Hwee-Xian Tan

Multipath Virtual Sink Architecture for Underwater Sensor Networks

The salient features of acoustic communications render many schemes that have been designed for terrestrial sensor networks unusable underwater. We therefore propose a novel virtual sink architecture for underwater sensor networks that aims to achieve robustness and energy efficiency under harsh underwater channel conditions. To overcome the long propagation delay and adverse link conditions in such environments, we make use of multipath data delivery. While conventional multipath routing tends to lead to contention near the sink, we avoid this caveat with the virtual sink design involving a group of spatially diverse physical sinks. Hence, we are able to exploit the reliability achieved from redundancy provided by multipath data delivery while mitigating the contention between the nodes.

Information Quality Aware Routing in Event-Driven Sensor Networks

Upon the occurrence of a phenomenon of interest in a wireless sensor network, multiple sensors may be activated, leading to data implosion and redundancy. Data aggregation and/or fusion techniques exploit spatio-temporal correlation among sensory data to reduce traffic load and mitigate congestion. However, this is often at the expense of loss in Information Quality (IQ) of data that is collected at the fusion center. In this work, we address the problem of finding the least-cost routing tree that satisfies a given IQ constraint. We note that the optimal least-cost routing solution is a variation of the classical NP-hard Steiner tree problem in graphs, which incurs high overheads as it requires knowledge of the entire network topology and individual IQ contributions of each activated sensor node. We tackle these issues by proposing: (i) a topology-aware histogram-based aggregation structure that encapsulates the cost of including the IQ contribution of each activated node in a compact and efficient way; and (ii) a greedy heuristic to approximate and prune a least-cost aggregation routing path. We show that the performance of our IQ-aware routing protocol is: (i) bounded by a distance-based aggregation tree that collects data from all the activated nodes; and (ii) comparable to another IQ-aware routing protocol that uses an exhaustive brute-force search to approximate and prune the least-cost aggregation tree.

Multiple-UUV approach for enhancing connectivity in underwater ad-hoc sensor networks

Underwater sensor networks typically comprise of sensor nodes that are deployed in sufficiently large numbers for data collection, monitoring and surveillance. The acquired data is relayed by the sensors over multihop wireless acoustic communications links to sinks and collection points. While the sensors are generally static, the adverse channel and harsh environmental conditions increase the chances of link breakages due to fading and ambient noise. Our proposed scheme utilizes multiple underwater unmanned vehicles (UUVs, e.g. seabed crawlers) to enhance connectivity. The UUVs patrol the areas where connectivity is likely to be poor to overcome temporal interference and if necessary deploy more sensors to repair the breaks in connectivity. In the event that network becomes partitioned, the UUVs can also serve as local sinks to the sensors in the isolated partitions, and ferry the data from the isolated sensors to the nearest connected part of the network.

Framework for statistical filtering against DDoS attacks in MANETs

A DDoS (distributed denial-of-service) attack is a distributed, large-scale attempt by malicious users to flood the victim network with an enormous number of packets. This exhausts the victim network of resources such as bandwidth, computing power, etc. The victim is unable to provide services to its legitimate clients and network performance is greatly deteriorated. There are many proposed methods in the literature which aim to alleviate this problem; such as hop-count filtering, rate-limiting and statistical filtering. However, most of these solutions are meant for the wired Internet, and there is little research efforts on mechanisms against DDoS attacks in wireless networks such as MANETs. In this paper, we study the vulnerability of MANETs to DDoS attacks and provide an overview of statistical filtering, which is commonly used as a security mechanism against DDoS attacks in wired networks. We then propose a framework for statistical filtering in MANETs to combat DDoS attacks.

A²-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks

Energy constraints in wireless sensor nodes necessitate the design and development of energy-efficient MAC protocols to arbitrate access to the shared communication medium. While there exists a plethora of sensor MAC protocols, these protocols do not individually vary the duty-cycle of each sensor according to local connectivity status, to maximize energy savings. In this paper, we propose A^2-MAC - an Adaptive, Anycast MAC protocol for low-powered wireless sensor networks. It utilizes: (i) a random wakeup schedule, such that each node can independently and randomly wakeup in each cycle without coordination and time synchronization; (ii) adaptive duty-cycles based on network topology; and (iii) adaptive anycast forwarders selection, which allows each node to transmit to any member in its forwarding set. There are two key adaptive mechanisms in A^2-MAC: (i) each node varies its duty-cycle and set of forwarding nodes such that energy consumption can be locally minimized for a given local delay performance objective; and (ii) nodes cooperatively reduce the duty-cycles required of their forwarding nodes, depending on local network connectivity. By allowing nodes to operate with different duty-cycles and forwarding sets, A^2-MAC achieves better energy-latency tradeoffs and extends node lifetime substantially, while providing good end-to-end latency.

