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Jamming and Anti-Jamming in IEEE 802.11 based WLANs
Ravi Teja C
4/9/2009
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Introduction
• Wireless communications are highly susceptible to interference
• To protect some devices from interference, frequency spectrum is allocated into– Licensed band: a service provider owns a part of
the spectrum.– Unlicensed band: everybody has equal right to use
it as long they conform to the regulations
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Introduction
• IEEE 802.11 based WLANs use unlicensed spectrum.
• Every user shares the channel with every other device operating in that band
• Every device has equal right to access the channel.
• Presence of other devices can severely affect the performance.
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Example: Interference from cordless phone
Phone turned OFFPhone turned ON
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Example: Interference from microwave oven
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And Interference from fifth floor lab
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Jamming
• Jamming is intentionally interfering to decrease the performance of a system.
• It can be done at different layers.• Physical (PHY) layer jamming– Continuous transmission of signal– Periodic transmission– Spread spectrum technologies are extensively
used to combat this type of jamming.
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• Jamming attacks are possible at other layers.• Here we focus on MAC layer of 802.11
standard.
• DCF* is the commonly used MAC technique in the IEEE 802.11 WLAN standard.
*DCF: Distributed Coordination Function
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Distributed Coordinated Function(DCF)
• A station that needs to transmit needs to– Sense idle channel for DIFS– Choose a random backoff timer between 0 and
contention window(CW), and counts down.– Transmit after the expiry of backoff timer
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Distributed Coordinated Function(DCF)
– On sensing a busy medium, backoff timer is frozen.
– Timer is restarted on sensing idle channel for DIFS– A failure will cause in exponential increase in CW
size.
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Jammer objective
• Jammer’s objective is to disrupt as much as possible.
• Compared to transmitting continuously, transmitting only when required is more effective in terms of power.
• For jamming devices with limited energy, jamming every packet might not be a good strategy.
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Markov Chain Model for DCF• This behavior can be modeled as a Markov
chain to study the effect of jamming on the system.
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Throughput
• From this model, the probability of a successful transmission(S), failed transmission (F) or the medium is idle(1-S-F) in a slot can be derived.
• Throughput can be defined as
SE p
1¡ S¡ F +STs F Tc
SEp1 ¡ S ¡ F + STs+ F Tc
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Ts and Tc
• Ts is the time required to transmit a frame successfully
• Tc is the time consumed due to a failure.
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Testbed jammer
• We built a jammer to observe the effect of jamming experimentally.
• Ath5k, a driver for WLAN cards, is modified to implement the jamming functionality.
• We used Atheros proprietary XR mode to stomp an ongoing transmission.
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Packet failure probabilityHere we assume that the jammer’s packet always destroys the target packet.
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ThroughputThroughput monotonically decreases with increase in jamming probability.
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Power expenditure of jammer
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DCF: shortcoming
Jammer’s power expenditure Reason• CW size is pushed to the
maximum value• Transmission probability
decreases.• Longer delays for
transmission• Smaller energy
consumption for jammer.
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ANTI JAMMING
DCF ModificationDetection of jamming
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DCF: modificationModified DCFMotivation
• Standard DCF reduces the transmission probability with collision
• DCF is modified to increase the jammer’s energy expenditure with jamming probability.
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Comparison of power expenditure of jammer for modified and standard DCF (3 stations)
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Comparison of power expenditure of jammer for modified and standard DCF (10 stations)
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Throughput comparison
10 stations 50 stations
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Probability of collisionPicture showing the collision probability versus the number of nodes in the system.
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Detection of jamming
• After transmitting K packets, each station looks at the number of times it has been in each transmitting state i.e., Markov Chain Model for DCF
• The problem of detecting jamming in the presence of normal collisions is a composite hypothesis testing problem.
• It is solved using Generalized Likelihood Ratio Test(GLRT).
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GLRT
• Hypotheses H0 and H1 are defined as follows
• The test takes the following form
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ROC curvePicture showing Receiver Operating Characteristics of the detector
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Conclusions
• A jammer is implemented to verify the effect of jamming in IEEE 802.11 wireless network.
• The Markovian model models the DCF very accurately.
• Standard DCF favors jamming probability of 1.• DCF is modified to increase the rate at which
jammer expends energy.
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Future Work
• Implement selective jamming.
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Thank you.
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Additional slides
Backup
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Power comparison of jammer w/ and w/o collision detection
10 stations 50 stations