Ka-Lok Hung

Tradeoff Between Lifetime and Rate Allocation in Wireless Sensor Networks: A Cross Layer Approach

This paper studies the tradeoff between energy consumption and application performance in wireless sensor networks by investigating the interaction between network lifetime maximization and rate allocation problems. To guarantee the individual performance of sensor nodes, we adopt the network utility maximization (NUM) framework to ensure certain fairness on source rates of sensor nodes. We formulate the network lifetime maximization problem and fair rate allocation problem as constrained maximization problems, and combine them by introducing a system parameter, which characterizes the tradeoff between the two problems. Using Lagrange dual decomposition, the original problem is vertically decomposed into three subproblems: a rate control problem at the transport layer, a contention resolution problem at the MAC Layer, and a cross-layer energy conservation problem. The first and second subproblems jointly solve the congestion problem in sensor networks via congestion prices, and fully distributed algorithms are derived. Furthermore, they are coupled with the cross layer energy conservation problem to solve the network lifetime maximization problem via energy prices. For the third subproblem, we first propose a partially distributed algorithm where network lifetime is a global information, and then by exploring the similarity between max-min rate allocation and network lifetime maximization in sensor networks, we approximate the latter by the NUM framework, and hence formulate the tradeoff problem in the unified NUM framework. As a result, a fully distributed algorithm is derived for the energy conservation problem.

Tradeoff between network lifetime and fair rate allocation in wireless sensor networks with multi-path routing

The network lifetime and application performance are two fundamental but conflicting desig objectives in wireless sensor networks. Hence there is an intrinsic tradeoff between network lifetime maximization and application performance maximization. Often application performance correlates to the application data rate obtained in sensor networks. We can thus study this tradeoff by investigating the interactions between the network lifetime maximization problem and the rate allocation problem. Severe bias on the allocated rates of some sensor nodes may exist if only the total throughput of the sensor network is maximized, hence we enforce fairness on source rates of sensor nodes by invoking the network utility maximization (NUM) framework. First we consider the network lifetime as global information shared by sensor nodes. We formulate the network lifetime maximization and fair rate allocation both as constrained maximization problems. By introducing a system parameter, we combine these two objectives into a single weighted objective, and characterize the tradeoff between them. Then we give the optimality condition, and derive a partially distributed algorithm. Also, we identify the similarity between network lifetime maximization and max-min rate allocation in networks. Since the latter one can be approximated using NUM framework, we adopt the same idea for the former one, and approximate the optimal solution in the unified NUM framework. Based on this, an efficient fully distributed algorithm is derived.

Rate-lifetime tradeoff for reliable communication in wireless sensor networks

The network lifetime and application performance are two fundamental, yet conflicting, design objectives in wireless sensor networks. There is an intrinsic tradeoff between network lifetime maximization and application performance maximization, the latter being often correlated to the rate at which the application can send its data reliably in sensor networks. In this paper we study this tradeoff by investigating the interactions between the network lifetime maximization problem and the rate allocation problem with a reliable data delivery requirement. Severe bias on the allocated rates of some sensor nodes may exist if only the total throughput of the sensor network is maximized, hence we enforce fairness on source rates of sensor nodes by invoking the network utility maximization (NUM) framework. To guarantee reliable communication, we adopt the hop-by-hop retransmission scheme. We formulate the network lifetime maximization and fair rate allocation both as constrained maximization problems. We characterize the tradeoff between them, give the optimality condition, and derive a partially distributed algorithm to solve the problem. Furthermore, we propose an approximation of the tradeoff problem using NUM framework, and derive a fully distributed algorithm to solve the problem.

O-MAC: An Organized Energy-Aware MAC Protocol for Wireless Sensor Networks

The efficient use of energy in wireless sensor networks is critical issue as the battery of a sensor node, in most cases, cannot be recharged or replaced after deployment. In order to detect an event, a sensor node spends most of the time in monitoring its environment, during which a significant amount of energy can be saved by placing the radio in the low power sleep mode when no reception and/or transmission of data is involved. In this paper, we discuss the design of a new MAC protocol for wireless sensor networks whose goal is to extend the lifetime of the network by avoiding major energy waste causes, such as collisions, overhearing and idle listening, without compromising other network performance measures such as network throughput. The performance of the protocol is studied by simulation and is compared to that of the well known S-MAC protocol which is designed to save energy and to the IEEE 802.11 protocol which is designed to maximize throughput.

Cross layer association control for throughput optimization in wireless LANs with inter-AP interference

In this paper, we study the problem of determining the optimal association in multi-cell WLANs in the presence of hidden terminals and inter-AP collisions. Unlike most work in this area which deal with networks without inter-AP interference, we reveal that association control alone is not sufficient to achieve fair throughput allocation and load balancing across APs. Instead, we advocate the joint association control, rate control and contention control to improve network performance. Based on this, we formulate a cross-layer optimization problem whose objective is to allocate downlink throughput according to the proportional fairness principle. As the problem turns out to be a non-convex mixed integer programming problem, which is known to be NP-hard, we relax it into a continuous convex problem and propose a distributed algorithm to solve it. We then design a simple yet effective distributed approximation algorithm to construct an solution that fulfills the discrete integral association constraint. The output of the algorithm provides the optimal association, the maximum achievable rate for each downlink flow and each APs optimal average backoff time. Numerical experiments and simulation results show that our algorithm converges rapidly and works effectively.

