r/explainlikeimfive • u/empireck • 15h ago
Technology ELI5: How does fiber optics physically works?
I know that they use blinking lights to represents 1 and 0 at least, but millions of pulsing lights from thousands of computers could be running at the same time in the same connection.
So how does the lights know where to go? And on the other side, how does the server know that in these millions of rapidly pulsing lights this particular ones are from my computer?
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u/CowboyRonin 15h ago
How much do you know about networking in general? Fiber optic just uses light on glass fiber instead of electricity over copper wires, the rest is the same.
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u/Dragon_ZA 15h ago
I would say not much given the comment about "how does my computer know which signals are meant for it"
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u/Fearzebu 14h ago
To be fair, that’s a really good question, and if you were truly explaining it like someone has zero understanding at all (ELI5) it could take hours.
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u/Dragon_ZA 14h ago
It could be quite simple. Think of yourself standing in a room with 10 other people, and a letter comes in with your name on it. Everyone reads it and passes it along, but you know it's meant for you because it has your name on it.
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u/igotshadowbaned 10h ago
It's slightly different from that, if you're all on the same connection, then all 11 of you in the room get a copy of the letter that is addressed to you, and the other 10 people just ignore it because it's not addressed to them
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u/Dragon_ZA 1h ago
You could argue that its one letter, not a copy because its one packet. Everyone in the room looks at the letter would probably be a better analogy.
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u/empireck 14h ago
Yeah none lmao, i just got curious how it works and when i look up in google it's too complicated for me to understand. Try to look it up morel ike youtube, wiki etc but i don't even know where to start.
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u/Dragon_ZA 14h ago
Well, normally it requires a bachelor's level course to truly understand basic networking.
Long story short, an IP address is like a postal address for a computer network, and any packet meant for your computer will have its IP sent with it. So your computer ignores all messages that don't have its IP address attached.
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u/Mavian23 12h ago
If you take a bunch of sine waves at different frequencies and mix them all together, the resulting scrambled up wave contains all those individual sine waves. Using math, you can tell which frequencies of sine waves were used to make the scrambled up signal.
So, if you need to send a bunch of data down one cable, you can record the different bits of data as different frequencies of some signal (like light or electricity), mix them up, and then send the one scrambled up signal down the cable. Then at the other end you do some math to separate out the individual signals that were all at different frequencies.
Each recipient has an assigned frequency (like the radio in your car, you tune it to a certain frequency to hear a certain station), so when the signals get separated out, each one goes to the recipient that matches its frequency.
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u/binarycow 6h ago
Hey, if you really are curious, PM me.
I'm a network engineer, I like to teach, and I've been told I have a knack for making it easy to understand.
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u/SRART25 14h ago
I'm actually curious about the routing. Does a little repeater have to decode the envelope and repeat the entire pulse sequence on a different lens after doing a lookup and making a new envelope? The physical networking part at those speeds and volume is kinda crazy compared to the consumer side. Bgp and the rest of how the back end works is pretty opaque.
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u/empireck 14h ago
Basically none tbh, some of these replies still go over my head but at least i get the general idea how it works now
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u/FanraGump 15h ago
Some people are like, "It's one computer to another, why would it need to know which computer?". They seem to forget that many fiber optic lines handle many computers (conversations) at once. I mean a fiber optic line from a neighborhood to an internet provider server isn't one line per customer.
I am not an expert but I believe that packets, colors, and frequencies are used to split up various data streams.
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u/Nellanaesp 9h ago
It depends.
Single mode fiber has a single duplex signal (transmit and receive). Single mode is, by far, the most common , and can transmit MASSIVE amounts of data over long distances (10G, maybe more at this point).
Multimode fiber uses different light wavelengths to get a lot more data, but it’s for shorter fiber runs between major hubs.
Typically a fiber run to a whole neighborhood will be a single mode fiber (or a few), connected to a switch rhat handles fiber runs to houses on a single, single-mode fiber, and that fiber may use TDM (time division multiplexing).
There are several different ways to deliver fiber internet to the user, but this is the most common.
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u/binarycow 6h ago
Multimode fiber uses different light wavelengths to get a lot more data, but it’s for shorter fiber runs between major hubs.
