Jeroen Mostert <[email protected]> wrote: > Even if you consider accurate detection of 1 cent to be good enough for > tuning > purposes, your measuring equipment still needs to be an order of magnitude > better. A 0.1 cent difference is 57.8 ppm. 12 ppm is 0.02 cent, which isn't > excessive if you're going for 0.2 cent accuracy.
Ok but when you want to measure to 12 ppm and you assume an average network jitter of 100 ms (which is better than reality when you are on UMTS) you will need two samples that are at least 2.5 hours apart when doing a calibration using SNTP. There will be the extra complication that a UMTS connection goes into a "sleep mode" when there is little or no traffic. To "wake it up" you need to send about 4 packets/second for some time. This is a very typical pattern on a UMTS connection: ping -i1 172.31.32.18 PING (172.31.32.18) 56(84) bytes of data. 64 bytes from (172.31.32.18): icmp_seq=1 ttl=127 time=461 ms 64 bytes from (172.31.32.18): icmp_seq=2 ttl=127 time=377 ms 64 bytes from (172.31.32.18): icmp_seq=3 ttl=127 time=390 ms 64 bytes from (172.31.32.18): icmp_seq=4 ttl=127 time=369 ms 64 bytes from (172.31.32.18): icmp_seq=5 ttl=127 time=446 ms 64 bytes from (172.31.32.18): icmp_seq=6 ttl=127 time=370 ms 64 bytes from (172.31.32.18): icmp_seq=7 ttl=127 time=380 ms 64 bytes from (172.31.32.18): icmp_seq=8 ttl=127 time=385 ms 64 bytes from (172.31.32.18): icmp_seq=9 ttl=127 time=383 ms 64 bytes from (172.31.32.18): icmp_seq=10 ttl=127 time=425 ms 64 bytes from (172.31.32.18): icmp_seq=11 ttl=127 time=402 ms 64 bytes from (172.31.32.18): icmp_seq=12 ttl=127 time=496 ms 64 bytes from (172.31.32.18): icmp_seq=13 ttl=127 time=410 ms 64 bytes from (172.31.32.18): icmp_seq=14 ttl=127 time=391 ms 64 bytes from (172.31.32.18): icmp_seq=15 ttl=127 time=448 ms 64 bytes from (172.31.32.18): icmp_seq=16 ttl=127 time=457 ms ping -i0.1 172.31.32.18 PING (172.31.32.18) 56(84) bytes of data. 64 bytes from (172.31.32.18): icmp_seq=1 ttl=127 time=552 ms 64 bytes from (172.31.32.18): icmp_seq=2 ttl=127 time=828 ms 64 bytes from (172.31.32.18): icmp_seq=3 ttl=127 time=809 ms 64 bytes from (172.31.32.18): icmp_seq=4 ttl=127 time=693 ms 64 bytes from (172.31.32.18): icmp_seq=5 ttl=127 time=593 ms 64 bytes from (172.31.32.18): icmp_seq=6 ttl=127 time=493 ms 64 bytes from (172.31.32.18): icmp_seq=7 ttl=127 time=448 ms 64 bytes from (172.31.32.18): icmp_seq=8 ttl=127 time=348 ms 64 bytes from (172.31.32.18): icmp_seq=9 ttl=127 time=246 ms 64 bytes from (172.31.32.18): icmp_seq=10 ttl=127 time=144 ms 64 bytes from (172.31.32.18): icmp_seq=11 ttl=127 time=50.8 ms 64 bytes from (172.31.32.18): icmp_seq=12 ttl=127 time=52.2 ms 64 bytes from (172.31.32.18): icmp_seq=13 ttl=127 time=102 ms 64 bytes from (172.31.32.18): icmp_seq=14 ttl=127 time=50.4 ms 64 bytes from (172.31.32.18): icmp_seq=15 ttl=127 time=51.4 ms 64 bytes from (172.31.32.18): icmp_seq=16 ttl=127 time=52.4 ms 64 bytes from (172.31.32.18): icmp_seq=17 ttl=127 time=47.6 ms 64 bytes from (172.31.32.18): icmp_seq=18 ttl=127 time=50.1 ms 64 bytes from (172.31.32.18): icmp_seq=19 ttl=127 time=51.4 ms 64 bytes from (172.31.32.18): icmp_seq=20 ttl=127 time=47.4 ms As you can see, totally unsuitable for accurate time calibration. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
