Towards energy efficient duty cycling in underwater wireless sensor networks

Multimedia Tools and Applications - Underwater Wireless Sensor Networks devices are usually battery powered and thereby their lifetime is limited. This issue leads to lose data measurements and...     

Performance improvement of TCP in ad hoc networks by mitigating channel contention

In ad hoc networks, the spatial reuse property limits the number of packets which can be spatially transmitted over a path. In standard Transmission Control Protocol (TCP), however, a TCP sender keeps transmitting packets without taking into account this property. This causes heavy contention for the wireless channel, resulting in the performance degradation of TCP flows. Hence, two techniques have been proposed independently in order to reduce the contention. First, a TCP sender utilizes a congestion window limit (CWL), by considering the spatial reuse property. This prevents the TCP sender from transmitting more than CWL number of packets at one time. Second, a delayed ack (DA) strategy is exploited in order to mitigate the contention between the TCP ACK and DATA packets. Recently, although TCP‐DAA (Dynamic Adaptive Acknowledgment) attempts to utilize a CWL‐based DA strategy, TCP‐DAA overlooks a dynamic correlation between these two techniques. This paper, therefore, reveals the dynamic correlation and also proposes a protocol which not only reduces the frequency of the TCP ACK transmissions but also determines a CWL value dynamically, according to network conditions. Simulation studies show that our protocol performs the best in various scenarios, as compared to TCP‐DAA and standard TCP (such as TCP‐NewReno). Copyright © 2009 John Wiley & Sons, Ltd.     

DFR: an efficient directional flooding‐based routing protocol in underwater sensor networks

Unlike terrestrial sensor networks, underwater sensor networks (UWSNs) have salient features such as a long propagation delay, narrow bandwidth, and high packet loss over links. Hence, path setup‐based routing protocols proposed for terrestrial sensor networks are not applicable because a large latency of the path establishment is observed, and packet delivery is not reliable in UWSNs. Even though routing protocols such as VBF (vector based forwarding) and HHVBF (hop‐by‐hop VBF) were introduced for UWSNs, their performance in terms of reliability deteriorates at high packet loss. In this paper, we therefore propose a directional flooding‐based routing protocol, called DFR, in order to achieve reliable packet delivery. DFR performs a so‐called controlled flooding, where DFR changes the number of nodes which participate in forwarding a packet according to their link quality. When a forwarding node has poor link quality to its neighbor nodes geographically advancing toward the sink, DFR allows more nodes to participate in forwarding the packet. Otherwise, a few nodes are enough to forward the packet reliably. In addition, we identify two types of void problems which can occur during the controlled flooding and introduce their corresponding solutions. Our simulation study using ns‐2 simulator proves that DFR is more suitable for UWSNs, especially when links are prone to packet loss. Copyright © 2011 John Wiley & Sons, Ltd.     

A Low-Complexity Routing Algorithm with Power Control for Self-Organizing Short-Range Wireless Networks

Mobile computing has become very pervasive, where the number of electronic devices equipped with wireless capabilities has increased significantly in recent years. This poses serious demands on wireless, mobile and self-organizing networks. Despite the fact that devices are getting smaller and more powerful, advances in battery technology have not yet reached the stage where devices can autonomously operate for days. Therefore, devices for self-organizing networks will strongly rely on the efficient use of their batteries. We present a cluster-based low-complexity routing algorithm for self-organizing networks of mobile nodes. Our proposed algorithm, called Cluster-based Energy-saving Routing Algorithm (CERA), allows mobile nodes to autonomously create clusters to minimize the power consumption. CERA is implemented as two separate protocols: the intra-cluster data-dissemination protocol, and the inter-cluster routing protocol. We present an extensive analysis of the overall protocol architecture by varying the critical factors related to protocol behavior. As a result, the CERA implementation generally saves up to 25% of energy, while keeping the overhead, in terms of energy consumption, acceptably low.     

