A wireless LAN, or WLAN
WLAN, is a local area network that does not
have wired Ethernet connections.
A WLAN can be either an extension to a current
wired network or an alternative to it.
Use of a WLAN adds flexibility to networking.
A WLAN allows users to move around while
keeping their computer connected, without
having to depend on Ethernet cables.
A wireless LAN, or WLAN
WLANs try to provide all the features of wired
LANs, but without the wires.
The only differences that is noticeable to the end
Isthe speed (ranging from 1 to 54Mbps, with some
manufacturers currently offering proprietary 108Mbps
And security (the wireless access point is shared
among everybody nearby, so security issues exist
with WLANs that don't exist for wired networks).
A wireless LAN, or WLAN
A small office
A large campus, with neighborhood and
city-wide ranges planned for the future.
Commonly, WLANs employ access points
that provide access within a radius of 65 to
The private home or small business
This consists of one or two access points
covering around a 100- to 200-foot radius.
The enterprise class WLAN:
This type has a larger number of individual access points covering a
The access points themselves have features not needed for a home
or small office, like better security, authentication, remote
management, and tools to help integrate with existing networks.
Each access point has a larger coverage area than home or small
office products, and all are designed to work together to cover a
much larger area.
The Wireless Metropolitan Area Network (WMAN):
A WMAN covers an area from multiple city blocks up to a city's boundaries.
The most common type of WMAN is a collection of individual enterprise
class wireless networks that collectively allow users to access all of them.
In most places, WMANs usually consist of wireless networks belonging to
several businesses or Internet service providers.
The WMAN is also the point where you start seeing different technologies
and standards. Again, the most common WMAN is basically a group of
individual access points and WLANs.
While you will see the term Metropolitan Area Network (MAN), don't confuse
it with a Wireless Metropolitan Area Network (WMAN). Aside from the fact
that MANs tend to be wired networks, they usually exist to provide
connectivity to local ISPs, or to business and enterprise class LANs.
In contrast, a WMAN exists to provide connectivity directly to an end user.
In other words, a MAN normally acts as a backbone, while a WMAN acts as
the "last mile" connection directly to a user's computer.
Wireless WAN (Wide Area Network): Although a WAN by definition is the exact
opposite of a LAN, Wireless WANs (WWANs) deserve brief mention here. Most
WANs exist to connect LANs that are not in the same geographical area, and until
recently this was also the case for WWANs.
But recently, cellular phone companies like AT&T have begun offering WWAN
technology that the end user can access directly.
Those WWANs use cellular data technology to cover extremely wide areas. While
they are considerably slower than wireless LAN speeds (most advertise between 50
to 144Kbps; compare this to dial-up speeds, which are around 56Kbps), they're still
better than the lowest end of DSL speeds (128Kbps), plus they allow far greater
mobility than standard 802.11a, b, or g wireless. Since they rely on coverage by the
major cellular network providers, coverage areas for wireless Internet access tend to
be more or less the same as they are for cell phones. There are many different
standards competing at this level (GSM, CDMA, GPRS, 3G, to name a few). Most of
them are mobile data standards that previously were used only on cell phones.
How does it work?
WLAN is a wireless local area network, which is the
linking of two or more computers without using wires.
WLAN utilizes spread-spectrum or OFDM modulation
technology based on radio waves to enable
communication between devices in a limited area, also
known as the basic service set.
This gives users the mobility to move around within a
broad coverage area and still be connected to the
For the home user, wireless has become
popular due to ease of installation, and location
freedom with the gaining popularity of laptops.
Public businesses such as coffee shops or malls
have begun to offer wireless access to their
customers; some are even provided as a free
Large wireless network projects are being put up
in many major cities.
The WLAN consists of nodes and access
A computer with an antenna/ network adapter
A peripheral with an antenna/ network adapter
Transmitters/receivers between the nodes
WLAN Data transfer
Data transfer is implemented by
FHSS is a method of transmitting radio signals by rapidly switching a carrier
among many frequency channels, using a pseudorandom sequence known
to both transmitter and receiver.
A spread-spectrum transmission offers three main advantages over a fixed-
Spread-spectrum signals are highly resistant to narrowband interference.
The process of re-collecting a spread signal spreads out the interfering
signal, causing it to recede into the background.
Spread-spectrum signals are difficult to intercept. A frequency-hop spread-
spectrum signal simply sounds like an increase in the background noise to a
Spread-spectrum transmissions can share a frequency band with many
types of conventional transmissions with minimal interference. The spread-
spectrum signals add minimal noise to the narrow-frequency
communications, and vice versa. As a result, bandwidth can be utilized
In telecommunications, direct-sequence
spread spectrum (DSSS) is a modulation
As with other spread spectrum technologies, the
transmitted signal takes up more bandwidth than
the information signal that is being modulated.
