Wireless NeTWOrKs
by
Ananth RavindranAssistant Professor
UNiT i
MUlTiPle rADiO ACCess
Medium Access Alternatives: Fixed-Assignment for Voice
Oriented Networks Random Access for Data Oriented Networks ,
Handoff and Roaming Support, Security and Privacy
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
WIRELESS MEDIUM ACCESS
ALTERNATIVES
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
WIRELESS MEDIUM ACCESS
1. Fixed-Assignment Access for Voice-Oriented Networks
a. Frequency Division Multiple Access (FDMA)
b. Time Division Multiple Access (TDMA)
c. Code-Division Multiple Access (CDMA)
2. Random Access for Data-Oriented Networks
a. ALOHA-Based Wireless Random Access Techniques
b. CSMA-Based Wireless Random Access Techniques
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Fixed-Assignment Multiple Access
TechniquesEC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• The available spectrum bandwidth for our wireless
communication is limited.
• Multiple access techniques enable multiple signals to
occupy a single communications channel.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Major Types
• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Frequency Division Multiple Access
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Frequency Division Multiple Access
• It assigns individual frequency to individual users.
(i.e ) accommodates one user at a time.
• Each user is separated by Guard Bands.
• The complexity of FDMA mobile systems is lower when
compared to TDMA systems
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• A guardband is a narrow frequency band betweenadjacent frequency channels to avoid interferencefrom the adjacent channels
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• The number of channels that can be simultaneouslysupported in a FDMA system is given by
• BT -> total spectrum allocation,
• BGUARD -> the guard band
• BC -> the channel bandwidth
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Key Features
• If an FDMA channel is not in use, then it sits idle and
cannot be used by other users
• The bandwidths of FDMA channels are narrow (30
kHz)
• Intersymbol interference is low
• It needs only a few synchronization bits
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
De Merits• FDMA systems are costlier because of the single channel per
carrier design,
• It need to use costly bandpass filters to eliminate spurious
radiation at the base station.
• The FDMA mobile unit uses duplexers since both the
transmitter and receiver operate at the same time. This
results in an increase in the cost of FDMA subscriber units and
base stations.
• FDMA requires tight RF filtering to minimize adjacent channel
interference.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Time Division Multiple Access
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Time division multiple access
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
TDMA vs FDMA
TDMA FDMA
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• Time division multiple access (TDMA) systems divide the radio
spectrum into time slots
• Each user occupies a cyclically repeat-ing time slot
• A set of ‘N’ slots form a Frame.
• Each frame is made up of a preamble, an information
message, and tail bits
• TDMA systems transmit data in a buffer-and-burst method
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• TDMA shares a single carrier frequency with several
users, where each user makes use of non-
overlapping time slots
• TDMA uses different time slots for transmission and
reception
• Adaptive equalization is usually necessary in TDMA
systems, since the transmission rates are generally
very high as compared to FDMA channelsEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• High synchronization overhead is required in
TDMA systems because of burst transmissions
• Guard Bands are necessary to ensure that
users at the edge of the band do not "bleed
over" into an adjacent radio service.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Frame Structure
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• The preamble contains the address and
synchronization information that both the
base station and the subscribers use to
identify each other.
• Trial bits specify the start of a data.
• Synchronization bits will intimate the receiver
about the data transfer.
• Guard Bits are used for data isolation.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Efficiency of TDMA
• The efficiency of a TDMA system is a measure
of the percentage of transmitted data that
contains information as opposed to providing
overhead for the access scheme
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
whereb0H – no over head bits per framebr - no of overhead bits per bp - no overhead bits per preamble in each slotbg - no equivalent bits in each guard time intervalNr - reference bursts per frame, Nt- traffic bursts per frame
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• The total number of bits per frame, bT, is
bT = TfR
• Tf is the frame duration, and R is the channel bit rate
• Then the frame efficiency is
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• And the no of frames
m - maximum number of TDMA users supported on each radio channel
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Spread spectrum multiple access (SSMA)
• Frequency Hopped Multiple Access (FHMA)
• Direct Sequence Multiple Access (DSMA)
Direct sequence multiple access is also called code division
multiple access (CDMA).
