offset data in peerstats shows the two stratum-1 GPS servers have a -4ms time offset relative to the Arduino server. Pretty much exactly the same offset in fact. Peerstats shows a very stable delay to these two servers -- 30ms on one and 65ms for the other. This data was taken over a 12-hour period.
So, here's my question: If the Arduino NTP server is accurate to better than 100us, is it believable that these two internet servers would show almost the same exact -4ms offset? Is this something to do with DSL? Or should I be digging into the Arduino server looking for bugs? Does anyone have suggestions for other experiments?
I've left out lots of details to keep the message shorter. I'll be glad to provide more info.
Thanks in advance if anyone can help out. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
