Wireless and Mobile Networks Reading: Sections 2.8 and 4.2.5Goals of Today’s LectureWidespread DeploymentWireless Links and Wireless NetworksWireless Links: High Bit Error RateSlide 6Slide 7Dealing With Bit ErrorsWireless Links: Broadcast LimitationsSlide 10Example 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., Keep Track of Friends on the MoveOption #1: Let Routing Protocol Handle ItExample: Boeing Connexion ServiceSlide 33Summary: Letting Routing Handle ItOption #2: Home Network and Home AgentVisited Network and Care-of AddressMobility: RegistrationMobility via Indirect RoutingIndirect Routing: Efficiency IssuesMobility via Direct RoutingMobility TodayImpact on Higher-Layer ProtocolsConclusions1Wireless and Mobile NetworksReading: Sections 2.8 and 4.2.5 COS 461: Computer NetworksSpring 2008 (MW 1:30-2:50 in COS 105)Jennifer RexfordTeaching Assistants: Sunghwan Ihm and Yaping Zhuhttp://www.cs.princeton.edu/courses/archive/spring08/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•Some specific examples–Wireless: 802.11 wireless LAN (aka “WiFi”)–Mobility: Boeing Connexion and Mobile IPMany slides adapted from Jim Kurose’s lectures at UMass-Amherst3Widespread Deployment•Worldwide cellular subscribers–1993: 34 million–2005: more than 2 billion–Now more than landline subscribers•Wireless local area networks–Wireless adapters built in to most laptops, and even PDAs–More than 220,000 known WiFi locations in 134 countries–Probably many, many more (e.g., home networks, corporate networks, …)4Wireless Links and Wireless Networks5Wireless Links: High Bit Error Rate•Decreasing signal strength–Disperses as it travels greater distance–Attenuates as it passes through matter6Wireless 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)7Wireless 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 receiverreceivertransmitter8Dealing 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 frames9Wireless 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.10Wireless 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.11Example 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 Kbps12Wireless Network: Wireless Linknetwork infrastructureWireless link•Typically used to connect mobile(s) to base station•Also used as backbone link •Multiple access protocol coordinates link access13Wireless Network: Wireless Hostsnetwork infrastructureWireless host•Laptop, PDA, IP phone•Run applications•May be stationary (non-mobile) or mobile14Wireless 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 points15Wireless 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 exist16Scenario #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 network17Scenario #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 themselves18Infrastructure 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 communication19Different 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
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