potential spectrum interference issues with 802.11b,
should you consider moving to 802.11a on 5 GHz? by Dr
most parts, the 802.11b specification has thrived on
the fact that the radio spectrum which it rides onthe
2.4-GHz bandis unlicensed. This meant that vendors,
installers and even DIY types can build WLAN networks
without needing to get clearance from regulatory bodies,
and consequently avoid paying exorbitant radio spectrum
But latitude is a double-edged sword. Plying in an unregulated
spectrum means that interference from other devices
becomes a potentially disastrous prospect.
For example, three signal-emitting devices which are
household staples and can potentially trip up an 802.11b
network are alarm systems, microwave ovens and cordless
Surveillance cameras equipped with on-board antennae,
including audio transmitters, can be mounted at any
corner within the home. Often, the cameras are secured
at the corner of each possible entry point within the
home, and provide a spread pattern overlapping all connected
intersections. Such wireless cameras usually come in
three common frequencies: 400 MHz, 900 MHz and 2.4 GHz.
900 MHz is illegal in most parts of Asia as it interferes
with the licensed GSM 900 MHz spectrum. 400 MHz is too
low a frequency and can be interfered with even the
likes of remote control toys. Therefore, the only useful
form of wireless camera surveillance systems are in
the 2.4 GHz range.
Microwave ovens also use the 2.4 GHz frequency range,
although most models have sufficient shielding. At the
same time, 802.11b WLANs are usually used in study rooms
which are normally situated away from the kitchen, where
microwave ovens are found. Because of the concrete walls
found in most modern architectures, the range of 802.11b
WLANs can be drastically reduced. If you observe the
range indicator for the WLAN, you may find that your
effective range is reduced to nearly half if your base
station needs to reach an adapter separated by two thick
concrete walls. Therefore, it is unlikely that the microwave
oven in the kitchen will interfere too much with the
WLAN in your study room, often separated by at least
two, if not three, concrete walls. If your study is
situated upstairs, and your kitchen downstairs, it is
also likely that the degree of separation is by atleast
2 walls (including the ceiling of the kitchen).
Cordless phone also come in 400-MHz, 900-MHz and 2.4-GHz
models. Again, 900-MHz models are banned in most parts
of Asia because they interfere with the legal GSM 900
MHz frequency range. 400-MHz models usually have low
sound fidelity and are not favored. This leaves the
2.4-GHz model, which is often touted by vendors to yield
"superb" sound quality. Since you do need
to use your cordless phones around the home, including
in your study, there will be possible significant interference
Another potentially phone-related interference culprit
is Bluetooth. Though not widely used, it is used by
a few cellular phone vendors to wirelessly-connect associated
gadgets together. Today, most cellular phones use wired
hands-free kits to allow users to talk without holding
the cellular phone close to their ears. Ericsson has
a Bluetooth-enabled wireless headset that communicates
with your cellular phone within 30 ft. These headsets
can potentially present interference issues should there
be users using 802.11b WLANs in the same vicinity.
you need to wire up your home or small office
(or workgroup) with 802.11b WLAN, be aware that
devices in the same frequency range should preferably
be excluded from the vicinity.
You may choose NOT to use 2.4 GHz cordless phones,
Bluetooth gadgets (which are showy but not that
useful anyway), and microwave ovens in the same
room. For phones, you can use corded phones, or
use good quality speakerphones or conference rooms
to free up your hands for your keyboard instead.
If you must have surveillance systems, install
a hard-wired setup instead. Incidentally, hard-wired
surveillance and alarm systems do have an added
benefitthey can run from line power with
a battery backup, rather than rely on battery
only. Although most surveillance and alarm systems
that run from battery sources (typically 9V DC)
can last at least six months to a year, it is
still safer to have a powered setup with a battery
backup. Also, failed batteries can give false
positives, creating havoc for you to travel home
to disable the alarm.
As for your 802.11b WLAN, you can also turn on
Interference Robustness, if your base station
presents such a choice in its setup. With Interference
Robustness, essentially the base station limits
its transmission range with a stronger signal
to recognize your network traffic.
You can also cycle through the available channels
to test which channel works best even if you do
have other conflicting 2.4 GHz devices around.
It is with the issue of interference that the 802.11a
IEEE specification came about, and became adopted as
the next generation of WLAN infrastructure technology.
With the 5 GHz Unlicensed National Information Infrastructure
(UNII), FCC regulations for 15.407 became more specific.
Therefore, the upper band from 5.725 to 5.825 GHz is
intended for outdoor fixed broadband wireless access
devices requiring higher power to reach longer ranges.
The lower 200 MHz band is intended for indoor, lower-powered
WLANs in the 5.15 to 5.35 GHz. The specification also
states that all UNII devices must be high data rate
communication devices, specifically excluding low-rate
devices such as cordless phones, Bluetooth gadgets,
microwave ovens, surveillance systems, and so on.
How 802.11a modulation works is through orthogonal frequency
division multiplexing (ODFM) to reduce the multipath
effect whereby the transmitted radio signal is reflected
from walls, fixtures and indoor objects, rather than
reaching the receiver in a single direct path. This
reduces the effective range, but also reduces interference
from other broadcasting signals at the same time.
There is also a technical advantage of the number of
usable channels for 802.11a versus 802.11b. 802.11b
technology has only three usable, non-overlapping channels
for use in data networking environments, which can present
proximity interference from adjacent base stations from
other subnets. On the other hand, 802.11a has more usable
non-overlapping channels (8 in the USA, 19 in the European
Union and four in Japan), which means that there will
be even fewer instances of proximity interference from
neighboring base stations belonging to other companies
with no business participating in your own setup.
Hold your horses
However, not all is green light for the cutting-edge
802.11a WLAN technology yet. Even in Japan, there is
some resistance for widespread adoption of 802.11a.
Likewise, in many parts of the world, including Asia-Pacific,
802.11a coincides with the primary use of the 5 GHz
band by radar and satellite applications.
There is a Dynamic Frequency Selection (DFS) and Transmit
Power Control (TPC) specification under review by spectrum
regulatory boards and hopefully, these will smooth the
path for universal adoption and acceptance of the 802.11a
WLAN technology, especially in more regulated markets
Beyond regulatory and governmental resistance issues,
802.11a technology is still quite expensive compared
to traditional 802.11b WLAN technologies, often at 50
percent or so higher retail prices. The lack of backward
compatibility with 802.11a, especially for corporations
that have already implemented partial or even complete
802.11b infrastructure, will be a major throw backward
as well for 802.11a's acceptance in the corporate environment.
Although there are vendors touting hybrid chipsets combining
both 802.11b and 802.11a in the same base stations,
these are merely stop-gap measures and the real technology
and migration challenges still lie ahead.
Until the dust settles with 802.11g (a backward compatible
high data rate specification with 802.11b), and vendors
can bring cheaper and more compatible 802.11a products
to the market, there will still be resistance to adopt
802.11a technology to transcend the interference gripes
with existing 2.4 GHz technologies.
Phan is research director at KnowledgeLabs News Center
(www.knowledgelabs.net), an independent technology news
bureau and writes for Network Computing-The Asian Edition.
He can be reached at email@example.com