Wi-Fi

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Wi-Fi

• Wireless Communications
• Sheldon Lou

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What is Wi-Fi?

• The standard for wireless local area networks
  (WLANs). Its 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.
• Its 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.

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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

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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
• 802.11n 600 Mbps 2.4/5 GHz

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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.

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U-NII (Unlicensed National Information
Infrastructure) Bands

• 802.11n can operate at the 5G U-NII bands
• U-NII Low (U-NII-1) 5.15-5.25 GHz.
• U-NII Mid (U-NII-2) 5.25-5.35 GHz.
• U-NII Worldwide 5.47-5.725 GHz.
• U-NII Upper (U-NII-3) 5.725 to 5.825 GHz.

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U-NII bands
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Wireless LAN Networks
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WLAN ArchitectureAd Hoc Mode

• Ad-Hoc mode Peer-to-peer setup where clients
  can connect to each other directly. Generally not
  used for business networks.

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Ad Hoc Structure

• Mobile stations communicate to each other
  directly.
• Its 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.

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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.

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WLAN ArchitectureInfrastructure Mode
To Wired Network
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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.

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Comparison of Two Structures

• Infrastructure Ad hoc
• Expansion X
• Flexibility X
• Control X
• Routing X
• Coverage X
• Reliability X

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Extended Service Area
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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.

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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.

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Antennas, continued

• Antennas come in all shapes and styles
• Omni-directional
• Vertical Whip
• Ceiling mount
• Directional
• Yagi (Pringles can)
• Wall mounted panel
• Parabolic dish

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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.

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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.) dont
  communicate simultaneously. Its 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.
• Its called Carrier Sensing Multiple Access with
  Collision Avoidance (CSMA/CA).

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How does CSMA/CA (Carrier Sensing Multiple Access
with Collision Avoidance) Work?
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RTS/CTS (Request-to-send/clear-to-send)

• Use Request-to-send/clear-to-send (RTS/CTS)
  mechanism 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.

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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.

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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).

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Complementary code keying (CCK)

• Used to increase the data rate to 11 Mbps
• Example 17, p. 119
• Sec. 11.3.4, p. 457

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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.

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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.

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Modulation

• Gaussian frequency shift keying (GFSK) is used.

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Wi-Fi network services

• Distribution and integration
• Association, re-association, and disassociation
• Authentication and deauthentication
• Providing privacy

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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.

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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.

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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.

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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.

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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.

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Authetication/deauthentication

• Physical security is a major component of a wired
  LAN security solution. Wired networks 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.