Jamming and Anti-Jamming in IEEE based WLANs Ravi Teja C 4/9/2009 TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAA A A A A

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 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 standard. DCF * is the commonly used MAC technique in the IEEE 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

T s and T c T s is the time required to transmit a frame successfully T c 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 probability Here we assume that the jammer’s packet always destroys the target packet.

Throughput Throughput monotonically decreases with increase in jamming probability.

Power expenditure of jammer

DCF: shortcoming Jammer’s power expenditureReason CW size is pushed to the maximum value Transmission probability decreases. Longer delays for transmission Smaller energy consumption for jammer.

ANTI JAMMING DCF Modification Detection of jamming

DCF: modification Modified DCF Motivation 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 collision Picture 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 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 H 0 and H 1 are defined as follows The test takes the following form

ROC curve Picture showing Receiver Operating Characteristics of the detector

Conclusions A jammer is implemented to verify the effect of jamming in IEEE 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 stations50 stations