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1.3 Wireless Alphabet Soup

While it is not the sole focus of this book, there are several
chapters that deal entirely with
"Wi-Fi," or Wireless
Fidelity. This phrase is
trademarked by the Wi-Fi Alliance, a group that consists
of nearly all 802.11 manufacturers. The Wi-Fi Alliance does product
testing and certification for interoperability.

802.11
was defined as a protocol by the Institute of Electrical
and Electronics Engineers (IEEE) in 1997. This protocol
specification allowed for 1 and 2 Mbps transfer rates using the
2.4 GHz ISM (Industrial, Scientific, and
Medical) band, which is open to unlicensed public use. Prior to the
adoption of this standard, there were various wireless network
vendors manufacturing proprietary equipment using both the 2.4 GHz
and the 900 MHz bands. The early adopters of the proprietary
technologies and 802.11 were primarily the manufacturing and health
care industries, which rapidly benefited from their
employees' mobile access to data. The 802.11
standard uses spread spectrum modulation to achieve high data rates.
Two types of modulation were specified: Frequency Hopping and
Direct
Sequence. 802.11 also uses the Carrier Sense Multiple
Access (CSMA), which was
developed for Ethernet in 1975 with the addition of Collision
Avoidance (CA)referred to as CSMA-CA.

In 1999, the IEEE adopted two supplements to the 802.11 standard:
802.11a and 802.11b. The 802.11b standard is also referred to as
High Rate DS
and is an extension of the Direct Sequence Spread Spectrum type of
modulation specified in 802.11. 802.11b uses 14 overlapping,
staggered channels, each channel occupying 22 MHz of the spectrum.
This standard's primary benefit is that it offers
data rates of 5.5 and 11 Mbps in addition to the 12 megabits provided
by 802.11. 802.11b has been widely adopted around the world, and its
products have been readily available since 1999.

However, 802.11a products did not begin shipping until 2001.
802.11a utilizes a
range in the 5 GHz frequency and operates with a theoretical maximum
throughput of 54 Mbps. It provides for 12 nonoverlapping channels.
Products based on this protocol have not seen the adoption rate of
802.11b products for several reasons. At higher frequencies, more
power is needed to transmit. The power of 802.11 radio types is
limited; therefore, 802.11 and 802.11b have longer range transmission
and reception characteristics than 802.11a. Because of its higher
frequency, 802.11a is absorbed more readily by obstacles, reducing
range and throughput.

In June of 2003, the IEEE ratified a third supplement to the 802.11
standard: 802.11g. This standard continues to operate in the 2.4 GHz
band with backward compatibility to 802.11b, but it raises the
theoretical maximum throughput to 54 Mbps. In early 2003, there were
many products released prior to the ratification of the standard. The
standard was delayed several times as the subcommittees in the IEEE
worked out interoperability issues between 802.11b and 802.11g.

1.3.1 Operating Modes

There are two main client operating modes in the
802.11 family of standards:
Infrastructure and Ad-Hoc. Two additional modes, Master and Monitor,
are discussed in later chapters.

Infrastructure
Mode requires the use of a
wireless access point. At a minimum, this
is a device with a radio that operates in Infrastructure Mode and has
a connection to a wired network. This is also known as the
Basic Service Set (BSS). There is also an
Extended Service Set (ESS) for use with
multiple access points.

A typical 802.11b access point consists of a radio, external antenna,
and at least one Ethernet port. There are many variations on this
theme, with models sporting 4-port Ethernet switches, connectors for
other external antennas, and higher-power radios.

When operating in Infrastructure Mode, an access point is the master
of any client radios that are associated with the access point. The
client radios are also operating in Infrastructure Mode, in a
different sub-mode. The access point is programmed with a
Service Set Identifier (SSID); this is the network
name for the access point. The access point broadcasts the SSID as an
advertisement of the network name.

Clients operating in Infrastructure Mode identify an access point by
these SSID broadcast frames. Once a client is associated with an
access point, the access point manages all communication over the
radio link. When multiple clients are associated with a single access
point, the access point has a set of algorithms for controlling
traffic to and from the access point radio.

Ad-Hoc Mode, or peer-to-peer mode, is designed specifically
for client-to-client communication. To use Ad-Hoc Mode, you need at
least two radio clients. In this example, let's say
we have two Linux notebooks with PCMCIA radio cards. Both cards are
configured to work in Ad-Hoc Mode, and both clients must use the same
SSID. Ad-Hoc clients do not advertise themselves with the same
broadcast frames used by an access point.

While Ad-Hoc Mode is very useful for
client-to-client communication, it
introduces a difficult situation known as the Hidden
Node problem. Ad-Hoc Mode does not provide
an access point to control communications between other client
machines, so any client using Ad-Hoc Mode may decide to transmit data
on its own rather than being told when it is clear to transmit. Figure 1-7 illustrates the problem.

Figure 1-7. A Hidden Node problem with three clients in Ad-Hoc Mode

As shown, node A can hear node B, but it cannot hear node C. Node C
can also hear node B, but it cannot hear node A. Because 802.11 is a
shared-access physical medium, only one device can transmit at any
given time. The Hidden Node problem is that node A and node C cannot
hear each other, and neither node will detect a collision. Hidden
Node issues reduce throughput in this example by at least 50%.

1.3.2 Wi-Fi Hardware

As discussed previously, to make a Wi-Fi
network, you need a minimum of two radios, whether you operate in
Ad-Hoc or Infrastructure Mode. For PC hardware, there are three
physical types of radio interface cards available:
PC Card,
PCI, and
MiniPCI.

