Determining the maximum range
for 802.11 communication
Three major qualities determine
whether an 802.11 station can receive
a signal: the signal's initial transmit
power, the distance between the transmitter
and the receiver, and the data rate
of the signal. These qualities are
interrelated, and a change in one
has direct effects on the others.
This month, we'll explore how these
qualities go together to determine
the range and data rate of an 802.11
every RF signal, an 802.11 signal
is transmitted at some power level
and then grows weaker as it travels
(propagates) away from the transmitter.
This power loss is called "free
space path loss," and is abbreviated
as "FSPL". Two conditions
will cause the signal to become unreceivable.
First, all 802.11 radios have a minimum
power level, called the receive sensitivity,
below which a signal cannot be received.
More sensitive cards can receive
weaker signals than less sensitive
cards. Second, at a certain point,
the signal will fade into the background
RF radiation, at which point the
receiver won't be able to make out
the signal, no matter how sensitive
the receiver is.
Free space path loss occurs even
when the signal is travelling through
open air. When the signal has to
pass through objects such as walls,
it loses additional energy. This
loss is known as absorption.
Understanding FSPL and absorption,
we can begin to understand the relationship
between transmit power and the signal's
range. The stronger the signal starts
out, the more power it can afford
to lose to FSPL before it becomes
too weak to be received. The more
power the signal can afford to lose,
the more distance it can travel and
the more obstacles it can push through.
Therefore, as transmit power goes
up, so does range.
Digital RF signals like 802.11
use various encoding methods to encode
1's and 0's onto an analog RF waveform.
More complex methods can squeeze
more 1's and 0's into the same space,
resulting in a higher data rate,
but the more complex the encoding
method is, the more susceptible the
signal is to corruption. Higher data
rates are more easily corrupted,
while lower data rates are more corruption-resistant.
The effect of this is that higher
data rates fade into the background
RF radiation sooner than lower data
rates. Since the lower data rates
remain recognizable longer, they
have greater range. Therefore, as
data rate goes up, range goes down.
In summary, transmit power, range,
and data rate have a reciprocal relationship.
If transmit power increases and data
rate remains the same, range also increases.
If data rate increases and transmit
power stays the same, range decreases.
If you want to increase data rate while
keeping range the same, you'll need
to increase transmit power accordingly.
do directional antennas become
directional? Why is it that amplifiers
have separate receive and transmit
gain, but antenna gain occurs equally
well for both transmission and reception.
The principle at work is called the "Law
of Antenna Reciprocity" and
it's what allows a high-gain antenna
to not only send RF energy out over
longer distances, but to pick up
weak signals arriving from long distances,
even though the weak transmitter
has no particularly special antenna.
Imagine an antenna that radiated
an equal amount of energy in all
directions. The coverage pattern
of this antenna could be visualized
as a sphere. The signal strength
that was perceived by a client would
be independent of the orientation
of the antenna relative to the client.
No matter which way the antenna was
pointed, the client would see the
same signal strength. The only thing
that would change the signal strength
perceived by the client would be
if the distance between the client
and the antenna changed.
type of antenna is called an isotropic
radiator. A good example of an
isotropic radiator is the sun.
The amount of energy the earth
receives from the sun doesn’t depend
on which “side” of the
sun we’re facing. There’s
no “front” or “back” side
to the sun. It puts out equal energy
in all directions.
antennas are never isotropic radiators.
They always “shape” or “focus” the
energy that they transmit, putting
more energy in some directions and
correspondingly less energy in other
directions. A “dipole” is
a very common type of antenna (the
antennas on most wireless access
points are dipoles). To get a mental
picture of how radiation propagates
outwards from a dipole antenna, consider
a fluorescent light. Light (electromagnetic
energy) propagates outwards from
the sides of the tube, and not from
the top or bottom. This is similar
to the way electromagnetic energy
propagates away from a dipole antenna.
Instead of a sphere the energy volume
of a dipole antenna more closely
resembles a 3-dimensional doughnut
shape, known as a toroid.
