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					Chapter 6
Wireless and Mobile
Networks


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represent a lot of work on our part. In return for use, we only ask the               A Top Down Approach
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that you mention their source (after all, we’d like people to use our book!)          Jim Kurose, Keith Ross
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note our copyright of this material.                                                  2009.
Thanks and enjoy! JFK/KWR

All material copyright 1996-2009
J.F Kurose and K.W. Ross, All Rights Reserved
                                                                             6: Wireless and Mobile Networks   6-1
Chapter 6: Wireless and Mobile Networks
 Background:
 r # wireless (mobile) phone subscribers now
   exceeds # wired phone subscribers!
 r computer nets: laptops, palmtops, PDAs,
   Internet-enabled phone promise anytime
   untethered Internet access
 r two important (but different) challenges
     m   wireless: communication over wireless link
     m   mobility: handling the mobile user who changes point
         of attachment to network




                                           6: Wireless and Mobile Networks   6-2
Chapter 6 outline
6.1 Introduction                Mobility
                                r 6.5 Principles:
Wireless                          addressing and routing
r 6.2 Wireless links,
                                  to mobile users
  characteristics               r 6.6 Mobile IP
    m   CDMA                    r 6.7 Handling mobility in
r 6.3 IEEE 802.11                 cellular networks
  wireless LANs (“wi-fi”)       r 6.8 Mobility and higher-
r 6.4 Cellular Internet           layer protocols
  Access
    m   architecture            6.9 Summary
    m   standards (e.g., GSM)

                                      6: Wireless and Mobile Networks   6-3
Elements of a wireless network

                                 wireless hosts
                                 r laptop, PDA, IP phone
                                 r run applications
                                 r may be stationary (non
                                    -mobile) or mobile
                 network             m wireless does not
              infrastructure             always mean mobility




                               6: Wireless and Mobile Networks   6-4
Elements of a wireless network
                                 base station
                                r typically connected to
                                   wired network
                                r relay - responsible
                                   for sending packets
                                   between wired
                 network           network and wireless
              infrastructure       host(s) in its “area”
                                    m e.g., cell towers,
                                      802.11 access
                                      points




                               6: Wireless and Mobile Networks   6-5
Elements of a wireless network
                                 wireless link
                                r typically used to
                                   connect mobile(s) to
                                   base station
                                r also used as backbone
                                   link
                 network        r multiple access
              infrastructure       protocol coordinates
                                   link access
                                r various data rates,
                                   transmission distance




                               6: Wireless and Mobile Networks   6-6
                      Characteristics of selected wireless link
                                                    standards

                   200       802.11n

                    54       802.11a,g               802.11a,g point-to-point                   data
Data rate (Mbps)




                   5-11      802.11b                           802.16 (WiMAX)

                     4                   UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO                    3G cellular
                                                                                              enhanced
                     1    802.15


                   .384                        UMTS/WCDMA, CDMA2000                           3G

                   .056                          IS-95, CDMA, GSM                             2G


                             Indoor        Outdoor         Mid-range            Long-range
                             10-30m        50-200m          outdoor               outdoor
                                                           200m – 4 Km          5Km – 20 Km

                                                                   6: Wireless and Mobile Networks     6-7
Elements of a wireless network
                                 infrastructure mode
                                r base station connects
                                   mobiles into wired
                                   network
                                r handoff: mobile
                                   changes base station
                 network           providing connection
              infrastructure       into wired network




                               6: Wireless and Mobile Networks   6-8
Elements of a wireless network
                         ad hoc mode
                         r no base stations
                         r nodes can only
                            transmit to other
                            nodes within link
                            coverage
                         r nodes organize
                            themselves into a
                            network: route among
                            themselves




                        6: Wireless and Mobile Networks   6-9
Wireless network taxonomy
                    single hop             multiple hops

                   host connects to        host may have to
infrastructure    base station (WiFi,    relay through several
   (e.g., APs)    WiMAX, cellular)         wireless nodes to
                  which connects to        connect to larger
                    larger Internet       Internet: mesh net

                                           no base station, no
                                          connection to larger
       no          no base station, no
 infrastructure                          Internet. May have to
                  connection to larger
                                         relay to reach other
                  Internet (Bluetooth,
                                         a given wireless node
                      ad hoc nets)
                                            MANET, VANET




