2001/10/26 Prof. Huei-Wen Ferng 1 Chapter 4 Wireless LAN Technologies and Products

2001/10/26 Prof. Huei-Wen Ferng 2 Preliminary A Brief Introduction to Network and Communication Terminology

2001/10/26 Prof. Huei-Wen Ferng 3 Wireless Media Microwave Most wireless networking traffic is carried in microwave frequency bands Infrared Infrared is used in short-range wireless communication Unlicensed primary: ISM bands and medical fields without the intent of communication such as microwave ovensSome equipment in the industry, scientific, and medical fields without the intent of communication such as microwave ovens

2001/10/26 Prof. Huei-Wen Ferng 4 Wireless Media Unlicensed Secondary usage: ISM bands Unlicensed Secondary usage: ISM bands Use the spread spectrum technologyUse the spread spectrum technology They don ’ t interfere with the primary userThey don ’ t interfere with the primary user Why do we want unlicensed bands for WLAN? Why do we want unlicensed bands for WLAN? Roaming is a problem for a WLAN userRoaming is a problem for a WLAN user Short-range communication: Short-range communication: Bluetooth and InfraredBluetooth and Infrared

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2001/10/26 Prof. Huei-Wen Ferng 6 Applications Classified by Media Radio: 100 meters Radio: 100 meters Conference, campus, universeConference, campus, universe Infrared: 10 meters, point-to – point Infrared: 10 meters, point-to – point Office, conferenceOffice, conference Bluetooth: 5 meters Bluetooth: 5 meters Small office small home (SOHO)Small office small home (SOHO)

2001/10/26 Prof. Huei-Wen Ferng 7 Spread Spectrum Technology Two major SS technology: Two major SS technology: Frequency Hopping SS (FHSS)Frequency Hopping SS (FHSS) Direct Sequence SS (DSSS)Direct Sequence SS (DSSS) These techniques resides at the physical layerThese techniques resides at the physical layer The responsibility of PHY layer is to transmit bits over the mediumThe responsibility of PHY layer is to transmit bits over the medium

2001/10/26 Prof. Huei-Wen Ferng 8 FHSS It is different with frequency division multiplexing (FDM) It is different with frequency division multiplexing (FDM) The typical bandwidth of the information signal is 1 MHz The typical bandwidth of the information signal is 1 MHz How does FHSS help to not interfere with the primary user? How does FHSS help to not interfere with the primary user?

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2001/10/26 Prof. Huei-Wen Ferng 14 DSSS The input data stream is transferred to a chip stream running x times faster The input data stream is transferred to a chip stream running x times faster A chip is a zero or a one, just like a bit, it is called a chip to distinguish it from a bit A chip is a zero or a one, just like a bit, it is called a chip to distinguish it from a bit The frequency spectrum is spread out and the spectral energy is x times lower The frequency spectrum is spread out and the spectral energy is x times lower It is so low that it does not interfere with the primary user It is so low that it does not interfere with the primary user

2001/10/26 Prof. Huei-Wen Ferng 15 DSSS (Cont’d) The transmitter and receiver use the same coded chip and synchronized The transmitter and receiver use the same coded chip and synchronized How to transfer each of x chips to 1 bit? How to transfer each of x chips to 1 bit? What is the effect of the interference? What is the effect of the interference? The degradation of the FHSS and DSSS in the presence of interference is quite different The degradation of the FHSS and DSSS in the presence of interference is quite different

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2001/10/26 Prof. Huei-Wen Ferng 18 Summary of FHSS and DSSS FHSS FHSS Can have up to 10 or 15 channelsCan have up to 10 or 15 channels Each channel can provide 1 or 2 MbpsEach channel can provide 1 or 2 Mbps Degrades in step of 1.25%Degrades in step of 1.25% DSSS DSSS Can have up to two or three channelsCan have up to two or three channels Each channel can provide 1 or 2 MbpsEach channel can provide 1 or 2 Mbps Degrade in steps of 50%Degrade in steps of 50%

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2001/10/26 Prof. Huei-Wen Ferng 20 C Hidden Nodes Problem A B

2001/10/26 Prof. Huei-Wen Ferng 21 CSMA/CA A Simple solution for hidden nodes problem A Simple solution for hidden nodes problem Carrier senseCarrier sense Sender broadcast RTS (request to send) to receiverSender broadcast RTS (request to send) to receiver Receiver broadcast CTS (clear to send) to senderReceiver broadcast CTS (clear to send) to sender Is CSMA/CD possible in wireless network? Is CSMA/CD possible in wireless network?

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2001/10/26 Prof. Huei-Wen Ferng 23 Digital Modulation Techniques Convert digital stream into the analog signal Convert digital stream into the analog signal The characteristics of this signal has: The characteristics of this signal has: Amplitude, frequency, and phaseAmplitude, frequency, and phase We can change the three characteristics to formulate the modulation techniques: We can change the three characteristics to formulate the modulation techniques: Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK)Frequency Shift Keying (FSK) Phase Shift Keying (PSK)Phase Shift Keying (PSK)

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