INTRODUCTION TO WIRELESS NETWORKS

INTRODUCTION TO WIRELESS NETWORKS
2
INTRODUCTION TO WIRELESS
NETWORKS
Up to a point, it’s quite possible to treat
your wireless network as a set of black boxes
that you can turn on and use without knowing
much about the way they work. That’s the way
most people relate to the technology that surrounds
them. You shouldn’t have to worry about the technical specifications just to
place a long-distance telephone call or heat your lunch in a microwave oven
or connect your laptop computer to a network. In an ideal world (ha!), the
wireless link would work as soon as you turn on the power switch.
But wireless networking today is about where broadcast radio was in
the late 1920s. The technology was out there for everybody, but the people
who understood what was happening behind that Bakelite-Dilecto panel
(Figure 2-1) often got better performance than the ones who just expected
to turn on the power switch and listen.
In order to make the most effective use of wireless networking technology, it’s still important to understand what’s going on inside the box
(or in this case, inside each of the boxes that make up the network). This
chapter describes the standards and specifications that control wireless
networks and explains how data moves through the network from one
computer to another.
Figure 2-1: Every new technology goes through the tweakand-fiddle stage.
When the network is working properly, you should be able to use it without thinking about all of that internal plumbing—just click a few icons and
you’re connected. But when you’re designing and building a new network,
or when you want to improve the performance of an existing network, it can
be essential to understand how all that data is supposed to move from one
place to another. And when the network does something you aren’t expecting
it to do, you will need a basic knowledge of the technology to do any kind of
useful troubleshooting.
How Wireless Networks Work
Moving data through a wireless network involves three separate elements:
the radio signals, the data format, and the network structure. Each of these
elements is independent of the other two, so you must define all three
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when you invent a new network. In terms of the OSI reference model, the
radio signal operates at the physical layer, and the data format controls
several of the higher layers. The network structure includes the wireless
network interface adapters and base stations that send and receive the radio
signals. In a wireless network, the network interface adapters in each computer
and base station convert digital data to radio signals, which they transmit to
other devices on the same network, and they receive and convert incoming
radio signals from other network elements back to digital data.
Each of the broadband wireless data services use a different combination
of radio signals, data formats, and network structure. We’ll describe each
type of wireless data network in more detail later in this chapter, but first,
it’s valuable to understand some general principles.
Radio
The basic physical laws that make radio possible are known as Maxwell’s
equations, identified by James Clerk Maxwell in 1864. Without going into the
math, Maxwell’s equations show that a changing magnetic field will produce
an electric field, and a changing electric field will produce a magnetic field.
When alternating current (AC) moves through a wire or other physical
conductor, some of that energy escapes into the surrounding space as an
alternating magnetic field. That magnetic field creates an alternating electric
field in space, which in turn creates another magnetic field and so forth until
the original current is interrupted.
This form of energy in transition between electricity and magnetic energy
is called electromagnetic radiation, or radio waves. Radio is defined as the radiation
of electromagnetic energy through space. A device that produces radio waves
is called a transmitter, and a complementary device that detects radio waves in
the air and converts them to some other form of energy is called a receiver.
Both transmitters and receivers use specially shaped devices called antennas to
focus the radio signal in a particular direction, or pattern, and to increase the
amount of effective radiation (from a transmitter) or sensitivity (in a receiver).
By adjusting the rate at which alternating current flows from each transmitter through the antenna and out into space (the frequency), and by adjusting
a receiver to operate only at that frequency, it’s possible to send and receive
many different signals, each at a different frequency, that don’t interfere
with one another. The overall range of frequencies is known as the radio
spectrum. A smaller segment of the radio spectrum is often called a band.
Radio frequencies and other AC signals are expressed as cycles per
second, or hertz (Hz), named for Heinrich Hertz, the first experimenter to
send and receive radio waves. One cycle is the distance from the peak of an
AC signal to the peak of the next signal. Radio signals generally operate at
frequencies in thousands, millions, or billions of hertz (kilohertz or KHz,
megahertz or MHz, and gigahertz or GHz, respectively).
The simplest type of radio communication uses a continuous signal that
the operator of the transmitter interrupts to divide the signal into accepted
patterns of long and short signals (dots and dashes) that correspond to
I n tr od uct io n t o Wir eles s N et wor ks
13
Amplitude
individual letters and other characters. The most widely used set of these
patterns was Morse code, named for the inventor of the telegraph, Samuel
F.B. Morse, where this code was first used.
