Hi,
I am currently studying computer networks and I have been reading about how data is transmitted via copper wires and fiber optics.
I understand the very basics of computers, data, logic gates and binary code.
However, currently I am struggling to wrap my head around how data can be transmitted by copper/light? It just does not make sense to me!
Can someone shed light on this?
In: Technology
Copper – electrical impulses that are decoded by the receiver on the other end and turned into a frame.
Fiber – laser/led light going down an extremely thin strand of glass. That light is decoded by the other end into a frame.
Remember that every single object in the universe has electrical properties. Some things have greater values for them, some of them smaller, to the point that we don’t notice them.
* inductance
* capacitance
* voltage
* resistance
We pretty much characterize everything based on those electrical properties. So, cabling systems and hardware are designed in such as way as to send a signal. The systems are designed to work within specific constraints of those properties. Examples of this that are easy to see:
* Limits on cable distance
* Twisted pairs in wiring
* Separation space for communications wiring
* voltage levels of the actual cables
The characteristics of those physical properties I mentioned above are the reasoning for this. Now, lets talk in general about signals. This is generally applicable to digital copper transmission systems (coax networks like cable use frequency modulation, which is different).
The network hardware will sit at some voltage. It varies based on the equipment, but it can often be at 0V. A cable connects two pieces of network hardware electrically. Signals are sent by applying a voltage to that cable relative to the 0V reference. That is just a fancy way of saying they will apply -5V, or +5V, for instance. There will then be some cut-off. The system will treat +/- 3V or greater as a 1 or 0. So, 3.5V is considered a 1. 4.5V is considered a 1, etc. -3.5V is considered a 0. -5V is considered a zero. This gives plenty of dead-band to help limit interference.
Next, there is some kind of language that they will speak at this hardware level. For instance, in electronics, there is often a pin that will either have a voltage applied or removed by one side or the other. When this voltage changes, one side knows that it needs to receive data, because the other informed it that it was about to send data. This is commonly called handshaking. It can also deal, at the hardware level, with data collisions on the line, where everyone waits for it to clear to send data.
Some important notes:
* I left out fiber here. It essentially works the same, but it does so with pulses of light
* It is important to remember that these networks switch VERY fast. Thousands of times a second. That is how you get your one’s and zeros for the binary transmission
* Networks have two main components: An electrical standard and a network standard. The electrical standard specifies things like cable geometry, voltage levels, power levels, pin-outs, etc so that hardware can be produced by many manufactures and won’t blow up. So that it will work together. There are lots of them. You can check some out if you want. RS232, RS485, DH+ (odd-ball early implementation of a vendor-specific electrical standard), ethernet. Those are all electrical standards that specify how the wiring is to be installed. Network standards often involve protocols, which are like languages. Examples of these are: Ethernet/IP (uses ethernet electrical standard, but is an industrial protocol), Modbus RTU (uses modbus protocol on RS485 or RS232 networks), Modbus ASCII (Like RTU but uses ASCII for data encoding), Modbus TCP (uses the ethernet electrical standard, but with modbus RTU protocol), Ethercat (uses ethernet standard, but with very high speeds), Canbus (serial bus-type electrical protocol AND standard used in some industrial drive applications and pretty much every automobile). The list goes on, and as you can see, it is a fucking mess.
Then, we move up a level. This is the protocol. The protocol is the language that they speak. Where the hardware gets the signals, it passes it up a level, where the information is read and actually processed. This is where you store that data you receive and manipulate it. Check it for errors, etc. It is important to remember that sometimes, this protocol level stuff is actually implemented in hardware for specific applications to speed it up. I’m not touch a whole lot on this, because it is very complex.
Now, in summation, the hardware is designed in such a way that it shares common voltage levels and circuitry. Then, voltage levels / light pulses “happen”, which is essentially just the circuit being switched very quickly. These events are then read, recorded, tracked and eventually stored and processed.
Other useful tidbits:
* FET-type transistors make these crazy switching speeds possible.
* OSI-model for data transfer. Covers the hardware on up to the actual application. Each layer is an abstraction on top of the other, with the lower numbers being more “basic” and fundamental.
* Remember to pay attention to install standards – they matter!
It is okay to be intimidated. This is a super-complex topic and even after years and years, no one knows all about all of it. Start by learning about basic Serial RS232 and wire it up, send data, play with it. Once you get an understanding there, you can move on to more involved stuff like ethernet and USB.
In principle, it is just turning the light on and off or the voltage from 0 to a positive voltage. Compare to morse code on a telegraph where you both used wires on land and blinking light between ships. A computer can read and transmit more code.
Morse is in a way a binary signal where a gap is a 0 a dot is a 1 and a dash is 111. It is not efficient for computer-like ASCII that gives you more charachters, is but it can be used. B is ” – . . .” = “111010101” or 9 bits
On a fiber is a laser or a led that just blinks on and off to send the data. On the other side a light-sensitive part, a photodiode if I am not mistaken, is used to convert the light to electricity.
It is not just the raw bits you send but encoded in a way so you always have som bits as zero and some as one. You could, for example, send 10 bits for every 8 bits of data. Look up [8b/10b_encoding](https://en.wikipedia.org/wiki/8b/10b_encoding) You have most of the time a pair of fibers one to send data and one to receive data
You can send multiple streams of data in the same fiber by using different colors of light then multiple lasers are used to send the and the signal optically combines. You also split the signal apart optically, a prism is a simple example of how to split light by color.
The hard part to do a simple receiver is not to convert light to electricity but to know when bist start and stop so you use designs like [Phase-locked_loop](https://en.wikipedia.org/wiki/Phase-locked_loop) to get a clock from the data.
For copper wires, it is a bit more complex. You could just turn the between 0V and 5V (or what is used) like inside a circuit with the bit pattern. The problem is the signal can change because of the wire and the environment especially and high speed for long-distance.
