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 _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