                                         6: Wireless and Mobile Networks   6-10
Wireless Link Characteristics (1)
Differences from wired link ….

   m   decreased signal strength: radio signal
       attenuates as it propagates through matter
       (path loss)
   m   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
   m   multipath propagation: radio signal reflects off
       objects ground, arriving ad destination at
       slightly different times

…. make communication across (even a point to point)
   wireless link much more “difficult”
                                    6: Wireless and Mobile Networks   6-11
Wireless Link Characteristics (2)
r   SNR: signal-to-noise ratio                 10-1

     m larger SNR – easier to                  10-2
       extract signal from noise (a
       “good thing”)                           10-3

r   SNR versus BER tradeoffs




                                         BER
                                               10-4

     m given physical layer:
                                               10-5
        increase power -> increase
        SNR->decrease BER                      10-6

    m   given SNR: choose physical             10-7
        layer that meets BER                                 10      20        30       40
                                                                  SNR(dB)
        requirement, giving highest
        thruput                                                QAM256 (8 Mbps)
         • SNR may change with
                                                               QAM16 (4 Mbps)
           mobility: dynamically adapt
           physical layer (modulation                          BPSK (1 Mbps)
           technique, rate)
                                                      6: Wireless and Mobile Networks        6-12
Wireless network characteristics
Multiple wireless senders and receivers create
 additional problems (beyond multiple access):

                                 A                      B          C
                 C

                                 A’s signal                   C’s signal
                        B        strength                     strength
    A

                                                space
Hidden terminal problem
r B, A hear each other           Signal attenuation:
r B, C hear each other           r   B, A hear each other
r A, C can not hear each other   r   B, C hear each other
means A, C unaware of their      r   A, C can not hear each other
  interference at B                  interfering at B

                                     6: Wireless and Mobile Networks   6-13
Code Division Multiple Access (CDMA)
r   used in several wireless broadcast channels
    (cellular, satellite, etc) standards
r   unique “code” assigned to each user; i.e., code set
    partitioning
r   all users share same frequency, but each user has
    own “chipping” sequence (i.e., code) to encode data
r   encoded signal = (original data) X (chipping
    sequence)
r   decoding: inner-product of encoded signal and
    chipping sequence
r   allows multiple users to “coexist” and transmit
    simultaneously with minimal interference (if codes
    are “orthogonal”)

                                    6: Wireless and Mobile Networks   6-14
 CDMA Encode/Decode
                                                                                                   .
                                                                                                         channel output Zi,m
                                                                              Zi,m= di cm
           data                                       d0 = 1
                                                                                                                  1        1 1 1 1 1 1        1
                    d1 = -1
           bits
sender
                                                                                                       -1 -1 -1       -1                 -1       -1 -1 -1

                  1 1 1        1              1 1 1            1                                           slot 1                  slot 0
           code                                                                                           channel                 channel
                          -1       -1 -1 -1               -1       -1 -1 -1
                                                                                                           output                  output
                      slot 1                         slot 0

                                                                                                         M
                                                                                                   Di = S Zi,m.cm
                                                                                                        m=1
                                                                                                                  M
         received                               1         1 1 1 1 1 1               1
                                                                                                                                                             d0 = 1
         input                       -1 -1 -1        -1                        -1       -1 -1 -1                                d1 = -1

                                      1 1 1          1                1 1 1         1                                           slot 1                   slot 0
                     code                                                                                                      channel                  channel
                                                -1        -1 -1 -1             -1       -1 -1 -1

         receiver                              slot 1                         slot 0
                                                                                                                                output                   output



                                                                                                       6: Wireless and Mobile Networks                                6-15
CDMA: two-sender interference




                    6: Wireless and Mobile Networks   6-16
Chapter 6 outline
6.1 Introduction                Mobility
                                r 6.5 Principles:
Wireless                          addressing and routing
r 6.2 Wireless links,
                                  to mobile users
  characteristics               r 6.6 Mobile IP
    m   CDMA                    r 6.7 Handling mobility in
r 6.3 IEEE 802.11                 cellular networks
  wireless LANs (“wi-fi”)       r 6.8 Mobility and higher-
r 6.4 cellular Internet           layer protocols
  access
    m   architecture            6.9 Summary
    m   standards (e.g., GSM)

