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Comment by walrus01 | Hacker News Reader

in terms of raw capacity, I do not think that the intersatellite laser links (if/when they do get that working, which is a long ways off, 10 out of the 1050+ satellites right now have them, and none in production use), will approach anywhere near the capacity of a 40 or 80 channel, coherent 100/200/400GbE DWDM submarine line terminal. it's a whole other order of magnitudes.

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Why couldn’t intersatellite laser links have similar capacity to fiber? In principle, if not practice?

Is it because ISLs will require more powerful lasers because the light is focused through a lens instead of guided through a fiber?

>> Why couldn’t intersatellite laser links have similar capacity to fiber? In principle, if not practice?

Because laser energies, signal paths, traveling through free space must literally share that space with each other. It is effectively one wire. An underground cable can have hundreds, thousands, even millions of wires lying beside each other without interference.

It is comparable to the difference between a wifi connection and bundle of ethernet cables. One can scale and one cannot.

I'll start by saying my understanding of the laser connection on the satellites is virtually nil.

Could you just add more beams on each satellite?

No. The beams would overlap, which is fine of they are on different frequencies, but eventually you run out of frequencies to use. Fiber confines the beam to inside the fiber. No overlap between one fiber and another means you can run multiple down the same pipe.

Why would they overlap?

Because a beam in an open medium always diverges due to wavefront errors and diffraction. A very good laser system might possibly have a divergence of only 1 microradians. For reference a typical laser pointer has a divergence angle of 1 to 2 milliradians. With the 2000km or so between satellites for Starlink the laser "spot" on the receiving satellite with the very good laser will be about 2 meter. Not enough to receive multiple beams on one satellite without overlapping.

Thanks for explaining. Would it work if they put receivers on a long pole, spaced 3m apart? Maybe they could mount them on the edge of the solar panel array, which is ~30m long [0].

[0] https://lilibots.blogspot.com/2020/04/starlink-satellite-dim...

The link estimates the surface of the panels to be 30m2. Since the panels are 3.1m wide the length would be 9.6 meters. But even at 30 meters it would be very very hard to engineer a system that can keep a laser beam aligned on a moving target with sub microradian precision. I'd guess Starlink laser are more divergent to make hitting the target easier. Keeps the exit pupil a manageable size too (narrower beams need bigger optics).

Ok, but I was asking about a single space laser vs a single fiber, not the theoretical limits of many of them.

Space is pretty big, lasers are pretty narrowly focused, and not many people have lasers in space yet.

High end fibre routers use multi-spectral bandwith, effectively allowing for multiple data streams to be transmitted on one fibre cable, and re-broken out at the other end. This significantly increases bandwidth.

I doubt the laser links support this, and I wonder if any could, when in a non-encapsulated environment. EG, spectral pollution from sunlight/etc.

probably less (literal) bandwidth, lasers are expensive

That and because the atmosphere is not clear (this is why 1Gbps free space laser network bridges have very limited range), the links from satellite to earth in any rf band will be very small in capacity as compared to a singlemode fiber cable. Space to space lasers, sure... Much clearer.

Starlink ground links are RF

That's what I said

oops my bad

Do the markets need lots of bandwidth? Or just-enough with the lowest possible latency

Markets don't, but, GP said:

"I can't imagine much appetite for any future under-sea cabling out of Australia"

And the reality is that satellites can not compete with the bandwidth of subsea cables for bulk data transfer, HFT is a whole other world.

I’d guess that high speed data transfer competes with overnight shipping a hard drive in most current cases. In my day to day life, for most cases I’d rather has 10 Mbps with 10 ms latency than 1 Gbps with 250 ms latency. Generally speaking I’m looking for a snappy user experience and fast enough for Netflix streaming to a non-premium television. But high bandwidth and low latency would be super nice!

A lot of serious HFT has moved to HF radio bands with giant aimed yagi uda or dipole antennas, already... London, new york, chicago, tokyo. It's very low bit rate but lower latency.

So Australian FinTech will definitely appreciate Starlink, then?

I suspect Australian FinTech would be more interested in colocating near exchanges instead of trying to beam their orders through space, but I don't work in the sector so I can't say for sure.

Totally different use cases.

If you want to observe events at one exchange and send orders to another, all that matters is total latency. For that use it doesn't matter whether your servers are in the same room as one of the exchanges or 200 miles away from either one.

It's true that total latency is what matters, and minimising latency by sitting near one of them and taking advantage of low-latency connections between exchanges sounds reasonable.

Does this have a name? Something like "a race to ridiculousness?" It's so bizarre that it matters that someone places an order a few ms before someone else. And it's probably ns now. No time to stop and think,... just be faster than someone else.

Capitalism? :P

Many wired leased lines that connect data vendors to exchanges are only ~2Mbps even in 2021.

You tend to get bandwidth spikes on most feeds at market open and close, but intraday rates are low

Check out this analysis of starlink ground relay routing vs fiber https://youtu.be/m05abdGSOxY

It's not exactly a long ways off if a dozen satellites are already using it (there were 2 previous ones).

Every time SpaceX makes a goal, someone claims it's really far away until it is completed.

If there was a need for Terabit/s laser capacity for Starlink, there's no particularly good reason it couldn't be developed.

Musk does tend to achieve his goals eventually although the timescales are generally 2-7 years too ambitious:

www.bloomberg.com/features/elon-musk-goals

Damn that doesn't bode well for me getting a Cybertruck any time soon.

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