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To improve the performance of wireless LAN in an error-prone radio channel, this paper proposes a new backoff counter chosen mechanism based on Noise Detection scheme, which is a method to distinguish between packets suffered by noise and collision. In this new mechanism, backoff counter can choose a lower value when the last transfer error is caused by noise, or choose a higher value when it meets a collision. The performance of the new access mechanism is evaluated by a modified Markov chain model. The numerical results show that this new mechanism can improve the saturation throughout, reduce the drop probability and shorten packet delay in a fading channel.

To improve the saturation throughput and decrease the packet drop probability of wireless LAN, this paper proposes a new backoff algorithm, named as hybrid backoff algorithm, which can be used with IEEE 802.11 DCF. In the hybrid backoff algorithm, stations increase their contention window linearly after experiencing several collisions. Based on the Markov chain model, a new method to calculate the packet delay for successfully transmitted packets considering retry limit is also derived, which can be applied to standard DCF as well. The numerical results are presented and the effects of various parameters in hybrid backoff algorithm are studied. The results show that the hybrid backoff algorithm can achieve better performance than binary exponential backoff scheme employed in DCF.

Stateful network services are a growing population of applications in the Internet, whose architecture relies more and more on COTS components. A good parallel programming model is the key to the success of network services. We argue that stage-based parallel programming model is the most suitable candidate, which takes the whole service logic as a set of stages interconnected by event queues, and in each stage, one or multiple threads reside to service messages. And we also reveal several key design principles for this model. 1) The number of stages should not be over 10 for a single CPU system. 2) For stages with blocking calls, a good estimation of the number of threads for that stage can be gotten by (call arrival rate blocking time) and moreover excessive threads give minor performance degradation. 3) To deliver message quickly, it is much better to locate this part of service in a separate nonblocking stage.

Cluster-based systems, a combination of interconnected individual computers, have become a popular solution to build the scalable and highly available Web servers. In order to reduce system outages due to aging phenomenon, software rejuvenation, a proactive fault-tolerance strategy has been introduced into cluster systems. Compared with clusters of a flat architecture, in which all the nodes share the same functions, we model and analyze the dispatcher-worker based cluster systems, which employ prediction-based rejuvenation both on the dispatcher and the worker pool. To evaluate the effects of rejuvenation, stochastic reward net models are constructed and solved by SPNP (stochastic Petri net package). Numerical results show that prediction-based software rejuvenation can significantly increase system availability and reduce the expected job loss probability.

Traditional automatic hardware testing deals mainly with stateless systems. With the rapidly increasing complexity of digital systems, software is more and more engaged in hardware testing. At present, such testing is still mainly carried out manually by engineers, consuming a lot of time and effort. In this paper, we propose an automatic hardware test system that can substantially save engineers' time in testing. It can download the test cases to the target system one by one, receive test output, verify it, and generate log files. The test case generation platform is also discussed, which supports multiple test sets.