George Athanasiou

A Cross-Layer Framework for Association Control in Wireless Mesh Networks

The user association mechanism specified by the IEEE 802.11 standard does not consider the channel conditions and the AP load in the association process. Employing the mechanism in its plain form in wireless mesh networks we may only achieve low throughput and low user transmission rates. In this paper we design a new association framework in order to provide optimal association and network performance. In this framework we propose a new channel-quality based user association mechanism inspired by the operation of the infrastructure-based WLANs. Besides, we enforce our framework by proposing an airtime-metric based association mechanism that is aware of the uplink and downlink channel conditions as well as the communication load. We then extend the functionality of this mechanism in a cross-layer manner taking into account information from the routing layer, in order to fit it in the operation of wireless mesh networks. Lastly, we design a hybrid association scheme that can be efficiently applied in real deployments to improve the network performance. We evaluate the performance of our system through simulations and we show that wireless mesh networks that use the proposed association mechanisms are more capable in meeting the needs of QoS-sensitive applications.

Dynamic Cross-Layer Association in 802.11-Based Mesh Networks

In IEEE 802.11-based wireless mesh networks a user is associated with an access point (AP) in order to communicate and be part of the overall network. The association mechanism specified by the IEEE 802.11 standard does not consider the channel conditions and the AP load in the association process. Employing the mechanism in its plain form in wireless mesh networks we may only achieve low throughput and low user transmission rates. In this paper, we propose an association mechanism that is aware of the uplink and downlink channel conditions. We introduce a metric that captures the channel conditions and the load of the APs in the network. The users use this metric in order to optimally associate with the available APs. We then extend the functionality of this mechanism in a cross-layer manner taking into account information from the routing layer. The novelty of the mechanism is that the routing QoS information of the back haul is available to the end users. This information can be combined with the uplink and downlink channel information for the purpose of supporting optimal end-to-end communication and providing high end-to-end throughput values. We evaluate the performance of our system through simulations and we show that 802.11-based mesh networks that use the proposed association mechanism are more capable in meeting the needs of QoS-sensitive applications.

Evaluation of localization methods in millimeter-wave wireless systems

Millimeter-wave communications are considered as one of the key technologies for enabling multi-gigabit wireless access and bandwidth demanding applications. An increasing number of these applications, such as smart environments, asset tracking, video surveillance, and specialized location based services, require knowledge related to the location of the wireless devices. Due to the special characteristics of millimeter-wave transmissions, such as high path loss attenuation, oxygen absorption, and antenna directivity, it is challenging to achieve accurate localization. In this overview paper, the performance of localization methods over the millimeter-wave medium is compared under realistic conditions. The prominent localization approaches based on received signal strength, time of arrival, and angle of arrival of millimeter-wave signals, are numerically evaluated by an extensive set of Monte Carlo simulations. It is suggested that localization based on angle of arrival is the most promising for millimeter-wave transmissions.

High throughput pipelined FPGA implementation of the new SHA-3 cryptographic hash algorithm

In this paper a two-staged pipelined architecture of the new SHA-3 (Keccak) algorithm is presented. The core can operate on both one-block and multi-block messages, realizing all possible modes of Keccak. Special effort has been paid and different design alternatives have been studied to derive efficient FPGA implementations in terms of throughput and throughput/area metrics. The proposed core has been implemented in Xilinx Virtex-5, Virtex-6, and Virtex-7 FPGA technologies and achieves significant improvements compared to existing FPGA implementations. Specifically, for Virtex-5 the proposed architecture achieves better throughput and throughput/area results from 45.8% to 248× and from 8.9% up to 17.9×, respectively. Regarding Virtex-6, the improvements in throughput and throughput/area are from 47.2% up to 18.1× and from 8% up to 27.3×, respectively.

An 802.11k compliant framework for cooperative handoff in wireless networks

In IEEE 802.11-based wireless networks, the stations (STAs) are associated with the available access points (APs) and communicate through them. In traditional handoff schemes, the STAs get information about the active APs in their neighborhood by scanning the available channels and listening to transmitted beacons. This paper proposes an 802.11k compliant framework for cooperative handoff where the STAs are informed about the active APs by exchanging information with neighboring STAs. Besides, the APs share useful information that can be used by the STAs in a handoff process. In this way, we minimize the delay of the scanning procedure. We evaluate the performance of our mechanisms through OPNET simulations. We demonstrate that our scheme reduces the scanning delay up to 92%. Consequently, our system is more capable in meeting the needs of QoS-sensitive applications.

