wifi , working, elements of wifi

7/27/2019 WiFi , working, elements of WiFi

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Wi-Fi and multi-Path Fading

Ppt. Made by Zohaib Jahan, fb.com/zohaibjahan, student at uettaxila.edu.pk


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Objectives

Introduction

Purpose

Wi

-

FiAlli

ance

Wi

-

FiT

echnology

Elements of a

WI

-

FIN

etwork

Working

Network

Topologies

Wi

-

FiConfig

urations

Security

Threats

Wi

-

Fi

Security

Wi

-

Fi

P

hysical

layer

Antenna

Advantages

Applications

Limitations


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Wi-Fi is short for wireless fidelity.

local area network which uses high frequencyradio signals to send and receive data.

A wireless network uses radio waves, just likecell phones, televisions and radios do.

Two-way radio communication.

Allow mobile devices, such as laptops andPDAs to connect to local area networks.


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Vic Haye

known as the father ofWi-Fi for his invention While working forAgere systems.

1990 IEEE (Institute of Electrical and Electronics Engineers) established

the 802.11 Group

1991 Wi-Fi was invented in 1991

1997 Specifications for standard ratified in 1997

1999 IEEE modified the standard:802.11b,802.11a

2000

Wi-Fi certified testing begins on 802.11b-based products delivering11Mbps raw data rates in 2.4GHz


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2002

Wi-Fi alliance membership reaches 100 companies

Wi-Fi certified testing begins for802.11a-based,delivering 54Mbps raw datarates in 5GHz

2003

Wi-Fi certified testing begins for802.11g-based, 54Mbps raw data rates in2.4GHz

Wi-Fi protected access certification launches

2011

Global Wi-Fi hotspots surpass 1million

Wi-Fialliance membership reaches 500 companies

2012

Wi-Fi certified lunches, solution for easier connection to Wi-Fi

Wi-Fi certified maracas lunches, for seamless display of HD video betweendevices.

2013

Wi-Fi Alliance programs continue to bring innovative solutions to the market,during seamless connectivity.

60GHz- thesolution todeliver multi-gigabit rawdata rates in60GHz

Wi-FiCERTIFIED-the solution todeliver1GB

raw data ratesin 5GHz

Future Wi-Fievolution


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MainPurpose

make access toinformation easier

eliminate complexcabling

eliminate switches,adapters, plugs, pins

and connectors

ensure compatibilityand co-existence of

devices

The purpose of Wi-Fi is to hide complexity by enabling wireless

access to applications and data, media and streams.


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AccessPoint (AP)

Acts as base station to the Users

Acts as bridge between wireless and wired networks

60 to 200 Users supports generally

Networkinterface

cards

Transmitting and receiving Change data from parallel to series.

Framing & make packets ready for sending

Bridge

Used to connect multiple LANs. Used in building to building connection.

Coverage 25miles(40Km).


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Serviceprovider place an access point at designated hot spot.

The access point transmits a wireless signal to the adapter card in userdevice

Users connect to the WLAN through a page in their internet browser.

Transmitter Receiver


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In 2001

Unlicensed 5-GHz by usingOFDM

Support 6Mbpsup to 54 Mbps

IEEE802.11a

Late 1999

Supports5.5Mbps and11Mbps

2.4 GHz radioband

IEEE802.11b

Providerequiredinformation toensure properbridgeoperations

IEEE 802.11c

Supportwidespreadadoption

For global

harmonization

IEEE802.11d

This works onQoS.

Optimizationtransmission ofvoice andvideo

IEEE802.11e

Ensure multi-vendor access-

pointinteroperabilitythrough Inter-Access PointProtocol. In 2003

IEEE 802.11f

First half of 2003 Supports 54Mbps

within 2.4 GHzusing OFDM

IEEE 802.11g End of 2003 5-GHz band with

radar and satelliteCommunication

Using DFS and TPC

IEEE 802.11h

In 2003 Effectively used for

enhancement toMAC Layer forEnhanced Security

IEEE 802.11i

IEEE 802.11n

Frequency = 5 GHz,2.4GHz

Modulation = OFDM

Addition of MIMO

Max. Speed = 150 Mbps

Range = 70 meters Encoding Scheme =

OFDM


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Independent Basic ServiceSet(IBSS)

AD HOC network

The Extended Service

Set(ESS)Infrastructure

Mode


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Within an IBSS,STAs with each other on a peer-to-peer level. this type of network is often formed ana temporary basis.

AD HOC networks allow for flexible

cost-effective arrangement.

Temporary setups such as group of laptops in aconference room.

Independent BasicService Set(IBSS)

Consists of a series of BSSs(each containing anAP) connected together.

Within an ESS, STAs can roam from one BSS toother, communicate with any mobile or fixed client.

The ESS enables coverage to extend beyond therange of a WLAN radio.

