计算机科学与技术专业无线局域网毕业论文外文文献翻译及原文

中英文对照外文翻译(文档含英文原文和中文翻译)PUTER NETWORKSDATE COMMUNICATIONSThe end equipment which either generates the digital information for transmission or uses the received digital data can be computer ,printers ,keyboards, CRTs, and so on. This equipment generally manipulates digital information internally in word units—all the bits that make up a word in a particular piece of equipment are transferred in parallel. Digital data, when transmitted, are in serial form. Parallel transmission of an 8-bit word require eight pairs of transmission lines—not at all cost-effective. Data terminal (DTE) is a general phrase encompassing all of the circuitry necessary to perform parallel-to-serial and serial-to-parallel conversions for transmission and reception respectively and for data link management. The UART (Universal Asynchronous Receiver/Transmitter) and USART (Universal Asynchronous/Asynchronous Receiver/Transmitter) are the devices that perform the parallel-to-serial and serial-to-parallel conversions. The primary DTE includes a line control unit (LCU or LinCo) which controls the flow of information in a multipoint data link system. A station controller (STACO) is the corresponding that belonged to the subscriber in a data link system. Between the DTEs, starting with the modems, was communications equipment owned and maintained by Telco property.Data communications equipment (DCE) accepts the serial data stream from the DTE and converts it to some form of analog signal suitable for transmission on voice-grade lined. At the receive end, the DCE performs the reverse function of converting the received analog signal to a serial digital data stream. The simplest form of DCE is a modem (modulator/demodulator) or data set. At the transmit end, the modem can be considered a form of digital-to-analog converter, while at the receive end, it can considered a form of analog-to-digital converter. The most common of modulation by modems are frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). This is a typically data transmission mode using the analog telephone lines. If you transmit data by digital channel (sometimes it is called “Digital T-carrier”), a pulse Code Modulation (PCM) equipment must be used. A microwave transmission system can also be used for the data communication. Finally, you can use the satellite communication system for data transmission.If the cables and signal levels used to interconnect the DTE and DCE were left unregulated, the variations generated would probably be proportional to the number of manufacturers. Electronics industries Association (EIA),an organization of manufactures with establishing the DTE and modem. This is a 25-pincable whose pins have designated functions and specified signal levels. The RS-232C is anticipated to be replaced by an update standard.2.ARCHITECTURE OF COMPUTER NETWORKSComputer network is a complex consisting of two or more conned computing units, it is used for the purpose of data communication and resource resource sharing. Design of a network and its logical structure should comply with a set of design principles, including the organization of functions and the description of data formats and procedure. This is the network architecture and so called as a set of layers and protocols, because the architecture is a layer-based.In the next two sections we will discuss two important network architectures, the OSI reference model and the TCP/IP reference model.1.The OSI Reference ModelThe OSI model is shown in Fig.14-2(minus the physical medium). This model is based on a proposal developed by the International Standards Organizations (OSI) as the first step toward international standardization of the protocols used in the various layers. The model is called the ISO OSI (Open System Interconnection) Reference Model because it deals with connecting open systems--that is, systems that are open for communication with other systems, We will usually just call it the OSI model for short.The OSI model has seven has seven layers. Note that the OSI model itself is not a network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do. However , However, ISO has also produced standards for all the layers, although these are not part of the reference model itself. Each one has been published as a separate international standard.2.The TCP/IP Reference ModelThe TCP/IP reference model is an early transport protocol which was designed by the US Department of Defence (DOD) around in 1978. It is often claimed that it gave rise the OSI “connectionless”mode of operation. TCP/IP is still usedextensively and is called as a industrial standard of internet work in fact, TCP/IP has two parts: TCP and IP. TCP means it is on the transport layer and IP means it is on the network layer separately.1.There are two end-to-end protocols in the transport layer, one of which is TCP (Transmission Control Protocol) , another is UDP (User Datagram Protocol). TCP is a connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. UDP is an unreliable, connectionless protocol for application that do not want TCP’s sequencing of flows control flow control and wish to provide their own.2.The network layer defines an official packet format and protocol called IP (Internet protocol). The job of the network layer is to deliver IP packets where they are supposed to go.The TCP/IP Reference Model is shown in Fig.14.3. On top of the transport layer is the application layer, It contains all the higher-level protocols. The early ones included virtual terminal (TELNET), file transfer (FTP), electronic mail (SMTP) and domain name service(DNS).3.WIDE AREA NETWORKA wide area network, or WAN, spans a large geographical area, often a country or continent . It contains a collection of machines intended for running user (i. e. , application) programs. We will follow traditional usage and call these machines hosts. By a communication subnet, or just subnet for short. The job of the subnet is to carry messages from host to host, just as the telephone system carries words from speaker to listener. By separating the pure communication aspects of the network (the subnet) from the application aspects (the hosts), the complete network design is greatly simplified. Relation between hosts and the subnet is shown in Fig.14-4.One of many methods that can be used to categorize wide area networks is with respect to the flow of information on a transmission facility. If we use this method to categorize wide area networks, we can group them into three basic types: circuit switched, leased line and packet switched.1.CIRCUIT SWITCHED NETWORKSThe most popular type of network and the one almost all readers use on a daily basis is a circuit switched network. The public switched telephone network, however,is not limited to the telephone company, By purchasing appropriate switching equipment, any organization can construct their own internal circuit switched network and, if desired, provide one or more interfaces to the public switched network to allow voice and data transmission to flow between the public network and their private internal network2．LEASED LINE NETWORKSThis is a dedicated network connected by leased lines. Leased line is a communications line reserved for the exclusive use of a leasing customer without inter-exchange switching arrangements. Leased or private lines are dedicated to the user. This advantage is that the terminal or computer is a always physically connected to the line. Very short response times are met with service.3.PACKET SWITCHING NETWORKSA packet network is constructed through the use of equipment that assembles and disassembles packets, equipment that routes packet, and transmission facilities used to route packets from the originator to the destination device. Some types of data terminal equipment (DTE) can create their own packets, while other types of DTE require the conversion of their protocol into packets through the use of a packet assembler / disassemble (PAD). Packets are routed through the network by packet switches. Packet switches examine the destination of packets as they flow through the network and transfer the packets onto trunks interconnecting switches based upon the packet destination destination and network activity.Many older pubic networks follow a standard called X.25. It was developed during 1970s by CCITT to provide an interface between public packet-switched network and their customers.CCITT Recommendation X.25 controls the access from a packet mode DTE, such as a terminal device or computer system capable of forming packets, to the DCE at a packet mode. CCITT Recommendation X.28 controls the interface between non-packet mode devices that cannot interface between the PAD and the host computer. CCITT Recommendation X.3 specifies the parameter settings on the PAD and X.75 specifies the interface between packet network.4.LOCAL AREA NETWORKLocal area data network , normally referred to simply as local area network or LANs, are used to interconnect distributed communities of computer-based DTEs located within a building or localized group of building. For example, a LAN may be used to interconnect workstations distributed around offices within a single building or a group of buildings such as a university campus. Alternatively, it may be complex. Since all the equipment is located within a single establishment, however, LANs are normally installed and maintained by the organization. Hence they are also referred to as private data networks.The main difference between a communication path established using a LAN and a connection made through a public data network is that a LAN normally offers much higher date transmission rates because of the relatively short physical separations involved. In the context of the ISO Reference Model for OSI, however, this difference manifests itself only at the lower network dependent layers. In many instances the higher protocol layers in the reference model are the same for both types of network.Before describing the structure and operation of the different types of LAN, it is perhaps helpful to first identify some of the selection issues that must be considered. It should be stressed that this is only a summary; there are also many possible links between the tips of the branches associated with the figure.1.TopologyMost wide area networks, such as the PSTN, use a mesh (sometimes referred to as a network) topology. With LANs, however, the limited physical separation of the subscriber DTEs allows simpler topologies to be used. The four topologies in common use are star, bus ,ring and hub.The most widespread topology for LANs designed to function as data communication subnetworks for the interconnection of local computer-based equipment is the hub topology, which is a variation a variation of the bus and ring. Sometimes it is called hub/tree topology.2.Transmission mediaTwisted pair, coaxial cable and optical fiber are the three main type of transmission medium used for LANs.3. Medium access control methodsTwo techniques have been adopted for use of the medium access control in the LANs. They are carrier-sense-multiple-access with collision detection (CSMA/CD), for bus network topologies, and control token, for use with either bus or ring networks.CSMA/CD is used to control multiple-access networks. Each on the network “listens” before attempting to send a message, waiting for the “traffic” to clear. If two stations try to sent their messages at exactly the same time, a “collision” is detected, and both stations are required to “step back” and try later.Control token is another way of controlling access to a shared transmission medium that is by the use of a control (permission) token. This token is passed form one DTE to another according to a defined set of rules understood and adhered to by all DTEs connected to the medium. ADTE may only transmit a frame when it is in possession of the token and, after it has transmitted the frame, it passed the token on to allow another DTE to access the transmission medium.1．计算机网络数据通信端设备可以是计算机、打印机、键盘、CRT等，它们可以产生要发送的数字信息，也可使用所接收的数字数据。

