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Make Your Wi-Fi Hi-Fi: The "Truth" About Wireless Signal
Interference
By Yousif Hassan
How many times have you fired up that laptop or PDA, merely a few feet
away from your wireless router, only to discover that the signal is so low
you have to dig for it? Maybe you bought your brand new 802.11(b)
wireless access point, set it up, and then found that it's not giving you
the signal range the protocol promises? You're not alone. Statistics
show that over half of residential or SO/HO wireless applications do not
function at optimal levels. The likely cause? Signal interference!
Categories of Signal Interference
Signal interference can be broken into three main categories, all of which
are very broad. The first kind of interference is the most easily
understood -- physical interference such as from walls, through
floors and ceilings, and other permanent fixtures in the vicinity. The
second kind of interference is "hidden" physical interference, like
from materials in your walls that cause enhanced signal degradation --
beyond the scope of what would normally be expected for a wall. Finally,
there is electromagnetic interference; in short, radio waves
interfering with other radio waves. This third category is often the
least well-understood.
Physical Obstacles Causing Interference
Remedying the first category is often the easiest. Keep in mind that even
though wireless radio signals are invisible, line-of-sight still applies.
Much like cellular towers, your wireless radio transducer (that's a
fancy physics term for something that converts one kind of energy into
another; in this case, electricity into radio signals) emits signals in a
360° sphere. This sphere's center is roughly near your transducer's
wireless antenna and its diameter is about equal to the range for the
Wi-Fi protocol you are using. So think of your transducer (wireless
router, wireless access point, etc.) as piece of equipment generating a
globe of radio waves. Anything listening for these waves and expecting to
receive them has to fall within the globe's three-dimensional area.
Also keep in mind that the closer you are to the center of this globe, the
stronger and better the signal.
What all this means is that maximizing your globe's usable area, closest
to its center, is a must. To do this, keep your access point off of the
floor. If your access point is on the floor, half of your signal globe is
now under you! This may be good if you have more than one story in your
house, but keep in mind that the signal will not be as strong on the other
side. (We'll talk more about that in the second category of
interference.) Don't even put your access point near the floor --
use a height that's about equivalent to the height at which you'll be
using your wireless devices, like your cell phones, PDAs, laptops, and
desktop computers. Avoid putting your access point directly adjacent to
any kind of wall. Walls block signals, to varying degrees. This means
that a small room, such as a study, isn't an ideal location either, unless
you don't plan to be connecting wirelessly all over your house. If your
wireless transducer is inside a room, open the door. You'll be surprised
at how a relatively small opening can make a difference. In summary,
choose a location that's as open as possible given the constraints of your
wiring and network location.
Hidden Physical Interference
The second category of interference is more difficult to mitigate, but
it's helpful to know about it, so you can decide where better to place
your wireless access point or whether you need other equipment. The
first place to look at is the paint on your walls. Is it lead-based? If
your home was built before the 1970's, it probably is. Remember that
there may be layers of paint underneath your current visible coat. Lead,
even in small amounts, is a notorious electromagnetic interferer.
Remember how Superman couldn't use his x-ray vision through lead boxes?
Wireless radio signals, in this regard, are no different from other forms
of electromagnetic radiation, like x-rays. Latex-based paint, on the
other hand, is almost 100% signal-transparent.
What else is inside those walls of yours? If it's an exterior wall, it's
probably stuff like masonry, mortar, brick, and thick insulation. None of
those are particularly conducive to a high-fidelity signal. Don't put
your wireless access point near an exterior wall. Similarly, expect that
any signal passing through such walls to the outside is likely to be
weakened. If it's an interior wall, you're probably only dealing with
wooden studs and drywall. Signals pass through these with only modest
degradation. If you're noticing a large drop-off in signal strength and
you know that your signal is passing through interior walls only,
something's probably inside those walls. In an apartment building or
condominium, even interior walls may be insulated. If you live outside of
the United States, drywall isn't a given for interior wall construction;
cinderblock and the like is possible. Those materials are more opaque to
radio signals.
