Fiber optics

Ever wonder how fiber optic cables work?

I have. I was here for the days of the dial-up modem, and boy, have things changed since then. They’ll be rolling out fiber in my neighborhood later this year, so I was thinking about it again today.

A digital communication primer

Before we start talking about copper and fiber, we need to have a short discussion about digital communication in general.

Digital communication means transporting bits (zeros and ones) from one digital device to another. This is different from analog communication, where you don’t transport bits, but continuous signals. Old-fashioned radios are analog: they translate sound waves to electrical waves, which are then broadcast using radio waves.

This distinction between digital and analog also applies to storage. A computer hard disk stores bits, while a vinyl record stores analog signals.

When we talk about modems, DSL, cable, and optic fiber, we’re talking about digital communication. Because it’s used in computer chips, which are also digital. Which means all these solutions transport bits from A to B, not wave forms.

The simplest way to transport bits, is to have a signal that distinguishes between two states: on (1) and off (0). That’s how a CPU works. It puts a current through a gazillion transistors, and the current is either high, meaning 1, or low, meaning 0. Of course, a transistor takes time to go from 0 to 1, so you also need a clock to synchronize them. A modern CPU has some 2 or 3 billion clock ticks per second (2 or 3 Gigahertz). Which is why your computer can do so much.


When you start sending bits through a modem, or DSL, copper or fiber, you could use ‘on’ and ‘off’, but the bit rate transported would be very low. You want to modulate that signal to pack more bits into your signal. A signal is a wave, meaning it has an amplitude (how high are the wave peaks), a frequency (how often are there peaks and valleys), and a phase (where in time are the peaks and valleys).

What you can do, is divide the amplitude, frequency and phase into discrete partitions, and use those to encode data. If you create 4 frequency steps, 4 amplitude steps, and 4 phases steps, you can encode 4x4x4=64 different values (6 bits). That’s 6 bits instead of 1, meaning 6 times the speed of a simple on/off signal.

Phone modems did exactly this, with increasing numbers of partitions, meaning more bits per second could be transported as modems got better. They sent the signals over normal phone lines. At the bottom range is the first modem, a 110 bits/second Bell 101. At the other end of the spectrum is the V.92 modem with 56.000 bits/second. The resulting modulated signal, when decoded back to sound by a phone causes that well-known modem sound from the nineties.


Okay, so there’s still more to tell before we get to fiber. So far, I’ve talked about transporting data from A to B. In practice, it gets more complicated. Because we don’t have a dedicated line going from A to B. We have a shared network, and we need to send multiple simultaneous signals. This goes for traditional phone lines, fiber optic cables, but also the air waves mobile phones use.

All these solutions require a way to mix signals. And luckily, that exists. In the previous paragraph I’ve talked about using a single signal and modulating it to allow more bits. You can use all the partitions you create for one signal, but what you can also do is use modulation to encode several signals into one. Most techniques are variants of TDM, FDM, and WDM.

Time Division Multiplexing (TDM) divides a single signal into time slices, and each connection its one time slot. So imagine I have 8 connections I need to maintain. I divide my signal up into 8 time slices, and each connection gets a slice. When each connection has received a slice, you start over, round-robin style.

Frequency Division Multiplexing (FDM) does the same as Time Division Multiplexing, but with the frequency. Frequency Division has been around for a long time: old-style radios also had (and have) a certain frequency band that you can tune into. This can also be used in cables to run multiple connections over a single line.

And with Wave Division Multiplexing (WDM) we get to an actual fiber thing. WDM uses light of various wave lengths to multiplex a signal. It works the same as those above, but uses different wave lengths (colors) of light to achieve it.

Fiber (finally)

I started with old phone lines, which used dial-up modems to send signals. Dial-up was succeeded by DSL connections. DSL (Digitial Subscriber Line) still uses the copper phone lines of yore, but digitizes both the data and phone calls over them. You have Asynchronous DSL (ADSL), High bitrate DSL (HSDL) and Very high bitrate DSL (VDSL), but they’re all basically the same. They send stuff over a copper cable. They’re also asynchronous, meaning the rate to the consumer (download) is higher than the rate away from the consumer (upload). This is useful because we usually download more than we upload (Netflix, game downloads, web pages, etc.). Of course, work-from-home video conferencing has changed the game recently.

The problem is, electricity going through copper loses quite a lot of energy through heat. Luckily, electricity isn’t the only thing that we can use. Light is also a wave. Fiberglass cables use light instead of electrons, and light loses less energy in transport.

When light travels through different materials, it has a different speed, and as a consequence it refracts when changing mediums. If the angle at which light hits such a barrier is within certain ranges, it bounces back instead of going through. That allows light to travels through a fiberglass cable and not escape, even when the cable isn’t straight.

Light can be modulated up the wazoo, and loses less energy to heat, meaning we can send a huge number of bits through. Even better, we can send light both ways through the cable, meaning it can transport bits two ways at the same speeds.

And that’s what we do, these days. With a fiber connection, you can send gigabits of data to and from consumers, where a copper VDSL connection can barely do a 10th of that.


Well, that’s it, fiber modulates light instead of electricity, and can transport a lot more data than our old copper-electron setup.

I hope I get it soon, making my Steam downloads even faster.

Martin Stellinga Written by:

I'm a science fiction and fantasy writer from the Netherlands