78 research outputs found
Is Our Model for Contention Resolution Wrong?
Randomized binary exponential backoff (BEB) is a popular algorithm for
coordinating access to a shared channel. With an operational history exceeding
four decades, BEB is currently an important component of several wireless
standards. Despite this track record, prior theoretical results indicate that
under bursty traffic (1) BEB yields poor makespan and (2) superior algorithms
are possible. To date, the degree to which these findings manifest in practice
has not been resolved.
To address this issue, we examine one of the strongest cases against BEB:
packets that simultaneously begin contending for the wireless channel. Using
Network Simulator 3, we compare against more recent algorithms that are
inspired by BEB, but whose makespan guarantees are superior. Surprisingly, we
discover that these newer algorithms significantly underperform. Through
further investigation, we identify as the culprit a flawed but common
abstraction regarding the cost of collisions. Our experimental results are
complemented by analytical arguments that the number of collisions -- and not
solely makespan -- is an important metric to optimize. We believe that these
findings have implications for the design of contention-resolution algorithms.Comment: Accepted to the 29th ACM Symposium on Parallelism in Algorithms and
Architectures (SPAA 2017
DinamiÄka distribucija sigurnosnih kljuÄeva i koalicijski protokol IP adresa za mobilne ad hoc mreĹže
In mobile adhoc networks (MANETs) a tree-based dynamic address auto-configuration protocol (T-DAAP) is one of the best protocols designed for address assignment as far as the network throughput and packet delays are concerned. Moreover, MANET security is an important factor for many applications given that any node can listen to the channel and overhear the packets being transmitted. In this paper, we merge the address assignment with the security key delivery into one protocol, such that a node in the MANET is configured with IP address and security key simultaneously. To the best of our knowledge, no single protocol provides concurrent assignment of IP addresses and security keys for MANET nodes. The proposed method, which is based on T-DAAP, shows significant enhancements in the required control packets needed for assigning network nodes IP addresses and security keys, MAC layer packets, total end-to-end delay, and channel throughput over those obtained when using separate protocols. Additionally, it provides not only efficient security keys to the nodes from the first moment they join the network, but also secure delivery of the address and security key to all participating nodes. It is noteworthy to mention that providing a complete security model for MANET to detect and countermeasure network security threats and attacks is beyond the scope of our proposed protocol.Kod mobilnih ad hoc mreĹža (MANET) dinamiÄki protokol za autokonfiguraciju adresa baziran na stablu (T-DAAP) je jedan od najboljih protokola dizajniranih za dodjelu adresa iz perspektive propusnosti mreĹže i i kaĹĄnjenja paketa. ĹĄtoviĹĄe, sigurnost MANET-a je vaĹžan faktor za mnoge aplikacije s obzirom da bilo koji Ävor moĹže osluĹĄkivati kanal i sluÄajno Äuti pakete koji se ĹĄalju. U ovom radu, dodjela adresa i dostava sigurnosnih kljuÄeva spojeni su u jedan protokol tako da je Ävor u MANET-u konfiguriran simultano s IP adresom i sigurnosnim kljuÄem. Prema saznanjima autora, niti jedan postojeÄi protokol ne pruĹža istovremeno dodjeljivanje IP adrese i sigurnosnog kljuÄa za MANET Ävorove. PredloĹžena metoda, koja se bazira na T-DAAP-u, pokazuje znaÄajna poboljĹĄanja u odnosu na metode koje koriste odvojene porotokole, kod traĹženih kontrolnih paketa koji su potrebni za dodjeljivanje IP adresa i sigurnosnih kljuÄeva Ävorovima mreĹže, MAC paketa, ukupnog end-to-end kaĹĄnjenja i propusnosti kanala. Dodatno pruĹža ne samo efikasne sigurnosne kljuÄeve Ävorovima od trenutka kad se prikljuÄe mreĹži, nego i sigurno dostavljanje adrese i sigurnosnog kljuÄa svim Ävorovima koji sudjeluju u mreĹži. VaĹžno je spomenuti da je pruĹžanje cjelokupnog sigurnosnog modela za MANET koji detektira dodatno i protumjere prijetnjama i napadima na sigurnost mreĹže izvan dosega predloĹženog protokola
Big Losses Lead to Irrational Decision-Making in Gambling Situations: Relationship between Deliberation and Impulsivity
In gambling situations, we found a paradoxical reinforcing effect of high-risk decision-making after repeated big monetary losses. The computerized version of the Iowa Gambling Task (Bechara et al., 2000), which contained six big loss cards in deck B', was conducted on normal healthy college students. The results indicated that the total number of selections from deck A' and deck B' decreased across trials. However, there was no decrease in selections from deck B'. Detailed analysis of the card selections revealed that some people persisted in selecting from the âriskyâ deck B' as the number of big losses increased. This tendency was prominent in self-rated deliberative people. However, they were implicitly impulsive, as revealed by the matching familiar figure test. These results suggest that the gap between explicit deliberation and implicit impulsivity drew them into pathological gambling
