[time-nuts] Project on precise timing over Ethernet

Magnus Danielson magnus at rubidium.dyndns.org
Thu Sep 10 22:30:05 UTC 2009

Javier Serrano wrote:
> 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
> Ethernet:
> http://www.ohwr.org/twiki/bin/view/OHR/WhiteRabbit/WhiteRabbit
> 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.

There is several types of "Synchronous Ethernet", but here is one where 
the bit-clock is being steered just as in a SDH network, and infact this 
variant is also supported in modern G.781 synchronisation.

The other form possible is by using counters and interchanging values. 
Ah well.

Synchronous Ethernet that steers bitclock will be tainted by 
inter-symbol interference, but it is not too bad for most purposes.

I have a thick WhiteRabbit document to read in detail, but other things 
have come inbetween.


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