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Anomaly detection is becoming a powerful and necessary component as wireless networks gain popularity. In this paper, we evaluate the efficacy of PCA based anomaly detection for wireless mesh networks (WMN). PCA based method [1] was originally developed for wired networks. Our experiments show that it is possible to detect different types of anomalies, such as Denial-of-service (DoS) attack, port scan attack [1], etc., in an interference prone wireless environment. However, the PCA based method is found to be very sensitive to small changes in flows causing non-negligible number of false alarms. This problem prompted us to develop an anomaly identification scheme which automatically identifies the flow(s) causing the detected anomaly and their contributions in terms of number of packets. Our results show that the identification scheme is able to differentiate false alarms from real anomalies and pinpoint the culprit(s) in case of a real fault or threat. Moreover, we also found that the threshold value used in [1] for distinguishing normal and abnormal traffic conditions is based on assumption of normally distributed traffic which is not accurate for current network traffic which is mostly self-similar in nature. Adjusting the threshold also reduced the number of false alarms considerably. The experiments were performed over an 8 node mesh testbed deployed in a suburban area, under different realistic traffic scenarios. Our identification scheme facilitates the use of PCA based method for real-time anomaly detection in wireless networks as it can filter the false alarms locally at the monitoring nodes without excessive computational overhead.

In this paper, we present a monitoring assisted routing scheme for wireless mesh networks which exploits the broadcast nature of wireless transmissions at special routers with added monitoring functionalities. These routers listen to the transmissions in neighborhood and compare the routing behavior against the routing state collectively maintained by them. If any inconsistency is found, as a result of software/hardware malfunction, these routers try to determine the node causing it and recover from it in a timely fashion. The scheme is developed for wireless mesh networks where the communication overhead is a critical issue. The performance evaluation of our scheme shows considerable improvement in reliability (i.e., delivery ratio) with minimal overhead under situations of malfunctions.

Wireless mesh networks (WMN) have attracted considerable interest in recent years as a convenient, flexible and low-cost alternative to wired communication infrastructures in many contexts. However, the great majority of research on metropolitan-scale WMN has been centered around maximization of available bandwidth, suitable for non-real-time applications such as Internet access for the general public. On the other hand, the suitability of WMN for mission-critical infrastructure applications remains by and large unknown, as protocols typically employed in WMN are, for the most part, not designed for real-time communications. In this paper, we describe the Smart Transport and Roads Communications (STaRComm) project at National ICT Australia (NICTA), which sets a goal of designing a wireless mesh network architecture to solve the communication needs of the traffic control system in Sydney, Australia. This system, known as SCATS (Sydney Coordinated Adaptive Traffic System)and used in over 100 cities around the world, connects a hierarchy of several thousand devices -- from individual traffic light controllers to regional computers and the central Traffic Management Centre (TMC)-- and places stringent requirements on the reliability and latency of the data exchanges. We discuss our experience in the deployment of an initial testbed consisting of 7 mesh nodes placed at intersections with traffic lights, and share the results and insights learned from our measurements and initial trials in the process.

Rural areas have very low revenue potential. The major issue in providing low-cost broadband to rural areas is to provide reliable backhaul connections that spread over tens or even hundreds of miles, connecting villages to the nearest service provider. Along with aerial networks of Google and Facebook, there has been a considerable amount of research toward long-distance terrestrial WiFi links. As a comparison, WiFi routers are easier to be deployed and maintained by non-technical people from the local communities, whereas the aerial networks require professional support to operate. Moreover, they are still in the experimentation phase. However, the long distance WiFi links require high-gain directional antennas and very expensive tall towers for high data rates. On the other hand, multihop paths with stronger links may provide better data rates without the need of tall towers. In this paper, we evaluated the concept of using such multihop WiFi links for long backhaul connections. Our simulation results show that these networks can possibly be a cost-effective and practical solution for rural connectivity. These initial results can serve as a first step to understand the comprehensive feasibility of using multihop WiFi networks for backhaul connections in rural area.

