COE 341: Data and Computer Communications
Armaghan Tawseef Khan (237871)
Fakhruddin Mahmood (246810)
Sadiq Jafar Hasan (237867)
Dr. Radwan Abdel-Aal
A wireless LAN or WLAN is a wireless local area network that enables client
computers and the server to communicate with one another without direct cable
connections. By using electromagnetic waves, WLANs transmit and receive data
over the air, and thus minimize the need for wired connections. There are several
different technologies by which these WLANs can be implemented depending on
the requirements of the users. Furthermore, several WLAN standards have been
developed in order to ensure interoperability between products from different
I) How They Work
In a typical WLAN configuration, the access point (a device that translates
between the wired LAN and the wireless LAN) is connected to the wired network
from a fixed location using standard Ethernet cable. 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 many hundred feet. End users
access the wireless LAN through the wireless-LAN adapters which are like PC
cards, PCI or ISA boards with radio transceivers to communicate with the Access
point, and these adapters provide an interface between the client network operating
system (NOS) and the airwaves. Also, there might be Extension Points which are
devices similar to the access point, but not connected to the wired LAN. These
extension points serve to extend the range of the wireless network by relaying
signals from client computers to the Access point.
II) Wireless LAN Technology
A) Infrared LANs
Infrared is common place in most homes, where it is used for a variety of remote
control devices. Recently attention has turned to the use of infrared technology to
construct wireless LANs. However, an individual cell of an IR LAN is limited to a
single room, because infrared light does not penetrate opaque walls. There are 3
types of transmission techniques:
1) Directed beam
A directed beam can be used to create point to point links. In this mode, the range
depends on the emitted power and on the degree of focusing. A focused IR data
link can have a range of kilometers. Such ranges are needed for constructing
outdoor wireless LANs. One indoor use of point to point IR links is to set up a
token ring LAN. A set of IR transceivers can be positioned so that data could
circulate around them in ring configuration.
2) Omni directional
It involves a single base station that is within the line of sight of all other stations
on the LAN. This station is usually mounted on a ceiling. The base station acts as
multiport repeater. The ceiling transmitter broadcasts an omnidirectional signal
that can be received by all of the other IR transceivers in the area. These other
transceivers transmit a directional beam aimed at the ceiling base unit.
In a diffused configuration, all of the IR transmitters are focused and aimed at a
point on a diffusely reflecting ceiling. IR radiation striking the ceiling is reradiated
omnidirectionally and picked up by all of the receivers in the area.
• Virtually unlimited bandwidth.
• Unregulated spectrum.
• IR light is diffusely reflected by light-colored objects. Can use this property
to provide coverage in the presence of obstacles.
• Does not penetrate walls or other opaque objects. This makes it easy to
secure and allows separate networks to operate without interference.
• There is interference in the form of background radiation from sunlight and
• Power needs to be restricted for eye safety.
B) Spread Spectrum LANs
Spread spectrum is a type of modulation that spreads data transmission across the
available frequency band, in excess of the minimum bandwidth required to send
the information. Spreading the data across the frequency spectrum makes the
signal resistant to noise, interference and eavesdropping. Spread spectrum
modulation schemes are commonly used with personal communication devices
such as digital cellular phones, as well as with wireless local area networks.
Two of the most popular spread spectrum techniques are:
1) Direct Sequence Spread Spectrum (DSSS)
This is probably the most widely recognized form of spread spectrum. A Direct
Sequence Spread Spectrum transmitter converts an incoming data (bit) stream into
a symbol stream where each symbol represents a group of one or more bits. Using
a phase-varying modulation technique such as quadrature phase-shift keying
(QPSK), the DSSS transmitter modulates or multiplies each symbol with a noise
like code called pseudorandom noise (pn) sequence. This is called a "chip"
sequence. The multiplication operation in a DSSS transmitter artificially increases
the used bandwidth based the length of the chip sequence.
2) Frequency Hopping Spread Spectrum (FHSS)
As the name implies the FHSS hops from narrow band to narrow band within a
wide band. More specifically, FHSS radios send one or more data packets and
continue this hop-transmit sequence. The hopping pattern or sequence appears
random but is actually a periodic sequence tracked by sender and receiver. The
FHSS systems can be susceptible to noise during any one hop but typically can
achieve better transmission during other hops within the wideband.