In order to transmit speech, music, and other sounds via radio, the transmitter alters, or modulates, the AC signal (the carrier wave) by either mixing
an audio signal with the carrier as shown in Figure 2-2 (this is called amplitude
modulation, or AM) or by modulating the frequency within a narrow range
as shown in Figure 2-3 (this is called frequency modulation, or FM). The AM or
FM receiver includes a complementary circuit that separates the carrier from
the modulating signal.
Time
Amplitude
Figure 2-2: In an AM signal, the audio modulates the carrier.
Time
Figure 2-3: In an FM signal, the audio modulates the radio frequency.
Because two or more radio signals using the same frequency can often
interfere with one another, government regulators and international agencies,
such as the International Telecommunication Union (ITU), have reserved
certain frequencies for specific types of modulation, and they issue exclusive
licenses to individual users. For example, an FM radio station might be
licensed to operate at 92.1 MHz at a certain geographical location. Nobody
else is allowed to use that frequency in close enough proximity to interfere
with that signal. On the other hand, some radio services don’t require a
license. Most unlicensed services are either restricted to very short distances,
to specific frequency bands, or both.
Both AM and FM are analog methods because the signal that comes out
of the receiver is a replica of the signal that went into the transmitter. When
we send computer data through a radio link, it’s digital because the content
has been converted from text, computer code, sounds, images or other information into ones and zeroes before it is transmitted, and it is converted back
to its original form after it is received. Digital radio can use any of several
different modulation methods: The ones and zeroes can be two different
audio tones, two different radio frequencies, timed interruptions to the
carrier, or some combination of those and other techniques.
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Wireless Data Networks
Each type of wireless data network operates on a specific set of radio frequencies. For example, most Wi-Fi networks operate in a special band of
radio frequencies around 2.4 GHz that have been reserved in most parts of
the world for unlicensed point-to-point spread spectrum radio services. Other
Wi-Fi systems use a different unlicensed band around 5 GHz.
Unlicensed Radio Services
Unlicensed means that anybody using equipment that complies with the technical requirements can send and receive radio signals on these frequencies
without a radio station license. Unlike most radio services (including other
broadband wireless services), which require licenses that grant exclusive use
of that frequency to a specific type of service and to one or more specific
users, an unlicensed service is a free-for-all where everybody has an equal
claim to the same airwaves. In theory, the technology of spread spectrum
radio makes it possible for many users to co-exist (up to a point) without
significant interference.
Point-to-Point
A point-to-point radio service operates a communication channel that carries
information from a transmitter to a single receiver. The opposite of point-topoint is a broadcast service (such as a radio or television station) that sends the
same signal to many receivers at the same time.
Spread Spectrum
Spread spectrum is a family of methods for transmitting a single radio signal
using a relatively wide segment of the radio spectrum. Wireless Ethernet
networks use several different spread spectrum radio transmission systems,
which are called frequency-hopping spread spectrum (FHSS), direct-sequence
spread spectrum (DSSS), and orthogonal frequency division multiplexing
(OFDM). Some older data networks use the slower FHSS system, but the first
Wi-Fi networks used DSSS, and more recent systems use OFDM. Table 2-1
lists each of the Wi-Fi standards and the type of spread spectrum modulation
they use.
Table 2-1: Wi-Fi Standards and Modulation Type
Wi-Fi Type
Frequency
Modulation
802.11a
5 GHz
OFDM
802.11b
2.4 GHz
DSSS
802.11g
2.4 GHz
OFDM
Spread spectrum radio offers some important advantages over other
types of radio signals that use a single narrow channel. Spread spectrum
is extremely efficient, so the radio transmitters can operate with very low
power. Because the signals operate on a relatively wide band of frequencies,
I n tr od uct io n t o Wir eles s N et wor ks
15
they are less sensitive to interference from other radio signals and electrical
noise, which means they can often get through in environments where a
conventional narrow-band signal would be impossible to receive and understand. And because a frequency-hopping spread spectrum signal shifts among
more than one channel, it can be extremely difficult for an unauthorized
listener to intercept and decode the contents of a signal.
