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Introduction
A wireless local area network (LAN) is a flexible data
communications system implemented as an extension to, or as an alternative for,
a wired LAN. Using radio frequency (RF) technology, wireless LANs transmit and
receive data over the air, minimizing the need for wired connections. Thus,
wireless LANs combine data connectivity with user mobility.
Wireless LANs have gained strong popularity in a number of vertical
markets, including health-care, retail, manufacturing, warehousing, and
academia. These industries have profited from the productivity gains of using
hand-held terminals and notebook computers to transmit real-time information to
centralized hosts for processing. Today wireless LANs are becoming more widely
recognized as a general-purpose connectivity alternative for a broad range of
business customers.
Why Wireless?
The widespread reliance on networking in business and the rapid growth of the
Internet and online services are strong testimonies to the benefits of shared
data and shared resources. With wireless LANs, users can access shared
information without looking for a place to plug in, and network managers can
set up or augment networks without installing or moving wires. Wireless LANs
offer the following productivity, convenience, and cost advantages over
traditional wired networks:
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Mobility: Wireless LAN systems can provide LAN
users with access to real-time information anywhere at work and in the
home.
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Installation Speed and Simplicity: Installing a
wireless LAN system can be fast and easy and can eliminate the need to pull
cable through walls and ceilings.
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Installation Flexibility: Wireless technology
allows the network to go where wire cannot go.
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Reduced Cost-of-Ownership: While the initial
investment required for wireless LAN hardware can be higher than the cost of
wired LAN hardware, overall installation expenses and life-cycle costs can be
significantly lower. Long-term cost benefits are greatest in dynamic
environments requiring frequent moves and changes.
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Scalability: Wireless LAN systems can be configured
in a variety of topologies to meet the needs of specific applications and
installations. Configurations are easily changed and range from peer-to-peer
networks suitable for a small number of users to full infrastructure networks
of thousands of users that enable roaming over a broad area.
How Wireless LANs Are Used in the Real World
Wireless LANs frequently augment rather than replace wired LAN networks—often
providing the final few meters of connectivity between a wired network and the
mobile user. The following list describes some of the many applications made
possible through the power and flexibility of wireless LANs:
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Doctors and nurses in hospitals are more productive because
hand-held or notebook computers with wireless LAN capability deliver patient
information instantly.
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Consulting or accounting audit teams or small workgroups increase
productivity with quick network setup.
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Students holding class on campus greens can access the Internet to
consult the catalog of the Library of Congress or class notes.
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Network managers in dynamic environments minimize the overhead
caused by moves, extensions to networks, and other changes with wireless LANs.
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Training sites at corporations and students at universities use
wireless connectivity to access information, information exchanges, and
learning.
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Trade show and branch office workers minimize setup requirements by
installing pre-configured wireless LANs needing no local MIS support.
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Warehouse workers use wireless LANs to exchange information with
central databases, thereby increasing productivity.
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Senior executives in meetings make quicker decisions because they
have real-time information at their fingertips.
Wireless LAN Technology
Manufacturers of wireless LANs have a range of technologies to choose from when
designing a wireless LAN solution. Each technology comes with its own set of
advantages and limitations.
Narrowband Technology
A narrowband radio system transmits and receives user information on a specific
radio frequency. Narrowband radio keeps the radio signal frequency as narrow as
possible just to pass the information. Undesirable crosstalk between
communications channels is avoided by carefully coordinating different users on
different channel frequencies.
A private telephone line is much like a radio frequency. When each
home in a neighborhood has its own private telephone line, people in one home
cannot listen to calls made to other homes. In a radio system, privacy and
noninterference are accomplished by the use of separate radio frequencies. The
radio receiver filters out all radio signals except the ones on its designated
frequency.
From a customer standpoint, one drawback of narrowband technology is
that the end-user must obtain an FCC license for each site where it is
employed.
