NMAM INSTITUTE OF TECHNOLOGY
NITTE
SEMINAR ON
WIRELESS LAN
BY
ROSHAN AGILLA
WIRELESS LAN
CONTENTS
1. Overview
2. Technology
3. WLAN configurations
4. IEEE 802.11 Standards
5. Hardware requirement
6. Setting up of a dummy LAN
7. Conclusion
8. Bibliography
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WIRELESS LAN
1.OVERVIEW
A wireless LAN (WLAN) is typically an extension of a wired LAN. WLAN components
convert data packets into radio waves or infrared (IR) light pulses and send them to other
wireless devices or to an access point that serves as a gateway to the wired LAN. Most
WLANs today are based on the IEEE 802.11 and 802.11b standards for wireless
communication between devices and a LAN. These standards permit data transmissions
at 1 to 2 Mbps or 5 to 11 Mbps, respectively, and specify a common architecture,
transmission methods, and other aspects of wireless data transfer to improve
interoperability among products.
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
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WIRELESS LAN
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.
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2.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,
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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.
Bluetooth technology
It is a forthcoming wireless personal area networking (WPAN) technology that has
gained significant industry support and will coexist with most wireless LAN solutions.
The Bluetooth specification is for a 1 Mbps, small form-factor, low-cost radio solution
that can provide links between mobile phones, mobile computers and other portable
handheld devices and connectivity to the Internet. It uses the FHSS technology.
CDMA vs. TDMA
Let's begin by learning what these two acronyms stand for. TDMA stands for "Time
Division Multiple Access", while CDMA stands for "Code Division Multiple Access".
Three of the four words in each acronym are identical, since each technology essentially
achieves the same goal, but by using different methods. Each strives to better utilize the
radio spectrum by allowing multiple users to share the same physical channel. You heard
that right. More than one person can carry on a conversation on the same frequency
without causing interference. This is the magic of digital technology.
Where the two competing technologies differ is in the manner in which users share the
common resource. TDMA does it by chopping up the channel into sequential time slices.
Each user of the channel takes turns transmitting and receiving in a round-robin fashion.
In reality, only one person is actually using the channel at any given moment, but he or
she only uses it for short bursts. He then gives up the channel momentarily to allow the
other users to have their turn. This is very similar to how a computer with just one
processor can seem to run multiple applications simultaneously.
CDMA on the hand really does let everyone transmit at the same time. Conventional
wisdom would lead you to believe that this is simply not possible. Using conventional
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modulation techniques, it most certainly is impossible. What makes CDMA work is a
special type of digital modulation called "Spread Spectrum". This form of modulation
takes the user's stream of bits and splatters them across a very wide channel in a pseudo-
random fashion. The "pseudo" part is very important here, since the receiver must be able
to undo the randomization in order to collect the bits together in a coherent order.
If you are still having trouble understanding the differences though, perhaps this analogy
will help you. This my own version of an excellent analogy provided by Qualcomm:
Imagine a room full of people, all trying to carry on one-on-one conversations. In TDMA
each couple takes turns talking. They keep their turns short by saying only one sentence
at a time. As there is never more than one person speaking in the room at any given
moment, no one has to worry about being heard over the background din. In CDMA,
each couple talk at the same time, but they all use a different language. Because none of
the listeners understand any language other than that of the individual to whom they are
listening, the background din doesn't cause any real problems.
CDMA
Now that we have a rudimentary understanding of the two technologies, let's try and
examine what advantages they provide. We'll begin with CDMA, since this new
technology has created the greatest "buzz" in the mobile communications industry.
One of the terms you'll hear in conjunction with CDMA is "Soft Handoff". A handoff
occurs in any cellular system when your call switches from one cell site to another as you
travel. In all other technologies, this handoff occurs when the network informs your
phone of the new channel to which it must switch. The phone then stops receiving and
transmitting on the old channel, and commences transmitting and receiving on the new
channel. It goes without saying that this is known as a "Hard Handoff".
