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infrastruc-
ture mode.



Ad hoc mode
Whenever two wireless stations are close enough to communicate with each
other, they are capable of establishing an ad hoc network: that is, a wireless
network that doesn™t use an AP. Theoretically, you could create a home net-
work out of wireless stations without the need for an AP. It™s more practical,
however, to use an AP; an AP is more effective because it facilitates communi-
cation between many stations at once (as many as 30 stations simultaneously
in a single wireless network segment). In addition, an AP can create a connec-
tion or bridge between a wireless network segment and a wired segment.

Ad hoc mode isn™t often used in wireless home networks, but it could be used
on occasion to connect two computers together to transfer files where there
is no AP in the vicinity to create a wireless infrastructure.




Your Wireless Network™s Power Station ”
the Antenna
The main interface between your access point or network interface card and
the network is the antenna. Signals generated and received by your wireless
gear is dependent on a high quality antenna interface. To be smart in wireless
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Chapter 2: From a to g and b-yond

networking, you need to know the basics about antennas. If you know how
they work, you can better optimize your network.

Access point antennas vary from manufacturer to manufacturer. Many APs
have a single external antenna about five inches in length. This type of
antenna is a dipole antenna. Some APs have two external dipole antennas.
Dual external antenna models should provide better signal coverage through-
out the house. APs with dual antennas might transmit from only one of the
antennas but receive through both antennas by sampling the signal and using
whichever antenna is getting the strongest signal ” a diversity antenna
system.

Typical omnidirectional dipole antennas attach to the AP with a connector
that enables you to position the antenna at many different angles; however,
omnidirectional dipole radio antennas send and receive best in the vertical
position.

The range and coverage of a Wi-Fi wireless AP used indoors is determined by
the following factors:

AP transmission output power: This is the power output of the AP™s
radio, usually referred to as transmission power or TX power. Higher
power output produces a longer range. Wi-Fi APs transmit at a power
output of less than 30 dBm (one watt). Government agencies around the
world regulate the maximum power output allowed. APs for home use
generally have power outputs in the range 13 dBm (20 mW) to 15 dBm
(31.6 mW). The higher the power rating, the stronger the signal and the
better range your wireless network will have. Some wireless networking
equipment manufacturers offer add-on amplifiers that boost the stan-
dard signal of the AP to achieve a longer range. We talk about boosters
in Chapter 18. (For more on TX power, see the sidebar, “TX power
output and antenna gain.”)
Antenna gain: The AP™s antenna and the antenna(s) on the other
device(s) on the network improve the capability of the devices to send
and receive radio signals. This type of signal improvement is gain.
Antenna specifications vary depending on vendor, type, and materials.
Adding a higher gain antenna at either end of the connection can
increase the effective range.
Antenna type: Radio antennas both send and receive signals. Different
types of antennas transmit signals in different patterns or shapes. The
most common type of antenna used in wireless home networks, the
dipole antenna, is described as omnidirectional because it transmits its
signal in all directions equally. In fact, the signal from a dipole antenna
radiates 360° in the horizontal plane and 75° in the vertical plane, creat-
ing a doughnut-shaped pattern. Consequently, the area directly above or
below the antenna gets a very weak signal.
40 Part I: Wireless Networking Fundamentals

Some types of antenna focus the signal in a particular direction and are
referred to as directional antennas. In special applications where you
want an AP to send its signal only in a specific direction, you could
replace the omnidirectional antenna with a directional antenna. In a
home, omnidirectional is usually the best choice, but that also depends
on the shape of the home; some antennas are better for brownstones
and multifloor buildings because they have a more spherical signal foot-
print rather than the standard flat-ish one.
Receive sensitivity: The receive sensitivity of an AP or other wireless
networking device is a measurement of how strong a signal is required
from another radio before the device can make a reliable connection and
receive data.
Signal attenuation: A radio signal can get weaker as a result of interfer-
ence caused by other radio signals because of objects that lie in the
radio wave path between radios and because of the distance between
the radios. The reduction in signal is attenuation. Read through Chapter
6 for a discussion of how to plan the installation of your wireless net-
work to deal with signal attenuation.

