[ntp:questions] Re: NTP4 has 3 different time formats! Namly (32, 64, 128) bits wide

Martin Burnicki martin.burnicki at meinberg.de
Mon Jul 17 07:41:02 UTC 2006


Terje Mathisen wrote:
> If you can get _repeatable_, at least some of the time, 4 us delays,
> then you can use the same statistical methods which NTP is already using
> to get absolute accuracy close to an order of magnitude better, i.e.
> half a us or so.
> 
> The real limiter will be the need for (a) a really good local clock
> source, i.e. better than the current 10 cent (?) quartz crystals, and
> (b) a hardware method to measure the interrupt latency.

Another way to get accuracy in the range of a few nanoseconds is what
IEEE1588 (PTP) does. It picks up a time stamp whenever a packet really
comes from the network cable, or goes onto the cable.

Thus the protocol can eliminate latencies of the IP stack and the
transmission time between 2 nodes can be reduced to the pure cable delay.
However, special hardware time stamp unit (TSU) is required in the NIC to
match the packet patterns and pick up a time stamp. Without that special
TSU the accuracy is also in the range achieved by NTP.

The accuracy is also degraded if there are switches involved which are not
PTP-aware and queue all packets internally for up to several milliseconds. 

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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