How to explain to a 10-year-old how information is transmitted through the air? Contains a lot of network knowledge!

How do you explain to a ten - year-old how information passes through thin air （WiFi、 data network 、3G etc. ） Delivered ？

Let's not talk about ten year olds , How would you explain this to an educated adult ？

When you send text messages to your friends on the other side of the ocean and they can receive messages immediately , How many of us really understand what happened ！

When you press... On your favorite messaging app “ send out ” when , Where on earth did your message go ？

You would say “ Fly into the air , Like radio waves ……”.

When you press send on wechat , In fact, your smart phone sends instructions to your mobile processor through the thin copper wire on the printed circuit board . These commands are essentially electrical pulses , An electric pulse is an electron that flows along a potential difference . How exactly does your information flow from the electrons in the copper wire “ jump ” Into the thin air ？

Imagine , You have a piece that you can touch and feel （ Even smell ） The circuit board , Then suddenly you have radio waves in the invisible part of the spectrum , Fly to the thin air at the speed of light .

iPhone 6 Plus Wi-Fi The antenna

This is an antenna , Translated in Latin as “ Pole ”. Simply speaking , An antenna is a metal device , It can absorb electrical pulses and emit radio waves . It is essentially a static existence , But it can spread several kilometers away .

say concretely , As shown in the picture Wi-Fi The antenna , With 2.4 GHz（ Per second 20 100 million times ！） The frequency emission length of is 12.5 Centimeter wave . Different from visible light , These waves can pass through walls , Even bend at the corner ！

How do antennas generate radio waves ？

I wonder if you know anything about the buzz of gravitational waves , They are basically waves of the gravitational field , Spread in the form of radiant energy , Einstein's general theory of relativity Their existence was predicted a hundred years ago , And we didn't find them until recently .

Like gravitational waves , Electromagnetic waves are electromagnetic waves (EM) Field fluctuation , Spread in the form of radiant energy . and , It's no different from gravitational waves , Maxwell's equations predicted the existence of invisible electromagnetic waves long before Heinrich Hertz's discovery ！

therefore , To generate radio waves , You need to generate waves in the electromagnetic field . To generate these fluctuations , You need electrons to move through a conductor ！

The behavior of radio wave synthesis is the rhythmic beating of carefully choreographed electrons on small copper wires . Just like the disturbance in still water radiates outward from the disturbing point , The flow of electrons in the antenna will cause the disturbance of electromagnetic field , Electromagnetic fields radiate into space like electromagnetic waves .

When you press... On your favorite messaging application “ send out ” when , Your mobile operating system triggers a series of events , Finally, the information is encoded into an elaborate electronic dance music . This dance produces rhythmic ebb and flow of tides in the electromagnetic field of the surrounding space , And radiate outward to the mobile signal tower .

The receiving antenna on the signal tower feels these tides on its conductive surface , This leads to an electronic dance very similar to the electronic dance on the transmitter . This electronic dance is also an electrical pulse in a group of thin copper wires , Decoded by the hardware of the signal tower . The decoded information is then transmitted for thousands of miles over high-throughput cables , Across countries 、 Continents and even oceans （ By transatlantic communication cable ） Or some other submarine communication cable ) Go to a cell phone tower near your friend on the other side of the ocean . From the tower to your friend's phone is another wireless jump . Last , Your friend hears a familiar voice .

Ask and answer quickly

1、 How does a cell phone know where the cell phone tower is ？

No, , There's no need ！ Your mobile phone will broadcast your message to all directions , For anyone to listen to . But don't worry , Your message will be encrypted , Only the cell phone tower can decode your message .

2、 How does the cell phone signal tower know that the message is sent by me ？

For every message you transmit , It also contains a code that uniquely identifies your device , That's how the cell phone tower knows it's you .

3、 How cell phone towers distinguish messages from different mobile phones ？

These calls follow a protocol , At different times 、 Different frequencies 、 Send messages to the tower from different locations or using different codes .

My best metaphor for this is the classroom , Imagine a person who has 100 A classroom with students and only one teacher . Now? , If all the students start talking at the same time , The teacher will not understand any of them . therefore , They agreed on an agreement . If students want to speak , They raised their hands and waited for the teacher's advice . Unless the teacher asks the students to speak , Otherwise the students won't speak . If multiple students raise their hands at the same time , The teacher will pick them out one by one , In this way, only one student is talking at any time .

Mobile tower is a teacher , The equipment is student . Although students can speak at different times to avoid interference , But the device can be at different times 、 Speak on different frequencies or different codes to avoid interference .

Interference is a basic challenge in wireless communication ！

4、 therefore , You're telling me that most communications are not really wireless ？

Yes ！ Unless you use a satellite phone , Wireless communication is only used for two steps along the road ：

 The sender -> tower 1

 tower 2-> The receiving party 

The communication between signal towers is carried out through underground ultra-high speed communication cables , If it weren't for the multi billion dollar telecommunications infrastructure on the back end , There will be no wireless ！

5、 however , Cable in 21 Isn't the century very out of date ？ Why not always use wireless ？

The cable we are talking about here （ For example, transatlantic communication cable ) It is absolutely high-tech . They can transmit a number of times per second TB The data of , And will not face interference problems like wireless devices .

Besides , If you want to use the wireless network all the time , You will need a lot of satellites to meet the needs of billions of users and their data . and , Launching a satellite is really expensive .

6、 How does the signal know which tower is closest to my friend ？

Each cell tower tracks all equipment currently in its coverage area . Again , Every device will track the nearest cell tower . therefore , Each device has a cell tower related “ Address ”.

When your message reaches the nearest cell phone tower , The signal tower will look for the receiver “ Address ” And send the message to the corresponding signal tower .