OverviewSwitches: dedicated accessMore on SwitchesInstitutional networkSwitches vs. RoutersSummary comparisonWireless and Mobile NetworksOutlineElements of a wireless networkSlide 10Slide 11Characteristics of selected wireless link standardsSlide 13Slide 14Wireless Link CharacteristicsWireless network characteristicsSlide 17IEEE 802.11 Wireless LAN802.11 LAN architecture802.11: Channels, associationIEEE 802.11: multiple accessIEEE 802.11 MAC Protocol: CSMA/CAAvoiding collisions (more)Collision Avoidance: RTS-CTS exchange802.11 frame: addressingThe Current Internet: Connectivity and ProcessingOverviewEthernetHubs, bridges, and switchesWireless links and LANsSwitches: dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions; full duplexSwitching: A-to-A’ and B-to-B’ simultaneously, no collisionsswitchAA’BB’CC’More on Switchescut-through switching: frame forwarded from input to output port without first collecting entire frameslight reduction in latencycombinations of shared/dedicated, 10/100/1000 Mbps interfacesInstitutional networkhubhubhubswitchto externalnetworkrouterIP subnetmail serverweb serverSwitches vs. Routersboth store-and-forward devicesrouters: network layer devices (examine network layer headers)switches are link layer devicesrouters maintain routing tables, implement routing algorithmsswitches maintain switch tables, implement filtering, learning algorithmsSummary comparisonWireless 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 networkOutlineIntroduction WirelessWireless links, characteristicsIEEE 802.11 wireless LANs (“wi-fi”)Cellular Internet Accessarchitecturestandards (e.g., GSM)Elements of a wireless networknetwork infrastructurewireless hostslaptop, PDA, IP phonerun applicationsmay be stationary (non-mobile) or mobilewireless does not always mean mobilityElements 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 pointsElements 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 distanceCharacteristics of selected wireless link standards384 Kbps384 Kbps56 Kbps56 Kbps54 Mbps54 Mbps5-11 Mbps5-11 Mbps1 Mbps1 Mbps802.15802.11b802.11{a,g}IS-95 CDMA, GSMUMTS/WCDMA, CDMA2000802.162G3GIndoor10 – 30mOutdoor50 – 200mMid rangeoutdoor200m – 4KmLong rangeoutdoor5Km – 20KmElements of a wireless networknetwork infrastructure infrastructure modebase station connects mobiles into wired networkhandoff: mobile changes base station providing connection into wired networkElements 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 themselvesWireless 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 ad destination at slightly different times…. make communication across (even a point to point) wireless link much more “difficult”Wireless network characteristicsMultiple wireless senders and receivers create additional problems (beyond multiple access):ABCHidden terminal problemB, A hear each otherB, C hear each otherA, C can not hear each othermeans A, C unaware of their interference at BABCA’s signalstrengthspaceC’s signalstrengthSignal fading:B, A hear each otherB, C hear each otherA, C can not hear each other interferring at BOutlineIntroduction WirelessWireless links, characteristicsIEEE 802.11 wireless LANs (“wi-fi”)Cellular Internet Accessarchitecturestandards (e.g., GSM)IEEE 802.11 Wireless LAN802.11b2.4-5 GHz unlicensed radio spectrumup to 11 Mbps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 versions802.11 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 routerAPAP802.11: 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 subnetIEEE 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goal: avoid collisions: CSMA/C(ollision)A(voidance)ABCABCA’s signalstrengthspaceC’s signalstrengthIEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for DIFS then transmit entire frame (no CD)2 if sense channel busy then start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff interval, repeat 2802.11 receiver- if frame received OK return ACK after SIFS (ACK needed due to hidden terminal