Throughput optimization in wireless local networks with inter-AP interference via a joint-association control, rate control, and contention resolution

The dense deployment of wireless access points (APs) either in wireless local area networks (WLANs) or in wireless mesh networks facilitates greatly ubiquitous Internet access, however, due to the limited number of orthogonal frequency channels allotted to the IEEE802.11-based networks, this also induces the inevitable problem of inter-AP interference. In this paper, we study the problem of determining the optimal association in multi-cell or extended wireless networks in the presence of hidden terminals and inter-AP collisions. Unlike most previous work in this area, which deals with networks without inter-AP interference, we first reveal that association control alone is not sufficient to achieve fair throughput allocation and load balancing across APs, then advocate a solution based on the joint association control, rate control and fair contention resolution as a means to improving network performance. Based on this, we formulate a cross-layer association control, throughput optimization and contention resolution problem whose objective is to allocate downlink throughput according to the proportional fairness principle. As the problem turns out to be a non-convex mixed integer programming problem, known to be NP-hard, we relax it first into a continuous problem, then transform the resultant into a convex problem and finally propose a distributed algorithm to solve it. We then design a simple yet effective approximation algorithm to recover an optimal solution that fulfills the discrete integral association constraints. The algorithm yields the optimal association, the maximum achievable rate for each downlink flow as well as each APs optimal average backoff time. Using these results as settings in a network, we can achieve the optimal operation point without any scheduling. Numerical experiments and simulation results show that our algorithm converges rapidly and works effectively.

TCP performance optimization in multi-cell wireless local area networks

In this paper, we consider the throughput allocation problem in an extended (or multi-cells) IEEE 802.11 wireless network. We first demonstrate the severe throughput imbalance that can take place between downlink TCP flows even in simple multi-cell WLANs. Then to solve this unfairness problem, we derive an analytical model that describes the interaction between different TCP flows at the MAC layer, and formulate the throughput allocation problem as a nonlinear optimization problem subject to certain fairness requirements. Real world complexity such as hidden terminals, packet transmission retry limit, and the unique characteristics of TCP traffic are considered. Unlike alternative approaches that rely on modifying the TCP sender or implementing active queue management at the network layer, solving our optimization problem yields the optimal MAC layer contention windows settings that can lead each TCP flow to its target end-to-end throughput. Among the practically appealing characteristics of this approach are: i) its locality and the possibility of implementing it at the AP and within the ESS, whereas alternative approaches rely on changing the TCP traffic source; and ii) its simplicity, i.e., it is achieved by controlling a single parameter per-node, whereas alternative approaches based on AQM require the tuning of many parameters and achieve mitigated results. Simulation results show that our approach can achieve a fair throughput allocation and attest to the accuracy of our proposed method.

Modeling Aggregate MAC Flow Throughput and Fairness in Non-Saturated IEEE 802.11 Based Wireless Mesh Networks

In this paper, we study the trade-off between the aggregate MAC flow throughput and the fairness in IEEE 802.11 based wireless mesh networks (WMNs) that utilize scheduling on top of the CSMA/CA access scheme. We use a toy WMN topology to allow us to understand easily this trade-off and propose an analytical model to study the interaction between contending links in this topology. Based on this model, we formulate the bandwidth scheduling problem as an aggregate MAC flow throughput maximization problem subject to the fairness requirements dictated by the scheduler. This study is dictated by the need to understand the limits of such scheduling algorithms which have proliferated in recent years to balance the throughput and fairness of WMNs without modifying the CSMA/CA protocol or the binary exponential backoff due to the non programmability of the MAC and backoff procedure in modern commercial Wi-Fi chipset. As an example, we evaluate our previously proposed bandwidth scheduling mechanism - the so-called distributed fair MAC scheduler (DFMS) to validate our model on one hand and demonstrate the efficiency of our scheduler on the other.

TCP performance optimization in multi-cell WLANs

Though significant attention has been given to understanding the performance of a single-cell WLAN, performance evaluation of a group of interfering basic service sets (BSSs) within an extended service set (ESS) is still an open area. In this paper, we first demonstrate that a severe throughput imbalance occurs between downlink TCP flows even in the simplest of multi-cell WLANs via simulation and real world experiments; then, to solve this unfairness problem, we derive an analytical model that describes the interaction between TCP flows at the MAC layer, and formulate a throughput allocation problem as a nonlinear optimization problem subject to certain fairness requirements. Our formulation considers real world complexity such as hidden terminals, packet transmission retry limit, and the unique characteristics of TCP traffic. Solving our optimization problem yields the optimal MAC layer contention window settings that can lead each TCP flow to its target end-to-end throughput without the need for any per-flow queuing nor modification of the TCP sender. Simulation results show that our approach can achieve a fair allocation on the end-to-end throughput and attest to the accuracy of our proposed method.

Bandwidth allocation in IEEE 802.11 WLANs in presence of inter-BSSs hidden terminals

Due to the limited number of orthogonal channels and the dense deployment of IEEE 802.11 access points (APs), interference between neighboring basic service sets (BSSs) are very common in indoor environment. One consequence of such neighboring interference is the inter-BSSs hidden terminal problem in which some mobile stations may suffer from throughput starvation. In this paper, we advocate and develop a methodology to allocate bandwidth as a mean of combating such starvation problem. We first propose an analytical model to study the interaction between contending links in the network, then based on this model, we demonstrate how the contention window (CW) can be adjusted in order to allocate the upload and download bandwidth to sustain some predefined bandwidth requirement with only the basic access scheme is used. Our approach does not require any hardware modification, and is compatible with most WLAN chipsets. The optimal window setting can be obtained by simply solving a nonlinear system once without any iterative algorithm. Simulation are invoked to support the effectiveness of the proposed methodology in achieving the target bandwidth even under saturation condition.