[time-nuts] Re: NTP via Passive Optical Network (PON)

[Magnus Danielson]

Hi,

Sure, it will slightly skew the offset, but it is quite small compared 
to jitter or usual performance so not really a problem. Asymmetries like 
these needs to be handled when you operate over infrastructure not 
supporting timing anyway, so PON is no major difference. PTP can be 
asymmetry compensated, and some implementations support that. You 
typically use GNSS to measure / estimate asymmetry and compensate. Turns 
out that WAN networks offer larger challenges than PON, since IP 
networks does not really have any guarantee for two directions between 
two nodes take the same path. In fact, for traffic engineering reasons 
it can vary quite a bit in some cases. NTP and PTP will suffer the same 
way from any such asymmetries / delay-differences in underlying network. 
PTP was not designed for it, NTP wasn't designed for it either, but 
achieved performane is lower so it is of less care.

Trying to address this require a multitude of solutions, first packet 
filtering to filter out measurements with less impact from jitter, then 
asymmery compensation and change that when there is reroutes. In ITU-T 
we work on that under the name of ePTS.

Cheers,
Magnus

On 2025-07-24 01:25, Kapp, Francois B. via time-nuts wrote:
> I was wondering if anyone has measured, or has seen results of measurements of the effect of a passive optical network on time transfer over NTP (or PTP). Intuitively, the fact that return communications are time division multiplexed should insert some jitter in the round trip time - any pointers to more concrete information?
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