Anytime we're going to start a wireless design, it's going to begin with us
taking a floor plan and our goal is to strategically place access points
throughout this space in order to provide proper coverage.
And so we draw the signals around these access points. This would not be a very
good cell design because we've got a lot of empty space in here, for example.
But again, each one of these access points is covering an area that we refer to
as a cell. But what exactly defines a cell? I mean, we've got a boundary around WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
as a cell. But what exactly defines a cell? I mean, we've got a boundary around
this access point.
But chances are I can not only communicate with this access point here, but
even if I go out into the space, I can probably still communicate with this
access point.
And so is the cell boundary arbitrary or we just drawing pretty circles around
these access points? But no, we actually do have a definition for what these
cells look like in the boundaries around them. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Specifically, we're going to set our cell boundaries based on that signal
strength concept.
What the wireless world will refer to as the received signal strength indicator
or SSI is going to be what we've been referring to as signal strength.
That dBm measurement of how strong a signal is. And recall that if we have
anything over, again, these are negative numbers to keep that in mind.
But anything over, I would say about negative 70 is going to be considered a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But anything over, I would say about negative 70 is going to be considered a
very good signal. If we get into the lower negative 60s and to the negative 50s
, it's a phenomenal signal. And you'll almost never see a signal that's as
strong as negative 40. If you do, then you know you are probably sitting right
underneath the access point.
So when we're talking about this signal strength and again, it's measured in dB
m, we've got to define what these cell boundaries look like based on what a good
signal strength is. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
The problem is that the definition of a good signal strength can change
depending on whether we're deploying access points for data only or
specifically whether we're going to deploy real time applications.
And when I say real time applications for the most part, we're talking about
especially voice voice over IP and video.
Now if we have other real time applications struggling even to think of some
examples, there are definitely examples out there. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Then we're going to need to design for real time even if we're not deploying
voice and video over wireless. But these days we pretty well expect that we
want to use the Wi-Fi for our voice calls.
So a lot of our designs are going to use real time metrics.
Now the difference between these has to do with what again we consider to be a
good enough signal strength with data only we can go as low as 72 dBm and say
that we should be able to connect at a really good data rate and have a strong WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that we should be able to connect at a really good data rate and have a strong
connection.
And if somebody wanders out of this cell and they're getting negative 74
negative 76 they can probably continue to communicate to that access point if
they need to and ideally at some point they find another access point and roam
over to it.
Now with real time applications are threshold shrinks a little bit we say
negative 67 dBm.
So if I am deploying the access points in the exact same space but I'm using a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So if I am deploying the access points in the exact same space but I'm using a
real time boundary metric then these cells are going to shrink and we might
expect them to look a little bit more like this at which point we see we're not
covering the entire space anymore.
So it does depend or does matter greatly what we're performing this access
point survey or really the wireless design the cell design what we're designing
it for is going to impact where we place the access points.
On top of all of this we need to also keep in mind that we do require some WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
On top of all of this we need to also keep in mind that we do require some
amount of overlap.
We may have noticed that we've got these areas in between the access points and
we've said that this overlap is important because if I'm roaming from one
access point to the other if the boundaries are barely coming up against one
another then if I'm trying to roam in here I might not have a good signal to
the first access point but I might not have a really good signal to the next
access point either. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
access point either.
Instead we want to overlap by enough so that by the time I recognize hey I'm on
this side this edge of the cell and I'm getting a little bit weak on this side
well look how close I am to the next AP and I can perform a seamless realm.
Now the question is how big of an overlap do we need well again it's going to
depend on whether we're talking about a data deployment or a real time
deployment with data we like to see a 20% overlap. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And with real time applications we like to see believe it or not 35% overlap we
want to ensure that our rooms are as seamless as possible with real time
applications because if we get a big hiccup when we're roaming and yeah
somebody might be on their phone and just walking through this whole facility
and they roam for different times well we don't want that phone call to get
disrupted for different times we wanted to be as seamless as possible.
So one thing we have to consider within the cell design concept is the idea of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So one thing we have to consider within the cell design concept is the idea of
bringing in or containing our cells a little bit because it certainly makes
sense on some level to say that hey I'm going to try to fill this space with
this few access points as possible if I need to deploy five access points in
here.
That's going to be a whole lot cheaper than deploying let's say 15 access
points and if I crank up the power and I say hey I want all these access points
to be broadcasting at 250 millawatts or something along those lines then yeah I WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to be broadcasting at 250 millawatts or something along those lines then yeah I
could probably cover this space with only the five APs.
So this is the concept of coverage and back when I was first starting to deploy
wireless coverage is what we cared about I'd go into a school building like
this and try to figure out the fewest number of APs required in order to hit
these boundaries these metrics but these days we care about not really coverage WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
these boundaries these metrics but these days we care about not really coverage
anymore what we really care about is capacity.
And when we have capacity requirements well this is where our large cells can
start to get us into trouble because again back when I was doing these coverage
based surveys we maybe had I don't know 50 wireless stations in the entire
building so maybe 10 stations on each access point we didn't have a lot of
wireless stations but these days we might have 500 wireless stations. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
wireless stations but these days we might have 500 wireless stations.
And so I can't have 100 stations associated to every single one of these access
points.
Number one the hardware might struggle with that but number two we have basic
wireless physics the physics of wireless defined for us how long it takes to
send a signal how long we have to wait for the signals to finish propagating in
the space and these physics are going to provide a limit to the number of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the space and these physics are going to provide a limit to the number of
devices that can associate with an AP.
And as you can imagine the physics of wireless signals they're not going to
change a whole lot if at all they're not going to change at all ultimately even
as technology develops I can have the best hardware in the world I can have the
best protocols in the world and the physics of light waves that's not going to
change.
