1. 6/2/05CS118/Spring051 Chapter 6: Wireless and Mobile Networks r Cover the following sections only:  6.3: 802.11 wireless LANs  6.5: mobility management: principles r two important (but different) new challenges  communication over wireless link  handling mobile user who changes point of attachment to network

2. 6/2/05CS118/Spring052 Elements of a wireless network Wireless host: may be stationary (non-mobile) or mobile wireless link r typically connecting mobile(s) to base station r can also be used as backbone link r multiaccess protocol: coordinates link access Infrastructure mode: basestations connect mobiles to wired networks when moving mobiles change basestations to keep Internet access (handoff) Wired network infrastructure Ad hoc mode: no basestations each node helps 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”

3. 6/2/05CS118/Spring053 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  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 A B C

4. 6/2/05CS118/Spring054 IEEE 802.11 Wireless LAN r 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 r 802.11a  5-6 GHz range  up to 54 Mbps r 802.11g  2.4-2.5 GHz range  up to 54 Mbps r All use CSMA/CA for multiple access r All have infrastructure and ad-hoc network versions

5. 6/2/05CS118/Spring055 802.11 LAN architecture r 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 r AP sends beacon frame periodically  SSID, MAC address r 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 BSS 1 BSS 2 Internet hub, switch or router AP BSS: Basic Service Set SSID: Service Set Identifier

6. 6/2/05CS118/Spring056 IEEE 802.11: multiple access r Like Ethernet, uses CSMA:  random access  carrier sense: don’t collide with ongoing transmission r 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 r 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 r Goal: avoid collisions: CSMA/C(ollision)A(voidance)

7. 6/2/05CS118/Spring057 IEEE 802.11 MAC Protocol: CSMA/CA (1) 802.11 sender 1 if sense channel idle for DIFS then - transmit entire frame (no CD) 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 2 802.11 receiver if frame received OK - return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK DIFS: distributed inter-frame spacing SIFS: short inter-frame spacing

8. 6/2/05CS118/Spring058 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  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  sender transmits data frame  other stations defer transmissions

9. 6/2/05CS118/Spring059 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

10. 6/2/05CS118/Spring0510 frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control 802.11 frame: addressing Address 2: MAC address of wireless host or AP transmitting this frame 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 3: used only in ad hoc mode

11. 6/2/05CS118/Spring0511 Internet router AP H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame R1 MAC addr H1 MAC addr dest. address source address 802.3 frame 802.11 frame: addressing

12. 6/2/05CS118/Spring0512 frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control Type From AP Subtype To AP More frag WEP More data Power mgt RetryRsvd Protocol version 2 2411111111 802.11 frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for reliable ARQ) frame type (RTS, CTS, ACK, data)

13. 6/2/05CS118/Spring0513 hub or switch AP 2 AP 1 H1 BBS 2 BBS 1 802.11: mobility within same subnet router r H1 detects weakening signal from AP1, scan and find AP2 to attach to r H1 remains in same IP subnet: IP address can remain same r 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

14. 6/2/05CS118/Spring0514 Mobility: Vocabulary home network: permanent “home” of mobile (e.g., 128.119.40/24) Permanent address: address in home network, can always be used to reach mobile e.g., 128.119.40.186 home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote wide area network correspondent correspondent: wants to communicate with mobile

15. 6/2/05CS118/Spring0515 Mobility: more vocabulary Care-of-address: address in visited network. (e.g., 79,129.13.2) wide area network visited network: network in which mobile currently resides (e.g., 79.129.13/24) Permanent address: remains constant ( e.g., 128.119.40.186) foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile

16. 6/2/05CS118/Spring0516 r 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 r 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 Mobility: approaches not scalable to millions of mobiles

17. 6/2/05CS118/Spring0517 Mobility: registration End result: r Foreign agent knows about mobile r Home agent knows location of mobile wide area network home network visited network 1 mobile contacts foreign agent on entering visited network 2 foreign agent contacts home agent home: “this mobile is resident in my network”

18. 6/2/05CS118/Spring0518 Mobility via Indirect Routing wide area network home network visited network 3 2 4 1 correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent Q: Which address will mobile use as source address?

19. 6/2/05CS118/Spring0519 Mobility via Indirect Routing: further movement wide area network home network visited network 2 3 1 4 Q: Will the correspondence be aware of mobile's move?

20. 6/2/05CS118/Spring0520 Indirect Routing: comments r 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 r foreign agent functions may be done by mobile itself r triangle routing: correspondent-home-network-mobile  inefficient when Correspondent & mobile are in same network

21. 6/2/05CS118/Spring0521 Indirect Routing: moving between networks r 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) r mobility, changing foreign networks transparent: on going connections can be maintained!

22. 6/2/05CS118/Spring0522 Mobility via Direct Routing wide area network home network visited network 4 2 4 1 correspondent requests, receives foreign address of mobile correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent 3

23. 6/2/05CS118/Spring0523 Mobility via Direct Routing: comments r overcome triangle routing problem r non-transparent to correspondent: correspondent must get care-of-address from home agent  what if mobile moves to another visited network?

24. 6/2/05CS118/Spring0524 wide area network 1 foreign net visited at session start anchor foreign agent 2 4 new foreign agent 3 5 correspondent agent correspondent new foreign network 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)