How Information Travels Wirelessly - Free Educational videos for Students in K-12

How Information Travels Wirelessly - By MITK12Videos

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00:05	All right , So in this video , we're going
00:07	to understand how information travels wirelessly . And what I
00:13	mean by this is that we're gonna understand how devices
00:15	like wifi , wireless internet work , how things like
00:20	radio saying , cars work , and how things like
00:24	cellphones , which definitely use wireless communication , how they
00:28	work . And the reason that we can understand all
00:30	these in one pretty short video is that they all
00:33	use the same principle of communication and they all use
00:36	communication via waves . So waves or something that we're
00:42	all intimately familiar with . We know , you know
00:44	from childhood that if we have say a lake and
00:47	someone throws a rock into this lake , then the
00:51	rock is going to cause ripples to form in the
00:54	lake . And the key point about these ripples ,
00:57	what makes them called waves is that they move ,
01:00	they propagate and because they propagate , they can carry
01:06	information out about the initial disturbance that created them .
01:10	For example , the way that you're listening to my
01:13	voice right now . Is it your speaker ? This
01:16	is a very poor triangle speaker but your speaker is
01:18	moving and your speaker is creating sound waves . And
01:21	what sound wave is is just regions of high and
01:24	low density in the air . These are supposed to
01:26	be air molecules . And so these sound waves reach
01:30	your ear and your ear does some complicated process to
01:35	transcribe that into sound . But again these waves this
01:39	information transmission is possible because the waves propagate now .
01:43	To understand how ah all of these devices work .
01:46	We're gonna have to talk about something called electromagnetic or
01:51	E . M . Waves and the exact nature of
01:55	these waves is too complicated to go into now .
01:57	And it's not important for understanding this . But let's
02:00	just let me just give you some examples of waves
02:01	that you already know about that are in fact E
02:04	. M . Waves . So light for example is
02:07	an electromagnetic wave , microwaves use electromagnetic waves to heat
02:12	up food X rays whenever you go to the doctor
02:15	and you get an X ray image taken the waves
02:17	that form that image are x rays . And lastly
02:21	of course radio waves are another special type of electromagnetic
02:26	waves . So how do we use waves to transmit
02:31	information ? That's what we're going to try to understand
02:33	now and before we do that , we need to
02:36	understand two characteristics of ways . Two ways that we
02:40	can quantify what makes one wave different from another .
02:43	So if I consider this wave that I have here
02:48	, there are two things that I can ask about
02:50	it . One I can ask , you know how
02:53	large is this wave and to answer such a question
02:58	, what I might do is I might simply measure
03:01	, you know , the difference between the maximum value
03:04	and the central value of the equilibrium point and that
03:07	is something that we call the amplitude in wave terminology
03:13	. So let's say that the amplitude is a .
03:15	So this distance is a and obviously this distance is
03:18	there and the difference between the peak and the trough
03:24	called the peak to peak , distance is equal to
03:29	obviously two times the amplitude or to a . So
03:31	amplitude is a very easy , very intuitive concept about
03:35	you know , how to characterize how big a wave
03:36	is . So the other concept that's important is called
03:40	frequency . And frequency is just the answer to the
03:43	question , how fast to cycles of the wave go
03:46	by . So by a cycle . I mean a
03:50	region like this , of a wave , the basic
03:52	unit of the wave that repeats each one of these
03:55	is a cycle . And basically frequency is equal to
04:00	the number of cycles that occur in a unit time
04:04	, let's say a second . So a wave that
04:08	looks like this has a much higher frequency than away
04:12	that looks like this even though they have the same
04:16	amplitude . So now that we understand what frequency and
04:20	amplitude means , we can use these concepts to transmit
04:23	information in the first way that I'm going to tell
04:25	you about how to do this is called amplitude modulation
04:28	. And this is what the A . M .
04:30	Stands for in AM radio . So the basic idea
04:33	is best seen by this example . So say I
04:36	have some signal . Maybe it's a sound signal or
04:38	something else . So I have some signal that I
04:40	want to send you and that signal looks like this
04:44	. So how do I send you the signal ?
04:45	Well , just for convenience , you'll see why I
04:47	do this in a second . But I want to
04:49	draw the negative of the signal . So this is
04:52	a signal that I want to send you and I
04:54	want to send it to you via an electromagnetic wave
04:56	. So what I do is I send you a
04:58	wave of particular frequency and I tell you to look
05:01	for waves of that frequency . What I'm gonna do
05:04	is I'm gonna send you this wave where I change
05:06	the wave based on the signal that I want to
05:08	send you . So whatever the signal is low ,
05:11	I make the amplitude of my way of loan where
05:13	the signals high make the amplitude of my wave .
05:15	Hi so here we have two distinct pieces . This
05:19	is the signal and this wave that I'm sending you
05:24	is called the carrier wave because it carries a signal
05:29	. So to decode this , all you have to
05:30	do is you have to look at how big the
05:32	waves at any given point and then you can get
05:35	the signal that I was trying to send you .
05:36	So this scheme is called amplitude modulation and it's the
05:39	one that's used in radio is more stuff . And
05:41	am radio specifically to get you the songs . Do
05:44	you hear say in a car when you're driving and
05:46	you're listening to the radio ? Now there's a m
05:49	there's also FM . And you've probably guessed by now
05:51	that FM means frequency modulation . So frequency modulation is
05:57	another way I can send you information and of course
05:59	we can we can use frequency modulation in the same
06:02	way that I used amplitude modulation to send you some
06:05	like sound file . But what I want to talk
06:07	about now is how to say things like cell phones
06:12	use frequency modulation to send digital information . And what
06:15	I mean by that is that cell phones want to
06:17	send to each other or to wireless towers and communicate
06:20	with them . They want to send information the forms
06:22	of ones and zeros and later they transmit some part
06:26	right on the cellphone , translate those ones and zeros
06:29	into texts or sounds or whatever . Ah But to
06:32	do that , what we can simply do is we
06:33	can agree that if your cell phone and you receive
06:37	a frequency that looks like this then you call that
06:40	a zero . But if you receive a frequency That
06:43	looks like this then you call that a one .
06:46	So if I want to send someone a message 1011
06:52	. All I do should I send them This signal
06:56	in which I changed the frequency so I want to
06:59	send a 1/1 . So here's the one . Then
07:03	I want to send a zero . You know send
07:06	another one . So the low frequency then I want
07:09	to send another one . So to decode this message
07:12	, all you have to do is you have to
07:14	look at different points and you have to see ,
07:17	okay , what's the frequency here ? Is it zero
07:19	or is it a one ? And so this idea
07:21	FM is the basis of how digital trends uh communication
07:25	takes place widely . It's called it's actually called any
07:28	more complex frequency shift . Q . So these two
07:35	concepts that I've outlined for you guys FM and am
07:39	constitute the majority of how information is traveled by electromagnetic
07:43	waves between wireless devices . Yeah .

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