Selection of timed token protocol parameters to guarantee messagedeadlines

Networks that use the timed token protocol (such as the 100 Mbit/s FDDI network) are well suited for real-time applications because they guarantee, to each node, an average bandwidth and a bounded access time to the communication network. This guarantee is necessary but not sufficient for the timely delivery of deadline-constrained messages; protocol parameters must be carefully selected to ensure that these messages meet their deadlines. This paper addresses the issue of selecting the protocol parameters TTRT (target token rotation time) and the synchronous capacities assigned to each node. The objective is to guarantee that each synchronous message is transmitted before its deadline. An upper bound is derived on the worst case achievable utilization (WCAU) of any parameter selection scheme. The WCAU of a scheme is defined as the maximum utilization U such that the scheme guarantees all synchronous messages as long as their utilization is less than U. An algorithm for selecting the above parameters is proposed, The algorithm is shown to have a WCAU that is very close to the upper bound     

An optimal synchronous bandwidth allocation scheme for guaranteeingsynchronous message deadlines with the timed-token MAC protocol

This paper investigates the inherent timing properties of the timed-token medium access control (MAC) protocol necessary to guarantee synchronous message deadlines in a timed token ring network such as, fiber distributed data interface (FDDI), where the timed-token MAC protocol is employed. As a result, an exact upper bound, tighter than previously published, on the elapse time between any number of successive token arrivals at a particular node has been derived. Based on the exact protocol timing property, an optimal synchronous bandwidth allocation (SBA) scheme named enhanced MCA (EMCA) for guaranteeing synchronous messages with deadlines equal to periods in length is proposed. This scheme is an enhancement on the previously published MCA scheme     

一个可变参数的内部消息处理系统

使用消息通讯是控制程序耦合的一种有效手段，在使用可视化工具Delphi开发Windows应用程序过程中，利用Delphi内嵌的消息处理系统在程序模块间传递内部消息(指不涉及外部程序和系统资源的消息)时，存在两个不足之处：消息在Delphi中传递过程过于复杂；格式受到windoWs标准消息格式的限制，不能满足参数形式多变的需求。为了弥补上述不足，设计了一个专门处理内部消息的消息处理系统，消息不经过Windows和delphi的分发机制，缩短了传递过程，另外消息的参数设计为可变类型的动态数组，可以满足各种形式参数的需要。由于将系统封装为一套控件，因此还具有使用方便的优点。     

线性令牌传递网络中消息实时保证概率分析

本文针对线性令牌传递网络,提出了一种网络带宽分配方法和基于实时保证概率的网络性能分析方法,导出了消息严格实是的保证概率表达式,并给出了实用的近似算法。     

Bayes estimators of exponential parameters utilizing a guessed guarantee with specified confidence

This paper proposes a procedure to modify the conjugate prior for the parameters of an exponential distribution in the light of an available guessed guarantee μ₀ with specified confidence c (0<c<1). The Bayes estimators under modified priors have been obtained. The proposed estimators have been compared with Bayes estimators under conjugate prior, shrinkage estimators and maximum likelihood estimators on the basis of a simulation study.     

浅谈现场认证测试仪的精度保证

在测试六类及以上布线应用时，相关标准规定测试仪表的精度必须至少达到Ⅲ级，认证七类链路必须达到Ⅳ级。线缆认证测试仪的测试精度不仅与测试仪主机设计与制造工艺有关，还与测试适配器的设计与制造工艺有关。在日常使用过程中，要使仪器始终保持厂家承诺的测试精度，则必须考虑测试仪表参数漂移与适配连接器磨损的补偿问题。     

Determination of InP HEMT noise parameters and S-parameters to 60GHz

A millimeterwave experimental technique is described for directly determining the noise parameters and scattering parameters of V-band InP HEMT's. The parameters are suitable for the design of monolithic millimeterwave integrated circuits since they represent the InP HEMT as it would appear in the monolithic environment. The method relies on careful characterization of the measurement system and the InP HEMT packages or test fixtures. Results are provided for an InP HEMT with 1.37 dB minimum noise figure and a maximum stable gain of 12.74 dB at 57 GHz. In addition, it is shown that noise parameters measured between 2 GHz and 26 GHz can be extrapolated to 60 GHz, and that consistent S-parameters can be obtained for InP HEMT's in precision packages and test fixtures     

