Saumitra M. Das

Ekta: an efficient DHT substrate for distributed applications in mobile ad hoc networks

Distributed hash tables (DHTs) have proven to be a novel and efficient platform for building a variety of scalable and robust distributed applications like content sharing and location in the Internet. Similar to those in the Internet, distributed applications and network services in mobile ad hoc networks (MANETs) can potentially benefit from the deployment of a DHT. However, bandwidth limitations, node mobility, and multi access interference pose unique challenges to deploying such DHTs in MANETs. In this paper, we first study how to efficiently implement DHTs in MANETs. We explore two disparate design options: the simple approach of directly overlaying a DHT on top of a MANET multi-hop routing protocol, and Ekta which integrates a DHT with a multi-hop routing protocol at the network layer. Second, we examine the efficiency of DHT substrates in supporting applications in MANETs by examining the performance of a resource discovery application built on top of Ekta with one that directly uses physical layer broadcast. Such a study answers the fundamental question of whether a DHT substrate can be more efficient in supporting applications than a physical layer broadcast-based protocol, since in MANETs, DHT protocols effectively rely on physical layer broadcast to discover and maintain routes.

Path Planning of Mobile Landmarks for Localization inWireless Sensor Networks

Many applications of wireless sensor networks require the sensor nodes to obtain their locations. The main idea in most localization methods has been that some nodes with known coordinates (e.g., GPS-equipped nodes) transmit beacons with their coordinates in order to help other nodes to localize themselves. A promising method that significantly reduces the deployment cost is to replace the set of statically deployed GPS-enhanced sensors with one mobile landmark equipped with a GPS unit. In this case, a fundamental research issue is the planning of the path that the mobile landmark should travel along in order to minimize the localization error. In this paper we first study the localization error of three different trajectories for the mobile landmark, namely SCAN, DOUBLE SCAN, and HILBERT. We further study the tradeoffs between the trajectory resolution and the localization accuracy in the presence of 2-hop localization, in which sensors that have already obtained an estimate of their positions help to localize other sensors. Our trajectories are practical and can be easily implemented in mobile robot platforms.

DMesh: Incorporating Practical Directional Antennas in Multichannel Wireless Mesh Networks

Wireless mesh networks (WMNs) have been proposed as an effective solution for ubiquitous last-mile broadband access. Three key factors that affect the usability of WMNs are high throughput, cost-effectiveness, and ease of deployability. In this paper, we propose DMesh, a WMN architecture that combines spatial separation from directional antennas with frequency separation from orthogonal channels to improve the throughput of WMNs. DMesh achieves this improvement without inhibiting cost-effectiveness and ease of deployability by utilizing practical directional antennas that are widely and cheaply available (e.g., patch and yagi) in contrast to costly and bulky smart beamforming directional antennas. Thus, the key challenge in DMesh is to exploit spatial separation from such practical directional antennas despite their lack of electronic steerability and interference nulling, as well as the presence of significant sidelobes and backlobes. In this paper, we study how such practical directional antennas can improve the throughput of a WMN. Central to our architecture is a distributed, directional channel assignment algorithm for mesh routers that effectively exploits the spatial and frequency separation opportunities in a DMesh network. Simulation results show that DMesh improves the throughput of WMNs by up to 231% and reduces packet delay drastically compared to a multiradio multichannel omni antenna network. A DMesh implementation in our 16-node 802.11b WMN testbed using commercially available practical directional antennas provides transmission control protocol throughput gains ranging from 31% to 57%