Spread Spectrum Technology
Most wireless LAN systems use spread-spectrum technology, a wideband
radio frequency technique developed by the military for use in reliable,
secure, mission-critical communications systems. Spread-spectrum is designed to
trade off bandwidth efficiency for reliability, integrity, and security. In
other words, more bandwidth is consumed than in the case of narrowband
transmission, but the tradeoff produces a signal that is, in effect, louder and
thus easier to detect, provided that the receiver knows the parameters of the
spread-spectrum signal being broadcast. If a receiver is not tuned to the right
frequency, a spread-spectrum signal looks like background noise. There are two
types of spread spectrum radio: frequency hopping and direct sequence.
Frequency-Hopping Spread Spectrum Technology
Frequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that changes
frequency in a pattern known to both transmitter and receiver. Properly
synchronized, the net effect is to maintain a single logical channel. To an
unintended receiver, FHSS appears to be short-duration impulse noise.
Direct-Sequence Spread Spectrum Technology
Direct-sequence spread-spectrum (DSSS) generates a redundant bit pattern for
each bit to be transmitted. This bit pattern is called a chip (or chipping
code). The longer the chip, the greater the probability that the original data
can be recovered (and, of course, more bandwidth is required). Even if one or
more bits in the chip are damaged during transmission, statistical techniques
embedded in the radio can recover the original data without the need for
retransmission. To an unintended receiver, DSSS appears as low-power wideband
noise and is rejected by most narrowband receivers.
Infrared Technology
A third technology, little used in commercial wireless LANs, is
infrared. Infrared (IR) systems use very high frequencies, just below visible
light in the electromagnetic spectrum, to carry data. Like light, IR cannot
penetrate opaque objects; it is either directed (line-of-sight) or diffuse
technology. Inexpensive directed systems provide limited range of approximately
3 feet and typically are used for personal area networks. Occasionally directed
systems are used in specific wireless LAN applications. High performance
directed IR is impractical for mobile users and is therefore used only to
implement fixed sub-networks. Diffuse or reflective IR wireless LAN systems do
not require line-of-sight, but cells are limited to individual rooms.
How Wireless LANs Work
Wireless LANs use electromagnetic airwaves (radio or infrared) to communicate
information from one point to another without relying on any physical
connection. Radio waves are often referred to as radio carriers because they
simply perform the function of delivering energy to a remote receiver. By
superimposing the transmitted data onto the radio carrier, data can be
accurately extracted at the receiving end. This is generally referred to as
modulation of the carrier by the information being transmitted. Once data is
superimposed (modulated) onto the radio carrier, the radio signal occupies more
than a single frequency, since the frequency or bit rate of the modulating
information adds to the carrier.
Multiple radio carriers can exist in the same space at the same time
without interfering with each other if the radio waves are transmitted on
different radio frequencies. To extract data, a radio receiver tunes in one
radio frequency while rejecting all other frequencies.
In a typical wireless LAN configuration, a transmitter/receiver
(transceiver) device, called an access point, connects to the wired network
from a fixed location using standard cabling. At a minimum, the access point
receives, buffers, and transmits data between the wireless LAN and the wired
network infrastructure. A single access point can support a small group of
users and can function within a range of less than one hundred to several
hundred feet.
End users access the wireless LAN through wireless-LAN adapters,
which are implemented as PC cards in notebook or palmtop computers, as cards in
desktop computers, or integrated within hand-held computers. Wireless LAN
adapters provide an interface between the client network operating system (NOS)
and the airwaves via an antenna. The nature of the wireless connection is
transparent to the NOS.
Wireless LAN Configurations
Wireless LANs can be simple or complex. At its most basic, two PCs
equipped with wireless adapter cards can set up an independent network whenever
they are within range of one another. This is called a peer-to-peer network.
On-demand networks, such as in this example, require no administration or
preconfiguration. In this case each client would only have access to the
resources of the other client and not to a central server.
Figure 1: A wireless peer-to-peer network
Installing an access point can extend the range of an ad hoc network,
effectively doubling the range at which the devices can communicate. Since the
access point is connected to the wired network, each client can have access to
server resources as well as to other clients. Each access point can accommodate
many clients; the specific number depends on the number and nature of the
transmissions involved. Many real-world applications exist where a single
access point services from 15-50 client devices.
Figure 2: Client and Access Point
Access points have a finite range, on the order of 500 feet indoor
and 1000 feet outdoors. In a very large facility such as a warehouse, or on a
college campus, it may be necessary to install more than one access point.
