In this video, we're going to discuss
wireless considerations that you need to think about
as you start to troubleshoot your wireless networks.
We're going to begin by discussing antennas, their placement,
their type, and their polarization.
Then we're going to cover channel utilization, site surveys,
and the types of information that those things
can help us with as we troubleshoot our networks.
Finally, we're going to talk about wireless access points,
or AP association times, and how association actually works.
First, let's talk all about antennas.
Antennas can come in multiple different types,
and each one has its own purpose.
By default, most wireless access points
are going to use omnidirectional antennas,
and normally they're going to be located
in a vertical form factor.
Now for a vertical antenna, the radiation pattern
for the radio frequency waves is going to extend outward
in all directions away from that antenna
and away from the wireless access point
at an equal power level.
As the radio waves travel further
and further from that antenna,
the signal is going to get lost a little bit
and the power and strength is going to decrease.
Now, if you're monitoring this from your client,
you're going to see that you have
more negative RSSIs as you go further
from the wireless access point and its antenna.
Now, for example, let's say you're sitting
right next to the antenna.
You might have an RSSI of -30 to -40 decibels,
but if you move out a hundred feet away,
that signal strength may now be at an RSSI
of -65 or -70 decibels.
Another type of antenna you might come across
is known as a dipole antenna, or a bi-directional antenna.
With a dipole antenna, the antenna
is going to produce a radio frequency wave
that extends outward in two directions
away from the antenna.
With a dipole antenna, you're normally going to see
a higher RSSI on the client device,
even when it's located further from the access point.
Because this antenna can now focus its power
in only two directions instead of the entire 360 degrees
around itself like an omnidirectional antenna produces.
So if you need the radio frequency waves
to only go in two directions,
a dipole is going to be a good option for you,
but they're not heavily used
in wireless networks for the most part.
Next, we have this thing known as a Yagi antenna.
Now, a Yagi antenna is a type of unidirectional antenna
that sends the radio frequency waves only in one direction.
Due to this focusing of the radio frequency
in a singular direction, a Yagi antenna
can send the radio frequency waves
further in a single direction, using less power
than an omnidirectional antenna will.
Normally, you're going to see a Yagi antenna used
when you're trying to connect two different sites
using a wireless link.
For example, you may want to connect two office buildings
on a college campus using a wireless link
instead of running a buried fiber optic cable.
In this case, you're going to use a Yagi antenna to do that.
Finally, we have a parabolic grid, or disc antenna.
These antennas are unidirectional antennas
just like a Yagi antenna is,
but they're built a little bit differently.
Now, parabolic grid antennas are most commonly used
for site to site applications, where you need to connect
buildings over a longer distance
than you would with a Yagi antenna.
Like a Yagi antenna, a parabolic antenna is going to allow
the radio wave to be transmitted in only one direction,
making it a unidirectional antenna.
Now, when it comes to placing your antennas,
this is going to be dependent on where you're going to place them,
inside or outside of your building,
to create the coverage that you desire.
If you're setting up a site to site
or building to building connection,
you're going to want to use a unidirectional antenna,
like a parabolic or a Yagi antenna.
Now you're going to take that and mount it
on the outside of your building,
and usually you're going to attach it to the roof
and make sure there's a clear line of sight
between both antennas on each of those buildings.
Now, one of the common issues you're going to experience
with wireless antennas for a site to site
or building to building connection
is that the connection may deteriorate and slow down
or completely stop over time.
If this occurs, you need to ensure that both antennas
still have a clear sight between them.
For example, maybe you have two office buildings
and they're only one or two stories high.
You've gone ahead and installed the antennas in the fall,
and there was no issues at all.
You had a perfectly clear line of sight.
But it's been about six months
and now the seasons have changed and spring appears,
and your wireless connection may start to be affected
by trees because they've sprouted new leaves,
and they are now blocking your line of sight clearly
between those two buildings.
Similarly, if in the wintertime there's a lot of snow,
this can also block your signal.
Or if it rains heavily in the summer,
you could also have a deteriorating signal
because the water droplets will block
the wireless communication between these two antennas.
