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Mạng lưới giao thông và đánh giá hiệu suất P11
In early literature about the performance of telecommunication systems, traf®c was generally modeled as memoryless Poisson streams of packets. In these models the packet arrival processes show no time interdependence. Recent measurements on Web traf®c show that this hypothesis is wrong and that Web traf®c actually experiences what we now call long-term dependence. Long-term dependence is interesting not only because it contradicts Poisson's law, but also because it signi®cantly impacts the performance of networks....
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Mạng lưới giao thông và đánh giá hiệu suất P12
In this chapter we present some of our results concerning source models for H.261 coded VBR (variable bit rate) video. Video services have been forecasted to be a substantial portion of the traf®c on emerging broadband digital networks. Statistical source models of video traf®c are needed to design networks that delivery acceptable picture quality at minimum cost, and to control and shape the output rate of the coder. For example, one issue is deciding whether a new video connection can be admitted to a network, and the consequent determination of the bandwidth that must be allocated to the connection to...
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Mạng lưới giao thông và đánh giá hiệu suất P13
To support multimedia applications, high-speed networks must be able to provide quality-of-service (QoS) guarantees for connections with drastically different traf®c characteristics. Some of the characteristics fall beyond the conventional framework of Markov traf®c modeling. For instance, recent studies have demonstrated convincingly that there exists long-range dependence or self-similarity in packet video, which is an important traf®c component in high-speed networks. Essentially, longrange dependence cannot be captured by Markov...
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Mạng lưới giao thông và đánh giá hiệu suất P14
Recent measurements of local-area and wide-area traf®c [14, 22, 28] have shown that network traf®c exhibits variability at a wide range of scales. Such scale-invariant variability is in strong contrast to traditional models of network traf®c, which show variability at short scales but are essentially smooth at large time scales; that is, they lack long-range dependence. Since self-similarity is believed to have a signi®cant impact on network performance [2, 15, 16], understanding the
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Mạng lưới giao thông và đánh giá hiệu suất P15
Packets are the basic unit of the Internet. Yet, most user operations involve more than one packet and user experience depends on the performance of the network on a set of packets. Thus it is not surprising that in today's Internet protocol (IP) networks sets of packets are starting to be used as the basis for network operations. The goals of the network operators include improving traf®c engineering by better load balancing and moving beyond best effort services. Operating on sets of packets is more ef®cient for network elements. In fact, per packet decisions may lead to undesired or even...
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Mạng lưới giao thông và đánh giá hiệu suất P16
The traditional role of traf®c engineering is to ensure that a telecommunications network has just enough capacity to meet expected demand with adequate quality of service. A critical requirement is to understand the three-way relationship between demand, capacity, and performance, each of these being quanti®ed in appropriate units. The degree to which this is possible in a future multiservice network remains uncertain, due notably to the inherent self-similarity of traf®c and the modeling dif®culty that this implies. The purpose of the present chapter is to argue that sound traf®c engineering remains the crucial element in providing quality of...
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Mạng lưới giao thông và đánh giá hiệu suất P17
In order to help design and control the emerging high-speed communication networks, we want source traf®c models (also called offered load models or bandwidth demand models) that can be both realistically ®t to data and successfully analyzed. Many recent traf®c measurements have shown that network traf®c is quite complex, exhibiting phenomena such as heavy-tailed probability distributions, longrange dependence, and self similarity; for example, see Caceres et al. [7], Leland  et al. [23], Paxson and Floyd [24], and Crovella and Bestavros [10]. In fact, the heavy-tailed distributions...
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Mạng lưới giao thông và đánh giá hiệu suất P18
Recent measurements of local-area and wide-area traf®c [8, 28, 42] have shown that network traf®c exhibits variability at a wide range of scales. What is striking is the ubiquitousness of the phenomenon, which has been observed in diverse networking contexts, from Ethernet to ATM, LAN and WAN, compressed video, and HTTPbased WWW traf®c [8, 15, 23, 42]. Such scale-invariant variability is in strong contrast to traditional models of network traf®c, which show burstiness at short time scales but are essentially smooth at large time scales; that is, they lack long-range dependence. Since...
