A comprehensive simulation study of slotted CSMA/CA for IEEE 802.15.4 wireless sensor networks

In this paper, we analyze the performance limits of the slotted CSMA/CA mechanism of IEEE 802.15.4 in the beacon-enabled mode for broadcast transmissions in WSNs. The motivation for evaluating the beacon-enabled mode is due to its flexibility for WSN applications as compared to the non-beacon enabled mode. Our analysis is based on an accurate simulation model of the slotted CSMA/CA mechanism on top of a realistic physical layer, with respect to the IEEE 802.15.4 standard specification. The performance of the slotted CSMA/CA is evaluated and analyzed for different network settings to understand the impact of the protocol attributes (superframe order, beacon order and backoff exponent) on the network performance, namely in terms of throughput (S), average delay (D) and probability of success (Ps). We introduce the concept of utility (U) as a combination of two or more metrics, to determine the best offered load range for an optimal behavior of the network. We show that the optimal network performance using slotted CSMA/CA occurs in the range of 35% to 60% with respect to an utility function proportional to the network throughput (S) divided by the average delay (D).

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in Portugal:

Methicillin-resistant Staphylococcus aureus (MRSA), a human pathogen confined to hospitals (HAMRSA) for over 30 years have been emerging worldwide in the last two decades as a leading cause of severe infections in healthy individuals in the community (CA-MRSA). Despite its clinical significance, in the beginning of our studies no information existed on the prevalence, and population structure of CA-MRSA in Portugal. Moreover, it remained to be clarified how CA-MRSA emerged in our country. In particular, it was not known if CA-MRSA emerged locally by acquisition of the staphylococcal cassette chromosome mec (SCCmec) by established methicillin-susceptible S. aureus (MSSA) in the community, if they were imported from abroad or have escaped from the hospital.(...)

Improving TCP Performance on CSMA/CA Connections

In IP networks, most of packets, that have been dropped, are recovered after the expiration of retransmission timeouts. These can result in unnecessary retransmissions and needless reduction of congestion window. An inappropriate retransmission timeout has a huge impact on TCP performance. In this paper we have proved that CSMA/CA mechanism can cause TCP retransmissions due to CSMA/CA effects. For this we have observed three wireless connections that use CSMA/CA: with good link quality, poor link quality and in presence of cross traffic. The measurements have been performed using real devices. Through tracking of each transmitted packet it is possible to analyze the relation between one-way delay and packet loss probability and the cumulative distribution of distances between peaks of OWDs. The distribution of OWDs and the distances between peaks of OWDs are the most important parameters of tuning TCP retransmission timeout on CSMA/CA networks. A new perspective through investigating the dynamical relation between one-way delay and packet loss ratio depending on the link quality to enhance the TCP performance has been provided.

Charles Clarke Chapman, ca. 1942

Original mounted photograph, signed by the photographer [Hartsook], as well as by C. C. Chapman [Affectionately yours, Father], ca. 1942.

Charles Clarke Chapman, Los Angeles, California, ca. 1895

Studio portrait photograph of Charles Clarke Chapman, Los Angeles, California, ca. 1895

Original Chapman home ranch packing house, Santa Ysabel, California, ca. 1900

Original Chapman home ranch packing house, Santa Ysabel, California, ca. 1900. [copy print]

Frank M. Chapman, Jr. son of Frank M. and Wilhelmina Chapman, Chicago, Illinois, ca. 1891.

Studio portrait of Frank M. Chapman, Jr. son of Frank M. and Wilhelmina Chapman, Chicago, Illinois, ca. 1891.

Frank C. Chapman, Chicago, Illinois, ca. 1890

Studio portrait of Frank C. Chapman, Chicago, Illinois, ca. 1890.

Grant Chapman, Clarke Chapman, and Frank Chapman Jr., Palmetto Grove, Covina, California, ca. 1915

Left to right: Grant Chapman, Clarke Chapman, and Frank Chapman Jr., sons of Frank Marion and Wilhelmina Zillen Chapman, on a porch at Palmetto Grove, Covina, California, ca. 1915.

Grounds of Palmetto Grove, estate of Frank Marion Chapman, Covina, California, ca. 1915

Grounds of Palmetto Grove, estate of Frank Marion Chapman, Covina, California, ca. 1915.

Charles C. Chapman with his daughter Ethel and son Stanley, ca. 1905.

Remnant of a portrait photograph of Charles C. Chapman with his daughter Ethel and son Stanley, ca. 1905.

Samuel James Chapman, ca. 1948

Samuel James Chapman, brother of Charles Clarke Chapman, ca. 1948.

Zella and Grant Chapman, ca. 1914

Zella and Grant Chapman, outside a hotel, ca. 1914.