On Wed, 4 Aug 2004, Randy Bush wrote:
How much is "low latency"? I have 6ms RTT over my 8M/800k ADSL, it's usually 6-8ms over an equivalent 2M g.shdsl line.
interesting question. i have two adsl lines. pinging the first hop router
verizon / lavanet (hawi to honolulu, 25 mins air time by plane)
64 bytes from 64.65.95.73: icmp_seq=0 ttl=64 time=20.637 ms 64 bytes from 64.65.95.73: icmp_seq=1 ttl=64 time=22.186 ms 64 bytes from 64.65.95.73: icmp_seq=2 ttl=64 time=21.965 ms 64 bytes from 64.65.95.73: icmp_seq=3 ttl=64 time=21.723 ms 64 bytes from 64.65.95.73: icmp_seq=4 ttl=64 time=21.538 ms
qwest / iinet (30 miles from bainbridge to hellview wa us)
64 bytes from 209.20.186.1: icmp_seq=0 ttl=63 time=67.008 ms 64 bytes from 209.20.186.1: icmp_seq=1 ttl=63 time=67.700 ms 64 bytes from 209.20.186.1: icmp_seq=2 ttl=63 time=56.696 ms 64 bytes from 209.20.186.1: icmp_seq=3 ttl=63 time=60.249 ms
i do not know why and can get no useful info on provisioning. i know iinet is redback.
Looks like Qwest are using data interleaving on their connection, while Verizon aren't. It helps reduce dataloss at the expense of increased latency, by interleaving bits over time so that a short burst of signal destroying noise can only remove part of any given larger block. Data blocks reserve some space for error-correction data, which can salvage a partially damaged block. I hear a lot of ISPs in the states are turning on interleaving by default these days, while in the UK I've never actually encountered it. Some ADSL modems have an option to disable it also. Sam