Reasoning about the Reliability of Diverse Two-Channel Systems in which One Channel is "Possibly Perfect"

This paper considers the problem of reasoning about the reliability of fault-tolerant systems with two "channels" (i.e., components) of which one, A, supports only a claim of reliability, while the other, B, by virtue of extreme simplicity and extensive analysis, supports a plausible claim of "perfection." We begin with the case where either channel can bring the system to a safe state. We show that, conditional upon knowing pA (the probability that A fails on a randomly selected demand) and pB (the probability that channel B is imperfect), a conservative bound on the probability that the system fails on a randomly selected demand is simply pA.pB. That is, there is conditional independence between the events "A fails" and "B is imperfect." The second step of the reasoning involves epistemic uncertainty about (pA, pB) and we show that under quite plausible assumptions, a conservative bound on system pfd can be constructed from point estimates for just three parameters. We discuss the feasibility of establishing credible estimates for these parameters. We extend our analysis from faults of omission to those of commission, and then combine these to yield an analysis for monitored architectures of a kind proposed for aircraft

Conservative reasoning about epistemic uncertainty for the probability of failure on demand of a 1-out-of-2 software-based system in which one channel is “possibly perfect”

In earlier work, (Littlewood and Rushby 2012) (henceforth LR), an analysis was presented of a 1-out-of-2 software-based system in which one channel was “possibly perfect”. It was shown that, at the aleatory level, the system pfd (probability of failure on demand) could be bounded above by the product of the pfd of channel A and the pnp (probability of non-perfection) of channel B. This result was presented as a way of avoiding the well-known difficulty that for two certainly-fallible channels, failures of the two will be dependent, i.e. the system pfd cannot be expressed simply as a product of the channel pfds. A price paid in this new approach for avoiding the issue of failure dependence is that the result is conservative. Furthermore, a complete analysis requires that account be taken of epistemic uncertainty – here concerning the numeric values of the two parameters pfdA and pnpB. Unfortunately this introduces a different difficult problem of dependence: estimating the dependence between an assessor’s beliefs about the parameters. The work reported here avoids this problem by obtaining results that require only an assessor’s marginal beliefs about the individual channels, i.e. they do not require knowledge of the dependence between these beliefs. The price paid is further conservatism in the results

Reasoning About the Reliability of Multi-version, Diverse Real-Time Systems

This paper is concerned with the development of reliable real-time systems for use in high integrity applications. It advocates the use of diverse replicated channels, but does not require the dependencies between the channels to be evaluated. Rather it develops and extends the approach of Little wood and Rush by (for general systems) by investigating a two channel system in which one channel, A, is produced to a high level of reliability (i.e. has a very low failure rate), while the other, B, employs various forms of static analysis to sustain an argument that it is perfect (i.e. it will never miss a deadline). The first channel is fully functional, the second contains a more restricted computational model and contains only the critical computations. Potential dependencies between the channels (and their verification) are evaluated in terms of aleatory and epistemic uncertainty. At the aleatory level the events ''A fails" and ''B is imperfect" are independent. Moreover, unlike the general case, independence at the epistemic level is also proposed for common forms of implementation and analysis for real-time systems and their temporal requirements (deadlines). As a result, a systematic approach is advocated that can be applied in a real engineering context to produce highly reliable real-time systems, and to support numerical claims about the level of reliability achieved

Conservative reasoning about epistemic uncertainty for the probability of failure on demand of a 1-out-of-2 software-based system in which one channel is "possibly perfect"

In earlier work, (Littlewood and Rushby 2011) (henceforth LR), an analysis was presented of a 1-out-of-2 system in which one channel was “possibly perfect”. It was shown that, at the aleatory level, the system pfd could be bounded above by the product of the pfd of channel A and the pnp (probability of non-perfection)of channel B. This was presented as a way of avoiding the well-known difficulty that for two certainly-fallible channels, system pfd cannot be expressed simply as a function of the channel pfds, and in particular not as a product of these. One price paid in this new approach is that the result is conservative – perhaps greatly so. Furthermore, a complete analysis requires that account be taken of epistemic uncertainty – here concerning the numeric values of the two parameters pfdA and pnpB. This introduces some difficulties, particularly concerning the estimation of dependence between an assessor’s beliefs about the parameters. The work reported here avoids these difficulties by obtaining results that require only an assessor’s marginal beliefs about the individual channels, i.e. they do not require knowledge of the dependence between these belief

Potentials and limits to generate employment and income by the National Programme for Production and Use of Biodiesel

This study analyses the National Programme for Production and Use of Biodiesel launched by the Brazil Federal Government in 2005 as a public policy to generate sustainable employment and income within the context of development of new alternative sources of energy. It also verifies the impact of PNPB on occupation and income rate of farmers participating in the projects of production of biodiesel through field research carried out on 93 family farms participating in projects already implemented in the State of Goiás. The choice of producers was made at random from a list of all producers who had already gone through a complete cycle of production and stretched across 33 municipalities in the second half of 2007. The survey data was obtained through a closed-ended questionnaire which was designed to ascertain: 1) the increase of occupation and income regarding producers participating in the projects, 2) ways of including these farmers into the programme, 3) technical assistance offered to them (according to the guidelines of the programme) and 4) the evaluation of the programme by participating farmers. The SPSS software was used for processing and data analysis. The results show that most of the objectives of the programme, such as generation of occupation and income by family farming, are being achieved.biofuels, biodiesel, family farm, public policy, Agribusiness, Agricultural Finance, Industrial Organization,

