Transcript Local Area Networks

Local Area Networks
part II
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 different received signals
• Relative phase shifts between them which,
at radio 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
requirement 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 the 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
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–
–
–
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
random but the same sequence is made much
larger than the source data rate
– Exclusive-ORed signal
• It occupies a proportionately wider frequency band
than the 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
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–
–
–
IEEE
IEEE
IEEE
IEEE
802.3
802.4
802.5
802.6
:
:
:
:
CSMA/CD bus
Token bus
Token ring
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 parameter of an L_DATA.request
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Network layer
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