Spatial reuse in 802.11 wireless networks is quite limited since neighbors of both the transmitter and the receiver must keep quiet for the entire duration of the communication. This is due to the bidirectional exchange of frames, i.e., nodes reverse their roles as transmitters and receivers, for transmitting a single DATA frame. To improve spatial reuse, we have proposed ways to reduce role reversals and enable concurrent transmissions. More recently, we are are exploring MAC schemes that leverage new physical layer capabilities such as Message-in-Message (MIM) for further improving spatial reuse in wireless networks.

• N. Santhapuri, J. Manweiler, S. Sen, R. R. Choudhury, S. Nelakuditi, K. Munagala, Message in Message (MIM): A Case for Shuffling Transmissions in Wireless Networks, HotNets, October 2008
• N. Santhapuri, S. Nelakuditi, and R. R. Choudhury, On Spatial Reuse and Capture in Ad Hoc Networks, WCNC, April 2008.
• N. Santhapuri, S. Nelakuditi, and R. R. Choudhury, Capture-Aware Staggering for Concurrent Transmissions, CoNEXT, December 2007.
• N. Santhapuri, J. Wang, Z. Zhong, and S. Nelakuditi, Piggybacked-Ack-aided Concurrent Transmissions in Wireless Networks, ICNP, November 2005.

In wireless networks, transmissions are broadcast, and all neighbors of a transmitting node can potentially receive the packet. Wireless network coding is a way to exploit broadcast transmissions and improve throughput by conveying more information in each transmission. A sender may encode multiple packets belonging to different flows (inter-flow) or to the same flow (intra-flow). We explore integration of intra-flow and inter-flow coding, route and rate selection to maximize coding gain.

• B. Ni, N. Santhapuri, C. Gray, and S. Nelakuditi, Selection of Bit-rate for Wireless Network Coding, WiNC, June 2008.
• Q. Chuan, Y. Xian, C. Gray, N. Santhapuri, and S. Nelakuditi, I2MIX: Integration of Intra-flow and Inter-flow Wireless Network Coding. WiNC, June 2008.
• B. Ni, N. Santhapuri, Z. Zhong, and S. Nelakuditi, Routing with Opportunistically Coded Exchanges in Wireless Mesh Networks. WiMesh, September 2006.

The successful reception of a packet in a wireless network depends on the bit-rate used for transmitting that packet. If the sender transmits at a low bit-rate, then its packet might be received by some far away nodes due to low SINR (signal to interference and noise ratio) requirements at lower rates. But it would tie up the wireless channel for many of its neighbors for a longer period of time due to the slower transmission rate. Conversely, if the sender transmits at a higher bit-rate, the packet loss probability would be high and the number of potential receivers might be few, but the channel would be occupied for less time. While several schemes have been proposed for selecting bit-rate for unicast transmission to a single receiver, none of them are suitable for broadcast transmission to multiple receivers under opportunistic routing or network coding. We study how the choice of bit-rate impacts the candidate selection under opportunistic routing and coding gain under network coding.

• C. Gray, N. Santhapuri, and S. Nelakuditi, On Bit-Rate Selection for Opportunistic Routing, WiMesh, June 2008.
• B. Ni, N. Santhapuri, C. Gray, and S. Nelakuditi, Selection of Bit-rate for Wireless Network Coding, WiNC, June 2008.

Traditional routing schemes select the best path for each destination and forward a packet to the corresponding next hop. While such best-path routing schemes are considered well-suited for networks with reliable point-to-point links, they are not necessarily ideal for wireless networks with lossy broadcast links. Consequently, opportunistic routing schemes that exploit the broadcast nature of wireless transmissions and dynamically select a next-hop per-packet based on loss conditions at that instant are being actively explored. We propose new metrics suitable for selection and prioritization of candidates for opportunistic routing, and investigate the scenarios where opportunistic routing is most effective.

• C. Gray, N. Santhapuri, and S. Nelakuditi, On Bit-Rate Selection for Opportunistic Routing. In Proceedings of WiMesh, June 2008.