Design of surface-active artificial enzyme particles to stabilize Pickering emulsions for high-performance biphasic biocatalysis

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Surface-active artificial enzymes (SAEs) are designed and constructed by a general and novel strategy. These SAEs can simultaneously stabilize Pickering emulsions and catalyze biphasic biotransformation with superior enzymatic stability and good re-usability; for example, for the interfacial conversion of hydrophobic p-nitrophenyl butyrate into yellow water-soluble p-nitrophenolate catalyzed by esterase-mimic SAE

Nerve Agent Hydrolysis Activity Designed into a Human Drug Metabolism Enzyme

Organophosphorus (OP) nerve agents are potent suicide inhibitors of the essential neurotransmitter-regulating enzyme acetylcholinesterase. Due to their acute toxicity, there is significant interest in developing effective countermeasures to OP poisoning. Here we impart nerve agent hydrolysis activity into the human drug metabolism enzyme carboxylesterase 1. Using crystal structures of the target enzyme in complex with nerve agent as a guide, a pair of histidine and glutamic acid residues were designed proximal to the enzyme's native catalytic triad. The resultant variant protein demonstrated significantly increased rates of reactivation following exposure to sarin, soman, and cyclosarin. Importantly, the addition of these residues did not alter the high affinity binding of nerve agents to this protein. Thus, using two amino acid substitutions, a novel enzyme was created that efficiently converted a group of hemisubstrates, compounds that can start but not complete a reaction cycle, into bona fide substrates. Such approaches may lead to novel countermeasures for nerve agent poisoning

Effectiveness of China's National Forest Protection Program and nature reserves

There is profound interest in knowing the degree to which China's institutions are capable of protecting its natural forests and biodiversity in the face of economic and political change. China's 2 most important forest-protection policies are its National Forest Protection Program (NFPP) and its national-level nature reserves (NNRs). The NFPP was implemented in 2000 in response to deforestation-caused flooding. We undertook the first national, quantitative assessment of the NFPP and NNRs to examine whether the NFPP achieved its deforestation-reduction target and whether the NNRs deter deforestation altogether. We used MODIS data to estimate forest cover and loss across mainland China (2000-2010). We also assembled the first-ever polygon dataset for China's forested NNRs (n = 237, 74,030 km(2) in 2000) and used both conventional and covariate-matching approaches to compare deforestation rates inside and outside NNRs (2000-2010). In 2000, 1.765 million km(2) or 18.7% of mainland China was forested (12.3% with canopy cover of >= 70%)) or woodland (6.4% with canopy cover = 40%). By 2010, 480,203 km(2) of forest and woodland had been lost, an annual deforestation rate of 2.7%. Forest-only loss was 127,473 km(2) (1.05% annually). In the NFPP provinces, the forest-only loss rate was 0.62%, which was 3.3 times lower than in the non-NFPP provinces. Moreover, the Landsat data suggest that these loss rates are overestimates due to large MODIS pixel size. Thus, China appears to have achieved, and even exceeded, its target of reducing deforestation to 1.1% annually in the NFPP provinces. About two-thirds of China's NNRs were effective in protecting forest cover (prevented loss 4073 km(2) unmatched approach; 3148 km(2) matched approach), and within-NNR deforestation rates were higher in provinces with higher overall deforestation. Our results indicate that China's existing institutions can protect domestic forest cover

Retrofitting the Brazilian Biodiesel Programme: Implications for Policy Design

In the context of oil price volatility and the need to reduce carbon emissions, biofuels are an emerging area of interest for many developing nations as alternative energy sources that, in some instances, can also enhance livelihoods in deprived agricultural areas. There are, however, a number of questions on this front: is it economically and environmentally feasible to incorporate small-scale family farmers into biofuel value chains? Can the production of biofuel feedstocks complement rather than compete with food crops? The experience of Brazil, a pioneer in the adoption of a socially inclusive approach to the production of feedstocks for biodiesel, has elicited much interest. This Policy Research Brief seeks to take stock of recent institutional developments and draw lessons as part of an ongoing learning process in an area where there are still no obvious sustainable business models or easy pathways to foster the inclusion of small-scale farmers. The Brief suggests that incorporation into the biodiesel value chain is both feasible and productive for family farmers. But the extent of the engagement required of intermediaries can be significant in the early stages of the programme in underserviced areas, particularly where farmers are dispersed and have not been extensively involved with market processes. Those embarking on these programmes thus have to consider such a production-support role. Further, the Brief suggests that intercropping (castor and beans, for example) can mitigate the food-fuel tradeoffs. However, the choice of optimal feedstocks from the point of view of equity and sustainability remains an open question.Retrofitting the Brazilian Biodiesel Programme: Implications for Policy Design