Adaptive TTL-Based Caching for Content Delivery

Content Delivery Networks (CDNs) deliver a majority of the user-requested content on the Internet, including web pages, videos, and software downloads. A CDN server caches and serves the content requested by users. Designing caching algorithms that automatically adapt to the heterogeneity, burstiness, and non-stationary nature of real-world content requests is a major challenge and is the focus of our work. While there is much work on caching algorithms for stationary request traffic, the work on non-stationary request traffic is very limited. Consequently, most prior models are inaccurate for production CDN traffic that is non-stationary. We propose two TTL-based caching algorithms and provide provable guarantees for content request traffic that is bursty and non-stationary. The first algorithm called d-TTL dynamically adapts a TTL parameter using a stochastic approximation approach. Given a feasible target hit rate, we show that the hit rate of d-TTL converges to its target value for a general class of bursty traffic that allows Markov dependence over time and non-stationary arrivals. The second algorithm called f-TTL uses two caches, each with its own TTL. The first-level cache adaptively filters out non-stationary traffic, while the second-level cache stores frequently-accessed stationary traffic. Given feasible targets for both the hit rate and the expected cache size, f-TTL asymptotically achieves both targets. We implement d-TTL and f-TTL and evaluate both algorithms using an extensive nine-day trace consisting of 500 million requests from a production CDN server. We show that both d-TTL and f-TTL converge to their hit rate targets with an error of about 1.3%. But, f-TTL requires a significantly smaller cache size than d-TTL to achieve the same hit rate, since it effectively filters out the non-stationary traffic for rarely-accessed objects

Emission Trading Systems and the Optimal Technology Mix

Cap and trade mechanisms enjoy increasing importance in environmental legislation worldwide. The most prominent example is probably given by the European Union Emission Trading System (EU ETS) designed to limit emissions of greenhouse gases, several other countries already have or are planning the introduction of such systems. One of the important aspects of designing cap and trade mechanisms is the possibility of competition authorities to grant emission permits for free. Free allocation of permits which is based on past output or past emissions can lead to inefficient production decisions of firms’ (compare for example B¨ohringer and Lange (2005), Rosendahl (2007), Mackenzie et al. (2008), Harstad and Eskeland (2010)). Current cap and trade systems grant free allocations based on installed production facilities, which lead to a distortion of firms’ investment incentives, however. It is the purpose of the present article to study the impact of a cap and trade mechanism on firms’ investment and production decisions and to analyze the optimal design of emission trading systems in such an environment.Emissions Trading; Free Allocation; Investment Incentives; Technology Mix

Near-Saturation Conditions at the Tropical Tropopause: Results from Ticosonde

The TTL lies between the neutral buoyancy level (NBL) at ~350 K and the tropopause. Within the the TTL radiative heating drives ascent and air parcel supersaturation. Ticosonde measurements since 2005 show that the incidence ofsupersaturation in the TTL over Costa Rica is ~60%. This is due to diabatic ascent. The frequency is highest (68%) in summer, when convection is frequent. The TSL was defined by Selkirk et al. (2010) as the upper edge of the TTL. It isIn this layer that the final saturation of air parcels rising into the stratosphere occurs and thus the water vapor minima which define the so-called "writehead" of the Atmospheric Tape Recorder

Long-term climatology of air mass transport through the Tropical Tropopause Layer (TTL) during NH winter

A long-term climatology of air mass transport through the tropical tropopause layer (TTL) is presented, covering the period from 1962–2005. The transport through the TTL is calculated with a Lagrangian approach using radiative heating rates as vertical velocities in an isentropic trajectory model. We demonstrate the improved performance of such an approach compared to previous studies using vertical winds from meteorological analyses. Within the upper part of the TTL, the averaged diabatic ascent is 0.5 K/day during Northern Hemisphere (NH) winters 1992–2001. Climatological maps show a cooling and strengthening of this part of the residual circulation during the 1990s and early 2000s compared to the long-term mean. Lagrangian cold point (LCP) fields show systematic differences for varying time periods and natural forcing components. The interannual variability of LCP temperature and density fields is found to be influenced by volcanic eruptions, El Niño Southern Oscillation (ENSO), Quasi-Biennial Oscillation (QBO) and the solar cycle. The coldest and driest TTL is reached during QBO easterly phase and La Niña over the western Pacific, whereas during volcanic eruptions, El Niño and QBO westerly phase it is warmer and less dry