Determination of gyrotron wave beam parameters

The output radiation of a gyrotron has the form close to the Gaussian beam. Knowing parameters of the wave beam along its way from a gyrotron to an object permits one to optimize the transmission losses. By this, the structure of the wave beam can be recorded with an infrared camera in several cross-sections of the beam. Real gyrotron wave beam is approximated with high precision by the Gaussian beam.     

How to signcrypt a message to designated group

In an open network environment, the protection of group communication is a crucial problem. In this article, a novel broadcast group-oriented signcryption scheme is presented for group communication scenarios in distributed networks. Anyone in this scheme can signcrypt a message and distribute it to a designated group, and any member in the receiving group can unsigncrypt the ciphertext. The ciphertext and public key in the scheme are of constant size. In addition, this new scheme offers public verification of the ciphertext. This property is very important to the large-scale group communication since the gateway can filter the incorrect ciphertext and alleviate the receiver's workload. Finally, a proof in the random oracle model is given to show that the scheme is secure against chosen ciphertext attack and existential forgery.     

简单加固无法保证网络安全

进入2003年的国外网络安全动态值得我们倍加关注。美国白宫网络空间安全通讯基础设施防护办公室主任Marcus H.Sachs,在去年6月的国际计算机网络安全技术论坛(FIRST)年会上,就十几年来美国的通讯设施及网络安全结合9·11事件作出了深刻的反思。本文借助《加固美国通讯基础设施的安全防务》这份特邀报告,来分析美国等国家当前对网络安全的新概念。 首先,这份报告提到一个历史的转折点,是1997年美国国防部作的一个有名的“合法接收者”的试验。该试验对美国国防部的一批主机进行网上攻击,居然发现在短时间内有多台主机的超级口令被破,并…     

Approximation to expected message delay of polling systems

The author discusses polling systems with single message queues and asymmetric exponential arrivals. A simple method is presented to obtain both an upper bound on the cycle length and an approximation to the expected message delay of these polling systems. Numerical examples show that the approximation obtains results which are very close to the simulation results     

Judicious use of redundant transmissions in multichannel ALOHAnetworks with deadlines

This paper shows how to improve the classic multichannel slotted ALOHA protocols by judiciously using redundant transmissions. The focus is on user-oriented requirements: a deadline along with a permissible probability of failing to meet it. Subject to satisfying those, maximization of capacity is the optimization goal. When there is no success/failure feedback prior to the deadline, the use of information dispersal with some redundancy provided by error-correcting codes for the data in conjunction with a replicated, separately transmitted synchronization preamble, is proposed. It is shown to sharply reduce the overhead resulting from the use of shorter packets and to significantly increase the capacity. When the deadline permits several transmission-feedback rounds, we propose a novel replication-based retransmission policy: all attempts except the final one entail the transmission of a single or very few copies, and a larger number of copies are transmitted in the final attempt. This sharply increases channel capacity, even with a single transmitter per station. The proposed approaches are particularly suitable for high-bandwidth satellites with on-board processing     

如何保证光缆的使用寿命

~~如何保证光缆的使用寿命@汪元桥$湖南电广传媒股份有限公司网络分公司     

精心备战 大展雄风——广东省迎亚运无线电安全应急保障技术演练纪实

为确保广州亚运会无线电安全．6月21日至24日,在广东省无线电管理办公室的统一部署和领导下．广东省无线电监测站、广州市科技和信息化局、佛山市信息产业局联合在佛山市三水区举行了”迎亚运2010年全省无线电安全应急保障技术演练”活动。     

Determination of material parameters from regions close to the collector using electron beam-induced current

The conventional method of extracting the minority carrier diffusion length using the electron beam-induced current (EBIC) technique requires that the electron beam be placed at region more than two diffusion lengths away from the collector. The EBIC signals obtained under this condition usually has low signal to noise ratio. In addition, the true diffusion length of the sample is initially unknown and hence it is difficult to estimate how close the beam can be placed from the collector. To overcome all these difficulties, a new method of extracting minority carrier diffusion length from the EBIC signal is proposed. It is shown that this method can be applied to EBIC signals obtained from regions close to the collector. It is also shown that the surface recombination velocity of the sample can also be obtained using this method. This theory is verified using EBIC data generated from a device simulation software.     

