×ØÖ ØBroadcasting is a common operation in a network to resolve many issues. In a mobile ad hoc network (MANET) in particular, due to host mobility, such operations are expected to be executed more frequently (such as finding a route to a particular host, paging a particular host, and sending an alarm signal). Because radio signals are likely to overlap with others in a geographical area, a straightforward broadcasting by flooding is usually very costly and will result in serious redundancy, contention, and collision, to which we refer as the broadcast storm problem. In this paper, we identify this problem by showing how serious it is through analyses and simulations. We propose several schemes to reduce redundant rebroadcasts and differentiate timing of rebroadcasts to alleviate this problem. Simulation results are presented, which show different levels of improvement over the basic flooding approach. Keywords: broadcast, communication, mobile ad hoc network (MANET), mobile computing, wireless network. Applications of MANETs occur in situations like battlefields or major disaster areas where networks need to be deployed immediately but base stations or fixed network infrastructures are not available. Unicast routing in MANET has been studied in several articles [6,7,14,15,23]. A working group called "manet" has been formed by the Internet Engineering Task Force (IETF) to study the related issues and stimulate research in MANET [21].This paper studies the problem of sending a broadcast message in a MANET. Broadcasting is a common operation in many applications, e.g., graph-related problems and distributed computing problems. It is also widely used to resolve many network layer problems. In a MANET in particular, due to host mobility, broadcastings are expected to be performed more frequently (e.g., for paging a particular host, sending an alarm signal, and finding a route to a particular host [6,14,15,23]). Broadcasting may also be used in LAN emulation [2] or serve as a last resort to provide multicast services in networks with rapid changing topologies.In this paper, we assume that mobile hosts in the MANET share a single common channel with carrier sense multiple access (CSMA), but no collision detection (CD), capability. Synchronization in such a network with mobility is unlikely, and global network topology information is unavailable to facilitate the scheduling of a broadcast. So one straightforward and obvious solution is broadcasting by flooding. Unfortunately, in this paper we observe that serious redundancy, contention, and collision could exist if flooding is done blindly. First, because the radio propagation is omnidirectional and a physical location may be covered by the transmission ranges of several hosts, many rebroadcasts are considered to be redundant. Second, heavy contention could exist because rebroadcasting hosts are probably close to each 1 other. Third, collisions are more likely to occur because the RTS/CTS dialogue is inapplicable and the timing of rebroadcasts is highly correlated.Coll...

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This paper develops a mobile learning system for scaffolding students learning about bird-watching. The aim is to construct an outdoor mobile-learning activity using up-to-date wireless technology. The proposed Bird-Watching Learning (BWL) system is designed using a wireless mobile ad-hoc network. In the BWL system, each learner has a PDA (Personal Digital Assistant) with a Wi-Fi-based (IEEE 802.11b) wireless network card. The BWL system offers a mobile learning system which supports the students learning through scaffolding. The aim of a formative evaluation was twofold: to explore the possible roles and scaffolding aids that the mobile learning device offers for bird-watching activities and to investigate whether student learning benefited from the mobility, portability, and individualisation of the mobile learning device.

In this paper, we investigate a mobicast, also called a mobile geocast, problem in three-dimensional (3-D) underwater sensor networks (USNs), which aims to overcome the hole problem and minimizes the energy consumption of the sensor nodes while maximizing the data collection. In this paper, all underwater sensor nodes are randomly distributed in a 3-D underwater environment in the sea to form a 3-D USN. Considered a mobile sink or an autonomous underwater vehicle (AUV), all possible sensor nodes near the AUV form a 3-D geographic zone called a 3-D zone of reference (3-D ZOR). The AUV travels a user-defined route and continuously collects data from sensor nodes within a series of 3-D ZORs at different times. The main problem is how to efficiently collect data from sensor nodes within a 3-D ZOR while those sensor nodes are usually in sleep mode for a long period. The routing protocol relies on two phases: the first phase consists of collecting data within a 3-D ZOR, and the second phase consists of waking up those sensor nodes in the next 3-D ZOR to be queried while trying to avoid topology holes. To save power, only sensor nodes in a 3-D ZOR are notified to enter the active mode in order to deliver sensed results to the AUV. The specific characteristics of USNs, including low communication bandwidth, large propagation delay, and ocean current, are significantly different from wireless sensor networks. To consider the characteristics of USNs, a new mobicast routing protocol is developed in 3-D USNs. The key design challenge is to develop a power-saving mobicast protocol in 3-D USNs to overcome the unpredictable 3-D hole problem. To solve the hole problem, an "apple slice" technique is used to build multiple segments to surround the hole and to assure routing path continuity. Finally, performance analysis is derived, and simulation results illustrate the performance improvement in successful delivery rate, power consumption, and message overhead.

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