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Comment by nodesocket | Hacker News Reader

Thanks for this explanation. I previously thought the distance fight was over being closest to the NASDAQ co-location building in New York, not connecting Chicago and New York.

I am still not clear why microwave is faster than fiber though. Is it simply that the fiber lines are multi-tenant, so there is not a direct hop?

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Light in glass is slower than light (or radio waves) in air. Also where they're unable to put point-to-point microwave towers, like the Atlantic ocean, they use HF/shortwave (as in 3-30MHz stuff) radio, because frequencies in that range bounce off the ionosphere. And apparently, the added lag from the increased path length is less than the added lag from glass's optical index, so HF wins out. HF is, however, very noisy and the bandwidth (amount of bulk bits per second you can send) is not too high (even though the end-to-end latency for a single given bit can be very low).

It's not pure HF -- there's a fibre link alongside the HF link where stuff like protocol negotiations, encryption keys, channel sounding, feedback on data received and RF conditions, and agreement on what kind of modulation/coding schemes to use for upcoming data (and what transmitted bits are to mean in terms of higher-layer protocol (something about price informations? I don't know much about HFT)).

Likewise, the availability of GPS makes time/frequency synchronisation easier (and combined with sounding signals and feedback over fibre), which very likely allows for eliminating latency-inducing complexity in the receiver.

It's a far cry from the traditional data-over-HF scenario, which is assumed only to be relevant when other (higher bandwidth) links such as fibre or satellite or point-to-point microwave are unavailable, so everything must be carried out over the RF channel (look at ALE).

Light is slower in glass than in air.

https://micro.magnet.fsu.edu/primer/java/speedoflight/index....

Also consider the path: you can pretty much do a great-circle with radio.

A cable is not going to be direct. If it's above ground, it needs some slack on the line to account for pole movement (you don't want your fibre holding two poles together...).

You'll need more slack for your splices to be done at surface level during construction. Splice trays every x meters will add more distance.

Sometimes the poles are overloaded and you need to run down and bury it for a while.

Your tunnels are never really straight. You need to follow existing easements, or get your own. When you do bury, you need to go around various physical obstructions, like rivers, buildings, cities, parks.

And if you want redundancy against back-hoe fade, your latency in those situations will shut you down, perhaps erasing your profits when you're down and your competitors are up.

The cost of running your own line point-to-point would be be monstrous, though I guess you could build in some latency and sell access to non-HFTers.

the index of refraction of a material is essentially a ratio to the speed of light in the material, where free space is 1. Glass, being around 1.5, has a lower speed of light.

So that's why my diamond optic cables never achieved product-market fit! http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/indrf.html

That, and how the fick do you machine diamond... regular glass fiber optics needs to be cut to a smooth surface and fused together. The devices to do this use diamond-coated blades.

Oh, and diamond has a large birefringence due to its crystal structure, which would cause temporal smear in signals.

Fiber is about 0.6c and the cables are much longer than direct line of sight.

Microwave signals travel from the transmitter to the receiver in a straight line. Whereas light bounces off the walls of the fibre optics cable, which increases the distance. Furthermore, the material that fibre optics are made out of attenuates the signal much stronger than air, so you have to place more repeaters and electronics in between your endpoints.

> light bounces off the walls of the fibre optics cable

This is true of multimode fibre, but not single mode. Long haul communications cables are single mode.

https://en.wikipedia.org/wiki/Single-mode_optical_fiber

Even for multimode fiber it's a huge simplification of the actual physics. The actual cause of the speed difference, for both single and multimode fibers, is the index of refraction of glass.

Look up refractory index. That’s all you need to know. The above is meaningless in this context.

Light signals in an optic fiber bounce off the walls (repeatedly, frequently) of the cable they're in -- the actual physical path the light is taking in this case is approx. 1.5x the path of the cable itself.

That's not true. The typical total reflection angle in single mode fiber optic cable is less than 4 degrees. The refractice index of glass, however, is about 1.45. So almost all the slowdown is due to the glass directly slowing down the signal.

Though you can still get a group delay (slower than free space) in vacuum filled RF waveguide. When you break down the single mode, it appears to be zig-zagging through the waveguide (bouncing off the walls), and slows down as cutoff is approached. Like in fiber, the modal dispersion is separate from the material dispersion.

AT&T was going to build out a cross country, single mode, circular waveguide network in the 60’s, but was obsoleted due to fiber. Maybe that’s what the HFT ought to due, or maybe air-line coax.

I think the distance is less of an issue than the temporal smear that the bouncing causes. A clean square pulse turns into a blob.

Temporal smear is caused by the varying refeactive index of glass with wavelength and the fact that a short time pulse must have finite (large) spectral width (by the uncertainty principle), not by the bouncing.

So, the different path lengths implied by internal reflections don't contribute anything to the smear?

By the way, the uncertainty principle doesn't say we can't have a narrow pulse. Frequency versus time can be used as an analogy to help explain the uncertainty principle. If we know a precise frequency, the signal is poorly localized in the time domain. If a signal is narrowly localized in the time domain, it is poorly localized in the frequency domain. The uncertainty principle is something else, though: it revolves around not being able to make certain related measurements simultaneously, at the quantum level. We can be quite certain about the frequency content of a transient pulse and its exact temporal shape, if it's not something from the quantum domain.

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