Performance behaviour of IEEE 802.11 distributed coordination function

The authors present an extensive investigation of the performance of the IEEE 802.11 medium access control (MAC) protocol, with respect to throughput and delay. For the protocol analysis, a new model, which describes the protocol's behaviour to a great extent by incorporating and extending the existing models, is proposed. The authors also present a detailed analysis of the end-to-end delay through the study of the MAC delay and the queueing delay. The authors use the Z-transform of backoff duration to obtain the mean value, the variance and the probability distribution of the MAC delay. For the queueing analysis, first the authors consider an M/G/l queue in order to provide a first look at the queueing delay. Second, the authors modify the input process of the queue so that the packet arrival process is described by an ON- OFF model, which expresses the bursty nature of traffic. In the investigations, data rates of 1, 5.5 and 11 Mbps are assumed to highlight the effect of the bit rate on network performance for both Basic and request-to-send/ clear-to-send access mechanisms. The throughput and delay analyses are validated by simulating the distributed coordination function, whereas the models are compared with the existing models based on their results. The accuracy of the analyses was found to be quite satisfactory.     

Reconceptualization of Hormetic Responses in the Frame of Redox Toxicology

Cellular adaptive mechanisms emerging after exposure to low levels of toxic agents or stressful stimuli comprise an important biological feature that has gained considerable scientific interest. Investigations of low-dose exposures to diverse chemical compounds signify the non-linear mode of action in the exposed cell or organism at such dose levels in contrast to the classic detrimental effects induced at higher ones, a phenomenon usually referred to as hormesis. The resulting phenotype is a beneficial effect that tests our physiology within the limits of our homeostatic adaptations. Therefore, doses below the region of adverse responses are of particular interest and are specified as the hormetic gain zone. The manifestation of redox adaptations aiming to prevent from disturbances of redox homeostasis represent an area of particular interest in hormetic responses, observed after exposure not only to stressors but also to compounds of natural origin, such as phytochemicals. Findings from previous studies on several agents demonstrate the heterogeneity of the specific zone in terms of the molecular events occurring. Major factors deeply involved in these biphasic phenomena are the bioactive compound per se, the dose level, the duration of exposure, the cell, tissue or even organ exposed to and, of course, the biomarker examined. In the end, the molecular fate is a complex toxicological event, based on beneficial and detrimental effects, which, however, are poorly understood to date.     

A review of the effects of pollution and water scarcity on the stream biota of an intermittent Mediterranean basin

This review presents the main results of a 10‐year research study conducted in a Mediterranean intermittent basin (Evrotas River). By assembling the main outcomes of past and ongoing research projects, this study provides an overview of multiple stressor effects, with emphasis on water scarcity, focusing on hydro‐biogeochemical processes, as well as on spatial and temporal variations in benthic macroinvertebrates and fish fauna. The major impact in the basin has been the over‐exploitation of surface and groundwater resources, which, in combination with droughts, has resulted in the recurrent artificial desiccation of large parts of the hydrological network. The response to intermittency of the macroinvertebrate fauna is characterized by high resilience through various drought‐resistant evolutionary mechanisms, with assemblages recovering successfully after recurrent droughts. However, when pollution is evident in combination with drought, effects on benthic species richness, abundance, and assemblage structure can be severe. Similarly, pollution and water stress may result in massive fish mortalities due to hypoxic conditions, with fish populations requiring long periods to recover. However, the fish fauna appears to be relatively resilient to drought‐driven reach‐scale desiccation, and ultimately recovers, provided that aquatic refugia are available to supply colonists and that there are no physical barriers impeding recolonisation. Appropriate conservation measures are urgently required to address the effects of recurrent bouts of water stress, as well as of other stressors on the freshwater communities of the Evrotas River, both at the level of water management and of water policy and at the local and the national level.     

Congestion probabilities in a batched Poisson multirate loss model supporting elastic and adaptive traffic

The ever increasing demand of elastic and adaptive services, where in-service calls can tolerate bandwidth compression/expansion, together with the bursty nature of traffic, necessitates a proper teletraffic loss model which can contribute to the call-level performance evaluation of modern communication networks. In this paper, we propose a multirate loss model that supports elastic and adaptive traffic, under the assumption that calls arrive in a single link according to a batched Poisson process (a more “bursty” process than the Poisson process, where calls arrive in batches). We assume a general batch size distribution and the partial batch blocking discipline, whereby one or more calls of a new batch are blocked and lost, depending on the available bandwidth of the link. The proposed model does not have a product form solution, and therefore we propose approximate but recursive formulas for the efficient calculation of time and call congestion probabilities, link utilization, average number of calls in the system, and average bandwidth allocated to calls. The consistency and the accuracy of the model are verified through simulation and found to be quite satisfactory.     

Wood availability and habitat heterogeneity drive spatiotemporal habitat use by riverine cyprinids under flow intermittence

Interspecies and intraspecies variation in the use of wooded habitats has been studied for salmonids, but little is known about nonsalmonid species. In this study, we explored spatiotemporal differences in the distribution of three cyprinid species at habitats of varying wood density and at habitats of varying heterogeneity in a Mediterranean intermittent river (Evrotas, southern Greece). Hydroecological data from 843 microhabitats across four sites and in five sampling periods were collected and analysed using generalized linear mixed effects models. Adult chubs, minnowroaches, and minnows and, to a lesser degree, juvenile chubs, preferred complex, wooded habitats, characterized by lower flow velocities, higher water depths, finer substrate, and increased canopy cover, compared with wood‐free and noncomplex habitats. At the advanced stage of the dry period, the deeper, wooded habitats served as drought refugia; all fish groups, except for the adult chub, resorted to these habitats during autumn. In contrast, no such shifts were evident in early summer at the onset of the dry period. In late spring, all fish groups had higher densities at wooded habitats, a shift probably associated with reproduction. Wood density did not affect fish habitat use, as fish densities were similar across wooded habitats. In contrast, habitat heterogeneity significantly affected fish densities; more adult chubs and minnows were observed in complex habitats, whereas juvenile chubs and minnowroaches clearly preferred less complex habitats. We conclude that the targeted maintenance of instream wood structures to ensure habitat heterogeneity is a key‐action for the successful conservation of riverine fish populations.     

Biomedical research productivity and economic crisis in Greece: a 22-year study

This article aims to study the effect of the current financial crisis (2010–2017) on biomedical productivity and impact of Greece-affiliated investigators. PubMed, Scopus and Web of Science were searched for articles published in biomedical journals with at least one Greek affiliation during the period 1995–2016 (date of last search October 19, 2017). The impact of Greek articles was the citations received by published articles during the first 2 years following the year of publication adjusted to the number of Greek and global articles. A discrepancy in the absolute article productivity between the databases was observed: a mean annual increase before the crisis was observed in all databases, while after the crisis the increase persisted in PubMed, in Scopus a decline was observed and in the Web of Science a smaller increase was observed. The changes in relative productivity were similar for both study periods in all databases (increasing before and decreasing after crisis, p?<?0.001 for trend in both periods). A continuous increase in total citations in both periods was observed (mean 2347?±?1622 before and 2627?±?3374 after the crisis). A continuous increase in the impact indices following adjustment for Greek (mean annual increase 0.27?±?0.30 before to 0.37?±?0.62 during the crisis, p?=?0.58) and global productivity (mean annual increase from 0.0024?±?0.0018 to 0.0007?±?0.004, p?=?0.95) was observed throughout the study period. In conclusion, the decline observed in the relative productivity of Greek affiliated articles during crisis compared to the period before was not reflected in their impact.