Hacker News Reader

Comment by upghost | Hacker News Reader

Can they splice these? Surely the don't have to run a whole new cable?

They can but it's an involved process. Modern cables manage their slack very precisely - a complicated job in and of itself - and usually cannot be hauled to the surface without snapping them. So if the cable is not already cut (an anchor can damage a cable in a variety of ways short of physically snapping it) it will be cut while still on the bottom. The ends are then winched up and a splice is patched in but now there's a rather lengthy section of extra cable to deal with. They deal with that by laying it down precisely on the seabed in a nice sideways loop.

So then you have extra latency for each repair.

And extra attenuation / reflections.

I've only done land based fiber design, but I would assume based on my experience in that field that one of two things happens.

First option, there's some slack somewhere nearby to do a pull nearby.

If you ever pay attention to aerial poles, you'll see at various intervals things that are vaguely reminiscent of a snowshoe or tennis racket, in the industry they are in fact informally referred to as a 'snowshoe' and the entire purpose is to have some fiber to 'release' if a cut happens and a resplice is needed. [0]

I'd -assume- there is an analogue for these in undersea cables...

If not, well then you'd have to splice a length in and probably have the right sort of mechanism to ensure the cladding/etc stays contiguous (maybe even weld? IDK) but regardless you're probably talking... Actually wait yeah they've got this all figured out; https://www.youtube.com/watch?v=QiwidMEv8CM

[0] - Real world example, for instance if someone uses a fiber splice can for target practice.

https://www.bbc.com/future/article/20241014-the-deep-sea-eme...

An article about such repairs (150-200 yearly)

> To repair the damage, the ship deploys a grapnel, or grappling hook, to lift and snip the cable, pulling one loose end up to the surface and reeling it in across the bow with large, motorised drums. The damaged section is then winched into an internal room and analysed for a fault, repaired, tested by sending a signal back to land from the boat, sealed and then attached to a buoy while the process is repeated on the other end of the cable.

I think they raise the cable to the surface, splice it and sink it again.

I've read (or perhaps watched on YouTube) the same thing.

Finding both ends of the cable to grab abd raise it took longer than the raise/splice operation.

I suspect a sufficiently malicious attacker could intentionally cut the cable twice, so when the repair crew drag up one end of the cut they know about, the section between the cuts drags its other end away from it's cut-point making the second repair significantly more difficult...

Cutting twice wouldn't be an issue, they can send a signal from both ends to get a reflection of how much of the cable is dead.

In most cases they can easily figure out approximately where the break is on the line by simply measuring the resistance in the cable's conductors (they have more than one but the big one caries a 10,000V feed to power the repeaters). A double cut would probably show up as resistances that didn't sum to the proper total.

You can only practically do that optically e.g through OTDR. You can't TDR steel conductors exposed to the ocean 500KM away with a ohm meter and a Murray loop, to even have a chance you need to put low voltage on it and it's not even close to as accurate.

Rubbish. TDR is not a Murray loop.

https://en.m.wikipedia.org/wiki/Murray_loop_bridge

  Spread-spectrum time-domain reflectometry (SS-TDR) is often used to locate and characterize a fault on active cables. Traditional time-domain reflectometry, which sends signals down a conductor to determine where any impedance discontinuities are by timing any reflections that come back from any impedance discontinuities along the way, can’t be used on in-use undersea cables thanks to the high voltages involved. SS-TDR, which was originally developed to detect faults in the wiring of airplanes using 400-Hz AC power, uses modulated pseudo-noise (PN) signals rather than a plain square wave pulse. The signals still bounce off any impedance changes introduced by damage, but an algorithm is used to correlate the returned PN codes with what was sent and when, making it easier to make measurements in a noisy environment. Optical TDR can also be used to locate fiber breaks

https://hackaday.com/2024/07/30/undersea-cable-repair/

A little knowledge goes a long way

Reply on news.ycombinator.com