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Chapter 6 Wireless and Mobile Networks


									          Chapter 6: Wireless and Mobile Networks

    r Cover the following sections only:
         v 6.3: 802.11 wireless LANs
         v 6.5: mobility management: principles

    r two important (but different) new challenges
         v communication   over wireless link
         v handling mobile user who changes point of
           attachment to network

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                    Elements of a wireless network
wireless link                                   Infrastructure mode:
r typically connecting                          • basestations connect mobiles to
mobile(s) to base station                       wired networks
r can also be used as                           • when moving mobiles change
backbone link
                                                basestations to keep Internet
r multiaccess protocol:
coordinates link access                         access (handoff)

                                                    Wired network    Ad hoc mode:
                                                    infrastructure   • no basestations
 Wireless host: may be
 stationary (non-mobile) or                                          • each node helps
 mobile                                                              forward packets
                                                                     to other node
    Base station:
    • typically connected to wired network
    • relay - responsible for sending packets
    between wired network and wireless
    host(s) in its “area”
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                     Wireless Link Characteristics
communication across a point to point wireless link is much more
  “difficult” than wired link ….
r decreased signal strength: radio signal attenuates as it propagates
  through matter (path loss)
r interference from other sources: standardized wireless network
  frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone);
  devices (motors) interfere as well
r multipath propagation: radio signal reflects off objects ground,
  arriving at destination at slightly different times
r Multiple wireless senders and receivers create additional problems
  (beyond multiple access): Hidden terminal problem
         v B, A hear each other
         v B, C hear each other
         v A, C can not hear each other                         C
         means A, C unaware of their interference at B
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             IEEE 802.11 Wireless LAN
                                   r 802.11a
    r 802.11b
                                      v 5-6 GHz range
       v 2.4-5 GHz unlicensed
                                      v up to 54 Mbps
         radio spectrum
       v Data rate up to 11 Mbps
                                   r 802.11g
                                      v 2.4-2.5 GHz range
       v direct sequence spread
                                      v up to 54 Mbps
         spectrum (DSSS) in
         physical layer            r All use CSMA/CA for
       v widely deployed, using      multiple access
         base stations             r All have infrastructure
                                     and ad-hoc network

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                      802.11 LAN architecture
                                           r   802.11b: 2.4GHz-2.485GHz
                                               spectrum divided into 11 channels
                                               at different frequencies; 3 non-
                                                   v   AP admin chooses frequency for AP
                                                   v   neighboring APs may choose same
                                 hub, switch
                 AP              or router r   AP sends beacon frame periodically
                                                   v   SSID, MAC address
         BSS 1
                                           r   host: must associate with an AP
                                 AP                v   scan channels, listening for beacon
                                                       frames containing AP’s name (SSID)
                                                       and MAC address
                            BSS 2                  v   selects AP to associate with; initiates
  BSS: Basic Service Set                               association protocol
  SSID: Service Set Identifier                     v   typically run DHCP to get IP address
6/2/05                                         5       in AP’s subnet                 CS118/Spring05
                 IEEE 802.11: multiple access
         r   Like Ethernet, uses CSMA:
             v   random access
             v   carrier sense: don’t collide with ongoing transmission
         r   Unlike Ethernet:
             v   no collision detection – transmit all frames to completion
             v   acknowledgment – because without collision detection, you don’t
                 know if your transmission collided or not
         r   Why no collision detection?
             v   difficult to receive (sense collisions) when transmitting due to weak
                 received signals (fading)
             v   can’t sense all collisions in any case: hidden terminal, fading
         r   Goal: avoid collisions: CSMA/C(ollision)A(voidance)

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         IEEE 802.11 MAC Protocol: CSMA/CA (1)
   802.11 sender
   1 if sense channel idle for DIFS then
        - transmit entire frame (no CD)                sender             receiver
   2 if sense channel busy then
        - start random backoff time
        - timer counts down while channel idle
        - transmit when timer expires
        - if no ACK, increase random backoff
           interval, repeat 2                                                   SIFS

   802.11 receiver                                                ACK
   if frame received OK
      - return ACK after SIFS (ACK needed due
       to hidden terminal problem)
                                                 DIFS: distributed inter-frame spacing
                                                 SIFS: short inter-frame spacing
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         IEEE 802.11 MAC Protocol: CSMA/CA (2)
    Dealing with hidden terminal:
    r idea: allow sender to “reserve” channel: avoid collisions of long
      data frames
    r sender first transmits small request-to-send (RTS) packets to AP
      using CSMA
         v   RTSs may still collide with each other (but they’re short)
    r AP broadcasts clear-to-send CTS in response to RTS
    r CTS heard by all nodes
         v   sender transmits data frame
         v   other stations defer transmissions

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         Collision Avoidance: RTS-CTS exchange
          A                                      B

              RTS(A)                             RTS(B)

                         reservation collision

              CTS(A)                  CTS(A)

              DATA (A)

              ACK(A)                  ACK(A)

6/2/05                         9                             CS118/Spring05
                      802.11 frame: addressing

         2        2       6     6        6          2    6       0 - 2312     4
    frame            address address address     seq address
            duration                                              payload    CRC
    control             1       2       3       control 4

                                                             Address 3: used only
                                                             in ad hoc mode
    Address 1: MAC address
    of wireless host or AP             Address 3: MAC address
    to receive this frame              of router interface to
                                       which AP is attached
             Address 2: MAC address
             of wireless host or AP
             transmitting this frame

