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What is a Wireless
LAN?
Contents
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 the 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. Business Research
Group, a market research firm, predicts a sixfold expansion of the
worldwide wireless LAN market by the year 2000, reaching more than $2
billion in revenues.
Why Wireless?
The widespread
reliance on networking in business and the meteoric 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 in their
organization. This mobility supports productivity and service
opportunities not possible with wired networks.
| | 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 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:
| 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 a campus greensward access the Internet to
consult the catalog of the Library of Congress.
| | 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 ease access to information, information
exchanges, and learning.
| | Network managers installing networked computers in older buildings
find that wireless LANs are a cost-effective network infrastructure
solution.
| | 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.
| | Network managers implement wireless LANs to provide backup for
mission-critical applications running on wired networks.
| | 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, the
more bandwidth 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 (ignored) 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 very limited range (3 ft) and typically are used
for personal area networks but occasionally 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. The data being transmitted is superimposed on the radio carrier
so that it 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. The access point (or the antenna
attached to the access point) is usually mounted high but may be mounted
essentially anywhere that is practical as long as the desired radio
coverage is obtained.
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 would 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 will probably 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 might 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 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 may 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, 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 and
IR 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 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, and access to multi-user databases and
applications.
As a point of comparison, it is worth noting that state-of-the-art V.90
modems transmit and receive at optimal data rates of 56.6 Kbps. In terms
of throughput, a wireless LAN operating at 1.6 Mbps is almost thirty times
faster.
Integrity and Reliability
Wireless data
technologies have been proven 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 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:
thought the use of the appropriate drivers. Once installed, the network
treats wireless nodes like any other network component.
Interoperability of Wireless Devices
Customers
should be aware that wireless LAN systems from different vendors might 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. This
issue is best addressed directly with the appropriate vendors.
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 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 very little new
information to take advantage of wireless LANs. Because the wireless
nature of a wireless LAN is transparent to a user's NOS, 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. 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.
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; access points range in price from $1,000
to $2000. 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, and range in
price from $300 to $1,000.
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
Wireless networks can be designed to 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
End-user wireless
products are designed to run off the AC or battery power from their host
notebook or hand-held computer, since they have no direct wire
connectivity of their own. wireless LAN vendors typically employ special
design techniques to maximize the host computer's energy usage and battery
life.
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 knowledge workers that use them.
Grateful acknowledgment is made to the Wireless LAN
Alliance, of which Proxim is a member, for use of their publication,
"Introduction to Wireless LANs," on which this primer was
based. Return
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