Of the three, the PC Card is by far the most common, because notebook
PCs are widely deployed, and most have at least one card slot;
notebook users are the most common users of 802.11 networks.

MiniPCI cards are the up-and-coming form factor. Many notebook
manufacturers have built MiniPCI cards into their motherboards, which
enables you to install network cards without using a PC Card slot.

At one time, PCI cards were not as common as the other types of
radios, but they are staging a comeback with new offerings from
Linksys and D-Link. Many manufacturers, such as Linksys and D-Link,
produce some PCI cards now, which actually consist of a MiniPCI or
PCMCIA
card on a larger PCI card.

There is a fourth option for a growing number of notebook and PDA
users: built-in Wi-Fi. Intel is marketing their
Centrino
chipset that integrates an 802.11b radio on the motherboard, and most
notebook manufacturers offer Centrino notebooks. Similarly, other CPU
manufacturers such as Via will be integrating wireless into their
chipsets. Finally, there are a number of notebook and PDA models that
feature built-in radios. Sony, for example, sells a Vaio notebook
with an Orinoco radio built in and also sells the Clie handheld PDAs
with optional Wi-Fi.

As of this writing, more and more dual- and
tri-mode cards are available. These cards
allow you to access 802.11a/b/g networks with a single radio. The
maker of a radio chipset decides the level of supportas of
this writing, support for these cards is still in flux under Linux.
We'll cover this in more detail in the next chapter.

Wireless access points are also available now in dual- and tri-mode.
There is a wide range of access points on the market, which range
from units geared specifically for home users with built-in
firewalls, 4-port switches, and web-based configuration to models
aimed at the corporate market with support for authentication
protocols such as
RADIUS and
LDAP, the ability to
run via Power Over
Ethernet (POE), and connectors for external
antennas.

Another category of access point is the
"hotspot in a box." With the
rising popularity of Wi-Fi hotspots in cafes, hotels, and airports,
many manufacturers have developed access points that are an
all-in-one solution. These boxes provide the radio and Ethernet of a
normal access point, but also have some form of authentication and
payment system, which range from a web-based login to a printed
coupon that the store clerk delivers to the customer.

1.3.2.1 Antennas

Although a discussion of the physics of antennas
is beyond the scope of this book, antennas are obviously a very
important part of any radio. Depending on the type of antenna, radio
coverage is narrowly focused or widely distributed, which makes a
great deal of difference when building or connecting to 802.11
networks.

Briefly, antennas are transducers that convert radio frequency
electric currents to electromagnetic waves that are then radiated
into space. Antennas are polarized according to
the plane of the electric field radiating from the antenna. A
vertically polarized antenna has an
electric field that is perpendicular to the Earth's
surface. Likewise, the electric field of a
horizontally polarized antenna is
parallel with the Earth's surface.

There are several types of antennas used for Wi-Fi networks. The most
common antenna is the integrated antenna,
followed by omnidirectional and
directional antennas

Integrated antennas

Most PC Card radios have integrated antennas inside the enclosure of
the card. A typical integrated antenna design has two very small
antennasreally just a solder trace or small piece of
foillocated at right angles to each other for
diversity. Diversity antennas are designed so
that one antenna or the other is used to transmit and receive, but
never at the same time. The card switches automatically between
antennas to choose the stronger signal. The antennas are horizontally
polarized, and this layout produces an antenna that has a somewhat
omnidirectional pattern in a horizontal beam.

Omnidirectional antennas

If you have a radio card or access point with a single external
antenna attached, you are likely looking at an omnidirectional, or
omni, antenna. Omnidirectional antennas, as the
name implies, are designed to send and receive signals 360 degrees
around the antenna. Figure 1-8, which is a sample
antenna gain pattern for a commercially produced omnidirectional
antenna, shows that the 360-degree pattern is not circular at all.
Notice that the antenna has pronounced gain at 0 and 180 degrees, but
hardly any gain at 90 and 270 degrees.

Figure 1-8. A sample omnidirectional antenna gain pattern

While the theoretical beamwidth of an omnidirectional antenna is 360
degrees horizontally, the vertical beamwidth of most omni antennas is
less than 8 degrees. See Figure 1-9 for a side view
of a typical omni antenna. Notice that if the antenna were mounted
high enough, someone directly under the antenna itself would have
very poor signal quality.

Figure 1-9. A side view of an omnidirectional antenna beamwidth

Most omnidirectional antennas are of the "rubber
ducky" typea rubber- covered antenna, which
ranges from a few inches long for a low-gain model to several feet
for high-gain types.

Directional antennas

Although patch antennas are similar to sector antennas, they are
considered directional antennas.
Patch antennas generally have
horizontal and vertical beamwidths that are similar. An example shown
in Figure 1-10 shows the gain patterns for a patch
antenna.

Figure 1-10. A sample patch antenna gain pattern

Yagi antennas are also directional
antennas and are designed for highly directional applications. They
typically have a beamwidth of less than 30 degrees; most of them look
like a PVC pipe or a "Christmas
tree" pointed at its target.

Finally, parabolic dish, or grid, antennas are
the most highly directional antennas used in the 802.11 world. If
you've seen a satellite dish,
you've seen a parabolic dish antenna. These antenna
types are suited for sending wireless network signals over several
miles. As shown in Figure 1-11, the gain pattern is
very tight.

Figure 1-11. A sample parabolic dish antenna gain pattern

Another antenna type widely used in outdoor applications is a
sector antenna. These antennas are
generally available with horizontal polarization and antenna patterns
from 90 to 180 degrees. They are rectangular with a flat profile.