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Frequency Hopped Multiple Access
• The carrier frequencies of the individual users
are varied in a pseudorandom fashion within a
wideband channel
• The digital data is broken into uniform sized
bursts which are transmitted on different
carrier frequenciesEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• Fast Frequency Hopping System -> the rate of
change of the carrier frequency is greater than
the symbol rate
• Slow Frequency Hopping -> the channel
changes at a rate less than or equal to the
symbol rate
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Code Division Multiple Access (CDMA)
• The narrowband message signal is multiplied by a
very large bandwidth signal called the spreading
signal (pseudo-noise code)
• The chip rate of the pseudo-noise code is much
more than message signal.
• Each user has its own pseudorandom codeword.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Message
PN sequence
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• CDMA uses CO-Channel Cells
• All the users use the same carrier frequency and may
transmit simultaneously without any knowledge of
others.
• The receiver performs a time correlation operation
to detect only the specific desired codeword.
• All other code words appear as noise
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• Multipath fading may be substantially reduced
because the signal is spread over a large spectrum
• Channel data rates are very high in CDMA systems
• CDMA supports Soft handoff MSC can
simultaneously monitor a particular user from two or
more base stations. The MSC may chose the best
version of the signal at any time without switching
frequencies.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• In CDMA, the power of multiple users at a receiver
determines the noise floor.
• In CDMA, stronger received signal levels raise the
noise floor at the base station demodulators for the
weaker signals, thereby decreasing the probability
that weaker signals will be received. This is called
Near- Far problem.
• To combat the Near- Far problem, power control is
used in most CDMAEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Random Access for Data-Oriented Networks
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Random Access for Data-Oriented Networks
• In all wireless networks such as cellular telephony or PCSservices all voice-oriented operations use fixed-assignmentchannel access.
• And data related traffic is carried out using Random AccessTechniques.
• Random access methods provide a more flexible and efficientway of managing channel access for communicating shortbursty messages.
• It provides each user station with varying degrees of freedomin gaining access to the network whenever information is tobe sent. EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
ALOHA-Based Wireless Random Access Techniques
• The original ALOHA protocol is also called pure ALOHA.
• ALOHA Protocol is developed by University of Hawaii. The
word ALOHA means "hello" in Hawaiian.
• The initial system used ground-based UHF radios to connect
computers on several of the island campuses with the
university's main computer center on Oahu, by use of a
random access protocol which has since been known as the
ALOHA protocolEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Basic Concept • A mobile terminal transmits an information packet when the packet
arrives from the upper layers of the protocol stack.
• A user accesses a channel as soon as a message is ready to be
transmitted.
• Each packet is encoded with an error-detection code.
• After a transmission, the user waits for an acknowledgment on
either the same channel or a separate feedback channel.
• The BS checks the parity of the received packet. If the parity checks
properly, the BS sends a short acknowledgment packet to the MS.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Collision
• The message packets are transmitted at arbitrary times, so there is
a possibility of collisions between packets.
• After sending a packet the user waits a length of time more than
the round-trip delay for an acknowledgment from the receiver.
• If no acknowledgment is received, the packet is assumed lost in a
collision, and it is transmitted again with a randomly selected delay
to avoid repeated collisions.
• As the number of users increase, a greater delay occurs because the
probability of collision increasesEC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Pure ALOHA
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Merits :• The advantage of ALOHA protocol is that it is very
simple, and it does not impose any synchronization
between mobile terminals
De-Merits:• Its has low throughput under heavy load conditions.
• The maximum throughput of the pure ALOHA is 18
percent. EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Slotted ALOHA• The maximum throughput of a slotted ALOHA is 36 percent.
• In slotted ALOHA, time is divided into equal time slots of
length greater than the packet duration t.
• The subscribers have synchronized clocks and each user will
be synchronized with the BS clock.
• The user message packet is buffered and transmitted only at
the beginning of a new time slot. This prevents partial
collisions. EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
New transmissions are started only at the beginning of new slot EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Application;
• In GSM the initial contact between BS and MS for
voice communication is carried out by slotted
ALOHA.
De-Merit;
• Even though the throughput is higher than pure
ALOHA it is still low for present day wireless
communication needs.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Reservation ALOHA
• Reservation ALOHA is the combination of slotted ALOHA and
time division multiplexing.
• In this certain packet slots are assigned with priority, and it is
possible for users to reserve slots for the transmission of
packets.
• For high traffic conditions, reservations on request offers
better throughput.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Packet Reservation Multiple Access (PRMA)
• PRMA is a method for transmitting a variable mixture of voice
packets and data packets.
• This allows each time slot to carry either voice or data, where
voice is given priority.
• PRMA merges characteristics of slotted ALOHA and TDMA
protocols.
