11-1©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. LouisDirectDirect--Sequence Spread SpectrumSequence 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OrthogonalFrequencyTime5μs01001011011011010010Data0111-10©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 walls101010111012101310141015101610171018109μwaveInfraredVisibleUltravioletx-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. Louis44--Way HandshakeWay 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 the duration. Destination sends: Clear to send (CTS) Can not detect collision ⇒ Each packet is acked. MAC level retransmission if not acked.11-22©2005 Raj JainCSE473sWashington University in St. LouisPeerPeer--toto--Peer or Base Stations?Peer or 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.11-23©2005 Raj