Frequently Asked Questions
about the White Rabbit switch.
Connecting the switch to an external GPS receiver
Q: What are the advantages of using external GPS signals or are there any disadvantages of not using them and leave the connectors open?
A: You mainly get two things:
- A more stable oscillator for the master switch and therefore for all the network.
- A GPS/UTC/TAI-related time scale. This is important if you want to relate your events to official time or if you want to avoid re-starting your time scale at 1 Jan 1970 (or something like that) every time you reboot the switch. Two events seen in a WR network at very different times could have very similar time stamps if there was a switch reboot in between. Syncing to GPS time on reboot avoids that.
Q: When connecting the GPS-PPS or the 10 MHz GPS clock to the WhiteRabbit Switch, do I have to change/configure anything?
A: Yes, it is documented in the Note on using WR Switch in Grandmaster mode.
Q: The last paragraph of the Note on using WR Switch in Grandmaster mode mentions HAL messages and RT log. I find HAL messages but what is meant by RT log and where to find it?
A: By the RT (i.e. realtime) log mentioned in the Note on using WR Switch in Grandmaster mode we mean the messages generated by the LM32 CPU inside the Virtex FPGA. They may provide useful information for debugging the timing subsystem. They can be viewed by connecting a serial terminal to the the front left USB port (up to v. 3.2 switches) or the "FPGA test" USB port in the rear (newer switches). RS232 parameters are 115200, 8N1.
Q: Is it possible to use White Rabbit systems in different locations and be able to have synchronised time via Satellite link?
A: Basically as long as the 10 MHz and PPS that you give to the
different White Rabbit systems are synchronised, WR will keep its
promises.
For example, you may synchronise two WR networks by at both ends letting
a GPS generate the PPS and 10MHz signals, but you will be limited by the
precision the GPS will give you on those two different locations. This
may be as good as 3 ns already or maybe better in combination with
augmentation systems.
Or you can synchronise your systems with Time and Frequency Transfer
using the phase of the GPS
Carrier, as CERN has done
with the CNGS experiment where we could, after the fact, synchronise
related events happening on both sites. The article Time transfer
techniques for the synchronisation between CERN and
LNGS explains this.
If you have Satellite links, you can use Two-Way Satellite Time Transfer
(TWSTT) and have on both sides a
system that will provide the PPS and 10 MHz signals. According to the
presentation Time transfer through optical
fibers,
page 3 from Mrs. Amy-Klein from LPL, Paris, this can provide an accuracy
better than 1 ns.
Using Time Transfer by Laser Link (T2L2) would give you even a sub-ns
accuracy.
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.
Inter-operability
Q: Is the WR Switch inter-operable with standard PTP (IEEE1588-2008) gear ?
A: Yes, although you would completely loose the sub-ns timing resolution of White Rabbit, a WR switch is interoperable with PTP and it has been tested for inter-operability on ISCPS PlugFest, but
- currently, WR devices (Switch/nodes) do not support standard-PTP slave mode. This means that a standard PTP device can "slave" to WR device (being master) but a WR device cannot slave to standard PTP device (WR device cannot be standard PTP-slave)
- it is important to remember that WR devices use a very specific PTP
configuration (2-step, delay request-response mechanism, multicast,
Layer 2) which is not the most popular configuration and it is not
the default setting for standard-PTP devices (which use IP instead
of Layer 2). So it is not just plug&play, it's more like
1. buy PTP switch which supports fully the standard
2. configure it properly (change from IP to Layer 2)
3. plug & play
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?
A: Yes, you can connect non-WR switches in a WR network. But it does not make sense as in that case you completely loose the sub-ns timing resolution of White Rabbit.
If the switch between the WR master (switch) and the WR end node is a
standard PTP one, your timing resolution will be reduced to the
relatively low PTP precision. The reason is that the White Rabbit
protocol is achieved by synchronizing each link between one end and the
other end of the fibre and the non-WR switch in the middle would not
support the WR protocol.
To say it in another way: you may mix non-WR and WR switches to build
your complete network (it will perfectly transfer your data) but to have
WR precision all switches between your WR timing master switch and your
WR nodes should be WR-compliant ones.
Ah, when you speak about different vendor, I assume you meant a non-White Rabbit switch. But you may want to know that as the design of the WR switch is open, there may in due time be more producers of White Rabbit switches.
General questions
Q: Should green and orange LEDs be lit when copper SFP is used even without Ethernet cable plugged in ?
A: Yes, that is the normal behavior with most of the copper SFPs. Even after unplugging Ethernet cable the copper SFP fakes an active connection so green LED is ON and orange may be blinking.
Q: There are two fans. Is it correct that both of them push the air out of the box on backside?
A: Yes, it is correct that both fans push the air out of the box on backside. The idea is that the cold air is just sucked in at the SFP cages at the front. Tests have been made that shows that this is the best way of cooling that is also compatible to racks that have a front-to-back airflow. In the White Rabbit Switch Hardware project one can find the WRS - Stress Tests report.
??Author: Benoit Rat, Seven Solutions
Q: Does the switch comply to CE regulations (EMC)?
A: It is. The company Seven Solutions has performed extensive EMC tests on the switch.
Q: The output from the SMC connector labeled at 125MHz is tuned at 62.5Mhz frequency?
A: This is perfectly normal, the design inside the FPGA has been
modified to be compatible with the speed of transceivers.
62.5Mhz is the new reference clock of the HDL design, the 125MHz will be
labeled differently in the next batch.
??Author: Benoit Rat, Seven Solutions
Q: Could the WR switch be used as Open Compute Project open source switch?
A: The article Facebook aims to knock Cisco down a peg with open
network
hardware
shows the need for an open source switch.
The OCP open source switch project seems to be all about being able to
run any OS on the switch and run software-defined networking (SDN) (a
kind of remote management) on it.
It opens some questions:
- Would one be able to run "any OS" on our switch?
- Yes. We have an ARM processor inside the switch that runs Linux, and any OS with ARM support may be run on it.
- Is our API to the firmware documented?
- Yes, see Documentation of Gateware-Software interface, a 101-page document.
- Is the hardware well documented?
- Everything we have is available on the White Rabbit switch
page.
Although most information is available, unfortunately it is not yet enough documented. Some description is in the Gateware-Software document and the hardware documentation should be completed (schematics, PCB layout, production documentation and mechanics) and it should be checked if everything is up-to-date. This is on the list of things to do already.
- Everything we have is available on the White Rabbit switch
page.
Likely the WR switch is not good enough for a final OCP product as with its size of only 18-ports it is less than what computer centers are used to, like the 1 Gbps speed instead of the now common 10 Gbps. The early implementation of the High Accuracy Option/Profile of IEEE1588 is a nice plus showing innovation for which Facebook or others may find good use, for example in Spanner.
So the WR switch project possibly could be used as a prototype platform. It is completely open and already commercially available and may quickly be used to port OpenFlow and OpenDaylight on it to prove the principle.
??Author: Erik van der Bij, CERN
See also Frequently Asked Questions about White Rabbit
10 May 2013