Wireless LAN 2 - PowerPoint by A6LxG4PK

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									            Wireless LANS
                Part 2
                                    Justin Champion
                               Room C208 - Tel: 3273
www.staffs.ac.uk/personal/engineering_and_technology/jjc1
              Wireless LANS
 Contents
     Speed
     HIPERLAN
     Wi-Fi
     Issues with radio communications
                Wireless LANS
 Speed
   Measure in bits per second (bps)
   Kilo (Kbps)
      1000 bits per second
   Mega
      1000 Kbps
   Giga (Gbps)
      1000 Mbps
   Terra (Tbps)
      1000 Gbps
      Limited optical networks work at this speed and certainly
        nothing which is wireless
   Remember that in data transfer a Kilo = 1000 not the
    1024 used for data storage
Wireless LANS
            Wireless LANS
 HIPERLAN
  HIgh PERformance Local Area Network
     HIPERLAN 1
        20 Mbps
     HIPERLAN 2
        54 Mbps
  Developed by ETSI
     European Telecommunication Standards Institute
                Wireless LANS
 Common terms
   Throughput
      Is how much data is passing through a network in a
       given time
   Bandwidth
      Is the amount of data that could be transferred in a
       given time
                 Wireless LANS
 HIPERLAN 2
   Features
      QOS
      Power Saving built into the technology
      Operates in the 5 GHz range
      Strong Security using per session keys or long term key
       usage
         DES or Triple-DES used
      Increased Throughput over other wireless technologies
      Allows convergence with other backbone technologies
         ATM – This was originally the primary use of this technology
         Ethernet
         3G
      Wireless LANS



Why is HIPERLAN 2 good?
                Wireless LANS
 Central Control
   This is referred to as the Access Point (AP)
   Responsible for
      Allows packets to be sent from a backbone to the
       wireless device
      Informs devices which frequency to operate on
         This allows for the optimum frequency to be selected, based
          on what else is happening with the interface
      Each channel will be divided by 20 MHz giving 19
      channels
         Each of the channels will be divided into 52 sub carriers
         48 of these are usable for data the other 4 are for
          synchronization
               Wireless LANS
 Communications
   Multicast and Broadcast are supported
   Communications are connection orientated
      This gives a short setup time for communications to
       take place
   Movement within the network is allowed
      A device which recognises a stronger signal form
       another AP will connect to that AP
      All connections will be moved from the 1st AP to the
       2nd.
         During this process packets may be lost and the application
          must request them again.
          Wireless LANS
 HIPERLAN 2 Layers
                        Higher OSI Layers



                       Convergence Layer



                      Data Link Control Layer



                             Physical
                       Wireless LANS
 Physical
      Different encoding methods used for different rates
         If the signal to noise ratio becomes higher a lower transmission
           speed will be selected
 Mode                 Modulation         Code Rate          Bit Rate Mbps
 1                    BPSK               ¾                  6
 2                    BPSK               ¾                  9
 3                    QPSK               ½                  12
 4                    QPSK               ¾                  18
 5                    16QAM              9/16               27
 6                    16QAM              ¾                  36
 7                    64QAM              ¾                  54
                      Wireless LANS
 Physical
   Data transferred in several different sub carriers
   Referred to as
      Orthogonal Frequency Division Multiplex (OFDM)
           The original bit pattern is re assembled at the destination
       This technique allows
           Better error handling
           Reduction of multi-path propagation
   Multi-Path Propagation
      When a signal is sent it may bounce off several items before getting
        to the receiver
       Indicating that the same signal can be received more than once or at
        a time when it would cause a corruption of a packet
       The further the distance travelled for the signal the higher the likely
        hood of this happening.
                 Wireless LANS
 Physical
     Encoding the bits for transmission
     Binary Phase Shift Keying (BPSK)
        Any change in the carrier wave indicates a 1 else 0
     The less data encoded the less chance there is of
      error


Carrier



BPSK
                       Wireless LANS
 Physical
    QPSK
       Quadrature Phase Shirt Keying
            Allows for streams of data to be encoded into the carrier wave
            Shifts the carrier by either 90 or 180 degrees
    16 QAM
       Quadtrative Amplitude Modulation
       By using Phase shifting and Amplitude changes encodes 4 bits at once
    64 QAM
       Same as 16 QAM but encoding 6 bits




                          bb.watch.impress.co.jp/column/infra/2001/09/26/16qam.gif
                  Wireless LANS
 Data Link Control Layer
   Responsible for
      MAC Access control
          Responsible for the sharing of the radio link
          Minimising the amount of time the link is required
          Ultimately the responsibility of this is controlled by the access point
      Logical Link Controller
          Carried out error detection
          Retransmission of lost packets
          Forward Error recovery FEC, with a number of redundant bits sent
           it is possible to recover small amounts of corrupted data
                HIPERLAN uses Reed Solomon codes to carry this out
          Essential on a unreliable interface like radio
             Wireless LANS
 Convergence Layer
   Adapts data sent from a lower/higher layer to
    the correct format.
   This layer is the reason why HIPERLAN, is
    able to communicate with other technologies
      As in transfer data from
        ATM – Cell based convergence
        Ethernet – Packet based convergence
        3G - Packet based convergence
            Wireless LANS
 Transmission Medium
   Uses Time Division duplex
 Transmission Packet Format
   Each packet which is transferred is a defined
    size of 2 ms in length
                   Wireless LANS
 Transmission Packet Format
                          2 ms

