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H O W W I - F I W O R K S
1
H O W W I - F I W O R K S
Up to a point, its 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. Thats the way
most people relate to all of the technology that sur-
rounds them you shouldnt have to worry about the
802.11b specification to connect your laptop com-
puter to a network. In an ideal world (ha!), it would
work just as soon as you turn on the power switch.
But wireless Ethernet today is about where broadcast radio was in 1923. The
technology was out there, but people spent a lot of time tweaking their equipment.
And the people who understood what was happening behind that Bakelite-Dilecto
panel were able to get better performance from their radios than the ones who
expected to just turn on the power switch and listen.
In order to make the most effective use of wireless networking technology,
its still important to understand whats going on inside the box (or in this case,
inside each of the boxes that makes up the network). This chapter describes the
standards and specications that control wireless networks, and it explains how
data moves through the network from one computer to another. 2
Chapter 1
When the network is working properly, you should be able to use it without
thinking about all of this internal plumbing just click a few icons on your
computers screen, and youre connected. But when youre designing and build-
ing a new network, or when you want to tweak the performance of an existing
network, it can be essential to know how all that data is supposed to move from
one place to another. And when the network does something you arent expect-
ing it to do, you will need a basic knowledge of the technology to do any kind of
useful troubleshooting.
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 its necessary to dene all three when you
invent a new network. In terms of the familiar OSI (Open Systems Interconnection)
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 interface
adapters and base stations that send and receive the radio signals.
In a wireless network, the network adapters in each computer convert digital
data to radio signals, which they transmit to other devices on the network, and
they convert incoming radio signals from other network elements back to digital
data. The IEEE (Institute of Electrical and Electronics Engineers) has produced a
set of standards and specications for wireless networks under the title IEEE
802.11 that denes the format and structure of those signals.
Figure 1.1: Every new technology goes through the tweak-and-ddle stage How Wi-Fi Works
3
The original 802.11 standard (without the b at the end) was released in 1997.
It covers several different types of wireless media: two kinds of radio transmissions
(which well explain later in this chapter) and networks that use infrared light.
The more recent 802.11b standard provides additional specications 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 moving toward public release.
The specication in widest use today is 802.11b. Thats the de facto standard
used by just about every wireless Ethernet LAN that you are likely to encounter in
ofces and public spaces and in most home networks. Its worth the trouble to
keep an eye on the progress of those other standards, but for the moment,
802.11b is the one to use, especially if youre expecting to connect to networks
where you dont control all the hardware yourself.
N O T E
The wireless networks described in this book follow the 802.11b standard, but much of the
same information also applies to other kinds of 802.11 networks.
You ought to know about two more names in the alphabet soup of wireless
LAN standards: WECA and Wi-Fi. WECA (Wireless Ethernet Compatibility
Alliance) is an industry group that includes all of the major manufacturers of
802.11b equipment. Their twin missions are to test and certify that wireless net-
work 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. WECAs marketing geniuses adopted the more friendly name of
Wi-Fi (short for Wireless Fidelity) for the 802.11 specications and changed their
own name to the Wi-Fi Alliance.
Once or twice a year, the Alliance conducts an interoperability bake-off
where engineers from many hardware manufacturers conrm that their hardware
will communicate correctly with equipment from other suppliers. Network equip-
ment that carries the Wi-Fi logo has been certied to meet the relevant
standards, and to pass those interoperability tests. Figure 1.2 shows the Wi-Fi logos
on network adapters from two different manufacturers.
Figure 1.2: Network adapters with the Wi-Fi logo 4
Chapter 1
Radio Signals
802.11b 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.
The unlicensed part means that anybody using equipment that complies with
the technical requirements can send and receive radio signals on these frequencies,
without the need for a radio station license. Unlike most radio services, which
require licenses that grant exclusive use of a frequency to a single user or group
of users, and which restrict the use of that frequency to a specic type of service,
an unlicensed service is a free-for-all, where everybody has an equal claim on the
same piece of the spectrum. In theory, the technology of spread-spectrum radio
makes it possible to coexist with other users (up to a point) without signicant
interference.
A point-to-point radio service operates a communication channel that carries
information from a transmitter to a single receiver. The opposite of point-to-point
is a broadcast service (such as a radio or television station) that sends the same sig-
nal to many receivers at the same time.
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 two different spread-spectrum radio transmission systems, called FHSS (fre-
quency-hopping spread spectrum) and DSSS (direct-sequence spread spectrum).
Some older 802.11 networks use the slower FHSS system, but the current genera-
tion of 802.11b and 802.11a wireless Ethernet networks use DSSS.
Spread-spectrum radio offers some important advantages over other types of
radio signals that use a single narrow channel. Spread spectrum is extremely ef-
cient, so the radio transmitters can operate with very low power. Because they
operate on a relatively wide band of frequencies, they are less sensitive to interfer-
ence from other radio signals and electrical noise, which means that the signals
are often able to 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 multiple channels, it can be
extremely difcult for an unauthorized listener to intercept and decode the con-
tents 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 subject 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 husbands customers.
Years later, during World War II, she came up with the concept of changing radio
frequencies to cut through interference.
Antheil turned out to be the ideal person to make this idea work. His most
famous composition was something called Ballet Mechanique, which was scored
for 16 player pianos, two airplane propellers, four xylophones, four bass drums,
and a siren. He used the same kind of mechanism that he had previously used to
synchronize the player pianos to change radio frequencies in a spread-spectrum How Wi-Fi Works
5
transmission. The original slotted pa