- [Instructor] As we delve into wireless technologies,
we need to have a basic understanding first,
about how that communication happens.
With wired networks,
this is maybe a bit easier to wrap your mind around.
We have some sort of physical medium,
maybe that's a copper wire or fiber optic cabling.
And we know that there is an actual electrical signal
being sent across that medium.
And that signal represents our data in transit.
With wireless communication, we have a similar
but slightly different principle occurring.
For a wireless communication to take place,
we need both a transmitter and a receiver.
Here, in this case,
you see that we have a wireless router as a transmitter
and a laptop as an example of a receiver.
That could be any device.
Could be a mobile phone, could be a wireless repeater,
any number of things could be receivers.
Both devices are capable of sending and receiving
an electrical data signal just as a wired device would.
Now normally if this were let's say,
we had a wired switch instead of a wireless router here,
the switch would interpret and forward
the electrical signal using a wired path.
But in this case the wireless sender or transmitter,
once it receives that electrical signal,
it's going to send that to an attached antenna.
And this essentially works as a physical medium.
Similar to a copper Ethernet cable, let's say.
The difference is that,
the electrical current that's being sent to the antenna,
is propagated out by the antenna over the open air,
using electromagnetic waves.
And electromagnetic waves consist of two components.
They have an electrical field,
and they have a magnetic field.
These fields that are represented here,
those fields will oscillate or in other words,
they'll vibrate at right angles to each other.
And that's going to push the wave along,
at right angles to these fields,
in what's called a transverse wave.
So that means that they travel and expand,
ideally in all directions away from the antenna,
in a spherical shape.
Electromagnetic waves do not need a medium of any sort,
to help transmit.
They don't need a gas, liquid or solids.
And that makes them really ideally suited
for wireless communication.
When we talk about,
the oscillation or the vibration of an electromagnetic wave,
one of the ways we measure their property,
is called the frequency.
And the frequency is very simply the number of times
the electromagnetic wave oscillates,
in one complete up and down motion,
over a period of one second.
We refer to one complete up and down motion of the wave,
as a cycle.
And so the frequency is measured in cycles per second,
or what we call hertz.
Let's say we have a wave as we see here on the screen,
and you can see the visualization of a wave,
is over the period of one second in this case.
If we lay out the complete up and down cycles,
we can see that there are two complete cycles
that take place during this one second interval.
And that means the frequency would be two cycles per second,
or in other words, two hertz.
With a typical wifi signal,
our electromagnetic waves are oscillating
at an incredible rate.
Take 2.4 gigahertz as an example of that.
If we break that down,
we know there are 1 billion hertz in a gigahertz.
So the 2.4 frequency would be equal to 2.4 billion hertz,
or in other words, 2.4 billion cycles per second.
Lots of things have frequencies.
Not only wifi waves, but also things like sounds and lights.
That includes sounds that are audible
and inaudible to humans.
As well as the visible light
and the light we cannot detect such as ultraviolet light.
The section of the spectrum that we want to focus on here,
is commonly referred to as the radio frequency range,
or RF range.
This is roughly between three kilohertz and 300 gigahertz.
And within that range, we find those devices
that we commonly use for wireless communication,
which are 2.4 and five gigahertz.
You've probably heard this referred to as
the 2.4, or five gigahertz band.
And a band is just a range of frequencies.
So the 2.4 gigahertz band will lie between,
2.4 and 2.4835 gigahertz.
While the five gigahertz band encompasses,
5.15 to 5.85 gigahertz.
These frequency ranges are also subdivided
into what we call channels.
If you've configured a wireless LAN before,
then you've most likely ran across
the channel selection option.
If we look at the 2.4 gigahertz channel band,
you can see that this is broken up
into 14 distinct channels.
Each one of those having its own frequency.
Which we can see visualized here.
Also, there are five megahertz of spacing,
between each channel or 0.005 gigahertz,
if you want to look at that in the same metric.
Now, you may be wondering why the spacing is necessary.
If we have specific frequencies for each channel,
it seems like we're wasting space.
And the reason is, an RF signal isn't a straight line,
when we visualize the frequency.
Although each of these channels has a frequency assigned,
this is what we call the center frequency of the signal.
The signal itself will actually look like an arch,
which you can see visualized here.
The bounds of this arch is what we call the bandwidth.
Which is simply the practical width of the space
that we need in the frequency band,
in order to have communication take place.
