# Transatlantic response times.

### On Mon, 25 Mar 2002 09:13:20 -0600, "Pistone, Mike"
### <Mike.Pistone@msfc.nasa.gov> casually decided to expound upon
### "'nanog@merit.edu'" <nanog@merit.edu> the following thoughts about
### "Transatlantic response times.":

I was curious if anybody would share what they consider to be average or
acceptable transatlantic ping response times over a T1.
I know there are tons of variables here, but I am looking for ballpark
figures.
Assume that utilization on the circuit is extremely low, and you are
measuring point to point across the line. You can also assume no other
bottlenecks effecting the response times (router performance, or what not).
Should you see a ~150ms trip? 250ms? 450ms???

Well, I've been seeing around 70ms (+/- 5ms) RTT pings from NYC to LON
across AC-1 (Global Crossing) as normal. Granted this is on an OC-48 but
bandwidth should not matter much to RTT if the load is light and all you're
measuring is ICMP ping.

Is there any equation to estimate response times? For example, if your
circuit from A to Z has a 500ms avg response, than that equates to a circuit
distance of aprox. 5000 miles or something?

Assuming you exclude switching latency in the hardware, latency induced by
regenerators, etc... spead of light in a medium is a simple
distance-rate-time equation with a slight twist: c = nL/t, where n is the
refractive index, L is the length, and t is the transmission time difference
(double this for RTT). The rest is just simple math. So expected one way
time should be: t = nL/c

Note -- I believe most fiber optic cables have a refractive index somewhere
on the order of 1.4.