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.
Switch management
Q: Can the WR Switch get its IP address on the Management Ethernet port from DHCP server ?
A: Yes, the WR Switch has a DHCP client software. Please remember though, if you modify the startup scripts to assign a static IP address, the configuration received from the DHCP server will be overwritten.
??Author: Grzegorz Daniluk, CERN
Q: Is it possible to forward received UDP packets to the switch's Management (copper) Ethernet port?
A: It could be done, but is not supported. It is a dangerous idea which is strongly discouraged as it can have very bad consequences including switch crashing. Therefore, this is not supported and no information on how to do it will be provided by the switch developers.
??Authors: Alessandro Rubini, Gnudd, Maciej Lipinski, CERN
Q: Why on older switches (up to Feb 2013) it was possible to access (e.g.: ssh) WR switch through any port (wr0-wr17 and management) and in the newer software releases (from July 2013, v3.3) it is not ?
A: Accessing the WR switch for management (e.g. ssh) through a "normal"
port (e.g. wr17) instead of through the copper Ethernet management port
is one of the dangerous features that you won't find on "industrial
switches" - as it would open a serious security hole. This is why it
shall not be supported out-of-the-box on WR switches and it is strongly
advised by WR developers to avoid doing it.
Here are some dangers of doing so:
- What it effectively does is enable any user in the network to break the network completely by ssh-ing to the switch and doing whatever. This is why only the management port should be used that should be connected to a separate network which is limited-access only by the administrator (we call it technical network at CERN). This is especially true for "normal networks" with public access, less in control&acquisition systems
- The management port will work regardless of FPGA binary and even operating system/daemons on the switch. This means that if you upgrade the FPGA binary or software on the WR switch and something goes wrong for some reason, chances are that you can still access the switch using the management port but you will surely have no access through wrPorts - if you limit your access to the switch only to wrPort (e.g. wr17) in a remote site... there are more chances you will need to go there physically to fix something.
- Using a wrPort (e.g. wr17) might degrade performance of frame forwarding as well as the daemons (e.g. WRPTP timing) are running on the CPU - remember that CPU is connected with all the wrPorts through a single "virtual" port which is multiplexed. Your ssh session (as well as scp and whatever you use over wr17) is shared the with the daemons that make sure your network works properly. If your management activities overwhelm the wrPort (and CPU), you might end up managing a misbehaving switch.
To sum up, using a non-management port (e.g. wr17) for ssh and management is possible on a WR switch but needs special configuration. It is something you might do for some reasons in the lab or while testing systems, but it should definitely be avoided in a deployed network.
??Authors: Maciej Lipinski, CERN
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. The full report can be found in White Rabbit Switch Hardware documentation.
Q: The output from the SMC connector labeled at 125MHz (OUT) is tuned at 62.5Mhz frequency?
A: In the initial version of the switch the reference frequency was set at 125MHz, but the gateware have been modify during the production. Therefore some old box are still labeled with 125 MHz. On the new box, you will find this output labeled as CLK (OUT).
??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
Q: I have bought WR switch v3.x is it compatible with the latest update of firmware?
A: You can look at the different version compatibility on the wr-switch-sw:WRS-versions page, you will also find which firmware version is the best to use according to your hardware.
??Author: Benoit Rat, Seven Solutions
Q: How fast is the overall performance within the WR switch fpga? Is it really 18*1Gbps? As an example: When I establish a connection between port1 and port2 with 1 Gb/s is it possible for port3 also to communicate with port4 with 1Gb/s ? Is it still possible for each connection to communicate at full speed?
A: A good question. It should be and it is meant to be but it's not yet possible. The current gateware have the following issues:
- It cannot handle well 100% load (so 1Gb/s) traffic even if you have only two ports connected, so if port1 talks with port2, only. It is ok up to ~90% of traffic load.
- It has problems with handling small frames (<128 bytes) at high loads
- If you use a large number of ports at the same time (more than 8 ports which create 4 pairs), the forwarding performance starts dropping (the load of traffic that the switch can forward between each directly connected pair of ports goes down as the number of used ports goes up).
(as for July 2013, concerning v3.3 and below of gateware, intended to be fixed for v4)
??Author: Maciej Lipinski, CERN
See also:
- Frequently Asked Questions about White Rabbit
- Frequently Asked Questions about the White Rabbit PTP Core
24 June 2013