Performance analysis of a fully-connected, full-duplex CDMA ALOHA network with channel sensing and collision detection

In cases where machines having bursty data are equally likely to transmit to one another, code-division multiple-access (CDMA) ALOHA which allows for an individual "virtual channel" for each receiving station may be a better multiple-access protocol than simple ALOHA. With the use of "receiver-based code" multiple-access protocol, it is also possible for a station to listen to the channel of the intended receiver before transmission, and also abort transmission when it detects others transmitting on the same channel. This paper describes a model for a fully-connected, full duplex, and slotted CDMA ALOHA network where channel sensing and collision detection are used. The model is analyzed using a discrete time Markov chain and some numerical results are presented. For a system with a large number of users, where Markov analysis is impractical, equilibrium point analysis is used to predict the stability of the system, and estimate the throughput as well as the delay performance of the system when it is stable. Finally, a comparison is made with a simple channel sense multiple-access with collision detection (CSMA-CD) network, showing that a substantial improvement in the performance is achieved by the proposed network.published_or_final_versio

Performance comparison of single and multi-channel CSMA-CD wireless networks using equilibrium point analysis

Conference theme: Mobile Technology for the Human RaceA fundamental defect of simple CSMA-CD multiple access scheme is that messages meant for different stations can collide in the common channel. In this paper, a multi-channel form of CSMA-CD network, created by CDMA signalling, is analyzed. Its performance is shown to be superior to that of a single channel CSMA-CD network.published_or_final_versio

A model for evaluating the performance of code phase assignment CDMA LAN

Conference Theme: Wireless: Merging onto the Information SuperhighwayThis paper describes a model for a fully-connected, full duplex, and synchronized CDMA LAN where the receivers use the same maximal length sequence with different chip phase offset for receiving messages. ALOHA medium access is used. The performance of the model is analyzed by considering the succession of states the system is in as a Markov process. The behaviour of a 5-station system is shown.published_or_final_versio

Performance analysis of a CDMA ALOHA network with channel sensing

It has been shown in [1] that the throughput-delay performance of a CDMA ALOHA network, where each station is assigned a spreading code so that each effectively has its own `virtual channel', and where channel sensing and collision detection are also used, is much better than that of a simple CSMA-CD network. Such a system, however, is difficult to implement. A simpler system that may provide good performance and that is easy to implement is a CDMA ALOHA network with just channel sensing. The channel sensing can be provided by a simple correlator and threshold device. In this paper, the performance of such a network is investigated, and some comparisons are made with those of the simple CSMA-CD network and the CDMA ALOHA network with both channel sensing and collision detection. It is shown that when the CDMA-CS network is stable, its performance is not much worse than that of a CDMA-CS/CD network, and much better than that of a simple single channel CSMA-CD system.published_or_final_versio

Delay-throughput comparison of single and multi-channel slotted ALOHA networks

In networks where stations are equally likely to transmit to one another, simple slotted ALOHA is not an appropriate form of multiple access protocol as messages meant for different stations can collide in the common channel. In this paper the performance of a single channel slotted ALOHA network is compared to that of a CDMA ALOHA network where each of the station is assigned a different spreading code with which to receive messages, and thus effectively possesses its own private 'virtual channel'.published_or_final_versio

Performance of multichannel CSMA networks

International Conference on Information, Communications and Signal Processing, Singapore, 9-12 September 1997Channel sense multiple access with collision detection (CSMA/CD) is a very simple and efficient way of allowing many stations to transmit messages to a central server down a shared channel. In wireless networks, however, collision detection is difficult to implement, and in such cases CSMA alone may have to be used. In this paper, it is shown that a multichannel CSMA network can be almost as efficient in utilizing the bandwidth available to the network as an equivalent single-channel CSMA/CD network. Furthermore, multichannel CSMA networks provide better throughput and delay performance than equivalent single channel CSMA systems, even when the message generation probability and the number of stations in the network are varied.published_or_final_versio

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Candidates registered for reasonable adjustments underperform compared to other candidates in the national undergraduate Prescribing Safety Assessment: Retrospective cohort analysis (2014-2018).

AIMS: Candidates with disabilities are eligible for reasonable adjustments (RA) while undertaking the national Prescribing Safety Assessment (PSA). The PSA is a novel open-book, time-constrained, multiformat assessment that may pose challenges to candidates with dyslexia and other disabilities. METHODS: Retrospective cohort analysis of 36 140 UK candidates undertaking first-sitting of the PSA (2014-2018). RESULTS: Of the 36 140 candidates, 9.1% (3284) were registered for RA. The RA group had lower pass rates (absolute difference 1.94%, 95% confidence interval 1.01-2.87%; P < .001) and assessment scores (1.16 percentage marks, 95% confidence interval 0.83-1.48; P < .001) compared with the non-RA group. This absolute difference is small relative to overall variability. This difference persists after adjusting for confounding factors (medical school and paper), and was present for all 8 different question types. The attainment gap within each medical school is negatively correlated with the school's overall performance, both in terms of pass rate (P < .001) and scores (P = .01). The RA group were also less likely to perceive the PSA as an appropriate test, having easy to follow layout/presentation or clear/unambiguous questions, even after adjusting for candidate performance. CONCLUSION: This analysis identifies slight differences in academic performance of candidates requiring RA in a national undergraduate assessment. The study is limited by the unavailability of data on ethnicity, sex, age, diagnosis and time of diagnosis. While further research is required to determine the cause of the attainment gap, this study emphasises the need to maintain a careful review on the fairness and validity of all aspects of the assessment