Parth H. Pathak

Impact of Power Control on Capacity of TDM-Scheduled Wireless Mesh Networks

In this paper, we consider power control as network layer problem in wireless mesh networks. The network connectivity between nodes is determined by their communication range which in turn can be controlled by adjusting the transmit power level. It is generally acknowledged that reducing transmit power levels of nodes to the minimum required to retain connectivity always increases network capacity. In this work, we show that though this is true for CSMA/CA based medium access, increasing power level of nodes can be beneficial in many cases when links are TDM-scheduled. Based on analysis and simulations, it is observed that increasing power levels of nodes (and decreasing number of hops in routing paths) results in increase of throughput in many representative traffic patterns and topologies. We characterize achievable spatial reuse and capacity with respect to power control in different topologies and traffic patterns. With increasing number of MAC protocols adopting TDMA approach, results presented here can be crucial in understanding how capacity is affected with varying levels of network connectivity.

Impact of power control on capacity of large scale Wireless Mesh Networks

Wireless Mesh Networks (WMNs) have emerged as low-cost alternative for broadband access networks in metropolitan area. In this paper, we revisit power control as a network layer problem in design of WMNs. Specifically, we show that increasing power levels of nodes in WMNs actually results into throughput improvements in node-to-gateway traffic pattern. It is proven that WMNs can achieve per node throughput of O(1=δn) where δ is factor dependent on hop-radius of the network, which in turn is reliant on power control. Also, it is shown that in case of multiple gateways, increasing power levels of nodes is more cost-effective because it results into increased throughput with fewer number of gateways.

Impact of Power Control on Relay Load Balancing in Wireless Sensor Networks

When shortest path routing is employed in large scale multi-hop wireless networks, nodes located near the center of the network have to perform disproportional amount of relaying for others. In energy-constrained networks like sensor, such unfair forwarding results into early depletion of batteries of these congested nodes. To solve the problem, various divergent routing schemes are used which route the data on centeravoiding divergent routing paths. Though they achieve better load balancing, overall relaying is increased significantly due to their longer routing paths which in turn results into reduced energy efficiency. In this paper, we propose power control as a way of achieving better load balancing in multi-hop wireless networks. We show that when communication range of nodes are properly controlled using power control, better load balancing can be achieved using shortest paths only. Such a strategy also decreases overall relaying in the network when compared to divergent routing schemes. We use the concept of centrality to achieve appropriate balance between relay burden of nodes and their power levels. Numerical results confirm that centrality based load balancing significantly improves network lifetime of sensor networks.

Loner links aware routing and scheduling in Wireless Mesh Networks

In wireless mesh networks (WMNs), TDMA based link schedulingcan allow multiple concurrent transmissions, resulting in throughput improvements. In this paper, we identify certain link characteristics which reduce achieved spatial reuse and increase schedule length. It is shown that certain links (called loners) based on their length and position, introduce inherent difficulty in scheduling them with other links in network. Effects of such links on scheduling reveals important limitations of any such scheduling algorithm and motivates need of joint routing, scheduling and power control. We then present joint routing, topology control and scheduling algorithm to alleviate effects of loner links. Simulation results confirm throughput improvement with shorter schedule length.

Using Centrality-Based Power Control for Hot-Spot Mitigation in Wireless Networks

When shortest path routing is employed in large scale multi-hop wireless networks, nodes located near the center of the network have to perform disproportionate amount of relaying for others. To solve the problem, various divergent routing schemes are used which route the data on center-avoiding divergent routing paths. Though they achieve better load balancing, overall relaying is increased significantly due to their longer routing paths. In this paper, we propose power control as a way for balancing relay load and mitigating hot-spots in wireless networks. Using a heuristic based on the concept of centrality, we show that if we increase the power levels of only the nodes which are expected to relay more packets, significant relay load balancing can be achieved even with shortest path routing. Different from divergent routing schemes, such load balancing strategy is applicable to any arbitrary topology. Also, it is shown that centrality based power control results into better throughput capacity in many different topologies.

Packet aggregation based back-pressure scheduling in multi-hop wireless networks

The back-pressure based scheduling policy originally proposed by Tassiulas et al. in [1] has shown the potential of solving many fairness and network utilization related problems of wireless multi-hop networks. Recently, the scheduling policy has been adapted in random medium access protocols such as CSMA/CA using prioritization of MAC layer transmissions. Here, MAC priorities are used to provide differentiated services to nodes depending on their queue backlogs. Even though these schemes work well in experiments to emulate back-pressure scheduling, they perform poorly with realistic Internet-type traffic where there is a large variation in packet sizes. In this paper, we propose packet aggregation based back-pressure scheduling which aggressively increases the rates at which back-logged queues are served. Different from other aggregation schemes, the presented scheme utilizes the back-pressure principles for determining when and how much aggregation is performed. We show that this results into increased service rates of back-logged queues which in turn results into high network throughput and utilization. We verify our scheme using simulations and testbed experiments, and show that it achieves significant performance improvements as compared to the original scheme.

