- [Instructor] Wifi's contention based media.
This means that only one device
is allowed to talk at a time.
Think of wifi communication as talking
to your friend with a walkie-talkie.
Only one of you can push the button and talk at a time.
If two devices attempt to send simultaneously
then traffic will collide.
Wifi has a mechanism called CMSA/CA
which is a contention based collision avoidance system
to help prevent collisions.
Wifi devices will sit, listen and wait
for the radio channel it is connected on
to be quiet before it sends.
The industrial scientific and medical or ISM bands
are set aside for all sorts of devices
like telecom and microwave ovens.
So yes, putting your access point
on top of the microwave is a bad idea.
Wifi uses two frequency ranges,
2.4 gigahertz and five gigahertz.
2.4 was the original frequency used in wifi
and is still the most prevalent.
The 2.4 gigahertz range is divided into 14 channels,
each spaced five megahertz apart.
Channel one starts at 2.412 gigahertz,
channel two is at 2.417 and so on.
Standard wifi devices use a 20 megahertz wide channel,
so in the 2.4 range we have channels one, six and 11
to work with without overlap.
The five gigahertz range
has much more spectrum to work with.
And available channels vary greatly by country.
In the US they range from channel 36,
which is 5.180 gigahertz,
to channel 165, which is 5.825 gigahertz,
giving us 25 usable channels.
Since all of the channels in the five gigahertz range
have at least 20 megahertz of separation
we can use each of them for wifi access.
This gives us 25 non-overlapping channels.
Signal distance is also an important factor
when choosing your frequency.
2.4 gigahertz has a wavelength of 4.92 inches.
This means that from peak to peak
there is a separation of around five inches.
Conversely, five gigahertz radios use a wavelength
of about 2.03 inches.
It's interesting information,
but how is this practical for us?
Well, the larger the wavelength the better the signal
will penetrate objects.
Objects like walls, doors and windows
will attenuate wireless signals.
Attenuation simply means your signal strength is lowered.
The lower the frequency you use
the less attenuation you get when passing through an object.
Therefore, two gigahertz will give you
better signal strength through walls than five gigahertz.
This means when planning for wireless deployment
we need to think about access point placement.
If you go straight through a wall at a 45 degree angle
there will be less attenuation
than if you were to move through
that same wall at a 120 degree angle.
At a 120 degrees the wall appears
to be much thicker to the radio signal.
Also remember, just because your powerful access point
can blast a signal to your smartphone
doesn't mean the smaller antenna on your smartphone
has enough power to be heard by the AP.
On another interesting note, radio waves and light waves
are both in the electromagnetic spectrum,
so they both behave in a lot of the same ways.
Just as light does if RF
hits a shiny metal surface they will reflect
while dense, black materials will help to absorb them.
Depending on your install
these can be strengths or weaknesses.
Another huge topic is channel utilization.
In environments where there are a lot of clients
on a single channel
they will experience degraded performance.
If you have a lot of members connected to a single AP
it can take a while for the channel
to clear enough for you to send.
Imagine that you have more than one access point
in your general area and both are configured
for the same channel,
all clients will hear each other communicating
on the same channel even if they aren't connected
to the same access point.
A simple solution is to set your APs to different channels.
The problem with the two gigahertz range
is there are only three non-overlapping channels.
This can be an obvious issue if you have a large
environment needing many access points.
It is best to space access points employing
the same channel as far apart as possible.
If you have a dense environment then this can pose issues.
This is where the five gigahertz range can really shine.
Since it has 25 channels to work with
you can have a dense environment
and not worry about overlapping clients.
So manufacturers will change antennas
or tweak some of the signal delivery methods
but the core principles remain the same.