Change in fish community composition following weir removal,field observations,and physical habitat simulations

The number of untended weirs has increased in Korea in recent years due to land use changes. The removal of abandoned weirs in streams has been attempted following an agreement between local government and residents. The Gongneung Weir‐2 was built for irrigation in the 1970s and was removed in 2006. The objective of this study was to investigate the impact of the removal of Gongneung Weir‐2 on the composition of the fish community. The study area was a 900‐m‐long reach in the Gongneung‐cheon River in Korea, and Gongneung Weir‐2 was located in the middle of the reach. Before weir removal, field monitoring revealed that five fish species were dominant and accounted for more than 80% of the entire fish community. The composition of the fish community changed significantly after weir removal. Lotic fish became dominant after weir removal, whereas lentic fish were dominant prior to weir removal. Physical habitat simulations (PHSs) for individual dominant fish species were carried out. For the PHS, the River2D model and the adaptive neuro‐fuzzy inference system method were used for hydraulic and habitat simulations, respectively. The distributions of the highly suitable portion for each fish species were identified before and after weir removal. The PHS successfully predicted changes in the composition of the fish community after weir removal. The PHSs for the entire fish community and for the lotic and lentic guilds were undertaken and the simulated results were compared with each other. The PHSs for the entire fish community could not account for the less dominant fish species in the fish community.     

Two-dimensional physical habitat modeling of effects of habitat structures on urban stream restoration

River corridors, even if highly modified or degraded, still provide important habitats for numerous biological species, and carry high aesthetic and economic values. One of the keys to urban stream restoration is recovery and maintenance of ecological flows sufficient to sustain aquatic ecosystems. In this study, the Hongje Stream in the Seoul metropolitan area of Korea was selected for evaluating a physically-based habitat with and without habitat structures. The potential value of the aquatic habitat was ...     

Propylene-induced plasticization in silver polymer electrolyte membranes

Silver polymer electrolyte membranes consisting of AgBF₄ or AgCF₃SO₃ dissolved in poly(2-ethyl-2-oxazoline) (POZ) or poly(N-vinyl pyrrolidone) (PVP) have previously shown outstanding separation performance for propylene/propane mixtures. The ideal separation factor (i.e. pure gas selectivity) of propylene over propane reached 15,000, but the actual selectivity (i.e. mixed gas selectivity) was approximately 50. In this study, the origin of large difference between pure and mixed gas selectivities has been elucidated in terms of membrane plasticization by propylene molecules. The membrane plasticization was confirmed by the decrease of selectivity with increasing propylene concentration in the feed, resulting from the increase of both propylene and propane permeances. The decrease of glass transition temperature (T_g) in silver polymer electrolytes upon the sorption of propylene also supported the membrane plasticization, determined by in situ high-pressure differential scanning calorimetry (HPDSC). Further analysis of configuration entropy model revealed that the interaction of propylene/silver ion in POZ/AgBF₄ was approximately 2-fold stronger than that in POZ/AgCF₃SO₃, which is in good agreement with the results of propylene solubility and FT-IR spectra.     

Repetition-based cooperative broadcasting for vehicular ad-hoc networks

In vehicular ad hoc networks, most of critical applications involved with safety rely on reliable broadcast communications with low latency. Recently, repetition-based protocols have been proposed to meet the requirements of timeliness and reliability for broadcasting. In these protocols, a sender repeatedly retransmits the broadcast message during the lifetime of the message. However, existing protocols face serious problems such as deterioration of the signal quality caused by wireless fading. In particular, since excessive repetitions might cause network congestion and waste channel resources, reliability of broadcasting should be achieved with as small a number of repetitions as possible. In this paper, we therefore propose a novel repetition-based broadcast protocol which exploits a cooperative diversity technique (called RB-CD) making a small number of repetitions robust for wireless fading. To support this cooperative diversity, neighboring nodes transmit the same message almost simultaneously (that is, using the same repetition pattern for each other) in order to form a virtual antenna array. The virtual antenna array achieves a diversity gain at the receivers. In the RB-CD protocol, the virtual antenna array consists of the source and some of its neighbors (called relays) which participate in repeating the transmission of a broadcast message. In addition, a new distributed relay selection algorithm is introduced in the RB-CD protocol. From the ns-2 simulation results, we verified that RB-CD provides a more reliable broadcasting service due to its capability of exploiting cooperative diversity.