2001/10/26 prof. huei-wen ferng 1 chapter 4 wireless lan technologies and products

2001/10/262001/10/26 Prof. Huei-Wen FerngProf. Huei-Wen Ferng 11

Chapter 4Chapter 4

Wireless LAN Technologies Wireless LAN Technologies and Productsand Products


PreliminaryPreliminary

A Brief Introduction to A Brief Introduction to Network and Communication Network and Communication

TerminologyTerminology


Wireless MediaWireless 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

• Some equipment in the industry, scientific, and and medical fields without the intent of medical fields without the intent of communication such as microwave ovenscommunication such as microwave ovens


Wireless MediaWireless Media Unlicensed Secondary usage: ISM Unlicensed Secondary usage: ISM

bandsbands• 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 Why do we want unlicensed bands for WLAN?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


Applications Classified by MediaApplications Classified by Media Radio: 100 metersRadio: 100 meters

• Conference, campus, universeConference, campus, universe Infrared: 10 meters, point-to –pointInfrared: 10 meters, point-to –point

• Office, conferenceOffice, conference Bluetooth: 5 metersBluetooth: 5 meters

• Small office small home (SOHO)Small office small home (SOHO)


Spread Spectrum TechnologySpread 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 These techniques resides at the physical

layerlayer• The responsibility of PHY layer is to The responsibility of PHY layer is to

transmit bits over the mediumtransmit bits over the medium


FHSSFHSS It is different with frequency division It is different with frequency division

multiplexing (FDM)multiplexing (FDM) The typical bandwidth of the The typical bandwidth of the

information signal is 1 MHzinformation signal is 1 MHz How does FHSS help to not interfere How does FHSS help to not interfere

with the primary user?with the primary user?


DSSSDSSS The input data stream is transferred to a The input data stream is transferred to a

chip stream running x times fasterchip stream running x times faster A chip is a zero or a one, just like a bit, it is A chip is a zero or a one, just like a bit, it is

called a chip to distinguish it from a bitcalled a chip to distinguish it from a bit The frequency spectrum is spread out and The frequency spectrum is spread out and

the spectral energy is x times lowerthe spectral energy is x times lower It is so low that it does not interfere with It is so low that it does not interfere with

the primary user the primary user


DSSS (Cont’d)DSSS (Cont’d) The transmitter and receiver use the The transmitter and receiver use the

same coded chip and synchronizedsame coded chip and synchronized How to transfer each of x chips to 1 How to transfer each of x chips to 1

bit?bit? What is the effect of the interference?What is the effect of the interference? The degradation of the FHSS and The degradation of the FHSS and

DSSS in the presence of interference DSSS in the presence of interference is quite differentis quite different


Summary of FHSS and DSSSSummary of FHSS and DSSS FHSSFHSS

• 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%

DSSSDSSS• 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%


C

Hidden Nodes ProblemHidden Nodes Problem

A B


CSMA/CACSMA/CA A Simple solution for hidden nodes A Simple solution for hidden nodes

problemproblem• Carrier senseCarrier sense• Sender broadcast RTS (request to send) Sender broadcast RTS (request to send)

to receiverto receiver• Receiver broadcast CTS (clear to send) Receiver broadcast CTS (clear to send)

to senderto sender Is CSMA/CD possible in wireless Is CSMA/CD possible in wireless

network?network?


Digital Modulation TechniquesDigital Modulation Techniques Convert digital stream into the analog Convert digital stream into the analog

signalsignal 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 We can change the three characteristics to

formulate the modulation techniques: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)

2001/10/26 prof. huei-wen ferng 1 chapter 4 wireless lan technologies and products

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