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So now that we've brainstormed a little bit about different ways that we could

have people share the same network meeting without causing too much

interference with one another. Which is called multiple access, we can

look at our first multiple access technology and that's called Frequency

Division Multiple Acts, or FDMA for short.

We just said what multiple access means. And the way that we go about

accomplishing that is by dividing the different lengths by assigning them

different frequencies. So each link is going to get a different

frequency or really a band of frequencies as we will see, in order to divide them

accordingly. So, really the idea is that, if we're

having some process occurring over time, so let's have this be time, everything

that's happening over time. And have this be frequency, where

assigning each person or each link gets a different frequency in which they're

going to communicate. So Anna, or whoever is receiving Anna's

message, knows what Anna was saying. Because they'll know and they'll see the

message come in at this frequency. And Ben, whoever's receiving Ben's

message will see that come in on Ben's message.

And Charlie, and Dana, accordingly. So the first time that FDMA was used was

in the multiple telegraph. Where we had a wire, which we would see

right here, and then we had different transmitters and receivers.

Let's draw the transmitters in this side, the transmitters could also be over here.

Every person could be a transmitter or receiver depending on what's currently

happening and the receivers. And so, if this is one link, this would

get one frequency band. Say this is Anna, and this is whoever is

receiving Anna's message. And this is Ben, and this is whoever is

receiving Ben's message. This would be on one frequency band, say

it's A. And this is on another one, we'll say

that's B, so now what really is a frequency how do we define that how we

define what it does. Well time you can't really feel and you

can't hear time you can't really perceive time other then the fact that it happens,

and frequency you can actually hear. If you sustain a frequency you get a

pitch so different musical instruments as we said before have different pitches.

So FDMA is really analogous to separating amongst pitches as we said before.

But it's separating them enough so that if you're listening in for this frequency

over here. That you will never be able to hear this

frequency over here because you're only listening at a select band.

And so, the way that we perceive and define what a frequency is, is by this

quantity called Hertz. And, what a Hertz means is it's looking

at a wave, or just some power, some electromagnetic signal.

As we can see right here and this would be one signal and we always draw things

as sinusoids. Because that's the definition that's

really how we define and come about the idea for frequencies in terms of the

sinusoid. And so this is one sinusoid and over

time, because we have time on this axis down here.

We see the level of the wave, over time and then it comes back and eventually it

hits zero again, so it's done one full cycle once it gets back over to here.

So, we define a Hertz, as being one cycle in one second.

So this wave we say is, has a frequency of 1 Hertz, and that's how we would

define it. And then so a frequency of 2 Hertz or 3

Hertz is higher. And that 2 Hertz would mean that we do 2

cycles in 1 second, 3 Hertz would mean that we do 3 cycles in one second.

Like this wave right here we can see it's done, it's come up and down, 1 time, and

then up and down 2 in 1 second. So we say that this wave would be a

frequency of 2 Hertz and so on and so forth.

Now the frequency bands that we look at in terms of communications are very much

higher than 1 Hertz or 2 Hertz. And very much higher then anything you

could ever hear. If your in the kHz range the human, the,

the human ear has a range up to about, somewhere between 12 and 14 kHz.

And kilo means times 10 to the 3, or 1,000.

So, 12 kHz is 12,000 hertz, which means that this cycle would have to repeat up

and down, up and down, back again, and so on.

12,000 times before we get to one second, very, very fast.

Now, on communications, we're speaking of frequencies that are way, way higher than

even what we can perceive. And instead of being in the kHz range,

we're actually in the MHz range, which is a million Hertz.

And so a frequency of 5 MHz means that the wave repeats 5 million times, in one

second. That it goes up and down, so, you can

think even faster than this will have to be doing 5 million times before it gets

to one second. You can barely ever even think of drawing

that out. So, as we said though, if we look at this

frequency access, it's not. Simply just one frequency that each of

these channels are on. They're actually on a band of

frequencies, so in between here and here, that's one channel.

In between here and here, that's another channel.

In between here and here, that's another channel, and so on.

So if we take this axis and rotate it down, remember, now we're not talking

about time anymore. Whereas up here we're looking at time now

this is frequency on this axis. So just make sure you remember which axis

we're speaking of. Suppose we have between 20 and 120 MHz or

as we've said, it's millions of Hertz. So this would be 120 million Hertz and

this is 20 million Hertz. So if we wanted to put, say, five

channels between 20 and 120 MHz, kind of, here we have four channels.

But suppose we had a fifth one, and we wanted to put five of them within this

range. It's really a simple math problem to

figure out how to divide that up. So at the end over here, you have 120

MHz, and at the beginning you're at a 20. So in between that we have 120 minus 20,

right? And then if we want to put five of those

channels in there, we would divide this by 5 to figure out how quick.

How much each needs to be spaced, and so if we do that division out, we get 20

MHz. So each of the channels would have to be

spaced by 20 MHz in order to put five of them in between.

So this first one's between 20 and 40, second one would be between 40 and 60,

third one between 60 and 80, 80 and 100, 100 and 120.

It fits just like a puzzle. So, just keep in mind that when we speak

of frequency, we're really meaning a frequency band.

That's really the proper term is that one of these conversations would be between

20 and 40 MHz. The other one would be between 40 and 60.

Between 16, 80, 80, and 100 and so on. And actually, technically we need some

space in between them so we can't have them intersecting or else we will have

some interference. So the easiest way to understand FDMA is

to think about you on your car radio. And if you're turning the dial to switch

stations really what you're doing is you're switching, you're changing your

receiver on your car to listen in for a different frequency.

And that's how the radio stations are distinguished is by what frequency

channel they're on. And so if this, if you're on, say, 95.5,

and you've probably heard on a radio station before, they'll say, you're now

listening to 95.5 WPLJ. What they really are saying is that your

receiver is tuned in to listen to the station of 95.5, and we're up in the MHz

range there. So, that's really saying that your car

radio is listening in for 95.5 MHz, and it's tuned in to listen to that station.

And there's another station out here, which if you turned your receiver you

would move up, and then listen in to this next one.

And that dial will switch between the channels for you, and listen in to what

that radio station is currently broadcasting.

And so right here we see one radio station second radio station and a third

radio station.