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10-01-2010, 11:35 PM

seminar and presentation report and ppt presentation on WLAN
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23-09-2010, 10:31 AM

please go through the following thread to get more information on 'WLAN'

seminar and presentationproject and implimentationsattachment.php?aid=1427
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06-11-2012, 02:07 PM

Wireless LAN – IEEE 802.11

.pdf   Wireless LAN.pdf (Size: 111.53 KB / Downloads: 13)


Local Area Networks have evolved over the past 20 or so years to become a
crucial ingredient in the success of businesses, large and small. From the smallest
office to the largest multinational corporation shared access to information resources
is an indispensable part of modern business processes. Local Area Networks (LAN)
have been traditionally connected with wired infrastructure and a multi-billion dollar
industry has grown up to supply customers needs for wired networking products.
Companies like Cisco, 3Com, Bay Networks and Cabletron have developed a vast
range of products to implement and manage Local Area Networks of all sizes and to
interconnect them throughout the enterprise. Over the past ten or so years an
alternative to wired LAN structures has evolved in the form of the Wireless LAN
(WLAN). In a manner analogous to the growth of the wired LAN, initial application
and market success of the WLAN was in specialized, vertical markets. Thus
applications that highly valued the mobile, untethered connectivity were the early
targets of the WLAN industry. These first generation products, which operated in the
unlicensed 902-928 MHz ISM (Industrial Scientific and Medical) band had limited
range and throughput, but proved useful in many factory floor and warehouse
applications. These systems took advantage of emerging semiconductor processes
developed for cellular telephone applications to enable inexpensive WLAN products.
Unfortunately these same inexpensive components also enabled a wide variety of
other 900 MHz products like cordless telephones. Consequently, the band quickly
became crowded with a variety of unlicensed products. Building upon technology
originally developed for military applications, spread spectrum techniques were
employed to minimize sensitivity to interference. This approach allowed the design
and manufacture of 900 MHz WLAN products having nominal data rates of 500
kilobits per second. Ultimately, the growing popularity of the band for a large range
of unlicensed products, aggravated by the limited bandwidth caused users of WLAN
to look to a different frequency band for growth in performance.

AD-HOC Network

In the ad-hoc network, computers are brought together to form a network "on
the fly." As shown in Figure 1, there is no structure to the network; there are no fixed
points; and usually every node is able to communicate with every other node. A
smallest Wireless LAN may consist of computers each equipped with wireless n/w
interface card. This mode of operation is possible when stations are able to
communicate directly and the network does not have an AP. This type of network is
often formed when a station is not able to locate an AP and starts communicating
with the peer stations directly. It can share printer, but cannot share resource of wired
LAN unless one of the computer act as bridge to the wired LAN using special s/w


To provide a basis for the further discussions of the technology and standards
issues related to WLAN, a brief review of network structures is in order. The first
concept to keep in mind is that networks represent an interactive collection of oftenpowerful
computers. The complexities of the interactions among these members of
the network are many. To provide a common framework for describing and
understanding, the International Standards Organization approved a standard called
ISO-7498 that defines a seven-layered model to describe the interconnection
processes between various members of a network. This model, which is officially
known at the Open System Interconnect model, is the basis for most discussions of
network function. The seven layers are shown in Figure 1. WLAN products, in
common with other networking products, typically work at the two bottoms most
layers of the 7-layered model. The Physical Layer (usually referred to as simply
PHY) is the actual physical method by which data is passed from one member of the
network to another. For a WLAN its description includes such things as frequency of
operation, data rate, modulation method, etc. In addition to the PHY, the lower half
of the Data Link layer, usually known as the Media Access Control (or MAC) layer
is defined by the WLAN product. The MAC layer is conventionally defined as the
protocol by which data is transferred between network members. In Figure 1, the
shaded areas represent the PHY and MAC layers.


The PHY layer, which actually handles the transmission of data between
nodes, can use either direct sequence spread spectrum, frequency hopping spread
spectrum, or infrared (IR) pulse position modulation. IEEE 802.11 makes provisions
for data rates of either 1 Mbps or 2 Mbps, and calls for operation in the 2.4 - 2.4835
GHz frequency band (in the case of spread-spectrum transmission), which is an
unlicensed band for industrial, scientific, and medical (ISM) applications, and 300 -
428,000 GHz for IR transmission.

Infrared (IR)

Infrared is generally considered to be more secure to eavesdropping, because
IR transmissions require absolute line-of-sight links (no transmission is possible
outside any simply connected space or around corners), as opposed to radio
frequency transmissions, which can penetrate walls and be intercepted by third
parties unknowingly. Infrared transmissions can be adversely Provide data rate
between 1Mbs and 2Mbps at a wavelength between 850nm and 950 nm. It is
immune to electrical interface. However, infrared transmissions can be adversely
affected by sunlight [5], and the spread-spectrum protocol of 802.11 does provide
some rudimentary security for typical data transfers.

Frequency Hopping Spread Spectrum (FHSS)

In a Frequency Hopping Spread Spectrum (FHSS) system, the data is
modulated on to the carrier in a manner identical to that employed for standard
narrow band communications. Most frequency hopping systems employ Gaussian
Frequency Shift Keyed modulation, either two or four level. The carrier frequency is
then changed (hopped) to a new frequency in accordance with a pre-determined
hopping sequence. If the receiver frequency is then hopped in synchronism with the
transmitter, data is transferred in the same manner as if the transmitter and receiver
were each tuned to a single fixed frequency. If different transmitter-receiver pairs
hop throughout the same band of frequencies, but using different hopping sequences,
then multiple users can share the same frequency band on a non-interfering basis.
The operation of a pair of frequency hopping transmitter-receiver pairs is shown
schematically in Figure 2. The obvious question arises: why not just assign a fixed
frequency to each user and share the bandwidth in that manner? The answer lies in
how a FHSS responds to interferors.

Direct Sequence Spread Spectrum (DSSS)

The second type of spread spectrum is known as Direct Sequence Spread
Spectrum (DSSS). In this system, the data stream is multiplied by a pseudo-random
spreading code to artificially increase the bandwidth over which the data is
transmitted. This is shown in Figure 3. The resulting data stream is then modulated
onto the carrier using either Differential Binary Phase Shift Keying or Differential
Quadrature Phase Shift Keying. By spreading the data bandwidth over a much wider
frequency band, the power spectral density of the signal is reduced by the ratio of the
data bandwidth to the total spread bandwidth. In a DSSS receiver the incoming
spread spectrum data is fed to a correlator where it is correlated with a copy of the
pseudo-random spreading code used at the transmitter. Since noise and interference
are by definition de-correlated from the desired signal, the desired signal is then
extracted from a noisy channel. While the block diagram of a DSSS WLAN product
is somewhat simpler than a FHSS product, there are some very subtle difficulties that
come into play in the presence of strong interfering

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