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Wireless and Mobile Networks

Wireless and Mobile Networks. Slides originally from Carey Williamson Notes derived from “ Computer Networking: A Top Down Approach” , by Jim Kurose and Keith Ross, Addison-Wesley. Slides are adapted from the book’s companion Web site, with changes by Anirban Mahanti and Carey Williamson.

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Wireless and Mobile Networks

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  1. Wireless and Mobile Networks Slides originally from Carey Williamson Notes derived from “Computer Networking: A Top Down Approach”, by Jim Kurose and Keith Ross, Addison-Wesley. Slides are adapted from the book’s companion Web site, with changes by Anirban Mahanti and Carey Williamson. CPSC 441: Wireless

  2. Outline • Introduction • Standards and Link Characteristics • IEEE 802.11 Wireless LANS • Mobility • Wireless/Mobility Performance Issues • Summary CPSC 441: Wireless

  3. The use of infra-red (IR) or radio frequency (RF) signals to share information and resources between devices Promises anytime, anywhere connectivity laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access Two important (but different) challenges communication over wireless link handling mobile user who changes point of attachment to network Lots of media buzzwords! Mobile Internet, Pervasive Computing, Nomadic Computing, M-Commerce, Ubiquitous Computing … What is Wireless Networking? CPSC 441: Wireless

  4. Wireless Networking Technologies • Mobile devices – laptop, PDA, cellular phone, wearable computer, … • Operating modes • Infrastructure mode (uses Access Point (AP)) • Ad hoc mode • Access technology • Bluetooth (1 Mbps, up to 3 meters) • IEEE 802.11 (up to 54 Mbps, 20 – 100 meters) CPSC 441: Wireless

  5. infrastructure mode • base station connects mobiles into wired network • handoff: mobile changes base station providing connection into wired network network infrastructure Infrastructure Mode CPSC 441: Wireless

  6. Ad hoc Mode Ad hoc mode • no base stations • nodes can only transmit to other nodes within link coverage • nodes organize themselves into a network: route among themselves CPSC 441: Wireless

  7. Outline • Introduction • Standards and Link Characteristics • IEEE 802.11 Wireless LANS • Mobility • Wireless/Mobility Performance Issues • Summary CPSC 441: Wireless

  8. Outdoor 50 – 200m Mid range outdoor 200m – 4km Long range outdoor 5km – 20km Indoor 10 – 30m Wireless link standards 54 Mbps 802.11{a,g} 5-11 Mbps 802.11b WiMAX 1 Mbps 802.15 3G 384 Kbps UMTS/WCDMA, CDMA2000 2G 56 Kbps IS-95 CDMA, GSM CPSC 441: Wireless

  9. IEEE 802.11 Fast (11 Mbps) High Power Long range Single-purpose Ethernet replacement Easily Available Apple Airport, iBook Cisco Aironet 350 Bluetooth Slow (1 Mbps) Low Power Short range Flexible Cable replacement Two Popular 2.4 GHz Standards: CPSC 441: Wireless

  10. Wireless Link Characteristics Differences from wired link …. • Decreasing 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 • Multi-path propagation: radio signal reflects off objects ground, arriving at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” CPSC 441: Wireless

  11. B A C C C’s signal strength A’s signal strength B A space Wireless Network Characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): Hidden terminal problem • A and B can hear each other • B and C can hear each other • A and C can’t hear each other • thus A and C are unaware of their interference at B Signal fading: • A and B hear each other • B and C hear each other • A and C can’t hear each other interfering at B CPSC 441: Wireless

  12. Outline • Introduction • Standards and Link Characteristics • IEEE 802.11 Wireless LANS • Mobility • Wireless/Mobility Performance Issues • Summary CPSC 441: Wireless

  13. IEEE 802.11 Organization Tree: PHYS: physical FHSS: Frequency-hopping spread spectrum DSSS: Direct-sequence spread specturm OFDM: Orthogonal frequency-division multiplexing CPSC 441: Wireless

  14. 802.11b 2.4 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations 802.11a 5 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All have infrastructure and ad hoc modes IEEE 802.11 Wireless LAN CPSC 441: Wireless

  15. AP AP Internet 802.11 LAN architecture • wireless host communicates with base station • base station = access point (AP) • Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: • wireless hosts • access point (AP) • ad hoc mode: hosts only hub, switch or router BSS 1 BSS 2 CPSC 441: Wireless

  16. 802.11 has 11 channels Channels 1, 6, and 11 are non-overlapping Each AP coverage area is called a “cell” Wireless nodes can roam between cells Channel 1 Channel 1 Channel 11 AP AP AP AP AP AP Channel 1 Channel 6 Channel 6 Wireless Cells CPSC 441: Wireless

  17. B A C C C’s signal strength A’s signal strength B A space IEEE 802.11: multiple access • avoid collisions: > 1 nodes transmitting at same time • 802.11: CSMA – (Carrier Sense Multiple Access / Collision Detection) sense before transmitting • don’t collide with ongoing transmission by other node • 802.11: no collision detection! • half-duplex: antenna can’t 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/CA (Collision Avoidance) CPSC 441: Wireless