Spread spectrum technology has an interesting history. It was invented
by the actress Hedy Lamarr and the American avant-garde composer George
Antheil as a “Secret Communication System” for directing radio-controlled
torpedoes that would not be vulnerable to enemy jamming. Before she came
to Hollywood, Lamarr had been married to an arms merchant in Austria,
where she learned about the problems of torpedo guidance at dinner parties
with her husband’s customers. Years later, shortly before the United States
entered World War II, she came up with the concept of changing radio frequencies to cut through interference. The New York Times reported in 1941
that her “red hot” invention (Figure 2-4) was vital to the national defense,
but the government would not reveal any details.
Figure 2-4: Hedy Lamarr and George Antheil received this patent in 1942 for the
invention that became the foundation of spread spectrum radio communication.
She is credited here under her married name, H.K. Markey. The complete document is accessible at http://uspto.gov.
Antheil turned out to be the ideal person to make this idea work. His
most famous composition was an extravaganza called Ballet Mechanique,
which was scored for sixteen player pianos, two airplane propellers, four
xylophones, four bass drums, and a siren. His design used the same kind of
mechanism that he had previously used to synchronize the player pianos to
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change radio frequencies in a spread spectrum transmission. The original
slotted paper tape system had 88 different radio channels—one for each of
the 88 keys on a piano.
In theory, the same method could be used for voice and data communication as well as guiding torpedoes, but in the days of vacuum tubes, paper
tape, and mechanical synchronization, the whole process was too complicated
to actually build and use. By 1962, solid-state electronics had replaced the
vacuum tubes and piano rolls, and the technology was used aboard US Navy
ships for secure communication during the Cuban Missile Crisis. Today,
spread spectrum radios are used in the US Air Force Space Command’s
Milstar Satellite Communications System, in digital cellular telephones, and
in wireless data networks.
Frequency-Hopping Spread Spectrum
Lamarr and Antheil’s original design for spread spectrum radio used a
frequency-hopping system (FHSS). As the name suggests, FHSS technology divides
a radio signal into small segments and “hops” from one frequency to another
many times per second as it transmits those segments. The transmitter and
the receiver establish a synchronized hopping pattern that sets the sequence
in which they will use different subchannels.
FHSS systems overcome interference from other users by using a narrow
carrier signal that changes frequency many times per second. Additional
transmitter and receiver pairs can use different hopping patterns on the same
set of subchannels at the same time. At any point in time, each transmission
is probably using a different subchannel, so there’s no interference between
signals. When a conflict does occur, the system resends the same packet
until the receiver gets a clean copy and sends a confirmation back to the
transmitting station.
For some older 802.11 wireless data services, the unlicensed 2.4 MHz
band is split into 75 subchannels, each of them 1 MHz wide. Because each
frequency hop adds overhead to the data stream, FHSS transmissions are
relatively slow.
Direct-Sequence Spread Spectrum
The direct-sequence spread spectrum (DSSS) technology that controls 802.11b
networks uses an 11-chip Barker Sequence to spread the radio signal through
a single 22 MHz–wide channel without changing frequencies. Each DSSS
link uses just one channel without any hopping between frequencies. As
Figure 2-5 shows, a DSSS transmission uses more bandwidth, but less power
than a conventional signal. The digital signal on the left is a conventional
transmission in which the power is concentrated within a tight bandwidth.
The DSSS signal on the right uses the same amount of power, but it spreads
that power across a wider band of radio frequencies. Obviously, the 22 MHz
DSSS channel is a lot wider than the 1 MHz channels used in FHSS systems.
A DSSS transmitter breaks each bit in the original data stream into a
series of redundant bit patterns called chips, and it transmits them to a receiver
that reassembles the chips back into a data stream that is identical to the
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17
original. Because most interference is likely to occupy a narrower bandwidth
than a DSSS signal, and because each bit is divided into several chips, the
receiver can usually identify noise and reject it before it decodes the signal.
Signal Strength
0.3mW
0.1mW
Frequency
2.41 GHz
2.425 GHz
2.41 GHz
2.5 GHz
Figure 2-5: A conventional signal (left) uses a narrow radio frequency bandwidth. A DSSS
signal (right) uses a wider bandwidth but a less powerful signal.
