A novel MAC protocol and layer two transmission scheduling algorithm for WLANs

It is known that hidden-terminal is one of the main problems in ad hoc wireless networks. In addition, there are scenarios where the desired destination is located in the range of other transmitters, so that the efforts on setting up communication with this terminal will fail due to collisions that may occur between transmitted control frames and undesired received control and data frames. This phenomenon becomes a bottleneck when most of data transmissions experience packet fragmentation. In such scenarios, the desired destination becomes unreachable during the data transfer of neighboring nodes. Furthermore, another problem that arises in such networks is in the case of mobile nodes with heterogeneous power capabilities where low power nodes have a disadvantage in accessing media due to higher levels of interference from the high power nodes. Conventional protocols can not address the aforementioned problems efficiently, resulting in throughput and channel utilization degradation. Using the same PHY of IEEE 802.11 and making slight modifications in its MAC, a new medium access control scheme is presented in this paper to address such problems. The performance of the proposed approach is compared with earlier schemes through simulation-based evaluations, showing performance enhancement due to better handling of unreachability issues in the proposed MAC scheme.     

Production of bacterial cellulose fibers in the presence of effective microorganism

In this study, effective microorganism (EM) was added into fermentation medium in static culture to enhance bacterial cellulose (BC) production by Acetobacter xylinum 23769 strain. According to SEM micrographs, BC pellicles from BC-Baikal EM1 show a smaller diameter and a relatively narrow diameter distribution compared to BC pellicles from Hestrin–Schramm (HS) medium. The BC-HS absorbed 90.5 times its dry weight of water. The water holding capacity increased to 132.5 for BC-Baikal EM1 medium compared to BC-HS. From the FT-IR spectra, BC samples exhibited a similar pattern. The crystalline indices of Baikal EM1-altered BC (66%) were lower than Baikal EM1-free BC (71%).     

Tabu search based algorithms for bandwidth-delay-constrained least-cost multicast routing

The advent of various real-time multimedia applications in high-speed networks creates a need for quality of service (QoS) based multicast routing. The Steiner tree problem, is a well-known NP-complete problem, provides the mathematical structure behind multicast communications. Two important QoS constraints are the bandwidth constraint and the end-to-end delay constraint. In this paper, we propose various algorithms to solve the bandwidth-delay-constrained least-cost multicast routing problem based on Tabu Search (TS), addressing issues of the selected initial solution and move type as two major building blocks in short-term memory version of Tabu Search and longer-term memory with associated intensification and diversification strategies as advanced Tabu Search techniques. We evaluate the performance and efficiency of the proposed TS-based algorithms in comparison with other existing TS-based algorithms and heuristics on a variety of random generated networks with regard to total tree cost. Finally we identify the most efficient algorithm uncovered by our testing.     

Effects of different surface treatments on shear bond strength between ceramic systems and metal brackets

The aim of this study was to evaluate the shear bond strength of orthodontic brackets bonded to different kinds of ceramic surfaces after different surface conditioning methods. A total of 120 ceramic disks were divided into two main groups in terms of feldspathic or lithium disilicate. Each ceramic group was further subdivided into six subgroups depending on surface treatment (n = 10). The ceramic surfaces were conditioned by one of the following methods: Group C: control group; Group P: %37.5 orthophosphoric acid; Group HF: %9.6 hydrofluoric acid; Group L: Nd-YAG laser irradiation; Group SB: sandblasting with 50 µm Al₂O₃ particles; and Group DB: grinding with a diamond bur. Surface roughness value was evaluated with a digital profilometer. Surface topographies of one specimen from each group were observed by atomic force microscopy (AFM) after surface treatments. All samples were primed with silane before the bracket bonding, including the control group. Metal brackets were bonded to the specimens with a light curing composite resin. The samples were stored in distilled water for 24?h and thermocycled 2500× at 5 and 55 ºC for 30?s. Shear bond strengths between the ceramic surface and the bracket were measured with a universal testing machine at a crosshead speed of 0.5 mm/min. Failure modes were classified as adhesive, cohesive, or mixed. Data were analyzed using ANOVA and Tukey's tests (α = .05). Group SB had significantly rougher surface compared with the other groups in each ceramic system (p < .05), and Group SB demonstrated significantly higher shear bond strengths than other groups as well. Within the limitations of this study, surface conditioning methods, except for sandblasting and grinding, were associated with lower shear bond strengths; however, thermocycling may have had negative effects on bond strengths of specimens. Furthermore, in each ceramic system, there was a significant difference between surface-conditioning methods and surface roughness with regard to shear bond strength.