• Z. Zhong, and S. Nelakuditi, On the Efficacy of Opportunistic Routing. In Proceedings of SECON'07, June 2007.

With the emergence of Voice over IP and other real-time business applications, there is a growing demand for an IP network with high service availability. Unfortunately, in today's Internet, transient failures occur frequently due to faulty interfaces, router crashes, etc., and current IP networks lack the resiliency needed to provide high availability. To improve failure resiliency without jeopardizing routing stability, we propose failure inferencing based fast rerouting (FIFR) approach that exploits the existence of a forwarding table per line-card, for lookup efficiency in current routers, to provide fast rerouting similar to MPLS, while adhering to the destination-based forwarding paradigm.

• J. Wang and S. Nelakuditi, IP Fast Reroute with Failure Inferencing. In Proceedings of INM'07, The 5-9's Workshop at ACM SIGCOMM, August 2007.
• S. Nelakuditi, S. Lee, Y. Yu, Z.-L. Zhang, and C.-N. Chuah, Fast Local Rerouting for Handling Transient Link Failures. IEEE/ACM Transactions on Networking, April 2007.
• J. Wang, Z. Zhong, and S. Nelakuditi, Handling Multiple Network Failures through Interface Specific Forwarding. In Proceedings of Globecom'06, November 2006.
• Z. Zhong, S. Nelakuditi, Y. Yu, S. Lee, J. Wang, and C. N. Chuah, Failure Inferencing based Fast Rerouting for Handling Transient Link and Node Failures. In Proceedings of IEEE Global Internet, March 2005.
• S. Lee, Y. Yu, S. Nelakuditi, Z.-L. Zhang, and C-N. Chuah, Proactive vs. Reactive Approaches to Failure Resilient Routing. In Proceedings of IEEE INFOCOM'04, Hong Kong, March 2004.
• S. Nelakuditi, S. Lee, Y. Yu, and Z.-L. Zhang, Failure Insensitive Routing for Ensuring Service Availability. In Proceedings of IWQoS'03, Lecture Notes in Computer Science 2707, June 2003.

Most of the wireless routing schemes have been designed primarily for mobile ad hoc networks with unpredictable topologies, and hence their route discovery and maintenance mechanisms are not ideal for fixed networks. We propose localized on-demand link state (LoLS) for scalable routing in fixed multihop wireless networks. The central idea behind the LoLS approach is to disseminate a base topology reflecting the long-term state of each link to all the nodes in the network, and propagate the short-term state of discrepant links with negative deviations w.r.t. the base topology to only the nodes in the neighborhood. Under LoLS, each link is assigned a long-term cost based on its usual quality and a short-term cost based on its current quality. The set of all links with their associated long-term costs forms the base topology, and a link is considered discrepant if its short-term cost is worse than the long-term cost. While the global base topology updates are performed infrequently, the localized discrepant link updates are triggered on-demand, i.e., a discrepant link's state is propagated only when needed and as far as necessary to enable loop-free forwarding.

• S. Nelakuditi, S. Lee, Y. Yu, J. Wang, Z. Zhong, G.-H. Lu and Z.-L. Zhang, Blacklist-Aided Forwarding in Static Multihop Wireless Networks. In Proceedings of SECON'05, September 2005.
• J. Wang, S. Lee, Z. Zhong and S. Nelakuditi, Localized On-Demand Link State Routing for Fixed Wireless Networks. In Poster Session of ACM SIGCOMM'05, August 2005.

Under link-state routing protocols such as OSPF and IS-IS, when there is a change in the topology, propagation of link-state announcements, path recomputation, and updating of forwarding tables (FIBs) will all incur some delay before traffic forwarding can resume on alternate paths. During this convergence period, routers may have inconsistent views of the network, resulting in transient forwarding loops. Previous remedies proposed to address this issue enforce a certain order among the nodes in which they update their FIBs. While such approaches succeed in avoiding transient loops, they incur additional message overhead and increased convergence delay.We propose an alternate approach, loopless interface-specific forwarding (LISF), that averts transient loops by forwarding a packet based on both its incoming interface and destination. LISF requires no modifications to the existing link-state routing mechanisms. It is easily deployable with current routers since they already maintain a FIB at each interface for lookup efficiency.

