[time-nuts] Timing on Ethernet

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Aug 3 08:54:08 EDT 2007

Pablo Alvarez Sanchez wrote:
> Dear timing colleges, 
> At CERN we are considering the possibility of using Ethernet as a real
> time field bus. 
> We may use IEEE 1588 to distribute precise UTC and Ethernet Powerlink or
> a similar home made product to guarantee real time. 
> In order to have full control over the network we are thinking of
> designing our own hardware using FPGAs (including the physical
> interface). It looks like we will end by implementing 100Base-TX or
> 10Base-T rather than gigabit Ethernet, just because it looks much more
> robust and simpler to design. 
> 1) Two ways calibration performed in IEEE 1588 needs a symmetric path
> between nodes. In a presentation made by 'Timing Solutions', they say
> there is a 10% of asymmetry on the path for 100Base-TX. I guess this
> comes from the fact of using one pair for the go and a different pair
> for the return path.
> Do you have some figures for the two ways asymmetry on a single pair? 

There should be none the propagation delay of a single pair should be 
the same in each direction.
The asymmetry between pairs is due to the slightly different lengths 
Each pair uses a slightly different number of twists per unit length.


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