By using an ESS campus-wide coverage is possible

The ExtendedService Set(ESS)


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Networks

Client &Access Point

Network

Peer-to-PeerNetwork

Point-to-Multipoint

bridgeNetwork

IEEE 802.11 operates in following

3 network configuration.


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Peer-to-Peer

A peer-to-peer network is a WLAN in its most basic form.

Two PCs equipped with wireless adapter cards are all that is needed toform a peer-to-peer network, enabling the PCs to share this type of networkrequires no administration or pre-configuration.

Applications

Small/branch offices sharing resources Remote control of another PC

Games for two or more players

Demos

Three mainconsideration

The stations must be arranged within the proper distance limits

All stations must send and receive on the same transmission frequency

The hidden node problem avoided so that each station can communicatewith all other stations


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Client &Access Point

In a client & Access Point network, users not only benefitfrom extended range capabilities, they are also able to

benefit from server resources, as the AP is connected to thewired backbone.

Benefits

Generally they can support between 15 and 50 users.

BSA-RF coverage provided by an AP.

ESA-It consists of 2 or more BSA.

ESA cell includes 10-15% overlap to allow roaming


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Wireless security is the prevention of unauthorized access or damage

to computers using wireless networks.

Categories

ControllingAccess

DeviceAuthentication

WiredEquivalent

Privacy (WEP)


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An APacts as the central base station for the wireless network.

By restricting access to the AP, only those devices that are authorized are

able to connect to the AP and become part of the wireless network.


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All wireless AP implement access control through Media Access Control(MAC) address filtering.

Access restrictions can limit a users access to the Internet, what days and

times it can be accessed, which Web sites can be visited, or the type of

traffic that passes

through the AP.


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Wired Equivalent Privacy (WEP) was designed to ensure that

only authorized parties can view transmitted wireless

information.

Each wireless device can

be use its default key to

encrypt text. The access point

must then use that same key

in order to decrypt it.


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There are two types of authentication supported by the 802.11 standard.

Open system authentication is the default method,

Shared key authentication in which the WEP default key is used.

Open system authentication

Shared key authentication


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Wirelesssecurity

Personal wirelesssecurity

WPA

PSK

TKIP

WPA2

PSK

AESEncryption

Enterprise wirelesssecurity


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Pre-shared key (PSK) authentication uses to generate the encryption key.Access points have a setting called Group Key Renewal. The Group Key

Renewal is the number of seconds between generating a new key.


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Temporal Key Integrity Protocol (TKIP), WPA replaces WEP. WEP uses

a 40-bit encryption key and does not change.

TKIP uses a longer128-bit key. Using TKIP, there are 280 trillionpossible keys that can be generated for a given data packet.

TKIP new key for each packet that is created.

TKIP provides an even greater level of security.


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PSKis intended for personal and small office home office users who do

not have advanced server capabilities.

PSKkeys are automatically changed (called rekeying) and authenticated

between devices after a specified period of time known as the rekeyinterval


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Both WPA and WPA2 provide a higher level of security than the original

IEEE 802.11b/a security protocols. WPA2 is preferred over WPA.


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how wireless media is accessed by the

User.

FHSS

DSSSOFDM

Sub-Layers

Physical

Layer

PMD Layer

PLCP Layer

defines methods for transmitting and

receiving data through the medium

reformats the data received from theMAC layer into packets

listens to medium to determinewhether data can be sent

concerned with rules for accessing the

wireless medium

MAC

Layer


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Combines bandwidth advantages of a narrow-band signal with security and

clarity advantages of a wide-band signal.

Uses narrow-band signal as little i.e. 1 MHz, this rate places signal on a

frequency for a very short period of time (dwell time) and then directs it to

hop to next frequency in the sequence.

More immune to interference

Highly ScalableAdvantage

Speed


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This Technique divides the 2.4 GHz band into 11 22-MHz channels.

Adjacent channels overlap one another partially, with three of the 11being

completely non-overlapping. Data is sent across one of these 22 MHz

channels without hopping to other channels.

Secure

Speed up to 11 MbpsAdvantage

More susceptible to interference

Limits number of overlapping cellsLimitation


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Sending device breaks transmissioninto pieces and sends it over channelsin parallel Receiving device combines signals tore-create the transmission

Enough bandwidth to handle

Usage of the 64 sub-carriers

12 zero sub-carriers on sides and center

48 data sub-carriers per symbol

4 pilot sub-carriers per symbol, for synchronizing / tracking


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Accessing

Medium DCF specifies

the use ofCSMA/CAalgorithm asmedia accessscheme.

Association

Wireless linksbetween clientsand accesspoints

Scanning thestation firstscans air to

know APs thenAssociationprocess begins.

Re-association

Concernedwith thehand-offofclients as theyroam innetwork.

Authentication

By default,standard isopen system

Securenetwork withWEP option

by aconfiguredshared keyinto APs.

