Performance Evaluation of a Modified Carrier Sense Multiple Access with Collision Detection Protocol

Using Carrier Sense Multiple Access with Collision Detection (CSMMCD), Ethernet Local Area Networks (LANs) suffers from capture effect in packet loss. As a result of capture effect, some nodes may be locked-out of using the medium for a period of time. Hence, CSMAICD based Ethernet is unsuitable for real-time multimedia traffic. It does not guarantee delay bound, behaves poorly under heavy load conditions. To overcome these shortcomings and enhance performance of CSMAKD based LAN, three new concepts are added to the conventional CSMMCD. Firstly, each node in the LAN has a finite buffer. A node competes for access to the medium after its buffer is full. It will transmit all packets in the buffer if access is permitted. To minimize the waiting delay of packets in the buffer prior to transmission, a time-out period is set, beyond which a node tries to transmit considering its buffer is full.Due to buffer, the number of nodes trying to transmit at a time is reduced, thereby collision rate is reduced. Capture effect, locked-out probability, bandwidth loss and backoff delays are also reduced. To support all types of traffic (mainly real-time traffic), the optimum buffer size obtained is 10 packetsibuffer. Using this buffer, multimedia traffic can be sent in a streamed fashion within a delay bound. Secondly, the maximum retransmission attempt limit and backoff limit are reduced to 10 and 8 times respectively to guarantee a tolerable delay for multimedia applications. A new special-jamming signal is introduced. It gives transmission priority to the node that already has finished its maximum retransmission attempt. This prevents packet loss and quality degradation of received normal data traffic and multimedia traffic. The final one is the priority scheduler, which is activated when multiple nodes send the special-jamming signal at a time. It gives permission to the node having either the lowest timestamp or the smallest source address (SA) to transmit while other nodes wait until their access is permitted accordingly. The proposed protocol is based on bus topology for a single channel LAN. Throughput, transmission efficiency, average delay and percentage of collision of the proposed network is evaluated against number of nodes, bus length and offered load within two environments, i.e. Fast Ethernet and Gigabit Ethernet. The results show significant performance improvement. Throughput, transmission efficiency are increased more than 10% in average. On the other hand, average delay and percentage of collision are reduced to less than 2 ms and 3.5% respectively compared to the conventional CSMA/CD based LAN

Observation of prolonged coherence time of the collective spin wave of atomic ensemble in a paraffin coated Rb vapor cell

We report a prolonged coherence time of the collective spin wave of a thermal 87Rb atomic ensemble in a paraffin coated cell. The spin wave is prepared through a stimulated Raman Process. The long coherence time time is achieved by prolonging the lifetime of the spins with paraffin coating and minimize dephasing with optimal experimental configuration. The observation of the long time delayed-stimulated Stokes signal in the writing process suggests the prolonged lifetime of the prepared spins; a direct measurement of the decay of anti-Stokes signal in the reading process shows the coherence time is up to 300 us after minimizing dephasing. This is one hundred times longer than the reported coherence time in the similar experiments in thermal atomic ensembles based on the Duan-Lukin-Cirac-Zoller (DLCZ) and its improved protocols. This prolonged coherence time sets the upper limit of the memory time in quantum repeaters based on such protocols, which is crucial for the realization of long-distance quantum communication. The previous reported fluorescence background in the writing process due to collision in a sample cell with buffer gas is also reduced in a cell without buffer gas.Comment: 4 pages, 4 figure

Throughput and Collision Analysis of Multi-Channel Multi-Stage Spectrum Sensing Algorithms

Multi-stage sensing is a novel concept that refers to a general class of spectrum sensing algorithms that divide the sensing process into a number of sequential stages. The number of sensing stages and the sensing technique per stage can be used to optimize performance with respect to secondary user throughput and the collision probability between primary and secondary users. So far, the impact of multi-stage sensing on network throughput and collision probability for a realistic network model is relatively unexplored. Therefore, we present the first analytical framework which enables performance evaluation of different multi-channel multi-stage spectrum sensing algorithms for Opportunistic Spectrum Access networks. The contribution of our work lies in studying the effect of the following parameters on performance: number of sensing stages, physical layer sensing techniques and durations per each stage, single and parallel channel sensing and access, number of available channels, primary and secondary user traffic, buffering of incoming secondary user traffic, as well as MAC layer sensing algorithms. Analyzed performance metrics include the average secondary user throughput and the average collision probability between primary and secondary users. Our results show that when the probability of primary user mis-detection is constrained, the performance of multi-stage sensing is, in most cases, superior to the single stage sensing counterpart. Besides, prolonged channel observation at the first stage of sensing decreases the collision probability considerably, while keeping the throughput at an acceptable level. Finally, in realistic primary user traffic scenarios, using two stages of sensing provides a good balance between secondary users throughput and collision probability while meeting successful detection constraints subjected by Opportunistic Spectrum Access communication

Collisional effects in the formation of cold guided beams of polar molecules

High fluxes of cold polar molecules are efficiently produced by electric guiding and velocity filtering. Here, we investigate different aspects of the beam formation. Variations of the source parameters such as density and temperature result in characteristic changes in the guided beam. These are observed in the velocity distribution of the guided molecules as well as in the dependence of the signal of guided molecules on the trapping electric field. A model taking into account velocity-dependent collisional losses of cold molecules in the region close to the nozzle accurately reproduces this behavior. This clarifies an open question on the parameter dependence of the detected signal and gives a more detailed understanding of the velocity filtering and guiding process

Design and Performance of the Data Acquisition System for the NA61/SHINE Experiment at CERN

This paper describes the hardware, firmware and software systems used in data acquisition for the NA61/SHINE experiment at the CERN SPS accelerator. Special emphasis is given to the design parameters of the readout electronics for the 40m^3 volume Time Projection Chamber detectors, as these give the largest contribution to event data among all the subdetectors: events consisting of 8bit ADC values from 256 timeslices of 200k electronic channels are to be read out with ~100Hz rate. The data acquisition system is organized in "push-data mode", i.e. local systems transmit data asynchronously. Techniques of solving subevent synchronization are also discussed.Comment: 14 pages, 13 figure

FA-SIFT study of the reactions of H₃O⁺∙(H₂O)n (n=0, 1, 2), NO⁺ and O₂˙⁺ with the terpenoid aldehydes citral, citronellal and myrtenal and their alcohol analogues

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Mobile Networking

We point out the different performance problems that need to be addressed when considering mobility in IP networks. We also define the reference architecture and present a framework to classify the different solutions for mobility management in IP networks. The performance of the major candidate micro-mobility solutions is evaluated for both real-time (UDP) and data (TCP) traffic through simulation and by means of an analytical model. Using these models we compare the performance of different mobility management schemes for different data and real-time services and the network resources that are needed for it. We point out the problems of TCP in wireless environments and review some proposed enhancements to TCP that aim at improving TCP performance. We make a detailed study of how some of micro-mobility protocols namely Cellular IP, Hawaii and Hierarchical Mobile IP affect the behavior of TCP and their interaction with the MAC layer. We investigate the impact of handoffs on TCP by means of simulation traces that show the evolution of segments and acknowledgments during handoffs.Publicad