无线局域网毕业论文中英文对照资料外文翻译文献中英文对照资料外文翻译文献WLANWhy use WLANFor one of the main local area network management, for the laying of cables, or check the cable is disconnected this time-consuming work, it is easy to upset, not easy to break in a short time to find out where. Furthermore, for the business and application environment constantly updating and development of enterprise network must be matched with the original re-layout, need to re-install the network lines, although the cable itself is not expensive, but requested the technical staff to the high cost of wiring, especially the old building, wiring project costs even higher. Therefore, the construction of wireless local area network has become the best solution.What conditions need to use WLANWLAN is not limited to alternative local area network, but to make up for lack of wired local area networks, in order to achieve the purpose of extending the network, the following circumstances may have wireless local area network.●no fixed workplace users●wired local area network set up by the environmental constraints●As a wired local area network backup systemWLAN access technologyCurrently manufacturers in the design of wireless local area network products, there are quite a variety of access design methods can be divided into three categories: narrowband microwave, spread spectrum (Spread Spectrum) technology, andinfrared have their advantages and disadvantages, limitations, and more, followed by detailed discussion of these techniques. (Infrared) technology, each technique has their advantages and disadvantages, limitations, and more, followed by detailed discussion of these techniques.Technical requirementsAs wireless local area network needs to support high-speed, burst data services, need to be addressed in the indoor use of multipath fading, as well as issues such as crosstalk subnets. Specifically, wireless local area network must achieve the following technical requirements:1)Reliability: Wireless LAN system packet loss rate should be lower than 10-5,the error rate should be lower than 10-8.2)Compatibility: For indoor use of wireless local area network, so as far aspossible with the existing wired LAN network operating system and networksoftware compatible.3)Data rate: In order to meet the needs of local area network traffic, wirelessLAN data transfer rate should be more than 1Mbps.4)The confidentiality of communications: As the data transmitted in the air viawireless media, wireless local area networks at different levels must takeeffective measures to improve communication security and data security.5)Mobility: support for all mobile networks or semi-mobile network.6)Energy Management: When receiving or sending data to the site when themachine is in sleep mode, when activated again when the data transceiver toachieve the savings in power consumption.7)small size and low price: This is the key to the popularity of wireless local areanetwork can be.8)Electromagnetic environment: wireless LAN should consider thehumanbodyand the surrounding electromagnetic environment effects.AndroidGoogle Android is a Linux-based platform for developing open-source phone operating system (registered trademark in China called "Achi;). It includes operating systems, user interface and applications - mobile phone work required by the software, but there is no past, the exclusive right to impede innovation and barriers to mobile industry, called mobile terminal is the first to create a truly open and complete mobile software. Google and Open Handset Alliance to develop the Android, the alliance by including China Mobile, Motorola, Qualcomm and T-Mobile, including more than 30 technology and the composition of a leader in wireless applications. Google with operators, equipment manufacturers, developers and other interested parties to form deep-level partnerships, hoping to establish a standardized, open software platform for mobile phones in the mobile industry to form an open ecosystem .It uses software stack layers (software stack, also known as the software stack) architecture, is divided into three parts: thecore of the underlying Linux-based language developed by the c, only basic functions. Middle layer consists of library. Library and Virtual Machine Virtual Machine, developed by the C +. At the top are a variety of applications, including the call procedures, SMS procedures, application software is developed by the companies themselves to write java.To promote this technology, Google, and dozens of other phone company has established the Open Handset Alliance (Open Handset Alliance).Characteristic●application framework to support component reuse and replacement●Dalvik virtual machine specifically for mobile devices i s optimized●Internal integrated browser, the browser-based open-source WebKit engine●optimization of 2D and 3D graphics library includes graphics library, 3Dgraphics library based on OpenGL ES 1.0 (hardware-accelerated optional)●# SQLite for structured data storage●Multimedia support includes the common audio, video and static image fileformats (such as MPEG4, H.264, MP3, AAC, AMR, JPG, PNG, GIF)●GSM phone (depending on hardware)●Bluetooth Bluetooth, EDGE, 3G, and WiFi (hardware dependent)●Camera, GPS, compass, and accelerometer (hardware dependent)●Rich development environment including a device emulator, debugger,memory and performance analysis charts, and the Eclipse integrateddevelopment environment plug-insApplicationsA core Android application package together with the release of the application package, including email client, SMS short messaging program, calendar, maps, browser, contact management procedures. A ll applications are written using JA V A.Android Application Framework Developers have full access to core applications used by the API framework. The application framework designed to simplify the reuse of software components; any application can publish its functional blocks and any other applications can use the function block its release (but must follow the framework of security restrictions). Reuse mechanism allows the application form can be user replaced.All of the following applications by the composition of a range of services and systems, including:●an expanded view (V iews) can be used to build applications, including a list of(lists), grid (grids), text boxes (text boxes), buttons (buttons), and even an embeddable web browser.●Content Manager (Content Providers) allows applications to access data fromanother application program (such as the contact database), or to share their own data.● A resource manager (Resource Manager) to provide access to non-coderesources, such as local strings, graphics, and hierarchical file (layout files).● A notification manager (Notif ication Manager) allows applications to customersin the status bar display notification information.●An activity class Manager (Activity Manager) to manage the application lifecycle and provides common navigation rollback feature.Ordering the systemOrdering the system information using automated software tools to achieve la carte, side dishes, stir fry vegetables to the transfer of all management processes; completion point, the computer management menu, point the menu and the kitchen, front-end checkout synchronization print; achieved without the menu paper-based operation; backstage manager of inquiry; warehouse inventory management and so on.In addition, ordering the system can also effectively manage customer data, archiving and future reference, put an end to the restaurant "leakage List", "run list" phenomenon; help restaurants using computer data processing capability and powerful ability to process optimization to achieve automated management, streamline workflow restaurant, reduce waste and man-made phenomenon of management oversight, re-optimal allocation of corporate resources, the operating costs to a minimum.Powerful addition to ordering the system to support the general application of stand-alone and LAN in addition to support head office / branch of multi-level framework used for remote network using the POS system to achieve front store sales cashier, sales of small-ticket instantly print sales day-end, reporting sales data and receive information of new featuresdishes.There are three currently ordering the system to achieve mode:First, the touch screen a la carte model: It uses the currently most popular touch-computer ordering process to achieve that members can to order the software screen prompts, simply click on the screen with your fingers can complete the entire ordering process and convenient This model applies to the practice of rich dishes and large restaurants, restaurants, and restaurant, etc..Second,the wireless PDA ordering mode: it uses a wireless WiFi technology, a la carte interface by PDA display, use touch pen to complete the ordering process, virtuallyanywhere, anytime to order real-time response, this model is more suitable for dishes and practices simple restaurant, features a restaurant and special mood of senior restaurants.Third, the wireless ordering Po mode: it uses the ISM band, can be a floor or other obstruction in the case of seamless coverage up to 10 meters away, while the signal remained stable, which is the ratio of the wireless PDA ordering model's greatest strength, this model applies to simple dishes and practices and other requirements with fewer fast food restaurants, pot shops.。

