Section: New Results

Wireless Networking

Participants : Anwar Al-Hamra, Chadi Barakat, Diego Dujovne, Mohamed Ali Kaafar, Mathieu Lacage, Katia Obraczka, Yongho Seok, Thierry Turletti, Jose Miguel Villalón.

• A New MAC Scheme for Very High-Speed WLANs

  We have studied how to improve the medium access control (MAC) layer for very high-speed Wireless LANs in order to support rich multimedia applications such as high-definition television (HDTV). We have proposed an Aggregation with Fragment Retransmission (AFR) scheme, which supports transmissions of very large frames and partial retransmissions in the case of errors. Aggregation allows increased performance despite per-transmission overhead while fragmentation alleviates the risk of losing the entire frame, a risk increases with transmission rate and frame size. Our simulations show that AFR greatly outperforms the DCF MAC protocol. In the best case we have tested, it is twice more efficient than DCF [27] . This work has been done in collaboration with several colleagues from the Hamilton Institute in Dublin, Ireland and Yang Xiao from the Univ. of Memphis, TN, USA.

• Multicast Transmission of Multimedia Streams over WiFi

  While the deployment of WiFi networks continue to grow at an explosive rate, the multicast multimedia delivery service on WiFi compliant devices is still in its early stage of development. The real culprit is the IEEE 802.11 MAC protocol, and in particular, the absence of feedback mechanism when multicast is used. Recently, the leader-based protocol (LBP) has been proposed to overcome this problem for reliable streams. We have measured the characteristics of the legacy multicast transmission mechanism and analyze its flaws. Then, we have studied the performance of the leader-based approach and compared its performance with the standard multicast service. The analysis has been done on a large set of measurements made with our wireless testbed. Such measurements are an important complement to previous simulation studies and help in the design of the best mechanism to replace the faulty legacy multicast mechanism. Our study [18] confirms that the leader-based mechanism outperforms the standard open-loop multicast mechanism while keeping fairness among other traffic.

  In parallel of this experimental study, we have designed an improved multicast transmission scheme for multimedia streams over WiFi WLANs. This scheme, called Auto Rate Selection Multicast (ARSM) aims to adapt the physical rate transmission to the varying conditions of the channel [31] . Our simulation results show that ARSF outperforms both the IEEE 802.11 standard multicast scheme and LBP.

• Topology-Aware Overlay Multicast for Mobile Ad-Hoc Networks

  AOMP (Ad-hoc Overlay Multicast Protocol) is a novel approach for application-layer multicast in ad-hoc networks [22] . We have designed a new algorithm that exploits a few properties of IP-routing to extract underlying topology information. The basic idea is to match path from nodes to the source in order to detect near neighbors in the physical topology. Then, in a dynamic and decentralized way, a minimum cost mobility-aware delivery tree is constructed, connecting nodes that are close to each other. We have designed a tree improvement algorithm in order to enhance the global performance of AOMP during data distribution. Our simulations results show that, compared to previously proposed application-layer multicast structures, AOMP yields trees with lower cost and traffic redundancy. In addition, it performs well in terms of packet losses, especially in case of node mobility.

• Network Coding for Wireless Mesh Networks

  Network coding is a new transmission paradigm that proved its strength in optimizing the usage of network resources. We have evaluated the gain from using network coding for file sharing applications running on top of wireless mesh networks. With extensive simulations carried out on a simulator we developed specifically for this study, we confirm that network coding can improve the performance of the file sharing application, but not as in wired networks. The main reason is that nodes over wireless cannot listen to different neighbors simultaneously. Nevertheless, one can get more from network coding if the information transmission is made more diverse inside the network. We support this argument by varying the loss rate over wireless links and adding more sources [19] . This work has been in collaboration with Anwar Al Hamra from Univ. of Oslo, Norway.

• Maximizing Transfer Opportunities in Bluetooth DTNs

  Devices in disruption tolerant networks (DTNs) must be able to communicate robustly in the face of short and infrequent connection opportunities. Unfortunately, one of the most inexpensive, energy-efficient and widely deployed peer-to-peer capable radios, Bluetooth, is not well-suited for use in a DTN. Bluetooth's half-duplex process of neighbor discovery can take tens of seconds to complete between two mutually undiscovered radios. This delay can be larger than the time that mobile nodes can be expected to remain in range, resulting in a missed opportunity and lower overall performance in a DTN. In this collaboration with the University of Massachusetts at Amherst (UMASS), we propose a simple, cost effective, and high performance modification to mobile nodes to dramatically reduce this delay: the addition of a second Bluetooth radio. We showed through analysis and simulation that this dual radio technique improves both connection frequency and duration. Moreover, despite powering two radios simultaneously, nodes using dual radios are more energy efficient, spending less energy on average per second of data transferred. We refer to [28] for more details.

• Heterogeneous Wireless networks

  In the context of the Divine project, we have been working on the design and evaluation through simulation of the protocols that will be developed to achieve efficient video distribution over a heterogeneous network. We have also been exploring new protocols for reliable message distribution in the face of episodic network connectivity.

  We also collaborate with Serge Fdida's group at Lip6, Paris on: (1) Self-localization in wireless sensor networks, and (2) a tool for prototyping wireless network protocols; as well as with UCSC, UCSB, and Uof Delaware on: (1) Energy-efficient MAC protocols for MANETs, (2) Sensor network systems for environmental monitoring, and (3) Robust routing for fault tolearnce and security.