Download - Week01 Fundamentals
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Advanced Networks
CC531Week 01
Fundamentals of short range wirelesscommunication
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What is wireless communication?
Any form of communication that does not
require the transmitter and receiver to be in
physical contact
Electromagnetic wave propagated through
free-space; Radar, RF, Microwave, IR, Optical
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Why do we use wireless communication?
Provides mobility
Enables communication without installing anexpensive infrastructure
Can easily set-up temporary LANs
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Wired versus Wireless
Wires
Attenuation is low Interference is nil: each wire is a separate medium
Clumsy, costly, no
Mobility
Signal is more difficult to be intercepted
No wires
Attenuation is high
Interference is high: single medium
No knots, no digging to lay cables
Allows mobility
Signal is easily intercepted
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Why Wireless Networks are spreading
quickly?
Faster installation when compared to cablednetworks
Adaptability in a dynamic environment.
The capability of sharing an Internet or otherWAN connection.
The continuing development of new electronicdevices (or newer versions) that use wirelesstechnology
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Electromagnetic Wave
Two important properties
Propagate : They travel in the space from the
sender to a receiver
Transfer energy: This energy can be used for data
transmission
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Antenna
Made of conducting material
Applying a current to an antenna creates an
electric field around the antenna
As the current of the antenna changes, so doesthe electric field
Radio waves hitting an antenna cause electronsto flow in the conductor and create current
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Spectrum
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Main issues in wireless communications
Fading: variation of signal strength due to:
Multipath fading : due to constructive &
destructive interference of multiple signal paths
between transmitter & receiver
Path loss: via distance attenuation
Shadowing: via obstacles
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Multipath fading
Multipath: the propagation phenomenon that
results in radio signals reaching the receiving
antenna by two or more paths
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Radio waves can be propagated and receivingpower is influenced in different ways:
Reflection at large obstacles
Scattering at small obstacles
Diffraction at edges
reflection
diffraction
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Path Loss
is the reduction in power density
(attenuation) of signal as it propagates
through space
Can be identified as the ratio of the power of
the transmitted signal to the power of the
same signal received by the receiver
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Shadowing effect
Shadowing is the effect that the received
signal power fluctuates due to objects
obstructing the propagation path between
transmitter and receiver.
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Interference
Adjacent channel interference: interfered by signals in nearby
frequencies.Solved by the guard bands.
Co-channel interference: narrow-band
interference due to other systems usingthe same frequency.Solved by
Multiuser detection mechanisms Directional antennas Dynamic channel allocation methods.
A l f M l i l A
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Analogy for Multiple Access
Techniques
people could take turnsspeaking (time division)
speak at different pitches(frequency division)
speak in different languages
(code division).
Code Multiplexing isanalogous to the lastexample where peoplespeaking the same languagecan understand each other,but other languages are
perceived as noise andrejected.
Similarly, each group ofusers is given a shared code.Many codes occupy thesame channel, but onlyusers associated with a
particular code cancommunicate
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Multiple Access Techniques
Multiplexing in 4
dimensions frequency (f)
time (t) code (c)
space (si)
Goal: multiple use of ashared medium
s2
s3
s1f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
channels ki
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1- Frequency Multiplexing
Separation of the whole spectrum into smallerfrequency bands
A channel gets a certain band of the spectrum for thewhole time
Advantages: no dynamic coordination
necessary
works also for analog signals
Disadvantages: waste of bandwidth
if the traffic isdistributed unevenly
inflexible
guard spaces
k2 k3 k4 k5 k6k1
f
t
c
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2- Time Multiplexing
A channel gets the whole spectrum for a certainamount of time
Advantages: only one carrier in themedium at any time
throughput high evenfor many users
Disadvantages: precise
synchronizationnecessary
f
t
c
k2 k3 k4 k5 k6k1
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Time and Frequency Multiplexing
Combination of both methods
A channel gets a certain frequency band for a
certain amount of time
Advantages: protection against frequency
selective interference
higher data rates compared
to code multiplex
but: precise coordination
required
f
t
c
k2 k3 k4 k5 k6k1
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3- Code Multiplexing
Each channel has a unique code
All channels use the same
spectrum at the same time Advantages:
bandwidth efficient
no coordination and synchronization necessary
good protection against interference
Disadvantages: lower user data rates
more complex signal regeneration
Implemented using spreadspectrum technology
k2 k3 k4 k5 k6k1
f
t
c
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Spread spectrum
spread-spectrum techniques are methods bywhich a signal generated with aparticular bandwidth is deliberately spread in
the frequency domain, resulting in a signal with awider bandwidth.
These techniques are used for a variety of
reasons including the establishment of secure communications
increasing resistance to natural interference, noise,jamming, and to prevent detection,
http://en.wikipedia.org/wiki/Signal_(electrical_engineering)http://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Frequency_domainhttp://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Interference_(communication)http://en.wikipedia.org/wiki/Noise_(electronics)http://en.wikipedia.org/wiki/Radio_jamminghttp://en.wikipedia.org/wiki/Radio_jamminghttp://en.wikipedia.org/wiki/Noise_(electronics)http://en.wikipedia.org/wiki/Interference_(communication)http://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Frequency_domainhttp://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Signal_(electrical_engineering) -
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Examples are:
Frequency-Hopping Spread Spectrum (FHSS)
Direct-Sequence Spread Spectrum (DSSS)
Frequency Hopping Spread Spectrum
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Frequency Hopping Spread Spectrum
(FHSS )
Data signal is modulated with a narrowbandcarrier signal that "hops" in a random butpredictable sequence from frequency tofrequency as a function of time over a wide bandof frequencies.
