13 - Wireless LANsSlide 2Chapter 6: Wireless and Mobile NetworksElements of a wireless networkSlide 5Slide 6Slide 7Slide 8Wireless Link CharacteristicsIEEE 802.11 Wireless LAN802.11 LAN architecture802.11: Channels, associationIEEE 802.11: multiple accessIEEE 802.11 MAC Protocol: CSMA/CACollision Detection vs. Collision AvoidanceSlide 16Avoiding collisions (optional scheme )Collision Avoidance: RTS-CTS exchangeSlide 19802.11 frame: addressingSlide 21Slide 22Cellular networks: the first hopCellular standards: brief surveySlide 25Slide 26What is mobility?Mobility: VocabularyMobility: more vocabularyHow do you contact a mobile friend:Mobility: approachesSlide 32Mobility: registrationMobility via Indirect RoutingIndirect Routing: commentsIndirect Routing: moving between networksMobility via Direct RoutingMobility via Direct Routing: commentsAccommodating mobility with direct routingMobile IPMobile IP: agent discoveryMobile IP: registration exampleMobile IP: indirect routingWireless, mobility: impact on higher layer protocolsChapter 6 Summary6: Wireless and Mobile Networks 6-113 - Wireless LANs6: Wireless and Mobile Networks 6-2Chapter 6Wireless and Mobile NetworksComputer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004. A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.Thanks and enjoy! JFK/KWRAll material copyright 1996-2004J.F Kurose and K.W. Ross, All Rights Reserved6: Wireless and Mobile Networks 6-3Chapter 6: Wireless and Mobile NetworksBackground: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!computer nets: 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 network6: Wireless and Mobile Networks 6-4Elements of a wireless networknetwork infrastructurewireless hostslaptop, PDA, IP phonerun applicationsmay be stationary (non-mobile) or mobilewireless does not always mean mobility6: Wireless and Mobile Networks 6-5Elements of a wireless networknetwork infrastructure base 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 points6: Wireless and Mobile Networks 6-6Elements of a wireless networknetwork infrastructure wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates, transmission distance6: Wireless and Mobile Networks 6-7Elements of a wireless networknetwork infrastructure infrastructure modebase station connects mobiles into wired networkhandoff: mobile changes base station providing connection into wired network6: Wireless and Mobile Networks 6-8Elements of a wireless networkAd hoc modeno base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network: route among themselves6: Wireless and Mobile Networks 6-9Wireless Link CharacteristicsDifferences from 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…. make communication across (even a point to point) wireless link much more “difficult”6: Wireless and Mobile Networks 6-10IEEE 802.11 Wireless LAN802.11b2.4-5 GHz unlicensed radio spectrum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2.4-5 GHz rangeup to 54 Mbps• All use CSMA/CA for multiple access• All have base-station and ad-hoc network versions6: Wireless and Mobile Networks 6-11802.11 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): base stationad hoc mode: hosts onlyBSS 1BSS 2Internethub, switchor routerAPAP6: Wireless and Mobile Networks 6-12802.11: Channels, association802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesAP admin chooses frequency for APinterference possible: channel can be same as that chosen by neighboring AP!host: must associate with an APscans channels, listening for beacon frames containing AP’s name (SSID) and MAC addressselects AP to associate withmay perform authenticationwill typically run DHCP to get IP address in AP’s subnet6-13IEEE 802.11: multiple accessavoid collisions: 2+ nodes transmitting at same time802.11: CSMA - sense before transmittingdon’t collide with ongoing transmission by other node802.11: 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transmitter is turned off to conserve powergoal: avoid collisions: CSMA/C(ollision)A(voidance)ABCABCA’s signalstrengthspaceC’s signalstrength6: Wireless and Mobile Networks 6-14IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for DIFS (Distributed Inter-Frame Space) 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