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
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:
|Mobility: Wireless LAN systems can
provide LAN users with access to real-time information anywhere at work
and in the home. |
|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. |
|Installation Flexibility: Wireless
technology allows the network to go where wire cannot go. |
|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.
|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
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
|Doctors and nurses in hospitals are more productive
because hand-held or notebook computers with wireless LAN capability
deliver patient information instantly. |
|Consulting or accounting audit teams or small
workgroups increase productivity with quick network setup. |
|Students holding class on campus greens can access
the Internet to consult the catalog of the Library of Congress or class
|Network managers in dynamic environments minimize
the overhead caused by moves, extensions to networks, and other changes
with wireless LANs. |
|Training sites at corporations and students at
universities use wireless connectivity to access information, information
exchanges, and learning. |
|Trade show and branch office workers minimize setup
requirements by installing pre-configured wireless LANs needing no local
MIS support. |
|Warehouse workers use wireless LANs to exchange
information with central databases, thereby increasing productivity. |
|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.
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
Frequency-Hopping Spread Spectrum
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
Direct-Sequence Spread Spectrum
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.
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
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.
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.
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.
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
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.
5: The use of directional antennas
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.
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.
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
LANs are in the 1.6 Mbps range. 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 operating at 1.6 Mbps is
almost thirty times faster than the state-of-the-art V.90 modem.
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.
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.
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.
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.
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.
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.
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. Complex encryption techniques make it
impossible for all but the most sophisticated to gain unauthorized access to
network traffic. In general, individual nodes must be security-enabled
before they are allowed to participate in network traffic.
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
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.
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.
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.
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.