In CDMA however, every site are on the SAME frequency. In order to begin listening to
a new site, the phone only needs to change the pseudo-random sequence it uses to decode
the desired data from the jumble of bits sent for everyone else. While a call is in progress,
the network chooses two or more alternate sites that it feels are handoff candidates. It
simultaneously broadcasts a copy of your call on each of these sites. Your phone can then
pick and choose between the different sources for your call, and move between them
whenever it feels like it. It can even combine the data received from two or more
different sites to ease the transition from one to the other.
This arrangement therefore puts the phone in almost complete control of the handoff
process. Such an arrangement should ensure that there is always a new site primed and
ready to take over the call at a moment's notice. In theory, this should put an end to
dropped calls and audio interruptions during the handoff process. In practice it works
quite well, but dropped calls are still a fact of life in a mobile environment. However,
CDMA rarely drops a call due to a failed handoff.
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A big problem facing CDMA systems is channel pollution. This occurs when signals
from too many base stations are present at the subscriber's phone, but none are dominant.
When this situation occurs, audio quality degrades rapidly, even when signal seem
otherwise very strong. Pollution occurs frequently in densely populated urban
environments where service providers must build many sites in close proximity. Channel
pollution can also result from massive multipath problems caused by many tall buildings.
Taming pollution is a tuning and system design issue. It is up to the service provider to
reduce this phenomenon as much as possible.
Supporters often cite capacity as one CDMA's biggest assets. Virtually no one disagrees
that CDMA has a very high "spectral efficiency". It can accommodate more users per
MHz of bandwidth than any other technology. What experts do not agree upon is by how
much. Unlike other technologies, in which the capacity is fixed and easily computed,
CDMA has what is known as "Soft Capacity". You can always add just one more caller
to a CDMA channel, but once you get past a certain point, you begin to pollute the
channel such that it becomes difficult to retrieve an error-free data stream for any of the
participants.
The ultimate capacity of a system is therefore dependent upon where you draw the line.
How much degradation is a carrier willing to subject their subscribers to before they
admit that they have run out of useable capacity? Even if someone does set a standard
error rate at which these calculations are made, it does not mean that you personally will
find the service particularly acceptable at that error rate.
TDMA
Let's move away from CDMA now and have a look at TDMA. Before we can go any
further though, I should note that there are actually three different flavors of TDMA in
the PCS market. Each of these technologies implements TDMA in a slightly different
way. The most complex implementation is, without a doubt, GSM. It overlays the basic
TDMA principles with many innovations that reduce the potential problems inherent in
the system.
To reduce the effects of co-channel interference, multipath, and fading, the GSM network
can use something known as Frequency Hopping. This means that your call literally
jumps from one channel to another at fairly short intervals. By doing this, the likelihood
of a given RF problem is randomized, and the effects are far less noticeable to the end
user. Frequency Hopping is always available, but not mandated. This means that your
GSM provider may or may not use it.
IS-136 is another form for TDMA, and it is this implementation that people generically
refer to as TDMA. IS-136 is probably the crudest implementation of TDMA. It will
suffer from various maladies far more easily than GSM, but it does have one unique
feature that compensate for its crudeness. It is the only technology that integrates with
existing analog systems. While CDMA can provide handoffs from digital to analog, there
is no way to send the call back to digital. In IS-136 you can go both ways at any time.
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iDEN is a proprietary Motorola technology that no other company seems to participate in.
Only Motorola makes iDEN phones, and only Motorola makes iDEN infrastructure
equipment. Perhaps the company guards its technology on purpose. iDEN performs
reasonably well, but its chosen CODEC is not quite as good as those on GSM or CDMA.
In my experience, the quality of iDEN depends a lot on which iDEN phone you use.
Some of Motorola's later models (such as the i85, i80, and i90) have improved things
markedly.
Each of the three TDMA technologies uses a different CODEC. GSM sports a CODEC
called EFR (short for Enhanced Full Rate). This CODEC is arguable the best sounding
one available in the PCS world. IS-136 used to sound horrible, but in the fall of 1997 they
replaced their old CODEC with a new one. This new CODEC sounds much better than
the old, but it doesn't quite match the GSM and CDMA entries.
