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Comment by adamcharnock | Hacker News Reader

Random fact: I did some planning around this for a client a while ago. While measuring the AWS latencies I found I could get approximate latencies (within 10%) by measuring the rough undersea cable length (km) and dividing by 150.

While not overly surprising, it was very consistent.

Edit: I think it was actually 155

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That reminds me of the story of the 500 mile email (https://www.ibiblio.org/harris/500milemail.html)

I read this yeaaaars ago. I'm about to re-read this, but before I do, I think this was the article that installed a little goblin in my brain that screams "TTS" in instances like this. I will edit this if the article confirms/denies this goblin.

EDIT: mostly, probably, sort of.

Funny story. He must thank the department of statistics for the quick turn around.

I think this is because of medium velocity of light

"Through LabVIEW the speed of light in the optical fiber is calculated to be ~ 2.054 x 108 m/s corresponding to a refractive index of n ≈ 1.4606 which is a typical value" https://web.phys.ksu.edu/posters/2009/juma-Adv-Lab-S09.pdf

There's a surprising amount of real-world modelling that can be done to satisfactory precision with just multiplication and addition.

This page is such a well executed interactive map. Really enjoyed it

Is the math-planation of your random fact basically

(thanks to https://news.ycombinator.com/user?id=Hikikomori for correcting the lightspeed in fibre medium from 3e5 to 2e5 !)

- lightspeed is 2e5 km/s ~ 2e2 km/ms, so/

- length (km) / 200 (km)/ms ~ K length (km) / 200 (km)/ms, so

- latency (ms) ~ K' length (km)

Where K is approximately 1.3 (K' is 1/155) and factors in things like:

- non straight line distance

- networking overhead / switching

- both ways / measurement error

Basically?

Speed of light in a medium like fiber is about 200 000km/s.

Oh shit! Thanks. Good point. That actually makes it more plausible, as K is smaller.

1/2 c in circuit boards (FR-4), 1/3 c in cables, two useful numbers to remember.

Thanks, nice! But wait - so we have

1/2 c ~ 150 km/ms in circuit board.

1/3 c ~ 100 km/ms in cable. And...

2/3 c ~ 200 km/ms in fiber?

I'm a bit confused about difference between cable and fiber heh :)

Sorry, it was a typo. I meant 2/3 (including common cables and fiber optics), not 1/3.

Depends on what kind of cable? As twisted pair network cable is at 2/3.

What an embarrassing typo! I was thinking of 0.66, and somehow I thought 0.66 = 1/3 (must've been distracted by the "2" in 1/2). I should've written 0.66 or 2/3.

It is possible I was measuring latency in a single direction, rather than round-trip-time. My memory is a little hazy now.

No I think you had it right. I was off on the speed. Anyway, it could have matched accounting K for other factors heh :)

Right, looking at the visualization most (all?) of the red lines are the longer ones - eg North America to South Africa.

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