Fabio Martignon

Bandwidth estimation schemes for TCP over wireless networks

The use of enhanced bandwidth estimation procedures within the congestion control scheme of TCP was proposed recently as a way of improving TCP performance over links affected by random loss. This paper first analyzes the problems faced by every bandwidth estimation algorithm implemented at the sender side of a TCP connection. Some proposed estimation algorithms are then reviewed, analyzing and comparing their estimation accuracy and performance. As existing algorithms are poor in bandwidth estimation, and in sharing network resources fairly, we propose TIBET (time intervals based bandwidth estimation technique). This is a new bandwidth estimation scheme that can be implemented within the TCP congestion control procedure, modifying only the sender-side of a connection. The use of TIBET enhances TCP source performance over wireless links. The performance of TIBET is analyzed and compared with other schemes. Moreover, by studying TCP behavior with an ideal bandwidth estimation, we provide an upper bound to the performance of all possible schemes based on different bandwidth estimates.

Joint Routing and Scheduling Optimization in Wireless Mesh Networks with Directional Antennas

Wireless Mesh Networks (WMNs) have recently emerged as a technology for next-generation wireless networking. WMNs partially replace wired backbone networks, and it is therefore reasonable to plan carefully radio resource assignment to provide quality guarantees to traffic flows. Directional transmissions allow to reduce radio interference, thus exploiting spatial reuse. Therefore, as a main contribution, in this paper we study the joint routing and scheduling optimization problem in Wireless Mesh Networks where nodes are equipped with directional antennas. To this aim, we assume a Spatial reuse Time Division Multiple Access (STDMA) scheme, a dynamic power control able to vary the emitted power slot-by-slot, and a rate adaptation mechanism that sets transmission rates according to the Signal- to-Interference-and-Noise Ratio (SINR). We provide column generation-based heuristic approaches for the proposed models in a set of realistic-size instances and discuss the impact of different parameters on the network performance. The results show that our schemes increase considerably the total traffic accepted by the network, providing bounds to the achievable performance.

EFW: A cross-layer metric for reliable routing in wireless mesh networks with selfish participants

Wireless mesh networks (WMNs) have emerged as a flexible and low-cost network infrastructure, where heterogeneous mesh routers managed by different users collaborate to extend network coverage. Several routing protocols have been proposed to improve the packet delivery rate based on enhanced metrics that capture the wireless link quality. However, these metrics do not take into account that some participants can exhibit selfish behavior by selectively dropping packets sent by other mesh routers in order to prioritize their own traffic and increase their network utilization.

Non-cooperative spectrum access in cognitive radio networks: A game theoretical model

Cognitive radio networks provide the capability to share the wireless channel with licensed (primary) users in an opportunistic manner. Primary users have a license to operate in a certain spectrum band; their access can only be controlled by the primary operator and is not affected by any other unlicensed (secondary) user. On the other hand, secondary users (SUs) have no spectrum license, and they attempt to exploit the spectral gaps left free by primary users. This work studies the spectrum access problem in cognitive radio networks from a game theoretical perspective. The problem is modeled as a non-cooperative spectrum access game where secondary users access simultaneously multiple spectrum bands left available by primary users, optimizing their objective function which takes into account the congestion level observed on the available spectrum bands. As a key innovative feature with respect to existing works, we model accurately the interference between SUs, capturing the effect of spatial reuse. Furthermore, we consider both non-elastic and elastic user traffic, to model real-time as well as data transfer applications. Finally, we consider an alternative formulation of the spectrum access problem, where players use replicator dynamics to adjust their strategies, and we derive convergence conditions to Nash equilibrium points. We demonstrate the existence of the Nash equilibrium, and derive equilibrium flow settings. Finally, we provide numerical results of the proposed spectrum access game in several cognitive radio scenarios, and study the impact of the interference between SUs on the game efficiency. Our results indicate that the congestion cost functions we propose in this paper lead to small gaps between Nash equilibria and optimal solutions in all the considered network scenarios, thus representing a starting point for designing pricing mechanisms so as to obtain a socially optimal use of the network.

Design and implementation of MobiSEC: A complete security architecture for wireless mesh networks

Wireless mesh networks (WMNs) have emerged recently as a technology for next-generation wireless networking. They consist of mesh routers and clients, where mesh routers are almost static and form the backbone of WMNs. WMNs provide network access for both mesh and conventional clients. In this paper we propose MobiSEC, a complete security architecture that provides both access control for mesh users and routers as well as a key distribution scheme that supports layer-2 encryption to ensure security and data confidentiality of all communications that occur in the WMN. MobiSEC extends the IEEE 802.11i standard exploiting the routing capabilities of mesh routers; after connecting to the access network as generic wireless clients, new mesh routers authenticate to a central server and obtain a temporary key that is used both to prove their credentials to neighbor nodes and to encrypt all the traffic transmitted on the wireless backbone links. A key feature in the design of MobiSEC is its independence from the underlying wireless technology used by network nodes to form the backbone. Furthermore, MobiSEC allows seamless mobility of both mesh clients and routers. MobiSEC has been implemented and integrated in MobiMESH, a WMN implementation that provides a complete framework for testing and analyzing the behavior of a mesh network in real-life environments. Moreover, extensive simulations have been performed in large-scale network scenarios using Network Simulator. Numerical results show that our proposed architecture considerably increases the WMN security, with a negligible impact on the network performance, thus representing an effective solution for wireless mesh networking.