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Mạng lưới giao thông và đánh giá hiệu suất P19
Network support for variable bit rate (VBR) video needs to consider (1) properties of workload induced (e.g., signi®cant autocorrelations into far lags and heterogeneous marginal distributions), and (2) application-speci®c bounds on delay-jitter and statistical cell-loss probabilities. This chapter presents a quality-of-service (QoS) solution for such traf®c at each multiplexing point in a network. Heterogeneity in both-offered workload and quality-of-service requirements are considered. The network is assumed to be cell-switched with virtual circuits (VCs)...
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Mạng lưới giao thông và đánh giá hiệu suất P20
Since the statistical analysis of Ethernet local-area network (LAN) traces in Leland et al. [20], there has been signi®cant progress in developing appropriate mathematical and statistical techniques that provide a physical-based, networking-related understanding of the observed fractal-like or self-similar scaling behavior of measured data traf®c over time scales ranging from hundreds of milliseconds to seconds and beyond. These techniques explain, describe, and validate the reported large-time scaling...
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Mạng lưới giao thông và đánh giá hiệu suất P21
Since the seminal study of Leland et al. [41] on the self-similar nature of network traf®c, signi®cant advances have been made in understanding the statistical properties of measured network traf®cÐin particular, Internet workloadsÐwhy self-similar burstiness is an ubiquitous phenomenon present in diverse networking contexts, mathematical models for their description and performance analysis based on queueing, and traf®c control and resource management under self-similar traf®c conditions. Chapter 1 gives a comprehensive overview including a...
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Cảm biến trong sản xuất P1
Tham khảo tài liệu 'cảm biến trong sản xuất p1', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả
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Cảm biến trong sản xuất P2
Soft computing techniques, such as fuzzy logic, artificial neural networks and genetic algorithms, which can to some extent imitate the human brain, can possibly contribute to making the monitoring system more intelligent.
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Cảm biến trong sản xuất P3
The role of sensor systems for mechanical manufacturing is generally composed of sensing, transformation/conversion, signal processing, and decision making, as shown in Figure 1.3-1. The output of the sensor system is either given to the operator via a human-machine interface or directly utilized to control the machine. Objectives, requirements, demands, boundary conditions, signal processing, communication techniques, and the human-machine interface of the sensor system are described in this section. ...
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Cảm biến trong sản xuất P4
Sensors in production systems such as machine tools or robots may be classified into four categories (Figure 2-1). They are activated either during operation or in the set-up phase. Three types of sensors may be applied during the operation: those which measure kinematic values such as position, velocity, orientation or angular velocity, sensors which are applied to control the process in adaptive control systems, and sensors which are used to monitor the production systems and to provide diagnostic functions to assure a high availability of the systems....
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Cảm biến trong sản xuất P5
Measurement of macro-geometric characteristic variables involves the acquisition of features of geometric elements that are defined in design by dimensions and tolerances for dimensional, form, and positional deviations (Figure 3.1-1). The term ‘dimension’ refers both to the diameter of rotationally symmetrical workpieces and to distances and angles between planes and straight lines and to cone angles. The sensors used for measurement can be classified according to the method used to acquire the measured value into mechanical, electrical, and optoelectronic sensors....
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Cảm biến trong sản xuất P6
Precision measurement of structures in the micrometer and sub-micrometer ranges is becoming more and more important. Because of the never-ending miniaturization it is central to the precision of production and metrology of microelectronics and micromechanics, but also to the measurement of the size distribution of microparticles, for example, in environmental protection. A number of measuring methods are available to perform these tasks.
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Cảm biến trong sản xuất P7
In this section, the possibilities of monitoring the physical properties of machined parts are discussed. Cutting processes with geometrically defined cutting edges such as hard turning have to be distinguished from abrasive processes such as grinding. In both cases workpiece material is removed in the form of chips due to the mechanical effect of the tool on the workpiece
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Cảm biến trong sản xuất P11
Abrasive processes, which are mostly applied for achieving high accuracy and high quality of mechanical, electrical, and optical parts, can be divided into two categories, fixed abrasive processes and loose abrasive processes. In fixed abrasive processes, grinding wheels or honing stones are used as tools and the abrasives are held together with bonding material, also providing sufficient pores for chip removal. In loose abrasive processes, the individual grains are not fixed and are usually supplied together with a carrier medium. Among various types of abrasive processes, grinding is most widely applied in the industry...