无线局域网技术外文翻译文献(文档含中英文对照即英文原文和中文翻译)翻译：无线局域网技术最近几年，无线局域网开始在市场中独霸一方。

越来越多的机构发现无线局域网是传统有线局域网不可缺少的好帮手，它可以满足人们对移动、布局变动和自组网络的需求，并能覆盖难以铺设有线网络的地域。

无线局域网是利用无线传输媒体的局域网。

就在前几年，人们还很少使用无线局域网。

原因包括成本高、数据率低、职业安全方面的顾虑以及需要许可证。

随着这些问题的逐步解决，无线局域网很快就开始流行起来了。

无线局域网的应用局域网的扩展在20世纪80年代后期出现的无线局域网早期产品都是作为传统有线局域网替代品而问世的。

无线局域网可以节省局域网缆线的安装费用，简化重新布局和其他对网络结构改动的任务。

但是，无线局域网的这个动机被以下一系列的事件打消。

首先，随着人们越来越清楚地认识到局域网的重要性，建筑师在设计新建筑时就包括了大量用于数据应用的预先埋设好的线路。

其次，随着数据传输技术的发展，人们越来越依赖于双绞线连接的局域网。

特别是3类和5类非屏蔽双绞线。

大多数老建筑中已经铺设了足够的3类电缆，而许多新建筑里则预埋了5类电缆。

因此，用无线局域网取代有线局域网的事情从来没有发生过。

但是，在有些环境中无线局域网确实起着有线局域网替代品的作用。

例如，象生产车间、股票交易所的交易大厅以及仓库这样有大型开阔场地的建筑；没有足够双绞线对，但又禁止打洞铺设新线路的有历史价值的建筑；从经济角度考虑，安装和维护有线局域网划不来的小型办公室。

