... | ... | @@ -38,25 +38,25 @@ USB port (up to v. 3.2 switches) or the "FPGA test" USB port in the rear |
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### Q: Is it possible to use White Rabbit systems in different locations and be able to have synchronised time via Satellite link?
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Basically as long as the 10 MHz and PPS that you give to the different
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White Rabbit systems are synchronised, WR will keep its promises.
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White Rabbit systems are synchronised, WR will keep its promises.
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For example, you may synchronise two WR networks by at both ends letting
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a GPS generate the PPS and 10MHz signals, but you will be limited by the
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precision the GPS will give you on those two different locations. This
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may be as good as 3 ns already or maybe better in combination with
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augmentation systems.
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Or you can synchronise your systems much later, as CERN has done with
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the CNGS experiment where we could, *after the fact*, synchronise
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related events happening on both sites (whe sent neutrinos through the
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earth's crust). The article [Time transfer techniques for the
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synchronisation between CERN and
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Or you can synchronise your systems with [Time and Frequency Transfer
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using the phase of the GPS
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Carrier](http://tf.nist.gov/time/carrierphase.htm), as CERN has done
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with the CNGS experiment where we could, *after the fact*, synchronise
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related events happening on both sites. The article [Time transfer
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techniques for the synchronisation between CERN and
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LNGS](https://www.ohwr.org/documents/112) explains this.
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If you have Satellite links, you can use Two-Way Satellite Time Transfer
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(TWSTT) and have on both sides a system that will provide the PPS and 10
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MHz signals. According to the presentation [Time transfer through
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optical
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([TWSTT](http://tf.nist.gov/time/twoway.htm)) and have on both sides a
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system that will provide the PPS and 10 MHz signals. According to the
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presentation [Time transfer through optical
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fibers](http://www.ptb.de/emrp/1393.html?&no_cache=1&cid=2274&did=2820&sechash=9d1ec729),
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page 3 from Mrs. Amy-Klein from LPL, Paris, you can have an accuracy
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page 3 from Mrs. Amy-Klein from LPL, Paris, this can provide an accuracy
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better than 1 ns.
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Using Time Transfer by Laser Link (T2L2) would give you even a sub-ns
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accuracy.
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... | ... | @@ -90,29 +90,28 @@ tested for inter-operability on ISCPS PlugFest, but |
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3\. plug &
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play
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### Q: Can you have a different vendor switch between it and another white rabbit device to distribute the time? Around a larger network for instance?
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A: No, you cannot do this and still expect to have the sub-ns timing
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resolution of White Rabbit. If the switch between the WR master (switch)
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and the WR end node is a standard PTP one, your timing resolution will
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be reduced to the relatively low PTP precision. The reason is that the
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White Rabbit protocol works on each link and there is no end-to-end
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synchronisation.
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If you want to build a larger distance network, you may want to know
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that there are several meteorological institutes working on extending
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the 10 km range. One may replace the standard SFP's by longer distance
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ones (see [Non-compliant SFP types](/Non-compliantSFP)), and there may
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be ways of multiplexing specific wavelengths on a dark channel in a
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Telecom network. Also light amplifiers may be used. See for example the
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data of *Vrije Universiteit Amsterdam and VSL* at [Users of WR
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technology](/WRUsers). At the moment of writing (April 2013) no links
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longer than 16 km have been made though.
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The [NEAT-FT Workshop on Optical Networks for Accurate Time and
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Frequency Transfer held in 2012](http://www.ptb.de/emrp/1393.html) may
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give you more ideas and you possibly can find partner institutes that
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can help you with such a
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system.
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### Q: Can you have a different vendor (non-WR) switch between it and another white rabbit device to distribute the time? Around a larger network for instance?
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A: Yes, you can connect non-WR switches in a WR network. But it does not
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make sense as in that case you completely loose the sub-ns timing
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resolution of White Rabbit.
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If the switch between the WR master (switch) and the WR end node is a
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standard PTP one, your timing resolution will be reduced to the
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relatively low PTP precision. The reason is that the White Rabbit
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protocol is achieved by synchronizing each link between one end and the
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other end of the fibre and the non-WR switch in the middle would not
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support the WR protocol.
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To say it in another way: you may mix non-WR and WR switches to build
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your complete network (it will perfectly transfer your data) but to have
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WR precision all switches between your WR timing master switch and your
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WR nodes should be WR-compliant ones.
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Ah, when you speak about different vendor, I assume you meant a
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non-White Rabbit switch. But you may want to know that as the design of
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the WR switch is open, there may in due time be more [producers of White
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Rabbit
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switches](/Switch#commercial-producer).
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-----
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... | ... | @@ -142,7 +141,11 @@ on the switch. |
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-----
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4 April 2013
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See also [Frequently Asked Questions about White Rabbit](/FAQwr)
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-----
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5 April 2013
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