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Essential Wi-Fi
The majority
of wireless installations are for
indoor networks, which means that
the average wireless network engineer
is unprepared when he or she encounters
an outdoor wireless network. In this
article, we’ll discuss how
outdoor wireless networks differ
from indoor ones and give you information
to assist you in designing and troubleshooting
an outdoor wireless network.
Indoor wireless networks seldom
have clear line-of-sight between
the access point and the clients.
There are usually walls or other
obstructions in the way. There
are some exceptions, such as
warehouses, convention halls,
and “cube farms”, but
it’s rare for an entire indoor
installation to consist of these
types of areas. When the line-of-sight
between the access point and the
client is obstructed, the range of
the signal is reduced significantly
and the number of access points needed
to cover an area increases.
In comparison, outdoor wireless
networks often allow for clear
line-of-sight between the clients
and the access points. Imagine
a courtyard, stadium, or parking
lot that requires RF coverage.
In all of these cases, it’s
likely that an antenna could be mounted
in a location that allowed for clear
line-of-sight to the entire coverage
area. When clear line-of-sight is
available between the access point
and the clients, the range of the
access point increases dramatically—sometimes
by a factor of three to five times,
and sometimes even more!
Increased coverage range due
to clear line-of-sight is the
first major difference between
outdoor and indoor RF installations.
Connect802’s
engineers report that they regularly
have to re-calibrate their expectations
of coverage distance when they go
from an indoor job to an outdoor
one. “The customer (who had
only done indoor installations) had
estimated three access points to
cover the parking lot, but after
doing the survey, it turned out that
one AP was more than enough,” said
one engineer. If you’re not
accustomed to doing outdoor installations,
get an access point and a client
device and take some measurements.
You might be surprised just how far
you can go before signal strength
drops to an unacceptable level.
This increased coverage range
is only present when line-of-sight
is clear. It doesn’t take very
much obstruction to dramatically
reduce range, so you have to be much
more aware of obstructions in outdoor
installations. Obstructions have
fundamentally the same effect in
indoor installations, but since they’re
pretty much always present, indoor
wireless networks are pretty much
always designed to deal with them.
With an outdoor network, you’re
usually designing based on the assumption
of clear line-of-sight and greatly
increased range. If that assumption
fails, even a little bit, you can
end up with big coverage gaps.
Trees are the main type of
object that reduces the range
of outdoor wireless networks.
The exact amount of reduction
depends on many factors, including
the type of leaf on the tree,
the season (and amount of leaves),
and the number of trees (a
thick stand obstructs more
than a single decorative plant).
The topic is too complex to
fully explore in a short article,
but suffice it to say that
whenever there are trees in
your coverage area, you should
pay close attention to areas
that are shadowed by the trees
in your survey. Connect802
has often found that a thick
band of trees essentially completely
blocks RF coverage. Additional
access points had to be added
to compensate.
The way around this problem is
to place access points such
that they have clear line-of-sight
to as much of the coverage
area as possible. For example,
if you’re installing
access points to cover a
street, place the antennas out
on the end of a lamp-post such
that they overhang the middle
of the street, as opposed to
on the poles, where the signal
might be blocked by trees
planted near the sidewalk.
A second major difference
between indoor and outdoor
wireless networks is the
effect of interference. Indoor
networks are somewhat protected
from outside interference
by the concrete, brick, or sheet
metal walls of the building.
Outdoor networks, on the other
hand, are totally susceptible
to whatever interference might
be present. It is sometimes the
case that there are not any interfering
devices near the outdoor coverage
area, but especially in heavily
populated urban areas, there
can be so much interference that
network performance can be significantly
compromised.
The only way to be sure is by
surveying the area, which
is why Connect802 recommends
surveying before installing
any outdoor network. A spectrum
analyzer can identify sources
of interference, but we also
recommend performing actual
data-transfer tests to determine
the practical effect of interference
on the wireless network.