在以上这些情况下，无线局域网向人们提供了一个有效且更具吸引力的选择。

其中大多数情况下，拥有无线局域网的机构同时也拥有支持服务器和某些固定工作站的有线局域网。

因此，无线局域网通常会链接到同样建筑群内的有线局域网上。

所以我们将此类应用领域成为局域网的扩展。

建筑物的互连无线局域网技术的另一种用途是邻楼局域网之间的连接，这些局域网可以是无线的也可以是有线的。

郑州轻工业学院本科毕业设计——文献翻译题目IP协议及IPSec协议安全分析学生姓名专业班级计算机科学与技术2003-1班学号56院(系) 计算机与通信工程学院指导教师)完成时间2007 年6 月6 日英文原文The Analysis Of IP and IPSec Protocol’s Security Problem1.OSI model in time for TCP/IP protocol Overview1.1OSI model in time for TCP/IP protocol introduceIt was repose International Or ganization for Standardization’ suggest to whereas extend arisen', it broke into seven layer into to that OSI model (open system interconnection reference model). The overdone bulkiness, intricacy incur know clearly heap criticism of the is ISO constitutive OSI reference model.As shown in the following chartThe practical application sense nope very large, thereof forsooth toward fathom network protocol interior wield did very avail out of the refer to network cannot but talk OSI reference model, notwithstanding OSI reference model. In reins network world liner, TCP/IP protocol suite obtain know clearly still for extensive application. These OSI seven layer model suffer, per layer big city provide thereon thickness with, combine one visit mouthpiece or interface.The homology hierarchy entitled peer layer of the differ mainframe of compartment. Stand for and mainframe B menses presentation layer each other forpeer layer, mainframe A menses session layer and mainframe B menses session layer each other for peer layer grade among as if mainframe A. Session layer and stand for (these double-deck function by merge to application layer realize) among at TCP/IP reference model suffer, take out know clearly OSI reference model.1.2TCP/IP presence frangibilityIt was run low of virtual security authentication and crypto system, there into up most factor namely IP address problem that IP layered major defect. R command , NFS, X window grade big city is repose IP address versus user proceed authentication and authorization to among that of TCP/IP protocol with IP address came by way of network node alone one identification, heap TCP/IP serve, include Berkeley. Source IP address estimation compact technique authenticity and security among both that of currently TCP/IP cellular security mechanism primarily repose IP address WRAPT filtration(packet filtering) and authentication (authentication) technology, its validity incarnate at could warranty IP WRAPT. The shield, sans versus IP fold misogyny IP address authenticity 'authentication mechanism and security measure in of whereas IP address lie heap problem, consultative maxima shortcoming namely sans versus IP address. The shield, sans versus IP fold misogyny IP address authenticity 'authentication mechanism and security measure in of whereas IP address lie heap problem, consultative maxima shortcoming namely sans versus IP address.It was repose IP protocol of last, TCP subsection and UDP protocol data packet is encapsulation be on the security menace of IP WRAPT suffer at network upper Tran missive, wherefore sameness be confronted with IP layer station encounter that owing to UDP. Now that people all the while in thought method set, yet still avoid less namely as per TCP tie hour in at rest "thrice handshake" Machine-made attack. Either these attack summarize arisen include：One: source address cheat(source address spoofing) or IP cheat(IP spoofing)；Two: source routing select cheat(source routing spoofing)；Three: rip attack(rip attacks)；Four: discriminate attack(authentication attacks)；Five: TCP serial number cheat(TCP sequence number spoofing)；Six: TCP/IP protocol data stream adopt plaintext transmission；Seven: TCP serial number BOMB attack(TCP SYN flooding attack), for short SYN attack；Eight: easy fraudulence (ease of spoofing).1.3Network SecurityBoth moiety end user versus thereof visit, furthermore enterprise network proper no more will exterior closed among be mainframe computer system suffer among it was one relatively walkway that it was resource that both that of one be indispensable to whereas important factor among Network security right through data network. Preparatory network suffer, none but calculator proper and application. Shield this resource. Confidential data by memory at one fit on glass hall. Past crypto guard make inspect weight, nothing but allowed at enterprise network. Versus heap government sector and academic institution came said, internet except one design to transponder electronic mail and proceed document transmission’ instrument.There be the resource completeness exposure at hacker, juggler and those hellion’ attack of down, there into likely to return include some immoral numerator in order to or certain still ugly motive station ongoing destroy of it was completeness visible that the revolution transfer know clearly possession all these of the both internet and computer technology. Now, calculator full high speed, cheapness combine possess resolvability, general design to depot private and confidential information. Internet in the range of get folk in an OSI to at global proceed without a hitch corresponding also, thereof unreliability no more will conceivable. These purport end user transmitting data station utilized network element likely to met absence their dam within sight back. If data proper yes confidential, it with met expose likely to burglar, by have no right limit user check or bowdlerize through to.In the meanwhile, special enterprise network too need for use internet combine therewith mutual to. Internet at advertisement and e-business aspect repose whereabouts huge business opportunity, versus user came said internet must. Figure full high speed, cheapness combine possess resolvability, general design to depotprivate and confidential information up lead folk in a OSI to at global within range proceed without a hitch corresponding also, thereof unreliability no more will conceivable to it means end user transmitting data station utilized network element likely to met absence their dam within sight back. But do so should gotten enterprise network T-number now that, figure full high speed, cheapness combine possesses resolvability, general design to depot private and confidential information up. In despite of cause how about, now versus network security requirements ratio anciently tighten up, too still necessity to know clearly.It was be on the foundation upward strain for security insure of IP-layer or still definitely said yes at each IP grouping that both data stream among as a matter of fact us has manifold means useful for protective network. Could through the medium of be on the fringe erect one fire wall, filter come off those undefeated data stream out for of dedicated network. Application and transport protocols command thereof own security mechanism. Other kind of technology, considering hereinafter several cause lead such approach possess definite meaning to：1. The Intranet big city yes repose IP 'of the both internet and enterprise.Proprietary data stream quantity big city must through IP-layer. Proprietarydata too big city yes by IP grouping came load-supporting;2. Both it could shield and isolation higher level application exempt meets withsafeness attack;3. It took the part of in being higher level security mechanism;4. It could took the part of above internet erect