How do internet cables that go under the ocean simultaneously handle millions or even billions of data transfers?

Question

I understand the physics behind how the cables themselves work in transmitting light. What I don’t quite understand is how it’s possible to convert millions of messages, emails, etc every second and transmit them back and forth using only a few of those transoceanic cables. Basically, how do they funnel down all that data into several cables?

In: Engineering

Answer 1

Hi! Here is a good link ( [https://twistedsifter.com/2012/07/the-undersea-cables-that-connect-the-world/](https://twistedsifter.com/2012/07/the-undersea-cables-that-connect-the-world/) ) on what the fiber looks like. It is super durable because, well, there is a lot of things that can go wrong at the bottom of the ocean. In essence, there are normally always one or two cable ships laying undersea cable at any given time, sometimes to fix lines that broke and sometimes to give more bandwidth. From what I have read before, to solve attenuation every 100KM or so there is a repeater device.

There isn’t only one or two wires in a cable bundle, there can be hundreds. The cable is huge, but the wires themselves are the width of a human hair. I can’t find the exact number of wires in the undersea cable but from looking at the picture they must have at least 48 pair.

So, quick explainer, on the internet you typically have one wire for send and one wire for receive. So if you want a 10Gb connection between two pieces of electronics, you would need two wires. So if you have a 48 wire cable, then you get 24 different send and receive pairs that can be put into electronics.

Technology has come along and we know how prisms work. For example, I have residential F/O service in my house. They gave me 1 wire. So how do I send and receive? The technical answer is ‘coarse and dense wave multiplexing’ which is a complicated way of saying you can send a communication in one direction on one wavelength (or ‘color’ for short) and receive on a different wavelength. The colors are a handy way of talking about it, but these electronics can use the non-visible spectrum as well. I think, the last time I purchased multiplexing optics, I was able to get 10 10Gb pairs on one wire. So for that undersea cable, assuming it is 48 pin, it is 48 x 10 send/receive pairs.

How the data actually flows? That is a topic called ‘digital signal processing’ (DSP) and you will need to ask another ELI5 for that.