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.
