Instantaneous Link Research Articles

An adaptive fuzzy-based CAC scheme for uplink and downlink congestion control in converged IP and DVB-S2 networks

This paper introduces a robust buffer occupancy-based connection admission control (CAC) mechanism to alleviate both uplink and downlink congestions in converged IP and broadcasting networks. The scheme also ensures a fair share of downlink bandwidth among competing satellite terminals (subnetworks) in the event of congestion. The proposed scheme is dubbed Weighted Fair CAC (W-FCAC). It accepts or rejects connections based on an adaptive fuzzy-based approach. The use of the fuzzy-based mechanism is for the purpose of overcoming issues related to instantaneous link capacity assessment, flow characterization and the associated high computational complexity, and use of traffic descriptors for new flows. Additionally, the adaptive fuzzy logic makes the proposed CAC approach robust to traffic dynamics. These features make the scheme highly suitable for DVB-S2 environments where the link capacity frequently fluctuates due to the adaptive coding/modulation of the physical layer during noisy periods. Simulation results elucidate that the proposed W-FCAC scheme prevents downlink congestion and fairly allocates network resources among satellite terminals. It also minimizes the frequency of congestion events while maintaining efficient utilization of network resources.

A Cooperation-Aware Routing Scheme for Fast Varying Fading Wireless Channels

In this letter, we propose a cooperation-aware routing scheme based on long-term statistics of cooperative links. The proposed scheme fully exploits cooperation on fading scenarios and proves to be more efficient than greedy cooperation-aware routing based on instantaneous link conditions. Moreover, the performance improvement is even higher when cooperative and non-cooperative nodes coexist.

Addressing conversion cascading constraint in OBS networks through proactive routing

The negative impact of cascaded wavelength conversions has been largely ignored in optical burst switching performance evaluations. When optical bursts are transmitted all optically from source to destination, each wavelength conversion performed along the lightpath may cause some signal-to-noise deterioration. If the distortion of the signal quality becomes significant enough, the receiver would not be able to recover the original data. In this paper we examine the performance degradation when an upper bound on the number of wavelength conversions that a signal can go through is enforced. We refer to this constraint as conversion cascading constraint. We propose a novel proactive routing scheme under this constraint that considers the instantaneous link congestion at the moment when the bursts arrive. It has three major advantages: (1) utilize the same offset times for the same node pairs while providing dynamic routing without using any fiber delay lines (FDL); (2) decrease burst loss probability to a great extent; (3) mitigate unfairness among the bursts with different hop counts, which is even worse under the conversion cascading constraint if without any special treatment. We also extend the proposed mechanism to embrace a variant of regulated deflection routing which can further improve network performance.

Nitrogen as a regulatory factor of methane oxidation in soils and sediments

The oxidation of methane by methane-oxidising microorganisms is an important link in the global methane budget. Oxic soils are a net sink while wetland soils are a net source of atmospheric methane. It has generally been accepted that the consumption of methane in upland as well as lowland systems is inhibited by nitrogenous fertiliser additions. Hence, mineral nitrogen (i.e. ammonium/nitrate) has conceptually been treated as a component with the potential to enhance emission of methane from soils and sediments to the atmosphere, and results from numerous studies have been interpreted as such. Recently, ammonium-based fertilisation was demonstrated to stimulate methane consumption in rice paddies. Growth and activity of methane-consuming bacteria in microcosms as well as in natural rice paddies was N limited. Analysing the available literature revealed that indications for N limitation of methane consumption have been reported in a variety of lowland soils, upland soils, and sediments. Obviously, depriving methane-oxidising bacteria of a suitable source of N hampers their growth and activity. However, an almost instantaneous link between the presence of mineral nitrogen (i.e. ammonium, nitrate) and methane-oxidising activity, as found in rice soils and culture experiments, requires an alternative explanation. We propose that switching from mineral N assimilation to the fixation of molecular nitrogen may explain this phenomenon. However, there is as yet no experimental evidence for any mechanism of instantaneous stimulation, since most studies have assumed that nitrogenous fertiliser is inhibitory of methane oxidation in soils and have focused only on this aspect. Nitrogen as essential factor on the sink side of the global methane budget has been neglected, leading to erroneous interpretation of methane emission dynamics, especially from wetland environments. The purpose of this minireview is to summarise and balance the data on the regulatory role of nitrogen in the consumption of methane by soils and sediments, and thereby stimulate the scientific community to embark on experiments to close the existing gap in knowledge.

Distributed MAC protocols and priority oriented scheduling for a PLC access network

AbstractIn this paper, we propose a distributed channel allocation scheme for the uplink data traffic and implement a priority oriented scheduling for the downlink data traffic for a powerline access type network. We study the performance of the uplink and downlink protocols in terms of the buffering probability, mean delay, statistical dropping probability and the number of subscriber stations supported. The distributed channel allocation scheme uses a modified carrier sensing mechanism and a centralized collision resolution scheme. We model the channel occupancy of uplink data calls using a continuous time Markov chain (CTMC). We derive the expressions for mean delay, statistical dropping probability, and utilization by solving the CTMC. We also perform extensive simulations to complement our analytical results. Our results indicate that, the distributed channel allocation approach achieves 10% improvement in spectrum utilization compared to a centralized reservation protocol. We also implement a priority oriented scheduling policy for the downlink data traffic. In the system we consider, the downlink channels are adaptively modulated based on their signal strength. We maximize the instantaneous link utilization by using MaxFlow algorithm. Simulation results for downlink scheduling show that the mean buffering delay for the real‐time variable bit rate traffic is zero and that the system supports 170 access stations. We also show that system with adaptive modulation achieves 26% savings in transmitted power compared to the system without adaptive modulation. Copyright © 2003 John Wiley & Sons, Ltd.

Electronic publication and data distribution for the five college astronomy department

The Star Formation Group at FCAD is making use of the World-Wide Web to: 1. (1) explore the advantages of hypermedia presentation for the distribution of preprints, Ph.D. theses and observatory publications; 2. (2) create hyperlinked catalogs of astronomical data which enable not only the recovery of tabular data but instantaneous links to abstracts of the original references; 3. (3) create on-line resources of spectroscopic and image data. We illustrate here the current status of this effort. As a by-product of this work, a method for displaying subscripts (e.g. M ⊙, Δν D ) and superscripts (e.g. T 4, C 18O) has been developed. This set of bitmaps is publically available.