It can be difficult to determine
just from looking at a spectrum
analyzer trace whether a
wireless network’s
performance will be compromised—after
all, 802.11 networks are
designed to work around interference
to some degree.
A data transfer test
can be performed with
an access point and two
laptops. One laptop plugs
into the AP’s
Ethernet port and the other laptop
connects wirelessly. Then an FTP
file transfer or an iperf test is
run between the two devices and the
overall throughput is measured. For
an 802.11b network, throughput should
max out around 6 Mbps, while 802.11a/g
networks should max out around 18-22
Mbps. Because interference can be
intermittent, you should especially
pay attention to sudden drops in
throughput. You might have an average
throughput of 15 Mbps, but if there
are occasional five-second periods
where the actual throughput drops
to 250 kbps, something is wrong!
You might be tempted to ignore the
intermittent dropouts as anomalies,
but don’t! Ask yourself whether
a user’s experience
of the network would be compromised
by those dropouts, and if
the answer is yes, design
the network to compensate
for the interference.
What
are the options for mitigating
the problem if an outdoor network
is experiencing interference?
Blocking the interference
is probably not feasible,
since that would require
building a wall around
the coverage area. One
option is to increase
signal strength (and therefore,
signal-to-noise ratio) by
installing more access points,
closer to the users. This
will only work if the interference
itself is sufficiently weak that
your signal can overpower it.
A second option is to use higher-gain
antennas with narrower, more
focused coverage patterns. This
will have the dual effect of
increasing signal strength in
the coverage area and of reducing
the degree to which the antenna
can “hear” the
interference. Finally,
the option of last resort is to move
to a different frequency
range. In some areas, the 2.4 GHz spectrum
is swamped with interference,
and the only feasible option is to
move to the 5 GHz band
with an 802.11a network. Networks that
are used for public safety
also have the option of moving
to the 4.9 GHz frequency
band.
Technology and Engineering
RF Engineers
usually think of solid objects as
creating “shadows” in
RF coverage. This analogy works
well for rough estimates of RF
coverage, but environmental effects
like reflection and diffraction
mean that the signal doesn’t
always go where a straight-line
approximation of its path would
predict. In this article, we’ll
explore how diffraction can actually
improve coverage in outdoor installations.
An example of how a signal might
diffract around the edges of a
building.
Put simply,
diffraction means that when the
RF signal encounters a sharp-edged
object, the signal seems to “bend” around
the edge of the object. This is illustrated
in the graphic above, which depicts
a top-down view of a building with
an 802.11 antenna on the left side
of the building. The RF signal will
diffract around each corner of the
building. In the zone marked “1”,
the receiver is in direct line-of-sight
to the antenna, and no diffraction
will affect the signal. In the zone
marked “2”, the signal
has diffracted around the lower-left-hand
corner of the building. The signal
is weaker than in line-of-sight,
but is still present. In the zone
marked “3”, the signal
that made it into zone 2 has further
diffracted around the lower-right-hand
corner of the building. This signal
is still weaker, but might still
be detectable by a receiver. In the
zone marked “4”, the
signal is too weak to be received;
this zone is considered to be in
shadow.
This example demonstrates that
RF shadows are not as simple
as they might at first seem.
If you were to consider only
line-of-sight, you would
conclude that zone 1 would
get signal and zones 2, 3, and
4 would be in shadow and
would receive no signal.
In reality, this is not the
case. Zones 2 and 3 do receive
some signal, albeit at much decreased
signal strength.
Diffraction means that the actual
shadow is much smaller than line-of-sight
would suggest.
Diffraction is usually beneficial
to RF networks, allowing signal
to propagate to locations where
it otherwise wouldn’t. For example, Connect802
once installed an antenna on the
south-most wall of a long rectangular
building. The building ran for approximately
300 feet to the north of the antenna.
The antenna was mounted approximately
six feet above the roof-line. If
you were to go just by line-of-sight,
you would conclude that the roof
would create a large shadow on the
north side of the building, but in
reality, this shadow was not very
pronounced. Effective throughput
of about 2 Mbps could be achieved
just 50 to 75 feet north of the north
wall of the building because diffraction “bent” the
signal down around the northern
edge of the roof, causing it to
hit the ground much closer to the
building than life-of-sight would
suggest.