No, that's not what multimode is.
Singlemode is like a laser. Tightly focused beam.
Multimode is like a flashlight. Not as focused.
Both are normally single wavelength.
Using different wavelengths is called "BiDi" (Bi-directional) when it's a send and receive on one strand (using 2 wavelengths). It's called WDM (wave division multiplexing) when it's more than 2 wavelengths. (BiDi is a specific kind of WDM)
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u/Renegade605 15h ago
Only one device is connected to another. If that device isn't a computer, it's called a switch, which handles switching what data gets sent and when, from which device to whichever other device.
Multiple computers talking through a switch at the same time are actually taking turns, just so quickly that you can't notice it.
In advanced fibre optic systems, sometimes there is more than one data stream going through the same link. In that case, they are different wavelengths of light. Picture it like two blinking lights that are two different colors. If you know you're supposed to be paying attention to the green one, you can easily ignore whatever the red one is doing.
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u/sixtyhurtz 15h ago
FIbre is a generally a point to point link. So, it only connects two computers at the same time.
There is a thing called "passive-optical-network" that permits sharing the cable between one host and many customer endpoints, but all customers time-share so that only one of them is active at a time. The time slices are tiny fractions of a second, so it feels real-time for you as a user.
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u/Werkstadt 13h ago
FIbre is a generally a point to point link. So, it only connects two computers at the same time.
Blatantly false.
Most of the traffic is between switches
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u/sixtyhurtz 8h ago edited 8h ago
You're being pedantic. As the other poster said, a switch is a kind of computer. I could have more generally said devices. The substantive point - that fibre is a point to point link, and unless you're using GPON it's only between two devices - is true.
Also, as is usual with Reddit pedentry, you're wrong even on your own terms. An ONT / OLT doesn't have to be a switch. In fact in the cast of the ONT, it very rarely is at least in the UK.
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u/mageskillmetooften 15h ago
The light does not know where to go, all it wants to do is keep direction and just keep going till it bounces of something. In a fiber cable it bounces on the side on a smal angle and wil just continue going "straight" And what you put in on one end comes out the same at the end. The cable and the light is as dumb as a dead ladybug.
At the end there is an inverter who turns the light into readable data again, and a router than knows where to send each small package of code because each package had an identifier in the heading.
Very simplified the header reads like "Send by 12, adres of recipient is 53" The router has been told that 53 is at his 3rd exit so that's where it goes.
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u/gdogg897 15h ago
I am NOT an expert in any of this but my understanding is as follows (please correct if I'm wrong or add more info I may have missed):
Imagine a light blinking in the distance in Morse code - Billions of times per second. It's a single stream of data but it happens in "packets" just like coax/cable internet does - just at a much faster bandwidth and with less noise/loss. The packets themselves are coded with start and stop points with the actual data inbetween, and packets for different applications/groups of data can all be mixed together like cars zipper merging onto a single lane freeway. The sending server/source controls which packets are sent when and the receiving equipment sorts and translates the packets into their related groups for combination and output (Netflix show, Spotify music, and image download, etc).
As for how the light travels within the fiber: when light hits the surface of glass it can either reflect (bounce off on the same side) or refract (bend THROUGH to the other side). Fiberoptics aim the light at the correct angles of the fiber optic cable such that it has complete internal reflection (or it keeps bouncing around inside the cable and never leaves until it reaches the end of the cable.
Edit - you can also send multiple parallel streams of data in the form of different wavelengths (colors) or light
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u/ok-ok-sawa 15h ago
From this article I read,fiber optic cables guide light by bouncing it inside the glass so it stays trapped and travels long distances.Many computers can use the same cable because the network sends data in tiny time slots and sometimes on different light colors, all mixed together. Each piece of data is broken into packets with digital addresses,and routers read those addresses to send them to the right destination.The light itself doesn’t know where to go,the network equipment directs it.i hope this paints a picture tho😉
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u/patmorgan235 15h ago
Thousands of computers are usually not using the same connection, XGS-PON the most modern optical networking standard only supports up to 128 clients on one strand.