基于RTT的TCP流带宽公平性保障机制

TCP端到端的拥塞控制机制使得TCP连接获得的瓶颈带宽反比于RTT(数据包往返时间)。为了缓解TCP对于RTT较小流的偏向,区分服务的流量调节机制在RTT较小的流取得目标速率且获得多余资源的情况下可以确保RTT较大流不至于饥饿。现有的基于RTT的流量调节机制在网络拥塞程度较轻时非常有效,但是当网络拥塞程度较重时,由于对RTT较大流的过分保护而导致RTT较小流饥饿。因此,通过引进自适应的思想提出了改进方法,其主要思想就是根据网络的拥塞程度自适应地调整对RTT较大流的保护程度。大量的仿真试验表明所提的机制能有效保障TCP流的带宽公平性并且比现有的方法具有更好的强壮性。     

Determination of equivalent network parameters of short-gate-lengthmodulation-doped field-effect transistors

The small-signal intrinsic Y-parameters of millimeter-wave MODFETs are examined. These parameters were extracted from S-parameter data after removal of parasitics through a method of successive Z-to-Y and Y-to-Z transformations. This approach has yielded accurate values of all the parameters of an equivalent network model for the MODFETs tested, with the exception of some uncertainties in the determination of the intrinsic Y₂₁ phase-delay time constant. It is shown that accurate determination of the intrinsic carrier transit time (τ) is possible through the unity extrapolation of the intrinsic current gain ∥h₂₁∥, and this yields the related carrier average velocity (ν_av). Results obtained from three 0.25-μm-gate-length MODFETs, based on different heterostructures, and results obtained from a 0.15-μm-gate-length lattice-matched structure are presented     

Determination of physical parameters of diffusion and drift transistors

The dynamic properties of drift and diffussion transistors are studied under both low- and high-level injection conditions in terms of the excess carrier charge in the base region. Subject to the assumptions of effectively one-dimensional device geometry with exponential impurity grading and collector conductivity much greater than that of the base, methods of determination of the main physical and high-frequency equivalent-circuit parameters of transistors are presented, utilizing the bias dependence of excess carrier charge in the base and space charge in the collector-depletion region. It is shown that measurement of the base transit time and the junction capacitances under low-level injection conditions enables the following physical parameters to be determined: base field parameterm = DeltaV/(kT/q), base width, base impurity distribution, emitter area, collector-depletion layer width. All the measurements are carried out at relatively low frequencies. Solutions for the base charge distribution, and hence charge-defined transit time, have been derived for the cases of exponential and erfc impurity grading under high-level injection conditions, assuming one-dimensional device geometry, constant base cross-sectional area, andDandmicroindependent of injection level. Experimental results have been found to differ greatly from the theoretical expectations based on these assumptions, the drift transistor showing greater and the diffusion transistor less than the predicted dependence on injection level. Explanations for the observed effects have been put forward in terms of reduction of emitter area and ofDandmicroat high injection levels.     

Multisim仿真软件中三极管模型参数的确定

Multisim仿真软件元件库中找不到特定型号三极管时,常常需要新建三极管模型并确定模型参数。从电流特性、输出特性等七个方面分析了三极管G-P模型参数的物理意义,并且通过实例说明了通常情况下三极管模型参数的估算方法。对于模型参数的准确性通过实验的方法加以验证,表明这种估算方法具有较好的仿真度和实用性。     

邻近效应校正函数及参数确定

对高斯圆束矢量扫描作图邻近效应校正的函数进行了深入探讨,并在此基础上确定了函数中的各参数的算法。