Lillian Lei Dai

A Comprehensive Evaluation of RPL under Mobility

This paper focuses on routing for vehicles getting access to infrastructure either directly or via multiple hops through other vehicles. We study routing protocol for low-power and lossy networks (RPL), a tree-based routing protocol designed for sensor networks. Many design elements from RPL are transferable to the vehicular environment. We provide a simulation performance study of RPL and RPL tuning in VANETs. More specifically, we seek to study the impact of RPLs various parameters and external factors (e.g., various timers and speeds) on its performance and obtain insights on RPL tuning for its use in VANETs. We then fine tune RPL and obtain performance gain over existing RPL.

RPL under mobility

This paper focuses on routing for vehicles getting access to infrastructure either directly or via multiple hops though other vehicles. We study Routing Protocol for Low power and lossy networks (RPL), a tree-based routing protocol designed for sensor networks. Many design elements from RPL are transferable to the vehicular environment. We provide a simulation performance study of RPL and RPL tuning in VANETs. More specifically, we seek to study the impact of RPLs various parameters and external factors (e.g., various timers and speeds) on its performance and obtain insights on RPL tuning for its use in VANETs.

Distributed Call Admission Control for VoIP over 802.11 WLANs Based on Channel Load Estimation

Call Admission Control (CAC) is recognized as one of the key strategies to achieve satisfactory QoS support for VoIP over IEEE 802.11 WLANs. However, most of the prior CAC solutions for VoIP over WLAN are centralized, and the few distributed CAC schemes proposed so far do not account for issues such as the loss of channel time due to medium contention and the coexistence of VoIP traffic with background traffic such as TCP data flows. In this paper, we describe a distributed CAC scheme which aims at addressing these issues by leveraging on channel monitoring techniques, thus being readily implementable in todays consumer devices. The proposed scheme is thoroughly evaluated by means of testbed experiments as well as network simulations in different scenarios, and it is shown to yield a better CAC performance with respect to prior solutions.

A reconfigurable test platform to experiment with wireless heterogeneous networks in a laboratory

A large number of wireless technologies and networks have been introduced in the telecommunication world during the last few years. Many protocols and applications (e.g., TCP, VoIP) are not specifically designed for such a variety of wireless protocols; therefore they require a deeper analysis to understand their behaviour over those radio protocols. Performance analysis in close-to-real environment represents a valid method to evaluate and model the behaviour of such protocols and applications for their improvement. This paper presents a wireless heterogeneous testbed that allows experimentation of WLANs and cellular networks in a laboratory. The paper describes also the assessment and the calibration of the testbed. It presents an extensive comparison with a commercial 3G network. Finally, the paper also presents two case studies in order to highlight the benefits of conducting experimental research in an in-lab fully controlled platform.

Intelligent interface switching among heterogeneous wireless networks for vehicular communications

As future vehicular communications will most likely involve multiple wireless networks, intelligent interface switching is of essential importance to support various user preferences across different performance metrics. In this paper, we present a general optimization framework for solving the interface switching problem, and develop a flexible and efficient solution based on stochastic dynamic programming (SDP). Our framework is designed to accommodate different performance metrics such as data transfer efficiency, monetary cost, and interface switching overhead. Accordingly, the proposed SDP-based policy can easily adapt its decision based on user-specified relative importance of the various metrics. Simulation study confirms the optimality of the SDP-based policy over a range of user preference choices, and shows that it consistent outperforms several heuristic schemes.

A real-time cellular system architecture to experiment with UMTS/HSDPA in a laboratory

Nowadays wireless wide area networks (e.g. cellular networks) are also used to provide access to Internet and its services. However, protocols and applications not specifically designed for wireless links (e.g., TCP, VoIP) often require a deeper analysis when applied to such links. Performance analysis represents a valid method to evaluate such protocols and applications, modeling their behavior and then working on their improvement. An appropriate solution to support performance evaluation of a wireless system and its support to protocols and applications consists in building a close-to-real test environments. This paper presents MUSA, a testbed that allows experimentation of cellular networks in a laboratory. The presented testbed combines emulation components and real equipments in a single experimentation platform, with the flexibility, scalability and repeatability features of an emulated environment and the possibility of testing real equipments, e.g., real mobile terminals or UMTS/HSDPA wireless cards and base stations. The paper describes also the assessment and calibration of the testbed. It presents an extensive comparison with a commercial 3G network and discusses the unique properties of MUSA. Finally, the paper also presents two case studies in order to highlight the benefits of conducting experimental research in an in-lab fully controlled platform rather than in an operator-controlled network.

A Probabilistic Approach to Identifying the Number of Frequency Hoppers for Spectrum Sensing

Characterizing the number and type of transmitters occupying a given frequency band is a critical aspect of spectrum sensing specifically and cognitive radio generally. We present an analytical framework based on probability to identify the number of frequency hopping transmitters of one specific type in a band of interest, and show that the probability mass functions associated with the different potential number of transmitters quickly becomes Gaussian as the number of channel observations increases. Simulation results confirm that the approach can lead with high probability to a correct decision regarding the number of interferers. Thus, the methods here can serve as a valuable complement to other spectrum sensing approaches.