We've focused a lot on walls because inside, walls are the number one
obstacle for wireless signals. However, there are other things that can
block the signal, such as your ceiling and floor, if you're trying to use
wireless access across multiple stories. Signals pass through hardwood a
little better than they do carpet and significantly better than they do
concrete. If you intend to use your wireless connectivity on a story
below, consider moving your access point over a wood-floor area of your
home, if you have one. Remember to keep it off of the floor itself.
Ceilings are another matter entirely. Depending on what is above your
head, ceilings can contain anything from studs to insulation to electrical
shielding. And of course, there's always the floor above it, which adds
thickness. Consider all of this when placing your wireless access point.
If you have glass walls or, more likely, glass sliding doors for a patio
or balcony, you need to be careful. The thickness of the glass plays a
large role. Generally, wireless signals are strong enough to pass through
glass, but this depends on the wireless protocol and the glass's
thickness. Just below, we'll discuss the protocols and their frequencies,
which will make all of this make a whole lot more sense!
There are many other potential sources of physical "hidden" interference.
There is a golden rule to keep in mind at all times: hardly any material
is 100% signal-transparent. There will always be some degradation as
radio waves pass through obstacles.
Electromagnetic Interference
The third category of interference is the trickiest; this is interference
from other devices that use electricity. This usually comes in two forms,
interference due to magnetic fields from devices that run on
electricity, and interference from the radio signals coming from
some electronic devices. In theory, anything running on
electricity can emit interference in the form of a magnetic field; other
competing radio waves; or both.
A little bit of a physics lesson might help here, but we promise this
won't be a physics lecture! All wireless signals are emitted over radio
waves. "Radio" is a catch-all term for a certain frequency of
electromagnetic waves which are relatively low-band and useful for data
transfer. This generally encompasses not only your AM/FM "radio," but
also your cordless phone, your cellular telephone, your television, your
microwave, and of course, your wireless network. All of these devices,
and numerous others, emit radio waves. Any electronic device which
transmits something in an "invisible" way over the air probably uses
radio waves.
Magnetic Interference from Electrical Devices
First, let's discuss magnetism. Electricity and magnetism are closely
tied; every electric field has a corresponding magnetic field. All of
this may sound like physics mumbo-jumbo, but it's important to understand
so you can avoid problems. For now, just keep in mind that magnetic
fields, regardless of their source, can interfere with radio waves. ANY
device that uses electricity generates a magnetic field. The power of the
magnetic field depends largely on the power of the device. An air
conditioning compressor, a microwave, and a clothes dryer all generate
larger magnetic fields than a blender, a lamp, or an electric train set
would. Avoid placing your wireless access point or wireless clients near
high-voltage devices. Yes, Virginia, this means that your dryer can
interfere with your wireless signal. Strange but true! Another source of
magnetic interference is -- naturally -- other magnets! Other transducers
in your home contain magnets, exactly because of what magnets do -- they
manipulate (polarize) electrical fields. Televisions contain magnets.
So do speakers, both big and small. Avoid putting your wireless access
point near speakers, unless you're really sure that they're magnetically
shielded. Never put a wireless access point near an unshielded TV; you
can discolor the TV picture tube and interfere with the wireless
signal. Luckily, most new TVs are magnetically shielded. TVs without
tubes (plasmas, for example) do not even contain large magnets. But it's
still best to stay away from TVs in general.
If you're looking for something tangible that puts all of this into
perspective, put your cell phone right up near a pair of unshielded
computer speakers, and then call it from another phone. You will probably
eventually hear weird noises coming out of your speakers as their magnetic
field collides with that of your cell phone. Both devices "suffer."
Your wireless access point would react in the same way your cell phone
did.
Two Kinds of Interference from Other Radio Waves
In addition to electricity and magnetism, the idea of radio waves
interfering with other radio waves is important to understand. There are
two main causes. One is our old friend magnetism, and the other is
a phenomenon known as "phasing."
Magnetic Interference from Other Radio Waves
We said that electrical equipment emits a magnetic field. Radio waves
also emit a magnetic field, which is why they are classified as
"electromagnetic." Even though this magnetic field is relatively weak,
radio waves are very small, sensitive things. Magnetic interference from
radio waves can interfere with other radio waves. Two radios next to one
another can interfere with one another; a radio near a television
receiving off-air broadcast signals may cause interference to one or both;
even two cell phones right next to one another can cause interference for
each other from time to time. Although this kind of interference isn't
the round-the-clock kind, it's best to put your wireless access point in a
place that is far from other sources of radio waves. Don't put your
wireless access point near your TV, a radio, a police scanner, microwave,
video game system, cordless phone, or anything else that can generate
radio waves when it's on -- including another wireless access point!
