RADIO TOPICSRADIO TOPICS

Wireless LAN Physical Wireless LAN Physical Channel Channel

• Infrared (IR) LANs:Infrared (IR) LANs: Individual Individual cell of IR cell of IR LAN limited to single roomLAN limited to single room– IR IR light does not penetrate opaque wallslight does not penetrate opaque walls

• Microwave : Spread spectrum LANs: Microwave : Spread spectrum LANs: MostlyMostly operate in ISM (industrial, operate in ISM (industrial, scientific, and medical) bandsscientific, and medical) bands– NoNo licensing is required in licensing is required in USAUSA

• Narrowband microwave: Narrowband microwave: MicrowaveMicrowave frequencies but not use spread spectrumfrequencies but not use spread spectrum– Some require licensingSome require licensing

Infrared LANsInfrared LANsStrengths and Strengths and WeaknessesWeaknesses• SpectrumSpectrum virtually unlimited virtually unlimited

– InfraredInfrared spectrum is unregulated worldwide spectrum is unregulated worldwide– ExtremelyExtremely high data rates high data rates

• Infrared Infrared shares some properties of visible shares some properties of visible lightlight– DiffuselyDiffusely reflected by light-colored objects reflected by light-colored objects

• UseUse ceiling reflection to ceiling reflection to cover cover entire room entire room

– Does Does not penetrate walls or other opaque not penetrate walls or other opaque objectsobjects• MoreMore easily secured against eavesdropping than easily secured against eavesdropping than

microwavemicrowave• Separate Separate installation in every room without installation in every room without

interferenceinterference

Infrared LANsInfrared LANsStrengths and WeaknessesStrengths and Weaknesses

• InexpensiveInexpensive and simple and simple– UsesUses intensity modulation, so receivers intensity modulation, so receivers

need to detect onlyneed to detect only amplitudeamplitude

• Background Background radiationradiation– Sunlight,Sunlight, indoor lighting indoor lighting– NoiseNoise, requiring higher power and , requiring higher power and

limiting rangelimiting range– PowerPower limited by concerns of eye safety limited by concerns of eye safety

and power consumptionand power consumption

Strengths of Infrared Over Strengths of Infrared Over Microwave RadioMicrowave Radio

• Spectrum for infrared virtually unlimitedSpectrum for infrared virtually unlimited– Possibility of high data ratesPossibility of high data rates

• Infrared spectrum unregulatedInfrared spectrum unregulated• Equipment inexpensive and simpleEquipment inexpensive and simple• Reflected by light-colored objectsReflected by light-colored objects

– Ceiling reflection for entire room coverageCeiling reflection for entire room coverage

• Doesn’t penetrate wallsDoesn’t penetrate walls– More easily secured against eavesdroppingMore easily secured against eavesdropping– Less interference between different roomsLess interference between different rooms

Drawbacks of Infrared MediumDrawbacks of Infrared Medium

• Indoor environments experience infrared Indoor environments experience infrared background radiationbackground radiation– Sunlight and indoor lightingSunlight and indoor lighting– Ambient radiation appears as noise in an infrared Ambient radiation appears as noise in an infrared

receiverreceiver– Transmitters of higher power requiredTransmitters of higher power required

• Limited by concerns of eye safety and excessive power Limited by concerns of eye safety and excessive power consumptionconsumption

– Limits rangeLimits range

Infrared LANsInfrared LANsTransmission TechniquesTransmission Techniques

• Directed-beam IRDirected-beam IR• Omni directionalOmni directional• DiffusedDiffused configuration configuration

Directed Beam InfraredDirected Beam Infrared

• Used to create point-to-point linksUsed to create point-to-point links• Range depends on emitted power and degree of Range depends on emitted power and degree of

focusingfocusing• Focused IR data link can have range of kilometersFocused IR data link can have range of kilometers

• UsedUsed for building interconnect for building interconnect within line of sight within line of sight

• Cross-building interconnect between bridges or Cross-building interconnect between bridges or routersrouters