In order to replace or add an antenna to an AP or other wireless device, you
need to have a place to plug it in ” as obvious a statement as that is, many
antennas are not detachable, and you can™t add another antenna. Some
access points use reverse TNC connectors that let optional antennas be used
in 802.11b/g products, but there™s a minor trend away from using detachable
antennas in 802.11a products because of potential conflict in the frequency
channels allocated to 802.11a. This potentially thwarts misuse, but also robs
those deploying access points of their ability to choose optimal antennas.




Industry Standards
One of the most significant factors that has led to the explosive growth of
personal computers and their impact on our daily lives has been the emer-
gence of industry standards. Although many millions of personal computers
are in use today around the world, only three families of operating system
software run virtually all these computers: Windows, Mac OS, and Unix
(including Linux). Most personal computers that are used in the home
employ one of the Microsoft Windows operating systems or one of the Apple
Macintosh operating systems. The existence of this huge installed base of
potential customers has enabled hundreds of hardware and software compa-
nies to thrive by producing products that interoperate with one or more of
these industry standard operating systems.
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Chapter 2: From a to g and b-yond

Computer hardware manufacturers recognize the benefits of building their
products to industry standards. To encourage the adoption and growth of
wireless networking, many companies that are otherwise competitors have
worked together to develop a family of wireless networking industry stan-
dards that build on and interoperate with existing networking standards. As a
result, reasonably priced wireless networking equipment is widely available
from many manufacturers. Feel safe buying equipment from any of these
manufacturers because they™re all designed to work together, with one impor-
tant caveat. The current three major flavors of this wireless networking tech-
nology for LAN applications are IEEE 802.11a, 802.11b, and 802.11g. You just
have to pick the flavor that best fits your needs and budget. (Note: There are
other wireless standards for other applications in the home, like Bluetooth
for short range communications. We talk about these in Chapter 3 and else-
where where their discussion is appropriate.)

The Institute for Electrical and Electronics Engineers
The Institute for Electrical and Electronics Engineers (IEEE) is a standards-
making industry group that has for many years been developing industry
standards that affect the electrical products that we use in our homes and
businesses every day. At present, the IEEE 802.11b standard is the over-
whelming market leader in terms of deployed wireless networking products.
Products that comply with this standard weren™t the first wireless networking
technology on the market . . . but they are now, by far, the dominant market
installed base. As you will soon see, the 802.11a and g products are coming
on strong.




TX power output and antenna gain
TX power output is measured in milliwatts (mW) gain an antenna has). An antenna with a 4 dBi
but is also often expressed by using dBm units gain increases the output power (the effective
of measurement. (dBm measures, in decibels, a isotropic radiated power, or EIRP) of the radio
radio™s amount of power.) The FCC permits an by 4 dBm. The FCC permits IEEE 802.11 radios a
AP to have a maximum power output of 1,000 maximum EIRP of 36 dBm when the device is
mW (1 watt), which is the same as 30 dBm. Wi- using an omnidirectional antenna. The antennas
Fi APs typically have maximum output power of included with home wireless networking equip-
100 mW (20 dBm) or less. APs for home use gen- ment are typically omnidirectional detachable
erally have power outputs in the range 13 dBm (20 dipole antennas with gains of from 2 dBi to 5
mW) to 15 dBm (31.6 mW). The higher the power dBi. Some manufacturers offer optional high-
rating, the stronger the signal and the better gain antennas. (Note: The maximum EIRP output
range that your wireless network will have. permitted in Japan is 100 mW; and the maximum
output in Europe is only 10 mW.)
Antenna gain is usually expressed in dBi units
(which indicate, also in decibels, the amount of
42 Part I: Wireless Networking Fundamentals

The Wi-Fi Alliance
In 1999, several leading wireless networking companies formed the Wireless
Ethernet Compatibility Alliance (WECA), a nonprofit organization (www.weca.
net). This group has recently renamed itself the Wi-Fi Alliance and is now a
voluntary organization of over 200 companies that make or support wireless
networking products. The Wi-Fi Alliances™ primary purpose is to certify that
IEEE 802.11 products from different vendors will interoperate (work together).
These companies recognize the value of building a high level of consumer con-
fidence in the interoperability of wireless networking products.

The Wi-Fi Alliance organization has established a test suite that defines how
member products will be tested by an independent test lab. Products that
pass these tests are entitled to display the Wi-Fi trademark, which is a seal of
interoperability. Although there is no technical requirement in the IEEE speci-
fications stating that a product must pass these tests, Wi-Fi certification
encourages consumer confidence that products from different vendors will
work together.

