Frequently Asked Questions
about White Rabbit.
White Rabbit general questions
Q: What's the easiest way to start playing with White Rabbit?
For starting to use WR technology a WR starting kit is available. The manual you can find there shows clearly what is possible with just two PCIe plug-in cards. Later on you can expand your kit with a White Rabbit switch.
Q: How can I test White Rabbit switch equipment? Or can I buy some test equipments from your company?
A: CERN is not a company, but a publicly funded intergovernemental
organization. We develop the technology but don't produce the equipment,
we buy WR switches from commercial vendors. Anyone can produce a WR
switch as all the sources are on our
pages (just have a careful
look at the wiki, sub-projects, ets... )
For example, there is wiki page about the WR switch on which
you can find the companies that are involved in switch development and
the companies from which you can buy a WR switch (Commercial producers,
at the bottom). On the wiki page about SPEC
card (an example of WR Node)
you will find companies producing SPEC
cards.
Q: If I don’t participate in WR workshop meetings, is the presentation material available?
A: All we do is public. All information from the workshops is directly accessible.
Q: How can I trust White Rabbit?
Most White Rabbit equipment is Open Hardware and Open Software. So if that's your thing, you have full access to all design data and can inspect and recompile the source code yourself.
White Rabbit use cases
Q: I have many GPS receivers on our company site. Any advantage to use White Rabbit here?
The cost of the GPS receiver, although even those cost over 10 K (well above the price of a simple WR installation), may not be your problem. It's the antenna installation and big cable for it that make it all a hassle to manage GPS installations. Everyone can nowadays install fibres and likely they are already installed in buildings, so within a day you may add WR timing capabilities to other buildings or offices. GPS receivers give you only the time at one single point.
Likely you can also manage the whole thing better, including backup GPS receivers on completely other places in the network as WR can do automatic switchover between timing sources. Redundancy is taken into account in the design, much in the same way as implemented by the Ethernet standards that it follows. Also for data transfer we make sure that high priority packets 'never' get lost.
White Rabbit technical questions
Q: Can I get links longer than 10 km?
If you want to build a larger distance network, you may want to know
that there are several meteorological institutes working on extending
the 10 km range. One may replace the standard SFP's by longer distance
ones (see Non-compliant SFP types), and there may
be ways of multiplexing specific wavelengths on a dark channel in a
Telecom network. Also light amplifiers may be used. See for example the
data of Vrije Universiteit Amsterdam and VSL at Users of WR
technology. At the moment of writing (April 2013) no links
longer than 16 km have been made though. The Finnish Metrology
Institute MIKES has set up a White Rabbit link between two PCs that are
connected with a 1000 (one thousand!) kilometer link
(news)
The NEAT-FT Workshop on Optical Networks for Accurate Time and Frequency Transfer held in 2012 may give you more ideas and you possibly can find partner institutes that can help you with such a system.
Q: Where can I put additional delays coming e.g. from optical amplifiers to be able to compensate them?
For compensating additional (constant) assymetry you can modify deltaTx and deltaRx parameters in the SFP database stored in EEPROM (for WR PTP Core) or in a config file (for Wr Switch). Please take a look on our calibration procedure draft (https://www.ohwr.org/project/white-rabbit/wikis/Documents/White-Rabbit-calibration-procedure) and the WR PTP Core User's Manual (https://www.ohwr.org/documents/192). Basically what you have to do is to use "sfp add" command to specify Tx and Rx delays for particular SFP transceiver. In fact our delay model says that those Tx/Rx delays are the sum of constant delays of transmission/reception path so you can add your correction value there.
Those values are loaded by the PTP daemon only at the initialization time, so if one wants to update them, restarting the daemon is required.
Q: What limits the timing performance for the long-distance WR links?
Basically, the timing performance of a WR network can be limited by any link asymmetry not taken into account in the White Rabbit link delay model:
- If, instead of WDM in a single fiber, two separate fibers are used for the transmission in both directions the latency of both links has to be measured and compensated (asymmetry caused by not equal fibers' length). Moreover, if those two fibers are not part of the same cable, their operation temperature will differ causing additional link asymmetry not foreseen in the WR link delay model.
- If optical amplifiers are used, their delays have to be measured and taken into account. Asymmetry caused by different operation temperature of each amplifier are not taken into account in WR link delay model.
- Constant delays (e.g. delays from optical amplifiers) can be taken into account during the calibration and summed together with Tx and Rx hardware delays (check White Rabbit Calibration).
- Large values of delays or asymmetry are not a problem. The PTP software uses 64-bit variables as well as 64-bit mathematical operations.
See also:
- Frequently Asked Questions about the White Rabbit Switch
- Frequently Asked Questions about the White Rabbit PTP Core
25 July 2013