Like other networking protocols, a DSSS wireless link exchanges handshaking messages within each data packet to confirm that the receiver can
understand each packet. For example, the standard data transmission rate in
an 802.11b DSSS WI-Fi network is 11Mbps, but when the signal quality won’t
support that speed, the transmitter and receiver use a process called dynamic
rate shifting to drop the speed down to 5.5Mbps. The speed might drop because
a source of electrical noise near the receiver interferes with the signal or
because the transmitter and receiver are too far apart to support full-speed
operation. If 5.5Mbps is still too fast for the link to handle, it drops again,
down to 2Mbps or even 1Mbps.
Orthogonal Frequency Division Multiplexing
Orthogonal frequency division multiplexing (OFDM) modulation, used in 802.11a
Wi-Fi networks, is considerably more complicated than DSSS technology.
The physical layer splits the data stream among 52 parallel bit streams that
each use a different radio frequency called a subcarrier. Four of these subcarriers carry pilot data that provides reference information about the
remaining 48 subcarriers, in order to reduce signal loss due to radio interference or phase shift. Because the data is divided into 48 separate streams
that move through separate subcarriers in parallel, the total transmission
speed is much greater than the speed of data through a single channel.
The subcarrier frequencies in an
OFDM signal overlap with the peak
of each subcarrier’s waveform matching the baseline of the overlapping
signals as shown in Figure 2-6. This
is called orthogonal frequency division.
The 802.11a standard specifies a
total of eight data channels that are
20 MHz wide. Each of these channels
is divided into 52 300 kHz
Figure 2-6: In OFDM, the peaks of oversubcarriers.
lapping frequencies don’t interfere with
one another.
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When a Wi-Fi radio receiver detects an 802.11a signal, it assembles the
parallel bit streams back into a single high-speed data stream and uses the
pilot data to check its accuracy. Under ideal conditions, an 802.11a network
can move data at 54Mbps, but like DSSS modulation, the OFDM transmitter
and receiver automatically reduce the data speed when the maximum transmission rate is not possible due to interference, weak signals, or other lessthan-perfect atmospheric conditions.
The more recent 802.11g specification was designed to combine the best
features of both 802.11b (greater signal range) and 802.11a (higher speed).
To accomplish this objective, it uses OFDM modulation on the 2.4 GHz frequency band.
Why This Matters
The great science fiction writer Arthur C. Clarke once observed that “Any
sufficiently advanced technology is indistinguishable from magic.” For most
of us, the technology that controls high-speed spread spectrum radio could
just as easily be a form of magic, because we don’t need to understand the
things that happen inside a transmitter and a receiver; they’re just about
invisible when we connect a computer to the Internet. As mentioned earlier
in this chapter, you don’t need to understand these technical details about
how a Wi-Fi transmitter splits your data into tiny pieces and reassembles
them into data unless you’re a radio circuit designer.
But when you know that there’s a well-defined set of rules and methods
that make the connection work (even if you don’t know all the details), you
are in control. You know that it’s not magic, and if you think about it, you
might also know some of the right questions to ask when the system doesn’t
work correctly. If knowledge is power, then knowledge about the technology
you use every day is the power to control that technology rather than just
use it.
Benefits of Wireless
Wireless broadband provides Internet access to mobile devices in addition to
allowing network operators to extend their networks beyond the range of
their wired connections. For our purposes, two-way radio is the most sensible
approach to wireless broadband, but other methods (such as infrared light
or visible signaling) are also possible. Connecting your computer to the
Internet (or a local network) by radio offers several advantages over connecting the same computer through a wired connection. First, wireless provides
convenient access for portable computers; it’s not necessary to find a cable or
network data outlet. And second, it allows a user to make a connection from
more than one location and to maintain a connection as the user moves from
place to place. For network managers, a wireless connection makes it possible
to distribute access to a network without the need to string wires or cut holes
through walls.
In practice, access without cables means that the owner of a laptop or
other portable computer can walk into a classroom, a coffee shop, or a library
and connect to the Internet by simply turning on the computer and running
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a communication program. Depending on the type of wireless network you’re
using, you might also be able to maintain the same connection in a moving
vehicle.
When you’re installing your own network, it’s often easier to use Wi-Fi
links to extend your network and your Internet connection to other rooms
because a wired system requires a physical path for the cables between the
network router or switch and each computer. Unless you can route those
cables through a false ceiling or some other existing channel, this almost
always means that you must cut holes in your walls for data connectors and
feed wires inside the walls and under the floors. A radio signal that passes
through those same walls is often a lot neater and easier.