one extendible, secure VPN.Both it is time for in order to met the needs of above IP-layer realize safeness, IETF came into existence know clearly IP security (IPSec) workgroup. Transit effort, to workgroup already fetch round robin at IPV four and IPV six upward strains for network layer safeness agreement, mechanism kimono devote one's efforts to.IPSec frame station took the part of serve include hereinafter content：1.access control;2.data origin authentication(demonstration every last IP grouping)；3.replay protection (Prevent attacker eavesdrop to certain grouping combineafter some hour playback)；4.data integrity (Test withal make IP grouping at transport process suffer haveno by distort certain out);5.data confidentiality & encryption (Part of past encrypt should grouping stash)6.limited traffic flow management (The IP address of the conceal originalitydispatcher)7.key managementIPSec frame initially definitive agreement include inspect weight head (AH), encapsulation security net lotus (ESP) and key management.2IPSec:IP layer protocol security2.1IPSec protocol bring necessity forthIPSec at IP layer endue safety service, it lead system be able to according to require select secure protocol, take serve station utilized algorithm in time for clap demand serve required key to relevant OSI in for to with. The path of the IPSec be used to shield a stick of or multiyear mainframe and mainframe compartment, safety net shut and safety net shut compartment, safety net shut and mainframe compartment. Both IPSec be able to submitted safety service multitude include access control, connectionless integrality, data source authentication, reject retransmitted packet(partial sequence integrality form), privacy and finitude transmission current privacy. For these serve equal at IP layer endue, so any higher level protocol use they, for instance TCP, UDP, ICMP, BGP and so on. It was through the medium of twain large transmission secure protocol, header authentication(AH)and encapsulation safe load(ESP), and key manager harmonize discuss 'use came finished as well that these object. It was by user, application, and / or site, organize versus security and systemic demand came decision that that of required IPSec protocol multitude content very utilized mode.Both the instant correct realize, use these mechanism, they ought not versus use these security mechanism shield tarn missive user, mainframe and rest hero specialnet part bring negative impact forth. This mechanism too by is designed for algorithm independent. Such modularity permit select different algorithm multitude instead of impact rest segmental realize for to. For example：In the event of, different user communication is available to different algorithm multitude.It was a sort of away hair homology serial number 'WRAPT lead system crash' method of attack grade means came attack to that of whereas IPv6 WRAPT proper without supply any security protection, hacker could past information packet detect, IP spoofing, joint captive, replay attack. The data packet be in existence hereinafter hazard of the wherefore, us receive：No came from legal dispatcher; data at transport process suffer by human amend; data content afterwards by human pick (for instance military secret equiponderance ask informational dialogue) for brains. Both that of that of IPSec purpose namely by way of realize data transfer integrality (source address demonstration and guarantee data have no modify) and confidentiality (without by human run over) and endue to a certainty degree 'versus replay offensive shield with as well to. IPSec usable it supplies security protection with IP very upper layer protocol (TCP and UDP grade). 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At the same time, in order to compose construct safety net T-number, return ought adopt rest safety precautions.(one)incorporation AAA authentication, nat-pt, two / three-ply mpls VPN, repose ACL standard visit list and static state spread visit list, phony sliver fold attack grade came realize safety precautions in.(two)through the medium of route filter , static route, policy route and route load-sharing came realize security route out.(three)through the medium of sshv2(secure shell beta two edition), snmpv3(Simple Network Management Protocol beta three edition), ex., endue tenor visit security, line visit security with.(four)through the medium of Hierarchical Direct, custom-tailor privilege class supervise grade instrument came realize Network security management.(five)through the medium of sophisticated alarm, log and audit function realize network clock' security.(six)endue visit list and critical event 'log, routing protocol event and error logging grade, for administration of networks personnel make fault analysis, orient and statistics with. put together station state, secure network.2.2IPSec basic structure analysesIt was avail authentication header (AH) and encapsulation melt security net lotus (esp.) came realize compact technique authentication and encrypt of that that of IPSec basic structure. And that be used to realize integrity of data, this be used to realize compact technique confidentiality. The transmission provision know clearly amphipods of the at the same time logarithm according to：Transmission mode and channel mode. Either at transmission mode suffer, IP head inscribe layer protocol head of compartment embed one new IPSec head (AH or esp.); at passage mode suffer, ask protective wholly IP fold big city encapsulation to the other one IP data packet liner, at the same time outwards and interior IP head of compartment embed one new IPSec head in for. IPSec head big city could at the same time withal transmission mode sum channel mode wrought.Both IPSec structure include a multitude of protocol and algorithm. The correlation as follows station notify of the protocols of compartment.IPSec structure includes a large number of agreements and algorithms. 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本科毕业论文外文翻译外文译文题目（中文）：具体数学：汉诺塔问题学院: 计算机科学与技术专业: 计算机科学与技术学号:学生姓名:指导教师:日期: 二○一二年六月1 Recurrent ProblemsTHIS CHAPTER EXPLORES three sample problems that give a feel for what’s to c ome. They have two traits in common: They’ve all been investigated repeatedly by mathe maticians; and their solutions all use the idea of recurrence, in which the solution to eac h problem depends on the solutions to smaller instances of the same problem.1.1 THE TOWER OF HANOILet’s look first at a neat little puzzle called the Tower of Hanoi,invented by the Fr ench mathematician Edouard Lucas in 1883. We are given a tower of eight disks, initiall y stacked in decreasing size on one of three pegs:The objective is to transfer the entire tower to one of the other pegs, movingonly one disk at a time and never moving a larger one onto a smaller.Lucas furnished his toy with a romantic legend about a much larger Tower of Brah ma, which supposedly has 64 disks of pure gold resting on three diamond needles. At th e beginning of time, he said, God placed these golden disks on the first needle and orda ined that a group of priests should transfer them to the third, according to the rules abov e. The priests reportedly work day and night at their task. When they finish, the Tower will crumble and the world will end.It's not immediately obvious that the puzzle has a solution, but a little thought (or h aving seen the problem before) convinces us that it does. Now the question arises：What' s the best we can do？