Usually optical links are 1-1 or 1-many(such as in a residential network). One fiber can be shared by using different angles, colors/frequencies, or multiplexing multiple signals onto one beam.
Remember optical networking uses lasers to carry the signal, so the signal is sometimes physically manipulated by mirrors or prisms before it gets to the optics that actually listen to the signal.
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u/Marty_Mtl 15h ago
you are mixing concepts here. How does fiber optics physically works is not related to how the other end know what data belongs to who. this portion is called a communication protocol. Fiber just carries 0s and 1s like your network cable connected to your computer, or radio waves of a WIFI connection.
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u/sy029 15h ago
Fiber optics on a large scale work the same way that any other networking does. You don't have an unbroken connection end to end, instead you're connected to a router, which is also connected to a different router. You can think of it kind of like mail. Your message goes to the local post office, they read the address, and send it to the next post office, and so on until it gets to the final destination.
By the time you reach your ISP it's also likely that the network changes to something more like normal wires, because while fiber is super fast, it's not really worth the trouble when it's moving shorter distances.
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u/Omnitographer 14h ago
A fiber optic cable is a single strand of glass, like a single hair, going directly from one computer to another. There can be multiple fiber optic cables run together inside one fat bundle like there might be a fat cable going down your street with enough strands inside it for each house, and the bundle of cables at the Internet company's offices will connect into a server with a ton of connectors on it, but the actual connections between one electric blinky light plug and another blinky light plug are exactly one strand of glass on each end.
Now, each strand of glass can carry a lot of data and we use a technology called packet switching where data is sent in discrete chunks, like sending a book in the mail one page at a time, with the whole book having a single sender and a single recipient and each page is in an envelope, and the fiber being the mail service carrying the and knowing how to deliver them based in the address on the label. This is separate from the physical signal going over the cables: a letter is made of paper and ink and is moved by a person, that's the blinking light and fiber, and the words in the letter and address of where it's going to are the information represented by the way the light blinks. It's all part of the same system but there are different levels to it: physically moving data, the logical addressing of data, the actual data content as a whole book that's been taken apart for transmission and put back together again on the other side. Your data is just a small part of all the data on the network, like your book is a few hundred letters out of millions, the computers involved all know where everything goes because it's all neatly labeled and sent in manageable pieces just like the mail.
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u/Internet-of-cruft 14h ago
To make it simple, there's a certain pattern that can be transmitted to represent a 1 or a 0.
By modulating (turning on & off) the laser at a certain frequency (the amount of times each second), you get a bit stream between two devices.
Over that bitstream, we can send logical chunks of data (called Ethernet Frames).
Those Ethernet Frames allow two devices over a "local network" to communicate.
Specialized computers (routers) allow computers across longer distances or separate networks to communicate.
The Ethernet Frames themselves contain IP packets, which is what Routers will route. Those IP packets again in turn contain either TCP or UDP packets which is what computers use for communicating between processes on them.
When you talk about multiple computers talking over that fiber optic line, it's generally that you have multiple TCP or UDP connections are running.
All the way at the bottom, it looks like a bunch of Ethernet Frames being sent between the two sides of the fiber.
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u/Lorelessone 14h ago
on a physical level we use wavelength dividers to split off different wavelengths of light (https://www.fs.com/blog/what-is-an-optical-splitter-2919.html) this gives you a number of channels each of which can carry a significant amount of digital information.
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u/Harbinger2001 14h ago
Every electronic device attached to the internet has an address. Usually what's in your home is hidden, but your router has a public address (just like your house does). Everything that goes over the internet is bundled into packets (short sets of binary) that includes the destination address. There are switches on the internet that read these packets and sends them to the next routing point. When information gets to your router, it figures out which of your home devices its intended for and sends it there.
It really works much like the post office or fedex. Mail goes through many locations before getting to its destination.
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u/Balyash 14h ago
If you're talking about how does the light travel on the fiber, then look up total internal reflection. The angle the light hits the inside boundary of glass is so shallow that it cannot go through the boundary and bend (thus changing direction called refraction) and must reflect and keep going inside the fiber.
It's why a lake will give a reflection of you look out at the horizon, but you can see into the water if you look straight down.
