Introduction

Citation

Zhang, L., Zhou, X., Chen, G. and Poellabauer, C. (2009), "Introduction", International Journal of Pervasive Computing and Communications, Vol. 5 No. 3. https://doi.org/10.1108/ijpcc.2009.36105caa.001

Introduction

Introduction

Article Type: Guest editorial From: International Journal of Pervasive Computing and Communications, Volume 5, Issue 3.

The time-varying and undeterministic nature of wireless communications makes resource allocation and management a challenging problem for wireless networks. Significant research efforts have been spent in the past aiming at efficient and fair utilization of the scarce wireless resources to better serve the ever-growing wireless applications. Recently, wireless mesh networks (WMNs) have emerged as a cost-effective networking approach for high-speed last mile connectivity and ubiquitous broadband access. Some characteristics of WMNs, e.g. the mixed (infrastructure and ad hoc) architecture and multi-hop, multi-radio, multi-channel, and multi-antenna technologies, have brought new research opportunities and challenges to the resource allocation and management problem.

This special issue of International Journal of Pervasive Computing and Communications is targeted at theoretical and experimental solutions to the resource allocation and management problem in WMNs. It invited three best papers from the second IEEE International Workshop on Wireless Mesh and Ad Hoc Networks (WiMAN 2008) held in conjunction with the 28th IEEE International Conference on Distributed Computing Systems (ICDCS 2008). Three additional papers were selected from a large amount of submissions in response to the call for papers. All submissions went through a rigorous peer review process. Below, we outline the contribution of each of the six papers that are included in this special issue.

In order to make efficient use of the scarce channel resource during topology formation, scheduling and routing, it is vital to understand the quality of the links in WMNs. Link assessment is a procedure of assessing the quality of all links (in terms of, e.g. probability of successful transmission, signal to noise ratio, distortion rate, or a combination of them) in a WMN using the exchange of a sufficient number of packets between neighboring nodes. In their paper “A non-constant weight code approach for fast link assessment in multi-hop wireless mesh networks”, Prasad, Nelavelli, and Wu propose a novel link assessment scheme where the link assessment packets are transmitted efficiently according to a set of non-constant weight codes, which achieves low collision probability within a short link assessment period. In their proposed approach, each node considers the actual nodal degree while choosing the codeword for its link assessment. In their paper, they design the link assessment procedure and establish a set of theorems that provide the necessary condition for successful link assessment. Their simulation results show that their proposed approach reduces link assessment delay by over 70 percent compared to the OOC-based approach, which is the best known approach for link assessment in literature.

A WMN's capacity can be greatly improved when mesh nodes are equipped with multi-radios, which can be tuned to different channels to avoid interference. One of the key issues is how to assign the available channels to these multi-radio nodes. In their paper “A new channel assignment algorithm for wireless mesh networks”, Yang and Fei propose a bipartite graph based channel allocation algorithm, which considers both bandwidth utilization and starvation problem. Their solution is based on using augmenting paths to find channel-node matching in the bipartite graph. It can deal with heterogeneous mesh nodes with diverse transmission types and bandwidths and it can make effective use of the radio spectrum. Compared with previous conflict-graph based algorithms, the proposed solution can reduce the starvation ratio and improve the bandwidth utilization.

The high capacity demand and the interference mitigation requirements of WMNs invite the adoption of multiple antenna systems to improve the network performance. In particular, in a low-rank outdoor environment, the characteristics of the channel enables the use of switched-beam and adaptive antennas. Switched-beam antennas are cheap systems having limited capabilities in interference suppression, while fully adaptive smart antennas are more expensive systems that are able to perform beam and null steering to reject undesired sources. In their paper “Performance comparison of advanced antenna systems for wireless mesh routers in an outdoor environment”, Babich, Comisso, and Mania present a theoretical and numerical comparison between switched-beam and fully adaptive smart antennas. The adopted performance metric is the number of communications that can be simultaneously sustained by the nodes forming the backbone of the mesh network. Besides, the relationship between antenna system and channel coding technique is investigated in order to determine the combination that may offer the best performance. The results are presented together with some practical considerations related to the tradeoff between the hardware complexity and the achievable performance.

The unfairness of TCP in wireless environments is a well-known problem. In their paper “Implementing an adaptive TCP fairness while exploiting 802.11e over wireless mesh networks”, Fowler, Eberhard and Blow propose the use of 802.11e MAC on a per class basis for TCP over a WMN. In a WMN, the transmission medium is shared locally and as a consequence geographical regions of the network tend to be overloaded, meaning congestion appears as a spatial phenomenon. As a result, the individual rates of multiple TCP flows do not necessarily converge to a value reflecting a fair sharing of the bandwidth. Furthermore, the MAC protocol in the widely deployed 802.11 networks is inflexible in terms of resource allocation and management on the shared media, which contributes to heavier unfairness of TCP. The main contribution of this work lies in taking advantage of the new features in the 802.11e MAC to mitigate the unfairness of TCP in WMNs. There are two key features in the proposed scheme: (1) the detection of congestion by measuring the fluctuation of RTT of the TCP ACK samples via the standard deviation and (2) the use of 802.11e MAC mechanisms to prioritize TCP acknowledgments.

In WiMax mesh networks, time slot allocation for end-to-end traffic flow between the base station and the subscriber stations is controlled by a centralized scheduling algorithm. To support high-quality multimedia services on the network, the scheduling algorithm should be able to minimize the total transmission time for all traffic flows. In their paper “Minimizing transmission time in 802.16-based multi-channel mesh networks”, Peng Du et al. investigate the scheduling problem in multi-channel single-transceiver WiMax mesh networks. In their scheme, each station has one transceiver and can be tuned between multiple channels, intending to eliminate the secondary interference for reducing the total transmission time. They first analyze how many channels are sufficient for the avoidance of secondary interference, and then present an efficient scheduling algorithm along with the channel assignment strategy for time slot allocation. The simulation results show that the multi-channel single-transceiver MAC can reduce the total transmission time substantially as compared with the single channel system, and the systems with double channels may provide performance comparable to systems with more than two channels.

Recently, emergency-deployed wireless networks have become a topic of great interest. In their paper, “Cross-layer scheduling over a heterogeneous opportunistic emergency-deployed wireless network”, Corvino et al. propose a new scheduling policy for a heterogeneous network composed of cameras and 802.15.4-based-sensors equipped with wireless transceivers, able to send data to a special node, namely the sink, whose aim is to collect all information and send them to a control unit where decisions are taken accordingly. The air interface considered uses Multi-Carrier Multiple Code Division Access, and the network is set up according to the recently emerging “opportunistic network” principle. Due to some specific characteristics of the devices involved, the network is organized in a hierarchical structure, and scheduling decisions are taken in a centralized way at the sink. The architecture and the scheduling strategy are novel, and simulation results show that a cross-layer strategy capable of taking channel and application-related information into account provides better performance than the well-known simple maximum throughput scheduling.

We are very grateful to Prof. Laurence T. Yang, the Editor-in-Chief of International Journal of Pervasive Computing and Communications for his encouragement, support, and guidance throughout the project. We thank the authors who submitted their work to this special issue. We would also like to express our deepest appreciation to the WiMAN 2008 technical program committee and the external reviewers who spent their valuable time providing professional reviews. Without their contributions and support, this special issue would not have been possible.

Liqiang Zhang, Xiaobo Zhou, Guihai Chen and Christian PoellabauerGuest Editors

This project was supported in part by US National Science Foundation under grant CNS-0834230, grant CNS-0720524, grant CNS-0834180, and National Grand Fundamental Research 973 Program of China under grant 2006CB303000.