That is，how many moves are necessary and suff i cient to perfor m the task?The best way to tackle a question like this is to generalize it a bit. The Tower of Brahma has 64 disks and the Tower of Hanoi has 8；let's consider what happens if ther e are TL disks.One advantage of this generalization is that we can scale the problem down even m ore. In fact, we'll see repeatedly in this book that it's advantageous to LOOK AT SMAL L CASES first. It's easy to see how to transfer a tower that contains only one or two di sks. And a small amount of experimentation shows how to transfer a tower of three.The next step in solving the problem is to introduce appropriate notation：NAME ANO CONQUER. Let's say that T n is the minimum number of moves that will t ransfer n disks from one peg to another under Lucas's rules. Then T1is obviously 1 , an d T2= 3.We can also get another piece of data for free, by considering the smallest case of all：Clearly T0= 0，because no moves at all are needed to transfer a tower of n = 0 disks! Smart mathematicians are not ashamed to think small,because general patterns are easier to perceive when the extreme cases are well understood(even when they are trivial).But now let's change our perspective and try to think big；how can we transfer a la rge tower? Experiments with three disks show that the winning idea is to transfer the top two disks to the middle peg, then move the third, then bring the other two onto it. Thi s gives us a clue for transferring n disks in general：We first transfer the n−1 smallest t o a different peg (requiring T n-1moves), then move the largest (requiring one move), and finally transfer the n−1 smallest back onto the largest (req uiring another T n-1moves). Th us we can transfer n disks (for n > 0)in at most 2T n-1+1 moves：T n≤2T n—1+1，for n > 0.This formula uses '≤' instead of '=' because our construction proves only that 2T n—1+1 mo ves suffice; we haven't shown that 2T n—1+1 moves are necessary. A clever person might be able to think of a shortcut.But is there a better way? Actually no. At some point we must move the largest d isk. When we do, the n−1 smallest must be on a single peg, and it has taken at least T moves to put them there. We might move the largest disk more than once, if we're n n−1ot too alert. But after moving the largest disk for the last time, we must trans fr the n−1 smallest disks (which must again be on a single peg)back onto the largest；this too re quires T n−1moves. HenceT n≥ 2T n—1+1，for n > 0.These two inequalities, together with the trivial solution for n = 0, yieldT0=0；T n=2T n—1+1 , for n > 0. (1.1)(Notice that these formulas are consistent with the known values T1= 1 and T2= 3. Our experience with small cases has not only helped us to discover a general formula, it has also provided a convenient way to check that we haven't made a foolish error. Such che cks will be especially valuable when we get into more complicated maneuvers in later ch apters.)A set of equalities like (1.1) is called a recurrence (a. k. a. recurrence relation or r ecursion relation). It gives a boundary value and an equation for the general value in ter ms of earlier ones. Sometimes we refer to the general equation alone as a recurrence, alt hough technically it needs a boundary value to be complete.The recurrence allows us to compute T n for any n we like. But nobody really like to co m pute fro m a recurrence，when n is large；it takes too long. The recurrence only gives indirect, "local" information. A solution to the recurrence would make us much h appier. That is, we'd like a nice, neat, "closed form" for Tn that lets us compute it quic kly，even for large n. With a closed form, we can understand what T n really is.So how do we solve a recurrence? One way is to guess the correct solution,then to prove that our guess is correct. And our best hope for guessing the solution is t o look (again) at small cases. So we compute, successively,T3= 2×3+1= 7; T4= 2×7+1= 15; T5= 2×15+1= 31; T6= 2×31+1= 63.Aha! It certainly looks as ifTn = 2n−1，for n≥0. (1.2)At least this works for n≤6.Mathematical induction is a general way to prove that some statement aboutthe integer n is true for all n≥n0. First we prove the statement when n has its smallest v alue,no; this is called the basis. Then we prove the statement for n > n0,assuming that it has already been proved for all values between n0and n−1, inclusive; this is called th e induction. Such a proof gives infinitely many results with only a finite amount of wo rk.Recurrences are ideally set up for mathematical induction. In our case, for exampl e，(1.2) follows easily from (1.1)：The basis is trivial，since T0 = 20−1= 0.And the indu ction follows for n > 0 if we assume that (1.2) holds when n is replaced by n−1：T n= 2T n+1= 2(2n−1−1)+1=2n−1.Hence (1.2) holds for n as well. Good! Our quest for T n has ended successfully.Of course the priests' task hasn't ended；they're still dutifully moving disks,and wil l be for a while, because for n = 64 there are 264−1 moves (about 18 quintillion). Even at the impossible rate of one move per microsecond, they will need more than 5000 cent uries to transfer the Tower of Brahma. Lucas's original puzzle is a bit more practical, It requires 28−1 = 255 moves, which takes about four minutes for the quick of hand.The Tower of Hanoi recurrence is typical of many that arise in applications of all kinds. In finding a closed-form expression for some quantity of interest like T n we go t hrough three stages：1 Look at small cases. This gives us insight into the problem and helps us in stages2 and 3.2 Find and prove a mathematical expression for the quantity of interest.For the Tower of Hanoi, this is the recurrence (1.1) that allows us, given the inc lination，to compute T n for any n.3 Find and prove a closed form for our mathematical expression.For the Tower of Hanoi, this is the recurrence solution (1.2).The third stage is the one we will concentrate on throughout this book. In fact, we'll fre quently skip stages I and 2 entirely, because a mathematical expression will be given tous as a starting point. But even then, we'll be getting into subproblems whose solutions will take us through all three stages.Our analysis of the Tower of Hanoi led to the correct answer, but it r equired an“i nductive leap”；we relied on a lucky guess about the answer. One of the main objectives of this book is to explain how a person can solve recurrences without being clairvoyant. For example, we'll see that recurrence (1.1) can be simplified by adding 1 to both sides of the equations：T0+ 1= 1;T n + 1= 2T n-1+ 2, for n >0.Now if we let U n= T n+1，we haveU0 =1；U n= 2U n-1，for n > 0. (1.3)It doesn't take genius to discover that the solution to this recurrence is just U n= 2n；he nce T n= 2n −1. Even a computer could discover this.Concrete MathematicsR. L. Graham, D. E. Knuth, O. Patashnik《Concrete Mathematics》，1.1 ，The Tower Of HanoiR. L. Graham, D. E. Knuth, O. PatashnikSixth printing, Printed in the United States of America1989 by Addison-Wesley Publishing Company，Reference 1-4 pages具体数学R.L.格雷厄姆，D.E.克努特，O.帕塔希尼克《具体数学》，1.1，汉诺塔R.L.格雷厄姆，D.E.克努特，O.帕塔希尼克第一版第六次印刷于美国，韦斯利出版公司，1989年，引用1-4页1 递归问题本章将通过对三个样本问题的分析来探讨递归的思想。

