Wireless LANsOverviewMobile vs WirelessFrequency Hopping Spread SpectrumSpectrumSlow Frequency Hop Spread SpectrumFast Frequency Hop Spread SpectrumDirect-Sequence Spread SpectrumDS SpectrumCode Division Multiple Access (CDMA)CDMA ExampleCDMA Encoding and DecodingWireless LAN RequirementsSlide 16Infrared LANsIEEE 802.11 FeaturesHidden Node Problem4-Way HandshakeIEEE 802.11 MACPeer-to-Peer or Base Stations?IEEE 802.11 ArchitectureArchitectureIEEE 802.11 PrioritiesTime Critical ServicesPower ManagementFrame FormatMAC Frame Fields802.11 Address FieldsStation LocationIEEE 802.11 PhyWhy 2.4 GHz?FHSS PhyDSSS PhyInfrared Phy802.11 Physical LayersIEEE 802.11 Protocol Architecture802.11a802.11b802.11gSummaryReading AssignmentHomework11-1©2005 Raj JainCSE473sWashington University in St. LouisWireless LANsWireless LANsRaj Jain Washington UniversitySaint Louis, MO [email protected] slides are available on-line at:http://www.cse.wustl.edu/~jain/cse473-05/Ohio Highway Patrol11-2©2005 Raj JainCSE473sWashington University in St. LouisMobile vs WirelessSpread Spectrum and Code Division Multiple AccessWireless LANsIEEE 802.11 Features, MAC, Architecture, Priorities, Power Management, Frame Format802.11 PHYs: 802.11, 802.11a, 802.11b, 802.11gOverviewOverview11-3©2005 Raj JainCSE473sWashington University in St. LouisMobile vs WirelessMobile vs WirelessMobile vs StationaryWireless vs WiredWireless media sharing issuesMobile routing, addressing issuesMobileWireless11-5©2005 Raj JainCSE473sWashington University in St. LouisFrequency Hopping Spread SpectrumFrequency Hopping Spread 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 ms11-6©2005 Raj JainCSE473sWashington University in St. LouisSpectrumSpectrumSignalNoiseSignalNoise(a) Normal (b) Frequency Hopping11-7©2005 Raj JainCSE473sWashington University in St. LouisSlow Frequency Hop Spread Spectrum Slow Frequency Hop Spread Spectrum Two bits/symbol 4 frequencies/symbol (Multi FSK)Two-bit PN Sequence 4 Carrier ChannelsTwo symbols/Hop Slow Frequency hopping11-8©2005 Raj JainCSE473sWashington University in St. LouisFast Frequency Hop Spread Spectrum Fast Frequency Hop Spread Spectrum 2 bits/symbolTwo-bit pseudo-random number sequenceTwo hops/symbol  Fast Frequency hopping11-9©2005 Raj JainCSE473sWashington University in St. LouisDirect-Sequence Spread SpectrumDirect-Sequence Spread SpectrumSpreading factor = Code bits/data bit, 10-100 commercial (Min 10 by FCC), 10,000 for militarySignal bandwidth >10 × data bandwidthCode sequence synchronizationCorrelation between codes InterferenceOrthogonalFrequencyTime5s01001011011011010010Data0 111-10©2005 Raj JainCSE473sWashington University in St. LouisDS SpectrumDS SpectrumTime Domain Frequency Domain(a) Data(b) CodeFrequencyFrequencyTime11-11©2005 Raj JainCSE473sWashington University in St. LouisCode Division Multiple Access (CDMA)Code Division Multiple Access (CDMA)Multiplexing Technique used with spread spectrumStart with data signal rate DCalled bit data rateBreak each bit into k chips according to fixed pattern specific to each userUser’s codeNew channel has chip data rate kD chips per secondE.g. k=6, three users (A,B,C) communicating with base receiver RCode for A = <1,-1,-1,1,-1,1>Code for B = <1,1,-1,-1,1,1>Code for C = <1,1,-1,1,1,-1>11-12©2005 Raj JainCSE473sWashington University in St. LouisCDMA ExampleCDMA Example11-13©2005 Raj JainCSE473sWashington University in St. LouisCDMA Encoding and DecodingCDMA Encoding and Decoding11-15©2005 Raj JainCSE473sWashington University in St. LouisWireless LAN RequirementsWireless LAN RequirementsLow power consumption: Need long battery life Must not expect nodes to be up all the timeTransmission robustness and security:Interference prone and easily eavesdroppedCollocated network operation:Two or more wireless LANs in same areaLicense-free operationHandoff/roaming: Move from one cell to anotherDynamic configuration: Addition, deletion, and relocation of end systems without disruption to users11-16©2005 Raj JainCSE473sWashington University in St. LouisWireless LANsWireless LANsInfrared RadioSpread Spectrum902 MHz2.4 GHzProxim RangeLANNCR WaveLANTelesystems ArLANMotorola ALTAIRWindata FreeportPhotonicsCollaborativeInfraLANProxim RangeLAN2FHDSDSFHNarrowband5.7GHzLine of SightDiffuse11-17©2005 Raj JainCSE473sWashington University in St. LouisInfrared LANsInfrared LANsDirected-beam IR: Point-to-point linksRange depends on power - Can be kilometersUsed for building interconnect within line of sightOmni-directional:Single base station within line of sight of all other stations Typically, mounted on ceiling. Acts as a repeaterOther transceivers use directional beam aimed at baseDiffused configuration: Reflections from walls101010111012101310141015101610171018109waveInfraredVisibleUltravioletx-rays11-18©2005 Raj JainCSE473sWashington 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 off11-19©2005 Raj JainCSE473sWashington 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 C11-20©2005 Raj JainCSE473sWashington University in St. Louis4-Way Handshake4-Way HandshakeAccessPointAccessPointMobileNodeMobileNodeReady to sendDataClear to sendAck11-21©2005 Raj JainCSE473sWashington 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