Channel width assignment using relative backlog: extending back-pressure to physical layer

With recent advances in Software-defined Radios (SDRs), it has indeed became feasible to dynamically adapt the channel widths at smaller time scales. Even though the advantages of varying channel width (e.g. higher link throughput with higher width) have been explored before, as with most of the physical layer settings (rate, transmission power etc.), naively configuring channel widths of links can in fact have negative impact on wireless network performance. In this paper, we design a cross-layer channel width assignment scheme that adapts the width according to the backlog of link-layer queues. We leverage the benefits of varying channel widths while adhering to the invariants of back-pressure utility maximization framework. The presented scheme not only guarantees improved throughput and network utilization but also ensures bounded buffer occupancy and fairness.

Designing for Network and Service Continuity in Wireless Mesh Networks

The paradigm of wireless mesh networks has matured in the last decade after extensive research and development. As opposed to the time of their inception, current mesh networks are expected to deliver carrier-grade access services to their users. With the increasing expectations, it is generally recognized that there is a lack of understanding of the level of service continuity that mesh networks can provide to its users. In this research, we address a set of network design problems and propose solutions to improve the service continuity of wireless mesh networks. First, we investigate power control as a network layer problem in mesh networks. It is commonly acknowledged that reducing transmit power levels of nodes to the minimum required to retain connectivity always increases network capacity. In this work, we show that increasing power level of nodes can be beneficial in many cases depending on network’s traffic and topological characteristics. With extensive analysis and simulations, we characterize achievable spatial reuse and capacity with respect to power control in various practical scenarios. Second, we address the problem of generation of traffic hot-spots in wireless networks, and propose a novel power control scheme for load balancing. Using a heuristic based on the concept of centrality, we show that if we increase the power levels of only the nodes which are expected to relay more packets, significant relay load balancing can be achieved even using shortest path routing. Different from divergent routing schemes, such load balancing strategy is applicable to any arbitrary topology and traffic pattern. With this understanding of power control and load balancing, we investigate the service continuity issues of an urban-scale mesh networks. We define and study two essential service continuity metrics - connection robustness (K-center availability) and performance robustness (K-center performability). K-center availability is the fraction of the time a mesh network is available to its users, while performability is a composite measure of performance (here - throughput) and robustness that captures aggregate experience of mesh users over a long period of time. We develop efficient algorithms for estimating these quantities for large urban-scale meshes and study their characteristics in various existing mesh networks. Our study reveals numerous novel insights that can help in designing mesh networks with high availability and performability. While evaluating the performability, we used ideal case throughput performance estimation that ensures fair network resource allocation among the flows. Although this allows faster computation which is especially necessary for large state space exploration, it is difficult to implement in real-world using CSMA/CA MAC. To address this, we target the issue of unfair and lower utilization of network resources in wireless networks. Specifically, in a mesh network, nodes closer to gateways often achieve a larger share of bandwidth resources while nodes farther away often starve for useful bandwidth. Using back-pressure utility maximization framework, we provide a solution to the issue of spatial bias that can be implemented using current 802.11 standards. To further increase the efficiency of back-pressure policy in mesh networks, we propose a dynamic packet aggregation scheme that is especially effective in Internet-like packet size distribution. Along the same lines, we use the back-pressure invariants to develop a variable channel width assignment scheme that can yield maximum benefits of using varying-sized channels while remaining fair in network-wide resource allocation.

Mesh Enabling Technology

While the mesh network paradigm does not constrain the physical and medium layer realizations, some technologies emerge over time as clearly being better suited. This becomes especially true when considering service continuity issues, and therefore robustness of enabling technologies. In recent times, Multiple Input Multiple Output (MIMO) technology at the lower layer is seen to hold promise to improve the predictability and dependability of mesh networks. At a higher level, various cellular approaches, including various 802.16 and 3GPP standards, as well as the newly emerging cognitive or “white space” networking approaches, are providing extensions to multi-hopping, and becoming more relevant than the 802.11 technology which was earliest put to work on WMNs. In this chapter, we briefly describe the relevant aspects of these, including references to some relevant literature. We start with a few remarks on 802.11 as background.

Mesh Design: Network Issues

We continue our discussion of traditional design problems in mesh networks, preparatory to the discussion of joint design in a following chapter. In this chapter, we focus on network-wide issues in mesh design, or issues that in some sense emerge at the network level. In particular, we focus on routing, deployment planning, performance modeling, rate control, and spectrum sensing and access. As before, we describe the problems, as well as discuss some representative approaches in recent literature.