How do Wireless Chains Behave? The Impact of MAC Interactions

In a Multi-hop Wireless Networks (MHWN), packets are routed between source and destination using a chain of intermediate nodes; chains are a fundamental communication structure in MHWNs whose behavior must be understood to enable building effective protocols. The behavior of chains is determined by a number of complex and interdependent processes that arise as the sources of different chain hops compete to transmit their packets on the shared medium. In this paper, we show that MAC level interactions play the primary role in determining the behavior of chains. We evaluate the types of chains that occur based on the MAC interactions between different links using realistic propagation and packet forwarding models. We discover that the presence of destructive interactions, due to different forms of hidden terminals, does not impact the throughput of an isolated chain significantly. However, due to the increased number of retransmissions required, the amount of bandwidth consumed is significantly higher in chains exhibiting destructive interactions, substantially influencing the overall network performance. These results are validated by testbed experiments. We finally study how different types of chains interfere with each other and discover that well behaved chains in terms of self-interference are more resilient to interference from other chains

A Cross Layer Solution to Address TCP Intra-flow Performance Degradation in Multihop Ad hoc Networks

Incorporating the concept of TCP end-to-end congestion control for wireless networks is one of the primary concerns in designing ad hoc networks since TCP was primarily designed and optimized based on the assumptions for wired networks. In this study, our interest lies on tackling the TCP instability and in particular intra-flow instability problem since due to the nature of applications in multihop ad hoc networks, connection instability or starvation even for a short period of time can have a negative impact on the Quality of Service and may not be acceptable for the end user. Through a detailed analysis, it will be shown that the main causes of TCP intra-flow instability lies in overloading the network by sending more packets than the capacity of the channel. Based on this, the paper proposes a novel cross layer solution called “TCP Contention Control” that dynamically adjusts the amount of outstanding data in the network based on the level of contention experienced by packets as well as the throughput achieved by connections. The simulation results show TCP Contention Control can drastically improve TCP stability over 802.11 multihop ad hoc networks

Security and Privacy Issues in Wireless Mesh Networks: A Survey

This book chapter identifies various security threats in wireless mesh network (WMN). Keeping in mind the critical requirement of security and user privacy in WMNs, this chapter provides a comprehensive overview of various possible attacks on different layers of the communication protocol stack for WMNs and their corresponding defense mechanisms. First, it identifies the security vulnerabilities in the physical, link, network, transport, application layers. Furthermore, various possible attacks on the key management protocols, user authentication and access control protocols, and user privacy preservation protocols are presented. After enumerating various possible attacks, the chapter provides a detailed discussion on various existing security mechanisms and protocols to defend against and wherever possible prevent the possible attacks. Comparative analyses are also presented on the security schemes with regards to the cryptographic schemes used, key management strategies deployed, use of any trusted third party, computation and communication overhead involved etc. The chapter then presents a brief discussion on various trust management approaches for WMNs since trust and reputation-based schemes are increasingly becoming popular for enforcing security in wireless networks. A number of open problems in security and privacy issues for WMNs are subsequently discussed before the chapter is finally concluded.Comment: 62 pages, 12 figures, 6 tables. This chapter is an extension of the author's previous submission in arXiv submission: arXiv:1102.1226. There are some text overlaps with the previous submissio

Smoothed Airtime Linear Tuning and Optimized REACT with Multi-hop Extensions

abstract: Medium access control (MAC) is a fundamental problem in wireless networks. In ad-hoc wireless networks especially, many of the performance and scaling issues these networks face can be attributed to their use of the core IEEE 802.11 MAC protocol: distributed coordination function (DCF). Smoothed Airtime Linear Tuning (SALT) is a new contention window tuning algorithm proposed to address some of the deficiencies of DCF in 802.11 ad-hoc networks. SALT works alongside a new user level and optimized implementation of REACT, a distributed resource allocation protocol, to ensure that each node secures the amount of airtime allocated to it by REACT. The algorithm accomplishes that by tuning the contention window size parameter that is part of the 802.11 backoff process. SALT converges more tightly on airtime allocations than a contention window tuning algorithm from previous work and this increases fairness in transmission opportunities and reduces jitter more than either 802.11 DCF or the other tuning algorithm. REACT and SALT were also extended to the multi-hop flow scenario with the introduction of a new airtime reservation algorithm. With a reservation in place multi-hop TCP throughput actually increased when running SALT and REACT as compared to 802.11 DCF, and the combination of protocols still managed to maintain its fairness and jitter advantages. All experiments were performed on a wireless testbed, not in simulation.Dissertation/ThesisMasters Thesis Computer Science 201

Study on Packet Forwarding Mechanism by Narrowing Relay Range in Ad Hoc Networks

早大学位記番号:新7461早稲田大

Performance evaluation of channel selection algorithm for multi-channel MAC protocol in ad hoc networks

This thesis aims to provide an approach that is to investigate channel selection algorithm for increasing the performance of ad hoc networks. Although our channel selection algorithms are very simple, multi-channel MAC protocol that employs our channel selection algorithms are effective for increasing the performance of ad hoc networks.学位記番号：工博甲47

IEEE 802.11 Ad Hoc Networks: Performance Measurements

In this paper we investigate the performance of IEEE 802.11b ad hoc networks by means of an experimental study. An extensive literature, based on simulation studies, there exists on the performance of IEEE 802.11 ad hoc networks. Our analysis reveals several aspects that are usually neglected in previous simulation studies. Firstly, since different transmission rates are used for control and data frames, different transmission ranges and carrier-sensing ranges may exist at the same time in the network. In addition, the transmission ranges are in practice much shorter than usually assumed in simulation analysis, not constant but highly variable (even in the same session) and depends on several factors. Finally, the results presented in this paper indicate that for correctly understanding the behavior of an 802.11b network operating in ad hoc mode, several different ranges must be considered. In addition to the transmission range, the physical carrier sensing range is very important. The transmission range is highly dependent on the data rate and is up to 100 m, while the physical carrier sensing range is almost independent from the data rate and is approximately 200 m. Furthermore, even though stations are outside from their respective physical carrier sensing range, they may still interfere if their distance is lower than 350 m