• IndoorIndoor use to set up token ring LAN use to set up token ring LAN

OminidirectionalOminidirectional

• Single base station within line of sight of all Single base station within line of sight of all other stations on LANother stations on LAN

• Station typically mounted on ceilingStation typically mounted on ceiling

• Base station acts as a multiport repeaterBase station acts as a multiport repeater– Ceiling transmitter broadcasts signal received by Ceiling transmitter broadcasts signal received by

IR transceiversIR transceivers– IR transceivers transmit with directional beam IR transceivers transmit with directional beam

aimed at ceiling base unitaimed at ceiling base unit

DiffusedDiffused

• All IR transmitters focused and aimed at a All IR transmitters focused and aimed at a point on diffusely reflecting ceilingpoint on diffusely reflecting ceiling

• IR radiation strikes ceiling IR radiation strikes ceiling – Reradiated omnidirectionally Reradiated omnidirectionally – Picked up by all receiversPicked up by all receivers

SPREAD SPECTRUM (SS) SPREAD SPECTRUM (SS) TECHNIQESTECHNIQES

• Spread Spectrum : spread information data randomly Spread Spectrum : spread information data randomly over a wide bandwidth to make jamming and over a wide bandwidth to make jamming and Interception difficult Interception difficult

• A random sequence algorithm is used by both of the A random sequence algorithm is used by both of the source and the receiver for signal spreading and source and the receiver for signal spreading and reception. reception.

• Two version of Spread Spectrum :Two version of Spread Spectrum :– Frequency Hopping Spread Spectrum FHSSFrequency Hopping Spread Spectrum FHSS

[ carrier Frequency is hopped randomly in the band [ carrier Frequency is hopped randomly in the band ]]

– Direct Sequence Spread Spectrum DSSS [ each Direct Sequence Spread Spectrum DSSS [ each data bit is replaced by a random code of bits (data bit is replaced by a random code of bits (chipchip ) ] ) ]

Spread Spectrum LANsSpread Spectrum LANsTransmission Issues Transmission Issues

• Licensing Licensing regulations differ from one country to anotherregulations differ from one country to another• UUSASA FCC authorized two unlicensed applications within the FCC authorized two unlicensed applications within the

ISM band:ISM band:– Spread Spread spectrumspectrum - up - up to 1 watt to 1 watt– VeryVery low power systems low power systems- up - up to 0.5 wattsto 0.5 watts– 902 - 928 MHz (915-MHz band)902 - 928 MHz (915-MHz band)– 2.4 - 2.4835 GHz (2.4-GHz band)2.4 - 2.4835 GHz (2.4-GHz band)– 5.725 - 5.825 GHz (5.8-GHz band)5.725 - 5.825 GHz (5.8-GHz band)– 2.4 GHz also in Europe and Japan2.4 GHz also in Europe and Japan– HigherHigher frequency frequency means means higher potential bandwidthhigher potential bandwidth

• InterferenceInterference– DevicesDevices at around 900 MHz, including cordless at around 900 MHz, including cordless

telephones, wireless microphones, and amateur radiotelephones, wireless microphones, and amateur radio– FewerFewer devices at 2.4 GHz; microwave oven devices at 2.4 GHz; microwave oven– LittleLittle competition at 5.8 GHz competition at 5.8 GHz

• Higher Higher frequency band, more expensive equipmentfrequency band, more expensive equipment

Spread Spectrum LAN Spread Spectrum LAN ConfigurationConfiguration

• Multiple-cell arrangementMultiple-cell arrangement

• Within a cell, either peer-to-peer or hubWithin a cell, either peer-to-peer or hub

• Peer-to-peer topologyPeer-to-peer topology– No hubNo hub– Access controlled with MAC algorithmAccess controlled with MAC algorithm

• CSMACSMA

– Appropriate for ad hoc LANsAppropriate for ad hoc LANs

Spread Spectrum LAN Spread Spectrum LAN ConfigurationConfiguration

• Hub topologyHub topology– Mounted on the ceiling and connected to backboneMounted on the ceiling and connected to backbone– May control accessMay control access– May act as multiport repeaterMay act as multiport repeater– Automatic handoff of mobile stationsAutomatic handoff of mobile stations– Stations in cell either:Stations in cell either:

• Transmit to / receive from hub onlyTransmit to / receive from hub only• Broadcast using omnidirectional antennaBroadcast using omnidirectional antenna

Narrowband Microwave LANsNarrowband Microwave LANs

• Use of a microwave radio frequency band for Use of a microwave radio frequency band for signal transmissionsignal transmission

• Relatively narrow bandwidthRelatively narrow bandwidth

• LicensedLicensed

• UnlicensedUnlicensed

Narrow Band Microwave Narrow Band Microwave LANsLANs

• Just Just wide enough to accommodate wide enough to accommodate signalsignal

• Until recently, all products used Until recently, all products used licensed bandlicensed band

• AtAt least one vendor has produced least one vendor has produced LAN product in ISM bandLAN product in ISM band

Licensed Narrowband RFLicensed Narrowband RF• Microwave frequencies usable for voice, data, and video Microwave frequencies usable for voice, data, and video

licensed within specific geographic areas to avoid licensed within specific geographic areas to avoid interferenceinterference– RadiumRadium 28 km 28 km – CanCan contain five licenses contain five licenses– EachEach covering two frequencies covering two frequencies– Motorola holds 600 licenses (1200 frequencies) in the 18-GHz rangeMotorola holds 600 licenses (1200 frequencies) in the 18-GHz range– CoverCover all metropolitan areas with populations of 30,000 or more all metropolitan areas with populations of 30,000 or more in in

USAUSA

• UseUse of cell configuration of cell configuration• Adjacent cells use non overlapping frequency bands Adjacent cells use non overlapping frequency bands • Motorola controls frequency bandMotorola controls frequency band

– CanCan assure nearby independent LANs do not interfere assure nearby independent LANs do not interfere

• AllAll transmissions are encrypted transmissions are encrypted• LicensedLicensed narrowband LAN guarantees interference-free narrowband LAN guarantees interference-free

communicationcommunication• LicenseLicense holder holder hashas legal right to interference-free data legal right to interference-free data

channelchannel

Unlicensed Narrowband Unlicensed Narrowband RFRF• 1995, Radio LAN 1995, Radio LAN introducedintroduced narrowband wireless LAN using narrowband wireless LAN using

unlicensed ISM spectrumunlicensed ISM spectrum– UsedUsed for narrowband transmission at low power for narrowband transmission at low power

• 0.5 watts or less0.5 watts or less– OperatesOperates at 10 Mbps at 10 Mbps– 5.8-GHz band5.8-GHz band– 50 m in semi open office and 100 m in open office50 m in semi open office and 100 m in open office

• PeerPeer-to-peer configuration -to-peer configuration • Elects Elects one node as dynamic masterone node as dynamic master

– Based Based on location, interference, and signal strengthon location, interference, and signal strength• MasterMaster can change automatically as conditions change can change automatically as conditions change• IncludesIncludes dynamic relay function dynamic relay function• Stations canStations can act as repeater to move data between stations act as repeater to move data between stations

that are out of range of each otherthat are out of range of each other

Wireless AntennasWireless Antennas

• An antenna is an An antenna is an electrical conductor or electrical conductor or system of conductorssystem of conductors– Transmission - radiates electromagnetic energy Transmission - radiates electromagnetic energy

into spaceinto space– Reception - collects electromagnetic energy from Reception - collects electromagnetic energy from

spacespace

• In two-way communication, the same antenna In two-way communication, the same antenna can be used for transmission and receptioncan be used for transmission and reception

Types of AntennasTypes of Antennas

• Isotropic antenna (idealized)Isotropic antenna (idealized)– Radiates power equally in all directionsRadiates power equally in all directions

• Dipole antennasDipole antennas– HHalf-wave dipole antenna (or Hertz antenna)alf-wave dipole antenna (or Hertz antenna)– Quarter-wave vertical antenna (or Marconi Quarter-wave vertical antenna (or Marconi

antenna)antenna)

• Parabolic Reflective AntennaParabolic Reflective Antenna