Tony Sun

A hybrid routing approach for opportunistic networks

With wireless networking technologies extending into the fabrics of our working and operating environments, proper handling of intermittent wireless connectivity and network disruptions is of significance. As the sheer number of potential opportunistic application continues to surge (i.e. wireless sensor networks, underwater sensor networks, pocket switched networks, transportation networks, and etc.), the design for an effective routing scheme that considers and accommodates the various intricate behaviors observed in an opportunistic network is of interest and remained desirable. While previous solutions use either replication or coding techniques to address the challenges in opportunistic networks, the tradeoff of these two techniques only make them ideal under certain network scenarios. In this paper, we propose a hybrid scheme, named H-EC, to deal with a wide variety of opportunistic network cases. H-EC is designed to fully combine the robustness of erasure coding based routing techniques, while preserving the performance advantages of replication techniques. We evaluate H-EC against other similar strategies in terms of delivery ratio and latency, and find that H-EC offers robustness in worst-case delay performance cases while achieving good performance in small delay performance cases. We also discuss the traffic overhead issues associated with H-EC as compared to other schemes, and present several strategies that can potentially alleviate the traffic overhead of H-EC schemes.

Reliable sensor networks for planet exploration

Wireless sensor networks will play a critical role in space and planet exploration, allowing remote monitoring of non-easily accessible areas in preparation of human or robotic missions. Sensors, however, are fragile and can fail, reporting erroneous measurements, for example. Decisions derived from flawed sensor measurements can adversely impact the correctness of the overall sensor network findings and may jeopardize the success of the mission. Unfortunately, failed sensors in space cannot be easily diagnosed and replaced. To improve the reliability of decisions and minimize the impact of faulty sensor measurements, the preferred approach is to exploit data redundancy. In this paper, we present confidence weighted voting (CWV), a distributed technique that can greatly improve the data reliability and fault tolerance of sensor network applications. We evaluate CWV against traditional approaches (e.g., majority voting (MV) and distance weighted voting (DWV)), in the presence of flawed sensors. The results show that CWV consistently outperforms the other schemes by providing as much as 49% more resiliency.

Impact of Node Heterogeneity in ZigBee Mesh Network Routing

Based on the IEEE 802.15.4 LR-WPAN standard, the ZigBee standard has been proposed to interconnect simple, low rate, and battery powered wireless devices. The deployment of ZigBee networks is expected to facilitate numerous applications, such as home-appliance networks, home healthcare, medical monitoring, consumer electronics, and environmental sensors. An effective routing scheme in a ZigBee network is particularly important in that it is the key to achieve resource (e.g., bandwidth and energy) efficiency in ZigBee networks. Routing in a ZigBee network is not exactly the same as in a MANET. In particular, while full function devices (FFD) can serve as network coordinators or network routers, reduced function devices (RFD) can only associate and communicate with FFDs in a ZigBee network. Therefore, different from traditional MANET routing algorithms, which only take into account node mobility to figure out a best route to a given destination, node heterogeneity plays an important role in ZigBee network routing. In this paper, we perform extensive evaluation, using NS-2 simulator, to study the impact of node heterogeneity on ZigBee mesh network routing. The results show that the ZigBee mesh routing algorithm exhibits significant performance difference when the network is highly heterogenous. We also reveal that the node type and the role of the node plays a critical role in deciding routing performances.

An Evaluation Study of Mobility Support in ZigBee Networks

Based on the IEEE 802.15.4 LR-WPAN specification, the ZigBee standard has been proposed to interconnect simple, low rate, and battery powered wireless devices. The deployment of ZigBee networks is expected to facilitate numerous applications, such as home healthcare, medical monitoring, consumer electronics, and environmental sensors. For many of the envisioned applications, device mobility is unavoidable and must be accommodated. Thus, providing ubiquitous connections to/from a mobile device is crucial for various future ZigBee applications. Knowledge of how nodal mobility affects the ZigBee routing protocol is important, but the lack of ZigBee simulator support has limited the amount of research, evaluation, and development in this area. Thus far, researchers have been unable to analyze and evaluate the impact of mobile applications via extensive simulations. In this paper, our contribution is threefold. First, we present an initial implementation of the ZigBee network layer in NS-2, which will allow further research and development to be conducted in this area. Second, we analyze the adequacy of current provisions for dealing with different mobility cases. Third, we provide a comprehensive set of simulation results that demonstrate the inefficacy of the current standard for handling mobility. Our results show that the ZigBee device plays a significant role in determining the routing performance in mobile scenarios.

Improving wireless link throughput via interleaved FEC

Wireless communication is inherently vulnerable to errors from the dynamic wireless environment. Link layer packets discarded due to these errors impose a serious limitation on the maximum achievable throughput in the wireless channel. To enhance the overall throughput of wireless communication, it is necessary to have a link layer transmission scheme that is robust to the errors intrinsic to the wireless channel. To this end, we present interleaved-forward error correction (I-FEC), a clever link layer coding scheme that protects link layer data against random and busty errors. We examine the level of data protection provided by I-FEC against other popular schemes. We also simulate I-FEC in Bluetooth, and compare the TCP throughput result with Bluetooths integrated FEC coding feature. We show that I-FEC consistently and significantly outperforms other link layer coding schemes by providing an impressive amount of protection against heavy channel burst errors.

