Conducted and Wireless Media (Part II)Learning ObjectivesThe Light spectrumWireless media used for data transmissionOmnidirectional and Dish AntennasWireless MediaWireless TransmissionIEEE 802.11 WLAN standards originally licensed by the Wi-Fi AllianceWireless LAN (802.11 standard)Slide 10Slide 11Slide 12Slide 13Slide 14Using Extensible Authentication Protocol (EAP) for SecurityUsing Extensible Authentication Protocol (EAP) for Security, ContinuedSummary QuestionsSlide 18Conducted and Wireless Media (Part II)School of BusinessEastern Illinois University © Abdou Illia, Spring 2007(Week 7, Tuesday 2/21/2007)2Learning ObjectivesOutline characteristics of wireless mediaUnderstand Wireless LAN3The Light spectrumSunEarthVisible lightInfrared raysMicrowavesShort radio wavesLong radio wavesUltraviolet raysX raysGamma raysCosmic rays4Wireless media used for data transmissionInfrared light–Has many of the same characteristics as visible light–Travels in straight lines–Cannot penetrate solid objectsRadio waves–Travel in straight lines–Can penetrate through nonmetallic objects–Can travel long distances5Omnidirectional and Dish AntennasDish AntennaConcentrates incoming and outgoingsignals in a narrow range-----Must point at receiverGood for fixed subscribersOmnidirectional AntennaSignal spreads as a sphereRapid signal attenuation-----No need to point at receiverGood for mobile subscribers6Wireless MediaUse electromagnetic waves or electromagnetic radiation for data transmissionPropagation through space, and indirectly, through solid objectsMany problems:LaptopComm. TowerShadowZoneRadio waves tend to bounce off objects. Receiver can receive 2 or more signals.Thick objects can block the direct path. So, Receiver will be in a Shadow zone where it cannot well receive.Insecure: Easier to “intercept” messagesElectromagneticInterference (EMI) from Other stations,Microwave ovens, etcMultipathInterference+ Much more attenuation: Inverse Square law7Using multiple antennas or using square or fan-shaped antennas reduces Multipath InterferenceWireless Transmission8802.11b 802.11a 802.11g2.4 GHz 5 GHz 2.4 GHzUnlicensed Band≤11 Mbps≤ 54 Mbps ≤ 54 MbpsRated SpeedIEEE 802.11 WLAN standardsoriginally licensed by the Wi-Fi Alliance802.11n*2.4 GHz or 5 GHz≤ 540 Mbps* Under development0 HzFrequencySpectrumInfinityAM Radio service band: 535 kHz-1705 kHzFM Radio service band: 88 MHz-108 MHz802.11b WLAN: 2.4 GHz-2.5 GHz3 12 12# of channels 14802.11g uses Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme to achieve higher speed than 802.11bService bands divided into channelsAM radio channels have a 10KHz bandwidthFM radio channels: 200KHz bandwidth35m/100m25m/75m 25m/75mRange (Indoor/Outdoor)50m/125m9Wireless LAN (802.11 standard)SwitchClient PCServerWired LANAccessPoint AAccessPoint BUTPRadio wavesHandoffIf mobile computermoves to anotheraccess point,it switches serviceto that access pointLaptopMAC protocol: CSMA/CA+ACK802.11 Frame802.3 FrameTypical physical typology: Point-to-Multipoint10The Wireless Station sends a frame to a server via the access pointThe access point converts the 802.11 frame into an 802.3 Ethernet frame and sends the frame to the serverMobileStationAccessPointEthernetSwitchServer802.11Frame802.3FrameWireless LAN (802.11 standard)11The server responds, sending a frame to the access pointThe access point converts the 802.3 frame into an 802.11 frame and sends the frame to the mobile station.MobileStationAccessPointEthernetSwitchServer802.11Frame802.3FrameWireless LAN (802.11 standard)12CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)–Sending Station or Access point listens for trafficIf there is no traffic, can send if there has been no traffic for a specified amount of timeIf the specified amount of time has not been met, must wait for the specified amount of time. Can send if the line is still clearWireless LAN (802.11 standard)13CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)–Sending Station or Access point listens for trafficIf there is traffic, the sender must wait until traffic stopsThe sender must then set a random timer and must wait while the timer is runningIf there is no traffic when the station or access point finishes the wait, it may sendWireless LAN (802.11 standard)14ACK (Acknowledgement)–Receiver immediately sends back an acknowledgement–If sender does not receive the acknowledgement, retransmits using CSMA/CA–Wireless NIC implements CSMA/CA+ACK when it sends–Access point implements CSMA/CA+ACK when it sendsWireless LAN (802.11 standard)15Using Extensible Authentication Protocol (EAP) for SecurityAuthenticationServerSwitchNotebookWired Ethernet LANClient PCEAP Data2.EAP Data3.Authentication is stored on an authentication server, not in access pointsThis simplifies access points, making them cheaperCentral authentication data is easier to manage and changeEAP Data1.AccessPoint A16Using Extensible Authentication Protocol (EAP) for Security, ContinuedAuthenticationServerSwitchNotebookAccessPoint AWired Ethernet LANClient PCOK4.OKIf an OK is sent back, the access point may accept an association request from the clientIf a bad report is sent back, the access point may decline an association request from the client5.Accept17Summary Questionsa) Wireless transmission can experience propagation problems due to shadow zones and multipath interference. Explain.b) Describe the elements in a typical 802.11 Wireless LAN.c) What does handoff refer to?d) Explain how attenuation occurs in wireless communications: Inverse square law.18Summary QuestionsCan we implement a completely wireless LAN not connected to a Wired LAN? Explain.Yes. Using a switch with built-in Access Point, we can implement a Wireless LAN not connected to a Wired LAN. We can even implement a small completely wireless LAN without Access Point or Switch. In this case, Wireless NICs operate automatically in ad hoc mode, in which they talk directly to other wireless stations. This only works for small