It’s important to realize
that, all other things being equal,
the dipole antenna doesn’t
put out any less energy than an isotropic
antenna would. The dipole antenna
is simply focusing the energy more
in one area and less in another.
Consider the analogy of an adjustable-spray
nozzle for a garden hose. The water
pressure coming through the hose
and the volume of water coming out
of the hose may be constant, but
you can have a wide, fine spray or
a narrow jet. This is, in essence,
how antennas work.
degree to which an antenna is “focused” can be expressed
in terms of gain. An antenna’s
gain is the strength of that antenna’s
output at its most focused point,
relative to the output of an isotropic
radiator. The unit used to measure
antenna gain is "dBi" (decibel
gain relative to isotropic). The
details of decibels are outside the
scope of this article, but suffice
it to say that the more focused an
antenna is, the higher its gain will
isotropic radiator would have 0
dBi of gain, but true isotropic
antennas aren’t practical to
build. Real antennas always have
some amount of gain relative to an
isotropic antenna. The most simple
dipole antenna (as would be found
on a typical 802.11 access point)
manifests 2.15 dBi. This means that,
at its most focused point, the antenna
is 2.15 dB stronger than an isotropic
antenna would be. The most focused
area of a dipole antenna is a plane
that extends perpendicular to the
antenna like a disc, with the antenna
stuck through the middle of the disc
like an axle. Correspondingly, at
its least focused point, the antenna
is 2.15 dB weaker than an isotropic
radiator would be. The least focused
area of a dipole antenna is directly
above and below the top and bottom
point of the antenna.
The Law of Antenna Reciprocity
defined the concept of gain, we
can now ask the question, “Where
does antenna gain come from?” The
answer is that antenna gain is directly
related to the physical shape and
structure of the antenna. Consider
a paddle dipped into the surface
of a lake. Imagine that the paddle
is waggled back and forth such that
it creates waves. This is analogous
to a transmitting antenna. Now, imagine
that there is a second paddle at
the other side of the lake. As the
waves pass the second paddle, they
cause it to waggle back and forth.
This is analogous to a receiving
antenna. The analogy is actually
much closer than you might think—the
main difference being that the antennas
are not physically “waggling”.
Rather, the electrons in the antennas
the “paddle” analogy,
we can imagine that different shapes
of paddle would make larger or smaller
waves. For example, a paddle with
teeth cut out of it like a comb would
make smaller waves than a solid one.
Here’s where things get interesting:
the solid paddle would also “waggle” more
than the comb-shaped one when waves
passed by it. In other words, the
same characteristics that would cause
the solid paddle to transmit big
waves would also cause it to be more
sensitive to the reception of waves.
principle holds true for RF antennas.
The Law of Antenna Reciprocity
that states that the same physical
characteristics that make an antenna “focus” its
transmissions in a given way also
cause the antenna’s reception
to be “focused” in the
same way. This is a real boon for
wireless network designers because
it’s usually the case that
the access point’s antenna
can be changed, but the client’s
antenna is fixed because it’s
built into the client device. If
you put a higher-gain antenna on
your access point in order to increase
its range, you don’t have to
worry about putting a corresponding
antenna on your client devices. The
Law of Antenna Reciprocity says that
the additional gain of the antenna
will make the AP talk louder and
hear better. The client device will
simply perceive a larger coverage
to fully understand the impact of the
Law of Antenna Reciprocity, consider
a different way of increasing a device’s
coverage area, increasing the transmit
power of the radio. If one radio has
a higher transmit power than another,
the weaker radio may hear the transmissions
of the stronger radio, but the stronger
radio will be too far from the weaker
radio to hear its responses. This situation,
which is known as the Unbalanced Power
Effect, is irrelevant in one-way transmission,
like television or radio, but in two-way
data transmission, the link is effectively
down when the Unbalanced Power Effect
is occurring. The benefit of the Law
of Antenna Reciprocity is that you
can use higher gain antennas to increase
the coverage of a radio without worrying
about creating an Unbalanced Power