Why do we use spread spectrum?
Spread spectrum has many different unique properties that cannot be found in any
other modulation technique. Below outlines the advantages and disadvantages for
a typical spread spectrum system.
• Has the ability to eliminate or alleviate the effect of multipath interference;
• Can share the same frequency band (overlay) with other users;
• Provides privacy due to unknown random codes;
• Involves low power spectral density since signal is spread over a large
• Bandwidth inefficient
• Implementation is somewhat complex.
C) Narrowband Microwave LANs
These LANs operate at microwave frequencies but do not use spread
spectrum. The term narrowband microwave refers to the use of a microwave radio
frequency band for signal transmission, with a relatively narrow bandwidth-just
wide enough to accommodate the signal. There are two types of narrowband
1) Licensed Narrowband RF
Within the United States, licensing is controlled by the FCC. Each geographic area
has a radius of 28 km and can contain five licenses, with each license covering two
frequencies. One advantage of the licensed narrowband LAN is that it guarantees
interference free communication. Unlike unlicensed spectrum, such as ISM,
licensed spectrum gives the license holder a legal right to an interference free data
communications channel. Users of an ISM-band LAN are at risk of interference
disrupting their communications, for which they may not have a legal remedy.
2) Unlicensed Narrowband RF
In 1995, Radio LAN became the first vendor to introduce a narrowband wireless
LAN using the unlicensed ISM spectrum. This spectrum can be used for
narrowband transmission at low power (0.5 watts or less). The Radio LAN product
operates at 10Mbps in the 5.8-GHz band. The product has a range of 50m in a
semi open office and 100m in an open office.
A) IEEE 802.11
When wireless networks initially started appearing in the market, there was no
unique standard to ensure interoperability between different products. In order to
combat this, the IEEE 802.11 committee was formed in 1990. The purpose of the
IEEE 802.11 standard (also known as the WiFi standard) was to define
specifications that allow wireless LAN devices from different vendors to operate
together. It does this by defining a single Medium Access Control protocol and
three different physical layer technologies (as depicted in fig.1):
• Frequency Hopping Spread Spectrum (FHSS) operating in the 2.4 GHz
band at 1 and 2 Mbps
• Direct Sequence Spread Spectrum (DSSS) operating in the 2.4 GHz band at
1 and 2 Mbps
• Infrared operating at 1 and 2 Mbps
Figure 1: 802.11 protocol
Further amendments were made to the protocol later on. These include the
802.11a, 802.11b and 802.11g amendments.
The 802.11a amendment to the original protocol was ratified in 1999. It uses a
technology called Orthogonal Frequency Division Multiplexing (OFDM) rather
than Spread Spectrum and operates in the 5 GHz band. Using this technique, data
rates of up to 54 Mbps are possible. Unfortunately it is not compatible with
802.11b devices unless the device implements both standards. The advantage of
802.11a is that instead of operating in the crowded 2.4 GHz band (which is also
used by cordless telephones, microwave ovens, Bluetooth devices and other
devices), it operates in the 5 GHz band which is relatively free of interference.
However, the higher frequency has less penetrating power thus reducing the
indoor range from 50m to 30m.
The 802.11b amendment was ratified in 1999 and is an extension of the original
standard, improving the DSSS technology. It uses a technique called
Complementary Code Keying (CCK) which increases the data rate to 11 Mbps.
Because it is similar to the original, it was easy for manufacturers to adapt to
802.11b and due to the higher data rate it became the most widespread technology
for wireless LANs.
In 2003 a new amendment called 802.11g was ratified. Like 802.11b, it operates in
the 2.4 GHz band using DSSS but allows data rates of up to 54 Mbps. This is done
by using Orthogonal Frequency Division Multiplexing (OFDM) similar to that
used by 802.11a. An advantage is that 802.11g is compatible with 802.11b
hardware. Vendors were quick to incorporate the new standard in their products
and currently most devices in the market can operate using 802.11a, b and g. As
mentioned before, the drawback of using 802.11b and 802.11g is that they operate
in the crowded 2.4 GHz band and thus are susceptible to interference.
