Stable Throughput and Delay Analysis of a Random Access Network With Queue-Aware Transmission

In this work we consider a two-user and a three-user slotted ALOHA network with multi-packet reception (MPR) capabilities. The nodes can adapt their transmission probabilities and their transmission parameters based on the status of the other nodes. Each user has external bursty arrivals that are stored in their infinite capacity queues. For the two- and the three-user cases we obtain the stability region of the system. For the two-user case we provide the conditions where the stability region is a convex set. We perform a detailed mathematical analysis in order to study the queueing delay by formulating two boundary value problems (a Dirichlet and a Riemann-Hilbert boundary value problem), the solution of which provides the generating function of the joint stationary probability distribution of the queue size at user nodes. Furthermore, for the two-user symmetric case with MPR we obtain a lower and an upper bound for the average delay without explicitly computing the generating function for the stationary joint queue length distribution. The bounds as it is seen in the numerical results appear to be tight. Explicit expressions for the average delay are obtained for the symmetrical model with capture effect which is a subclass of MPR models. We also provide the optimal transmission probability in closed form expression that minimizes the average delay in the symmetric capture case. Finally, we evaluate numerically the presented theoretical results.Comment: Submitted for journal publicatio

Adaptive authentication and key agreement mechanism for future cellular systems

Since the radio medium can be accessed by anyone, authentication of users is a very important element of a mobile network. Nowadays, in GSM/GPRS a challenge response protocol is used to authenticate the user to the mobile network. Similarly, in third generation mobile systems [3] a challenge response protocol was chosen in such a way as to achieve maximum compatibility with the current GSM security architecture. Both authentication mechanisms use symmetric key cryptography because of the limited processing power of the mobile devices. However, recent research [6] has shown that asymmetric, or public, key cryptography can be enabled successfully in future mobile terminals. In this paper, we propose a new adaptive authentication and key agreement protocol (AAKA) for future mobile communication systems. The novelty of AAKA and its main advantage over other challenge response protocols is that can be adaptive to the mobile environment and use symmetric and/or public key cryptography for user and network authentication

Social media: a guide for researchers

This guide has been produced by the International Centre for Guidance Studies, and aims to provide the information needed to make an informed decision about using social media and select from the vast range of tools that are available. One of the most important things that researchers do is to find, use and disseminate information, and social media offers a range of tools which can facilitate this. The guide discusses the use of social media for research and academic purposes and will not be examining the many other uses that social media is put to across society. Social media can change the way in which you undertake research, and can also open up new forms of communication and dissemination. It has the power to enable researchers to engage in a wide range of dissemination in a highly efficient way.Research Information Networ

Multi-Party Trust Computation in Decentralized Environments

In this paper, we describe a decentralized privacy-preserving protocol for securely casting trust ratings in distributed reputation systems. Our protocol allows n participants to cast their votes in a way that preserves the privacy of individual values against both internal and external attacks. The protocol is coupled with an extensive theoretical analysis in which we formally prove that our protocol is resistant to collusion against as many as n-1 corrupted nodes in the semi-honest model. The behavior of our protocol is tested in a real P2P network by measuring its communication delay and processing overhead. The experimental results uncover the advantages of our protocol over previous works in the area; without sacrificing security, our decentralized protocol is shown to be almost one order of magnitude faster than the previous best protocol for providing anonymous feedback

Performance and sustainability of short-rotation energy crops treated with municipal and industrial residues

The sustainability of short-rotation willow coppice (SRWC) as a multifunctional system for phytoremediation—the use of plants for treatment of contaminated air, soil or water—and for producing energy biomass, was studied. SRWC is grown commercially in Sweden to produce energy biomass, nutrient-rich residues being applied as cost-efficient fertiliser to increase production. The principal residues used are municipal wastewater, landfill leachate, industrial wastewater (e.g. log-yard runoff), sewage sludge and wood-ash. Small- and large-scale experiments with residues aimed to quantify the extent of potential hazards and the performance of SRWC in reducing them. Lysimeter experiments with willow plants, intensively irrigated with N-rich municipal wastewater, showed that N-leaching is a potential threat when high N loads are applied. Experimental data from SRWC fields irrigated with municipal wastewater in central Sweden suggest that in practice, N-leaching is significantly lower, even when the N load applied is greater than the N requirements of SRWC. Growth of willow plants of five different clones in pot experiments irrigated with landfill leachate was reduced by comparison with that of control plants. The reduction was attributed to saline stress or P deficiency, and indicates that, when hazardous compounds are present in wastewater, irrigation rates should be adjusted to avoid growth reduction. Genetic differences were observed between willow clones in salt tolerance and growth performance. The careful selection of clones to suit specific situations is therefore recommended. Leaf length can be used for rapid diagnosis of stress, to permit adjustment of the irrigation rate, and thus to avoid growth reduction. Phytoremedial efficiency of SRWC is satisfactory when the concentration of hazardous compounds in wastewaters is low, as in log-yard runoff, and depends on irrigation intensity. Application of sludge–ash mixtures to SRWC is not a substantial threat to sustainability, in terms of heavy metals. Total Cd in the soil is expected to decrease after harvest, but other metals and P loads, must also be considered. Any decision concerning residue application must take into account factors such as residue composition, soil type, climate, and species or clone characteristics, if both a high growth rate of SRWC and sustainability are to be attained

Constant Factor Approximation for Balanced Cut in the PIE model

We propose and study a new semi-random semi-adversarial model for Balanced Cut, a planted model with permutation-invariant random edges (PIE). Our model is much more general than planted models considered previously. Consider a set of vertices V partitioned into two clusters $L$ and $R$ of equal size. Let $G$ be an arbitrary graph on $V$ with no edges between $L$ and $R$. Let $E_{random}$ be a set of edges sampled from an arbitrary permutation-invariant distribution (a distribution that is invariant under permutation of vertices in $L$ and in $R$). Then we say that $G + E_{random}$ is a graph with permutation-invariant random edges. We present an approximation algorithm for the Balanced Cut problem that finds a balanced cut of cost $O(|E_{random}|) + n \text{polylog}(n)$ in this model. In the regime when $|E_{random}| = \Omega(n \text{polylog}(n))$, this is a constant factor approximation with respect to the cost of the planted cut.Comment: Full version of the paper at the 46th ACM Symposium on the Theory of Computing (STOC 2014). 32 page