Hierarchical geographic multicast routing for wireless sensor networks

Wireless sensor networks comprise typically dense deployments of large networks of small wireless capable sensor devices. In such networks, multicast is a fundamental routing service for efficient data dissemination required for activities such as code updates, task assignment and targeted queries. In particular, efficient multicast for sensor networks is critical due to the limited energy availability in such networks. Multicast protocols that exploit location information available from GPS or localization algorithms are more efficient and robust than other stateful protocols as they avoid the difficulty of maintaining distributed state (multicast tree). Since localization is typically already required for sensing applications, this location information can simply be reused for optimizing multicast performance at no extra cost. Recently, two protocols were proposed to optimize two orthogonal aspects of location-based multicast protocols: GMR (Sanchez et al. GMR: Geographic multicast routing for wireless sensor networks. In Proceedings of the IEEE SECON, 2006) improves the forwarding efficiency by exploiting the wireless multicast advantage but it suffers from scalability issues when dealing with large sensor networks. On the other hand, HRPM (Das et al. Distributed hashing for scalable multicast in wireless ad hoc networks. IEEE TPDS 47(4):445–487, 2007) reduces the encoding overhead by constructing a hierarchy at virtually no maintenance cost via the use of geographic hashing but it is energy-inefficient due to inefficacies in forwarding data packets. In this paper, we present HGMR (hierarchical geographic multicast routing), a new location-based multicast protocol that seamlessly incorporates the key design concepts of GMR and HRPM and optimizes them for wireless sensor networks by providing both forwarding efficiency (energy efficiency) as well as scalability to large networks. Our simulation studies show that: (i) In an ideal environment, HGMR incurs a number of transmissions either very close to or lower than GMR, and, at the same time, an encoding overhead very close to HRPM, as the group size or the network size increases. (ii) In a realistic environment, HGMR, like HRPM, achieves a Packet Delivery Ratio (PDR) that is close to perfect and much higher than GMR. Further, HGMR has the lowest packet delivery latency among the three protocols, while incurring much fewer packet transmissions than HRPM. (iii) HGMR is equally efficient with both uniform and non-uniform group member distributions.

Sensor replacement using mobile robots

Sensor replacement is important for sensor networks to provide continuous sensing services. Upon sensor node failures, holes (uncovered areas) may appear in the sensing coverage. Existing approaches relocate redundant nodes to fill the holes and require all or most sensor nodes to have mobility. However, mobility equipment is expensive while technology trends are scaling sensors to be smaller and cheaper. In this paper, we propose to use a small number of mobile robots to replace failed sensors for a large-scale static sensor network. We study algorithms for detecting and reporting sensor failures and coordinating the movement of robots that minimize the motion energy of mobile robots and the messaging overhead incurred to the sensor network. A manager receives failure reports and determines which robot to handle a failure. We study three algorithms: a centralized manager algorithm, a fixed distributed manager algorithm, and a dynamic distributed manager algorithm. Our analysis and simulations show that: (a) the centralized and the dynamic distributed algorithms have lower motion overhead than the fixed distributed algorithm; (b) the centralized algorithm is less scalable than the two distributed manager algorithms, and (c) the two distributed algorithms have higher messaging cost than the centralized algorithm. Hence, the optimal choice of the coordination algorithm depends on the specific scenarios and objectives being optimized.

Distributed Hashing for Scalable Multicast in Wireless Ad Hoc Networks

Several multicast protocols for mobile ad hoc networks have been proposed, which build multicast trees by using location information that is available from the Global Positioning System (GPS) or localization algorithms and use geographic forwarding to forward packets down the multicast trees. These stateless multicast protocols carry encoded membership, location, and tree information in each packet and are more efficient and robust than stateful protocols (for example, ADMR and ODMRP), as they avoid the difficulty of maintaining distributed state in the presence of frequent topology changes. However, current stateless multicast protocols are not scalable to large groups because of the per-packet encoding overhead, and the centralized group membership and location management. We present the hierarchical rendezvous point multicast (HRPM) protocol, which significantly improves the scalability of stateless multicast with respect to the group size. HRPM consists of two key design ideas: 1) hierarchical decomposition of a large group into a hierarchy of recursively organized manageable-sized subgroups and 2) the use of distributed geographic hashing to construct and maintain such a hierarchy at virtually no cost. Our detailed simulations demonstrates that HRPM achieves significantly enhanced scalability and performance due to hierarchical organization and distributed hashing.

Characterizing multi-way interference in wireless mesh networks

Wireless mesh networks (WMNs) have been proposed as a solution for ubiquitous last-mile broadband access. A critical limiting factor for many WMN protocols in realizing their throughput potential is the interference between nodes in the WMN. Understanding and characterizing such interference is important for a variety of purposes such as channel assignment, route selection, and fair scheduling. Instead of using ad hoc heuristics, a recent study proposed characterizing interference in a WMN by measuring two-way interference, i.e., interference between each pair of communicating links.In this paper, we study the extent of multi-way interference, i.e., the interference caused by multiple transmitters to a communicating link. We find through simulations and through measurements of a 32-node wireless testbed that even if these transmitters individually do not interfere significantly with a given communicating link, simultaneous transmissions of them have the potential to significantly affect the throughput of the communicating link. This implies that pairwise interference measurements may be optimistic when used to drive protocols in wireless mesh networks. Encouragingly, we find that this phenomenon, although significant when it occurs, is not widespread. In particular, multi-way interference caused significant additional throughput degradation compared to pairwise interference to a small fraction of the links in the testbed over our measurement period. In addition, we find that there is a strong correlation between the impact of multi-way interference and the quality of the link under consideration. We conclude with recommendations on how protocols should take multi-way interference into account.

