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
Gee, everyone talking about DWDM and stuff when that is really almost irrelevant to the question. Yeah, sure, having n wavelengths on a fiber reduces the number of fibers you need by a factor of n, which is great, but if you have hundreds of millions of “connections”, it having a thousand “channels” rather than a hundred “channels” does not really answer the question.
**The real answer is: Packets!**
What you perceive as a “connection” of sorts, at the network level, just isn’t. You might be thinking of how traditional telephone networks worked, where a pair of copper wires was (more or less) connected up between caller and callee to establilsh a connection. That is not how modern computer networks work. Modern computer networks are what is called **packet switched** (as opposed to **circuit switched**).
**As far as the network is concerned, there simply are no connections.** In the particular case of IP (Internet protocol–the thing you use for websites and email and instant messaging and what have you …), how that works is that every device connected to the internet has a unique number identifying that device, also known as the IP address. And that really means every device, your laptop or smartphone just as any one of Google’s or reddit’s servers. Now, when you(/your device) wants to send some piece of data to any other device on the internet, all it has to do is to label it with the address of the device it wants to send it to (that’s then called a packet) and transmit it to its upstream internet router. That router will look at the address to determine which of its available links (“cables”) would be the best (fastest/cheapest/whatever) choice to get the packet to the device that has that address. At the other end of that link, there will usually be another router that does the same thing. And then another one. And another one. … until, at some point, it reaches the router that the destination device is connected to, so the router will transmit it to that device, which will then, presumably, somehow do something useful with it–and also, oftentimes, send another packet in reply. For that to work, the sender of a packet also adds its own address to any packet it sends, so the recipient can use that address to send back a response.
However, the size of packets on the internet is limited, commonly to around 1500 bytes. So, if you want to transmit something that is larger than that, what you have to do is to split it up into small pieces and transmit those as individual packets–and you have to add some information to the packets that allows the receiver to put them back together, of course. But the important part here is that, as far as all those routers are concerned, there is no connection. They see a packet with a destination address, select a next-hop link to transmit it to, and forget about it. If you do some hour-long download, the routers don’t know and don’t care, all they see is individual packets, millions of them. And really, there is no reason all those packets would even necessarily take the same path through the same cables or routers. All that matters is that all those packets get to their destination, somehow. Or, well, most of them at least, because even that isn’t guaranteed, some fraction of packets do just get dropped for various reasons, in which case the sender will simply have to retransmit if they don’t hear back from the recipient in time.
So, the answer to the question is that those fiber links simply have an extremely high speed, and the routers connected to them simply push through them any packets that arrive in whatever order they arrive in. If your submarine cable has a capacity of 100 Terabit/second, say, that means it is capable of transmitting about 8 billion packets of the typical maximum size per second–and as far as the routers and cables are concerned, those could all be from the same “connection”, or they could be 8 billion different “connections”. In practice, usually, there would be a few million “connections” using that link during any second: Some people transfering data between datacenters or if you have a gigabit connection at home, you might be transferring ~ 80000 packets per second, someone else using a dialup modem in a rural area might be transferring only ~ 4 packets per second. All of those are just mixed as they arrive and transmitted through the cable, one after the other.
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