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Cảm biến trong sản xuất P12
The field of laser manufacturing has been expanding rapidly in the last 20 years. New materials and new laser sources have considerably increased the potential of laser applications. This expansion has led to a necessity for higher quality and reproducibility when using lasers. To satisfy this demand, a considerable amount of research has been expended into the use of sensing for laser applications.
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Cảm biến trong sản xuất P13
The main problem is that the cause and effect of certain faults are still unclear. More research is required to find out which sensors are more adequate for monitoring specific faults so that redundant information can be minimized. Furthermore, more work has to be done to reduce the size and cost of the sensors so that a ‘complete sensing’ approach can be seriously taken into account.
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Cảm biến trong sản xuất P14
Machining system by moving the workpiece back to the machining position and performing additional machining. In other words, the dimensional error of machining can be corrected before demounting the workpiece from the machine. Concerning the VS method, applications for the measurement of practical parts and improvements in reproducibility and accuracy are currently being investigated.
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Cảm biến trong sản xuất P15
One of the most promising, generally applicable and well-adopted methods to enhance the surface performance of materials is the use of coatings. The reason is that it is extremely difficult and expensive to meet homogeneous materials having on their surface the ensemble of desired properties, such as high hardness, wear resistance, adequate stiffness, increased ductility, chemical inertness, stainlessness, controllable electrical and thermal conductivity behavior, etc.
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Cảm biến trong sản xuất P16
After mechanical manufacturing, heat treatment is mainly applied in one of the final steps of production, to adjust the workpiece properties to the later mechanical, tribological, and corrosive load. The lifetime of these components is defined through the raw material used, the construction geometries, and the quality of heat treatment. The great demands for quality control and the documentation of process parameters have led to the increasing importance of sensor applications in heat treatment.
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Cảm biến trong sản xuất P17
Sensors used for ultra-precision machines include position and velocity sensors, which are mostly part of a control loop, and for some special applications acceleration sensors for active vibration control, and thermal sensors for thermal error compensation [1]. An essential component of each positional feedback loop in a machine tool is the displacement measurement system for detecting the actual position of moving machine parts. It is the performance of such measurement systems that limits the accuracy of machine tools and hence directly affects the quality of the machined part...
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Cảm biến trong sản xuất P18
Several developments made high-speed cutting (HSC) possible. HSC became an important trend in machining (Figure 5.2-1). The development of tool materials was a prerequisite to higher wear resistance under high temperatures. Up to the 1960s, the dualism of hardness and wear resistance on the one hand and toughness on the other were dominant limiting factors in tool materials. The functional separation of the ability to carry static and dynamic loads and of tribological functions was established by introducing coatings....
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Cảm biến trong sản xuất P19
Wear may be critical and therefore wear sensors are of interest. They should be able to determine the end of tool life reliably. There are several approaches, as discussed in Section 4.3. One of the main accuracy problems with automated machine tools is derived from the thermal stability. The temperature field in the machine structure changes according to the effect of several heat sources. The most important heat sources are very often the spindle bearings.
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Cảm biến trong sản xuất P20
Ecological issues are assuming increasing importance in many areas of the economy as a result of legislation and growing public awareness. Manufacturing, characterized by a chain of resource-intensive processes and by large quantities of waste materials and emissions, is frequently the focus of interest.
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Tín hiệu trong các mạng viễn thông P1
There are two types of communication networks: circuit-switched networks and packed-switched networks. In circuit-switched networks, a dedicated physical circuit between the calling and called party is set up at the start of a call, and released when the call has ended. Telephone networks are circuit-switched networks. Today, these networks are used for speech and other purposes, such as facsimile, and are usually referred to as telecommunication networks.
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Tín hiệu trong các mạng viễn thông P2
This section presents a brief historical outline of signaling. The earliest telephone exchanges were “manual” switchboards, in which all calls were set up and taken down by operators. Signaling between subscribers and operators was limited to tivtging. To make a call, the subscriber would send a ringing signal. This alerted an operator, who would connect her telephone to the calling line, and ask for the called number. The operator then would connect her telephone to the called line, and ring the line. After answer by the called party, the operator would establish the connection...
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