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Comment by thirdlamp | Hacker News Reader

How does that compete with starlink? I suppose the bandwidth of a subsea cable is much higher, but latency is worse?

Starlink as designed or the current system?

As designed oceanic latencies will be lower than direct fibre, only microwave would be faster.

As currently deployed, starlink latency isn’t much to write home about as you only get a few hindered km to the ground station and then you’re on fibre.

There's no reason for a subsea cable latency to be worse than starlink, both don't go in the straightest line possible but close to that.

Starlink gets the speed of light in a vaccuum, which is roughly 50% faster than the speed of light in glass.

From what I understand Starlink satellites do not communicate between each other yet, maybe that's outdated info. I know there will be inter-sat comm eventually. But they are 500km in altitude and the atmosphere is 100km so they only have about 400km of vacuum from obit to Earth stations. So it's a 1,000km round trip and then through ground stations, fibre cabling.

> From what I understand Starlink satellites do not communicate between each other yet, maybe that's outdated info.

It’s always been wrong. By design the Starlink satellites communicated with each other. Too cost prohibitive and infeasible in other ways to have a ground station covering every satellite.

While the long term plan was for Starlink satellites to communicate with each other, only the last four batches of v1.5 satellites have the actual hardware to do so, and the vast majority of Starlink satellites in orbit right now are only able to talk with ground stations and do not have the inter-satellite laser links.

You phrased it like it's no longer the long-term plan, but your next sentence indicates it is being progressed to. Older starlink sats without this laser link will eventually fall and burn up and be replaced.

Yes but I think the plan is to have any satellite that is over a ground station to be the relay for others. Send up to a sat then sat to sat via laser and then down to the ground.

I'm curious how far each satellite is from the other, do they need line-of-sight. And how much it adds to latency since signals received and sent have to go through networking equipment within each satellite.

> do they need line-of-sight.

Yes, they do.

> And how much it adds to latency since signals received and sent have to go through networking equipment within each satellite.

Negligible vs the impact “speed of light” (aka distance) already has added to the latency.

Fiber gets closer to the speed of light, even when accounting for retransmission delays, than just 50%. The index of refraction for glass is commonly quoted as 1.5. That makes it 33%. Also, Starlink isn’t vacuum speeds either (unless you are talking about the inter satellite laser links and not the downlink to CPE, which even they I’m not sure operate at full “speed of light in a vacuum”).

The refractive index of air is very close to that of vacuum (which is 1), so most Starlink transmissions between nodes (ground-satellite, and satellite-satellite) will travel at near light speed.

> The refractive index of air is very close to that of vacuum

Yeah, 1.0003 vs 1.0. Very close, but not the same was my point.

The refractive index is only one part of the story, light does not follow a straight path through fiber it bounces around. I'd assume this to be about the newer inter-sattelite links as comparing fiber to fiber plus some seems obvious.

> light does not follow a straight path through fiber it bounces around.

this is only relevant for multimode fiber, right? in singlemode fiber the light must propagate parallel with the fiber. even in multimode fiber, some light is parallel, so it doesn’t necessarily limit latency — it creates dispersion.

Singlemode is designed to minimized modal dispersion but it still occurs, especially over the longer distances including from stresses in the core rather than traditional bouncing. First photon isn't as important, it's the signal peak that matters as the receiver will try to decode from that. Typically this latency is hidden by needing repeaters every ~60km anyways due to both dispersion and loss. I'm not sure how far apart Starlink inter-satellite can repeat and if they can "skip" satellites as long as there is a clear direct path to another farther along the path, I haven't been in contact with their engineering folks since I changed jobs last year and am no longer a corporate customer.

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