Enhancing QoS support for vertical handoffs using implicit/explicit handoff notifications

Vertical handoffs between different wireless technologies usually lead to dramatic changes in the link capacity. A successful QoS solution for vertical handoffs must be able to fast track the capacity changes and agilely adapt the delivery rates and qualities of the ongoing applications. Though traditional AIMD-based source adaptation schemes (as found in TCP, TFRC, etc.) have been well designed for mild, gradual rate adjustments required by load fluctuations and network congestion, their response time is inadequate when the rate must be adjusted to the drastic network capacity changes that are typical in vertical handoff scenarios. To expedite the response to such changes, we propose in this paper two adaptive algorithms, named the fast rate adaptation (FRA) and early rate reduction (ERR), that are launched when the handoff is from low to high capacity (LOW-to-HIGH) or from high to low capacity (HIGH-to-LOW), respectively. We also propose two vertical handoff notification mechanisms to work with FRA and/or ERR, i.e. the implicit handoff notification (IHN) and explicit handoff notification (EHN). We show by simulation that our proposed schemes are able to provide better QoS support than the traditional AIMD based schemes during vertical handoffs

Evaluating mobility support in ZigBee networks

The deployment of ZigBee networks is expected to facilitate numerous applications, such as home healthcare, medical monitoring, consumer electronics, and environmental sensors. For many envisioned applications, device mobility is unavoidable and must be accommodated. Therefore, providing ubiquitous connection to/from a mobile ZigBee device is crucial for future ZigBee applications. In particular, knowledge of how nodal mobility affects ZigBee routing protocol is of significance. In this paper, our contributions are twofold. First, we dissect ZigBee routing and its support for device mobility, and we analyzed the current provisions in dealing with different mobility cases. Second, we performed a rich set of preliminary tests, illustrating the inefficacy of current standard. Our results indicate that ZigBee device type plays a significant role in determining the routing performance in most mobile scenarios.

Ad-hoc Storage Overlay System (ASOS): A Delay-Tolerant Approach in MANETs

Mobile ad-hoc networks (MANETs) are most useful in unprepared emergencies where critical applications must be launched quickly. However, they often operate in an adverse environment where end-to-end connectivity is highly susceptible to disruption. Adjusting the motion of existing nodes or deploying additional nodes can improve the connectivity under some circumstances, but for scenarios where connectivity cannot be immediately improved, disruption must be coped with properly. In this paper we propose the ad-hoc storage overlay system (ASOS). ASOS is a self-organized overlay of storage-abundant nodes to jointly provide distributed and reliable storage to data flows under disruption. ASOS is a delay-tolerant networking (DTN) approach that significantly improves the applicability of MANETs in practice

Measuring effective capacity of IEEE 802.15.4 beaconless mode

IEEE 802.15.4 is an emerging wireless standard addressing the needs of low-rate wireless personal area networks with a focus on enabling various pervasive and ubiquitous applications that require interactions with our surrounding environments. In view of the application potential of IEEE 802.15.4, knowing the fundamental network properties soon becomes essential in fasten the interactivity between these devices. Among all, knowing effective capacity of a path in wireless networks is of particular importance in routing and traffic management. In this paper, we implement SenProbe, a recently proposed path capacity estimation tool specially designed for the multi-hop ad hoc wireless environment. We present an implementation of SenProbe in sensor operating system (SOS), and evaluate the behavior/effectiveness of SenProbe in various testbed setups; including an interfered setting that cannot be simulated. Experiment results validate the workings of SenProbe and offer insights into how the capacity of a wireless path changes in real wireless environments. Our efforts provide a basis for realistic results that can be of assistance in activities such as capacity planning, protocol design, performance analysis, and etc

Modeling Channel Conflict Probabilities between IEEE 802.15 based Wireless Personal Area Networks

With the increasingly deployed Wireless Personal Area Network (WPAN) devices, channel conflict has become very frequent and severe when one WPAN technology coexists with other WPAN technologies in the same interfering range. In this paper, we study the coexistence issue between various IEEE 802.15 based WPAN technologies. We present analytical models on the non-conflicting channel allocation probabilities, focusing on the coexistence scenarios of one WPAN technology coexisting with another. The results show that channel allocation conflicts occurs frequently in all cases, and is especially severe between IEEE 802.15.3 and IEEE 802.15.4 networks. On the other hand, the probability of non-conflict channel allocation is less dramatic between a single IEEE 802.15.1 and coexisting IEEE 802.15.4 networks. In addition, the proposed models in this paper are also applicable to other wireless technologies, as long as the channel allocation mechanisms are known.