Wireless Local Area Networks (WLANs)WiFiIEEE 802.11 FeaturesHidden Node Problem4-Way HandshakeIEEE 802.11 MACIEEE 802.11 ArchitectureIEEE 802.11 Architecture (Cont)IEEE 802.11 PrioritiesTime Critical ServicesNorth American ChannelsIEEE 802.11 Physical LayersIEEE 802.11 ActivitiesIEEE 802.11 Activities (Cont)802.11n802.11nMIMOSmart AntennasIEEE 802.11n StatusTGnSync vs WWiseIEEE 802.11 DCF BackoffDFSDFS: Example (Cont)IEEE 802.11e QoSEnhanced DCFECDF BurstingSummaryReading AssignmentHomework 55-1©2006 Raj JainCSE574sWashington University in St. LouisWireless Local Area Wireless Local Area Networks (WLANs)Networks (WLANs)Raj JainProfessor of CSE Washington University in Saint LouisSaint Louis, MO [email protected] slides are available on-line at:http://www.cse.wustl.edu/~jain/cse574-06/5-2©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 1. Features2. MAC3. Physical Layers4. Current Activities5. Next Generation: 802.11n6. Enhanced Quality of Service: 802.11eOverviewOverview5-3©2006 Raj JainCSE574sWashington University in St. LouisWiFiWiFi! Almost all wireless LANs now are IEEE 802.11 based! Competing technologies, e.g., HiperLAN can’t compete on volume and cost! 802.11 is also known as WiFi = “Wireless Fidelity”! Fidelity = Compatibility between wireless equipment from different manufacturers! WiFi Alliance is a non-profit organization that does the compatibility testing (WiFi.org)5-4©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 FeaturesIEEE 802.11 Features! Original 802.11 at 1 and 2 Mbps! Supports both Ad-hoc and base-stations! Spread Spectrum ⇒ No licensing required.Three Phys: Direct Sequence, Frequency Hopping, 915-MHz, 2.4 GHz (Worldwide ISM), 5.2 GHz, and Diffused Infrared (850-900 nm) bands. ! Supports multiple priorities! Supports time-critical and data traffic! Power management allows a node to doze off5-5©2006 Raj JainCSE574sWashington University in St. LouisHidden Node ProblemHidden Node Problem! C cannot hear A. It may start transmitting while A is also transmitting ⇒ A and C can't detect collision.! Only the receiver can help avoid collisionsA B C5-6©2006 Raj JainCSE574sWashington University in St. Louis44--Way HandshakeWay HandshakeAccessPointAccessPointMobileNodeMobileNodeReady to sendDataClear to sendAck5-7©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 MACIEEE 802.11 MAC! Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)! Listen before you talk. If the medium is busy, the transmitter backs off for a random period.! Avoids collision by sending a short message: Ready to send (RTS)RTS contains dest. address and duration of message.Tells everyone to backoff for the duration.! Destination sends: Clear to send (CTS)Other stations set their network allocation vector (NAV) and do not transmit for that duration! Can not detect collision ⇒ Each packet is acked.! MAC level retransmission if not acked.5-8©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 ArchitectureIEEE 802.11 ArchitectureStationStationStationStationStationStationStationStationAd-hocStationAd-hocStationAd-hocStationAd-hocStationAccessPointAccessPointAccessPointAccessPointBasic Service Set2nd BSSServerServerAd-hocnetworkIBSSDistribution System5-9©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 Architecture (Cont)IEEE 802.11 Architecture (Cont)! Basic Service Area (BSA) = Cell! Each BSA may have several wireless LANs! Distribution System (DS) - wired backbone! Extended Service Area (ESA) = Multiple BSAs interconnected via Access Points (AP)! Basic Service Set (BSS) = Set of stations associated with an AP! Extended Service Set (ESS) = Set of stations in an ESA! Independent Basic Service Set (IBSS): Set of computers in ad-hoc mode. May not be connected to wired backbone.! Ad-hoc networks coexist and interoperate with infrastructure-based networks5-10©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 PrioritiesIEEE 802.11 Priorities! Initial interframe space (IFS)! Highest priority frames, e.g., Acks, use short IFS (SIFS)! Medium priority time-critical frames use “Point Coordination Function IFS” (PIFS)! Asynchronous data frames use “Distributed coordination function IFS” (DIFS)BusySIFSPIFSDIFSContention WindowRandom BackoffCarrier SensedTimeFrame5-11©2006 Raj JainCSE574sWashington University in St. LouisContention-Free PeriodTime Critical ServicesTime Critical Services! Timer critical services use Point Coordination Function! The point coordinator allows only one station to access! Coordinator sends a beacon frame to all stations.Then uses a polling frame to allow a particular station to have contention-free access! Contention Free Period (CFP) varies with the load.TimeBeaconDCF AccessPCF AccessSuper FrameContention Period5-12©2006 Raj JainCSE574sWashington University in St. LouisNorth American ChannelsNorth American Channels5 GHz Band: 12 non-overlapping channelsChannel 1Channel 3Channel 5Channel 6Channel 7Channel 9Channel 112402 2422 2432 2442 2452 2462 2472 248224122483.524002.4 GHz Band: Only 3 non-overlapping channels5180 5200 5220 5240 5260 5280 5300 5320 5350515036 40 44 48 52 56 60 645725 5745 5765 5785 5805 5825149 153 157 1615-13©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 Physical LayersIEEE 802.11 Physical Layers! Issued in four stages! First part in 1997: IEEE 802.11 " Includes MAC layer and three physical layer specifications" Two in 2.4-GHz band and one infrared" All operating at 1 and 2 Mbps! Two additional parts in 1999:" IEEE 802.11a-1999: 5-GHz band, 54 Mbps/20 MHz, OFDM" IEEE 802.11b-1999: 2.4 GHz band, 11 Mbps/20 MHz! Fourth part:" IEEE 802.11g-2003 : 2.4 GHz band, 54 Mbps/20 MHz, OFDM5-14©2006 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 ActivitiesIEEE 802.11 Activities! 802.11c: Bridge Operation (Completed. Added to IEEE 802.1D)! 802.11d: Global Harmonization (PHYs for other countries. Published as IEEE Std 802.11d-2001)! 802.11e: Quality of Service. IEEE Std 802.11e-2005 ! 802.11f: Inter-Access Point Protocol (Published as IEEE Std Std 802.11F-2003)! 802.11h: Dynamic Frequency Selection and transmit power control to satisfy 5GHz band operation in Europe. Published as IEEE Std 802.11h-2003! 802.11i: MAC Enhancements for Enhanced Security. Published as IEEE Std 802.11i-2004! 802.11j: 4.9-5 GHz operation in Japan. IEEE Std 802.11j-2004! 802.11k: Radio Resource Measurement interface to
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