In 2004, the IEEE announced plans for a new amendment to the 802.11 standard:
802.11n. This amendment will incorporate Multiple-Input Multiple-Output
(MIMO) technology into 802.11 to allow a proposed data rate of 540 Mbps. The
amendment is expected to appear some time after July 2007.
The IEEE 802.11’s architecture is made up of cells, each one known as a Basic
Service Set (BSS). A BSS is made up of clients known as stations in close vicinity
of each other and sharing the same medium. The BSS can be isolated or it can be
connected to a Distribution System (DS) via an Access Point (AP). A distribution
system is like a backbone which connects several BSSs and it can be either a
wired network, a wireless network or simply a switch. Several BSSs that are
connected through a DS form an Extended Service Set (ESS). Furthermore, an
ESS can be connected to another LAN using a Portal which is usually a bridge or
router. This is shown in figure 2.
Figure 2 : IEEE 802.11 Architecture
B) Other Standards:
Some other standards also exist but they are not as popular as the IEEE 802.11.
HiperLAN is a standard that was developed by a team of researchers at the
European Telecommunications Standards Institute (ETSI) but did not gain
popularity. HiperLAN II was also developed by ETSI and is used in some parts of
Europe. There are also standards like OpenAir, HomeRF and SWAP which have
been developed by private corporations.
IV) Benefits and Drawbacks of WLANs:
A) Benefits of WLANs:
1) Long-Term Cost Savings:
The cost of installing and maintaining a wireless LAN is generally lower than it
for wired LAN. This is because that a WLAN eliminates the direct costs of cabling
and the labor associated with installing and repairing it. Also, because WLANs
simplify moves, adds, and changes, they reduce the indirect costs of user
downtime and administrative overhead. Moreover, companies usually reorganize,
and this results in movement of people, new floor plans, office partitions, etc. This
reorganization often requires recabling the network, incurring both labor and
material costs. In some cases, the recabling costs of organizational changes are
substantial, especially with large networks.
2) Reduced Installation Time
Wireless LANs eliminate the time needed with wired LANs for cable installation
which might include digging in the streets and on the walls. So, WLAN makes the
network available for use much sooner. Moreover, it can reach places that cannot
be reached by wires. Thus, many countries lacking a network infrastructure have
turned to wireless networking as a method of providing connectivity among
computers without the expense and time associated with installing physical media.
Wireless LAN systems can provide LAN users with access to network information
anywhere in their organization. Thus, users can physically move while using an
appliance, such as a handheld PC or data collector. In fact, many jobs require
workers to be mobile such as healthcare workers, policemen, and emergency care
specialists. So, this freedom of movement results in significant advantages due to
gains in efficiency.
Wireless LANs can be designed to be extremely simple or quite complex. These
systems grow easily with the need by adding more access points. It is a good
solution if you need to connect several buildings without installing a wired
5) Easy access to the Internet in public places:
Wireless infrastructure providers are enabling wireless connectivity in public areas
around the world. Thus, conference centers and hotels can provide wireless access
to the Internet for their visitors. So, for example, when a traveling worker reaches
his destination, he can do his job easily using this WLAN.
B) Drawbacks of WLANs:
As mentioned before, the frequency at which 802.11b&g operate at is the 2.4 GHz
radio band, which is also used by cordless phones and most microwave ovens.
Thus, when WLAN is used near other devices sharing the same frequency band,
Interference might occur.
Most commonly used wireless LANs have speeds much less than those offered by
wired LANs. So, larger offices with high network traffic and demands for speed,
should take this into consideration.
Because wireless LANs use microwaves similar to those in phones, they have the
same health concerns as these. However, the transmission power of a typical
wireless access point is less than 100 mW while it might be up to 2 watts in
mobile phones. Moreover, WLAN is typically used at much greater distance from
At a wired network, the access to the network can be restricted by physical means.
Usually, the geographical range of a wireless network covers greater than the area
it should cover. As a result, any neighbor may be able to gain an unauthorized
access to internal network resources as well as to the Internet, possibly sending
spam or doing illegal actions using the owner's IP address.
Wireless LANs allow computers to communicate without the need for wires. This
can be achieved through several technologies as we have seen. Standards have also
been developed to allow interoperability between products from different vendors.
Due to their many advantages, wireless LANs are quickly gaining popularity and
are being developed further to provide improved performance.