• It is used for short-range voice transmission where a small
delay is acceptable.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• The transmission format in PRMA is organized into frames,
each containing a fixed number of time slots.
• Each slot as named as either "reserved" or "available“
• Only the user terminal that reserved the slot can use a
reserved slot.
• Other terminals not holding a reservation can use an
‘available’ slot.
• Terminals can send two types of information, referred to as
periodic and random.
• Speech packets are always periodic. Data packets can be
random. EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Reservation ;
• A terminal having periodic information to send starts
transmitting in contention for the next available time slot.
• After completion of transmission the base station grants the
sending terminal a reservation for exclusive use of the same
time slot in the next frame.
• This frame is reserved till the terminal completes its
transmission.
• The reservation status is reverted when the terminal sends
nothing in that frameEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
CSMA-Based Wireless Random Access
Techniques
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
De-Merits of ALOHA
1. ALOHA protocols do not listen to the channel before
transmission, the users will start transmitting as
soon as the message is ready.
2. Efficiency is reduced by the collision and
retransmission process.
3. There are no mechanisms to avoid collisions.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
CSMA- Carrier Sense Multiple Access
• In this each terminal will monitor the status of the channel before
transmitting information.
• If there is another user transmitting on the channel, it is obvious
that a terminal should delay the transmission of the packet.
• If the channel is idle, then the user is allowed to transmit data
packet without any restrictions.
• The CSMA protocol reduces the packet collision significantly
compared with ALOHA protocol. But not eliminate entirely.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Parameters in CSMA protocols
1. Detection delay - is a function of the receiver
hardware and is the time required for a terminal to
sense whether or not the channel is idle
2. Propagation delay- is a relative measure of how fast
it takes for a packet to travel from a base station to
a mobile terminal.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• Propagation delay is important, since just after a user begins
sending a packet, another user may be ready to send and may
be sensing the channel at the same time.
• If the transmitting packet has not reached the user who is
poised to send, the latter user will sense an idle channel and
will also send its packet, resulting in a collision between the
two packets.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Propagation delay (td)
where
• tp -> propagation time in seconds,
• Rb -> channel bit rate
• m -> expected number of bits in a data packet EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Various strategies of the CSMA
1. NON-PERSISTENT CSMA — In this type of CSMA strategy, afterreceiving a negative acknowledgment the terminal waits arandom time before retrans-mission of the packet.
2. 1-PERSISTENT CSMA — The terminal senses the channel andwaits for transmission until it finds the channel idle. As soonas the channel is idle, the terminal transmits its messagewith probability one.
3. p-PERSISTENT CSMA —When a channel is found to be idle,the packet is transmitted with probability p . It may or maynot be immediate.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
4. CSMA/CD – In this the user monitors the channel for possible
collisions. If two or more terminals start a transmission at the
same time the transmission is immediately aborted in midway.
5. Data sense multiple access (DSMA) - is a special type of CSMA
that is used to serve the hidden terminals. Cellular networks uses
different frequencies for forward and reverse channel. Each MS
may not have the knowledge about other MS operating in that
area. So it may not know when the channel is idle. For this the BS
can announce the availability of the reverse channel through the
forward control channel. The BS uses Busy-Idle bit to announce.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
6. Busy tone multiple access (BTMA)- this is a special type of
technique where the system bandwidth is divided into
message channel and busy channel. Whenever a terminal
sends data through message channel it will also transmits a
busy-tone in busy channel. If another terminal senses the
busy channel it will understand that the message channel is
busy and it will also turns its busy tone. This acts as an alarm
for other terminals.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Handoff
• Process of transferring a moving active user
from one base station to another without
disrupting the call.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Handoff Strategies
1. Ist generation handoff2. MAHO (Mobile Assisted HandOff)3. Inter system handoff4. Guard channel concept5. Queuing 6. Umbrella approach7. Soft and hard handoff8. Cell dragging.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Ist generation handoff-
• In this almost all the work were carried out by
MSC with the help of Base Station.
• Using the Locator Receiver the MSC will measure
the signal strength of the moving mobile.
• If the level decreases it will perform handoff by
its own.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
MAHO (Mobile Assisted HandOff)
• In this every mobile station measures the received power
from surrounding base stations and continually reports the
results of these measurements to the serving base station.
• When the power received from the base station of a
neighboring cell begins to exceed the power received from
the current base station by a certain level or for a certain
period of time a handoff is initiated.