Discrete changes of carrier frequency The total bandwidth is split into many channels of smaller bandwidth.
Transmitter and receiver stay on one of these channels for a certain timeand hop to another channel.
The pattern of channel usage is called the hopping sequence
sequence of frequency changes determined via pseudo random numbersequence
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user data
slow
hopping
(3 bits/hop)
fast
hopping
(3 hops/bit)
0 1
tb
0 1 1 tf
f1
f2
f3
t
td
f
f1
f2
f3
t
td
Two versions Slow Hopping: several user bits per frequency
Fast Hopping: several frequencies per user bit
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Advantages frequency selective fading and interference limited to short
period
simple implementation
uses only small portion of spectrum at any time
Disadvantages not as robust as DSSS
simpler to detect
Direct Sequence Spread Spectrum
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Direct Sequence Spread Spectrum
(DSSS)
Data signal at the sending station is combined with a higher data rate bit sequence,or chipping code. The chipping code which increases the signal's resistance tointerference.
user data
chipping
sequence
resulting
signal
0 1
0 1 1 0 1 0 1 01 0 0 1 11
XOR
0 1 1 0 0 1 0 11 0 1 0 01
=
tb
tc
tb: bit period
tc: chip period
XOR of the signal with pseudo-random number(chipping sequence)
Each chip has a much shorter duration thanan information bit That is, each information bit is modulated
by a sequence of much faster chips.Therefore, the chip rate is much higherthan the information signal bit rate
the sequence of chips produced by the
transmitter is already known by the receiver The receiver can then use the same
sequence to counteract the on the receivedsignal in order to reconstruct theinformation signal
http://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Chip_(CDMA) -
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Advantages reduces frequency selective fading
in cellular networks base stations can use the same
frequency range several base stations can detectand recover the signal
Disadvantages
Requires synchronization
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4- Space Division Multiple Access
Space division multiple access (SDMA) uses
directional transmitters/antennas to cover
angular regions.
Different areas/regions can be served using the
same frequency channel. This method is suited
to
Satellite system: a narrowly focused beam to prevent the signal fromspreading too widely.
Cellular phone system: base station covers a certain transmission area
(cell). Mobile devices communicate only via the base station
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Wireless Networking Challenges
Challenge-1: Mobility
User moves out of range, or obstacle comes in-
between
Network address has to change
Challenge-2: Low Bandwidth
Result of shared channel, high attenuation
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Challenge-3: Variable Bandwidth
Sources of variability:
Moving from wired to wireless Moving from one wireless network to another When
changing location
Challenge-4: Security Risks Shared medium
Device can be stolen
Challenge-5: Power Consumption
Portable devices cannot have large batteries
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Basic wireless parameters
1. Range
2. Throughput
3. Interference and coexistence
4. Power consumption
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1- Range
Transmission range communication possible
low error rate
Detection range
detection of the signalpossible
no communicationpossible
Interference range
signal may not bedetected
signal adds to thebackground noise
distance
sender
transmission
detection
interference
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Transmission range is closely tied up with
throughput and output (transmit) power.
Wireless connection maintains a link that
supports a data rate close to the maximum
the transmitter and receiver can support
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There is no strict definition of where the
extreme of range is determined
Increasing transmission power can increase
the range, but it is not the best solution
Fading, interference and reflections will
reduce the range significantly
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2- Throughput
Throughput falls as the range increases and
the Bit Error Rate (BER) rises
Range Vs Throughput for 802.11g
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Is usually measured in bits/second
To get highest throughput wireless standards
tries to gather a lot of information into each
transmitted bit by using complex coding
schemes
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3- Interference and coexistence
If two radios within range of each other both transmit at the sametime and at the same frequency, so signals arriving are likely to becorrupted.
Since no acknowledgement of reception is received, they will
attempt to retransmit the data.
Different standards use different techniques to try and ensure thatthey dont clash with each other on the retransmission
but even if the next transmissions do not overlap and aresuccessful, it means that the throughput will have decreased as aresult of retransmission.
idd d bl
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Hidden node problem
A and C can each communicate with the hub B,
but are hidden from each other
The problem is when nodes A
and C start to send packetssimultaneously to the access
point B
Since the nodes cannot sense the carrier, Carrier
sense multiple access with collision avoidance
(CSMA/CA) does not work, and collisions occur,
corrupting the data at the access point
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Exposed node problem
occurs when a node is prevented from sending packets to other
nodes due to a neighboring transmitter
the two receivers are
out of range of each
other, yet the two
transmitters in themiddle are in range of
each other.
Here, if a transmission between S1 and R1 is taking place, node
S2 is prevented from transmitting to R2 as it concludes aftercarrier sense that it will interfere with the transmission by its
neighbor S1.
However note that R2 could still receive the transmission of S2
without interference because it is out of range of S1
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4- Power consumption
Many wireless products are designed to run
on batteries
There are different modes
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Discussion
What is your biggest complaint about current
wireless technology?
In what application areas do you see wireless
networks succeeding?
In what application areas do you see wireless
networks failing?
What do you see as the motivating factors for
using wireless as opposed to wired networks?