                                      6: Wireless and Mobile Networks   6-17
    IEEE 802.11 Wireless LAN
r   802.11b                              r   802.11a
    m   2.4-5 GHz unlicensed spectrum        m   5-6 GHz range
    m   up to 11 Mbps                        m   up to 54 Mbps
    m   direct sequence spread          r    802.11g
        spectrum (DSSS) in physical          m   2.4-5 GHz range
        layer                                m   up to 54 Mbps
          • all hosts use same chipping
            code
                                        r    802.11n: multiple antennae
                                             m   2.4-5 GHz range
                                             m   up to 200 Mbps


            r   all use CSMA/CA for multiple access
            r   all have base-station and ad-hoc network versions
                                             6: Wireless and Mobile Networks   6-18
802.11 LAN architecture
                               r   wireless host communicates
              Internet             with base station
                                    m base station = access
                                      point (AP)
                               r   Basic Service Set (BSS)
                                   (aka “cell”) in infrastructure
                 hub, switch
                 or router         mode contains:
         AP
                                    m wireless hosts
                                    m access point (AP): base
 BSS 1
                                      station
                AP
                                    m ad hoc mode: hosts only




               BSS 2
                                   6: Wireless and Mobile Networks   6-19
802.11: Channels, association
r   802.11b: 2.4GHz-2.485GHz spectrum divided into
    11 channels at different frequencies
     m AP admin chooses frequency for AP
     m interference possible: channel can be same as
       that chosen by neighboring AP!
r host: must associate with an AP
   m scans channels, listening for beacon frames
     containing AP’s name (SSID) and MAC address
   m selects AP to associate with
   m may perform authentication [Chapter 8]
   m will typically run DHCP to get IP address in AP’s
     subnet
                                  6: Wireless and Mobile Networks   6-20
  802.11: passive/active scanning
        BBS 1             BBS 2                   BBS 1                  BBS 2


      AP 1                        AP 2         AP 1           1               AP 2
                1         1                               2          2
                         2                                          3
                              3                                           4

                    H1                                        H1

Passive Scanning:                        Active Scanning:
(1) beacon frames sent from APs          (1) Probe Request frame broadcast
(2) association Request frame sent:          from H1
    H1 to selected AP                    (2) Probes response frame sent from
(3) association Response frame sent:         APs
    H1 to selected AP                    (3) Association Request frame sent:
                                             H1 to selected AP
                                         (4) Association Response frame
                                             sent: H1 to selected AP
                                                6: Wireless and Mobile Networks   6-21
IEEE 802.11: multiple access
r avoid collisions: 2+ nodes transmitting at same time
r 802.11: CSMA - sense before transmitting
    m   don’t collide with ongoing transmission by other node
r   802.11: no collision detection!
    m   difficult to receive (sense collisions) when transmitting due
        to weak received signals (fading)
    m   can’t sense all collisions in any case: hidden terminal, fading
    m   goal: avoid collisions: CSMA/C(ollision)A(voidance)

                                     A                B       C
                      C
                                     A’s signal            C’s signal
                            B        strength              strength
           A
                                                  space
                                              6: Wireless and Mobile Networks   6-22
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle for DIFS then             sender               receiver
    transmit entire frame (no CD)
                                               DIFS
2 if sense channel busy then
    start random backoff time
    timer counts down while channel idle                     data
    transmit when timer expires
    if no ACK, increase random backoff
                                                                             SIFS
       interval, repeat 2
802.11 receiver                                              ACK

- if frame received OK
 return ACK after SIFS (ACK needed due to
  hidden terminal problem)
                                           6: Wireless and Mobile Networks   6-23
Avoiding collisions (more)
idea:   allow sender to “reserve” channel rather than random
    access of data frames: avoid collisions of long data frames
r   sender first transmits small request-to-send (RTS) packets
    to BS using CSMA
     m RTSs may still collide with each other (but they’re short)
r   BS broadcasts clear-to-send CTS in response to RTS
r   CTS heard by all nodes
     m sender transmits data frame
     m other stations defer transmissions


           avoid data frame collisions completely
              using small reservation packets!