LAC: Load-aware channel selection in 802.11 WLANs

Dense deployments of hybrid WLANs result in high levels of interference and low end-user throughput. Many frequency allocation mechanisms for WLANs have been proposed by a large body of previous studies. However, none of these mechanisms considers the load that is carried by APs in terms of channel conditions, number of affiliated users as well as traffic-load, in conjunction. In this paper, we propose LAC, a load-aware channel allocation scheme for WLANs, which considers all the above performance determinant factors. LAC incorporates an airtime cost metric into its channel scanning process, in order to capture the effects of these factors and select the channel with the maximum long-term throughput. We evaluate LAC through extensive OPNET simulations, for many different traffic scenarios. Our simulations demonstrate that LAC outperforms other frequency allocation policies for WLANs in terms of total network throughput by up to 135%.

Dynamic Optimization of Generalized Least Squares Handover Algorithms

Efficient handover algorithms are essential for highly performing cellular networks. These algorithms depend on numerous parameters, whose settings must be appropriately optimized to offer a seamless connectivity. Nevertheless, such an optimization is difficult in a time varying context, unless adaptive strategies are used. In this paper, a new approach for the handover optimization is proposed. Three dynamical optimization approaches are presented, where the probability of outage and the probability of handover are considered. Since it is shown that these probabilities are difficult to compute, simple approximations of adequate accuracy are developed. A distributed optimization algorithm is then developed to maximize handover performance. Numerical results show that the proposed algorithm improves the performance of the handover considerably when compared to more traditional approaches.

Overcoming misbehavior in mobile ad hoc networks: an overview

In the recent years, wireless networks have experienced an enormous growth which has given rise to new research challenges. Ad hoc networks are composed of autonomous nodes that are independent of any fixed infrastructure. Mobile ad hoc networks have a fully decentralized topology and they are dynamically changing. Besides these challenges, the wireless transmission medium introduces limitations in communication. For these reasons, providing security guarantees is particularly difficult. In a mobile ad hoc network every node acts as a router for its neighbors. The routing protocols that have been proposed assume that the nodes will fully participate. Unfortunately, node misbehavior is a common phenomenon. Misbehavior is due to selfish or malicious reasons. Another reason, which is rarer, is a faulty link due to the wireless medium. Misbehavior can take place at all layers. At the Physical layer a misbehaving node can increase its transmitting power, adversely affecting the network performance. At the MAC (Medium Access Control) layer a node may choose to avoid waiting for its turn to access the medium, taking an unfair advantage of the shared medium. The basic threat at the Network layer is the non-cooperative behavior as far as packet forwarding is concerned. The proper execution of a routing protocol demands that the intermediate nodes in a path forward the packets correctly to the intended receivers. Misbehaving nodes do not forward these packets. A routing protocol for MANETs should give incentives for cooperative action or at least it should be able to detect misbehaving nodes and correct them.

Distributed association control and relaying in millimeter wave wireless networks

Millimeter wave (mmWave) spectrum is one of the frontiers in the evolution towards the next generation of the wireless communication systems, which can provide great performance benefits at the variform access and backbone networks. However, at the access level the typical rapidly fading behavior of the mmWave channel imposes the careful design of client association to access points (APs), as well as relaying to other clients, which can act as bridge toward the APs. This challenge is hereby addressed by a distributed approach that optimally solves the joint client association and relaying problem. The problem is posed as a novel multi-dimensional assignment problem, for which an original solution method is established by a series of transformations that lead to a tractable minimum cost flow problem. The method allows to design distributed auction algorithms where the clients and relays act asynchronously to achieve optimal client-relay-AP association. It is shown that the algorithms converge to a solution that maximizes the total network throughput within a desired bound.

Routing-Aware Channel Selection in Multi-Radio Mesh Networks

Efficient channel selection is essential in 802.11 mesh deployments, for minimizing contention and interference among co-channel devices and thereby supporting a plurality of QoS-sensitive applications. In this paper, we propose ARACHNE, a routing-aware channel selection protocol for wireless mesh networks. ARACHNE is distributed in nature, and motivated by our measurements on a wireless testbed. The main novelty of our protocol comes from adopting a metric that captures the end-to-end link loads across different routes in the network. ARACHNE prioritizes the assignment of low-interference channels to links that (a) need to serve high-load aggregate traffic and/or (b) already suffer significant levels of contention and interference. Our protocol takes into account the number of potential interfaces (radios) per device, and allocates these interfaces in a manner that efficiently utilizes the available channel capacity. We evaluate ARACHNE through extensive, trace-driven simulations. We observe that our protocol improves the total network throughput, as compared to three other channel allocation strategies.