RADIO TOPICS

Wireless LAN Physical Channel Infrared (IR) LANs: Individual cell of IR LAN limited to single roomInfrared (IR) LANs: Individual cell of IR LAN limited to single room –IR light does not penetrate opaque walls Microwave : Spread spectrum LANs: Mostly operate in ISM (industrial, scientific, and medical) bandsMicrowave : Spread spectrum LANs: Mostly operate in ISM (industrial, scientific, and medical) bands –No licensing is required in USA Narrowband microwave: Microwave frequencies but not use spread spectrumNarrowband microwave: Microwave frequencies but not use spread spectrum –Some require licensing

Infrared LANs Strengths and Weaknesses Spectrum virtually unlimitedSpectrum virtually unlimited –Infrared spectrum is unregulated worldwide –Extremely high data rates Infrared shares some properties of visible lightInfrared shares some properties of visible light –Diffusely reflected by light-colored objects Use ceiling reflection to cover entire roomUse ceiling reflection to cover entire room –Does not penetrate walls or other opaque objects More easily secured against eavesdropping than microwaveMore easily secured against eavesdropping than microwave Separate installation in every room without interferenceSeparate installation in every room without interference

Infrared LANs Strengths and Weaknesses Inexpensive and simpleInexpensive and simple –Uses intensity modulation, so receivers need to detect only amplitude Background radiationBackground radiation –Sunlight, indoor lighting –Noise, requiring higher power and limiting range –Power limited by concerns of eye safety and power consumption

Strengths of Infrared Over Microwave Radio Spectrum for infrared virtually unlimitedSpectrum for infrared virtually unlimited –Possibility 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 coverage Doesn’t penetrate wallsDoesn’t penetrate walls –More easily secured against eavesdropping –Less interference between different rooms

Drawbacks of Infrared Medium Indoor environments experience infrared background radiationIndoor environments experience infrared background radiation –Sunlight and indoor lighting –Ambient radiation appears as noise in an infrared receiver –Transmitters of higher power required Limited by concerns of eye safety and excessive power consumptionLimited by concerns of eye safety and excessive power consumption –Limits range

Infrared LANs Transmission Techniques Directed-beam IRDirected-beam IR Omni directionalOmni directional Diffused configurationDiffused configuration

Directed Beam Infrared Used to create point-to-point linksUsed to create point-to-point links Range depends on emitted power and degree of focusingRange depends on emitted power and degree of focusing Focused IR data link can have range of kilometersFocused IR data link can have range of kilometers Used for building interconnect within line of sightUsed for building interconnect within line of sight Cross-building interconnect between bridges or routersCross-building interconnect between bridges or routers Indoor use to set up token ring LANIndoor use to set up token ring LAN

Ominidirectional Single base station within line of sight of all other stations on LANSingle base station within line of sight of all other 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 IR transceivers –IR transceivers transmit with directional beam aimed at ceiling base unit

Diffused All IR transmitters focused and aimed at a point on diffusely reflecting ceilingAll IR transmitters focused and aimed at a point on diffusely reflecting ceiling IR radiation strikes ceilingIR radiation strikes ceiling –Reradiated omnidirectionally –Picked up by all receivers

SPREAD SPECTRUM (SS) TECHNIQES Spread Spectrum : spread information data randomly over a wide bandwidth to make jamming and Interception difficultSpread Spectrum : spread information data randomly over a wide bandwidth to make jamming and Interception difficult A random sequence algorithm is used by both of the source and the receiver for signal spreading and reception.A random sequence algorithm is used by both of the source and the receiver for signal spreading and reception. Two version of Spread Spectrum :Two version of Spread Spectrum : –Frequency Hopping Spread Spectrum FHSS [ carrier Frequency is hopped randomly in the band ] –Direct Sequence Spread Spectrum DSSS [ each data bit is replaced by a random code of bits (chip ) ]

