Delay distributions of slotted ALOHA and CSMA

We derive the closed-form delay distributions of slotted ALOHA and nonpersistent carrier sense multiple access (CSMA) protocols under steady state. Three retransmission policies are analyzed. We find that under a binary exponential backoff retransmission policy, finite average delay and finite delay variance can be guaranteed for G<2S and G<4S/3, respectively, where G is the channel traffic and S is the channel throughput. As an example, in slotted ALOHA, S<(ln2)/2 and S<3(ln4-ln3)/4 are the operating ranges for finite first and second delay moments. In addition, the blocking probability and delay performance as a function of r/sub max/ (maximum number of retransmissions allowed) is also derived

Maximally Flexible Assignment of Orthogonal Variable Spreading Factor Codes for Multi-Rate Traffic

In universal terrestrial radio access (UTRA) systems, orthogonal variable spreading factor (OVSF) codes are used to support different transmission rates for different users. In this paper, we first define the flexibility index to measure the capability of an assignable code set in supporting multirate traffic classes. Based on this index, two single-code assignment schemes, nonrearrangeable and rearrangeable compact assignments, are proposed. Both schemes can offer maximal flexibility for the resulting code tree after each code assignment. We then present an analytical model and derive the call blocking probability, system throughput and fairness index. Analytical and simulation results show that the proposed schemes are efficient, stable and fair

Investigation of variations and trends in TSP concentrations in the Klang Valley region, Malaysia

Air pollution in the atmosphere by TSP is a problem that has been growing for the last few years due to rapid industrialization, population growth and urbanization in the Klang Valley Region. Suspended particle levels are monitored in ambient air quality network of their potential impact on human health, visibility and climate. The objective of this study is to investigate variations and trends in TSP in the Klang Valley Region and to identify the main factors affecting the variation in TSP concentrations. This analysis used daily data collected from Malaysian Meteorological Services for Petaing Jaya station monitoring TSP data for the period 1977-2002. On daily average basis, annual, seasonal, monthly and weekly variations and trends in TSP concentrations are presented. Linear regressions were performed of the annual average TSP over the 1977 through 2002. The slope of the regression line in annual average decreased in TSP in order of -0.977 and statistically significant at 0.05 level. This suggests that whatever control measures have been applied are effective in reducing TSP concentrations. TSP concentrations in the Klang Valley Region showed seasonal variation, with the higher concentration during southwest monsoon season (dry season) and lower concentrations during rainy season. The causes of highest TSP concentrations during southwest monsoon season due to lack of rainfall and the stable atmospheric conditions, which reduce the ability of atmosphere to disperse the pollutants. The weekly pattern of TSP concentration exhibits a downward trend from Tuesday (maximum) to Sunday (minimum). This weekly cycle in TSP gives the indication that the main sources of the particulates in this area are from human activities rather than natural source

Edge Elimination in TSP Instances

The Traveling Salesman Problem is one of the best studied NP-hard problems in combinatorial optimization. Powerful methods have been developed over the last 60 years to find optimum solutions to large TSP instances. The largest TSP instance so far that has been solved optimally has 85,900 vertices. Its solution required more than 136 years of total CPU time using the branch-and-cut based Concorde TSP code [1]. In this paper we present graph theoretic results that allow to prove that some edges of a TSP instance cannot occur in any optimum TSP tour. Based on these results we propose a combinatorial algorithm to identify such edges. The runtime of the main part of our algorithm is $O(n^2 \log n)$ for an n-vertex TSP instance. By combining our approach with the Concorde TSP solver we are able to solve a large TSPLIB instance more than 11 times faster than Concorde alone

Effects of Transit Signal Priority on Traffic Safety: Interrupted Time Series Analysis of Portland, Oregon, Implementations

Transit signal priority (TSP) has been implemented to transit systems in many cities of the United States. In evaluating TSP systems, more attention has been given to its operational effects than to its safety effects. Existing studies assessing safety effects of TSP reported mixed results, indicating that the safety effects of TSP vary in different contexts. In this study, TSP implementations in Portland, Oregon, were assessed using interrupted time series analysis (ITSA) on month-to-month changes in number of crashes from January 1995 to December 2010. Single-group and controlled ITSA were conducted for all crashes, property-damage-only crashes, fatal and injury crashes, pedestrian-involved crashes, and bike-involved crashes. Evaluation of the post-intervention period (2003 to 2010) showed a reduction in all crashes on street sections with TSP (-4.5 percent), comparing with the counterfactual estimations based on the control group data. The reduction in property-damage-only crashes (-10.0 percent) contributed the most to the overall reduction. Fatal and injury crashes leveled out after TSP implementation but did not change significantly comparing with the control group. Pedestrian and bike-involved crashes were found to increase in the post-intervention period with TSP, comparing with the control group. Potential reasons to these TSP effects on traffic safety were discussed.Comment: Published in Accident Analysis & Preventio