So if you're working with antennas,
you need to think about these things
when you're working outside.
Now, on the other hand,
if you're working inside your building,
you're going to be more likely to use omnidirectional
or unidirectional patch antennas
to control the signal being radiated
by your wireless networks to each of your clients.
With omnidirectional antennas,
it's going to be common to place this on the ceiling
in the middle of the office or classroom.
For a unidirectional patch antenna,
you're usually going to place this on an outer wall
of the building facing inward,
thereby directing all those radio frequency waves
back inward towards the office or the classrooms.
Now, the last thing we need to discuss with antennas
is the concept of polarization.
Polarization is the orientation of the electric field
or transmissions that are occurring from that antenna.
Basically, as the radio frequency waves leave the antenna,
how is it going to be oriented?
Every antenna has a polarization associated with it,
and for Wi-Fi networks,
our antennas usually are going to have a vertical
or horizontal polarization associated with them.
Now, most Wi-Fi clients that have external antennas
are going to use vertical polarization,
but your wireless access point could use vertical
or horizontal polarization,
depending on what type of antenna it has.
Now, if you see a problem with some clients
getting a poor RSSI, even though they're pretty close
to your access point, this may indicate
you have a polarization issue.
If this is the case, I want you to try flipping
the wireless access point antenna
to allow the clients to better connect to that device
if they support bending the antenna upwards.
Or if you're using wireless access points
that are sitting on the floor,
you want to make sure you're using vertical polarization
and radiating antennas to increase your coverage area.
If you're using horizontal,
it's going to keep sending those radio waves out horizontally,
right along the floor, and add up to the desk level
where everybody else is working.
Remember, most Wi-Fi networks use vertical polarization,
so your antennas should be sticking upward
if you're using an omnidirectional antenna
with your Wi-Fi networks.
All right, next we have channel utilization.
Channel utilization is a statistic
or measure of the amount of airtime utilization
that occurs for a particular frequency or channel.
If there's a higher rate of channel utilization,
then that means there's more traffic
being transmitted over that particular frequency.
In order to have a fast wireless network,
you want to keep your channel utilization under around 30%.
Now, channel utilization is not something
you alone can control though,
because there's lots of these different channels
that overlap with other people in that area,
and so these same channels and frequencies
can be used by your network and other networks too.
For example, let's say you're running a wireless B,
or a wireless G, or a wireless N network,
and you're operating in the 2.4 gigahertz spectrum.
We are going to be using the same channels
that everybody else is, channels one, six and 11.
Now, if you have a large office building,
this won't be a big problem for you
because nobody else is going to be
in the same general area as we are.
But if you're in a crowded office building
or a shopping mall or an apartment building,
this can easily start to have a lot
of different wireless networks
operating on the same channels as ours in the same location.
Now, when wireless access points
and wireless clients are operating on the same channel
and all of those things are in the same range of each other,
they're going to begin to form a single broadcast domain,
similar to an ethernet hub.
All of the devices can hear each other's transmissions,
and if any two devices transmit at the same time,
their radio signals will collide and it becomes garbled,
which results in data corruption or a complete frame loss.
If there's an excessive amount of collisions,
data will never be remitted successfully
and the wireless network could become unusable.
So to avoid collisions, 802.11 wireless devices
use a "listen before they speak" approach
when they're accessing the wireless medium,
which is the radio frequency,
following the CSMA/CA, or collision avoidance techniques.
Now with this, devices are going to perform
a Clear Channel Assessment, or CCA, by first listening
to see if another device is actively transmitting
on the channel before they attempt to send their own frames
on that given frequency.
Now, when a device detects another transmission in progress,
it's going to perform a random back off
for a short period of time,
after which it's going to perform another CCA check
before it attempts to transmit.
If the channel is clear after the check,
that device will be able to access the channel
and send some data.
Now, as the number of devices need to transmit frames
starts increasing on a channel,
this is going to cause congestion to occur
to the point where devices can spend a lot more time
waiting to be able to send something.