The Wi-Fi interoperability tests are designed to ensure that hardware from
different vendors can successfully establish a communication session with an
acceptable level of functionality. The test plan includes a list of necessary fea-
tures. The features themselves are defined in detail in the IEEE 802.11 stan-
dards, but the test plan specifies an expected implementation.

IEEE 802.11b: The defending champ
In 1990, the IEEE adopted a document entitled “IEEE Standards for Local and
Metropolitan Area Networks” that provided an overview of the networking
technology standards used in virtually all computer networks in prevalent
use today. The great majority of computer networks use one or more of the
standards included in IEEE 802; the most widely adopted is IEEE 802.3, which
covers Ethernet.

IEEE 802.11 is the section that defines wireless networking standards and is
often called wireless Ethernet. The first edition of the IEEE 802.11 standard,
adopted in 1997, specified two wireless networking protocols that can trans-
mit at either one or two megabits per second (Mbps) using the 2.4 GHz radio
frequency band, broken into 14 5-MHz channels (11 in the United States).
IEEE 802.11b-1999 was a supplement to IEEE 802.11 that added subsections to
IEEE 802.11 that specify the protocol used by Wi-Fi-certified wireless network-
ing devices.

The IEEE 802.11b protocol is backward compatible with the IEEE 802.11 pro-
tocols adopted in 1997, using the same 2.4 GHz band and channels as the
slower protocol. The primary improvement of the IEEE 802.11b protocol is a
technique that enables data transmission at either 5.5 Mbps or 11 Mbps.
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Chapter 2: From a to g and b-yond

Because 11 Mbps is as fast as standard Ethernet (10 Mbps) and is faster than
most broadband Internet connections, it is quite adequate for use in most
home networks. However, 11 Mbps is still a bit slow for transmission of DVD-
quality streaming video.

Several vendors offer IEEE 802.11b products with a “turbo” setting that pro-
vides data transmission speeds up to 22 Mbps, double the normal maximum
rate. Be aware that this feature is proprietary, which means that it might only
work with other wireless networking equipment from the same manufacturer.

IEEE 802.11a: Fast, faster, and fastest
IEEE adopted 802.11a-1999 at the same time that it adopted 802.11b. IEEE
802.11a specifies a wireless protocol that operates at higher frequencies than
the IEEE 802.11b protocol and uses a variety of techniques to provide data
transmission rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. 802.11a has 12 non-
overlapping channels in the United States and Canada, but most deployed
products use only 8 of these channels.

Some wireless networking vendors offer proprietary enhancements to IEEE
802.11a-compliant products that double the top speed to over 100 Mbps.

An increasing number of products based on the IEEE 802.11a standard has
reached the market. In addition to the higher transmission speeds, IEEE
802.11a offers the following advantages over IEEE 802.11b:

Capacity: 802.11a has about four times as many available channels,
resulting in about eight times the network capacity: that is, the number
of wireless stations that can be connected to the AP at one time and still
be able to communicate. This isn™t a significant advantage for a wireless
home network because you™ll almost certainly never use all the network
capacity available with a single access point (approximately 30 stations
simultaneously).
Less competition: Portable phones, Bluetooth, and residential
microwave ovens use portions of the same 2.4 GHz radio frequency band
used by 802.11b, which sometimes results in interference. By contrast,
very few devices other than IEEE 802.11a devices use the 5 GHz radio
frequency band.
Improved throughput: Tests show as much as four to five times the data
link rate and throughput of 802.11b in a typical office environment.
Throughput is the amount of data that can be transferred over the con-
nection in a given period of time. (See the sidebar elsewhere in this
chapter, “Gauging your network™s throughput.”)
44 Part I: Wireless Networking Fundamentals

When does a + b = g?
The last of the IEEE standards-based products to hit the street is 802.11g, and
these products are selling like hotcakes. Although the g standard is still being
finalized as we write ” it™s expected to be final in mid-2003 and will enter
interoperability testing soon thereafter ” the appeal of 802.11g is so great
that many vendors aren™t waiting for the final standard to be adopted before
they release their first products based on this technology. Instead, they will

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