• S. Nelakuditi, Z. Zhong, J. Wang, R. Keralapura, and C.-N. Chuah, Mitigating Transient Loops through Interface-specific Forwarding. Computer Networks, Volume 52, Issue 3, February 2008, Pages 593-609.

• Z. Zhong, R. Keralapura, S. Nelakuditi, Y. Yu, J. Wang, C.-N. Chuah, S. Lee, Avoiding Transient Loops through Interface-Specific Forwarding. In Proceedings of IWQoS'05, June 2005.

Wireless networks are susceptible to simple forms of radio interference attacks (i.e. jamming) that can prevent other wireless devices from even being to transmit or receive. The traditional approach to coping with radio interference is to employ more sophisticated physical-layer technologies (such as spread spectrum). Such methods, however, imply more expensive transceivers and, with the exception of some military systems, most commodity sensors, like the Berkeley Mica2 or the Zigbee, do not employ sufficiently strong spreading techniques to survive jamming or to achieve multiple accesses. Our approach to this problem has been to design a light-weight defense approach that can be employed on commodity wireless devices.

• W. Xu, On Adjusting Power to Defend Wireless Networks from Jamming. In proceedings of the First Workshop on the Security and Privacy of Emerging Ubiquitous Communication Systems (speucs), 2007.

• W. Xu, W. Trappe, and Y. Zhang, Anti-Jamming Timing Channels for Wireless Networks. In proceedings of the 1st ACM Conference on Wireless Security (WiSec), pg. 203-213, 2008. (Acceptance ratio: 16.7%)

Wireless communication is susceptible to radio interference and jamming attacks, which prevent the reception of communications. Most existing anti-jamming work does not consider the location information of radio interferers and jammers. However, this information can provide important insights for networks to manage its resource and to defend against radio interference. For instance, one can cope with a jammer by localizing it and neutralize it through human intervention, or a routing protocol can choose a route that does not traverse the jammed region to avoid wasting resources due to failed packet delivery. In this project, we explore methods to localize radio interferers in wireless networks. Most existing localization algorithms are not applicable in jamming scenario, since they rely on the cooperation of the target. Thus, we proposed a Virtual Force Iterative Localization (VFIL) algorithm, which estimates the location of a jammer iteratively by utilizing the network topology.

• H. Liu, W. Xu, Y. Chen, and Z. Liu, Localizing Jammers in Wireless Networks. In Proceedings of IEEE PerCom International Workshop on Pervasive Wireless Networking (IEEE PWN) (Held in conjunction with IEEE PerCom), Texas, March 2009.

A first line of defense for protecting sensor communications involves applying encryption to provide confidentiality. However, these methods cannot address all of the contextual privacy issues that will arise in sensor networks. One type of sensitive contextual information that needs to be protected is the location provacy of the sink. Due to the many-to-one communication pattern, many communication statistics near the sink are typically different from the ones at other spots. For instance, the traffic rate near the sink is higher than the one in other places. The objective of sink location privacy is to make it difficult for an adversary to infer the location of the sink by merely monitoring the encrypted traffics crossing the network.

Tracking problems for mobile robots have received substantial attention in recent years. Informally, a robot tracker seeks to maintain close proximity to an unpredictable target. Effective target tracking algorithms have many important applications, including monitoring and security. Existing methods for robotic tracking are hampered by two primary limitations. Existing target tracking algorithms require the tracker to have access to information-rich sensors, and may have difficulty recovering when the target is out of the tracker’s sensing range. In this project, we consider a target tracking architecture that combines an extremely simple mobile robot with a networked collection of wireless sensor nodes, each of which is equipped with an unreliable, limited-range, boolean sensor for detecting the target. The tracker maintains close proximity to the target using only information sensed by the network, and can effectively recover from temporarily losing track of the target.

• J. M. O'Kane, and W. Xu, Energy-efficient Target Tracking with a Sensorless Robot and a Network of Unreliable One-bit Priximity Sensors. In Proceedings of IEEE International Conference on Robotics and Automation (ICRA), 2009.