[ntp:questions] Frequency adjustments in a local network

Martin Burnicki martin.burnicki at meinberg.de
Thu Apr 3 14:37:30 UTC 2014


Maximilian Brehm wrote:
> Hey NTP community,
> I need to synchronize only the frequency of a
> destination systems based on the frequency of a reference system in the
> same network. This is because the reference system does not supply
> timestamps and they are not important for the destination system. So, I
> would say I need (a) only frequency and no clock adjustments for the
> destination system and (b) a method to synchronize clocks via a local
> network.
> Are there ways to achieve this using NTP that are implemented
> or may be implemented? If yes, can you recommend me starting points for
> either way?

I think it depends. ;-)

You can't transport frequency directly over a standard LAN/WAN, so you 
would have to use some timestamps on both sides, and see if the time 
drifts away over time, in which case your frequency is off the reference 

How good this works depends on

1.) how accurately you can generate timestamps at both ends (e.g. 
resolution of the system time)

2.) how accurately you can transfer and thus compare time over the 
network (constant or varying network delay, execution time in the 
network protocol stacks, etc.)

3.) the stability of your oscillator at the client side, assuming the 
oscillator at the server side is stable anyway.

For example, to minimize the error caused by varying latency you would 
want to increase the measurement interval, but if you don't have a 
stable oscillator the frequency will drift more if the the measurement 
interval is longer, so you need to find a good compromise and see if the 
results are good enough for your application.

If you need high accuracy and have a local network with known network 
nodes (server, client, and switches) you could use PTP instead of NTP.

PTP supports hardware timestamping, so if all your involved nodes also 
support this you can do *very* accurate time measurement over the 
network, and thus implement a control loop which can discipüline the 
frequency at the client side very accurately.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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