Local Area Networkspart II

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Wireless LANs

• Why wireless LAN– Cost with wired LANs is that of installing the

physical wire cable• If the layout of the interconnected computers

changes,– Then a cost similar to the initial installation coast

can be incurred as the wiring plan is changed

– The advent of handheld terminals and portable computers

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Wireless LANs (cont’d)

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Wireless LANs (cont’d)

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Wireless media

• Two type of media– Radio-frequency wave– Infrared optical signal

• Radio– Be used extensively for many

applications• Radio and television broadcasting• Cellular telephony networks

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Radio

• Radio waves can readily propagate through objects such as walls and doors

• Radio bandwidth is scarce– For a particular applications

• A specific frequency band must be officially allocated

• The circuitry associated with radio-based system– is more sophisticated that that used in

infrared optical system– Reasonable cost

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Radio (cont’d)

• Path loss– SNR (signal-to-noise)

• The radio power of the received signal to the power of the receiver noise signal must not fall below the specified value

– Receiver noise• Temperature• Bandwidth of the received signal

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Radio (cont’d)

– Signal power• The power of the transmitted signal• The distance between the transmitter and

receiver– In free space

» The power of a radio signal decays inversely with the square of the distance from the source

– In an indoor environment» The decay is increased» Because of the presence of objects such as

furniture and people» Because of destructive interference of the

transmitted signal caused by the reflected signals from the these objects (Path loss)

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Radio (cont’d)

– With portable computers• The power of the transmitted signal is

limited by the power consumption of the radio

• Adjacent channel interference– The available bandwidth can be

divided into a number of sub-bands• So that the area of coverage of adjacent

sub-bands utilize a different frequency

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Radio (cont’d)

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Radio (cont’d)

• Multipath– Multipath dispersion (delay spread)

• The receiver receives multiple signals originating from the same transmitter

– each of which has followed a different path between the transmitter and receiver

– Intersymbol interference(ISI)• The signals relating to a previous bit/symbol to

interfere with the signals relating to the next bit/symbol

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Radio (cont’d)

– Frequency-selective fading• Caused by the variation in path lengths of the di

fferent received signals• Relative phase shifts between them which, at ra

dio frequencies, can cause the various reflected signals to significantly attenuate the direct path signal and in the limit, to cancel each other out

• Rayleigh fading

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Infrared

• Infrared emitter and detector– It include optical fiber transmission

systems and various remote control applications• Such as television sets, CD players, VCRs

• Very much higher than radio frequency waves– Greater than 1014Hz

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Infrared (cont’d)

• Wavelength = c / f

• c is the speed of light• f is the signal frequency in Hz

• Advantage– The lack of regulations relating to its use– A similar wavelength to visible light

• Be reflected from shiny surfaces• It will pass through glass but not through walls or other

opaque objects– Be limited to a single room which, in wireless LAN

applications, reduces the level of adjacent channel interference

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Infrared (cont’d)

• When using infrared as the physical medium is the interference caused by background light– The noise power can be high, which leads to a req

uirement for a high signal power to obtain an acceptable SNR

• It can lead to a high power demand on a battery source• To reduce the level of noise

– Optical bandpass filter» Attenuates those infrared signals that are outside of th

e frequency band of the transmitted signal

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Infrared (cont’d)

• Topology– Point-to-point mode

• The emitter is pointed directly at the detector

– Much lower power emitters– Less sensitive detectors can be used

– Diffused mode (broadcast mode)• The output of the infrared source is

optically diffused so that the light is spread over a wide angular area

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Infrared (cont’d)

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Transmission schemes

• Radio propagation characteristics– Direct sequence spread spectrum– Frequency-hopping spread spectrum– Single-carrier modulation– Multi-subcarrier modulation

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Direct sequence spread spectrum

• ISM band– Free radio spectrum available– The frequency bands set aside for general

industrial, scientific, and medical applications• Heating equipment, microwave ovens, amateur

radio operator

– In order to coexist with such applications• It is essential that the transmission scheme selected

has a high level of co-channel interference rejection• Spread spectrum

– Direct-sequence– Frequency-hopping

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DSSS (cont’d)

• Pseudorandom binary sequence– The source data to be transmitted is first exclusive

-ORed– That is the bits making up the sequence are rando

m but the same sequence is made much larger than the source data rate

– Exclusive-ORed signal• It occupies a proportionately wider frequency band than th

e original source data bandwidth

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DSSS (cont’d)

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Frequency hopping spread spectrum

• Channel– The allocated frequency band is divided into a

number of lower-frequency sub-bands– A transmitter uses each channel for a short period of

time before moving/hopping to a different channel– Hopping sequence

• The pattern of usage of the channel is pseudorandom

– Chip period• The time spent on each channel

– Chipping rate• The hopping rate

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FHSS (cont’d)

• Fast frequency-hopping– When the chipping rate is higher than the

data rate

• Slow frequency-hopping– When the chipping rate is lower than the

data rate

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FHSS (cont’d)

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Single-carrier modulation

• Single-carrier modulation– Signal located in the center of the

allocated band is modulated with the data

– It is simply an extension of the modulation schemes for transmitting data over an analog switched telephone network

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Multi-subcarrier modulation

• Multi-subcarrier modulation– First to divide the high bit rate binary signal

to be transmitted into a number of lower bit rate streams

– Each lower bit rate stream is then used to modulate a separate subcarrier – from the allocated frequency band – as with a single-carrier scheme

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Protocols

• The protocols standards for LANs– IEEE 802, ISO 8802

• IEEE standards– IEEE 802.3 : CSMA/CD bus– IEEE 802.4 : Token bus– IEEE 802.5 : Token ring– IEEE 802.6 : Wireless

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Protocols (cont’d)

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MAC sublayer services• For CSMA/CD,

confirm primitive indicates the successful(or not) transmission of requests

• For token LAN, confirm primitive indicates the successful(or not) delivery of requests

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LLC sublayer

• L_DATA.request– The only user service primitive– Because this is a best-try protocol

• All data is transferred using the unnumbered information(UI) frame

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Network layer

• The primary role– To route the messages associated with the higher

protocol layers above it

• It involves– Creating an NPDU from the parameters associated

with the incoming N_UNITDATA.request primitive – Passing this to the LCC sublayer in the user data p

arameter of an L_DATA.request

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Network layer