      Mac Frame         Mac Frame     Mac Frame       Mac Frame




BCH     FCH       ACH      DL Phase        UL Phase       RCH’s
                    Wireless LANS
 Transmission Packet Format
    Broadcast Channel (BCH)
       Transmission Power
       Starting point of the FCH and length
       Starting point and Length RCH
       Identifier for the AP and network
    Frame Control Channel (FCH)
       Description of how transmit and receive resources are allocated in this
         frame
    Access Feedback Channel (ACH)
       Give information on previous attempts at accessing the RCH
    Downlink (DL) Phase & Uplink (UL) Phase
       Allows the transmission/receipt of packets of 54 bytes in size
    Random Access Channel (RCH)
       Used to request transmission resources – uplink and downlink
       Transmission at this time takes place based on the fact that nothing
         else is transmitting
                   Wireless LANS
 Summary of HIPERLAN
   Quality of service is given by controlling the amount of
    transmissions
       Each device must request permission to transmit or receive
          This is given on the basis of the quality of service required
   Issues
       Expensive at the moment due to the radio technology
       Not widely available
   Future
       HiperMan
          High speed access on a metropolitan wide basis
      HiperPan
          High speed access on a personal basis
                Wireless LANS
 Wireless Fidelity (Wi-Fi)
    Is a generic name for a set of IEEE standards namely
        IEEE 802.11
        IEEE 802.11A
        IEEE 802.11B
        IEEE 802.11G
    Any product with this logo is able to interoperate with
     each other
    The products are certified by the Wi-Fi Alliance
               Wireless LANS
 Standards
   IEEE 802.11A
      54 Mbps
      5 GHz range
      Modulation OFDM
   IEEE 802.11B
      11 Mbps
      2.4 GHz Range
      Modulation of DSSS
   IEEE 802.11G
      20 Mbps
      2.4 GHz Range
      Modulation of OFDM
                  Wireless LANS
 Frequency 2.4 GHz
   More commonly known as the ISM band
      Industrial Scientific Medical (ISM)
      Intended as worldwide free usage radio band
           No license required
   Widely used for
      Wireless LAN technology
   Actual usage will differ between countries
   Usage must be none commercial
   Same frequency as
      Microwave Ovens
      Cordless Phones
      Other wireless Devices
                   Wireless LANS
 Wireless Fidelity (Wi-Fi)
    Operates
       In the same manner as the Ethernet networks
       A device waits for silence on the radio frequency
            Carrier Sense Multiple Access (CSMA)
    Operates in
       AD-Hoc mode or
       Uses a Access point (AP) to allow connection to wired
         infrastructure
              Wireless LANS
 Wireless Fidelity (Wi-Fi)

              CSMA             Access Point
                       RTS

                       CTS
                                   RTS = Request to Send
                        Data       CTS = Clear to Send
                                    Data =
                       ACK         ACK = Acknowledgment
                   Wireless LANS
 Wi-Fi 802.11b transmissions
   Carried out using Direct Sequence Spread Spectrum
    (DSSS)
      The original signal is combined with a Pseudo random Number
       (Code Word)
          The code word is referred to as the Baker Code
          This allows improved reliability in data transmission rather than just
           sending data.
              A wider bandwidth is required for the transmission
          Each part of the packet is then sent on a different frequency
              Total required bandwidth is 22 MHz
          The receiver puts this information back together again
      This method is used due to the ability to send large amounts of
       data at once
                  Wireless LANS
 Wi-FI Data Packet
   Only the Data part is sent at full speed
      The rest is sent at 1 Mbps
                      Wireless LANS
 Wi-Fi
   Wired Equivalent Privacy (WEP)
      Uses either 40 or 128 bit RS4 symmetric encryption
           The standard does not define how to distribute the keys!
           Discussion of cracking the encryption algorithm real-time!
               http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html, 2003)
           When used 40 Bit encryption reduces throughput by 20%
      Trying to avoid the Pringle situation, which was embarrassing for the
       technology and users of it
           news.bbc.co.uk/1/hi/sci/tech/1860241.stm, 2002)
           War Driving, users accessing wireless networks without permission
           Wi-Fi is targeted with wall chalking indicating locations to connect
               The technology is targeted as it is widely used and available
      Radio Signal Propagation
           Radio signals can not be restricted to a geographic area without
            expensive specially built buildings
          Wireless LANS
 Wi-Fi
                 Wireless LANS
 Wi-Fi - Future
   IEEE 802.16 (WiMax)
      802.16a is intended as a wireless metropolitan technology
      First devices should be appearing in Late 2004
      Allows 120 Mbps
      Uses the 10 to 66 GHz frequency
          This will require no interference, so transmitters and receivers will
           need to be placed on the roof
      Transmission up to 30 Miles
          Currently a limit on receivers which is the hundreds
      Standards Group
          grouper.ieee.org/groups/802/16/index.html
            Wireless LANS
 Summary
  HiperLan2
  ISM
  Wi-fi

								
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