                                         6: Wireless and Mobile Networks   6-24
  Collision Avoidance: RTS-CTS exchange
         A                                             B
                        AP

             RTS(A)                                  RTS(B)

                        reservation collision
             RTS(A)


             CTS(A)                   CTS(A)



             DATA (A)
                                                           defer


time
             ACK(A)                   ACK(A)


                              6: Wireless and Mobile Networks   6-25
  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 4: 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



                                                 6: Wireless and Mobile Networks   6-26
802.11 frame: addressing

                                                                 Internet
            H1                         R1 router
                             AP



                                        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
                                                   6: Wireless and Mobile Networks   6-27
802.11 frame: more
                                                              frame seq #
                   duration of reserved
                                                              (for RDT)
                   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)

                                                      6: Wireless and Mobile Networks      6-28
802.11: mobility within same subnet

r H1 remains in same IP                         router
  subnet: IP address
  can remain same                                 hub or
r switch: which AP is                             switch

  associated with H1?            BBS 1
    m   self-learning (Ch. 5):
        switch will see frame            AP 1
        from H1 and                                        AP 2
        “remember” which
        switch port can be                H1                 BBS 2
        used to reach H1



                                   6: Wireless and Mobile Networks   6-29
802.11: advanced capabilities
Rate Adaptation                      10-1

r   base station, mobile             10-2


    dynamically change
                                     10-3




                               BER
                                     10-4
    transmission rate                10-5
    (physical layer                  10-6

    modulation technique)            10-7
                                               10    20   30   40
    as mobile moves, SNR                            SNR(dB)

    varies                           1. SNR decreases, BER
                                     increase as node moves
             QAM256 (8 Mbps)         away from base station
             QAM16 (4 Mbps)
             BPSK (1 Mbps)           2. When BER becomes too
                                     high, switch to lower
             operating point


                                     transmission rate but with
                                     lower BER

                                            6: Wireless and Mobile Networks   6-30
802.11: advanced capabilities
Power Management
r node-to-AP: “I am going to sleep until next
  beacon frame”
   m AP knows not to transmit frames to this
     node
   m node wakes up before next beacon frame
r beacon frame: contains list of mobiles with AP-
  to-mobile frames waiting to be sent
   m node will stay awake if AP-to-mobile frames
     to be sent; otherwise sleep again until next
     beacon frame


                                  6: Wireless and Mobile Networks   6-31
802.15: personal area network
r less than 10 m diameter
r replacement for cables
  (mouse, keyboard,                            S               P

  headphones)                                      P               radius of
                                                       M
r ad hoc: no infrastructure                                        coverage

r master/slaves:                           S               S       P
                                                       P
    m   slaves request permission to
        send (to master)
    m   master grants requests
r   802.15: evolved from                  M Master device

    Bluetooth specification                S Slave device
    m   2.4-2.5 GHz radio band             P Parked device (inactive)
    m   up to 721 kbps
                                       6: Wireless and Mobile Networks    6-32
 802.16: WiMAX
                                         point-to-point
r like 802.11 & cellular:
  base station model
   m   transmissions to/from
       base station by hosts
       with omnidirectional
       antenna
   m   base station-to-base     point-to-multipoint
       station backhaul with
       point-to-point antenna
r unlike 802.11:
   m range ~ 6 miles (“city
     rather than coffee
     shop”)
   m ~14 Mbps
                                   6: Wireless and Mobile Networks   6-33
802.16: WiMAX: downlink, uplink scheduling

r            transmission frame
              m down-link subframe: base station to node
              m uplink subframe: node to base station


                                             …                                           …
    pream.




              DL- UL-      DL        DL            DL      Initial request
                                                                           SS #1 SS #2       SS #k
              MAP MAP    burst 1   burst 2       burst n   maint. conn.
                                             …                                           …
                        downlink subframe                            uplink subframe

                   base station tells nodes who will get to receive (DL map)
                   and who will get to send (UL map), and when


r            WiMAX standard provide mechanism for
             scheduling, but not scheduling algorithm
                                                              6: Wireless and Mobile Networks        6-34
Chapter 6 outline
6.1 Introduction                Mobility
                                r 6.5 Principles:
Wireless                          addressing and routing
r 6.2 Wireless links,
                                  to mobile users
  characteristics               r 6.6 Mobile IP
    m   CDMA                    r 6.7 Handling mobility in
r 6.3 IEEE 802.11                 cellular networks
  wireless LANs (“wi-fi”)       r 6.8 Mobility and higher-
r 6.4 Cellular Internet           layer protocols
  Access
    m   architecture            6.9 Summary
    m   standards (e.g., GSM)

