Instructor: Wireless frequencies.
We've talked about the basics of wireless.
Now it's time for us to dig a little bit deeper
into the specific frequencies
that are being used in this wireless spectrum.
First, I want to talk about spread spectrum
wireless transmission.
There are three main ways that we can do this.
The first is DSSS,
or direct sequence spread spectrum.
The next is FHSS,
or frequency hopping spread spectrum.
And the third is OFDM,
or orthogonal frequency division multiplexing.
Now in today's networks
we don't rely as much on frequency hopping.
Instead, we like to use direct sequence
or orthogonal frequency division.
Now we're going to talk about this
as we go through each of the next parts of this lesson.
First, we have DSSS or direct sequence spread spectrum.
This is going to modulate your data
over the entire range of frequencies
using a series of signals which are called chips.
Now, these chips are more susceptible
to electrical interference and environmental interference,
and that's going to cause us to have slower bandwidth.
For this reason, we don't use it very often.
Also, it's going to use the entire frequency of the spectrum
to transmit signal.
This is very suboptimal for us.
So for example,
if I'm using channel 1 or channel 6 or channel 11,
you could see here on the screen
that I have large portions
of that frequency band being used.
Now to have no overlapping channels
and prevented interference,
I have to use channels 1, 6 and 11.
But that means I'm giving up all the other channels;
2, 3, 4, 5, 7, 8, 9, and 10.
You can see this is a ton of wasted space here
because we're using DSSS.
On the other hand, FHSS
or frequency hopping spread spectrum
is going to allow devices
to hop between predetermined frequencies.
Now, this makes it harder to guess
where the frequency actually is,
depending on the algorithm
that's being used by your protocol.
Now, frequency hopping is used
as a security measure in some networks.
But in most commercial grade wireless networks
we're not going to be using it
because it slows down our ability to use all the bandwidth
and reduces the amount of spectrum
you have available to use for bandwidth.
And so this is going to start slowing down your network,
although it does increase security.
So using it is a trade off
if you decide to use it.
Our next and our most common one that we use nowadays
is known as OFDM,
Orthogonal Frequency Division Multiplexing.
Now, OFDM is going to use a slow modulation rate
with simultaneous transmissions
over 52 different data streams.
By doing this with these small chunks,
we're able to actually take
a larger piece of the spectrum
and give us more bandwidth.
Now, this gives us higher data rates
while at the same time resisting interference,
because these data streams are small little chunks.
Now, if we compare OFDM
that's used by wireless G and wireless N,
we can see how these differ.
When we use it with wireless G,
we're going to be using it with the 22 MHz spectrum.
And these chunks are going to take place
on channels 1, 6 and 11.
Now, if I move into wireless N
in the 5 GHz spectrum,
we're now going to have a 40 MHz chunk.
That's going to give us the ability
and additional bandwidth
to increase our speeds in wireless N
and follow on protocols
like wireless AC and wireless AX.
Now, before we go further,
I do want to point out that for the exam,
you do not need to go in depth in DSSS,
frequency hopping or orthogonal division.
Instead, you really just need to know these three terms.
And when you see them,
they're referring to something
in the wireless networking world.
If you know that you'll be able to pick out
the right answer on test day.
Just recognizing those three terms
are relating to wireless networking
is really as in depth as you need
for this particular exam.
Next, let's talk about frequencies and channels.
Now, we've already touched a little bit on this
as I started talking about 2.4 GHz and 5 GHz.
These are two different spectrums
that are used by wireless networks today.
The 2.4 GHz band actually isn't 2.4 GHz,
it's 2.4 and 2.5 GHz.
But for the exam
and anything else you see in real life,
people are just going to say 2.4 GHz and that's sufficient.
Now the same thing holds true with 5 GHz.
Technically it's 5.75 to 5.875 GHz,
but everyone just calls it 5 GHz.
And for the exam that's what they'll call it as well.
So each band here between 2.5 GHz and 5 GHz
has specific frequencies and channels
that are going to be used.
And this helps us to avoid overlapping with other signals
and causing interference.
Now, when I talk about a channel,
I'm really talking about something that's analogous
to a physical medium.
Now, when we think about a channel
it's essentially how we're going to transmit information
over our wireless networks.