So we will always have this limit to the number of stations that can or should
be associated to a single AP. And so now that we care more about the wireless WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
be associated to a single AP. And so now that we care more about the wireless
capacity what we're really focusing on is the number of wireless stations per
access point.
So let's pause for a moment and think why this matters so much I mean why do we
need fewer stations per access point beyond the concept of wireless physics
because we might look at this situation say okay yeah but I've
really only got I don't have 500 maybe I've got 150 stations so I could
probably throw 30 wireless devices on every access point and that would work WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
probably throw 30 wireless devices on every access point and that would work
right well number one that assumes they're evenly distributed throughout the
space but we're also going to have other issues with these large cells so if we
're talking about large cells we're going to find that we have greater chances
of having wireless issues.
First and foremost we're going to have a little bit more likelihood that we
have poor wireless signal in places so the concept of shadows. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
have poor wireless signal in places so the concept of shadows.
It would be like saying I've got a room and I put one gigantic bright light in
the middle well that's going to create shadows.
And throughout the room based on where there are maybe pieces of furniture and
the like whereas if I put a lot of lights which is typically how we do lighting
in a space well now there's fewer shadows it's better lit throughout the whole
area and this exact same premise is going to apply to us in the wireless space WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
area and this exact same premise is going to apply to us in the wireless space
so we want to have good wireless signal everywhere in as few shadows as
possible.
Remember wireless signal is light so there's a lot of these analogies we can
use to kind of think along those terms furthermore we're going to find that
wireless stations are sticky.
They do not like to roam because roaming is has the potential to be disruptive
and so if we are looking at large cells like we have up in the upper left at
this point we're assuming those are large cells we might have a client that WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
this point we're assuming those are large cells we might have a client that
gets out into the middle of nowhere.
And is still trying to hang on to its access point far away whereas there might
be actually a closer access point nearby and so it'd be better for us to roam
to that but if the access point is further away because of our large cells and
we just happen to be sitting in one of these shadows we talked about is going
to make it so that it's very difficult for us to seamlessly roam.
We're also going to have that problem of co-channel contention remember the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We're also going to have that problem of co-channel contention remember the
more stations we have in the space the more waiting there's going to be because
we speak one at a time.
Everybody's going to have to take turns and so if we have 50 or 60 stations on
one access point versus 20 there's going to be a whole lot more contention for
that wireless space because of the half duplex nature.
So instead what we're really talking about here is we want to have smaller WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So instead what we're really talking about here is we want to have smaller
cells for our wireless deployments in most cases and yes this will require more
access points it might come at a greater cost but it's going to be a more
robust wireless design.
So there's a number of ways of accomplishing this first of all we can lower the
transmit power of our devices.
Now for access points this is pretty simple I just get into the access point
and turn down the transmit power for our wireless stations this is going to
depend on the capabilities of the station there have been some protocols that WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
depend on the capabilities of the station there have been some protocols that
have been put out that allow access points and wireless networks to dictate the
transmit power of a station but not every station supports that not every
wireless network configuration supports it and so ideally we can make that
happen.
But we don't always have the best control over it but if we do have that
capability we should absolutely take advantage of it.
We should also consider deploying lots of overlapping cells which is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We should also consider deploying lots of overlapping cells which is
interesting we can do a multi cell overlap and we can do this using either a
band based method or a channel based method.
So if I've got an access point here we mentioned this previously in this course
that this access point let's say it's a Cisco AP it might have four different
radios well those radios might be a 2.4 gigahertz a 5 gigahertz another 5 giga WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
radios well those radios might be a 2.4 gigahertz a 5 gigahertz another 5 giga
hertz and a 6 gigahertz radio and so when I deploy this AP I might see that I've
got a 2.4 gigahertz cell but then I've got a 5 gigahertz.
So I can draw that about the same size and then the 6 gigahertz cell and we can
maybe even adjust the transmit power per radio to try to make them overlap a
little bit more.
But if we think about it this physics space the half duplex nature of all the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But if we think about it this physics space the half duplex nature of all the
contention we've been talking about this all applies within the concept of a
channel.
And so if I have four different bands and therefore four different channels or
for that matter if I've got I guess this you know it'd be three different bands
but either way I've got the potential to even deploy dual channels within the 5
gigahertz space well at this point I've got four different spaces so I could
have 120 different wireless stations all in this physical space and yet I've WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
have 120 different wireless stations all in this physical space and yet I've
got a rough distribution of 30 stations per radio.
And that would be perfectly fine. That's very useful. We can also lower the
threshold of what makes a packet valid.
And this uses a technique known as receiver start of packet or RxSOP and
essentially what we're saying is if a station gets too far away from the AP and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
essentially what we're saying is if a station gets too far away from the AP and
again remember they're sticky they don't like to roam.
So if this is too far away and I'm getting a signal that exceeds the threshold
or maybe misses the threshold either way we can just ignore this wireless
station and say hey you no longer have a connection.
Whereas the Wi-Fi standard says yeah you're allowed to stay connected but
because they're connected they have a low data rate.
They're going to have a low success rate on their transmit so they're going to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
They're going to have a low success rate on their transmit so they're going to
have to do a lot of retransmissions and those transmissions will be very slow
because of the amount of bits and bytes that it has to send
or well I should say the number of transmissions it needs to send to
communicate the same number of bytes and ultimately this is going to slow the
entire cell down.
So we can lower the transmit power to shrink the cells we can deploy multiple
cells on top of each other and we can call it raise the threshold of what a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
cells on top of each other and we can call it raise the threshold of what a
valid signal should be.
So by leveraging these techniques we can create a network that has smaller
cells which will be again a little bit more capacity focused than coverage
focused it will be a more expensive wireless design because of the number of AP
s that will be required but it will be a far more robust experience for wireless
users.