The name 'spread spectrum' comes from the
fact that the carrier signals occur over the full
bandwidth (spectrum) of a device's transmitting
It phase-modulates a sine wave pseudorandomly with a
continuous string of pseudonoise (PN) code symbols called
"chips", each of which has a much shorter duration than an
Each information bit is modulated by a sequence of much faster
chips. Therefore, the chip rate is much higher than the information
signal bit rate.
It uses a signal structure in which the sequence of chips produced
by the transmitter is known a priori by the receiver. The receiver
can then use the same PN sequence to counteract the effect of
the PN sequence on the received signal in order to reconstruct
the information signal.
Direct-sequence spread-spectrum transmissions multiply the data
being transmitted by a "noise" signal. This noise signal is a
pseudorandom sequence of 1 and −1 values, at a frequency much
higher than that of the original signal, thereby spreading the energy
of the original signal into a much wider band.
The resulting signal resembles white noise, like an audio recording
However, this noise-like signal can be used to exactly reconstruct
the original data at the receiving end, by multiplying it by the same
pseudorandom sequence (because 1 × 1 = 1, and −1 × −1 = 1). This
process, known as "de-spreading", mathematically constitutes a
correlation of the transmitted PN sequence with the receiver's
For de-spreading to work correctly, the transmit and receive
sequences must be synchronized.
This requires the receiver to synchronize its sequence with the
transmitter's sequence via some sort of timing search process.
However, this apparent drawback can be a significant benefit: if the
sequences of multiple transmitters are synchronized with each
other, the relative synchronizations the receiver must make between
them can be used to determine relative timing, which, in turn, can be
used to calculate the receiver's position if the transmitters' positions
are known. This is the basis for many satellite navigation systems.
The resulting effect of enhancing signal to noise ratio on the channel is called
processing gain. This effect can be made larger by employing a longer PN sequence
and more chips per bit, but physical devices used to generate the PN sequence
impose practical limits on attainable processing gain.
If an undesired transmitter transmits on the same channel but with a different PN
sequence (or no sequence at all), the de-spreading process results in no processing
gain for that signal. This effect is the basis for the code division multiple access
(CDMA) property of DSSS, which allows multiple transmitters to share the same
channel within the limits of the cross-correlation properties of their PN sequences.
As this description suggests, a plot of the transmitted waveform has a roughly bell-
shaped envelope centered on the carrier frequency, just like a normal AM
transmission, except that the added noise causes the distribution to be much wider
than that of an AM transmission.
In contrast, frequency-hopping spread spectrum pseudo-randomly re-tunes the
carrier, instead of adding pseudo-random noise to the data, which results in a uniform
frequency distribution whose width is determined by the output range of the pseudo-
random number generator.
Uses high frequencies for transmission just
below visible light.
Cannot penetrate opaque objects.
Either directed or diffused technology is used.
In directed it is the line of sight where the emitter
directs the IR to the detector.
In diffused the Emitter transmits the IR signal it
bounces off the ceiling and the detectors are
capable of detecting the reflected signal. WLANs
typically use this IR.
WLAN modes of operation
It has two basic modes of operation:
Adhoc (distributed control)
Infrastructure LAN (Centralized control)
Ad hoc mode enables peer-to-peer transmission
between mobile units.
A peer-to-peer (P2P) allows wireless devices to directly
communicate with each other. Wireless devices within range of
each other can discover and communicate directly without
involving central access points. This method is typically used by
two computers so that they can connect to each other to form a
If a signal strength meter is used in this situation, it may not read
the strength accurately and can be misleading, because it
registers the strength of the strongest signal, which may be the
Infrastructure mode in which mobile units
communicate through an access point that
serves as a bridge to a wired network
A bridge can be used to connect networks,
typically of different types. A wireless Ethernet
bridge allows the connection of devices on a
wired Ethernet network to a wireless network.
The bridge acts as the connection point to the
This is a hardware component used to connect two or more network
segments which are physically separated.
Consumers have been presented with wireless bridges operating in
different frequencies and licensing models.
Wireless bridges usually work only in pairs or more, and can be
used in two types of implementations. They are the point-to-point
link, or the point to multipoint link.
In point to point link, there are a pair of bridges which are used to
connect two network segments, typically in two separate buildings.
In a point to multipoint scenario, one bridge is installed as the "root
bridge", and multiple non-root bridges connect to this root bridge.
With this arrangement, if one non-root network segment wants to
pass data to the other non-root segment, it passes it through the