A Novel Approach to Analyzing the Occupancy of the Geo/Geo/∞ Queueing System

Discrete time queueing systems have been successfully used to model packet queues in computer and digital communications systems. In this article we introduce a novel approach to analyzing the Geo^X/Geo^Y/∞ queueing system, which is suitable for modelling high speed computer and communications systems. The approach is based on characterizing the system by a functional equation which is then solved recursively to give identical results to those obtained using other approaches.     

最新MDAC中的安全特征

本文描述数据访问组件MDAC(Microsoft Data Access Components)的结构与编程接口,分析了MDAC缓冲区溢出安全漏洞的原因,最后阐述了最新MDAC2.8的安全特征.     

无线收、发信机的设计

主要讲述移动通信中收、发信机的工作原理、基本结构和组成.在给出收、发信机原理框图的同时,着重分析了锁相环路的设计.     

Seeking the truth about ad hoc join costs

In this paper, we re-examine the results of prior work on methods for computing ad hoc joins. We develop a detailed cost model for predicting join algorithm performance, and we use the model to develop cost formulas for the major ad hoc join methods found in the relational database literature. We show that various pieces of “common wisdom” about join algorithm performance fail to hold up when analyzed carefully, and we use our detailed cost model to derive op timal buffer allocation schemes for each of the join methods examined here. We show that optimizing their buffer allocations can lead to large performance improvements, e.g., as much as a 400% improvement in some cases. We also validate our cost model's predictions by measuring an actual implementation of each join algorithm considered. The results of this work should be directly useful to implementors of relational query optimizers and query processing systems. Edited by M. Adiba. Received May 1993 / Accepted April 1996     

An accurate and practical buffer allocation model for the buffer cache based on marginal gains

A new dynamic buffer allocation strategy based on the notion of marginal gains is presented for the buffer cache that is used by the operating system to store frequently accessed disk blocks in main memory, and the performance of the proposed strategy is compared with those of previous allocation strategies. In the proposed strategy, marginal gain values are predicted by exploiting functions that approximate the expected number of buffer hits per unit time. Experimental results from both trace-driven simulation and an actual implementation in the FreeBSD operating system show that the proposed strategy accurately predicts the marginal gain values for various workloads resulting in significantly improved buffer hit ratios.     

局域网服务器缓冲区容量的Petri网设计方法

本文在分析局域网服务器信息缓冲区、数据包接受和发送缓冲区的相互关系基础上提出一种新的局域网服务器缓冲区设计方法─—广义随机Ｐｅｔｒｉ网方法，并给出了局域网服务器信息缓冲区的广义随机Ｐｅｔｒｉ网模型。该设计方法理论完备、严谨实用，且不再局限于追求某一边界值（上限或下限），并且避免了排队理论处理多级服务的繁琐公式。最后本文用一例题说明了这种方法的应用。     

Performance of in-call buffer/window allocation schemes for short intermittent file transfers over broadband packet networks

While static open loop rate controls may be adequate for handling continuous bit rate (CBR) traffic, relatively smooth data traffic, and relatively low speed bursty data traffic over broadband integrated networks, high speed bursty data sources need more dynamic controls. Burst level resource allocation is one such dynamic control. Potential benefits and other issues for burst level resource parameter negotiations for bursty data traffic over high speed wide area packet networks have been discussed earlier.^1–6 A detailed analysis of an adaptive buffer/window negotiation scheme for long file transfers using these concepts is presented in Reference 1. In this paper we discuss two burst level buffer/window negotiation schemes for short intermittent file transfers, focusing on the specific needs of such traffic streams. We develop closed network of queues models to reflect the behaviour of the proposed schemes. These models, while being simple, capture essential details of the control schemes. Under fairly general assumptions, the resulting network of queues is of product form and can be analysed using the mean value analysis. We use such an analysis to compare the proposed schemes and to determine appropriate sizes of trunk buffers to achieve the desired balance between bandwidth utilization and file transfer delay. The effects of other parameters on the performance of these schemes as well as on the buffer sizing rules are also discussed. Burst level (in-call) parameter negotiation may be carried out by the end system with the network elements or by an interface system (access controller) with the broadband network elements. We discuss implications of this location as well as the needed protocol features. Finally, the service discrimination capabilities desired at the trunk controllers in switching nodes are briefly discussed.     

兰尖铁矿边帮控制爆破的实践

兰尖铁矿需要用预裂爆破、光面爆破、刷帮爆破等不同的控制爆破技术来减小爆破震动及后冲对固定边坡的危害。文中概述了这些控制爆破的设计及施工中的关键问题 ;对预裂爆破布孔参数和单孔药量的选取以及缓冲孔的参数、超深、装药量和主爆孔药量与预裂爆破的关系进行了较详细的阐述。此外 ,讨论了光面爆破、刷帮爆破的布孔参数、装药量和装药结构。作者认为 ,预裂爆破可作为降震措施 ;在破碎带中可用光面爆破来保证边坡的平整和稳定 ;特殊情况下才采用刷帮爆破来清理边坡     

Monomer recycling for vulcanized silicone rubbers in the form of cyclosiloxane monomers. Role of acid buffers

The KOH-catalyzed depolymerization of vulcanized silicone rubbers to reproduce cyclosiloxane monomers was studied. First, the depolymerization was carried out in toluene with varying amounts of KOH to find that the yield of monomers first increased and then decreased with the increment of KOH, the highest yield was 65% at the molar ratio KOH/(Si–O) UNITS=0.08. At the molar ratio 0.13 in the absence of solvent, the monomers yield was 46%. However, when an acid buffer such as KH₂PO₄ and KCOOC₆H₄COOH was added after the KOH-catalyzed depolymerization was over, the product yield was increased remarkably to more than 80%.     

基于VME总线的高速A／D缓冲系统的研制

本文介绍一种基于ＶＭＥ总线的高速Ａ／Ｄ缓冲系统的原理和结构。着重阐述了高速数据采集技术，高速ＦＩＦＯ缓冲技术，存储器总线切换技术，ＶＭＥ及ＰＣ／ＩＳＡ总线接口技术在本系统中的成功应用。