Distributed CDMA code assignment for wireless sensor networks

Wireless sensor networks are deployed for many collaborative tasks such as monitoring, target tracking and surveillance. They communicate via multi-hops to one or more sinks over a shared wireless medium. Code division multiple access (CDMA) allows many sensor nodes to transmit simultaneously over the common shared medium without excessive collisions that often result in packet loss and lowered throughput. However, this requires the proper assignment of orthogonal codes to the sensor nodes to minimize interference among adjacent neighbors. Due to the distributed and autonomous nature of sensor networks, CDMA codes cannot be assigned in the same way as cellular networks. We propose a distributed methodology of assigning CDMA codes to the nodes, which has low message overheads, allows for spatial reuse of the channel and does not depend on the availability of neighbor information. Our approach is also able to handle topological changes that could be induced by nodes entering or leaving the network. We study the effectiveness of our algorithm through simulations and analysis.

Performance of Slotted-Aloha over TH-UWB

In the IEEE 802.15.4a UWB-PHY standard for Low Rate WPANs (LR-WPANs), the UWB-PHY provides Time-Hopping (TH) to enable multiple users to transmit simultaneously, thereby potentially increasing the overall system throughput. However, the intrinsic properties of the impulse-based UWB renders existing narrowband MAC protocols which make use of carrier sensing unsuitable for use in UWB systems. In the standard, Aloha has been proposed as the MAC protocol. In this paper, we study the throughput performance of slotted-Aloha, an enhanced version of the Aloha MAC protocol, over the TH-UWB physical layer, using both theoretical analysis and simulations. Our results show that random time-hopping can be detrimental to the throughput performance of the network. An optimal method of assigning TH codes to the different symbols in each packet is necessary in order to exploit the benefits of time-hopping, which can potentially allow concurrent transmissions in multiuser systems.

A medium access control protocol for UWB sensor networks with QoS support

Ultra-wideband (UWB) is a physical (PHY) layer technology that promises high transmission rates, as well as high resistance to noise and multipath effects. However, the impulse-based nature of UWB, coupled with its low transmission power, makes it difficult to enable efficient detection of the signals. Consequently, conventional carrier-sensing based MAC protocols cannot be used with a UWB PHY. In this paper, we propose SASW-CR - a slotted Aloha MAC protocol for UWB networks with sliding contention window and cooperative retransmissions, which provides QoS support without the use of carrier sensing. SASW-CR utilizes the slotted-Aloha technique to avoid carrier sensing and reduce packet collisions. In addition, it makes use of differentiated contention windows to provide varying classes of QoS for different traffic classes. A cooperative retransmission technique is also introduced to improve the overall traffic throughput and reduce end-to-end delay. The efficacy of our protocol is demonstrated through simulations.

A Resource Allocation Scheme for TH-UWB Networks with Multiple Sinks

In this work, we study the time-slot allocation problem in a multi-sink single-hop TH-UWB network scenario, where the traffic from a sensor node is anycasted via a single hop to any one of multiple sinks. The slot allocation problem is formulated as an optimization problem and shown to be NP-hard. We then present a heuristic to increase network throughput and fairness as compared to a random allocation. In the proposed heuristic, nodes that are of similar distances to any sinks are grouped together to utilize the same set of TH slots for transmissions. Simulations show that the proposed heuristic improves both throughput and fairness, scales with multiple sinks and can be used as a simple admission control mechanism.

Impact of ATM cell delay on multimedia applications

The impact of ATM cell delays on multimedia application performance is studied through measurements using a broadband network analyser. A method is developed to measure the ATM cell delays of an application using the analyser. It is found that the cell delay distribution becomes continuous as the traffic load on a connection becomes lighter. The effect of background traffic on cell delay for a reference connection is studied. Though the cell delay of the reference connection is unaffected by the presence of background traffic, the cell-delay auto-correlation is found to decrease. A gamma distribution is used to approximate the cell delay distribution. The measurement results closely match the delay prediction using the approximation. In a network with multiple ATM switches, the gamma distribution only slightly overestimates the delay. The frame level statistics and the cell delay performance of a video conferencing application based on motion JPEG are measured. Using the gamma cell delay distribution, the frame loss of an MPEG-I video source is predicted and compared against the measured values based on MPEG-I traces.