Week01 Fundamentals

7/30/2019 Week01 Fundamentals

1/43

Advanced Networks

CC531Week 01

Fundamentals of short range wirelesscommunication


2/43

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


3/43

Why do we use wireless communication?

Provides mobility

Enables communication without installing anexpensive infrastructure

Can easily set-up temporary LANs


4/43

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


5/43

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


6/43

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


7/43

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


8/43

Spectrum


9/43

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


10/43

Multipath fading

Multipath: the propagation phenomenon that

results in radio signals reaching the receiving

antenna by two or more paths


11/43

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


12/43


13/43

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


14/43

Shadowing effect

Shadowing is the effect that the received

signal power fluctuates due to objects

obstructing the propagation path between

transmitter and receiver.


15/43


16/43

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


17/43

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


18/43

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


19/43

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


20/43

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


21/43

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


22/43

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


23/43

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)


24/43

Examples are:

Frequency-Hopping Spread Spectrum (FHSS)

Direct-Sequence Spread Spectrum (DSSS)

Frequency Hopping Spread Spectrum


25/43

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


26/43

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


27/43

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


28/43

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)


29/43

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


30/43

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
http://en.wikipedia.org/wiki/Chip_(CDMA)http://en.wikipedia.org/wiki/Chip_(CDMA)


31/43

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


32/43

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


33/43

Basic wireless parameters

1. Range

2. Throughput

3. Interference and coexistence

4. Power consumption


34/43

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


35/43

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


36/43

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


37/43

2- Throughput

Throughput falls as the range increases and

the Bit Error Rate (BER) rises

Range Vs Throughput for 802.11g


38/43

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


39/43

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


40/43

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
http://en.wikipedia.org/wiki/CSMA_CAhttp://en.wikipedia.org/wiki/CSMA_CA


41/43

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


42/43

4- Power consumption

Many wireless products are designed to run

on batteries

There are different modes


43/43

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?

Week01 Fundamentals

Documents

Transcript of Week01 Fundamentals