外文翻译Birth of the NetThe Internet has had a relatively brief, but explosive history so far. It grew out of an experiment begun in the 1960's by the U.S. Department of Defense. The DoD wanted to create a computer network that would continue to function in the event of a disaster, such as a nuclear war. If part of the network were damaged or destroyed, the rest of the system still had to work. That network was ARPANET, which linked U.S. scientific and academic researchers. It was the forerunner of today's Internet.In 1985, the National Science Foundation (NSF) created NSFNET, a series of networks for research and education communication. Based on ARPANET protocols, the NSFNET created a national backbone service, provided free to any U.S. research and educational institution. At the same time, regional networks were created to link individual institutions with the national backbone service.NSFNET grew rapidly as people discovered its potential, and as new software applications were created to make access easier. Corporations such as Sprint and MCI began to build their own networks, which they linked to NSFNET. As commercial firms and other regional network providers have taken over the operation of the major Internet arteries, NSF has withdrawn from the backbone business.NSF also coordinated a service called InterNIC, which registered all addresses on the Internet so that data could be routed to the right system. This service has now been taken over by Network Solutions, Inc., in cooperation with NSF.How the Web WorksThe World Wide Web, the graphical portion of the Internet, is the most popular part of the Internet by far. Once you spend time on the Web,you will begin to feel like there is no limit to what you can discover. The Web allows rich and diverse communication by displaying text, graphics, animation, photos, sound and video.So just what is this miraculous creation? The Web physically consists of your personal computer, web browser software, a connection to an Internet service provider, computers called servers that host digital data and routers and switches to direct the flow of information.The Web is known as a client-server system. Your computer is the client; the remote computers that store electronic files are the servers. Here's how it works:Let's say you want to pay a visit to the the Louvre museum website. First you enter the address or URL of the website in your web browser (more about this shortly). Then your browser requests the web page from the web server that hosts the Louvre's site. The Louvre's server sends the data over the Internet to your computer. Your web browser interprets the data, displaying it on your computer screen.The Louvre's website also has links to the sites of other museums, such as the Vatican Museum. When you click your mouse on a link, you access the web server for the Vatican Museum.The "glue" that holds the Web together is called hypertext and hyperlinks. This feature allow electronic files on the Web to be linked so you can easily jump between them. On the Web, you navigate through pages of information based on what interests you at that particular moment, commonly known as browsing or surfing the Net.To access the Web you need web browser software, such as Netscape Navigator or Microsoft Internet Explorer. How does your web browser distinguish between web pages and other files on the Internet? Web pages are written in a computer language called Hypertext Markup Language or HTML.Some Web HistoryThe World Wide Web (WWW) was originally developed in 1990 at CERN, the European Laboratory for Particle Physics. It is now managed by The World Wide Web Consortium, also known as the World Wide Web Initiative.The WWW Consortium is funded by a large number of corporate members, including AT&T, Adobe Systems, Inc., Microsoft Corporation and Sun Microsystems, Inc. Its purpose is to promote the growth of the Web by developing technical specifications and reference software that will be freely available to everyone. The Consortium is run by MIT with INRIA (The French National Institute for Research in Computer Science) acting as European host, in collaboration with CERN.The National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign, was instrumental in the development of early graphical software utilizing the World Wide Web features created by CERN. NCSA focuses on improving the productivity of researchers by providing software for scientific modeling, analysis, and visualization. The World Wide Web was an obvious way to fulfill that mission. NCSA Mosaic, one of the earliest web browsers, was distributed free to the public. It led directly to the phenomenal growth of the World Wide Web.Understanding Web AddressesYou can think of the World Wide Web as a network of electronic files stored on computers all around the world. Hypertext links these resources together. Uniform Resource Locators or URLs are the addresses used to locate thesefiles. The information contained in a URL gives you the ability to jump from one web page to another with just a click of your mouse. When you type a URL into your browser or click on a hypertext link, your browser is sending a request to a remote computer to download a file.What does a typical URL look like? Here are some examples:/The home page for study english.ftp:///pub/A directory of files at MIT* available for downloading.news:rec.gardens.rosesA newsgroup on rose gardening.The first part of a URL (before the two slashes* tells you the type of resource or method of access at that address. For example:∙http - a hypertext document or directory∙gopher - a gopher document or menu∙ftp - a file available for downloading or a directory of such files∙news - a newsgroup∙telnet - a computer system that you can log into over the Internet∙WAIS* - a database or document in a Wide Area Information Search database∙file - a file located on a local drive (your hard drive)The second part is typically the address of the computer where the data or service is located. Additional parts may specify the names of files, the port to connect to, or the text to search for in a database.You can enter the URL of a site by typing it into the Location bar of your web browser, just under the toolbar.Most browsers record URLs that you want to use again, by adding them to a special menu. In Netscape Navigator, it's called Bookmarks. In Microsoft Explorer, it's called Favorites. Once you add a URL to your list, you can return to that web page simply by clicking on the name in your list, instead of retyping the entire URL.Most of the URLs you will be using start with http which stands for Hypertext Transfer Protocol*. http is the method by which HTML files are transferred over the Web. Here are some other important things to know about URLs:∙ A URL usually has no spaces.