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Chapter 6: Wireless and Mobile Networks

Chapter 6: Wireless and Mobile Networks. Cover the following sections only: 6.3: 802.11 wireless LANs 6.5: mobility management: principles two important (but different) new challenges communication over wireless link handling mobile user who changes point of attachment to network.

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Chapter 6: Wireless and Mobile Networks

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  1. Chapter 6: Wireless and Mobile Networks • Cover the following sections only: • 6.3: 802.11 wireless LANs • 6.5: mobility management: principles • two important (but different) new challenges • communication over wireless link • handling mobile user who changes point of attachment to network CS118/Spring05

  2. Elements of a wireless network wireless link • typically connecting mobile(s) to base station • can also be used as backbone link • multiaccess protocol: coordinates link access Wired network infrastructure Wireless host: may be stationary (non-mobile) or mobile • Base station: • typically connected to wired network • relay - responsible for sending packets between wired network and wireless host(s) in its “area” • Infrastructure mode: • basestations connect mobiles to wired networks • when moving mobiles change basestations to keep Internet access (handoff) • Ad hoc mode: • no basestations • each node helps forward packets to other node CS118/Spring05

  3. Wireless Link Characteristics C B A communication across a point to point wireless link is much more “difficult” than wired link …. • decreased signal strength: radio signal attenuates as it propagates through matter (path loss) • 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 • multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times • Multiple wireless senders and receivers create additional problems (beyond multiple access): Hidden terminal problem • B, A hear each other • B, C hear each other • A, C can not hear each other means A, C unaware of their interference at B CS118/Spring05

  4. IEEE 802.11 Wireless LAN 802.11b 2.4-5 GHz unlicensed radio spectrum Data rate up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-2.5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All have infrastructure and ad-hoc network versions CS118/Spring05

  5. 802.11 LAN architecture AP AP Internet • 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies; 3 non-overlapping • AP admin chooses frequency for AP • neighboring APs may choose same channel–interference • AP sends beacon frame periodically • SSID, MAC address • host: must associate with an AP • scan channels, listening for beacon frames containing AP’s name (SSID) and MAC address • selects AP to associate with; initiates association protocol • typically run DHCP to get IP address in AP’s subnet hub, switch or router BSS 1 BSS 2 BSS: Basic Service Set SSID: Service Set Identifier CS118/Spring05

  6. IEEE 802.11: multiple access • Like Ethernet, uses CSMA: • random access • carrier sense: don’t collide with ongoing transmission • Unlike Ethernet: • no collision detection – transmit all frames to completion • acknowledgment – because without collision detection, you don’t know if your transmission collided or not • Why no collision detection? • difficult to receive (sense collisions) when transmitting due to weak received signals (fading) • can’t sense all collisions in any case: hidden terminal, fading • Goal: avoid collisions: CSMA/C(ollision)A(voidance) CS118/Spring05

  7. IEEE 802.11 MAC Protocol: CSMA/CA (1) DIFS data SIFS ACK 802.11 sender 1 if sense channel idle for DIFSthen - transmit entire frame (no CD) 2 ifsense 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 2 802.11 receiver if frame received OK - return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS: distributed inter-frame spacing SIFS: short inter-frame spacing CS118/Spring05

  8. IEEE 802.11 MAC Protocol: CSMA/CA (2) Dealing with hidden terminal: • idea: allow sender to “reserve” channel: avoid collisions of long data frames • sender first transmits small request-to-send (RTS) packets to AP using CSMA • RTSs may still collide with each other (but they’re short) • AP broadcasts clear-to-send CTS in response to RTS • CTS heard by all nodes • sender transmits data frame • other stations defer transmissions CS118/Spring05

  9. Collision Avoidance: RTS-CTS exchange RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) A B AP defer time CS118/Spring05

  10. 802.11 frame: addressing 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control Address 3: used only in ad hoc mode Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame CS118/Spring05

  11. 802.11 frame: addressing router AP Internet R1 MAC addr H1 MAC addr source address dest. address 802.3frame AP MAC addr H1 MAC addr R1 MAC addr address 3 address 2 address 1 802.11 frame H1 R1 CS118/Spring05

  12. 802.11 frame: more 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To AP From AP More frag Retry Power mgt More data WEP Rsvd frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) frame type (RTS, CTS, ACK, data) CS118/Spring05

  13. H1 detects weakening signal from AP1, scan and find AP2 to attach to H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning: switch will see frame from H1 and “remember” which interface can be used to reach H1 802.11: mobility within same subnet router hub or switch BBS 1 AP 1 AP 2 H1 BBS 2 CS118/Spring05

  14. Mobility: Vocabulary home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote home network: permanent “home” of mobile (e.g., 128.119.40/24) wide area network Permanent address: address in home network, can always be used to reach mobile e.g., 128.119.40.186 correspondent correspondent: wants to communicate with mobile CS118/Spring05

  15. Mobility: more vocabulary visited network: network in which mobile currently resides (e.g., 79.129.13/24) Permanent address: remains constant (e.g., 128.119.40.186) Care-of-address: address in visited network. (e.g., 79,129.13.2) wide area network foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile CS118/Spring05

  16. Mobility: approaches not scalable to millions of mobiles • Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. • routing tables indicate where each mobile located • no changes to end-systems • Let end-systems handle it: • indirect routing: correspondent sends packets to to home agent, which forwards to mobile • direct routing: correspondent gets foreign address of mobile, sends directly to mobile CS118/Spring05

  17. Mobility: registration mobile contacts foreign agent on entering visited network foreign agent contacts home agent home: “this mobile is resident in my network” 1 2 End result: • Foreign agent knows about mobile • Home agent knows location of mobile visited network home network wide area network CS118/Spring05

  18. Mobility via Indirect Routing foreign agent receives packets, forwards to mobile home agent intercepts packets, forwards to foreign agent correspondent addresses packets using home address of mobile mobile replies directly to correspondent Q: Which address will mobile use as source address? 3 2 4 1 visited network home network wide area network CS118/Spring05

  19. Mobility via Indirect Routing: further movement 4 1 2 3 visited network home network wide area network Q: Will the correspondence be aware of mobile's move? CS118/Spring05

  20. Indirect Routing: comments • Mobile uses two addresses: • permanent address: used by correspondent (hence mobile location is transparent to correspondent) • care-of-address: used by home agent to forward datagrams to mobile • foreign agent functions may be done by mobile itself • triangle routing: correspondent-home-network-mobile • inefficient when Correspondent & mobile are in same network CS118/Spring05

  21. Indirect Routing: moving between networks • suppose mobile user moves to another network • registers with new foreign agent • new foreign agent registers with home agent • home agent update care-of-address for mobile • packets continue to be forwarded to mobile (but with new care-of-address) • mobility, changing foreign networks transparent: on going connections can be maintained! CS118/Spring05

  22. Mobility via Direct Routing foreign agent receives packets, forwards to mobile mobile replies directly to correspondent 4 2 4 1 3 correspondent forwards to foreign agent visited network home network wide area network correspondent requests, receives foreign address of mobile CS118/Spring05

  23. Mobility via Direct Routing: comments • overcome triangle routing problem • non-transparent to correspondent: correspondent must get care-of-address from home agent • what if mobile moves to another visited network? CS118/Spring05

  24. Accommodating mobility with direct routing 1 2 4 3 5 • anchor foreign agent: FA in first visited network • data always routed first to anchor FA • when mobile moves: new FA arranges to have data forwarded from old FA (chaining) foreign net visited at session start anchor foreign agent wide area network new foreign network correspondent agent new foreign agent correspondent CS118/Spring05

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