Think about it like a virtual pipe.
It's very much like the physical cables
we use in our wired networks.
But instead of a physical copper or fiber cable,
we're using a portion of the wireless frequency
that exists to create these channels
and center data over these virtual pipes over the airwaves.
Now, depending on which frequency band you're using,
you're going to have more or less channels available.
When we deal with the 2.4 GHz spectrum,
there are 11 channels or 14 channels.
Now, the reason there's a difference
is because of regulation.
Depending on where you are in the world,
you'll either have access to 11 channels or 14 channels.
All wireless frequencies are regulated
by the country that you're operating in.
So if you live in the United States,
you can only use 11 channels
within the 2.4 GHz spectrum.
This goes from 2401 MHz up to 2473 MHz.
Now, if you're operating
in the rest of the world except Japan,
you can operate from 2401 MHz just like the United States,
up to 2483 MHz.
If you're operating in Japan,
you can go all the way up to 2495 MHz.
So this means in the US we only have 11 channels.
The rest of the world gets 13 channels
and Japan has 14 channels.
Now, each of these channels is only around 22 MHz wide
within the 2.4 GHz spectrum.
This is going to limit the amount of data
that we could send at any given time.
The other problem we have for these channels
is that they actually overlap a lot
because we only have 72 MHz of total frequency
inside the 2.4 GHz spectrum
that's been allocated to us by the FCC
and other regulatory authorities
within our 80211 wireless standards.
So if you're dealing with 2.4 GHz for instance,
there are going to be three channels
that you have to memorize
and use these three channels to prevent interference.
These are channels 1, 6 and 11.
Those three channels are truly important
because they are far enough apart from each other
to prevent any kind of interference
by giving you 22 MHz for each of those three channels
and still fitting within the 72 MHz total spectrum provided.
So if you're ever asked
about how to prevent wireless interference,
and somebody asks you what channels you should use,
the answer is always going to be 1, 6 and 11
if we're talking about using wireless B, wireless G
or wireless N within the 2.4 GHz spectrum.
Because of this limitation,
newer wireless networks are going to operate
in the 5 GHz spectrum instead.
In the 5 GHz spectrums,
regulators have given us from 5.725 GHz
all the way up to 5.875 GHz.
This allows us to run
our wireless networks within that range.
Now, if we keep with the 20 MHz wide channels
that we are using with 2.4 GHz,
we are now going to have 24 non overlapping channels.
Which is a huge improvement
over the older 2.4 GHz networks,
which only had the three non overlapping channels
of 1, 6 and 11.
Now, inside of our 5 GHz networks
we can also make wider channels
than just 20 MHz though.
Starting with wireless N networks,
there's an option to perform
what's known as channel bonding.
And this was increased in wireless AC
to allow for 80 MHz channels
and 160 MHz channels too.
So what is channel bonding?
Well, bonding a channel allows you to create a wider channel
by merging neighboring channels into one.
Think about it as if we have these virtual pipes
and we put them all together,
that's going to allow us to push more data through
at the same time.
So instead of only taking up one 20 MHz area
for a single channel,
we can now take two 20 MHz channels
to give us a 40 MHz bonded channel.
Or I can combine eight of these channels
and get 160 MHz channel.
By having this wider channel,
I can push more data across the network at one time
leading to increased speeds and additional bandwidth.
Now, the only challenge with channel bonding
is that it now increases the probability
that you can experience interference.
Because you're now reducing the number
of non overlapping channels,
because you've taken up more of the spectrum
by combining these channels together.
Remember. with 5 GHz networks
we have 24 non overlapping channels of 20 MHz each.
But if I created a bonded channel of 160 MHz,
I just took up the equivalent
of 8 of those 24 non overlapping channels.
This could lead other wireless network devices
near my access point
to start causing interference with my network.
Now for the exam you don't need to memorize
all the different frequencies for the different channels.
Instead, you should be aware
of the standard channel size being 20 MHz
for both 2.4 GHz and 5 GHz networks.
But if you use channel bonding with the 5 GHz network,
you can make them larger.
You can make them two times, four times,
or eight times as wide.
Now when you do that,
you can reach higher network speeds,
but you also risk more interference too.
So it is a balancing act between these.