Access point positioning is accomplished by means of a site survey. The goal is
to blanket the coverage area with overlapping coverage cells so that clients
can range throughout the area without ever losing network contact. The ability
of clients to move seamlessly among a cluster of access points is called roaming.
Access points hand the client off from one access point to another in a way
that is invisible to the client, ensuring unbroken connectivity.
Figure 3: Multiple access points and roaming
To solve particular problems of topology, the network designer might
choose to use Extension Points to augment the network of access points.
Extension Points look and function like access points, but they are not
tethered to the wired network as are APs. EPs function just as their name
implies: they extend the range of the network by relaying signals from a client
to an AP or another EP. EPs can be strung together in order to pass along
messaging from an AP to far-flung clients (just as humans in a bucket brigade
pass pails of water hand-to-hand from a water source to a fire).
Figure 4: Use of an extension point
One last item of wireless LAN equipment to consider is the
directional antenna. Let’s suppose you had a wireless LAN in your building A
and wanted to extend it to a leased building, B, one mile away. One solution
might be to install a directional antenna on each building with each antenna
targeting the other. The antenna on A is connected to your wired network via an
access point. The antenna on B is similarly connected to an access point in
that building, which enables wireless LAN connectivity in that facility.
Figure 5: The use of directional antennas
Customer Considerations
While wireless LANs provide installation and configuration flexibility
and the freedom inherent in network mobility, customers should be aware of the
following factors when considering wireless LAN systems.
Range and coverage
The distance over which RF waves can communicate is a function of
product design (including transmitted power and receiver design) and the
propagation path, especially in indoor environments. Interactions with typical
building objects, including walls, metal, and even people, can affect how
energy propagates, and thus what range and coverage a particular system
achieves. Solid objects block infrared signals, which imposes additional
limitations. Most wireless LAN systems use RF because radio waves can penetrate
most indoor walls and obstacles. The range (or radius of coverage) for typical
wireless LAN systems varies from under 100 feet to more than 300 feet. Coverage
can be extended, and true freedom of mobility via roaming, provided through
microcells.
Throughput
As with wired LAN systems, actual throughput in wireless LANs is product- and
set-up-dependent. Factors that affect throughput include the number of users,
propagation factors such as range and multipath, the type of wireless LAN
system used, as well as the latency and bottlenecks on the wired portions of
the LAN. Data rates for the most widespread commercial wireless 802.11b LANs
are in the 2-11 Mbps range. For 802.11g and 802.11a data rates can be
expected up to 54 Mbps. Users of traditional Ethernet or Token Ring LANs
generally experience little difference in performance when using a wireless
LAN. Wireless LANs provide throughput sufficient for the most common LAN-based
office applications, including electronic mail exchange, access to shared
peripherals, Internet access, file transfer, and access to multi-user databases
and applications.
As a point of comparison, state-of-the-art V.90 modems transmit and
receive at data rates of less than the advertised 56.6 Kbps. In terms of
throughput, a wireless LAN is significantly faster than the state-of-the-art
V.90 modem.
Integrity and Reliability
Wireless data technologies have been proven reliable through more than
fifty years of wireless application in both commercial and military systems.
While radio interference can cause degradation in throughput, such interference
is rare in the home or workplace. Robust designs of proven wireless LAN
technology and the limited distance over which signals travel result in
connections that are far more robust than cellular phone connections and
provide data integrity performance equal to or better than wired networking.
Compatibility with the Existing Network
Most wireless LANs provide for industry-standard interconnection with
wired networks such as Ethernet or Token Ring. Wireless LAN nodes are supported
by network operating systems in the same fashion as any other LAN node through
the use of the appropriate drivers. Once installed, the network treats wireless
nodes like any other network component.
Interoperability of Wireless Devices
Wireless LAN systems from different vendors may not be interoperable.
For three reasons. First, different technologies will not interoperate. A
system based on spread spectrum frequency hopping (FHSS) technology will not
communicate with another based on spread spectrum direct sequence (DSSS)
technology. Second, systems using different frequency bands will not
interoperate even if they both employ the same technology. Third, systems from
different vendors may not interoperate even if they both employ the same
technology and the same frequency band, due to differences in implementation by
each vendor.