                                      6: Wireless and Mobile Networks   6-35
Components of cellular network architecture
                          MSC
                          q   connects cells to wide area net
                          q   manages call setup (more later!)
                          q   handles mobility (more later!)
 cell
q covers geographical
region
q base station (BS)                   Mobile
analogous to 802.11 AP               Switching
                                      Center
q mobile users attach                               Public telephone
to network through BS                               network, and
q   air-interface:                                  Internet
physical and link layer             Mobile
protocol between                   Switching
mobile and BS                       Center



                                                 wired network

                                  6: Wireless and Mobile Networks   6-36
Cellular networks: the first hop
Two techniques for sharing
  mobile-to-BS radio
  spectrum
r combined FDMA/TDMA:
  divide spectrum in                           time slots
  frequency channels, divide
  each channel into time
  slots                  frequency
r CDMA: code division
                           bands

  multiple access



                                     6: Wireless and Mobile Networks   6-37
Cellular standards: brief survey
2G systems: voice channels
r IS-136 TDMA: combined FDMA/TDMA (north
  america)
r GSM (global system for mobile communications):
  combined FDMA/TDMA
    m   most widely deployed
r   IS-95 CDMA: code division multiple access

                 0   TDMA/FDMA
        CDMA-200
              GPRS EDGE UMTS     Don’t drown in a bowl
           IS-136
                   GSM IS-95     of alphabet soup: use this
                                 for reference only


                                        6: Wireless and Mobile Networks   6-38
Cellular standards: brief survey
2.5 G systems: voice and data channels
r for those who can’t wait for 3G service: 2G extensions
r general packet radio service (GPRS)
    m   evolved from GSM
    m   data sent on multiple channels (if available)
r   enhanced data rates for global evolution (EDGE)
    m   also evolved from GSM, using enhanced modulation
    m   data rates up to 384K
r   CDMA-2000 (phase 1)
    m   data rates up to 144K
    m   evolved from IS-95

                                            6: Wireless and Mobile Networks   6-39
Cellular standards: brief survey
3G systems: voice/data
r Universal Mobile Telecommunications Service (UMTS)
   m data service: High Speed Uplink/Downlink packet
     Access (HSDPA/HSUPA): 3 Mbps
r CDMA-2000: CDMA in TDMA slots
   m data service: 1xEvolution Data Optimized (1xEVDO)
     up to 14 Mbps




  ….. more (and more interesting) cellular topics due to mobility (stay
  tuned for details)

                                         6: Wireless and Mobile Networks   6-40
2G (voice) network architecture
       Base station system (BSS)
                                   MSC
           BTS            BSC                         G        Public
                                                               telephone
                                                               network
                                                    Gateway
                                                    MSC


                                         Legend

                                                  Base transceiver station (BTS)

                                                  Base station controller (BSC)

                                                  Mobile Switching Center (MSC)

                                                  Mobile subscribers


                                         6: Wireless and Mobile Networks   6-41
2.5G (voice+data) network architecture

                                      MSC
                           BSC                           G        Public
                                                                  telephone
                                                                  network
                                                        Gateway
                                                        MSC


                                                          G        Public
Key insight: new cellular data        SGSN                         Internet
network operates in parallel
(except at edge) with existing                           GGSN
cellular voice network
q voice network unchanged in core              Serving GPRS Support Node (SGSN)
q data network operates in parallel
                                                Gateway GPRS Support Node (GGSN)


                                             6: Wireless and Mobile Networks   6-42
Chapter 6 outline
6.1 Introduction                Mobility
                                r 6.5 Principles:
Wireless                          addressing and routing
r 6.2 Wireless links,
                                  to mobile users
  characteristics               r 6.6 Mobile IP
    m   CDMA                    r 6.7 Handling mobility in
r 6.3 IEEE 802.11                 cellular networks
  wireless LANs (“wi-fi”)       r 6.8 Mobility and higher-
r 6.4 Cellular Internet           layer protocols
  Access
    m   architecture            6.9 Summary
    m   standards (e.g., GSM)