Spread Spectrum LANs Transmission Issues Licensing regulations differ from one country to anotherLicensing regulations differ from one country to another USA FCC authorized two unlicensed applications within the ISM band:USA FCC authorized two unlicensed applications within the ISM band: –Spread spectrum - up to 1 watt –Very low power systems- up to 0.5 watts – MHz (915-MHz band) – GHz (2.4-GHz band) – GHz (5.8-GHz band) –2.4 GHz also in Europe and Japan –Higher frequency means higher potential bandwidth InterferenceInterference –Devices at around 900 MHz, including cordless telephones, wireless microphones, and amateur radio –Fewer devices at 2.4 GHz; microwave oven –Little competition at 5.8 GHz Higher frequency band, more expensive equipmentHigher frequency band, more expensive equipment

Spread Spectrum LAN Configuration 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 hub –Access controlled with MAC algorithm CSMACSMA –Appropriate for ad hoc LANs

Spread Spectrum LAN Configuration Hub topologyHub topology –Mounted on the ceiling and connected to backbone –May control access –May act as multiport repeater –Automatic handoff of mobile stations –Stations in cell either: Transmit to / receive from hub onlyTransmit to / receive from hub only Broadcast using omnidirectional antennaBroadcast using omnidirectional antenna

Narrowband Microwave LANs Use of a microwave radio frequency band for signal transmissionUse of a microwave radio frequency band for signal transmission Relatively narrow bandwidthRelatively narrow bandwidth LicensedLicensed UnlicensedUnlicensed

Narrow Band Microwave LANs Just wide enough to accommodate signalJust wide enough to accommodate signal Until recently, all products used licensed bandUntil recently, all products used licensed band At least one vendor has produced LAN product in ISM bandAt least one vendor has produced LAN product in ISM band

Licensed Narrowband RF Microwave frequencies usable for voice, data, and video licensed within specific geographic areas to avoid interferenceMicrowave frequencies usable for voice, data, and video licensed within specific geographic areas to avoid interference –Radium 28 km –Can contain five licenses –Each covering two frequencies –Motorola holds 600 licenses (1200 frequencies) in the 18-GHz range –Cover all metropolitan areas with populations of 30,000 or more in USA Use of cell configurationUse of cell configuration Adjacent cells use non overlapping frequency bandsAdjacent cells use non overlapping frequency bands Motorola controls frequency bandMotorola controls frequency band –Can assure nearby independent LANs do not interfere All transmissions are encryptedAll transmissions are encrypted Licensed narrowband LAN guarantees interference-free communicationLicensed narrowband LAN guarantees interference-free communication License holder has legal right to interference-free data channelLicense holder has legal right to interference-free data channel

Unlicensed Narrowband RF 1995, Radio LAN introduced narrowband wireless LAN using unlicensed ISM spectrum1995, Radio LAN introduced narrowband wireless LAN using unlicensed ISM spectrum –Used for narrowband transmission at low power 0.5 watts or less0.5 watts or less –Operates at 10 Mbps –5.8-GHz band –50 m in semi open office and 100 m in open office Peer-to-peer configurationPeer-to-peer configuration Elects one node as dynamic masterElects one node as dynamic master –Based on location, interference, and signal strength Master can change automatically as conditions changeMaster can change automatically as conditions change Includes dynamic relay functionIncludes dynamic relay function Stations can act as repeater to move data between stations that are out of range of each otherStations can act as repeater to move data between stations that are out of range of each other

Wireless Antennas An antenna is an electrical conductor or system of conductorsAn antenna is an electrical conductor or system of conductors –Transmission - radiates electromagnetic energy into space –Reception - collects electromagnetic energy from space In two-way communication, the same antenna can be used for transmission and receptionIn two-way communication, the same antenna can be used for transmission and reception

Types of Antennas Isotropic antenna (idealized)Isotropic antenna (idealized) –Radiates power equally in all directions Dipole antennasDipole antennas –Half-wave dipole antenna (or Hertz antenna) –Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective AntennaParabolic Reflective Antenna