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                     802.11 frame: addressing

                     H1                         R1 router

                                                 R1 MAC addr H1 MAC addr
                                                 dest. address   source address

                                                                        802.3 frame

         AP MAC addr H1 MAC addr R1 MAC addr
         address 1        address 2        address 3

                                             802.11 frame

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                              802.11 frame: more
                                                                       frame seq #
                            duration of reserved
                                                                       (for reliable ARQ)
                            transmission time (RTS/CTS)

             2      2          6         6        6         2          6        0 - 2312        4
         frame            address address address           seq address
                 duration                                                       payload     CRC
         control             1       2       3             control 4

             2          2          4         1        1     1      1        1        1      1        1
         Protocol                            To   From More                Power More
                    Type       Subtype                           Retry                     WEP      Rsvd
         version                             AP    AP   frag                mgt  data

                            frame type
                            (RTS, CTS, ACK, data)

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          802.11: mobility within same subnet

    r H1 detects weakening
      signal from AP1, scan and                           router
      find AP2 to attach to
                                                             hub or
    r H1 remains in same IP                                  switch

      subnet: IP address can
                                           BBS 1
      remain same
                                                   AP 1
    r switch: which AP is
                                                                      AP 2
      associated with H1?
                                                    H1                  BBS 2
                        switch will
         v self-learning:
          see frame from H1 and
          “remember” which
          interface can be used to
          reach H1                    13                                     CS118/Spring05
                           Mobility: Vocabulary
                               home agent: entity that
                               will perform mobility
home network:                  functions on behalf of
permanent “home” of            mobile, when mobile is
mobile (e.g., 128.119.40/24)   remote

                                              wide area
    Permanent address:
    address in home network,
    can always be used to
    reach mobile
    e.g.,                  correspondent

                  correspondent: wants to
                  communicate with mobile
6/2/05                                         14         CS118/Spring05
                 Mobility: more vocabulary
                                                                visited network: network in
                        Permanent address: remains              which mobile currently resides
                                                                (e.g., 79.129.13/24)
                        constant (e.g.,

                              Care-of-address: address in
                              visited network. (e.g., 79,129.13.2)

                                                wide area

                                                                       foreign agent: entity in
                                                                       visited network that
                                                                       performs mobility
         correspondent: wants to                                       functions on behalf of
         communicate with                                              mobile.
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                      Mobility: approaches
    r Let routing handle it: routers advertise permanent
         address of mobile-nodes-in-residence via usual
         routing table exchange.
         v routing         to millions of
                  tables indicate where each mobile located
         v no changes to end-systems

    r Let end-systems handle it:
         v indirect routing: correspondent sends packets to to
           home agent, which forwards to mobile
         v direct routing: correspondent gets foreign address of
           mobile, sends directly to mobile

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                   Mobility: registration
                                                visited network
             home network

                              wide area

                                                      mobile contacts
                  foreign agent contacts home         foreign agent on
                  agent home: “this mobile is         entering visited
                  resident in my network”             network

         End result:
         r Foreign agent knows about mobile
         r Home agent knows location of mobile
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                     Mobility via Indirect Routing
                                                   foreign agent receives
                                                   packets, forwards to
                   home agent intercepts           mobile
                   packets, forwards to foreign                        visited
                   agent                                               network

         home                        wide area                        3
         network                     network
correspondent addresses                                     4
packets using home
                                                                  mobile replies
address of mobile                                                 directly to
                                     Q: Which address will
                                     mobile use as source

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Mobility via Indirect Routing: further movement


         home                   wide area
         network                network



                   Q: Will the correspondence be aware of mobile's move?
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                Indirect Routing: comments
    r Mobile uses two addresses:
         v permanent address: used by correspondent (hence
          mobile location is transparent to correspondent)
       v care-of-address: used by home agent to forward
          datagrams to mobile
    r foreign agent functions may be done by mobile itself
    r triangle routing: correspondent-home-network-mobile
       v inefficient when
       Correspondent & mobile
       are in same network

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           Indirect Routing: moving between networks
         r suppose mobile user moves to another
           v registerswith new foreign agent
           v new foreign agent registers with home agent
           v home agent update care-of-address for mobile
           v packets continue to be forwarded to mobile (but
             with new care-of-address)
         r mobility, changing foreign networks
           transparent: on going connections can be

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                     Mobility via Direct Routing
                                                      foreign agent
                                                      receives packets,
                          correspondent forwards      forwards to mobile
                          to foreign agent                                 visited
         network                                                     4
                                          wide area
            correspondent             1                       4
            requests, receives
                                                                  mobile replies
            foreign address of
                                                                  directly to

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             Mobility via Direct Routing: comments

         r overcome triangle routing problem
         r non-transparent to correspondent:
           correspondent must get care-of-address from
           home agent
           v what   if mobile moves to another visited network?

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             Accommodating mobility with direct routing
         r anchor foreign agent: FA in first visited network
         r data always routed first to anchor FA
         r when mobile moves: new FA arranges to have data
           forwarded from old FA (chaining)

                                                                          foreign net visited
                                                                          at session start
                       wide area             agent
                                   1                    4
                             correspondent                                      foreign
                                                            new foreign
                             agent                                              network
             correspondent                                  agent

6/2/05                                             24                                           CS118/Spring05

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