                                      6: Wireless and Mobile Networks   6-43
What is mobility?
r     spectrum of mobility, from the network perspective:


    no mobility                                       high mobility




mobile wireless user, mobile user,           mobile user, passing
using same access     connecting/            through multiple
point                 disconnecting          access point while
                      from network           maintaining ongoing
                      using DHCP.            connections (like cell
                                             phone)


                                      6: Wireless and Mobile Networks   6-44
  Mobility: Vocabulary
  home network: permanent     home agent: entity that will
  “home” of mobile            perform mobility functions on
  (e.g., 128.119.40/24)
                              behalf of mobile, when mobile
                              is remote



                               wide area
                               network
Permanent address:
address in home
network, can always be
used to reach mobile
e.g., 128.119.40.186        correspondent




                                            6: Wireless and Mobile Networks   6-45
Mobility: more vocabulary
                                                 visited network: network
         Permanent address: remains              in which mobile currently
         constant (e.g., 128.119.40.186)         resides (e.g., 79.129.13/24)

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


                                    wide area
                                    network

                                                           foreign agent: entity
                                                           in visited network
                                                           that performs
correspondent: wants                                       mobility functions on
to communicate with                                        behalf of mobile.
mobile                                          6: Wireless and Mobile Networks   6-46
How do you contact a mobile friend:

Consider friend frequently changing         I wonder where
  addresses, how do you find her?           Alice moved to?
r search all phone
  books?
r call her parents?
r expect her to let you
  know where he/she is?




                                 6: Wireless and Mobile Networks   6-47
Mobility: approaches
r   Let routing handle it: routers advertise permanent
    address of mobile-nodes-in-residence via usual
    routing table exchange.
     m routing tables indicate where each mobile located
     m no changes to end-systems
r   Let end-systems handle it:
     m indirect routing: communication from
        correspondent to mobile goes through home
        agent, then forwarded to remote
    m   direct routing: correspondent gets foreign
        address of mobile, sends directly to mobile

                                   6: Wireless and Mobile Networks   6-48
Mobility: approaches
r   Let routing handle it: routers advertise permanent
    address of mobile-nodes-in-residence via usual
                          not
                        scalable
    routing table exchange.
                     to millions of
     m routing tables indicate where each mobile located
                         mobiles
     m no changes to end-systems
r   let end-systems handle it:
     m indirect routing: communication from
        correspondent to mobile goes through home
        agent, then forwarded to remote
    m   direct routing: correspondent gets foreign
        address of mobile, sends directly to mobile

                                   6: Wireless and Mobile Networks   6-49
Mobility: registration
                                         visited network
     home network


                                                    1
                          2
                       wide area
                       network

                                                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
                                   6: Wireless and Mobile Networks   6-50
Mobility via Indirect Routing
                                            foreign agent
                                            receives packets,
                home agent intercepts       forwards to mobile
                packets, forwards to                                    visited
                foreign agent                                           network
home
network
                                                                3
                               wide area
                               network
                                        2
                        1
   correspondent                                      4
   addresses packets
                                                             mobile replies
   using home address
                                                             directly to
   of mobile
                                                             correspondent


                                            6: Wireless and Mobile Networks   6-51
Indirect Routing: comments
r Mobile uses two addresses:
   m permanent address: used by correspondent (hence
     mobile location is transparent to correspondent)
   m 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
   m inefficient when
   correspondent, mobile
   are in same network

                                6: Wireless and Mobile Networks   6-52
Indirect Routing: moving between networks
 r suppose mobile user moves to another
   network
   m   registers with new foreign agent
   m   new foreign agent registers with home agent
   m   home agent update care-of-address for mobile
   m   packets continue to be forwarded to mobile (but
       with new care-of-address)
 r mobility, changing foreign networks
   transparent: on going connections can be
   maintained!

                                  6: Wireless and Mobile Networks   6-53
Mobility via Direct Routing
                                             foreign agent
                                             receives packets,
                 correspondent forwards      forwards to mobile
                 to foreign agent                                        visited
                                                                         network
home
network                                                          4
                                 wide area
                         2       network
                                             3
   correspondent             1                         4
   requests, receives
                                                              mobile replies
   foreign address of
                                                              directly to
   mobile
                                                              correspondent


                                             6: Wireless and Mobile Networks   6-54
Mobility via Direct Routing: comments
r overcome triangle routing problem
r non-transparent to correspondent:
  correspondent must get care-of-address
  from home agent
  m   what if mobile changes visited network?




