Rusty Dekema wrote on 18-9-2007 0:50: >> Does anyone on the list know if such a connection has an asymmetric >> latency? How much? I can't think of a way to measure it (it's my only > > Yes, an ADSL line, if the upstream and downstream sync rates differ > (which they generally do), will have asymmetric latency. Although > other factors always affect network latency, I think the main cause of > asymmetric latency on an ADSL line is due to serialization delay, > meaning the amount of time it takes for the entire packet to be > transmitted, one bit at a time, at the bit rate of the line. > > Say for instance that your ADSL line syncs at 6000kbit (6,000,000 bits > per second) down and 384kbit (384,000 bits per second) up. A typical > NTP packet is approximately 80 bytes (640 bits). Assuming the NTP > packets are not queued behind other traffic on the line, it > will/should take approximately 1.67ms (640 bits / 384000 bits/sec) to > transmit an NTP packet on the line's upstream link, but only 0.11ms > (640 bits / 6000000 bits/sec) to receive an NTP packet on the line's > downstream link. > > These numbers will vary depending on the ratio of one's upstream and > downstream speeds, but in general, ~2ms or less of additional > uncertainty caused by asymmetric DSL rates in a somewhat ad-hoc time > service operating over the public Internet is probably not a very > significant factor in the overall uncertainty of the time information > you are providing to your users. > > Rusty
That makes perfect sense, it must be asymmetric. Someone answered me off list that the systematic offset of a stratum 2 (or higher) server on an adsl is cancelled out when another host gets time from that server. I think that was a smart and interesting remark too. Jan _______________________________________________ timekeepers mailing list [email protected] https://fortytwo.ch/mailman/cgi-bin/listinfo/timekeepers