Phasing Interference from Other Radio Waves
Phasing is a much nastier problem because it may happen even if your
wireless access point isn't close to the source of the interference. To
understand phasing, we have to discuss a little bit of the properties of a
wave, since we've established now that your wireless signals are nothing
more than radio waves. Just about all energy, electromagnetic or
otherwise, can be characterized as waves (there are exceptions, but we
promised this wasn't a physics lesson!). Each wave has characteristics;
it has a direction, a size (wavelength), and a period of time in which its
pattern repeats (frequency).
Generally speaking, the smaller a wave is, the faster it repeats
itself. This sort of makes sense, right? This means smaller waves have
higher frequency. But since higher frequency waves are smaller, they
don't travel as far. This effectively lowers their operating range.
Wave frequency is measured using an SI (metric) unit called the Hertz, or
Hz.
Armed with that knowledge, you can now figure out why the different
Wi-Fi protocols have different operating ranges -- the frequencies differ.
Suddenly, the chart below makes sense!
| Protocol | | Frequency | |
Speed | |
Indoor Operating Range Max (approx.) |
| 802.11b | | 2.4 GHz | |
11Mbps | | 150-300 feet |
| 802.11g | | 2.4 GHz | |
54Mbps | | 100-200 feet (due to 54 Mbps rate) |
| 802.11a | | 5.0-5.8 GHz | |
54Mbps | | 80 feet |
Wave frequency is the property of interest in phasing. When two waves
of the same frequency but from different sources collide, they can cancel
each other out, since the timely repetition gets interrupted. You can
demonstrate the effects of phasing by using another common energy wave --
sound! If you wire speakers to an amplifier such that the positive
terminal is receiving the ground signal, and the ground terminal is
receiving the positive signal, your speakers' sound will be "out-of-phase"
-- unclear and very non-directional. The sound is out-of-phase because
the wave-like signals from the amp, when changed into wave-like sound
energy by the speakers, are canceling each other out due to the backwards
wiring. By the way, don't try this -- you can damage your equipment!
Also, since different waves of the same frequency all have the same
characteristics, the devices that receive them can't really tell them
apart. So, if you have your Wi-Fi capable laptop sitting near your
microwave, and the oven is on, it's receiving signals from both the
microwave and the wireless access point! Many microwaves, cordless
telephones and Bluetooth devices operate in the 2.4 GHz range, as do baby
monitors and certain high-powered walkie-talkies. All of these
devices, when on, can interfere with anything else listening in the 2.4
GHz range -- which can include your 802.11b wireless network!
The best way to avoid these kinds of problems is to do your homework. If
you already have a 2.4 GHz phone, you probably don't want an 802.11b or g
network. Then again, you may not be on the phone often enough to care.
If you've already invested in an 802.11b network, don't buy a 2.4 GHz
cordless phone. A little research will yield similar products that
operate in different frequency ranges. For example, the old 900 MHz
cordless phones will not interfere with your wireless network. If
your network is 802.11b or g, a 5 GHz cordless is also ok.
So, What Do I Do?
In summary, there are lots of different options with wireless networks and
just as many potential sources of interference. A little strategic
planning in your placement of the wireless access point will go a long way
towards solving your physical interference problems and your
electromagnetic interference problems. Another key is to understand that
any electronic device that works without wires uses radio frequencies,
more than likely. If it can send something "invisibly" over the air, it's
using radio waves. This includes cordless phones, baby monitors,
remote-controlled devices (and the remotes), wireless network equipment,
walkie-talkies, Bluetooth devices, and the like. Many of these devices,
like TV remotes and remote-controlled toys, operate at such a low
frequency (infrared, for instance) that it's never an issue. But newer,
more powerful devices can be problematic. Before buying any electronic
device that transmits radio frequency data, look into its frequency range
and make sure it does not conflict with your wireless network plans.
In a future article, we'll explore what to do if you run out of options
for frequency range device compatibility or strategic placement of your
wireless network devices.
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