                                 6: Wireless and Mobile Networks   6-55
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
                                     anchor
                                     foreign
               wide area             agent
                                                                   2
               network
                           1                   4
                                                             3
                                               5
                                                                       new
                     correspondent                                     foreign
                                                   new foreign
                     agent                                             network
     correspondent                                 agent

                                                      6: Wireless and Mobile Networks   6-56
Chapter 6 outline
6.1 Introduction                Mobility
                                r 6.5 Principles:
Wireless                          addressing and routing
r 6.2 Wireless links,
                                  to mobile users
  characteristics               r 6.6 Mobile IP
    m   CDMA                    r 6.7 Handling mobility in
r 6.3 IEEE 802.11                 cellular networks
  wireless LANs (“wi-fi”)       r 6.8 Mobility and higher-
r 6.4 Cellular Internet           layer protocols
  Access
    m   architecture            6.9 Summary
    m   standards (e.g., GSM)

                                      6: Wireless and Mobile Networks   6-57
Mobile IP
r RFC 3344
r has many features we’ve seen:
   m home agents, foreign agents, foreign-agent
     registration, care-of-addresses, encapsulation
     (packet-within-a-packet)
r three components to standard:
   m indirect routing of datagrams
   m agent discovery
   m registration with home agent



                                 6: Wireless and Mobile Networks   6-58
  Mobile IP: indirect routing

                                                 foreign-agent-to-mobile packet
      packet sent by home agent to foreign            dest: 128.119.40.186
      agent: a packet within a packet

      dest: 79.129.13.2   dest: 128.119.40.186




Permanent address:
128.119.40.186

                                                               Care-of address:
                                                                    79.129.13.2
           dest: 128.119.40.186
           packet sent by
           correspondent

                                                 6: Wireless and Mobile Networks   6-59
 Mobile IP: agent discovery
  r   agent advertisement: foreign/home agents advertise
      service by broadcasting ICMP messages (typefield = 9)



H,F bits: home
and/or foreign agent

R bit: registration
required




                                     6: Wireless and Mobile Networks   6-60
Mobile IP: registration example




                    6: Wireless and Mobile Networks   6-61
Components of cellular network architecture

     recall:                               correspondent
                           wired public
                           telephone
                           network

                   MSC                          MSC

                          MSC
                                                       MSC
                                          MSC




               different cellular networks,
               operated by different providers

                                            6: Wireless and Mobile Networks   6-62
Handling mobility in cellular networks

r   home network: network of cellular provider you
  subscribe to (e.g., Sprint PCS, Verizon)
   m home location register (HLR): database in home
     network containing permanent cell phone #,
     profile information (services, preferences,
     billing), information about current location
     (could be in another network)
r visited network: network in which mobile currently
  resides
   m visitor location register (VLR): database with
     entry for each user currently in network
   m could be home network


                                  6: Wireless and Mobile Networks   6-63
  GSM: indirect routing to mobile
                                        home
                        HLR
                                        network                       correspondent
                              2
                                    home
                                    Mobile
home MSC consults HLR,             Switching
gets roaming number of              Center
mobile in visited network
                                                               1               call routed
                                                                               to home network
                                                       3           Public
                                  VLR                              switched
                                         Mobile
                                                                   telephone
                                        Switching
                                                                   network
                                         Center
                              4
                                                       home MSC sets up 2nd leg of call
                                                       to MSC in visited network
             mobile
             user                                   MSC in visited network completes
                                   visited          call through base station to mobile
                                   network
                                                           6: Wireless and Mobile Networks   6-64
  GSM: handoff with common MSC

                                         r   Handoff goal: route call via
                                             new base station (without
                                             interruption)
          VLR Mobile                     r   reasons for handoff:
               Switching                     m   stronger signal to/from new
                Center                           BSS (continuing connectivity,
                                                 less battery drain)
          old           new
                     routing
                                             m   load balance: free up channel
          routing
old BSS
                                                 in current BSS
                               new BSS
                                             m   GSM doesn’t mandate why to
                                                 perform handoff (policy), only
                                                 how (mechanism)
                                         r   handoff initiated by old BSS