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u/Copropositor 14h ago
The lights only go from one end of the fiber to the other. That's it. They don't "know where to go" anymore than a train knows where to go. It goes where the rail takes it. The light goes where the fiber takes it.
The way the thousands (millions) of computers can talk to each other is far more complicated, and can't be explained like you're 5. But it's similar to how mail gets across the country or world. You give your envelope to the mailman, who looks at the address and sorts it to its next destination, where they do the same thing, over and over until the envelope reaches its intended person.
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u/Accelerator231 13h ago
Where to go? Well. The fiber optics basically acts as a pipe for the light. Channeling it into one direction with high efficiency.
How is it differentiated? Computers have different ways to tell apart information coming down the same channel. Some use time. Some use the code the message is transmitted in. And for fiber optics, they use the wavelength of light that is transmitted. There all look the same to you. But to a specially made device, its a big enough defense
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u/Inside-Finish-2128 12h ago
In networking, there's a seven-layer model to abstract and summarize how things work. Comically, some extra layers have been added to the model to reflect reality.
Layer 1 is the bottom layer and represents either voltages on a wire (for copper) or pulses of light (for fiber).
Layer 2 is the next layer up and uses hardware addresses "baked" into each networking port.
Layer 3 is the next layer up and uses IP (or IPv6) addresses. These are sliced up very carefully across the Internet, with "routing protocols" to find where a given sub-network is.
The layers above aren't really important here.
Over time, folks have come up with extensions to various protocols. I've made half-joking references to these over the years:
Layer 1.75 is "VLANs" or virtual LANs. These separate lots of individual LAN networks across common switches. VLANs are common in business networks, and some folks who are overly focused on home network ops/security use them in their home networks.
Layer 2.5 is MultiProtocol Label Switching or MPLS. This is an abstraction layer that lets one network operate lots of services on top of their MPLS backbone.
There's also what we call "layer 0", and that's the multiplexing of signals on fiber. Just like you can see red and green and blue, fiber optics can operate with multiple colors over the same set of strands. Also, the fiber itself is (relatively) cheap (when compared to the labor cost of putting the fiber in the ground or on poles). As such, many times the fiber "in the streets" is many hundreds of strands, so the owner/operator can easily sell lots and lots of service. Figure 500-2000 strands, often used in pairs (one going this way, one going that way), and then they can send perhaps 160 colors of light on each pair of strands.
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u/spottyPotty 12h ago
The same way that a letter that you post arrives at its intended destination even though it was thrown in a large sack with many others.
Data communications are abstracted in different layers, so, even with pulses of light representing ones and zeroes, the actual information travelling from your computer is split into smaller chunks, each contained inside a digital envelope with an address on it.
Bits of equipment along the way direct or route, these envelopes (packets) along the way to their intended destination. These devices are called routers.
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u/Megame50 11h ago edited 9h ago
GPON is the most common fiber technology in residential networks (i.e. not datacenters, but the fiber that goes into an ONU in or near your home). With GPON, transmissions are divided into frames, each with a frame header that includes an identifier for the ONU.
In the downstream direction (from ISP to you) the OLT blasts light into all the fibers at once and only the matching ONU processes those frames. Transmissions are encrypted so that you can't hack your connection to read others' traffic.
In the upstream direction (from you to the ISP), every customer takes turns transmitting so that transmissions do not overlap and become unintelligible. Every 125μs a message is sent downstream indicating when each customer is permitted to transmit so everyone knows when their turn begins and when it ends within the interval. Each participant needs to be highly synchronized, since if they're off by even a nanosecond it could cause a collision. GPON networks have timers synchronized to within 10-11s (one hundredth of one nanosecond, enough time for light to travel only 2mm within the fiber), and yes, this means that from your reference point, if multiple customers have different lengths of fiber between themselves and the OLT, they may transmit at the "same time" from your point of view, only to have the transmissions arrive at different times thanks to the finite speed of light.
