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The IEEE 802.11 standard defines access categories (AC) and differentiated medium access control mechanisms for wireless local area networks. The preferential or deferral treatment of frames is achieved using configurable Arbitration Inter-Frame Spaces (AIFS) and customizable Contention Window (CW) sizes. In this paper, we address the problem of determining when a station, being a part of wireless communication, will access the medium. We present an algorithm calculating the probability of winning the contention by a given station, characterized by its AIFS and CW values. The probability of collision is calculated by similar means. The results were verified by simulations in Matlab and OPNET Modeler tools. We also introduce a web applet implementing and interactively demonstrating the results. [ABSTRACT FROM AUTHOR]

In recent times, there has been increasing interest in storing data securely in the cloud environment. To provide owners of data stored in the cloud with flexible control over access to their data by other users, we propose a role-based encryption (RBE) scheme for secure cloud storage. Our scheme allows the owner of data to store it in an encrypted form in the cloud and to grant access to that data for users with specific roles. The scheme specifies a set of roles to which the users are assigned, with each role having a set of permissions. The data owner can encrypt the data and store it in the cloud in such a way that only users with specific roles can decrypt the data. Anyone else, including the cloud providers themselves, will not be able to decrypt the data. We describe such an RBE scheme using a broadcast encryption algorithm. The paper describes the security analysis of the proposed scheme and gives proofs showing that the proposed scheme is secure against attacks. We also analyse the efficiency and performance of our scheme and show that it has superior characteristics compared with other previously published schemes. [ABSTRACT FROM AUTHOR]