This results in slower speeds
because devices have to wait longer
before they can send their data.
This is the negative effect
of having high channel utilization,
and at least a slower throughput for our network devices.
Now, how do we solve this problem
of over channel utilization?
Well, first we're going to start by conducting a site survey.
Now, a site survey, also known as a wireless survey,
is the process of planning
and designing a wireless network
to provide a wireless solution
that will deliver the required wireless coverage,
the data rates, network capacity, roaming capability,
and quality of service that your organization desires.
As part of the wireless survey, you need to determine
where each access point is located,
what power level it's using to transmit
by using its effective isotropic radiated power, or EIRP,
overlapping coverage areas for your wireless access points,
and the other wireless access points in clients
that may be operating in the same,
or general area or channels is you.
Now, by using this site survey information,
you can determine if you need
to change the channels you're using to a less busy,
less utilized channel,
or if you may need to upgrade your networks
to a new frequency band altogether.
After all, if you decide to upgrade
to a wireless N, wireless AC, or wireless AX network,
and you start using the five gigahertz spectrum,
this gives you 24 non-overlapping channels
instead of the three in the 2.4 gigahertz spectrum.
So there's going to be a lot less overall utilization
on each of those 24 channels in the five gigahertz spectrum.
By conducting a site survey, you can better understand
the wireless environment in and around your networks,
and this is going to allow you to configure your devices
to use the less utilized channels,
ensure there's proper coverage for all of your work areas,
and ensure your wireless network
is not being blocked or interfered with
by physical obstacles within the building.
Finally, we need to talk
about wireless access point association times.
Now, when a wireless client attempts to connect
to a wireless network, it goes through a seven step process.
First, the wireless client is going to send a probe request
to discover any 802.11 wireless networks
in the general location that it's in.
Essentially, it's going to send a broadcast message
to the BSSID of ff:ff:ff:ff:ff:ff,
and all the access points in that area
that are using the same frequency, either 2.4 gigahertz
or five gigahertz, will respond to that client.
Second, any access points that receive this probe,
will check to see if they can support the data rate
that the client is now requesting in that probe.
For example, if I'm a wireless AC access point
and you just requested to communicate using wireless B,
I'm going to ignore your request
because I'm operating at five gigahertz
and you're operating at 2.4 gigahertz.
But if we both support wireless N, then I can respond
with a probe response that provides my SSID,
my supported data rates, my encryption types
if I'm using wireless security,
and other capabilities of my particular access point.
Third, our wireless client will then send
a low level 802.11 authentication frame to the access point,
and this will begin the authentication handshake
between my client and the access point.
Fourth, the access point receives this authentication frame
and responds with an acknowledgement
to continue the handshake.
If the wireless access point receives
anything other than an authentication
or a probe request from this client though,
it's going to send a deauthentication frame
and break the communication with this client,
because this client is not yet part
of my authenticated network.
Fifth, the wireless client is going to choose
the access point it wants to associate with
and sends an association request
using the encryption and 802.11 capabilities
that are supported by that access point.
Sixth, the access point is going to process
the association request if the information
sent matches its own capabilities,
and then an association ID is being created
for this wireless client and this access point.
And the access point is going to respond
with an association response of success.
This way, the client knows it is now fully connected
to this wireless access point
Seven, now that the client
is fully connected and associated,
it can begin to conduct any data transfer it needs to
and utilize the wireless network and its associated devices.
Now, that's how everything is supposed to operate,
but sometimes you're going to see a wireless network
that has a really long association time
for an end user's client.
This is because the client has to scan the airwaves,
find the access points it might want to connect to,
request the association, authenticate to the access point,
and then contact the DHCP server to get an IP address
prior to being able to actually use
that wireless network connection that we just established.
This process can take just a few seconds
in a not heavily loaded network,
but it can take up to 30 to 60 seconds
if you have a busier network.
To speed up the association process,
clients should be located in a high signal strength area.
Those with the highest RSSIs during the association process
are going to give us a stronger strength,
and that's going to drastically reduce
the wait time during this association.