Wireless Data Services
Because radio signals move through the air, you can set up a network connection from any place within range of the network base station’s transmitter;
it’s not necessary to use a telephone line, television cable, or some other
dedicated wiring to connect your computer to the network. Just turn on the
radio connected to the computer and it will find the network signal. Therefore, a radio (or wireless) network connection is often a lot more convenient
than a wired one.
This is not to say that wireless is always the best choice. A wired network
is usually more secure than a wireless system because it’s a lot more difficult
for unauthorized eavesdroppers and other snoops to monitor data as it
moves through the network, and a wired link doesn’t require as many complex
negotiations between the sender and receiver on protocols and so forth. In
an environment where your computer never moves away from your desk and
there are no physical obstacles between the computer and the network access
point, it’s often easier to install a data cable between the computer and a
modem.
So now we have a bunch of radio transmitters and receivers that all
operate on the same frequencies and all use the same kind of modulation.
(Modulation is the method a radio uses to add some kind of content, such as
voice or digital data, to a radio wave.) The next step is to send some network
data through those radios. Several different wireless data systems and services
are available to connect computers and other devices to local networks and
to the Internet, including Wi-Fi, WiMAX, and a handful of services based on
the latest generations of cellular mobile telephone technology.
Wi-Fi
The IEEE (Institute of Electrical and Electronics Engineers) has produced a
set of standards and specifications for wireless networks under the title IEEE
802.11 that define the formats and structures of the relatively short-range
signals that provide Wi-Fi service. The original 802.11 standard (without any
letter at the end) was released in 1997. It covers several types of wireless media:
two kinds of radio transmissions and networks that use infrared light. The
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802.11b standard provides additional specifications for wireless Ethernet
networks. A related document, IEEE 802.11a, describes wireless networks
that operate at higher speeds on different radio frequencies. Still other
802.11 radio networking standards with other letters are also available or
moving toward public release.
The specifications in widest use today are 802.11a, 802.11b, and 802.11g.
They’re the de facto standards used by just about every wireless Ethernet
LAN that you are likely to encounter in offices and public spaces and in most
home networks. It’s worth the trouble to keep an eye on the progress of those
other standards, but for the moment, 802.11a and 802.11g are the ones to
use for short-range wireless networks, especially if you’re expecting to connect
to networks where you don’t control all the hardware yourself.
NOTE
Many first-generation 802.11b Wi-Fi network adapters are still compatible with
today’s networks, but their manufacturers don’t offer the device drivers that are
necessary to make them work with the latest operating systems (such as Windows XP
or Windows Vista).
The 802.11n standard is the next one in the pipeline, and when it’s
released, it will replace both 802.11b and 802.11g because it’s faster, more
secure, and more reliable. The older standards will still work, so new Wi-Fi
equipment will support all three (often along with 802.11a, which uses
different radio frequencies) and automatically match your network interface
to the signals it detects from each base station.
NOTE
Until the new 802.11n standard is formally approved and released, some manufacturers offer “pre-n” network adapters and access points that include many of the features
that will be in the final 802.11n standard. These preliminary versions generally work
best on networks that are limited to equipment (adapters and access points) made by a
single manufacturer, although they all generally work with any existing 802.11b or
802.11g network. Your best bet is to wait until the final standard is released before you
upgrade your system, but if you do buy a pre-n device, the manufacturer will probably
offer a free firmware upgrade to the final 802.11n specifications.
There are two more names in the alphabet soup of wireless LAN
standards that you ought to know about: WECA and Wi-Fi. WECA (Wireless
Ethernet Compatibility Alliance) is an industry group that includes all of the
major manufacturers of wireless Ethernet equipment. Their twin missions
are to test and certify that the wireless network devices from all of their
member companies can operate together in the same network, and to
promote 802.11 networks as the worldwide standard for wireless LANs.
WECA’s marketing geniuses have adopted the more friendly name of Wi-Fi
(short for wireless fidelity) for the 802.11 specifications.
Once or twice per year, the Wi-Fi Alliance conducts an “interoperability
bake-off ” where engineers from many hardware manufacturers confirm
that their hardware will communicate correctly with equipment from other
suppliers. Network equipment that carries a Wi-Fi logo has been certified
by the Wi-Fi Alliance to meet the relevant standards and to pass interoperability tests. Figure 2-7 shows one version of the Wi-Fi logo.