Sensitivity and Bias in Decision-Making under Risk: Evaluating the Perception of Reward, Its Probability and Value
BACKGROUND: There are few clinical tools that assess decision-making under risk. Tests that characterize sensitivity and bias in decisions between prospects varying in magnitude and probability of gain may provide insights in conditions with anomalous reward-related behaviour. OBJECTIVE: We designed a simple test of how subjects integrate information about the magnitude and the probability of reward, which can determine discriminative thresholds and choice bias in decisions under risk. DESIGN/METHODS: Twenty subjects were required to choose between two explicitly described prospects, one with higher probability but lower magnitude of reward than the other, with the difference in expected value between the two prospects varying from 3 to 23%. RESULTS: Subjects showed a mean threshold sensitivity of 43% difference in expected value. Regarding choice bias, there was a 'risk premium' of 38%, indicating a tendency to choose higher probability over higher reward. An analysis using prospect theory showed that this risk premium is the predicted outcome of hypothesized non-linearities in the subjective perception of reward value and probability. CONCLUSIONS: This simple test provides a robust measure of discriminative value thresholds and biases in decisions under risk. Prospect theory can also make predictions about decisions when subjective perception of reward or probability is anomalous, as may occur in populations with dopaminergic or striatal dysfunction, such as Parkinson's disease and schizophrenia
A qualitative study exploring midlife womenâs stages of change from domestic violence towards freedom
Gold OABackground Domestic Violence (DV) remains a significant global health problem for women in contemporary society. Existing literature on midlife womenâs experiences of domestic violence is limited and focuses on health implications. Leaving a violent relationship is a dynamic process that often requires multiple attempts and separations prior to final termination. The aim of this study was to explore the process of leaving a violent relationship for midlife women. Methods This qualitative study involved fifteen women aged between 40â55 who had accessed residential and non-residential community support services for domestic violence within the UK. Community-based support agencies provided these women with access to letters of invitation and participant information sheet explaining the study. The women notified agency staff who contacted the research team to arrange a mutually convenient time to meet within a safe place for both the women and researchers. It was stressed to all potential participants that no identifiable information would be shared with the agency staff. Women were considered survivors of DV if they defined themselves as such. Data were gathered through semi structured interviews, transcribed verbatim and thematically analysed. Results Midlife women appear to differ from younger women by transitioning quickly though the stages of change, moving rapidly through the breaking free onto the maintenance stage. This rapid transition is the resultant effect of living with long-term violence causing a shift in the womenâs perception towards the violent partner, with an associated reclamation of power from within the violent relationship. A realisation that rapid departure from the violence may be critical in terms of personal safety, and the realisation that there was something âwrongâ within the relationship, a âday of dawningâ that had not been apparent previously appears to positively affect the trajectory of leaving. Conclusions Midlife women appeared to navigate through the stages of change in a rapid linear process, forging ahead and exiting the relationship with certainty and without considering options. Whilst these findings appear to differ from younger womenâs process of leaving, further research is needed to explore and understand the optimum time for intervention and support to maximise midlife womenâs opportunities to escape an abusive partner, before being reflected appropriately in policy and practice.This study received funding from The Research and Knowledge Transfer Office, The University of Chester, and from the Western Australian Health Promotion Foundation â âHealthway
NON INVASIVE FINGER BLOOD PRESSURE: PRELIMINARY STUDIES USING A MATHEMATICAL MODEL.
The specification of devices using the vascular unloading technique of blood pressure measurement until now has been based primarily on direct in-vivo testing. Here, the entire system is described mathematically as a set of finite-difference equations so that the relationship between the finger and sensor can optimized. The resulting model is a 9th-order set of nonlinear equations. The model has been validated by comparison to in-vivo measurements of intra-arterial pressure and optically measured finger vascular volume in humans. In general the theoretical predictions are in agreement with experimental results, indicating that the model will be useful in further work with such devices
Comparison of Simpson's method and three-dimensional reconstruction for measurement of right ventricular volume in patients with complete or corrected transposition of the great arteries
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