RF networks should not be
designed to rely heavily
on diffraction to achieve
RF coverage. Connect802 has
found that the difference
between “in
the shadow” (zone 4 in the
graphic above) and “in the
diffracted signal” (zone 3
in the graphic above) can be as small
as five or ten feet. A client can
be achieving 24 or 36 Mbps data rates
in the diffracted signal area and
then, after moving a relatively short
distance, suddenly lose connection
altogether because they’re
in the shadow. When you have line-of-sight
and you move away from the access
point, a graceful, slow decrease
in signal strength and data rates
occurs. With diffraction, the drop-off
can be much more sudden and unexpected.
Diffraction can be most
helpful when clients are
roaming between two access
points. For example, imagine
a wireless network covering
a grid of city blocks. Through
diffraction, an access point
on a north/south street will
have a little bit of coverage
around the corner onto each cross
street, and vice versa. This
means that there will be a small
zone of overlap between the APs
on the north/south streets and
the APs on the east/west streets,
assisting in roaming.
Finally,
what about indoor networks? Yes,
diffraction does occur in indoor
networks too, but it is less of
a factor than in outdoor networks,
because indoor networks have so
many objects in the signal path
between the access point
and the client device. This
means that there are many edges around
which the signal diffracts,
and the net effect is much less discrete
than it is in an outdoor
network where there are likely
to be only a few objects
in the signal path. In indoor
networks, the entire signal
path is likely to be obstructed
by walls or other objects,
and so the effect of diffraction
is insignificant compared
to the absorption of the
signal by the obstructions.
Ask the Expert
I
was called in to help troubleshoot
another engineer’s wireless
installation. The network has a dual-radio
access point and both radios are
set to use 802.11g channel 6. Is
this actually helping performance
at all? I think not, but I want to
be sure.
Dual-radio
access points can increase a wireless
network’s capacity,
but not when they’re configured
as you describe. CSMA/CA, the network
access method that’s used in
802.11, prohibits two radios from
transmitting at the same time on
the same channel. If the two radios
in your access point were set to
different channels, then they could
transmit and receive at the same
time, basically doubling the total
capacity of the network. If both
radios are set to the same channel,
however, only one of them can operate
at a time, and there’s basically
no benefit. (Okay, in theory, if
one radio failed, the other one could
still operate as a backup, but realistically,
if both radios are in a single access
point, it seems unlikely that one
would fail and the other wouldn’t.)
To Infinity... and Beyond!
One of
the goals of Wi-Fi certification
is to ensure compatibility between
devices, but the Wi-Fi Alliance’s
Draft-802.11n certification might
not achieve that goal. Devices certified
as Draft-802.11n compliant will only
be tested in 20 MHz channel mode.
In this mode, 802.11n channels are
the same width as 802.11b/g channels,
and so no additional steps need to
be taken to avoid interference between
802.11n and 802.11b/g networks. But
802.11n networks can also use a 40
MHz wide channel—and indeed
they have to do so in order to get
the highest data rates. A 40 MHz
wide channel centered on channel
6 of the 802.11b/g channel set will
interfere with channels 1 through
11, effectively disrupting communications
for all transmitters in the 2.4 GHz
range. Of course, the 802.11n standard
includes provisions to avoid this
interference. For example, an 802.11n
device that detects “legacy” 802.11b/g
devices will switch to 20 MHz mode
in order to avoid interfering. But
without the Wi-Fi Alliance’s
certification, there’s less
certainty that this functionality
is working properly in a given device.
The Alliance stated that there’s
still too much debate over how backwards
compatibility between 20 MHz and
40 MHz devices will be handled, and
thus they aren’t testing for
it. We can hope that, once the final
standard is approved, this testing
will be added to the Wi-Fi 802.11n
certification.
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