[time-nuts] Project on precise timing over Ethernet

Javier Serrano javier.serrano.pareja at gmail.com
Fri Sep 11 07:04:52 UTC 2009

Yes, exchanging data over a link will always harm its timing
accuracy/precision performance, irrespective of whether you steer clocks
(our case) or you exchange values or both. In fact, as I said to Jeremy in
another thread I tend to see clock steering (i.e. PLL) as a limit case of
interchanging values, where this exchange would happen at every single clock
tick. So you end up with lots more information, up to you to low-pass filter
to get rid of high frequency phase noise.



On Fri, Sep 11, 2009 at 12:30 AM, Magnus Danielson <
magnus at rubidium.dyndns.org> wrote:

> 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.
> Cheers,
> Magnus
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