Power

ManagementTwo separatemodes:

Active mode:when client istransmitting orreceiving

Power savemode: when nocommunication

This layer is concerned whit the rules for accessing the wireless medium


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All WLAN equipment comes with a built-in Omni-directional antenna, but

some select products will let you attach secondary antennas that willsignificantly boost range.

Antennas come in all shapes and styles:

Omni

directional

VerticalWhip

MastRadiator

Directional

Yagi

Wallmountedpanel

Parabolic

dish


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Advantages

Wi-Fi Applications

Limitations


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Multi-Path Fading


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Line of sight (LOS) is the fastest component reaching to

destination.
http://tele-information.com/wp-content/uploads/2012/08/multipath-propagation.jpg


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Reflectionoccurs when signal encounters a surface that is large relative to the

wavelength of the signal Diffractionoccurs at the edge of an impenetrable body that is large compared to

wavelength of radio wave Scatteringoccurs when incoming signal hits an object whose size in the order of the

wavelength of the signal or less


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Large-scale fading (shadowing)

Long term variation in the mean signal level caused by the

mobile unit moving into the shadow of surrounding objects

Small-scale fading (multipath fading)

Short term fluctuation in the signal amplitude caused by the

local multipaths.


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Small scale fading is used to describe the rapid Fluctuation in

the amplitude of radio signal over a short period of time or travel

distance.

The actual power received over a much smaller distance vary due to thedestructive/constructive interference of multiple signals that followmultiple paths to the receiver


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The direct ray is actually made up of many rays due to scattering multipletimes by obstructions along its path, all travelling about the samedistance

Each of these rays appearing at the receiver will differ randomly inamplitude and phase due to the scattering


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Small-scale fading can be further classified into

flat(or non-selective) fading

frequency selective fading

Fast fading

Slow fading


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It is defined as: If the delay of the multipath components with respect to

the main component is smaller than the symbols

duration time, a channel is said to be subject to flat

fading.Characteristics are:

In a flat fading channel inter-symbol interference

(ISI) is absent

(ISI is a type of distortion)


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The channel has a constant gain and a linear phase response

over a bandwidth that is greater than the bandwidth of the

transmitted signal.

The spectral characteristics of the transmitted signal are

preserved at the receiver


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The strength of the received signal generally changes slowly in

time due to fluctuations caused by multipath.

Typical values of delay spread are

0.2s (rural area),0.5s (suburban area),

3-8s (urban area)

S(f)C(f) R(f)


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different frequencies across the channel to differentdegrees.

It will mean that the phases and amplitudes of the

signal will vary across the channel

Sometimes relatively deep nulls may be experienced,

and this can give rise to some reception problems.

Simply maintaining the overall amplitude of the

received signal will not overcome the effects ofselective fading, and some form of equalization may

be needed.


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The spectral characteristics of the transmitted signal

are not preserved at the receiver

Certain frequency components have larger gains than

others


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The Rayleigh propagation model is most applicable to instances

where there are many different signal paths(urban areas), none of

which is dominant.

In this way all the signal paths will vary and can have an impact on

the overall signal at the receiver.

There is no dominant propagation along a LOS between the

transmitter and receiver.

When the signals reach the receiver, As some will be in phase and

others out of phase.


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In many instances cellular telephones being used in a dense

urban environment fall into this category.


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If the environment is such that, in addition to the scattering, there is astrongly dominant signal seen at the receiver, usually caused by a LOS.

Such a situation may be better modelled as Rician fading.


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http://tele-information.com/wp-content/uploads/2012/08/Doppler-frequence.gif


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Doppler proposed

The change in frequency of a wave for a receiver moving

relative to the transmitter.

How rapidly the channel fades will be affected by how fast the receiver

and/or transmitter are moving

Motion causes Doppler shift in the received signal components.

The Doppler effect occurs due to the distance affects the time you need to

receive sound wave


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The terms slowand fast fading refer to the rate at which the magnitude and

phase change imposed by the channel on the signal changes

Slow fading

is a long-term fading effect changing the mean value of

the received signal. Slow fading is usually associated with movingaway from the transmitter and experiencing the expected reduction in

signal strength.

Fast fading

is also called multi-path fading,Rapid changes in the strength overhalf wavelength, the constructive and destructive phases create a variation in

signal strength.


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Multipath fading may be minimized by

1. Space Diversity

2. Frequency Diversity

Space DiversityIn space diversity, two or more receiving

antennas are spaced some distance apart.

Fading does not occur simultaneously at both antennas.

Therefore, enough output is almost always available from one of the

antennas to provide a useful signal.


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Therefore, enough output is almost always available from

one of the antennas to provide a useful signal.


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In frequency diversity

Two transmitters and two receivers are used, each pair tuned to a

different frequency, with the same information being transmitted

simultaneously over both frequencies.

One of the two receivers will almost always produce a useful signal.


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