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					WLAN
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
    user
     Isthe speed (ranging from 1 to 54Mbps, with some
      manufacturers currently offering proprietary 108Mbps
      solutions)
     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
 Areas covered
 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
  300 feet.
WLAN types
 The private home or small business
  WLAN:
 This consists of one or two access points
  covering around a 100- to 200-foot radius.
WLAN types

   The enterprise class WLAN:
   This type has a larger number of individual access points covering a
    wider area.
   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.
WLAN Types
   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.
WLAN Types
   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
    network.
Prospects
   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
    service.
   Large wireless network projects are being put up
    in many major cities.
WLAN
 The WLAN consists of nodes and access
  points.
 Node
    A  computer with an antenna/ network adapter
     A peripheral with an antenna/ network adapter

   Access points
     Transmitters/receivers   between the nodes
WLAN Data transfer
   Data transfer is implemented by
     FHSS
     DSSS
     Infrared
Frequency-hopping spread
spectrum (FHSS)
   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-
    frequency transmission:
   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
    narrowband receiver.
   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
    more efficiently.
Direct-sequence spread
spectrum (DSSS)
   In telecommunications, direct-sequence
    spread spectrum (DSSS) is a modulation
    technique.
   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
    frequency.
Direct-sequence spread
spectrum (DSSS)
   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
    information bit.
   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.
Transmission method
   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
    of "static".
   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
    assumed sequence.
Transmission method
   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.
Transmission Method
   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.
Infrared Technology
   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)
Adhoc Mode
   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
        network.
       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
        closest computer.
Infrastructure Mode
   Infrastructure mode in which mobile units
    communicate through an access point that
    serves as a bridge to a wired network
    infrastructure
   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
    Wireless LAN.
    Wireless bridge
   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
    root bridge.

				
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posted:10/1/2011
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