White Rabbit at MIKES
The Centre for metrology and accreditation in Finland (MIKES, http://www.mikes.fi ) is evaluating White Rabbit for time & frequency transfer.
Grand Master
GM hardware is SPEC + DIO.
Input 10 MHz and 1 PPS signals to grand master.
attachment:mikes_gm_oscilloscope_signals.jpg
Note that the 1 PPS signal serves only to mark which rising-edge of the
10 MHz waveform is considered the inter-second boundary. We use a
Stanford Research DG645 delay generator to delay our UTC PPS pulse
appropriately for the GrandMaster. Our 10 MHz input for the GM is
derived from the UTC PPS using a phase locked loop.
Setting time
After a "mode gm" command to lock the PLL to the external 10 MHz signal the PTP time is reset starting at 1970. If the system time is kept roughly accurate using NTP we can use a python script that issues a "time setsec XX" command with the integer UTC seconds.
Stability measurement
An experiment to measure the stability of time-transfer over short (<10 km) distances.
- GrandMaster locked to UTC (MIKE)
- 500m fiber to WR-switch, 500m fiber back to WR-Slave
- Slave 1PPS output measured compared to GrandMaster input timebase UTC (MIKE).
- Measurement with Agilent 53230A (20ps) time-interval-counter.
Data was collected over ca 30 days. The result is similar to that of
Lipinski2012, showing no drift and a 1/tau dependence.
Long distance time transfer
MIKES has set up a White Rabbit link between two computers connected
with a 1000 (one thousand!) kilometer link.
This work is a collaboration between MIKES and CSC (http://www.csc.fi) -
IT Center for Science Ltd that operates Funet (the Finnish National
Research and Education Network).
The White Rabbit console reports a mean round-trip-time (RTT) delay of 10.4 ms with a daily variations of ca +/- 40 ns. 10.4 ms corresponds to a ca 2000 km round trip, assuming a refractive index of 1.5. The WR protocol is compensating for the daily variations, but this could not yet be verified as both points are physically spaced at a 567 km driving distance.
This is a first step to evaluate WR for comparison and synchronization of geographically distant atomic clocks in the White Rabbit time-transfer experiment between Espoo and Kajaani in Finland (pdf). The next step is to verify the accuracy of the transferred white-rabbit timebase by moving a Cs-clock and a GPS receiver to Kajaani in the near future.
This experiment shows that the White Rabbit hardware and protocols are much more flexible than the original design goal for use at maximum distance of ten kilometer. Also the fact that the designs are "Open", made that MIKES could easily replace the original SFP lasers by longer reaching types.
Typical RTT variations over a period of ca 48 hours. Here an older
firmware version was used and the data shows a few 8ns jumps when the
RTT crosses a critical value.
We are now (December 2013) using wr-nic-v1.0-243 firmware which so far
has proved
stable.
This graph shows RTT variations over a period of one week. The average
RTT is 10.4 milliseconds with temperature changes causing up to 50 ns
drift. Any small interruptions or instabilities in the RTT are because
problems with the clock used as input to the GrandMaster node and not
related to White Rabbit.
Hardware
- GrandMaster (Espoo): SPEC + DIO
- SFP 1000BASE-BX TX1490nm/RX1310nm (came with starter-kit)
- Switch: WRS
- WR-switch - GM link: SFP 1000BASE-BX TX1310nm/RX1490nm (3rd party)
- WR-switch - slave2 link: SFP 1000BASE-BX TX1490nm/RX1310nm (3rd party)
- WR-Switch - slave1(Kajaani) link: SFP 1000BASE-DWDM 196.000THz (Eoptolink EOLS-1612-2860D)
- Slave1 (Kajaani): SPEC + DIO
- SFP 1000BASE-DWDM 196.000THz (Eoptolink EOLS-1612-2860D)
- Slave2 (Espoo): SPEC + FDELAY
- SFP 1000BASE-BX TX1310nm/RX1490nm (starter-kit SFP)
- DWDM system (Espoo-Kajaani): Coriant hiT7300, "2,5Gbps alienwave" 196.000THz
NTP shared memory refclock driver
By reading the WR time from shared memory (see specmem.c in spec-sw) and
writing the time to a specific place&format in shared memory (where NTP
looks) it is possible to use WR as a refclock for NTP.
This allows placing NTP servers anywhere in your WR network that
distribute WR time.
For the experimental code see
https://github.com/aewallin/ptp2ntpd
Preliminary observation suggests that running the ptp2ntpd refclock
driver keeps the system clock offset below 0.05 ms from WR time. Note
that this data was collected with "minpoll 4 maxpoll 4" settings in
ntp.conf - smoother/better performance may be achieved by tuning these
parameters. The constant negative offset may also be tuned out with an
additional "time1" parameter in ntp.conf.
Documents
- Slides, 2013 August, White Rabbit time-transfer experiment between Espoo and Kajaani in Finland (pdf).
- more coming soon...
Contacts
- Mikko Merimaa, MIKES
- Anders Wallin, MIKES
- Jani Myyry, CSC
Status
Date | Event |
01-04-2013 | Start of project. |
20-06-2013 | Stability test for short-distance link |
12-08-2013 | 1000 km (thousand kilometer) link up and running! |
20-08-2013 | experimental NTP shared memory refclock driver |
01-12-2013 | thanks to Gregorz for fixing bugs and producing wr-nic-v1.0-243 which seems stable |
31-12-2013 | updated RTT figure with one week of jump-free data. |
9 September 2013