[time-nuts] Project on precise timing over Ethernet

Javier Serrano javier.serrano.pareja at gmail.com
Thu Sep 10 11:29:06 UTC 2009

Dear nuts,

We have this ongoing project whose aim is to synchronize roughly one
thousand stations (typical distances around 10 km) to within 1 ns using
The idea is basically to use Synchronous Ethernet and something PTP-like in
combination. Sync Ethernet means from a timing point of view there is
hierarchy: there is a master source and its frequency is used to encode data
in the uppermost Ethernet switch in the hierarchy. This frequency then
propagates everywhere because switches extract it off the incoming data on
their "uplink" port and use it to encode data on every "downlink" port.
Fiber delays are evaluated using a two-way scheme, and used in phase
shifters to produce accurate PPS pulses everywhere. Our first Ethernet
switch prototype proved the concept works, although we are far from a final
system covering all our specs, and now we are thinking about making this a
more serious project by applying for EU funding. In the proposal document
there is a very important chapter on "Impact" and I would be very interested
in reading about potential applications you might have for such a network.
Here is our list so far:

- Synchronization of different parts of large experimental physics
facilities (the original purpose). The advantage wrt current approaches is
you could do both data and timing on the same fiber. I am not saying nobody
has done timing over Ethernet so far, we have done our homework and none of
the existing projects fulfilled our constraints concerning clock quality,
accuracy, number of nodes, distances, Ethernet compliance and
"transparency", latency, determinism, design openness, etc.

- Distributed data acquisition: you can have ADCs feeding data to rolling
buffers, and all connected to this White Rabbit (WR) network. When one of
them receives a trigger we want to produce the effect in the control room
that all of them were triggered at the same time. So the trigger pulse gets
a precise UTC time tag and then all the other ADCs are informed of it
(through the same network) and also requested to freeze their buffers and
return the appropriate subset of data (again through WR) for coherent
display on operational consoles. The  1 ns spec comes mainly from this type
of application, applied to analog signal acquisition in particle
accelerators. Other examples are distributed sound measurements, structural
monitoring (vibration), power monitoring (detection of partial discharge and
dielectric losses in high voltage power cables: by taking synchronized
measurements over a segment of cable, defects can be pinpointed and repaired
before complete failure of the transmission line occurs), distributed RF
Time Division Multiple Access (TDMA) and meteorological event measurements
(e.g. lightning).

- Backbone for wireless positioning networks. There are projects to build
wireless networks in factories where you can position a mobile wireless node
by measuring two-way delays wrt fixed wireless points, which need to be very
well synchronized among themselves.

I am very interested in hearing about any potential application you could
imagine for White Rabbit, and also of course about any remarks on the
project in general.



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