Let's say we choose channel six on our wifi router,
as we see visualized here.
This means the center frequency,
you can see that lies at 2.437 gigahertz.
Because of the bandwidth required here,
we see spillover into other channels.
That spans from channel four to channel eight, roughly.
And so this is the reason,
why we typically only use channels one, six and 11,
for our center frequencies within the 2.4 gigahertz band.
Those are the channels
that will give us the separation needed,
so that there's no overlap between the signals.
Although if you're in a really congested area,
maybe a really busy neighborhood,
with lots of wifi routers around,
all the channels might be used.
If your neighbor next door,
has a router running on channel six,
that's within the range of your own wireless devices,
and you try to run your router on channel six,
you're going to have a bad time.
I've actually had this happen before.
Way back in the day
when I was using 2.4 gigahertz primarily.
I had a neighbor that was interfering with my wifi signal.
They were pretty a close neighbor.
And so that's definitely something that you have
to be aware of within the 2.4 gigahertz channel.
Because you're going to get overlapped,
and it's going to cause communication interference
and performance issues.
When we're talking about
the actual strength of the RF signal,
it's important to know that this is measured
in something called decibel milliwatts.
If you've ever used a wifi signal analysis tool,
with a visualization feature,
such as this one that we see here on screen,
you've probably noticed the dBm indicator on the left side.
And that measures decibel milliwatts.
When we measure the RF signal,
we usually do that in watts, such as microwaves.
And there are varying ranges of microwave ovens
that you can buy that output between 600
and 1200 watts.
But with wireless communication, our transmitters will have
a very comparably low signal strength,
that's going to range between 0.001 watts
and 0.1 watts.
Which is the equivalent of one milliwatts to 100 milliwatts.
A milliwatt corresponds to 1/1000 of a watt.
And the decibel milliwatts is relative to this value.
So we use that as just a much more convenient nomenclature.
It's much easier to use a relative dBm value,
than to have the state a large numerical decimal value
in milliwatt values.
As an example of this,
here, we can see how this relationship works.
One milliwatt would correspond to zero decibel milliwatts.
10 milliwatts corresponds to 10 decibel milliwatts,
and so on.
And what we have here played out, is what we call,
the rule of 10s and threes.
We need this rule because as you can see,
this does not scale in a linear fashion.
So we need to know these rules.
So let's elaborate on that a bit.
Basically, what's happening here,
is that for each gain of 10 decibel milliwatts,
the power in milliwatts is multiplied by 10.
Likewise, if we have a loss of 10 decibel milliwatts,
that means that our power in milliwatts is divided by 10.
For every gain of three decibel milliwatts,
the power in milliwatts is doubled.
And likewise the loss of three decibel milliwatts
means that the milliwatt power is cut in half.
It's not super intuitive.
There's lots of complicated math functions
behind the scenes.
But knowing the rule of 10s and threes,
is going to give you a general idea about your RF power.
Also note that the decibel milliwatt values,
are represented as negative numbers oftentime,
in our analysis charts.
With the closer you are to zero being a stronger signal.
And this is what's referred to as,
the received signal strength indicator or RSSI.
This is measured by most wireless vendor receivers,
as a value between -30 decibel milliwatts
to -100 decibel milliwatts.
- 30 would probably only be seen,
if you're just a few feet away from an access point.
While -90 would essentially be unusable.
- 70 decibel milliwatts,
is going to be the minimum signal strength required
in most cases for reliable packet delivery.
While more sensitive applications,
maybe something like voice-over IP,
they may require even better signal strength.
Another important term to understand
when we're looking at signal strength,
is the signal to noise ratio or SNR.
This is essentially the difference in decibels,
between our received signal and any other background noise.
Remember that there are many other items
that share the RF range with our wifi.
And so anything other than our desired signal,
that we are receiving, we refer to that as noise.
The average signal strength of this noise, is what we call,
the noise floor.
So in this example, our noise floor is -90 dBm.
So in other words,
we have very little interference taking place.
And our receiver is obtaining a signal strength
measured at -65 dBm.
So that means our signal to noise ratio, our SNR,
is 25 decibel milliwatts.
Now obviously, the higher our SNR is,
the better it is for us.
A higher SNR value means that we have more space between
the noise floor and our signal.
And therefore we have less chance
of our signal being interfered with in any way.
So that's an overview of wireless communication theory
and many important terms and concepts related
to wireless LAN function.