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Vladimír Smotlacha, CESNET [email protected] Accurate Time Transfer over Optical Network 6 th CEF...
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Transcript of Vladimír Smotlacha, CESNET [email protected] Accurate Time Transfer over Optical Network 6 th CEF...
Vladimír Smotlacha, [email protected]
Accurate Time Transfer over Optical Network
6th CEF Networks Workshop
Prague
13 September 2010
Time and frequency standards
• pendulum – frequency standard until 1930s
• quartz crystal
• Cesium (Rubidium) clock – elements of IA periodical table group (single electron in level s)
– hyperfine levels transition emits microwave frequency photon
• Hydrogen maser
• near future: quantum logical clock– transition (in optical frequency) of isolated ion kept in
electromagnetic field trap (Al, Hg, Sr, ...)
Time transfer
• 1 ns represents 30 cm light path in vacuum, ~20 cm in cable or fiber
• cable, optical link– short distance, negligible environmental influence (e.g. thermal
dilatation)
• radio broadcast systems• satellite navigation systems (GPS, ...)
– time broadcast – tens of nanoseconds noise– timestamping of local clock – “common-view”, “all-in-view”,
similar signal propagation in geographically close localities, accuracy below 1 ns (e.g. GTR50)
• two-way satellite transfer (TWSTFT)– assumes equal propagation delay in both directions
• two-way optical link time transfer
Time transfer over fibre
Goal
• Design device and method for accurate time transfer – alternative to satellite based methods
• Comparable or better accuracy and stability on range ~1000 km
• Use existing DWDM all-optical networks
Adapters
Features
• Two-way transfer
• Optical signal modulation for 1-pps encoding
• Uses SFP transceivers
• Based of FPGA Virtex-5
• Requires two time interval counters
Time transfer principle
xA = TrA – TA xB = TrB – TB
εA = TsA – TA εB = TsB – TB
δAB = (xA – εB – ΘAB ) δBA = (xB – εA + ΘAB ) assume δAB = δBA
ΘAB = ((xA – xB ) + (εA – εB )) / 2
Adapter prototype
Cesnet DWDM network
Experiments
We made 3 experiments:• Optical loop measurement
• Time transfer Cesnet – BEV (Prague – Vienna)
• Comparison with GPS time transfer
Participants:• IPE (Institute of Photonics and Electronics), Czech national time and
frequency laboratory, Prague
• BEV (Bundesamt für Eich - und Vermessungswesen), Austrian national time and frequency laboratory, Vienna
• ACOnet, Austrian NREN, Vienna
• West Bohemian University, Plzen
Optical loop experiment
• Both endpoints in one laboratory, common clock
• Bidirectional optical loop length 744 km
• DWDM production network
• 4 segments, 3 fiber providers
• 12 optical amplifiers
• Various optical elements (Cisco, CzechLight)
• Segment Praha – Hradec Kralove on top of electricity distribution poles
Optical loop - geography
Praha - Brno 284 km Brno - Olomouc 113 km
Hradec Králové - Olomouc 197 kmPraha - Hradec Králové 150 km
------------
744 km
Optical loop - results
One-way delay in both directions• fluctuation ~130 ns (temperature changes about 12 °C)• aerial fiber on top electricity distribution poles • residual asymmetry < 2 ns (resp. TDEV 8.7 ps / 500 s)
Optical loop – results II
Prague – Vienna experiment
Time transfer between Cesnet and BEV
• Site A: Rb clock in Cesnet, Prague, GPS disciplined
• Site B: Rb clock, BEV (resp. Vienna university), free
running
• 506 km, DWDM in production network, (Prague – Brno –
Vienna)
Prague – Vienna results
Step in one-way delay (2.3.2010 10:13 UTC)– direction to Prague +72 ns (cca +14 m)– direction to Vienna +16 ns (cca +3 m)
Free running Rb clock: relative frequency offset 8.08 *10-12
Optical x GPS time transfer
Comparison with GPS time transfer
• Site A: free running Rb clock in Plzen (West Bohemian
University)
• Site B: Cs clock in IPE – UTC(TP)
• 150 km of fibre, WDM, production network
• GTR50 installed in both sites
• 95 km geographical distance
Optical x GPS transfer - results
• 10 days measurement• comparison with GTR50 – exact calibration system based on GPS • difference in range ± 2 ns• short time TDEV (time stability) 120 ps
Optical x GPS transfer – results II
• Optical transfer is more stable for averaging time up to 100 s then “common-view“ method
• Equal stability for longer averaging time interval (influenced by Rb clock rather then the measurement method)
Conclusions
• Adapters prototypes successfully tested and method verified
• No influence to other DWDM traffic
• Fiber length dilatation cancels in two-way transfer (residual asymmetry less than 1 ns at 744 km fiber in 6 day test)
• TDEV 8.7 ps / 500 s at 744 km fiber
• Better TDEV then “common-view” method (averaging interval up to 100 s, distance about 100 km)
Future work
• Build all-optical path between BEV and IPE
• Convert experimental method to service – timescale
comparison
• TIC integration into FPGA structure
• Design adapters suitable for time distribution – no data
processing at server side
Thank you