Often you use two wires where then one is high the other is low and vise versa. So you drive one wire with the signal and one with the inverted signal. This is to reduce the effect of external interference, this is called differential signaling.
If you signal like that after you encode bits a bit like the 8b/10 format, have 3 parallel data pairs and apart for the clock you have an HDMI cable. The system only allows for communication in one direction on the wires
For connection like gigabit ethernet, you have multiple voltage levels, two negative, tow positive and a zero. You have 4 pairs of wires and there will be interference between them so you need complex filters on the receiver to interpret the signal.
It also sends and receives data on the same pairs that make it a lot more complex.
Consider that you and a friend are on two hills far enough that your voices cannot carry but within sight of each other. You both have flashlights and know morse code. So by flashing the lights at each other you can send messages (ie data).
Instead of flashlights, assume you have a wire between the hills and one side has a switch and the other a lamp operated by that switch. You can now send the same data by flicking the switches on and off according to morse code.
Now replace the wire by a long internally reflective tube. By flashing a light on one end, the flash appears on the other, so you can send messages again.
This is how copper wire and fibre optics work – just an agreed upon method to interpret the on-off signals. As long as both parties agree what certain sequences mean, then you can send information.
Copper is used to send electrical impulses that get translated into data.
Same.. with light for fibre optic. One end transfers data to a series of impulses and sends them, and then the other side receives them and re transmits them back to their original form (audio, video, file, etc)
This is the entire basis of how computers work, if you dont mind me asking, have you JUST started reading about computer networks and data transmission, or have you just ignored this part of it for a while?
You have the essence of the problem correct. We need to somehow send a signal or message out and have the other end be able to receive it.
Signal on=1
Signal off=0
Next question then is how do we get it there?
Through some sort of channel or medium is the answer of course so for us here we are talking about wires or cables.
With wires or cables we suddenly are hit with the problem of how far can we send some message down the wire before it starts to lose strength and becomes just noise. Attenuates. This would be because of the resistance of the wire or cable.
Let’s talk about how is the signal sent and received. If it’s a fibre cable, fibre optic, then the sending side will have a really fast transmitter that just sends out photons, pulses of light, that represent the bits. Usually a laser or LED will do the job here.
On the receiving side there will be a photo diode (?) that gets that pulse and turns it back into electrical pulses which then are turned into bits.
For copper it’ll be the same concept except the pulses will be whether the electrical signal is on or off.
Fibre optic is much faster because the energy you need to send photons down it is way less than having to shunt electrons and current around. Also the resistance is way less. Photons are massless so you can send a ton of them without heating up the wire. Electrons, not so much.
In both these case though you will also need to have repeaters set up along the cables to boost the signal when is weakens too much. Like for undersea cables it’s every 30kms or so?
Lastly, if we remember that no pulse or signal means bit=0 then how are we meant to know what to do if there is no traffic? No signal? Well this is where protocols come in. Communications protocols. Think of them as being able to understand wake words like “Ok Google” or “Hey Alexa” for when to start/stop expecting a signal.
[fibre optics](https://www.otelco.com/resources/a-guide-to-fiber-optic-internet/)
think of morse code, replace the tone you are now imagining with a flash of light or a voltage being high or low, make machines that create the dots/dashes and machines that decode them back to the original data then connect one of each type of machine to the other type of machine with fiber or copper, connect 1/2 of each pair to a computer.
substitute morse code with binary and this is effectively a serial connection.
shrink the machines on either end as much as possible and add a way to share a single connection for bi-directional traffic through an agreed upon signal or time slice to switch directions on and you have the basis for many other kinds of connections.
at the protocol level it gets a bit more complicated but if there is a separate signal or a clock to indicate which ones and zeroes go together you then have a way to get back to bytes. and everything in a computer is encoded to or from bytes anyway.
Think about morse code. It’s a series of long and short tones / beeps. And, people can known what someone is saying in morse code, by listening to those beeps, and assembling it into a message.
* beep, beeeeeeep = “A”
* beeeeeeep, beep, beep, beep = “B”
* etc…
Networks work in the same way. But instead of using sound, they use electricity (copper) and light (fiber optic).
And, instead of beep / beeeeeeep, they use (on / off). But, they still rely on a pattern on / off / on / off messages coming in. And, instead of morse code and English, they use a computer code and language.
Both are very simple, but very different.
For copper wire transmission, you actually have two wires: the data wire and ground. The two together make a circuit.
Imagine you and a friend took two copper wires and stretched them across the street, then attached one end of each wire to a battery and the other to a light bulb. The person with the battery can connect and disconnect the wire, and the bulb will switch on and off. If the person with the battery connects and disconnects at a fixed rate (eg 10 seconds), the person at the other end could take a stopwatch and write down the state of the bulb every 10 seconds… voila, you’re transmitting binary. Now imagine you’re doing it a million times a second.
That’s fundamentally what our networking equipment does: one end turns the circuit on and off very very quickly, and the other measures the state by testing for an electrical signal. On or off, 1 or 0.
Fiber optic works in much the same way, but instead of sending an electrical signal over a wire, we literally shine a light down a long, flexible, internally reflective tube. Like a fiber optic christmas tree or [these lights](https://i.imgur.com/5YscgHG.png)
It works in much the same way – the sender turns a light on and off very fast, and the receiver has a light sensor that detects the presence of the light.
Wire is cheap but the electrical signal degrades and we can only send a simple on/off signal.
Optical fiber is more expensive but the signal doesn’t degrade anywhere near as easily, and we can “multiplex” signals by sending different wavelengths of light: eg we can send 3 signals via red, blue, and green light, and use a sensor tuned to those colours to receive three signals instead of one.
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