Effects of radiation on spray flame characteristics and soot formation

Two-dimensional numerical simulations are applied to spray flames formed in a laminar counterflow and the effects of radiation on spray flame characteristics and soot formation are studied. N-Decane (C₁₀H₂₂) is used as the liquid fuel, and the droplet motion is calculated by the Lagrangian method. A single-step global reaction is employed for the combustion reaction model. A kinetically based soot model with a flamelet model is used to predict soot formation. Radiation is taken into account using the discrete ordinate method. The results show that radiation strongly affects the spray flame behavior and soot formation. Without the radiation model, flame temperature and soot volume fraction are greatly overestimated. The soot is formed in the diffusion flame regime, and its radiation emission increases with the increase in the equivalence ratio of the droplet fuel. This trend is in good agreement with that of the luminous flame behavior observed in the experiments.     

Push-to-Peer Video-on-Demand System: Design and Evaluation

We propose Push-to-Peer, a peer-to-peer system to cooperatively stream video. The main departure from previous work is that content is proactively pushed to peers, and persistently stored before the actual peer-to-peer transfers. The initial content placement increases content availability and improves the use of peer uplink bandwidth. Our specific contributions are: (i) content placement and associated pull policies that allow the optimal use of uplink bandwidth; (ii) performance analysis of such policies in controlled environments such as DSL networks under ISP control; (iii) a distributed load balancing strategy for selection of serving peers.     

Performance evaluation of connection rerouting schemes forATM-based wireless networks

Supporting mobility in asynchronous transfer mode (ATM)-based broad-band networks with wireless access links poses many technical challenges. One of the most important of these challenges is the need to reroute ongoing connections to/from mobile users as these users move among base stations. Connection rerouting schemes must exhibit low handoff latency, maintain efficient routes, and limit disruption to continuous media traffic while minimizing reroute updates to the network switches. In this paper we propose, describe an implementation for, and experimentally evaluate the performance of five different connection rerouting schemes. We show that one of these schemes, which operates in two phases, executes very fast reroutes (with a measured latency of 6.5 ms) in a real-time phase and, if necessary, reroutes again in a nonreal-time phase to maintain efficient routing. The scheme also results in negligible disruption to both audio (e.g., a 1-in-100 chance of a single packet loss at CD-quality audio rates of 128 kb/s) and low-bit-rate video (e.g., a 2-in-100 chance of a single packet loss for 1-Mb/s video) traffic during connection rerouting. Based on these results, we conclude that simple handoff schemes coupled with a connection management architecture are sufficient for supporting low-bit-rate continuous media applications over ATM-based wireless networks     

A Comparison of Hard-State and Soft-State Signaling Protocols

One of the key infrastructure components in all telecommunication networks, ranging from the telephone network to VC-oriented data networks to the Internet, is its signaling system. Two broad approaches towards signaling can be identified: so-called hard-state and soft-state approaches. Despite the fundamental importance of signaling, our understanding of these approaches-their pros and cons and the circumstances in which they might best be employed-is mostly anecdotal (and, occasionally, religious). In this paper, we compare and contrast a variety of signaling approaches ranging from "pure" soft state to soft-state approaches augmented with explicit state removal and/or reliable signaling, to a "pure" hard state approach. We develop an analytic model that allows us to quantify state inconsistency in singleand multiple-hop signaling scenarios, and the "cost" (both in terms of signaling overhead and application-specific costs resulting from state inconsistency) associated with a given signaling approach and its parameters (e.g., state refresh and removal timers). Among the class of soft-state approaches, we find that a soft-state approach coupled with explicit removal substantially improves the degree of state consistency while introducing little additional signaling message overhead. The addition of reliable explicit setup/update/removal allows the soft-state approach to achieve comparable (and sometimes better) consistency than that of the hard-state approach     

A four-input beam-forming downconverter for adaptive antennas

A four-input beam-forming downconverter for adaptive antennas is described. It consists of 2-bit variable gain amplifiers (VGAs), 5-bit local oscillator (LO) signal phase shifters using double RC-bridge circuits, and mixers. The VGAs adjust gain deviation between signal paths. A differential-signal-to-eight-phase-signal converter is employed as a part of the LO phase shifter to reduce the chip size. A maximum phase error of 4.1/spl deg/, which is less than 1/2 LSB, is achieved. This error value indicates that the required phase shifter accuracy and the necessary isolation between the VGAs has been achieved. This beam-forming IC is applicable to receivers with adaptive antennas, and is expected to help to reduce the costs of adaptive antenna systems.     

Smoothing, statistical multiplexing, and call admission control forstored video

Variable bit-rate (VBR) compressed video is known to exhibit significant, multiple-time-scale rate variability. A number of researchers have considered transmitting stored video from server to a client using smoothing algorithms to reduce this rate variability. These algorithms exploit client buffering capabilities and determine a “smooth” rate transmission schedule, while ensuring that a client buffer neither overflows nor underflows. We investigate how video smoothing impacts the statistical multiplexing gains available with such traffic, and we show that a significant amount of statistical multiplexing gains can still be achieved. We then examine the implication of these results on network resource management and call admission control when transmitting smoothed stored video using VBR service with statistical quality-of-service (QoS) guarantees. Specifically, we present a uniform call admission control scheme based on a Chernoff bound method that uses a simple, novel traffic model requiring only a few parameters. This scheme provides an easy and flexible mechanism for supporting multiple VBR service classes with different QoS requirements. We evaluate the efficacy of the call admission control scheme over a set of MPEG-1 coded video tracts     

Hypercholesterolemia enhances oxidant production in mesenteric venules exposed to Ischemia/Reperfusion

BACKGROUND: Three-dimensional echocardiography allows calculation of left ventricular ejection fraction without geometric assumption on the ventricular shape. Our aim was to validate this technique in a paediatric population with distorted ventricles. METHODS: Twenty-one patients aged 6 months to 17 years underwent equilibrium radionuclide angiography and three-dimensional echocardiography. Fourteen patients had dilated cardiomyopathy and seven had univentricular hearts. A new, easy to handle, transthoracic rotational probe was used and motion artefacts were limited during the rotation (3 degrees intervals with ECG and respiratory gating). Left ventricular volumes and ejection fraction were calculated using the Simpson's rule with 12 slices. RESULTS: Three-dimensional echocardiography correlated well with equilibrium radionuclide angiography for ejection fraction measurement (r = 0.90; the mean difference between the two methods being 3.8 +/- 6%). Intra-observer and inter-observer variabilities for 3D echocardiography were 2.4% and 4.5%. CONCLUSIONS: Three-dimensional echocardiography is an accurate, non-invasive, and reproducible methods to measure left ventricular ejection fraction in children.     

Stack algorithms for random multiple-access networks in thepresence of asymmetric feedback

The operation of stack splitting random-access protocols in multiaccess networks in which individual stations may receive asymmetric feedback from the channel (i.e. different stations may observe different, possibly erroneous, outcomes on the channel) is examined. Several possible modifications to the basic stack algorithm are proposed for such environments, and the performances of the various alternatives are reviewed. An approximate Markov chain model is developed to analytically study the time delay versus throughput performance of the various alternatives, and the analytic results are validated through simulation. Representative performance results are given for the alternative stack algorithms. It is found that those algorithms which tend to treat the receipt of corrupted feedback by a station as a collision show superior performance for throughput values greater than approximately 0.2, whereas, at low throughput values, there is relatively little difference between the performances of the various approaches studied. It was noted during the simulation studies that, with an error rate of up to 5%, the algorithms remained stable up to an arrival rate of approximately 0.3 or higher