The performance impact of traffic patterns on routing protocols in mobile ad hoc networks

In this paper, we examine the communication model widely used in simulation studies of mobile ad hoc networks (MANETs). We find that the communication model uses an overly simplistic traffic pattern which restricts the number of connections that originate from each source node to be one or two, and thus the communication model may not represent traffic patterns in many potential applications in MANETs. We then propose a new communication model which extends the previous communication model to include a more general traffic pattern that varies the number of connections per source node. We study the performance impact of traffic patterns on various routing protocols via detailed simulations of a MANET of 112 mobile nodes. Our simulation results show that many of the conclusions drawn in previous protocol comparison studies no longer hold under the new communication model. These results motivate the need for performance evaluation of MANETs to not only include rich and diverse mobility models as has been done in the past but also include diverse traffic patterns that stress a wide set of protocol design issues.

Mitigating the gateway bottleneck via transparent cooperative caching in wireless mesh networks

Wireless mesh networks (WMNs) have been proposed to provide cheap, easily deployable and robust Internet access. The dominant Internet-access traffic from clients causes a congestion bottleneck around the gateway, which can significantly limit the throughput of the WMN clients in accessing the Internet. In this paper, we present MeshCache, a transparent caching system for WMNs that exploits the locality in client Internet-access traffic to mitigate the bottleneck effect at the gateway, thereby improving client-perceived performance. MeshCache leverages the fact that a WMN typically spans a small geographic area and hence mesh routers are easily over-provisioned with CPU, memory, and disk storage, and extends the individual wireless mesh routers in a WMN with built-in content caching functionality. It then performs cooperative caching among the wireless mesh routers. We explore two architecture designs for MeshCache: (1) caching at every client access mesh router upon file download, and (2) caching at each mesh router along the route the Internet-access traffic travels, which requires breaking a single end-to-end transport connection into multiple single-hop transport connections along the route. We also leverage the abundant research results from cooperative web caching in the Internet in designing cache selection protocols for efficiently locating caches containing data objects for these two architectures. We further compare these two MeshCache designs with caching at the gateway router only. Through extensive simulations and evaluations using a prototype implementation on a testbed, we find that MeshCache can significantly improve the performance of client nodes in WMNs. In particular, our experiments with a Squid-based MeshCache implementation deployed on the MAP mesh network testbed with 15 routers show that compared to caching at the gateway only, the MeshCache architecture with hop-by-hop caching reduces the load at the gateway by 38%, improves the average client throughput by 170%, and increases the number of transfers that achieve a throughput greater than 1Mbps by a factor of 3.

On the scalability of rendezvous-based location services for geographic wireless ad hoc routing

Geographic routing protocols allow stateless routing by taking advantage of the location information of mobile nodes and thus are highly scalable. A central challenge in geographic routing protocols is the design of scalable distributed location services that track mobile node locations. A number of location services have been proposed, but little is known about the relative performance of these location services. In this paper, we perform a detailed performance comparison of three rendezvous-based location services that cover a range of design choices: a quorum-based protocol (XYLS) which disseminates each nodes location to O(N) nodes, a hierarchical protocol (GLS) which disseminates each nodes location to O(logN) nodes, and a geographic hashing-based protocol (GHLS) which disseminates each nodes location to O(1) nodes. We present a quantitative model of protocol overheads for predicting the performance tradeoffs of the protocols for static networks. We then analyze the performance impact of mobility on these location services. Finally, we compare the performance of routing protocols equipped with the three location services with two topology-based routing protocols, AODV and DSR, for a wide range of network sizes. Our study demonstrates that when practical mobile ad hoc network (MANET) sizes are considered, the constants matter more than the asymptotic costs of location service protocols. In particular, while GLS scales better asymptotically, GHLS transmits fewer control packets and delivers more data packets than GLS in MANETs of sizes considered practical today and in the near future. Additionally, in contrast to the complex GLS design, the simplicity of GHLS provides significant resilience to performance degradation from mobility. Finally, although XYLS has a comparable packet delivery ratio to GHLS, it achieves this ratio with a higher overhead.