wi fi .ppt by lovkeshkumar1

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									                Wi-Fi
Wireless Communications
            Sheldon Lou
What is Wi-Fi?
 The standard for wireless local area networks
  (WLANs). It’s like a common language that all
  the devices use to communicate to each other.
  If you have a standard, people can make all
  sorts of devices that can work with each other.
 It’s actually IEEE 802.11, a family of standards.
  The IEEE (Eye-triple-E, Institute of Electrical and Electronics
  Engineers Inc.) is a non-profit, technical professional association of
  more than 360,000 individual members in approximately 175
  countries. The Wireless Ethernet Compatibility Alliance started the
  Wi-Fi--wireless fidelity--certification program to ensure that
  equipment claiming 802.11 compliance was genuinely interoperable.
US Frequency Bands
Band                            Frequency range
UHF ISM                         902-928 MHz
S-Band                          2-4 GHz
S-Band ISM                      2.4-2.5 GHz
C-Band                          4-8 GHz
C-Band satellite downlink       3.7-4.2 GHz
C-Band Radar (weather)          5.25-5.925 GHz
C-Band ISM                      5.725-5.875 GHz
C-Band satellite uplink         5.925-6.425 GHz
X-Band                          8-12 GHz
X-Band Radar (police/weather)   8.5-10.55 GHz
Wi-Fi Standards

Standard Speed    Freq band   Notes
802.11    2 Mbps   2.4 GHz   (1997)
  802.11a 54 Mbps   5 GHz     (1999)
  802.11b 11 Mbps   2.4 GHz
  802.11g 54 Mbps   2.4 GHz
ISM Band
ISM stands for industrial, scientific, and medical.
  ISM bands are set aside for equipment that is
  related to industrial or scientific processes or is
  used by medical equipment. Perhaps the most
  familiar ISM-band device is the microwave oven,
  which operates in the 2.4-GHz ISM band. The
  ISM bands are license-free, provided that
  devices are low-power. You don't need a license
  to set up and operate a wireless network.
Wireless LAN Networks
WLAN Architecture—Ad Hoc Mode
 Ad-Hoc mode: Peer-to-peer setup where
 clients can connect to each other directly.
 Generally not used for business networks.
Ad Hoc Structure

Mobile stations communicate to each
 other directly.
It’s set up for a special purpose and for a
 short period of time. For example, the
 participants of a meeting in a conference
 room may create an ad hoc network at the
 beginning of the meeting and dissolve it
 when the meeting ends.
WLAN Architecture--Mesh

 Mesh: Every client in
  the network also acts
  as an access or relay
  point, creating a “self-
  healing” and (in
  theory) infinitely
  extensible network.
   Not yet in widespread
   use, unlikely to be in
   homes.
WLAN Architecture—Infrastructure Mode




                       To Wired Network
Infrastructure network
 There is an Access Point (AP), which becomes the
  hub of a “star topology.”
 Any communication has to go through AP. If a
  Mobile Station (MS), like a computer, a PDA, or a
  phone, wants to communicate with another MS, it
  needs to send the information to AP first, then AP
  sends it to the destination MS
 Multiple APs can be connected together and handle
  a large number of clients.
 Used by the majority of WLANs in homes and
  businesses.
Comparison of Two Structures

              Infrastructure   Ad hoc
Expansion          X
Flexibility                       X
Control            X
Routing            X
Coverage           X
Reliability                       X
Extended Service Area
Roaming
 In an extended service area, a mobile station (MS) can
  roam from one BSS (Basic Service Set) to another.
 Roughly speaking, the MS keeps checking the beacon
  signal sent by each AP and select the strongest one and
  connect to that AP.
 If the BSSs overlap, the connection will not be
  interrupted when an MS moves from one set to another.
  If not, the service will be interrupted.
 Two BSSs coverage areas can largely overlap to
  increase the capacity for a particular area. If so, the two
  access points will use different channels, as we will
  explain later.
Antennas
All WLAN equipment comes with a built-in
  omni-directional antenna, but some select
  products will let you attach secondary
  antennas that will significantly boost range.
Antennas, continued
 Antennas come in all
  shapes and styles:
  Omni-directional:
      Vertical Whip
      Ceiling mount
  Directional:
      Yagi (“Pringles can”)
      Wall mounted panel
      Parabolic dish
How Can Several Users Communicate
Simultaneously?
As we have discussed, there is a difference
  between a network designed for voice
  conversation and one for data exchange.
  For voice conversations, like telephone and cell phone
   calls, each person has a dedicated channel during the
   entire conversation. (3G and 4G cell phones are
   somewhat different, as we will explain later.)
  For data exchange, many users can share one channel.
   A user sends information when no one else is sending.
  New technologies try to accommodate both voice and
   data transmissions, as we will discuss in this course.
Share one channel in data
communication
 In data communication, data are grouped into packets/frames. Each
  packet/frame contains a number of bits of information.
 Devices (phones, computers, etc.) don’t communicate
  simultaneously. It’s like they are sharing one single cable (the air in
  this case), only one person can use it at one time.
 Before an MS (mobile station) sends its packets, it checks to see if
  someone else is sending information. Only when the medium is free
  can an MS sends packets.
 If some station is sending or receiving signal, the MS that intends to
  send will generate a random waiting time and wait for its turn. If
  several MSs are all waiting for their turns, since their waiting times
  are randomly generated and thus not equal, they will not start
  sending simultaneously. Thus collision (two or more MSs sending
  signals simultaneously) is avoided.
 It’s called Carrier Sensing Multiple Access with Collision Avoidance
  (CSMA/CA).
How does CSMA/CA (Carrier Sensing
Multiple Access with Collision Avoidance)
Work? (p. 189, Example 4.18)
RTS/CTS (Request-to-send/clear-to-send)
 Use Request-to-send/clear-to-send (RTS/CTS)
  mechanism (p. 191-192, Fig. 4.17 & p. 462, Fig. 11.14)
  to avoid collision when two MSs cannot hear each other
  (blocked by a wall …).
 A terminal ready for transmission sends an RTS packet
  identifying the source address, destination address, and
  the length of the data to be sent.
 The destination station responds with CTS packet.
 The source terminal receives the CTS and sends the
  data.
 Other terminals go to the virtual carrier-sensing mode
  (NAV signal on), therefore the source terminal sends its
  packet with no contention.
 After completion of the transmission, the destination
  station sends an ACK, opening contention for other
  users.
Spread spectrum in 802.11