                                                  6: Wireless and Mobile Networks   6-65
   GSM: handoff with common MSC
                                             1. old BSS informs MSC of impending
                                                handoff, provides list of 1+ new BSSs
                                             2. MSC sets up path (allocates resources)
                                                to new BSS
              VLR Mobile                     3. new BSS allocates radio channel for
                   Switching
                    Center 2                    use by mobile
                      4                      4. new BSS signals MSC, old BSS: ready
          1
                           7
               8                             5. old BSS tells mobile: perform handoff to
                                     3
old BSS   5                    6
                                                new BSS
                                   new BSS
                                             6. mobile, new BSS signal to activate new
                                                channel
                                             7. mobile signals via new BSS to MSC:
                                                handoff complete. MSC reroutes call
                                             8 MSC-old-BSS resources released

                                                        6: Wireless and Mobile Networks   6-66
GSM: handoff between MSCs

                                       r   anchor MSC: first MSC
                                           visited during cal
home network
                       correspondent       m   call remains routed
 Home
 MSC
                                               through anchor MSC
                                       r new MSCs add on to end
anchor MSC
                    PSTN                 of MSC chain as mobile
        MSC
                                         moves to new MSC
                               MSC
                                       r IS-41 allows optional
                MSC

                                         path minimization step
                                         to shorten multi-MSC
                                         chain
               (a) before handoff


                                               6: Wireless and Mobile Networks   6-67
GSM: handoff between MSCs

                                       r   anchor MSC: first MSC
                                           visited during cal
home network
                       correspondent       m   call remains routed
 Home
 MSC
                                               through anchor MSC
                                       r new MSCs add on to end
anchor MSC
                    PSTN                 of MSC chain as mobile
        MSC
                                         moves to new MSC
                               MSC
                                       r IS-41 allows optional
                MSC

                                         path minimization step
                                         to shorten multi-MSC
                                         chain
               (b) after handoff


                                               6: Wireless and Mobile Networks   6-68
Mobility: GSM versus Mobile IP
     GSM element                 Comment on GSM element                Mobile IP element
Home system            Network to which mobile user’s permanent           Home
                       phone number belongs                               network
Gateway Mobile         Home MSC: point of contact to obtain routable      Home agent
Switching Center, or   address of mobile user. HLR: database in
“home MSC”. Home       home system containing permanent phone
Location Register      number, profile information, current location of
(HLR)                  mobile user, subscription information
Visited System         Network other than home system where               Visited
                       mobile user is currently residing                  network
Visited Mobile         Visited MSC: responsible for setting up calls      Foreign agent
services Switching     to/from mobile nodes in cells associated with
Center.                MSC. VLR: temporary database entry in
Visitor Location       visited system, containing subscription
Record (VLR)           information for each visiting mobile user
Mobile Station         Routable address for telephone call segment        Care-of-
Roaming Number         between home MSC and visited MSC, visible          address
(MSRN), or “roaming    to neither the mobile nor the correspondent.
number”
                                                   6: Wireless and Mobile Networks    6-69
Wireless, mobility: impact on higher layer protocols

r logically, impact should be minimal …
   m best effort service model remains unchanged
   m TCP and UDP can (and do) run over wireless, mobile
r … but performance-wise:
   m packet loss/delay due to bit-errors (discarded
     packets, delays for link-layer retransmissions), and
     handoff
   m TCP interprets loss as congestion, will decrease
     congestion window un-necessarily
   m delay impairments for real-time traffic
   m limited bandwidth of wireless links


                                   6: Wireless and Mobile Networks   6-70
Chapter 6 Summary
Wireless                        Mobility
r wireless links:               r principles: addressing,
    m   capacity, distance        routing to mobile users
    m   channel impairments         m   home, visited networks
    m   CDMA                        m   direct, indirect routing
r   IEEE 802.11 (“wi-fi”)           m   care-of-addresses
    m   CSMA/CA reflects        r   case studies
        wireless channel            m   mobile IP
        characteristics             m   mobility in GSM
r   cellular access             r   impact on higher-layer
    m   architecture                protocols
    m   standards (e.g., GSM,
        CDMA-2000, UMTS)

                                         6: Wireless and Mobile Networks   6-71

				
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