Modeling the 802.11 Distributed Coordination Function in Nonsaturated Heterogeneous Conditions

Analysis of the 802.11 CSMA/CA mechanism has received considerable attention recently. Bianchi presented an analytic model under a saturated traffic assumption. Bianchi's model is accurate, but typical network conditions are nonsaturated and heterogeneous. We present an extension of his model to a nonsaturated environment. The model's predictions, validated against simulation, accurately capture many interesting features of nonsaturated operation. For example, the model predicts that peak throughput occurs prior to saturation. Our model allows stations to have different traffic arrival rates, enabling us to address the question of fairness between competing flows. Although we use a specific arrival process, it encompasses a wide range of interesting traffic types including, in particular, VoIP     

A randomized community trial to increase mammography utilization among low-income women living in public housing

BACKGROUND: A randomized trial was conducted to evaluate the impact of a community-based intervention on mammography use among low-income women living in public housing. METHODS: All 41 public housing high-rise buildings were randomized to treatment and delayed treatment (control) conditions. After a cross-sectional baseline survey, an intervention called Friend to Friend was conducted in the treatment buildings by American Cancer Society and building resident volunteers. The intervention consisted of a health professional talk, small group discussions, and an opportunity to request assistance in obtaining a mammogram or mammogram reminder. A second cross-sectional survey was conducted to measure differences in screening rates between the study groups. RESULTS: Participation in the intervention averaged 27%. The study groups were equivalent at baseline. At follow-up, the proportion of women age 50-79 years who reported mammography screening in the previous 15 months was significantly higher in the treatment group (64%) than in the control group (52%). Breast cancer knowledge, attitudes, and beliefs did not differ between groups. CONCLUSIONS: These findings suggest that a multidimensional intervention which reaches women within their social environment and uses community volunteers can increase mammography utilization among women in public housing.     

Plant cell suspension cultures: some engineering considerations

Higher plants are the source of a vast array of biochemicals which are used as drugs, pesticides, flavourings and fragrances. For some of these compounds, plant cell culture can provide a potential production alternative to traditional cultivation methods or chemical synthesis routes. Many systems have been patented and the last 20 years have seen considerable industrial and academic interest in the development of large scale cultures to produce pharmaceutically active, high value substances. However, the industrial application of plant cell suspension cultures has, to date, been limited. Commercialisation has essentially been impeded by economic feasibility, arising from both biological and engineering considerations. This paper reviews the commercial development of the technology to date and focuses on the impact of specific engineering-related factors, in particular, the shear sensitivity of plant cell suspension cultures. Evidence of sensitivity to hydrodynamic shear in bioreactors has generally been attributed to the physical characteristics of the suspended cells. Recent studies indicate that shear sensitivity may not be as important, in some cases, as initially anticipated.     

Mucinous adenocarcinoma arising in a neovagina

The development of carcinoma in a neovagina is a rare occurrence. A case of mucinous adenocarcinoma of a neovagina is presented. A neovagina was formed in 1945 to correct a congenital anomaly. Subsequently the patient developed a chronic rectovaginal fistula. It is thought that the mucinous adenocarcinoma developed as a result of chronic inflammation secondary to the fistula. Although rare, development of a malignancy in ectopic tissue can occur. Clinicians need to be aware of this as they follow these patients.     

DNA-damaging steroidal alkaloids from Eclipta alba from the suriname rainforest1

Bioassay-guided fractionation of the MeOH extract of Eclipta alba using three yeast strains (1138, 1140, and 1353) resulted in the isolation of eight bioactive steroidal alkaloids (1-8), six of which are reported for the first time from nature. The major alkaloid was identified as (20S)(25S)-22,26-imino-cholesta-5,22(N)-dien-3beta-ol (verazine, 3), while the new alkaloids were identified as 20-epi-3-dehydroxy-3-oxo-5,6-dihydro-4,5-dehydroverazine (1), ecliptalbine [(20R)-20-pyridyl-cholesta-5-ene-3beta,23-diol] (4), (20R)-4beta-hydroxyverazine (5), 4beta-hydroxyverazine (6), (20R)-25beta-hydroxyverazine (7), and 25beta-hydroxyverazine (8). Ecliptalbine (4), in which the 22,26-imino ring of verazine was replaced by a 3-hydroxypyridine moiety, had comparable bioactivity to verazine in these assays, while a second alkaloid (8) showed good activity against Candida albicans. All the alkaloids showed weak cytotoxicity against the M-109 cell line.     

Representation of single-pole open conditions in stability studies[of power systems]

The general practice in representing a power transmission line with one phase open in stability studies is to assume that the phase is open at one end only. The error involved in such a representation, especially at EHV voltages with a long line section that is not fully compensated, can be appreciable. It is shown that to represent the effect of single phase open conditions in stability studies accurately the equivalent line parameters should be calculated by considering the line to be open at both ends instead of at one end only. The exact method of representing the phase open condition in positive-sequence network simulations in stability studies is derived. A method of calculating the two-port parameters, as seen from the two openings, given the four-port parameters with ground as a return node, is derived     

The role of defects in the quantum size effect

Many macroscopic properties of films exhibit an oscillatory dependence on thickness when the thickness is comparable with the de Broglie wavelength of an electron at the Fermi surface. This behavior is called the quantum size effect. In this paper the influence of defects in thin semimetal films on the electrical conductivity is explored by extending the work of Sandomirskiǐ to include a scattering potential of finite range. An isotropic sample bounded by infinitely high potential walls is assumed, and the Greenwood-Peierls formula is the starting point for this calculation. The potential selected and the approach taken allow the conductivity to be calculated with correction terms which have not previously been given. Within this model the calculations show that for δ-function scattering, Sandomirski?'s results are exact. Compared with Sandomirski?'s work, the calculations in this paper predict a reduction in the amplitude of the oscillations in addition to an overall decrease in the conductivity. A physical explanation of the results is given.