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Wi-Fi was originally intended to be a wireless
extension of a wired LAN, so the distances between
Wi-Fi base stations and the computers that communicate through them are limited to about 100 feet
(35 meters) indoors or up to 300 feet (100 meters)
outdoors, assuming there are no obstructions
between the access point and the computer. When
802.11n equipment becomes available, it will support connections between computers and base
stations at least as far apart as the older Wi-Fi
versions. There are ways to extend the range
of a Wi-Fi signal, but those techniques require
special equipment and careful installation.
NOTE
Figure 2-7: A Wi-Fi logo
See “Extending the Network” on page 167 for more about long-range Wi-Fi operation.
Because most Wi-Fi signals have such a limited range, you must find a
new access point, or hot spot, and set up a new connection every time you
move your computer to a new location. And because many Wi-Fi access
points don’t permit strangers to connect through them, you may have to
establish a separate account for each location.
The Wi-Fi networks described in this book follow the 802.11a, b, and g
standards, but much of the same information will also apply to the new
802.11n networks when they become available.
Metropolitan Wi-Fi Services
In some metropolitan areas, a large number of interconnected Wi-Fi base
stations are being installed by either local government agencies or private
businesses to provide wireless service throughout an entire region or in
selected neighborhoods as an economical alternative to cable and telephone
(DSL) services. The base stations for these services are often mounted on
utility poles or rooftops.
These same networks might also provide a variety of special data services
to the local government and major subscribers. For example, the local natural
gas, electric, and water utilities could add small Wi-Fi adapters to their meters
and use the system to send readings once a month. And city buses might
have transponders that report their locations to a central tracking system,
like the one in Seattle at http://busview.org/busview_launch.jsp, as shown in
Figure 2-8.
It’s not yet clear whether these city-wide Wi-Fi services will be able to
overcome possible interference problems and competition from other
wireless data alternatives, or whether they will attract enough business to
remain viable. But if they do, any computer within the coverage area that
has a Wi-Fi adapter should detect the signal and have access to a broadband
Internet connection.
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Figure 2-8: Wireless technology tracks city buses in Seattle and
reports locations on a website.
Cellular Mobile Wireless Services
Several broadband wireless data services are extensions of cellular mobile
telephone technology. You might see them described as 3G services because
they’re based on the third generation of cellular telephone technology. If
you have been using a mobile telephone for more than a year or two, you
probably remember that the earliest phones were only good for voice calls,
but as each new generation was introduced, your mobile carrier offered
more and better features. Table 2-2 describes the various generations.
For people who use their computers away from their home or office, the
great advantage of a mobile broadband service is that it covers a much wider
territory than any Wi-Fi base station; you can connect your computer to the
Internet without the need to search for a new hot spot and use a different
access account in each new location, and you can even keep the same connection alive in a moving vehicle. Each of the major wireless broadband services
offers coverage in most metropolitan areas and much of the countryside
between cities.
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Table 2-2: Cellular Mobile Telephone Generations
Name
Features
1G
Analog voice communication only
2G
System can handle more calls
Digital voice
Uses less power
Less background noise
Digital data
Simple text messages
Email
2.5G
Packet-switched signaling
Faster data transfer (up to 144Kbps)
Supports relatively slow Internet connections
3G
Even more calls at the same time
Much faster data transfer rates (up to 2.4Mbps)
Broadband Internet
Video and music
4G (not yet available)
Based on Internet technology
Packet signaling
Very high speed (100Mbps–1Gbps)
Will combine telephone, computer, and other technologies
Of course, computer technology has also been improving at the same
time, so today’s 2.5G and 3G mobile telephones often incorporate enough
computing power to allow them to double as pocket-size Internet terminals
(as well as cameras and media players). And equally important, from the
perspective of this book, broadband data adapters that use 2.5G and 3G
technology can attach to a laptop or other portable computer and provide a
direct wireless connection to the Internet through the same cellular telephone
company that offers mobile telephone service.
Today, most cellular broadband wireless services offer credit card–size
adapters that connect to your computer through the PC Card socket on the
side of a laptop or into the front or back panel of a desktop computer. In
another year or two, many new laptops will come with internal adapters and
integrated antennas for both Wi-Fi and 3G wireless or WiMAX that mount
directly on the motherboard, just as they contain internal Wi-Fi adapters and
dial-up modems today.
