On the impact of IEEE 802.11 MAC on traffic characteristics

IEEE 802.11 medium access control (MAC) is gaining widespread popularity as a layer-2 protocol for wireless local-area networks. While efforts have been made previously to evaluate the performance of various protocols in wireless networks and to evaluate the capacity of wireless networks, very little is understood or known about the traffic characteristics of wireless networks. In this paper, we address this issue and first develop an analytic model to characterize the interarrival time distribution of traffic in wireless networks with fixed base stations or ad hoc networks using the 802.11 MAC. Our analytic model and supporting simulation results show that the 802.11 MAC can induce pacing in the traffic and the resulting interarrival times are best characterized by a multimodal distribution. This is a sharp departure from behavior in wired networks and can significantly alter the second order characteristics of the traffic, which forms the second part of our study. Through simulations, we show that while the traffic patterns at the individual sources are more consistent with long-range dependence and self-similarity, in contrast to wired networks, the aggregate traffic is not self-similar. The aggregate traffic is better classified as a multifractal process and we conjecture that the various peaks of the multimodal interarrival time distribution have a direct contribution to the differing scaling exponents at various timescales.     

Crystallization of mixed rare earth (didymium) molybdates in silica gel

Experiments on the growth of mixed rare earth (didymium—a combination of La, Nd, Pr and Sm) molybdates in silica gel medium are reported. The optimum conditions conducive for the growth of these crystals are described and discussed. Concentration programming is reported to enhance the size of crystals by two-fold; the maximum size obtained being about 1 mm³. EDAX results suggest the crystals to be heptamolybdates of type R₂Mo₇O₂₄, bearing composition La_1.23Nd_0.43Pr_0.29 Sm_0.05Mo₇0₂₄. The didymium molybdate crystals assume morphologies corresponding to those of spherulites, platelets, cuboids and coalesced crystals. Twinned structure in didymium molybdate crystals are also reported. It is explained that spherulitic morphologies result from aggregates of crystals joining in a spherical envelope. It is suggested that the crystals of didymium molybdates grow by two-dimensional spreading and piling up of layers.     

VM3: Measuring,modeling and managing VM shared resources

With cloud and utility computing models gaining significant momentum, data centers are increasingly employing virtualization and consolidation as a means to support a large number of disparate applications running simultaneously on a chip-multiprocessor (CMP) server. In such environments, contention for shared platform resources (CPU cores, shared cache space, shared memory bandwidth, etc.) can have a significant effect on each virtual machine’s performance. In this paper, we investigate the shared resource contention problem for virtual machines by: (a) measuring the effects of shared platform resources on virtual machine performance, (b) proposing a model for estimating shared resource contention effects, and (c) proposing a transition from a virtual machine (VM) to a virtual platform architecture (VPA) that enables transparent shared resource management through architectural mechanisms for monitoring and enforcement. Our measurement and modeling experiments are based on a consolidation benchmark (vConsolidate) running on a state-of-the-art CMP server. Our virtual platform architecture experiments are based on detailed simulations of consolidation scenarios. Through detailed measurements and simulations, we show that shared resource contention affects virtual machine performance significantly and emphasize that virtual platform architectures is a must for future virtualized datacenters.     

Evaluation of Irrigation-Service Utility from the Perspective of Farmers

This paper highlights an alternative approach for the partial evaluation of an irrigation system considering irrigation as a service to farmers. It avoids the need of quantitative data on water delivery and concentrates on performance evaluation from the perspective of farmers. The concept of fuzzy set theory was applied to analyse the linguistic responses of farmers with respect to irrigation service provided to them. The present study was carried out in Nimapara Branch Canal under Puri Main Canal irrigation system of Mahanadi Delta Irrigation Project in the State of Orissa, India. The utility of irrigation water supply schedule was assessed on the basis of three factors i.e. tractability, convenience and predictability and 10 sub-factors under three factors. The analyses revealed that farmers' level of satisfaction with the factors in an increasing order was predictability, convenience and tractability. Head reach of the system recorded relatively higher utility values as compare to middle and tail reach of irrigation command. The most important factor is found to be predictability followed by tractability and convenience. There are differences in the opinions of the farmers about the water delivery system. These opinions are, however, closer in the importance attached by them to the individual sub-factors and factors.