  18. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA If the medium is continuously idle for DCF Interframe Space (DIFS) duration then only it is allowed to transmit a frame. DIFS = SIFS + (2 * Slot time) 802.11 sender 1 if sense channel idle for DIFSthen transmit entire frame 2 ifsense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires 3 if no ACK then increase random backoff interval, repeat step 2 802.11 receiver - if frame received OK return ACK after SIFS (service model is connectionless, acked) sender receiver Short Interframe Space (SIFS), is the small time interval between the data frame and its acknowledgment. SIFS are found in IEEE 802.11 networks. CPSC 441: Wireless

  19. Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames • sender first transmits small request-to-send (RTS) packets to base station using CSMA • RTS may still collide with each other (but they’re short) • BS broadcasts clear-to-send CTS to host in response to RTS • RTS heard by all nodes because of broadcast property • sender transmits (large) data frame • other stations defer transmissions until it is done Avoid data frame collisions completely using small reservation packets! CPSC 441: Wireless

  20. RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) Collision Avoidance: RTS-CTS exchange B A AP defer time CPSC 441: Wireless

  21. 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 802.11 frame: addressing 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 CPSC 441: Wireless

  22. router AP Internet R1 MAC addr AP 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 802.11 frame: addressing H1 R1 CPSC 441: Wireless

  23. 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 802.11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) frame type (RTS, CTS, ACK, data) CPSC 441: Wireless

  24. Outline • Introduction • Standards and Link Characteristics • IEEE 802.11 Wireless LANS • Mobility • Wireless/Mobility Performance Issues • Summary CPSC 441: Wireless

  25. no mobility high mobility What is mobility? • spectrum of mobility, from thenetwork perspective: mobile wireless user, using same access point mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) mobile user, connecting/ disconnecting from network using DHCP. CPSC 441: Wireless

  26. 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 switch port can be used to reach H1 router 802.11: mobility within same subnet hub or switch BBS 1 AP 1 AP 2 H1 BBS 2 CPSC 441: Wireless

  27. Mobility: Vocabulary home network: permanent “home” of mobile (e.g., 128.119.40/24) home agent: entity that will perform mobility functions on 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 CPSC 441: Wireless

  28. 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 home agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile CPSC 441: Wireless

  29. How do you contact a mobile friend: • search all phone books? • call her parents? • expect her to let you know where he/she is? I wonder where Alice moved to? Consider friend frequently changing addresses, how do you find her? CPSC 441: Wireless

  30. Mobility: approaches • 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: communication from correspondent to mobile goes through home agent, then forwarded to remote • direct routing: correspondent gets foreign address of mobile, sends directly to mobile CPSC 441: Wireless

  31. Mobility: approaches • 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: communication from correspondent to mobile goes through home agent, then forwarded to remote • direct routing: correspondent gets foreign address of mobile, sends directly to mobile not scalable to millions of mobiles CPSC 441: Wireless

  32. mobile contacts foreign agent on entering visited network foreign agent contacts home agent home: “this mobile is resident in my network” 1 2 Mobility: registration visited network home network End result: • Foreign Agent (FA) knows about mobile • Home Agent (HA) knows location of mobile wide area network CPSC 441: Wireless

  33. 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 3 2 4 1 Mobility via Indirect Routing visited network home network wide area network CPSC 441: Wireless

  34. 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 CPSC 441: Wireless

  35. 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: ongoing connections can be maintained! CPSC 441: Wireless

  36. foreign agent receives packets, forwards to mobile mobile replies directly to correspondent 4 2 4 1 3 Mobility via Direct Routing correspondent forwards to foreign agent visited network home network wide area network correspondent requests, receives foreign address of mobile CPSC 441: Wireless

  37. 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 changes visited network? CPSC 441: Wireless

  38. 1 2 4 3 5 Accommodating mobility with direct routing • 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 CPSC 441: Wireless

  39. Mobile IP • RFC 3220 • has many features we’ve seen: • home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) • three components to standard: • indirect routing of datagrams • agent discovery • registration with home agent CPSC 441: Wireless

  40. Outline • Introduction • Standards and Link Characteristics • IEEE 802.11 Wireless LANS • Mobility • Wireless/Mobility Performance Issues • Summary CPSC 441: Wireless

  41. Wireless, mobility: impact on higher layer protocols • logically, impact should be minimal … • best effort service model remains unchanged • TCP and UDP can (and do) run over wireless, mobile • … but performance-wise: • packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoffs from mobility and transient connectivity • TCP interprets loss as congestion, will decrease congestion window un-necessarily • delay impairments for real-time traffic • limited bandwidth of wireless links CPSC 441: Wireless

  42. Wireless wireless links: capacity, distance channel impairments CDMA IEEE 802.11 (“WiFi”) CSMA/CA reflects wireless channel characteristics Mobility principles: addressing, routing to mobile users home, visited networks direct, indirect routing care-of-addresses Mobile IP impact on higher-layer protocols Summary CPSC 441: Wireless

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