NOTE
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Some cellular service providers also offer mobile telephones that can connect a computer
to the Internet through a USB cable linked to the phone, but separate PC Card adapters
are a lot more convenient and easy to use.
WiMAX
Worldwide Interoperability for Microwave Access (WiMAX) is yet another
method for distributing broadband wireless data over wide geographic areas.
It’s a metropolitan area network service that typically uses one or more base
stations that can each provide service to users within a 30-mile radius. The
IEEE 802.16 specification contains the technical details of WiMAX networks.
In the United States, the earliest WiMAX services were offered by
Clearwire as a wireless alternative to DSL and cable broadband Internet
access in fixed locations (such as homes and businesses), but mobile
WiMAX access is not far behind. By early 2008, Clearwire plans to offer
access to their wireless networks through an adapter on a PC Card. When
those adapters become available, WiMAX, 3G cellular data services, and
metropolitan Wi-Fi networks will compete for the same commercial niche:
wireless access to the Internet through a service that covers an entire metropolitan area.
Each WiMAX service provider uses one or more licensed operating frequencies somewhere between 2 GHz and 11 GHz. A WiMAX link can transfer
data (including handshaking and other overhead) at up to 70Mbps, but most
commercial WiMAX services are significantly slower than that. And as more
and more users share a single WiMAX tower and base station, some users
report that their signal quality deteriorates.
Unlike the cellular broadband wireless data services that piggyback on
existing mobile telephone networks, WiMAX is a separate radio system that is
designed to either supplement or replace the existing broadband Internet
distribution systems. In practice, WiMAX competes with both 3G wireless
services and with Internet service providers that distribute Internet access to
fixed locations through telephone lines and cable television utilities. Home
and business subscribers to a WiMAX service usually use either a wired LAN
or Wi-Fi to distribute the network within their buildings. Figure 2-9 shows a
typical WiMAX network.
Mobile
laptop user
Internet
backbone
Line-of-sight
relay
WiMax
transmitter
Home LAN
ISP
WiMax
transmitter
Last mile
Figure 2-9: WiMAX provides last mile Internet connections to homes and businesses.
I n tr od uct io n t o Wir eles s N et wor ks
25
What About Bluetooth?
Bluetooth is the other type of wireless networking technology that we ought
to describe. Bluetooth uses radio signals to replace the wires and cables that
connect a computer or a mobile telephone to peripheral devices, such as a
keyboard, a mouse, or a set of speakers. You can also use Bluetooth to transfer
data between a computer and a mobile telephone, smartphone, BlackBerry, or
other PDA (personal digital assistant).
Bluetooth is an FHSS system that splits the radio signal into tiny pieces.
It moves among 79 different frequencies 1,600 times per second in the same
unlicensed 2.4 GHz range as 802.11b and 802.11g Wi-Fi services.
Bluetooth is not practical for connecting a computer to the Internet
because it’s slow (the maximum data transfer rate is only about 700Kbps),
and it has a very limited signal range (most often about 33 feet, or 10 meters,
or less).
In order to prevent interference between Bluetooth and Wi-Fi signals,
many computers that use both technologies (including the widely used Intel
Centrino chip set) coordinate the two services. When either module is active,
it notifies the other, and the active service takes priority. This coordinated
operation is slightly slower than either service operating alone, but the
difference is insignificant.
Frequency Allocations
Each type of broadband wireless service uses a specific set of radio frequencies.
Some of these frequencies are reserved for the exclusive use of a specific
licensed service provider while others are free-for-all bands that are open
for anybody to use.
Wi-Fi Services
The 802.11b, 802.11g, and 802.11n Wi-Fi services all operate in a frequency
range at or slightly above 2.4 GHz. The 802.11a signal uses a band close to
5.3 GHz. The specific center frequencies of each Wi-Fi channel are listed in
Table 2-3.
Unless you’re a radio engineer, the important things to know about
the different Wi-Fi services are the maximum data transmission rate and the
signal range. Table 2-3 shows the important characteristics of each Wi-Fi
specification.
The differences between the maximum data speeds and the typical speeds
are caused by the handshaking and other nondata information that must
attach itself to each data packet. Obviously, there’s a tremendous amount
of overhead involved in moving information through any kind of Wi-Fi
network.