• Since all the measurements were done by the mobile, the
load of the MSC is reduced considerablyEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• Inter system handoff -occurs if a mobile moves from
one cellular system to a different cellular system
controlled by a different MSC (service provider) or
while roaming
• Guard channel concept – In this some channels are
reserved only for handoff.
• Queuing – If more number of users request handoff the
they will be placed in queue before allotting channels
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Umbrella approach
• Speed of the user is a main factor in deciding a
successful handoff.
• In urban areas the cell size will be very small and high
speed users will cross quickly.
• To perform handoff on these high speed users we
use Micro and Macro cells concurrently.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Umbrella approach
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Cell dragging
• Cell dragging occurs in an urban environment when
there is a line-of-sight (LOS) radio path between the
pedestrian subscriber and the base station.
• Even after the user has traveled well beyond the
designed range of the cell, the received signal at the
base station does not decay rapidly resulting in Cell
DraggingEC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Soft and hard handoff
• Hard handoff- when the user moves to a new cell, he
will be assigned with a new set of channels.
• Soft Handoff- when the user moves to a new cell, the
channel itself will be switched to the new base
station. CDMA uses soft Handoff.
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
WIRELESS SECURITY AND
PRIVACY
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• The broadcast nature of wireless communications
renders it very susceptible to malicious interception and
wanted or unintentional interference.
• Analog techniques are extremely easy to tap.
• Digital systems such as TDMA and CDMA are much
harder to tap.
• Wireless security is necessary to prevent the
unauthorized access or damage to computers
using wireless networks.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
There are two names you need to know in a wireless
network:
• Station (STA) -> is a wireless network client—a desktop
computer, laptop, or PDA
• Access point (AP)-> is the central point (like a hub) that
creates a basic service set to bridge a number of STAs
from the wireless network to other existing networks.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Modes of unauthorized access
1. Accidental association2. Malicious association3. Ad-hoc networks4. Non-traditional networks5. Identity theft (MAC spoofing)6. Man-in-the-middle attacks7. Denial of service8. Network injection9. Caffe Latte attack
http://en.wikipedia.org/wiki/Wireless_securityEC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
1. Accidental association
– Violation of security perimeter of corporate
network unintentionally.
2. Malicious association
– when wireless devices can be actively made by attackers to
connect to a company network through their cracking company
access point (AP).
– These types of laptops are known as “soft APs” and are created
when a cyber criminal runs some software that makes his/her
wireless network card look like a legitimate access point.
– Once access is gained, he/she can steal passwords, launch
attacks on the wired network, or plant TrojansEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
3. Ad-hoc networks
– Ad-hoc networks are defined as peer-to-peer networks
between wireless computers that do not have an access point
in between them. While these types of networks usually have
little protection, encryption methods can be used to provide
security.
4. Non-traditional networks
– Non-traditional networks such as personal
network Bluetooth devices are not safe from cracking and
should be regarded as a security risk. Even barcode readers,
handheld PDAs, and wireless printers and copiers should be
secured EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
5. Identity theft (MAC spoofing)
– Identity theft occurs when a cracker is able to listen
in on network traffic and identify the MAC address of
a computer with network privileges.
6. Man-in-the-middle attacks
– In this the hacker will include a soft AP in to a
network. Once this is done, the hacker connects to a
real access point through another wireless card
offering a steady flow of traffic through the
transparent hacking computer to the real networkEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
7. Denial of service
– A Denial-of-Service attack (DoS) occurs when an
attacker continually bombards a targeted Access
Point or network with bogus requests, premature
successful connection messages, failure messages,
and other commands.
– These cause legitimate users to not be able to get
on the network and may even cause the network
to crashEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
8. Network injection
– In a network injection attack, a cracker can make use
of access points that are exposed to non-filtered
network traffic.
– The cracker injects bogus networking re-configuration
commands that affect routers, switches, and
intelligent hubs.
– A whole network can be brought down in this manner
and require rebooting or even reprogramming of all
intelligent networking devicesEC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
9. Caffe Latte attack
– The Caffe Latte attack is another way to defeat WEP.
– It is not necessary for the attacker to be in the area of
the network using this exploit.
– By using a process that targets the Windows wireless stack,
it is possible to obtain the WEP key from a remote client
– By sending a flood of encrypted Address Resolution
Protocol (ARP) requests, the assailant takes advantage of
the shared key authentication and the message
modification flaws in WEP.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
The Attack Methodology
1. Footprint the wireless network- Locate and understand
your target.