Granular activated carbon (GAC) biofilter in water and wastewater treatment

University of Technology, Sydney. Faculty of Engineering.Biofilters can effectively remove organic matters from water and wastewater. Basically, the success of operation of a biofilter depends mainly on the activities of the microbial community in the filter. Organic substances are adsorbed onto filter media and then biodegraded by those microbes. The experimental investigation of the granular activated carbon (GAC) adsorption indicated that GAC exhibited a high organic removal rate by adsorption in both the batch and column experiments. The GAC adsoiption equilibrium with synthetic wastewater fitted better with association theory (Talu) than with Freundlich and Sips models. Adsorption kinetics and fixed bed of GAC with organic matter was well described and predicted by the Linear Driving Force Approximation (LDFA) model. Long term GAC biofiltration was mathematically described using a simple model which incorporated both adsoiption and biodegradation. The model was mainly successful in describing the biological phase. Biomass accumulation onto GAC was evaluated in terms of dry mass and number of viable cells both in the batch and column experiments. The attachment of micro-organisms onto the GAC surface depended on the hydraulic loading rate and influent concentration. The amount of dry mass retained on GAC in the column was double than that in batch test in the steady state with 44mg/g GAC and 85mg/g GAC respectively. More viable cells were enumerated from GAC in the experiments with synthetic wastewater than with river water. The prominent bacteria isolated from GAC biofilter for synthetic wastewater and river water included: Pseudomonas Aeruginos, Pseudomonas Alcaligenes, Brevibacterium Otitidis, Enterobacter Cloacae, Enterobacter Aerogenes, Staphylococcus Epidermidis and Kocuria Rosea. The long term performance of the GAC biofilter with synthetic wastewater which has similar characteristics to biologically treated sewage effluent and river water was experimentally evaluated. The result showed that the GAC biofilter could maintain high organic removal efficiency after a long filtration time without any regeneration of activated carbon. Even after 42 days of continuous run, the biofilter of very short depth of 15cm GAC bed depth maintained a consistent organic removal efficiency of 40-50% with synthetic wastewater and 55% with river water. GAC biofilter also removed 60-98% of total coliforms from synthetic wastewater and river water. Especially, no fecal coliforms were detected in effluent from the GAC biofilter for river water. The daily backwash adopted to avoid the physical clogging of the biofilter did not have significant effect on the performance of the filter. The change of filter bed depths, filtration rates and influent concentrations affected the performance of the GAC biofilter. Total organic carbon (TOC) removal efficiency in the higher filter bed was significantly better than that with shallow bed depth (60% TOC removal with a 30cm- bed depth while only 30% with a 5 cm - bed depth). As expected, the efficiency in organic removal decreased with an increase in the filtration rate. The difference was more significant in the lower bed. TOC removal efficiency of the biofilter was also affected by the concentration of influent. It increased with the increase in the concentration of influent, but the TOC removal pattern with time was almost the same. Further, the increase of organic removal rate was not proportional to the rise of influent concentration. A study with different filter media showed that GAC as filter media was superior to plastic bead, anthracite and sponge in terms of organic removal. In addition, GAC was combined with sponge or polypropylene as medium to enhance the effectiveness of biofilter in removing organic matter. The use of sponge and floating media can eliminate further 10-20% of TOC from the effluent of GAC biofilter with less effort in installation, operation and maintenance. In summary, GAC biofilter can be used as an economical treatment system in removing organic matters and pathogens from biologically treated wastewater and surface water. The merits of GAC biofilter are the consistent of TOC removal efficiency, long life cycle, and simplicity in operation. The combined system of GAC - sponge filter media biofilter can be an effective and a good practical solution for improving the organic removal from water and wastewater

Water Vapor and Cloud Formation in the TTL: Simulation Results vs. Satellite Observations

Driven by analyzed winds and temperature, domain-filling forward trajectory calculations are used to reproduce water vapor and cloud formations in the tropical tropopause layer (TTL). As with most Lagrangian models of this type, excess water vapor is instantaneously removed from the parcel to keep the relative humidity with respect to ice from exceeding a specified (super) saturation level. The dehydration occurrences serve as an indication of where and when cloud forms. Convective moistening through ice lofting and gravity waves are also included in our simulations as mechanisms that could affect water vapor abundances and cloud formations in the TTL. Our simulations produce water vapor mixing ratios close to that observed by the Aura Microwave Limb Sounder (MLS) and are consistent with the reanalysis tropical tropopause temperature biases, which proves the importance of the cold-point temperature to the water vapor abundances in the stratosphere. The simulation of cloud formation agrees with the patterns of cirrus distribution from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). It proves that the trajectory calculations fed by the analyzed wind and temperature could produce reasonable simulations of water vapor and cloud formation in the TTL

Atmospheric temperature tides in the tropical upper troposphere and lower stratosphere

Atmospheric thermal tides are global-scale waves with periods that are harmonics of a solar day, mainly excited by diurnally varying diabatic heating in the troposphere and the stratosphere. Some recent studies suggested that the tidal temperature variations in the TTL might affect the appearance of cirrus clouds and, thus, the dehydration process. It should be noted, however, that the global pattern of diurnal temperature variations in the TTL still remains unclear. In this study, we aim at revealing the 3D structure of diurnal temperature variations around the TTL, including its seasonal variations, by using data from global reanalyses for the period of 2002-2006. It is found that the Sun synchronous tides have amplitudes of ~0.3 K (~0.5 K) at 100 hPa (70 hPa) in January. Superposed on these components, the non-Sun-synchronous tides are strong over the continent (South America, Africa); these may be excited by latent heat release associated with deep convections there. The total (i.e., Sun-synchronous plus non-Sun-synchronous) diurnal temperature amplitudes reach ~0.5 K (~1 K) at maxima at 100 hPa (70 hPa) in January. The seasonality and the impact on the dehydration will be discussed in the presentation

Investment Incentives and Electricity Spot Market Design

In liberalized electricity markets strategic firms compete in an environment characterized by fluctuating demand and non-storability of electricity. While spot market design under those conditions by now is well understood, a rigorous analysis of investment incentives is still missing. Existing models, as the peak-load-pricing approach, analyze welfare optimal investment and find that optimal investment is higher with more competitive spot markets. In this article we want to extend the analysis to investment decisions of strategic firms that anticipate competition on many consecutive spot markets with fluctuating (and possibly uncertain) demand. We study how the degree of spot market competition affects investment incentives and welfare and provide an application of the model to electricity market data. Our results show that more competitive spot market prices strictly decrease investment incentives of strategic firms. The reduction of investment incentives can be so intense to even offset the beneficial impact of more competitive spot market design. Those results obtain with and without free entry. Our analysis thus demonstrates that investment incentives necessarily have to be taken into account for a meaningful assessment of proper electricity spot market design.Investment; demand fluctuation; cost fluctuation; spot market design