How Does Wireless Fidelity (Wi-Fi) Work?
A networked desktop computer is connected
to a larger network (a LAN, wide area network [WAN],
or the Internet) via a network cable to a hub, router, or
switch. The computer's network interface card (NIC)
sends zeros and ones down the cable by changing the voltage
on the wires from +5 volts to -5 volts in a prearranged cadence.
Wi-Fi simply replaces these cables with small, low-powered
two-way radios. Instead of changing the voltage on a wire,
it encodes the zeros and ones by laying an alternating radio
signal over a constant existing signal in a prearranged cadence.
The alternating signal encodes zeros and ones on the radio
waves. The 802.11b specification allows for the wireless transmission
of approximately 11 Mbps of raw data at distances up to a
few hundred feet over the 2.4 GHz unlicensed band. The distance
depends on impediments, materials, and line of sight.
Many people might ask, "So what?" It
means a PC user can install a $70 PC card in his or her laptop
or personal digital assistant (PDA) and be connected
to the Internet or corporate network as if he or she was still
tied to a desk. Enterprises have been quick to adopt this
technology based on the following factors:
- Wiring
a building for voice and data is expensive.
- It
improves worker productivity by allowing mobility within
a building or corporate campus.
- It
does not require right-of-way agreements to bring service
to a business.
- It
is independent of distance limitation to the central office.
- It
is relatively free of federal, state, and local regulations.
A wireless local area network (WLAN) installation
usually uses one or more access points (APs), which
are dedicated stand-alone hardware with typically more powerful
antennas. Figure 2-1 illustrates
a WLAN. In addition to servicing enterprise networks, 802.11b
has become the most popular standard for public short-range
networks, known as hot spots, found at airports, hotels,
conference centers, and coffee shops and restaurants. Several
companies currently offer paid hourly, session-based, or unlimited
monthly access via their deployed networks around the United
States and internationally.[1]
Figure 2-1: A WLAN on an enterprise network
How Is Data Transmitted via Wireless Technology?
The 802.11 standard provides for two radio
frequency (RF) variations (as opposed to infrared) of
the physical (PHY) layer. These include direct sequence
spread spectrum (DSSS) and frequency-hopping spread
spectrum (FHSS). Both of these were designed to comply
with the Federal Communications Commission (FCC) regulations
(FCC 15.247) for operation in the 2.4 GHz band, which is an
unlicensed spectrum. 802.11b uses DSSS.
DSSS systems use technology similar to Global
Positioning System (GPS) satellites and some types of
cell phones. Each information bit is combined with a longer
pseudorandom numerical (PN) in the transmission process.
The result is a high-speed digital stream, which is then modulated
onto a carrier frequency using differential phase-shift
keying (DPSK). Figure 2-2 illustrates
how data is modulated with a PN sequence for wireless transmission.[2]
Figure 2-2: Digital modulation of data with PN sequence
As illustrated in Figure 2-2, DSSS works
by taking a data stream of zeros and ones and modulating it
with a second pattern—the chipping sequence. The sequence
is also known as the Barker code, which is an 11-bit
sequence (10110111000). The chipping or spreading code is
used to generate a redundant bit pattern to be transmitted,
and the resulting signal appears as wideband noise to the
unintended receiver. One of the advantages of using spreading
codes is that even if one or more of the bits in the chip
are lost during transmission, statistical techniques embedded
in the radio can recover the original data without the need
for retransmission. The ratio between the data and width of
spreading code is called processing gain. It is 16
times the width of the spreading code and increases the number
of possible patterns to 64,000 (216), reducing
the chances of cracking the transmission.
The DSSS signaling technique divides the 2.4 GHz
band into fourteen 22 MHz channels, of which 11 adjacent channels
overlap partially and the remaining 3 do not overlap. Data
is sent across one of these 22 MHz channels without hopping
to other channels, causing noise on the given channel. To
reduce the number of retransmissions and noise, chipping is
used to convert each bit of user data into a series of redundant
bit patterns called chips. The inherent redundancy
of each chip, combined with spreading the signal across the
22 MHz channel, provides the error checking and correction
functionality to recover the data. Spread spectrum products
are often interoperable because many are based on the IEEE
802.11 standard for wireless networks. DSSS is used primarily
in interbuilding LANs, as its properties are fast and far
reaching.[3]
At the receiver, a matched filter correlator is
used to remove the PN sequence and recover the original data
stream. At a data rate of 11 Mbps, DSSS receivers use different
PN codes and a bank of correlators to recover the transmitted
data stream. The high-rate modulation method is called complimentary
code keying (CCK).
As illustrated in Figure 2-2 the PN
sequence spreads the transmitted bandwidth of the resulting
signal (hence the term spread spectrum) and reduces
peak power. The total power remains unchanged. Upon receipt,
the signal is correlated with the same PN sequence to reject
narrowband interference and recover the original binary data.
Regardless of whether the data rate is 1, 2, 5.5, or 11 Mbps,
the channel bandwidth is about 20 MHz for DSSS systems.
Figure 2-3: The spreading of spectrum for transmission in DSSS or
FHSS
The Significance of Spread Spectrum Radio
One of the basic technologies underlying
the IEEE 802.11 series of standards is spread spectrum radio.
The fundamental concept of spread spectrum radio is the use
of a wider frequency bandwidth than that needed by the information
that is transmitted. Using extra bandwidth would seem to be
wasteful, but it actually results in several benefits, including
reduced vulnerability to jamming, less susceptibility to interference,
and coexistence with narrowband transmissions. Several spread
spectrum techniques are available, including time hopping,
frequency modulation, FHSS, DSSS, and hybrids of these.
FHSS and DSSS are not modulation techniques,
but methods of distributing a radio signal across bandwidth.
In addition to spreading the signal across a frequency band,
spread spectrum systems modulate the signal. Modulation is
the variation of a radio signal to convey information. The
base signal is called the carrier. The variation may
be based on the strength (amplitude modulation [AM]),
frequency, or phase (frequency offset) of the signal. The
modulation technique directly affects the data rate. Higher
data rate modulations are generally more complex and expensive
to implement. Modulations resulting in higher data rates pack
more information in the same bandwidth. Small disruptions
in the signal cause the degradation of more data. This means
that the signal must have a higher signal-to-noise ratio
(SNR) at the receiver to be effectively processed. Because
a radio signal is stronger the closer it is to the source,
the SNR decreases with distance. This is why higher-speed
systems have less range. Examples of modulation techniques
used in the IEEE 802.11 series of specifications include binary
phase-shift keying (BPSK), quadrature phase-shift keying
(QPSK), Gaussian frequency-shift keying (GFSK), and
CCK. |