Wireless and Mobile Networks Reading: Sections 2.8 and 4.2.5Goals of Today’s LectureWireless Links and Wireless NetworksWireless Links: High Bit Error RateSlide 5Slide 6Dealing With Bit ErrorsWireless Links: Broadcast LimitationsSlide 9Example Wireless Link TechnologiesWireless Network: Wireless LinkWireless Network: Wireless HostsWireless Network: Base StationWireless Network: InfrastructureScenario #1: Infrastructure ModeScenario #2: Ad Hoc NetworksInfrastructure vs. Ad HocDifferent Types of Wireless NetworksWiFi: 802.11 Wireless LANs802.11 LAN ArchitectureChannels and AssociationMobility Within the Same SubnetCSMA: Carrier Sense, Multiple AccessCA: Collision Avoidance, Not DetectionMedium Access Control in 802.11Host MobilityVarying Degrees of User MobilityMaintaining Ongoing TransfersE.g., Keeping Track of Mobile FriendsLetting Routing Protocol Handle ItExample: Boeing Connexion ServiceSlide 32Summary: Letting Routing Handle ItHome Network and Home AgentVisited Network and Care-of AddressMobility: RegistrationMobility via Indirect RoutingIndirect Routing: Efficiency IssuesMobility via Direct RoutingImpact on Higher-Layer ProtocolsConclusions1Wireless and Mobile NetworksReading: Sections 2.8 and 4.2.5 COS 461: Computer NetworksSpring 2007 (MW 1:30-2:50 in Friend 004)Jennifer RexfordTeaching Assistant: Ioannis Avramopoulos http://www.cs.princeton.edu/courses/archive/spring07/cos461/2Goals of Today’s Lecture•Wireless links: unique channel characteristics–High, time-varying bit-error rate–Broadcast where some nodes can’t hear each other•Mobile hosts: addressing and routing challenges–Keeping track of the host’s changing attachment point–Maintaining a data transfer as the host moves•Two specific technologies–Wireless: 802.11 wireless LAN (aka “WiFi”)–Mobility: Mobile IPMany slides adapted from Jim Kurose’s lectures at UMass-Amherst3Wireless Links and Wireless Networks4Wireless Links: High Bit Error Rate•Decreasing signal strength–Disperses as it travels greater distance–Attenuates as it passes through matter5Wireless Links: High Bit Error Rate•Interference from other sources–Radio sources in same frequency band–E.g., 2.4 GHz wireless phone interferes with 802.11b wireless LAN–Electromagnetic noise (e.g., microwave oven)6Wireless Links: High Bit Error Rate•Multi-path propagation–Electromagnetic waves reflect off objects–Taking many paths of different lengths–Causing blurring of signal at the receiverreceivertransmitter7Dealing With Bit Errors•Wireless vs. wired links–Wired: most loss is due to congestion–Wireless: higher, time-varying bit-error ate•Dealing with high bit-error rates–Sender could increase transmission powerRequires more energy (bad for battery-powered hosts)Creates more interference with other senders–Stronger error detection and recoveryMore powerful error detection codesLink-layer retransmission of corrupted frames8Wireless Links: Broadcast Limitations•Wired broadcast links–E.g., Ethernet bridging, in wired LANs–All nodes receive transmissions from all other nodes•Wireless broadcast: hidden terminal problemABC•A and B hear each other•B and C hear each other•But, A and C do notSo, A and C are unawareof their interference at B.9Wireless Links: Broadcast Limitations•Wired broadcast links–E.g., Ethernet bridging, in wired LANs–All nodes receive transmissions from all other nodes•Wireless broadcast: fading over distanceABCA’s signalstrengthspaceC’s signalstrength•A and B hear each other•B and C hear each other•But, A and C do notSo, A and C are unawareof their interference at B.10Example Wireless Link Technologies•Data networks–Indoor (10-30 meters)802.11n: 200 Mbps802.11a and g: 54 Mbps802.11b: 5-11 Mbps802.15.1: 1 Mbps–Outdoor (50 meters to 20 kmeters)802.11 and g point-to-point: 54 MbpsWiMax: 5-11 Mbps•Cellular networks, outdoors–3G enhanced: 4 Mbps–3G: 384 Kbps–2G: 56 Kbps11Wireless Network: Wireless Linknetwork infrastructureWireless link•Typically used to connect mobile(s) to base station•Also used as backbone link •Multiple access protocol coordinates link access12Wireless Network: Wireless Hostsnetwork infrastructureWireless host•Laptop, PDA, IP phone•Run applications•May be stationary (non-mobile) or mobile13Wireless Network: Base Stationnetwork infrastructureBase station•Typically connected to wired network•Relay responsible for sending packets between wired network and wireless host(s) in its “area”•E.g., cell towers, 802.11 access points14Wireless Network: Infrastructurenetwork infrastructureNetwork infrastructure•Larger network with which a wireless host wants to communicate•Typically a wired network•Provides traditional network services•May not always exist15Scenario #1: Infrastructure Modenetwork infrastructureInfrastructure mode•Base station connects mobiles into wired network•Network provides services (addressing, routing, DNS)•Handoff: mobile changes base station providing connection to wired network16Scenario #2: Ad Hoc NetworksAd hoc mode•No base stations•Nodes can only transmit to other nodes within link coverage•Nodes self-organize and route among themselves17Infrastructure vs. Ad Hoc•Infrastructure mode–Wireless hosts are associated with a base station–Traditional services provided by the connected network–E.g., address assignment, routing, and DNS resolution•Ad hoc networks–Wireless hosts have no infrastructure to connect to–Hosts themselves must provide network services•Similar in spirit to the difference between–Client-server communication–Peer-to-peer communication18Different Types of Wireless NetworksInfrastructure-based Infrastructure-lessSingle hop Base station connected to larger wired network (e.g., WiFi wireless LAN, and cellular telephony networks)No wired network; one node coordinates the transmissions of the others (e.g., Bluetooth, and ad hoc 802.11)Multi-hop Base station exists, but some nodes must relay through other nodes (e.g., wireless sensor networks, and wireless mesh networksNo base station exists, and some nodes must relay through others (e.g., mobile ad hoc networks, like vehicular ad hoc networks)19WiFi: 802.11 Wireless LANs20802.11 LAN Architecture•Access Point (AP)–Base station that communicates with the wireless hosts•Basic Service Set (BSS)–Coverage of one AP–AP acts as the master–Identified by an
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