From: Sam Stickland (no email)
Date: Fri Aug 06 2004 - 09:10:05 EDT
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
> verizon / lavanet (hawi to honolulu, 25 mins air time by plane)
> 64 bytes from 22.214.171.124: icmp_seq=0 ttl=64 time=20.637 ms
> 64 bytes from 126.96.36.199: icmp_seq=1 ttl=64 time=22.186 ms
> 64 bytes from 188.8.131.52: icmp_seq=2 ttl=64 time=21.965 ms
> 64 bytes from 184.108.40.206: icmp_seq=3 ttl=64 time=21.723 ms
> 64 bytes from 220.127.116.11: icmp_seq=4 ttl=64 time=21.538 ms
> qwest / iinet (30 miles from bainbridge to hellview wa us)
> 64 bytes from 18.104.22.168: icmp_seq=0 ttl=63 time=67.008 ms
> 64 bytes from 22.214.171.124: icmp_seq=1 ttl=63 time=67.700 ms
> 64 bytes from 126.96.36.199: icmp_seq=2 ttl=63 time=56.696 ms
> 64 bytes from 188.8.131.52: 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.