mercredi 27 novembre 2019

How WiFi and Cell Phones Work - Wireless Communication

How WiFi and Cell Phones Work - Wireless Communication

How many of us really understand what goes on when you text your friend across the ocean
and they get the ping, almost instantaneously! Allow me to entertain you with a little thought experiment. When you press “send” on your favourite messaging app, wheres you message really going?

How WiFi and Cell Phones Work

“Into the air,” you’d say, “like radio waves..”

But let’s back up a bit. When you press send on whatsapp, you’re essentially sending instructions to your mobile processor via tiny copper wires on a printed circuit board in your smartphone. Now, these instructions are essentially electrical impulses, right?

And electrical impulses are just electrons flowing along the potential difference.
How exactly does your message “jump” into thin air from being electron flows in copper
wires? At one stage you had a circuit board that you could touch and feel (and even smell)
and then suddenly you have radio waves in the invisible part of the spectrum, flying away into thin air at light speed.

What’s in between?

What’s in between is this odd looking device It’s an antenna, which translates to a “pole”

in Latin.An antenna is a metal-tongued voodoo device that swallows electrical impulses and spits out radio waves.It is silent as the dead, but its screams can be heard formiles.Specifically, a Wi-Fi antenna like in the picture, screams at . GHz ( billion beats
per second!) and spits out waves of length . centimeters.Unlike visible light, these waves can pass through walls, and even bend around the corners!

How WiFi and Cell Phones Work

How does an antenna produce radio waves?

Before we answer this question, let’s take a small detour.Do you remember the recent buzz in pop-science about gravitational waves?They’re basically fluctuations in gravitational field, propagating as radiant energy.Einstein’s general relativity predicted their existence a hundred years ago, and we’ve only recently discovered them.

Just like gravitational waves, electromagnetic waves are fluctuations in electromagnetic
field, propagating as radiant energy. And, not much unlike the story of gravitational waves, the existence of invisible electromagnetic waves was predicted by Maxwell’s equations well before their discovery by Heinrich Hertz! So, to produce radio waves, you need to create fluctuations in electromagnetic field. And to create those fluctuations, you need electrons moving around in a conductor! The act of radio wave synthesis is a carefully choreographed, rhythmic dance of electrons in tiny copper wires. Like perturbations in still water that radiate outward from the point of disturbance, the electron flows in an antenna cause perturbations in electromagnetic field which radiate out into space likeelectro-magnetic waves.

When you press “send” on your favourite messaging app, your mobile OS sets off a chain
of events that ultimately encode the message as a careful choreography of electron dance.
This dance results in rhythmic ebbs and flows in the electromagnetic field in the surrounding
space, which radiate outward towards a cell tower.The receiving antenna on the cell tower feels these ebbs and flows on its conducting surface,inducing an electron dance very similar to the one at the transmitter.This electron dance is again a set of electric impulses in tiny copper wires, which are decoded by the hardware at the cell tower.

The decoded information is then carried on high-throughput cables for thousands of miles
across countries, continents and even oceans through Transatlantic communications cables
or some other Submarine communications cables, to a cell tower near your friend across the ocean. From the tower to your friend’s phone is another wireless jump.And finally, your friend hears the familiar ping.

How does the phone know where the cell tower is?

It doesn’t, and it doesn’t need to!Your phone broadcasts your message in all directions for anyone to listen to.But don’t worry, your message will be encrypted and only the cell tower can decode your message.How does the cell tower know that the message is from me?
With every message you transmit, you also include a code that uniquely identifies your device.That is how the cell tower knows it’s you.

How does the cell tower differentiate between messages from different phones?

The phones all agree on a protocol to send messages to the tower either at different
times, different frequencies, different locations, or using different codes.The best analogy I have for this is that of a classroom.Imagine a classroom with a  students and just one teacher.Now, if all the students start speaking at once, the teacher cannot understand any of
them.So, they agree on a protocol.If a student wants to speak, they raise their hand and wait for the teacher to point to them.

The student does not speak unless they are asked by the teacher to speak.If multiple students raise their hands at once, the teacher picks them out one by one so that only one student is speaking at any point in time.

The cell tower is the teacher, and the devices are students.While students can speak at different times to avoid interference, devices can speak at different times, different frequencies or different codes to avoid interference.So, you’re telling me that the majority of the communication is not really wireless?

Unless you’re using a Satellite phone, wireless communication is only used for two steps along the road sender to tower-, and tower- to receiver. The communication between the cell towers happens through ultra high speed communication cables underground.

But, aren’t cables so out of fashion for st century?

Why not use wireless all the way? Well, the cables we’re talking about here like the Transatlantic communications cable are definitely very high-tech.They can carry terabytes of data every second, and do not face the problem of interference nearly as much as their wireless counterparts. Also, you’ll need a lot of satellites to cater to billions of users and their data needs if you want to go wireless all the way. And, launching satellites is really really expensive.

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