Interference and Coexistence
The unlicensed nature of radio-based wireless LANs means that other
products that transmit energy in the same frequency spectrum can potentially
provide some measure of interference to a wireless LAN system. Microwave ovens
are a potential concern, but most wireless LAN manufacturers design their
products to account for microwave interference. Another concern is the
co-location of multiple wireless LANs. While wireless LANs from some
manufacturers interfere with wireless LANs, others coexist without
interference.
Licensing Issues
In the United States, the Federal Communications Commission (FCC) governs radio
transmissions, including those employed in wireless LANs. Other nations have
corresponding regulatory agencies. Wireless LANs are typically designed to
operate in portions of the radio spectrum where the FCC does not require the
end-user to purchase a license to use the airwaves. In the U.S. most wireless
LANs broadcast over one of the ISM (Instrumentation, Scientific, and Medical)
bands. These include 902-928 MHz, 2.4-2.483 GHz, 5.15-5.35 GHz, and 5.725-5.875
GHz. For wireless LANs to be sold in a particular country, the manufacturer of
the wireless LAN must ensure its certification by the appropriate agency in
that country.
Simplicity/Ease of Use
Users need little new information to take advantage of wireless LANs. Because
the wireless nature of a wireless LAN is transparent to a user's network
operating system, applications work the same as they do on wired LANs. Wireless
LAN products incorporate a variety of diagnostic tools to address issues
associated with the wireless elements of the system; however, products are
designed so that most users rarely need these tools.
Wireless LANs simplify many of the installation and configuration
issues that plague network managers. Since only the access points of wireless
LANs require cabling, network managers are freed from pulling cables for
wireless LAN end users. Lack of cabling also makes moves, adds, and changes
trivial operations on wireless LANs. Finally, the portable nature of wireless
LANs lets network managers preconfigure and troubleshoot entire networks before
installing them at remote locations. Once configured, wireless LANs can be
moved from place to place with little or no modification.
Security
Because wireless technology has roots in military applications, security has
long been a design criterion for wireless devices. Security provisions are
typically built into wireless LANs, making them more secure than most wired
LANs. It is extremely difficult for unintended receivers (eavesdroppers) to
listen in on wireless LAN traffic. In general, individual nodes must be
security-enabled before they are allowed to participate in network traffic.
Cost
A wireless LAN implementation includes both infrastructure costs, for the
wireless access points, and user costs, for the wireless LAN adapters.
Infrastructure costs depend primarily on the number of access points deployed.
The number of access points typically depends on the required coverage region
and/or the number and type of users to be serviced. The coverage area is
proportional to the square of the product range. Wireless LAN adapters are
required for standard computer platforms.
The cost of installing and maintaining a wireless LAN generally is
lower than the cost of installing and maintaining a traditional wired LAN, for
two reasons. First, a wireless LAN eliminates the direct costs of cabling and
the labor associated with installing and repairing it. Second, because wireless
LANs simplify moves, adds, and changes, they reduce the indirect costs of user
downtime and administrative overhead.
Scalability
The design of wireless networks can be extremely simple or quite complex.
Wireless networks can support large numbers of nodes and/or large physical
areas by adding access points to boost or extend coverage.
Battery Life for Mobile Platforms
Since end-user wireless products are designed to run off the AC or battery
power from their host notebook or hand-held computer, wireless products have no
direct wire connectivity of their own.
Safety
The output power of wireless LAN systems is very low, much less than that of a
hand-held cellular phone. Since radio waves fade rapidly over distance, very
little exposure to RF energy is provided to those in the area of a wireless LAN
system. Wireless LANs must meet stringent government and industry regulations
for safety. No adverse health affects have ever been attributed to wireless
LANs.
Summary
Flexibility and mobility make wireless LANs both effective extensions and
attractive alternatives to wired networks. Wireless LANs provide all the
functionality of wired LANs, without the physical constraints of the wire
itself. Wireless LAN configurations range from simple peer-to-peer topologies
to complex networks offering distributed data connectivity and roaming. Besides
offering end-user mobility within a networked environment, wireless LANs enable
portable networks, allowing LANs to move with the workers that use them.
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