> 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 _______________________________________________ timekeepers mailing list [email protected] https://fortytwo.ch/mailman/cgi-bin/listinfo/timekeepers