TDMA systems still rely on the switch to determine when to perform a handoff. Unlike
the old analog system however, the switch does not do this in a vacuum. The TDMA
handset constantly monitors the signals coming from other sites, and it reports this
information to the switch without the caller being aware of it. The switch then uses this
information to make better handoff choices at more appropriate times.
Perhaps the most annoying aspect of TDMA system to some people is the obviousness of
handoffs. Some people don't tend to hear them, and I can only envy those individuals.
Those of us who are sensitive to the slight interruptions caused by handoffs will probably
find GSM the most frustrating. Its handoffs are by far the most messy. When handoffs
occur infrequently (such as when we are stationary or in areas with few sites), they really
don't present a problem at all. However, when they occur very frequently (while traveling
in an area with a huge number of sites) they can become annoying.
What’s different?
CDMA really comes into its element when you are out in the countryside with few sites
covering large expanses of land. Under these conditions CDMA provides extremely
stable audio with few frame errors to mess things up. This is because Channel Pollution is
almost non-existent in these situations. Under similar conditions TDMA suffers too
readily from interference and it will often blank the audio. Many people who use CDMA
systems in sparsely populated areas have given these technology extremely high marks.
TDMA systems also have great difficulties in open regions just outside densely populated
areas. In this situation your phone is exposed to signals coming from countless sites in the
densely populated areas, but there are no dominant signals from a close-by site. CDMA
can suffer under these conditions too (due to channel pollution), but not quite so badly.
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GPRS
General Packet Radio Service (GPRS) enabled networks offer 'always-on', higher
capacity, Internet-based content and packet-based data services. This enables services
such as color Internet browsing, e-mail on the move, powerful visual communications,
multimedia messages and location-based services.
With General Packet Radio Service (GPRS) you can enjoy a continuous wireless
connection to data networks and access your favorite information and entertainment
services. GPRS technology allows mobile phones to be used for sending and receiving
data over an Internet Protocol (IP)-based network. GPRS as such is a data bearer that
enables wireless access to data networks like the Internet. The applications using GPRS
are WAP, MMS, SMS, Java and the PC dial-up (for example, Internet and e-mail).
EDGE
Once GPRS implementation has been completed, the next step on the road to 3G will be
a technology known as enhanced data rates for GSM evolution. EDGE encompasses a
series of hardware and software enhancements and has the potential to deliver speeds as
high as 384 Kbps. Again, as with other technologies, some analysis is skeptical. A recent
article on ZDNet predicted EDGE speeds would come in around 56 Kbps , so there’s
some debate about whether it should be considered a 2G, 2.5G and 3G technologies.
EDGE also has the potential to bring back together forking technologies, as an EDGE-
compliant phone may be able to work on GSM and TDMA networks, greatly improving
one’s ability to roam.
UMTS
The Universal Telecommunications System is the promise of fat pipes over the ether,
constant connection at speeds that can deliver entertainment content. UMTS is
broadband, packet-based digital wireless technology, based on GSM and GPRS systems.
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3.WLAN CONFIGURATIONS
Peer to peer network (ad-hoc mode)
The most basic wireless LAN consists of two PCs equipped with wireless adapter cards
that form an independent network whenever they are within a range of one another. On-
demand networks, such as this example, require no administration or pre configurations.
In this case, each client would only have access to the resources of the other client and
not to a central server. The wireless LAN setup is sometimes called an ad-hoc network.
Client and access point (infrastructure mode)
Installing an access point allows each client to have shared resources as well as to other
clients. The access point connects can accommodate many clients ( up to 16 with the
multitech route finder RF802EW); the specific number depends on the number and nature
of the transmissions involved. This wireless LAN setup is sometimes called Infrastructure
Mode.
Multiple access points and roaming
Access points have a finite range for transmission – around 100 meters (328 feet) indoors
and 300 meters (984 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
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throughout the area without ever losing network contact. The ability of clients to move
seamlessly among a cluster of access points is called roaming.
4.IEEE 802.11 STANDARDS
Introduction
802.11 is a set of specifications for LANs (local area networks) from the institute of
electrical and electronic engineers (IEEE) . 802.11 define the standard for wireless LANs
encompassing three incompatible (non-interoperable) technologies: frequency hopping
spread spectrum (FHSS), direct sequence speed spectrum (DSSS) and infrared. The
standard promises multi vendor interoperability among products utilizing the same
technology. 802.11 has been moving rapidly to provide continuous improvements. The
latest step in the evolution was the ratification of the 802.11b or high rate, providing data
rate of 11 Mbps.