It is a requirement imposed by the
 regulatory authorities for devices in ISM
 band in order to reduce interference.
There is also limitations on transmitted
 power.
We discuss two methods specified in
 802.11, FHSS and DSSS.
DSSS in 802.11
 Used by 802.11b
 Symbol transmission rate = 1Mbps
 Multipath spread of up to 1/1 Mbps = 1 µs does
  not cause ISI. For indoor applications this
  ensures that the system does not suffer from ISI.
 Chip rate = 11 Mcps
 Resolution is on the order of 1/11 Mcps = 90 ns.
 Use Barker code (Example 3.16, p. 116).
Complementary code keying (CCK)

Used to increase the data rate to 11 Mbps
Example 17, p. 119
Sec. 11.3.4, p. 457
Frequency Hopping in 802.11
 The frequency can hop over 78 hopping channels each
  separated by 1 MHz. The first channel, Channel 0,
  starts at 2.402 GHz. Channel 1 is at 2.403 GHz,
  Channel 2, 2.404 GHz, and so on up to Channel 77 at
  2.479 GHz (US, Canada, and Europe standards).
 These frequencies are divided into three patterns of 26
  hops each corresponding channel numbers (0, 3, 6, 9, …,
  75), (1, 4, 7, 10, …, 76), (2, 5, 8, 11, …, 77), see p. 454,
  Fig. 11.5.
 Three APs can coexist without any hop collision, that
  results in a threefold increase in the capacity of the cell.
 Hop rate = 2.5 hops per second.
Frequency bands for DSSS
 FHSS uses 1 MHz bandwidth (narrowband), but the
  center frequency hops over 76 MHz. DSSS uses a chip
  rate of 11 Mcps which occupies around 26 MHz of
  bandwidth (wideband).
 The ISM band at 2.4 GHz is divided into 11 overlapping
  channels spaced by 5 MHz (see Fig. 11.6, P. 455).
 APs located close to each other can choose different
  channels to mitigate interference.
 The coverage areas of two access points (Basic Service
  Sets, BSS) may overlap to increase capacity. For
  example, up to 8 users can use VoIP simultaneously
  through one access point. With two overlapping APs, 16
  users can talk simultaneously. But the two APs have to
  use non-overlapping channels.
Modulation

Gaussian frequency shift keying (GFSK) is
 used (Sec. 3.6.1, p. 97).
Wi-Fi network services

Distribution and integration
Association, re-association, and
 disassociation
Authentication and deauthentication
Providing privacy
Distribution
 This service is used by mobile stations in an
  infrastructure network every time they send data.
  Once a frame has been accepted by an access
  point, it uses the distribution service to deliver
  the frame to its destination. Any communication
  that uses an access point travels through the
  distribution service, including communications
  between two mobile stations associated with the
  same access point.
Integration

Integration is a service provided by the
 distribution system; it allows the
 connection of the distribution system to a
 non-IEEE 802.11 network. The integration
 function is specific to the distribution
 system used and therefore is not specified
 by 802.11, except in terms of the services
 it must offer.
Association

Delivery of frames to mobile stations is
 made possible because mobile stations
 register, or associate, with access points.
 The distribution system can then use the
 registration information to determine which
 access point to use for any mobile station.
Reassociation
 When a mobile station moves between basic
  service areas within a single extended service
  area, it must evaluate signal strength and
  perhaps switch the access point with which it is
  associated. Reassociations are initiated by
  mobile stations when signal conditions indicate
  that a different association would be beneficial;
  they are never initiated by the access point. After
  the reassociation is complete, the distribution
  system updates its location records to reflect the
  reachability of the mobile station through a
  different access point.
Disassociation
 To terminate an existing association, stations
  may use the disassociation service. When
  stations invoke the disassociation service, any
  mobility data stored in the distribution system is
  removed. Once disassociation is complete, it is
  as if the station is no longer attached to the
  network. Disassociation is a polite task to do
  during the station shutdown process. The MAC
  is, however, designed to accommodate stations
  that leave the network without formally
  disassociating.
Authetication/deauthentication
 Physical security is a major component of a wired LAN
  security solution. Wired network’s equipment can be
  locked inside offices. Wireless networks cannot offer the
  same level of physical security, however, and therefore
  must depend on additional authentication routines to
  ensure that users accessing the network are authorized
  to do so. Authentication is a necessary prerequisite to
  association because only authenticated users are
  authorized to use the network. (In practice, though, many
  access points are configured for "open-system" mode
  and will authenticate any station.)
 Deauthentication terminates an authenticated
  relationship. Because authentication is needed before
  network use is authorized, a side effect of
  deauthentication is termination of any current association.

								
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