1Wireless Data NetworkingWireless Data NetworkingIEEE 802.11 &IEEE 802.11 &Overview of IEEE 802.11bOverview of IEEE 802.11bDr. Arian DurresiThe Ohio State UniversityColumbus, OH [email protected]❑Wireless Application Market❑Wireless WANs❑Wireless LANs❑ISM band❑Spread Spectrum❑Wireless LAN standard: IEEE 802.11❑Overview of IEEE 802.11b.OverviewOverview3Mobile Application Market by 2005Mobile Application Market by 200545%26%16%8%5%Mobile OfficeField SalesField ServicesTransportationPersonal Communications[Dayem 97]4WirelessWireless WANs WANs❑Data over Analog and Digital Cellular, ARDIS, RAMMobile Data, Cellular Digital Packet Data (CDPD)❑4.8 kbps to 19.2 kbps nominal,❑Packetized short transmission, Email, stock quotes,weather, Wired backbone❑3G Goals:❍Multi-rate: 2Mbps indoor, 384 kbps pedestrian, 144kbps mobile❍Multi-service: Mobile Internet, Multimedia, packetand circuit switched services❍Multi-cell: Seamless coverage across pico-, micro-,and macro-cells5Wireless LANsWireless LANs❑High speed: > 1Mbps❑Real time voice not supported❑About 50 m coverage radius❑Pedestrian speed❑Industry Scientific Medical (ISM) band LANs❍Use Spread Spectrum not to interfere with primaryusers❍IEEE 802.11, IEEE 802.11a, IEEE 802.11b,Hiperlan❑ Infrared LANs: Limited applications❑ Unlicensed Personal Communication Services(UPCS): will use dedicated bandwidth: 1910-1930MHz6 Wireless Wireless WANs WANs versus LANs versus LANs❑Different from wired WANs versus LANs❍In wireless direct competition❍Wired LANs: high bandwidth, cheap, everywhere❑Business issues❍Services❍Coverage❍Price❑Technical Issues❍Bandwidth, capacity, mobility❍Security❍Software applications7ISM band in USISM band in USBand(GHz)BandwidthMHzPowerLevelSpreadSpectrum0.902 26 1W FHSS,DSSS2.4 83.5 1W FHSS,DSSS5.725 125 1W FHSS,DSSS24 250 50mv/m@3mNA8Why 2.4 GHz?Why 2.4 GHz?5.2 GHz Hiperlan2.4 GHz (ISM)Europe5.2 GHz (Future?)2.4 GHz (ISM)JapanU.S.5.8 GHz (ISM)5.2 GHz (Future?)2.4 GHz (ISM)1.9 GHz UPCS915 MHz (ISM)IEEE 802.119Frequency HoppingFrequency HoppingSpread SpectrumSpread Spectrum❑Pseudo-random frequency hopping❑Spreads the power over a wide spectrum⇒ Spread Spectrum❑Developed initially for military❑Patented by actress Hedy Lamarr❑Narrowband interference can't jamFrequencyTime50 ms10SpectrumSpectrumSignalNoiseSignalNoise(a) Normal (b) Frequency Hopping11Direct-SequenceDirect-SequenceSpread SpectrumSpread Spectrum❑Spreading factor = Code bits/data bit, 10-100commercial (Min 10 by FCC), 10,000 for military❑Signal bandwidth >10 × data bandwidth❑Code sequence synchronization❑Correlation between codes ⇒Interference⇒ OrthogonalFrequencyTime5µs01001011011011010010Data0112DS SpectrumDS SpectrumTime Domain Frequency Domain(a) Data(b) CodeFrequencyFrequencyTime13IEEE 802.11 FeaturesIEEE 802.11 Features❑Standard for WLANs approved by IEEE 802.11Working Group in June 1997❑1 and 2 Mbps❑Asynchronous, connectionless service❑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 off14Hidden 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 C154-Way Handshake4-Way HandshakeAccessPointAccessPointMobileNodeMobileNodeReady To SendDataClear To SendAck• Check if carrier is present• If no carrier, check the table of CTS if anyone is going to transmit at that time• If no carrier and the table indicates the medium free then transmit16IEEE 802.11 MACIEEE 802.11 MAC❑Two access methods:❍Distributed Coordination Function❍Point Coordination Function❑Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA)❑Ethernet CSMA/CD Collision Detection❑Collision Detection not suitable for wireless:❍Need full duplex radio => increase the price❍Not all stations can hear each other❑CA: Listen before you talk. If the medium is busy, thetransmitter backs off for a random period.❑CA: Can not detect collision ⇒ Each packet is acked17IEEE 802.11 MAC (IEEE 802.11 MAC (contcont.).)❑MAC level retransmission if not acked❑Virtual Carrier Sense: Avoids collision by sending ashort message: Ready To Send (RTS):❑Contains source and dest. addresses + duration ofmessage. Tells everyone to backoff for theduration.❑Destination sends: Clear To Send (CTS)❑All stations receiving RTS and/or CTS set their timer:NAV (Network Allocation Vector) for the givenduration❑RTS/CTS short=>reduced overhead of collisions18IEEE 802.11 MAC (IEEE 802.11 MAC (contcont.).)❑Why shorter packets than wired?❍Higher BER=>increased probability of packetcorruption❍In case of packet corruption, smaller the packet-less overhead by retransmission❍In Frequency Hopping (jump every 20 msec) bettershort packets❑To be able to deal with Ethernet packet=> Fragmentation and Reassembly❍Send and wait for each fragment❍Can transmit to others while waiting19Peer-to-Peer orPeer-to-Peer orBase Stations?Base Stations?❑Ad-hoc (Autonomous) Group:❍Two stations can communicate❍All stations have the same logic❍No infrastructure, Suitable for small area❑Infrastructure Based: Access points (base units)❍Stations can be simpler than bases.❍Base provide connection for off-network traffic❍Base provides location tracking, directory,authentication ⇒ Scalable to large networks❑IEEE 802.11 provides both.20Architecture of 802.11Architecture of 802.11MAC LayerPhysical Layer ConvergenceProcedure (PLCP)Physical Medium Dependent(PMD sublayerPHYManagement• PLCP simplifies the interface to MAC• PMD provides a clear channel assessment mechanism, a transmission and a reception mechanism21IEEE 802.11 ArchitectureIEEE 802.11 ArchitectureStationStationStationStationStationStationStationStationAd-hocStationAd-hocStationAd-hocStationAd-hocStationAccessPointAccessPointAccessPointAccessPointBasic Service Set2nd BSSServerServerAd-hocnetworkDistribution System22BSS ServicesBSS Services❑Coordination Function: distributed in ad hoc or in AP❑Join a BSS, needs to get synchronization informationfrom AP:❍Passive Scanning: waits to receive a Beacon Framesent
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