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C ha pt er 2
Table 2-3: Wi-Fi Characteristics
Type
Radio
Frequency
802.11b
Signal Range
Maximum Data Speed
Typical Speed
2.4 GHz
~30 meters (indoor)
~100 meters (outdoor)
11Mbps
4Mbps
802.11a
5 GHz
~35 meters (indoor)
~110 meters (outdoor)
54Mbps
23Mbps
802.11g
2.4 GHz
~35 meters (indoor)
~110 meters (outdoor)
54Mbps
20Mbps
802.11n
(proposed)
2.4 GHz
~70 meters (indoor)
~160 meters (outdoor)
300Mbps
120Mbps
Other Broadband Services
National broadband wireless data service providers use a different variation
on spread spectrum technology and a different range of radio frequencies.
The broadband wireless services provided by Sprint, AT&T, and Verizon
all share the frequencies around 800 MHz and 1,900 MHz used by those
companies’ digital mobile telephone networks. WiMAX services, such as
Clearwire, use signals in the 2.3 to 2.5 GHz and 3.5 GHz bands.
Many new radio frequencies may open up for mobile telephone and
data services in the United States after February 17, 2009, when all the
existing analog television stations move to new digital channels, and the
old VHF channels will close down. The newly vacant radio spectrum will
become available for new services, including broadband wireless data.
NOTE
Don’t panic. All your favorite television stations will still be available after the
changeover. You will need either an inexpensive converter box or a new digital
television set to receive them, but they’ll still be there for you.
The exact frequencies that the WiMAX and broadband wireless data
services use are less important to you as a user than the frequencies of Wi-Fi
signals because service providers control the base stations and access points.
The network interface device in your computer automatically finds the right
signal and sets up the connection without forcing you to choose a specific
channel.
Choosing a Service
Each type of wireless access to the Internet offers a different combination
of cost, coverage areas, reliability, ease of use, and security. Your choice will
depend on your particular needs and the availability of signals in the locations
where you need wireless Internet access.
I n tr od uct io n t o Wir eles s N et wor ks
27
For example, if you use your computer in just a few places and all of those
places are within range of Wi-Fi hot spots, the built-in Wi-Fi adapter (or an
inexpensive plug-in adapter) is probably your best choice. It’s likely that Wi-Fi
hot spots already exist at your workplace and in the libraries, coffee shops,
schools, and conference centers where you regularly spend time, and it’s
relatively easy and inexpensive to install one or more access points at home.
However, you will probably need a separate account to log in to each Wi-Fi
network. Some of these Wi-Fi services are free, but others charge for access
by the hour, by the day, or by the month; if you need paid accounts at several
locations, the total cost can be more than a single account with a cellular
service.
Wi-Fi also allows you to add portable computers to an existing LAN at
home or in your school or workplace. And if cost is a primary concern, you
will probably choose to use free public Wi-Fi hot spots instead of a cellular or
WiMAX service that charges a monthly fee.
On the other hand, if you want constant Internet access wherever you
go, the cellular data and WiMAX metropolitan area network services are
better choices. Both systems provide coverage throughout large geographic
regions, and both allow you to maintain a connection while you’re moving
from one place to another. You can use the same account and the same login
and password every time you set up a connection. However, it’s important to
make sure that there’s a usable cellular or WiMAX signal in all the places
where you expect to use them before you commit to a long-term contract.
Most wireless data service providers offer a free or low cost trial period that
you can use to test the system.
As WiMAX and cellular data services become more common, many
laptop computers and add-on network adapters will operate with both types
of wireless services. When the computer detects a high-speed Wi-Fi signal, it
will automatically try to establish a connection to that network. But when
there is no local Wi-Fi signal, or if you haven’t configured your computer to
use any of the local signals, it will automatically shift over to your WiMAX or
cellular data account and use that service to connect to the Internet.
All three types of wireless Internet services—Wi-Fi, cellular, and
WiMAX—offer fast and reliable connections, but each has a different set of
strengths and weaknesses. For short-range coverage and for access to local
area networks, Wi-Fi is the obvious choice. If you are outside of the service
areas of a DSL or cable Internet service, WiMAX is a huge improvement over
a slow dial-up service. But when you carry your computer to many locations, a
single account with a cellular or WiMAX service will allow you to connect to
the Internet without the need to search for a new hot spot and set up a new
account.
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C ha pt er 2
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