2. Passive attack - Analyze the network traffic or break the
WEP.
3. Authentication and authorization - Determine what
methods are enforced and how they can be circumvented.
4. Active attack - Launch denial of service (DoS) attacks.
http://technet.microsoft.com/en-us/library/bb457019.aspxEC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Defense Mechanisms
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• Wired Equivalent Privacy (WEP)
• Wi-Fi Protected Access (WPA)
• Wi-Fi Protected Access- 2 (WPA-2)
• ……. EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Wired Equivalent Privacy (WEP)
• WEP is a standard network protocol that adds security to
wireless networks at the data link layer.
• WEP utilizes a data encryption scheme called RC4 for
data protection.
• RC4 (also known as ARC4 or ARCFOUR ) is the most
widely used software stream cipher and is used in
popular protocols.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
• RC4 generates a pseudorandom stream of bits.
• Standard 64-bit WEP uses a 40 bit key (WEP-40) and
a 24 bit initialization vector .
• 128-bit WEP protocol using a 104-bit key size (WEP-
104) and a 24 bit initialization vector.
• Initialization vector (IV) is a fixed-size input which is
used for randomization of key. The purpose of an IV
is to prevent any repetition.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Authentication1. The client sends an authentication request to the
Access Point.
2. The Access Point replies with a clear-text challenge.
3. The client encrypts the challenge-text using the
configured WEP key, and sends it back in another
authentication request.
4. The Access Point decrypts the response. If this matches
the challenge-text the Access Point sends back a
positive reply.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Dis Advantages
• The same traffic key must never be used twice.
• But a 24-bit IV is not long enough to ensure this on a busy network.
• In August 2001, Scott Fluhrer, Itsik Mantin, and Adi
Shamir published a cryptanalysis of WEP that decodes the way the
RC4 cipher and IV is used in WEP.
• Using a passive attack they were able to recover the RC4key after
eavesdropping on the network.
• A successful key recovery could take as little as one minute
depending on the traffic.
• WEP is replaced by WPA(Wi-Fi Protected Access)EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Wi-Fi Protected Access(WPA)
• The Wi-Fi Alliance intended WPA as an
intermediate measure to take the place of
WEP.
• WPA uses Temporal Key Integrity Protocol
(TKIP) to bolster encryption of wireless
packets.EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Wi-Fi Protected Access(WPA)
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
TKIP
• TKIP encryption replaces WEP's 40-bit or 104-bit
encryption key that must be manually entered on
wireless access points and devices and does not change
• TKIP uses a 128-bit per-packet key, it dynamically
generates a new key for each packet and prevents
collisions
• It has an extended initialization vector (IV) with
sequencing rules, and a re-keying mechanism.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
WPA with TKIP provides 3 levels of security
1. TKIP implements a key mixing function that
combines the secret root key with the initialization
vector before passing it to the RC4 initialization.
2. WPA implements a sequence counter to protect
against replay attacks. Packets received out of order
will be rejected by the access point.
3. TKIP implements a 64-bit Message Integrity Check
(MIC)EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Merits and Demerits
• TKIP uses the same underlying mechanism as WEP,and consequently is vulnerable to a number ofsimilar attacks.
• But the message integrity check, per-packet keyhashing, broadcast key rotation, and a sequencecounter prevents many attacks.
• The key mixing function also eliminates the WEP keyrecovery attacks
• Beck-Tews attack has successfully extracted thekeystream EC2043 W
IRELESS N
ETWORKS
Visit www.agniece.blog.com for further details Ananth.R
Ohigashi-Morii attack
• Japanese researchers Toshihiro Ohigashi andMasakatu Morii reported a simpler and fasterimplementation of a similar attack.
• It utilizes a similar attack method, but usesa man-in-the-middle attack
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
WPA 2
• WPA2 (Wireless Protected Access 2) replaced the
original WPA technology on all certified Wi-Fi hardware
since 2006.
• WPA2 uses Pre-Shared Key (PSK) instead of TKIP
• WPA2 Pre-Shared Key (PSK) utilizes keys with 256 bits
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
There are two versions of WPA2
1. WPA2-Personal- protects unauthorized
network access by utilizing a set-up password
2. WPA2-Enterprise- verifies network users
through a server. WPA2 is backward
compatible with WPA.EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R
EC2043 WIR
ELESS NETWORKS
Visit www.agniece.blog.com for further details Ananth.R