Jamming and Anti-Jamming in IEEE 802.11 based WLANs

Ravi Teja C

4/9/2009

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

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.

Example: Interference from cordless phone

Phone turned OFFPhone turned ON

Example: Interference from microwave oven

And Interference from fifth floor lab

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.

• 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

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

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.

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.

Markov Chain Model for DCF• This behavior can be modeled as a Markov

chain to study the effect of jamming on the system.

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

Ts and Tc

• Ts is the time required to transmit a frame successfully

• Tc is the time consumed due to a failure.

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.

Packet failure probabilityHere we assume that the jammer’s packet always destroys the target packet.

ThroughputThroughput monotonically decreases with increase in jamming probability.

Power expenditure of jammer

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.

ANTI JAMMING

DCF ModificationDetection of jamming

DCF: modificationModified DCFMotivation

• Standard DCF reduces the transmission probability with collision

• DCF is modified to increase the jammer’s energy expenditure with jamming probability.

Comparison of power expenditure of jammer for modified and standard DCF (3 stations)

Comparison of power expenditure of jammer for modified and standard DCF (10 stations)

Throughput comparison

10 stations 50 stations

Probability of collisionPicture showing the collision probability versus the number of nodes in the system.

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).

GLRT

• Hypotheses H0 and H1 are defined as follows

• The test takes the following form

ROC curvePicture showing Receiver Operating Characteristics of the detector

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.

Future Work

• Implement selective jamming.

Thank you.

Additional slides

Backup

Power comparison of jammer w/ and w/o collision detection

10 stations 50 stations