Razvan Stanica

Large-Scale Mobile Traffic Analysis: A Survey

This article surveys the literature on analyses of mobile traffic collected by operators within their network infrastructure. This is a recently emerged research field, and, apart from a few outliers, relevant works cover the period from 2005 to date, with a sensible densification over the last three years. We provide a thorough review of the multidisciplinary activities that rely on mobile traffic datasets, identifying major categories and sub-categories in the literature, so as to outline a hierarchical classification of research lines. When detailing the works pertaining to each class, we balance a comprehensive view of state-of-the-art results with punctual focuses on the methodological aspects. Our approach provides a complete introductory guide to the research based on mobile traffic analysis. It allows summarizing the main findings of the current state-of-the-art, as well as pinpointing important open research directions.

Simulation of vehicular ad-hoc networks: Challenges, review of tools and recommendations

Vehicular communications are considered the next step in increasing transportation safety and comfort. The prohibitive price of operational test beds means that computer simulations are the only viable solution for analysing the performance of different protocols and architectures. However, simulation frameworks used in vehicular ad-hoc networks research are still highly heterogeneous and, as a consequence, many of the proposed ideas can not be compared and validated. In this paper, we focus on the challenges faced when modelling the vehicular environment and the solutions adopted in the main simulation tools. As the research community is concerned with many different problems, from safety related issues to traffic efficiency and from intersection management to Internet access, we consider that every study should chose the appropriate simulator based on its requirements. Consequently, we make some recommendations which take into account the scope of the simulated scenario and the properties of the simulation frameworks.

Offloading Floating Car Data

Floating Car Data (FCD) is currently collected by moving vehicles and uploaded to Internet-based processing centers through the cellular access infrastructure. As FCD is foreseen to rapidly become a pervasive technology, the present network paradigm risks not to scale well in the future, when a vast majority of automobiles will be constantly sensing their operation as well as the external environment and transmitting such information towards the Internet. In order to relieve the cellular network from the additional load that widespread FCD can induce, we study a local gathering and fusion paradigm, based on vehicle-to-vehicle (V2V) communication. We show how this approach can lead to significant gain, especially when and where the cellular network is stressed the most. Moreover, we propose several distributed schemes to FCD offloading based on the principle above that, despite their simplicity, are extremely efficient and can reduce the FCD capacity demand at the access network by up to 95%.

Enhancements of IEEE 802.11p Protocol for Access Control on a VANET Control Channel

Adding communication capabilities to vehicles and road infrastructure has become a major goal in the intelligent transportation systems industry. The IEEE 802.11p amendment has specially been conceived for the Wireless Access in Vehicular Environments (WAVE) architecture. In this paper we study the performance of this standard by the means of extensive simulations and we argue that the current version of the protocol can not cope with high vehicular densities. We propose a simple but efficient modification of the back off mechanism which has an important impact on the quality of communications on the control channel.

Properties of the MAC layer in safety vehicular Ad Hoc networks

With intervehicle communications becoming a more and more popular research topic recently, the medium access control layer of the vehicular network has also received a considerable amount of attention. However, this increased interest has not always translated into a careful analysis of the properties exhibited by the MAC protocol when used by vehicular safety applications. This article tries to fill this gap by providing a comprehensive discussion on a number of important characteristics of the link layer in vehicular communications.

Local density estimation for contention window adaptation in vehicular networks

The medium access control protocol of a future vehicular ad-hoc network is expected to cope with highly heterogeneous conditions. An essential parameter for protocols issued from the IEEE 802.11 family is the minimum contention window used by the backoff mechanism. While its impact has been thoroughly studied in the case of wireless local area networks, the importance of the contention window has been somehow neglected in the studies focusing on vehicle-to-vehicle communication. In this paper we show that the adjustment of the minimum contention window depending on the local node density can notably improve the performance of the IEEE 802.11 protocol. Moreover, we compare through simulation in a realistic framework five different methods for estimating the local density in a vehicular environment, presenting the advantages and the shortcomings of each of them.

Physical Carrier Sense in Vehicular Ad-Hoc Networks

Enhancing road safety using vehicle-to-vehicle communication has become an important goal for the automotive industry. A lot of effort has been put in the design of an efficient medium access control protocol, capable to function correctly even under heavy congestion. The solutions proposed so far focus on data rate or transmission power control. In this paper, we argue that the most important parameter for congestion control in vehicular ad hoc networks is the carrier sense threshold. We support this theory with analytical and simulation results and we demonstrate that the optimal threshold depends on the vehicular density. Furthermore, we propose an adaptive mechanism for physical carrier sense control and analyse its performance, showing an important increase in message reception probability.

Broadcast communication in Vehicular Ad-Hoc Network safety applications

Contention-based protocols for Medium Access Control (MAC) in Vehicular Ad-Hoc Networks (VANET) are currently under development in several standardization organizations. The availability and maturity of the IEEE 802.11 technology makes it the first choice for the future vehicle-to-vehicle communications. At the same time, safety applications in a vehicular environment are expected to intensively use broadcast messages. However, the IEEE 802.11 standard has not been designed for broadcast communication and a number of problems arise from this. In this paper, we analyze the impact of the minimum Contention Window (CW) on the MAC layer performance in a realistic vehicular environment and we propose a simple solution for adapting CW to the network density in order to improve the reception probability of broadcast messages.

Congestion control in CSMA-based vehicular networks: Do not forget the carrier sensing

Inter-vehicular communications are considered to be an efficient proactive approach for reducing the number and the consequences of road accidents. After a series of remarkable standardisation efforts, one of the last points needing to be addressed in order for safety vehicular networks to become a reality is the scalability problem of the CSMA-based medium access control layer. With node densities that can range from very sparse to several hundred contending stations, the MAC protocol needs the capacity to adapt to the state of the vehicular network without compromising the performance of the safety applications. While previous studies focused on individual mechanisms for data rate selection or transmission power control from a global point of view, this paper proposes a complete congestion control framework aiming to increase the message reception probability in the immediate neighbourhood under heavy congestion conditions. We propose a new concept for physical carrier sensing, which takes into account the location of the transmitter, and we combine it with transmission power control and a recently proposed backoff mechanism to obtain an important improvement over the original protocol. Several implementation problems are discussed, showing the feasibility of the solution using existing hardware, and a simulation study confirms the performance of this enhanced channel access method.

Capillary networks: a novel networking paradigm for urban environments

In this paper, we present our vision of the networking challenges that are yielded by the rise of Smart Cities. Smart Cities leverage massive data collected by sensors, connected devices, social applications,... for proving a whole set a new services to the citizens. However, there is a lack of reflexion on the networking solutions that enable these services, from the gathering of sensed data to the dissemination of digital services. We identify the emerging needs of Smart Cities, focus on the capillary networks paradigm which unify the wealth of wireless connectivity available in urban environment, and present the research issues it yields.