A bandwidth trading marketplace for mobile data offloading

The Radio Access Network (RAN) infrastructure represents the most critical part for capacity planning, which usually accounts for peak traffic conditions. A promising approach to increase the RAN capacity and simultaneously reduce its energy consumption is represented by the opportunistic utilization of third party Wi-Fi access devices. In order to foster the utilization of unexploited Internet connections, we propose a new and open market, where a mobile operator can lease the bandwidth made available by third parties (residential users or private companies) through their access points to increase the network capacity and save large amounts of energy. We formulate the offloading problem as a reverse auction considering the most general case of partial covering of the traffic to be offloaded. We discuss the conditions (i) to offload the maximum amount of data traffic according to the capacity of third party access devices, (ii) to foster the participation of access point owners (individual rationality), and (iii) to prevent market manipulation (incentive compatibility). Finally, we propose a greedy algorithm that solves the offloading problem in polynomial time, even for large-size network scenarios.

Directional MAC and routing schemes for power controlled Wireless Mesh Networks with adaptive antennas

Wireless Mesh Networks (WMNs) have emerged recently as a technology for next-generation wireless networking. Several approaches that exploit directional and adaptive antennas have been proposed in the literature to increase the performance of WMNs. However, while adaptive antennas can improve the wireless medium utilization by reducing radio interference and the impact of the exposed nodes problem, they can also exacerbate the hidden nodes problem. Therefore, efficient MAC protocols are needed to fully exploit the features offered by adaptive antennas. Furthermore, routing protocols that were designed for omnidirectional communications can be redesigned to exploit directional transmissions and the cross-layer interaction between the MAC and the network layer. In this paper we first propose a novel Power-Controlled Directional MAC protocol (PCD-MAC) for adaptive antennas. PCD-MAC uses the standard RTS-CTS-DATA-ACK exchange procedure. The novel difference is the transmission of the RTS and CTS packets in all directions with a tunable power while the DATA and ACK are transmitted directionally at the minimal required power. We then propose the Directional Deflection Routing (DDR), a routing algorithm that exploits multiple paths towards the destination based on the MAC layer indication on channel availability in different directions. We measure the performance of PCD-MAC and DDR by simulation of several realistic network scenarios, and we compare them with other approaches proposed in the literature. The results show that our schemes increase considerably both the total traffic accepted by the network and the fairness among competing connections.

A Multi-Commodity Flow Model for Optimal Routing in Wireless MESH Networks

We propose a mathematical programming model of the routing problem in multi-hop wireless networks that takes into account quality of service requirements considering bandwidth constraints. The proposed approach is suitable for Wireless MESH Networks (WMN) where topology is almost fixed and routes can be optimized based on global objectives. We then consider and solve the scheduling problem, illustrating how routing and scheduling models can be combined to route flows with guaranteed bandwidth. As an interesting application of the proposed approach, we present some numerical examples that show how our model can be used to estimate the impact of transmission range on network capacity.

Enhanced loss differentiation algorithms for use in TCP sources over heterogeneous wireless networks

Loss differentiation algorithms (LDA) are used to provide TCP with an estimate of the cause of packet losses, to improve performance over heterogeneous networks including wired and wireless links. In this work, we compared by simulation the accuracy of several LDA schemes in various realistic scenarios. We experienced that LDA schemes originally proposed in literature exhibit poor performance in estimating the cause of packet losses. Thus, we propose enhancements to the noncongestion packet loss detection (NCPLD) and Vegas schemes, achieving higher accuracy in all network scenarios. We shown that our proposed enhanced schemes approach reasonably ideal accuracy of LDA having perfect knowledge of the cause of packet losses. These results entail the possibility to adopt such algorithms within TCP congestion control, to achieve higher performance over heterogeneous wireless networks.

Bandwidth Estimates in the TCP Congestion Control Scheme

Many bandwidth estimation techniques, somehow related to the TCP world, have been proposed in the literature and adopted to solve several problems. In this paper we discuss their impact on the congestion control of TCP and we propose an algorithm which performs an explicit and effective estimate of the used bandwidth. We show by simulation that it efficiently copes with the packet clustering and ACK compression effects without leading to the biased estimate problem of existing algorithms.We present numerical results proving that TCP sources implementing the proposed scheme with an unbiased used-bandwidth estimate fairly share the bottleneck bandwidth with classical TCP Reno sources. Finally, we point out the benefits of using the proposed scheme compared to TCP Reno in networks with wireless links.