IEEE 802.11b
The standard’s 11Mbps PHY layer uses Complementary Code Keying (CCK)
technology. This latest standard is based on DSSS technology and provides speeds up to
11 Mbps with fallback rates of 5.5 mbps, 2 Mbps and 1 Mbps. It uses the same bandwidth
as the 2 Mbps.
DSSS standard and thus interoperates with legacy IEEE DSSS systems. As in the wired
world, higher speeds are continuously desired for applications such as streaming video,
telephony and multimedia. Moreover, faster peak rates will allow more nodes to
effectively connect to WLAN via single channel.
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5.HARDWARE REQUIREMENT
Wireless cards (wireless adapters)
This is perhaps the most essential component of a wireless LAN. These perform the same
functions as Ethernet cards in a wired network. Almost all the wireless cards now
available use PCMCIA to connect, and thus cannot be used on desktop computers.
However, PCI adapters are available that accommodate these cards and connect to a free
PCI slot in an ordinary desktop PC.
Access points
Three kinds of access points are currently available. The first are those that don’t
communicate with other access points and do not perform any bridging junctions. The
next kind is access points that act as bridges, which also come with different options.
These include:
Point-to-point: these are used to connect two LAN segments together. These
either use the Master/Slave configuration, or let you control which bridge unit you
connect to using its MAC address.
Point-to multipoint: Here all bridge units communicate with each other wirelessly.
Repeater: In the first two modes, bridging units talk only to each other, and do not
connect to wireless clients.
Mixed media router
Instead of bridging units, one has the option of using MMR (mixed media router) for
connecting different networks to each other. This is usually the cheapest solution, since it
is mostly implemented in software on a machine that is connected to all the networks.
Other products
Some other 802.11 based products have also started appearing in the market. These
include wireless internet cameras, presentation gateways and print servers. Some of the
other components are
Wireless antennae
Wireless router
Wireless bridges
Wireless modem gateway
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6.SETTING UP OF A DUMMY WIRELESS LAN
The following dummy network makes use of D-Link India’s equipments for the setting
up of the wireless network. It provides for the best possible option for a cost effective and
reliable setting up of a wireless LAN.
Hardware used:
D-Link DW900AP+ access point
D-Link 650+ PCMCIA based adapter card
Cost:
Rs.18,400/-
Steps to connect and configure an access point to set up a hybrid LAN
Connecting the access point to the existing LAN using a RJ45 cable.
Connecting the power cords of the access point and then configures the access
point by entering the IP address of the built-in web server in the browser of the
master system.
Configuration includes setting up the SSID and assigning the network key. The
network mode is setup in infrastructure mode along with the WEP.
Configuring the client
Once the adapter drivers are installed, the adapter card is plugged into
PCI/PCMIA slot.
Once the adapter is initialized the adapter card is configured.
The signal strengths are tested to determine the maximum accessibility.
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7.CONCLUSION
For the proper functioning of the network, the individual components should be
acquired from the same vendor.
The cost of setting up a wireless network is much higher than that of its wired
counterpart.
Efficiency of the network is diminished due to the presence of obstacles in its path
like metal objects, concrete walls and electrical equipments like microwave
owens, speakers.
Wireless networks have been an essential part of communication in the last century. Early
adopters of wireless technology primarily have been the military , emergency services,
and law enforcement organizations.
As society moves toward information centricity, the need to have information accessible
at any time and anywhere takes on a new dimension. With the rapid growth of mobile
telephony and networks, the vision of a mobile information society is slowly becoming a
reality.
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8.BIBLIOGRAPHY
1. IEEE wireless communications journal
2. IEEE personnel communications journal
3. A text book on communication networks- leon, Garcia, widjaja
4. www.smarthomeforum.com
5. www.umts.com
6. www.ABCs of spreadspectrum.com
7. www.palowireless.com
8. www.streampacketradio.com
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