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Local Area Networkspart II
2
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
3
Wireless LANs (cont’d)
4
Wireless LANs (cont’d)
5
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
6
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
7
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
8
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)
9
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
10
Radio (cont’d)
11
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
12
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
13
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
14
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
15
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
16
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
17
Infrared (cont’d)
18
Transmission schemes
• Radio propagation characteristics– Direct sequence spread spectrum– Frequency-hopping spread spectrum– Single-carrier modulation– Multi-subcarrier modulation
19
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
20
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
21
DSSS (cont’d)
22
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
23
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
24
FHSS (cont’d)
25
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
26
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
27
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
28
Protocols (cont’d)
29
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
30
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
31
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
32
Network layer
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