Ananth Rao

Geographic routing without location information

For many years, scalable routing for wireless communication systems was a compelling but elusive goal. Recently, several routing algorithms that exploit geographic information (e.g. GPSR) have been proposed to achieve this goal. These algorithms refer to nodes by their location, not address, and use those coordinates to route greedily, when possible, towards the destination. However, there are many situations where location information is not available at the nodes, and so geographic methods cannot be used. In this paper we define a scalable coordinate-based routing algorithm that does not rely on location information, and thus can be used in a wide variety of ad hoc and sensornet environments.

Load Balancing in Structured P2P Systems

Most P2P systems that provide a DHT abstraction distribute objects among “peer nodes” by choosing random identifiers for the objects. This could result in an O(log N) imbalance. Besides, P2P systems can be highly heterogeneous, i.e. they may consist of peers that range from old desktops behind modem lines to powerful servers connected to the Internet through high-bandwidth lines. In this paper, we address the problem of load balancing in such P2P systems. We explore the space of designing load-balancing algorithms that uses the notion of “virtual servers”. We present three schemes that differ primarily in the amount of information used to decide how to re-arrange load. Our simulation results show that even the simplest scheme is able to balance the load within 80% of the optimal value, while the most complex scheme is able to balance the load within 95% of the optimal value.

Estimation of link interference in static multi-hop wireless networks

We present a measurement-based study of interference among links in a static, IEEE 802.11, multi-hop wireless network. Interference is a key cause of performance degradation in such networks. To improve, or to even estimate the performance of these networks, one must have some knowledge of which links in the network interfere with one another, and to what extent. However, the problem of estimating the interference among links of a multi-hop wireless network is a challenging one. Accurate modeling of radio signal propagation is difficult since many environment and hardware-specific factors must be considered. Empirically testing every group of links is not practical: a network with n nodes can have O(n2) links, and even if we consider only pairwise interference, we may have to potentially test O(n4) pairs. Given these difficulties, much of the previous work on wireless networks has assumed that information about interference in the network is either known, or that it can be approximated using simple heuristics. We test these heuristics in our testbed and find them to be inaccurate. We then propose a simple, empirical estimation methodology that can predict pairwise interference using only O(n2) measurements. Our methodology is applicable to any wireless network that uses omni-directional antennas. The predictions made by our methodology match well with the observed pairwise interference among links in our 22 node, 802.11-based testbed.

An overlay MAC layer for 802.11 networks

The widespread availability of 802.11-based hardware has made it the premier choice of both researchers and practitioners for developing new wireless networks and applications. However, the ever increasing set of demands posed by these applications is stretching the 802.11 MAC protocol beyond its intended capabilities. For example, 802.11 provides no control over allocation of resources, and the default allocation policy is ill-suited for heterogeneous environments and multi-hop networks. Fairness problems are further exacerbated in multi-hop networks due to link asymmetry and hidden terminals. In this paper, we take a first step towards addressing these problems without replacing the MAC layer by presenting the design and the implementation of an Overlay MAC Layer (OML), that works on top of the 802.11 MAC layer. OML uses loosely-synchronized clocks to divide the time in to equal size slots, and employs a distributed algorithm to allocate these slots among competing nodes. We have implemented OML in both a simulator and on a wireless testbed using the Click modular router. Our evaluation shows that OML can not only provide better flexibility but also improve the fairness, throughput and predictability of 802.11 networks.

Troubleshooting wireless mesh networks

Effective network troubleshooting is critical for maintaining efficient and reliable network operation. Troubleshooting is especially challenging in multi-hop wireless networks because the behavior of such networks depends on complicated interactions between many factors such as RF noise, signal propagation, node interference, and traffic flows. In this paper we propose a new direction for research on fault diagnosis in wireless mesh networks. Specifically, we present a diagnostic system that employs trace-driven simulations to detect faults and perform root cause analysis. We apply this approach to diagnose performance problems caused by packet dropping, link congestion, external noise, and MAC misbehavior. In a 25 node mesh network, we are able to diagnose over 10 simultaneous faults of multiple types with more than 80% coverage.

Troubleshooting multihop wireless networks

Effective network troubleshooting is critical for maintaining efficient and reliable network operation. Troubleshooting is especially challenging in multihop wireless networks because the behavior of such networks depends on complicated interactions between many unpredictable factors such as RF noise, signal propagation, node interference, and traffic flows. In this paper we propose a new direction for research on fault diagnosis in wireless networks. Specifically, we present a diagnostic system that employs trace-driven simulations to detect faults and perform root cause analysis. We apply this approach to diagnose performance problems caused by packet dropping, link congestion, external noise, and MAC misbehavior. In a 25 node multihop wireless network, we are able to diagnose over 10 simultaneous faults of multiple types with more than 80% coverage. Our framework is general enough for a wide variety of wireless and wired networks.

Adaptive Distributed Time-Slot Based Scheduling for Fairness in Multi-Hop Wireless Networks

Recent research indicates that multi-hop wireless networks can suffer from extreme imbalances in the throughput achieved by simultaneous competing flows. We address this problem by designing a practical distributed algorithm to compute a time-slot based schedule that provides end-to-end max-min fairness. Our system uses randomized priorities based on local weights to arbitrate access between nodes that directly compete with each other (we call this weighted slot allocation or WSA). The local weights are in turn computed by a higher layer called end-to-end fairness using local weights (EFLoW). EFLoW implements an additive-increase multiplicative-decrease (AIMD) algorithm that can automatically adapt to changes in traffic demands and network conditions. In each iteration, EFLoW only uses state obtained from within a given nodes contention region. We have implemented WSA and EFLoW in both a simulator and a real system by using the overlay MAC layer (OML). Unlike previous work on end-to-end fairness, our approach does not use a centralized coordinator and works for traffic patterns with any number of sources and sinks. Also, since we compute both the fair allocation and a schedule to achieve it, we do not make any assumptions about the efficiency of carrier-sense (CS) based MACs - this is very important in the light of recent work which shows that current CS-based MACs can be very unfair even when all nodes are limited to sending at their fair rate. Our results show that WSA and EFLoW can prevent starvation of flows and improve fairness without sacrificing efficiency for a wide variety of traffic patterns.

End-host controlled multicast routing

The last decade has seen a deluge of proposals for supporting multicast in the Internet. These proposals can be categorized as either infrastructure-based, with the multicast functionality provided by specialized network nodes, or host-based, with the multicast functionality provided by the members of the multicast group itself. In this paper, we present the design and evaluation of a hybrid multicast architecture wherein the infrastructure provides packet forwarding, and the end-hosts implement the control plane. End-hosts build multicast trees by setting up forwarding state in the infrastructure. This division of functionality enables our architecture to combine the efficiency of infrastructure-based solutions and the flexibility and deployability of host-based solutions. We present scalable and efficient algorithms for distributed tree construction and maintenance, and for reliable packet delivery. We have implemented the algorithms using i3 as the forwarding infrastructure. We evaluate our techniques using a combination of event-driven packet-level simulations, and our implementation over the PlanetLab testbed.