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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.
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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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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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