13 -Wireless LANs13 Wireless LANs6: Wireless and Mobile Networks 6-1Chapter 6pWireless and Mobile NetworksNetworksC mp t N t kin : A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). Computer Networking: A Top Down Approach Featuring the Internet, 3rdedition. gy(y )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, th t ti th i ( ft ll ’d lik l t b k!)Jim Kurose, Keith RossAddison-Wesley, July 2004. 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/KWR6: Wireless and Mobile Networks 6-2Thanks and enjoy! JFK/KWRAll material copyright 1996-2004J.F Kurose and K.W. Ross, All Rights ReservedChapter 6: Wireless and Mobile Networkshapt r 6 W r ss an Mo N twor sBackground: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!computer nets: laptops palmtops PDAs 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 handling mobile user who changes point of attachment to network6: Wireless and Mobile Networks 6-3Elements of a wireless networkwireless hosts laptop, PDA, IP phone run applications may be stationary (bl ) blnetwork infrastructure(non-mobile) or mobile wireless does notalways mean mobility6: Wireless and Mobile Networks 6-4Elements of a wireless networkbase stationtypically connected to typ cally connected to wired network relay - responsible for sending packets b id network infrastructurebetween wired network and wireless host(s) in its “area”e g cell towers e.g., cell towers 802.11 access points 6: Wireless and Mobile Networks 6-5Elements of a wireless networkwireless linktypically used to typ cally used to connect mobile(s) to base station also used as backbone li k network infrastructurelink  multiple access protocol coordinates link access link access  various data rates, transmission distance6: Wireless and Mobile Networks 6-6Elements of a wireless networkinfrastructure modebase station connects base stat on connects mobiles into wired network handoff: mobile h b i network infrastructurechanges base station providing connection into wired network6: Wireless and Mobile Networks 6-7Elements of a wireless networkAd hoc modeno base stationsno base stat ons nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among network: route among themselves6: Wireless and Mobile Networks 6-8Wireless Link CharacteristicsWreless Lnk CharacterstcsDifferences from wired link …. decreased signal strength: radio signal attenuates as it propagates through matter (path loss)(path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e g phone); devices shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving at destination at objects ground, arriving at destination at slightly different times make communication across (even a point to point) 6: Wireless and Mobile Networks 6-9…. make communication across (even a point to point) wireless link much more “difficult”IEEE 802.11 Wireless LANIEEE 80 . W reless LAN 802.11b  802.11a 2.4-5 GHz unlicensed radio spectrumup to 11 Mbps 5-6 GHz range up to 54 Mbps802 11up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer802.11g 2.4-5 GHz range up to 54 Mbpsphysical layer widely deployed, using base stations• All use CSMA/CA for multiple access• All have base-station and ad-hoc network versions6: Wireless and Mobile Networks 6-10versions802.11 LAN architecture wireless host communicates with base station base station = access point (AP)Internet Basic Service Set (BSS)(aka “cell”) in infrastructure mode contains:hub, switchor routerAP wireless hosts access point (AP): base stationBSS 1or routerAPAP ad hoc mode: hosts onlyAP6: Wireless and Mobile Networks 6-11BSS 2802.11: Channels, association80 . Channels, assoc at on 802.11b: 2.4GHz-2.485GHz spectrum divided into p11 channels at different frequencies AP admin chooses frequency for APinterference possible: channel can be same as interference possible: channel can be same as that chosen by neighboring AP! host: must associatewith an AP scans channels, listening for beacon framescontaining AP’s name (SSID) and MAC addressselects AP to associate withselects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in AP’s b6: Wireless and Mobile Networks 6-12subnetIEEE 802.11: multiple access avoid collisions: 2+nodes transmitting at same time 802.11: CSMA - sense before transmittingfg don’t collide with ongoing transmission by other node 802.11: nocollision detection!diffi l i ( lli i ) h i i d 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)BCABCA’s signalstrengthC’s signalstrength6-13ABstrengthspacestrengthIEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idlefor DIFS (Distributed 1 if sense channel idlefor DIFS (Distributed Inter-Frame Space) then- transmit entire frame (no CD)2 if hl b thsenderreceiverDIFS2 ifsense channel busy then- start random backoff time- timer counts down while channel idledata- transmit when timer expires- if no ACK, increase random backoff interval, repeat 2SIFSACK,p802.11 receiver- if frame received OKreturn ACK after SIFS (Short InterACK6: Wireless and Mobile Networks 6-14return ACK after SIFS (Short