In this paper, reliability of WiFi multihop backhaul connections, in terms of probability of successful packet delivery, is analyzed in detail. The major issue in providing a cost-effective rural broadband network is to connect the remote sites to the existing network and with each other over distances of tens or hundreds of miles. There has been a lot of research activity around long distance WiFi links. These links require high-gain directional antennas and very expensive tall towers to maintain LoS (Line-of-Sight). In this paper, we evaluated, using empirical models, the idea of using multiple hop connections in linear topology, also termed as multihop chains, for rural backhaul. Reliability of WiFi chains is evaluated in adverse conditions of radio propagation and in the presence of node failures. This analysis shows promising potential of cost-effective WiFi chain networks as backhauls with reasonable reliability. Considerable improvement in reliability and robustness can be achieved by providing some redundancy in network topology. Networks with redundant nodes can tolerate larger separation between adjacent nodes, thus reducing the overall deployment cost.

HELLO messages are widely used for neighbor discovery in routing protocols for wireless multihop networks. In this paper, we provide mathematical and experimental proofs that the current strategy of declaring a link down based on observations that a specific number of HELLO messages are lost, is not a correct strategy. In our mathematical analysis, we characterized the error bursts over wireless links by extending the Gilbert bit error model, and validate our model using HELLO loss data from our wireless mesh testbed. Our analysis shows that error burst lengths follow a geometric distribution, i.e. the probability of additional losses in the burst does not depend on the observed losses. We propose an alternative link break detection strategy where a link break is declared when the observed mean error burst length for the link is longer than the mean route recovery time. Our testbed results show that running the AODV routing protocol with our proposed strategy yields better throughput and lower control message overhead, compared to the case where AODV relies on its default settings of declaring link breaks when two consecutive HELLO messages are lost.

ETX (Expected Transmission Count) is the best routing metric for single radio wireless mesh network (WMN) in terms of throughput as it selects the links with highest packet delivery paths. In our experiments, however, we found that under some scenarios which commonly occur in normal practice, e.g., when data rates approach channel capacity, ETX could yield lower throughput than other metrics, such as, minimum hop-count or RTT (Round Trip Time). The paper presents the experimental evidence of performance anomaly in ETX, identifies the operating conditions when anomaly occurs, and discusses the results of our investigation regarding potential causes, such as, probing overhead, load-sensitivity, etc., using simulations and testbed experiments. Results show that probing, though incurs small overhead, plays a major role in performance degradation of ETX. When node buffers are full because of higher data rate, prioritized control packets, including probes, push data packets outside the queue. When alternative paths are not very much different to each other, ETX appears worse than hop-count because of this additional buffer overflow loss. Although, this loss becomes insignificant when penalty of selecting a lossy path is higher. We also observed that ETX exhibits route oscillations due to load-sensitivity but strong correlation of its load-sensitivity with degraded performance is not found.

In this paper, we present experimental results from testbed implementation of a simple monitoring assisted local route recovery scheme for wireless mesh networks (WMN). The scheme exploits the broadcast nature of wireless transmissions at special routers with added monitoring functionalities. These routers passively listen to the transmissions in the neighborhood and compare the forwarding behavior against pre-specified thresholds. If forwarded traffic exceeds the threshold, as a result of a malfunction, e.g., Denial-of-service (DoS) attack or node failure, these routers try to isolate the node causing the problem by routing the traffic around it. The performance evaluation of our scheme shows considerable improvement in data delivery rates over standard routing schemes.

We present a mobility-aware handoff trigger mechanism that can predict the occurrence of a handoff well in advance of the actual event. Such a scheme could be used to provide smoother handoffs by enabling handoff procedures and pre-allocation of resources to be initiated prior to the actual handoff event. The handoff trigger scheme uses mobility information to predict the occurrence of a handoff event, i.e. the event when the pilot signal strength received from the target base station exceeds that received from the current base station by a given hysteresis level. The handoff trigger scheme is derived from a mobility estimation and prediction scheme, which is based on an autoregressive model or mobility. Simulation results show that the handoff trigger scheme can provide a non-trivial trigger interval with low probabilities of false alarm and missed detection.