Infoscience

Thesis

On uncoordinated wireless ad-hoc networks: data dissemination over WIFI and cross-layer optimization for ultra wide band impulse radio

Emerging pervasive wireless networks, pocket switched networks, Internet of things, vehicular networks and even sensor networks present very challenging communication circumstances. They might involve up to several hundreds of wireless devices with mobility and intermittent connectivity. Centralized coordination in such networks is practically unfeasible. We deal with these challenge using two potential technologies: WIFI and Ultra Wide Band (UWB) Impulse Radio (IR) for medium and short communication range, respectively. Our main goal is to improve the communication performance and to make these networks sustainable in the absence of a centralized coordination. With WIFI, the goal is to design an environment-oblivious data dissemination protocol that holds in highly dynamic unpredictable wireless ad-hoc networks. To this end, we propose a complete design for a scope limited, multi-hop broadcast middleware, which is adapted to the variability of the ad-hoc environment and works in unlimited ad-hoc networks such as a crowd in a city, or car passengers in a busy highway system. We address practical problems posed by: the impossibility of setting the TTL correctly at all times, the poor performance of multiple access protocols in broadcast mode, flow control when there is no acknowledgment and scheduling of multiple concurrent broadcasts. Our design, called "Self Limiting Epidemic Forwarding" (SLEF), automatically adapts its behavior from single hop MAC layer broadcast to epidemic forwarding when the environment changes from being extremely dense to sparse, sporadically connected. A main feature of SLEF is a non-classical manipulation of the TTL field, which combines the usual decrement-when-sending to many very small decrements when receiving. Then, we identify vulnerabilities that are specific to epidemic forwarding. We address broadcast applications over wireless ad-hoc networks. Epidemic forwarding employs several mechanisms such as forwarding factor control and spread control, and each of them can be implemented using alternative methods. Thus, the existence of vulnerabilities is highly dependent on the methods used. We examine the links between them. We classify vulnerabilities into two categories: malicious and rational. We examine the effect of the attacks according to the number of attackers and the different network settings such as density, mobility and congestion. We show that malicious attacks are hard to achieve and their effects are scenario-dependent. In contrast, rational attackers always obtain a significant benefit. The evaluation is carried out using detailed realistic simulations over networks with up to 1000 nodes. We consider static scenarios, as well as vehicular networks. In order to validate our simulation results, we build a solid and widely adaptable experimental testbed for wireless networks. It is composed of 57 mobile wireless nodes equipped with WIFI interface. The adopted platform is OpenWrt, a Linux-like firmware, which makes the testbed robust and easily configurable. With UWB IR, the main problem we deal with is the presence of uncontrolled interference. Indeed, similarly to Code Division Multiple Access (CDMA) systems, signal acquisition with UWB IR signaling requires power control in the presence of interferers, which is very expensive in an uncoordinated system. We solve this problem through a cross-layer optimization: We propose a new signal acquisition method that is independent of the received signal power and we adapt the MAC layer accordingly. Our signal acquisition method is designed to solve the IUI (Inter-User Interference) that occurs in some ad-hoc networks where concurrent transmissions are allowed with heterogeneous power levels. In such scenarios, the conventional detection method, which is based on correlating the received IR signal with a Template Pulse Train (TPT), does not always perform well. The complexity of our proposal is similar to that of the conventional method. We evaluate its performance with the Line Of Sight (LOS) and the Non-LOS (NLOS) office indoor-channel models proposed by the IEEE P802.15.4a study group and find that the improvement is significant. We also investigate the particular case where the concurrent transmissions have the same time-hopping code, and we show that it does not result in collision, such scenarios appear in ad-hoc networks that employ a common code for control or broadcast purposes. At the MAC level, we focus only on one component of a MAC layer, which is the sleeping mode that could be added to any MAC layer proposal adequate to UWB IR. We are motivated by the low power consumption constraint required by the potential applications. We identify the design elements that should be taken into account for an optimal design for a sleeping protocol for UWB-IR such as the possibility of transmitting concurrently without collision and the power consumption model of the hardware behind which is completely different than with the narrow-band signaling. Then, we design two sleeping protocols for centralized and decentralized ad-hoc networks, respectively. We evaluate their performance analytically with the adopted metric being the average life-time of the wireless nodes.

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