Asynchronous low-power MACs based on preamble sampling are considered some of the most promising low-power protocols for wireless sensor networks. Although many such protocols have been suggested, they suffer from the inefficiency of essential features such as a means for preamble collision avoidance. We suggest a multiple preamble transmission scheme rather than the previous exclusive method in which only one sender can transmit a series of preambles to prevent preamble collisions and other neighbors should wait until the sender finishes all preamble and data transmissions. In the proposed scheme, the preamble interval that was stationary in previous works becomes variable. Since the sampling duration must be longer than the preamble interval, we calculate its most appropriate value from an analysis of preamble interval based on the renewal theory. Our simulation and experiment demonstrate the correctness of the analysis and the performance improvement.

Redundant arrays of inexpensive disks (RAID) offer fault tolerance against disk failures. However a storage system having more disks suffers from less reliability and performance. A RAID architecture tolerating multiple disk failures shows severe performance degradation in comparison to the RAID Level 5 due to the complexity of implementation. We present a new RAID architecture that tolerates at least three disk failures and offers similar throughout to the RAID Level 5. We call it the hierarchical RAID, which is hierarchically composed of RAID Levels. Furthermore, we formally introduce the mean-time-to-data-loss (MTTDL) of traditional RAID and the hierarchical RAID using Markov process for detailed comparison.

Most distributed shared memory systems use point-to-point networks in conjunction with directory-based cache coherence protocols. A cache invalidation transaction generates a number of unicast invalidation messages and as many acknowledgment messages. This results in heavy network traffic, high latency, and high occupancy at home nodes. This paper introduces a fast cache invalidation method, called collective cache invalidation (CCI), and its simple and cost effect implementation method, called virtual bus based collective cache invalidation (VCCI). The simulation results show that we can reduce the total network traffic up to 45% and the overall execution time up to 11% by VCCI. The proposed method keeps the system scalable-the growth rate of the implementation cost if O(N/spl radic/N), and, VCCI can reduce the complexity of coherence protocol and make directory controllers simple since it does not require acknowledgment messages.

This paper presents a mesh with virtual buses as the bandwidth-efficient implementation of the mesh with multiple broadcasting on which many computational problems can be solved with reduced time complexity. The new system provides a low latency and high bandwidth communication mechanism without the disadvantage of the dual network approach: the possible bandwidth waste. The virtual buses are built using those communication links of the base point-to-point mesh network only upon request. Even if bus requests are not made, the full network bandwidth is harnessed by the point-to-point communication. On the contrary, the bandwidth assigned to the conventional real buses will be wasted when there is little bus request. This paper introduces a wormhole router design equipped with the virtual bus functions, describes the connections among these routers, and presents various virtual bus transactions. We prove the effectiveness of the virtual bus by showing that a representative semigroup computation can be solved very efficiently on it: a vector maxima finding is accelerated by 2.66 times in the experiment. We also explore the network characteristics by the distribution-driven simulation of the system. These evaluations convinced us that the mesh with virtual buses is a promising approach to low latency and high bandwidth communication which has applications in many parallel computations such as in parallel simulations, computer graphics, real time systems, and many others.