This paper investigates time-efficient implementations of atomic read-write registers in message-passing systems where the number of readers can be unbounded. In particular we study the case of a single writer, multiple readers, and S servers, such that the writer, any subset of the readers, and up to t servers may crash. A recent result of Dutta et al. [3] shows how to obtain fast implementations in which both reads and writes complete in one communication round-trip, under the constraint that the number of readers is less than S t − 2, where t < S 2 . In that same paper the authors pose a question of whether it is possible to relax the bound on readers, and at what cost, if semifast implementations are considered, i.e., implementations that have fast reads or fast writes.This paper provides an answer to this question. It is shown that one can obtain implementations where all writes are fast, i.e., involving a single round-trip communication, and where reads complete in one to two communication rounds under the assumption that no more than t < S 2 servers crash. Simulated scenarios included in this paper indicate that only a small fraction of reads require a second communication round. Interestingly the correctness of the implementation does not depend on the number of concurrent readers in the system. The solution is obtained with the help of non-unique virtual ids assigned to each reader, where the readers sharing a virtual id form a virtual node. For the proposed definition of semifast implementations it is shown that implementations satisfying certain assumptions are semifast if and only if the number of virtual ids in the system is less than S t − 2. This result is proved to be tight in terms of the required communication. It is shown that only a single complete two-round read operation may be necessary for each write operation. It is furthermore shown that no semifast implementation exists for the multi-reader, multi-writer model.
Underwater Sensor Networks (UWSNs) are significantly different from terrestrial sensor networks in the following aspects: low bandwidth, high latency, node mobility, high error probability, and 3-dimensional space. These new features bring many challenges to the network protocol design of UWSNs. In this paper, we tackle one fundamental problem in UWSNs: robust, scalable, and energy efficient routing. We propose vector-based forwarding (VBF), a geographic routing protocol. In VBF, the forwarding path is guided by a vector from the source to the target, no state information is required on the sensor nodes, and only a small fraction of the nodes is involved in routing. To improve the robustness, packets are forwarded in redundant and interleaved paths. Further, a localized and distributed self-adaptation algorithm allows the nodes to reduce energy consumption by discarding redundant packets. VBF performs well in dense networks. For sparse networks, we propose a hop-by-hop vector-based forwarding (HH-VBF) protocol, which adapts the vector-based approach at every hop. We evaluate the performance of VBF and HH-VBF through extensive simulations. The simulation results show that VBF achieves high packet delivery ratio and energy efficiency in dense networks and HH-VBF has high packet delivery ratio even in sparse networks.EURASIP Journal on Wireless Communications and Networking 40 km×kbps as the maximum attainable Range×Rate product.(ii) UWSNs Are Highly Dynamic. The underwater sensor networks we target are highly mobile networks where sensor nodes are not fixed and they will float with water currents. From empirical observations, underwater objects may move at the speed of 2-3 knots (or 3-6 kilometers per hour) in a typical underwater condition [6,7]. This kind of mobility results in a highly dynamic network topology.(iii) UWSNs Are Highly Error-Prone. Underwater acoustic communication channels are significantly affected by many factors such as signal attenuation, noise, multipath, Doppler spread, and even water temperature. All these factors cause high bit-error and delay variance. Thus, communication links in UWSNs are highly error-prone. Moreover, sensor nodes are more vulnerable in harsh underwater environments. Compared with their terrestrial counterparts, underwater sensor networks have a higher node-failure rate and packet-loss probability.(iv) UWSNs Are Three-Dimensional. UWSNs are usually deployed in a three-dimensional space. This is different from the 2-dimensional deployment of most terrestrial sensor networks.These characteristics of UWSNs bring up many new challenges and make the existing routing protocols for terrestrial sensor networks unsuitable here. For UWSNs, the routing protocols should be able to handle the node mobility and the unreliable communication links with high energy efficiency. Routing Challenges in UWSNs.UWSNs are highly dynamic networks, which makes existing routing protocols for stationary or quasistationary networks unsuitable. In UWSNs, the mobility speed of nodes is around 1-3 mete...
BackgroundThe prevalence of childhood asthma and allergies in Cyprus was significantly higher in urban compared to rural areas back in the year 2000, against a background of an overall low prevalence (e.g. current wheeze 6.9%) by comparison to northern European countries. In this study we aimed to assess temporal changes in the prevalence of asthma and allergies in Cyprus after an 8-year interval and to examine whether any differential changes have occurred in urban and rural parts of the island.MethodsDuring the academic years 1999-2000 and 2007-2008, the parents of 7-8 year old children residing in the same set of urban and rural areas completed the ISAAC core questionnaire. In addition to providing prevalence estimates of allergic diseases in 2000 and 2008, changes between the two periods were expressed as odds ratios estimated in multiple logistic regression models adjusting for survey participants' characteristics.ResultsThe prevalence of current wheeze was higher in 2008 (8.7%, 95% confidence interval 7.5%-9.9%, n = 2216) than the previously recorded figure in 2000 (6.9%, 95% CI 6.2%-7.6%, OR = 1.25, 95% CI: 1.02-1.53, n = 4944). Significant increases were also seen in the prevalence of lifetime asthma (11.3% vs. 17.4%, OR = 1.59, CI: 1.36-1.86), eczema (6.8% vs. 13.5%, OR = 1.91, CI: 1.59-2.29) and allergic rhinoconjuctivitis (2.6% vs. 5.2%, OR = 1.82, CI: 1.39-2.41). The prevalence of current wheeze nearly doubled between 2000 and 2008 in rural areas (5.4% vs. 9.7%, OR 1.81, CI: 1.24-2.64) while no significant change was observed in urban areas (7.5% vs. 8.4%, OR 1.08, CI: 0.84-1.37); p value for effect modification = 0.04. Rises in asthma and rhinitis prevalence, but not eczema were also more pronounced in rural compared to urban areas.ConclusionsThe prevalence of allergic diseases in Cyprus is still on the rise; recent increases appear more pronounced among children living in rural areas possibly indicating recent environmental and lifestyle changes in these communities
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