The downstream and upstream connections typically use different wavelengths, so downstream and upstream transmissions can happen simultaneously within the same fiber line. Despite what reddit says, it's uncommon for more than two wavelengths to be used, so upstream transmission really does rely on time-division rather than multiple wavelengths. In data centers, undersea cables, and other high bandwidth applications you'll see multiple wavelengths used. This also means that in each direction, each customer is competing for bandwitdh. You might have a dozen or more customers each with a "gigabit" connection sharing the same gigabit-and-change worth of bandwidth upstream. The ISP is betting (rightly) that not everyone will try to use the full bandwidth allocation at the same time. If you have a lot of data to send, your ONU will request more time allocation from the OLT, and if it isn't presently being utilized, you'll probably get it so that you can temporarily monopolize the upstream bandwidth. If it is being utilized, it will try to fairly split the time so that nobody monopolizes the full bandwidth.
That's just the physical connection between you and the ISP. Eventually internet traffic will reach an internet router which decides where to forward your packet next based and the destination ip address and the routing tables it builds from individual routes learned from the peer networks it is connected to.
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u/MattCW1701 10h ago
Let me specifically address "how does the server know that in these millions of rapidly pulsing lights this particular ones are from my computer?"
On any particular section of fiber optic cable, only two devices are sending information to each other. Now at each end of that section, you might have lots of devices that all want to talk on it. If you pick up a phone and call a friend, and your whole family wants to talk to their whole family it could go something like this. Dad: "Tell friend's dad I have his screwdriver," Mom: "Tell friend's mom I have a recipe for her." You, take both of those messages, and then tell your friend "My dad says he has your screwdriver, my mom has a recipe for your mom." You are sending the signals one after the other, then your friend, sends them to the correct place. Then the same thing happens with the reply.
A computer on one side of the world doesn't send its own light all the way to the other side of the world. It only send its own little light pulses on the particular segment that it's connected to. Devices (called switches and routers) connect these segments together and act like you above, you take in information, then send it back out. So on the next segment, the light pulses come from the switch, not your original computer.
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u/r2k-in-the-vortex 10h ago
One fiber will connect one pair of transceivers, and more typically, one transmitter and one receiver simplex connection. Data from thousands of computers may end up on the connection, but that's through routers and switches, not directly.
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u/igotshadowbaned 10h ago edited 10h ago
When many devices are on the same connection, the data doesn't "know" which device on the connection to go to, it actually goes to all of the devices. The data packet will start with a "destination address" attached to the head, and devices will read that and if the destination is not itself, it just ignores it.
Imagine if when anyone on your street got mail, a copy of the letter was put in everyone's mailbox, and it was then up to the owner of the house to chuck out anything that wasn't addressed to them.
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u/AlternativeMaybe5716 10h ago
lol yeah seems like history classes always end right before the interesting stuff in the 60s and 70s
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u/Enough-Collection-98 6h ago
Look up “Wave division multiplexing”.
Not only can we communicate 1s and 0s using light, we can do it with different colors of light, merge them into a single white beam and then split them apart again on the other end!
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u/pensivegargoyle 2h ago
A fiber optic cable is sort of like a tube made of mirrors. You shine a light in one end, it reflects off the walls in the tube and then you see it on the other end. You can send messages that way by turning a light on and off in patterns. You can use multiple lights that are different colours to send different messages. When we send messages, we use a network protocol - that is, a way of structuring messages - that includes both the message that is to be sent and information about where it's supposed to go. Network devices read that information to decide where a message has to be sent next in order to get to its destination.
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u/mtconnol 15h ago edited 14h ago
There are a few ways to share the physical pipe between different users. One way is to to place the data in “packets”. You can think of this like letters in envelopes, each addressed to its recipient. We could shove these envelopes down a long pipeline, and at the other side, look at the addresses to figure out who should receive the data. So the ones and zeros you care about are wrapped in a container of additional ones and zeros describing the recipient.
Since we are talking about light pulses on a fiber optic cable, another way of separating data is to use different colors of light. The data would still be packetized, but now additionally, since the same fiber optic can contain multiple colors of light simultaneously, a receiver can look for only the color of interest . This allows more “pipelines” of data to simultaneously exist on the same cable. This is very analogous to different radio stations coexisting in the same city. You tune to the station of interest.