∙ A URL always uses forward slashes (//).If you enter a URL incorrectly, your browser will not be able to locate the site or resource you want. Should you get an error message or the wrong site, make sure you typed the address correctly.You can find the URL behind any link by passing your mouse cursor over the link. The pointer will turn into a hand and the URL will appear in the browser's status ba r, usually located at the bottom of your screen.Domain NamesWhen you think of the Internet, you probably think of ".com." Just what do those three letters at the end of a World Wide Web address mean?Every computer that hosts data on the Internet has a unique numerical address. For example, the numerical address for the White House is198.137.240.100. But since few people want to remember long strings of numbers, the Domain Name System (DNS)* was developed. DNS, a critical part of the Internet's technical infrastructure*, correlates* a numerical address to a word. To access the White House website, you could type its number into the address box of your web browser. But most people prefer to use "." In this case, the domain name is . In general, the three-letter domain name suffix* is known as a generictop-level domai n and describes the type of organization. In the last few years, the lines have somewhat blurred* between these categories..com - business (commercial).edu - educational.org - non-profit.mil - military.net - network provider.gov - governmentA domain name always has two or more parts separated by dots and typically consists of some form of an organization's name and the three-letter suffix. For example, the domain name for IBM is ""; the United Nations is "."If a domain name is available, and provided it does not infringe* on an existing trademark, anyone can register the name for $35 a year through Network Solutions, Inc., which is authorized to register .com, .net and .org domains. You can use the box below to see if a name is a available. Don't be surprised ifthe .com name you want is already taken, however. Of the over 8 million domain names, 85% are .com domains.ICANN, the Internet Corporation for Assigned Names and Numbers, manages the Domain Name System. As of this writing, there are plans to add additional top-level domains, such as .web and .store. When that will actually happen is anybody's guess.To check for, or register a domain name, type it into the search box.It should take this form: In addition to the generic top-level domains, 244 national top-level domains were established for countries and territories*, for example:.au - Australia.ca - Canada.fr - France.de - Germany.uk - United KingdomFor US $275 per name, you can also register an international domain name with Net Names. Be aware that some countries have restrictions for registering names.If you plan to register your own domain name, whether it's a .com or not, keep these tips in mind:The shorter the name, the better. (But it should reflect your family name, interest or business.)The name should be easy to remember.It should be easy to type without making mistakes.Remember, the Internet is global. Ideally, a domain name will "read" in a language other than English.Telephone lines were designed to carry the human voice, not electronic data from a computer. Modems were invented to convert digital computer signals into a form that allows them to travel over the phone lines. Those are the scratchy sounds you hear from a modem's speaker. A modem on theother end of the line can understand it and convert the sounds back into digital information that the computer can understand. By the way, the word modem stands for MOdulator/DEModulator.Buying and using a modem used to be relatively easy. Not too long ago, almost all modems transferred data at a rate of 2400 Bps (bits per second). Today, modems not only run faster, they are also loaded with features like error control and data compression. So, in addition to converting and interpreting signals, modems also act like traffic cops, monitoring and regulating the flow of information. That way, one computer doesn't send information until the receiving computer is ready for it. Each of these features, modulation, error control, and data compression, requires a separate kind of protocol and that's what some of those terms you see like V.32, V.32bis, V.42bis and MNP5 refer to.If your computer didn't come with an internal modem, consider buying an external one, because it is much easier to install and operate. For example, when your modem gets stuck (not an unusual occurrence), you need to turn it off and on to get it working properly. With an internal modem, that means restarting your computer--a waste of time. With an external modem it's as easy as flipping a switch.Here's a tip for you: in most areas, if you have Call Waiting, you can disable it by inserting *70 in front of the number you dial to connect to the Internet (or any online service). This will prevent an incoming call from accidentally kicking you off the line.This table illustrates the relative difference in data transmission speeds for different types of files. A modem's speed is measured in bits per second (bps). A 14.4 modem sends data at 14,400 bits per second. A 28.8 modem is twice as fast, sending and receiving data at a rate of 28,800 bits per second.Until nearly the end of 1995, the conventional wisdom was that 28.8 Kbps was about the fastest speed you could squeeze out of a regular copper telephoneline. Today, you can buy 33.6 Kbps modems, and modems that are capable of 56 Kbps. The key question for you, is knowing what speed modems your Internet service provider (ISP) has. If your ISP has only 28.8 Kbps modems on its end of the line, you could have the fastest modem in the world, and only be able to connect at 28.8 Kbps. Before you invest in a 33.6 Kbps or a 56 Kbps modem, make sure your ISP supports them.Speed It UpThere are faster ways to transmit data by using an ISDN or leased line. In many parts of the U.S., phone companies are offering home ISDN at less than $30 a month. ISDN requires a so-called ISDN adapter instead of a modem, and a phone line with a special connection that allows it to send and receive digital signals. You have to arrange with your phone company to have this equipment installed. For more about ISDN, visit Dan Kegel's ISDN Page.An ISDN line has a data transfer rate of between 57,600 bits per second and 128,000 bits per second, which is at least double the rate of a 28.8 Kbps modem. Leased lines come in two configurations: T1 and T3. A T1 line offers a data transfer rate of 1.54 million bits per second. Unlike ISDN, a T-1 line is a dedicated connection, meaning that it is permanently connected to the Internet. This is useful for web servers or other computers that need to be connected to the Internet all the time. It is possible to lease only a portion of a T-1 line using one of two systems: fractional T-1 or Frame Relay. You can lease them in blocks ranging from 128 Kbps to 1.5 Mbps. The differences are not worth going into in detail, but fractional T-1 will be more expensive at the slower available speeds and Frame Relay will be slightly more expensive as you approach the full T-1 speed of 1.5 Mbps. A T-3 line is significantly faster, at 45 million bits per second. The backbone of the Internet consists of T-3 lines. Leased lines are very expensive and are generally only used by companies whose business is built around the Internet or need to transfer massiveamounts of data. ISDN, on the other hand, is available in some cities for a very reasonable price. Not all phone companies offer residential ISDN service. Check with your local phone company for availability in your area.Cable ModemsA relatively new development is a device that provides high-speed Internet access via a cable TV network. With speeds of up to 36 Mbps, cable modems can download data in seconds that might take fifty times longer with a dial-up connection. Because it works with your TV cable, it doesn't tie up a telephone line. Best of all, it's always on, so there is no need to connect--no more busy signals! This service is now available in some cities in the United States and Europe.The download times in the table above are relative and are meant to give you a general idea of how long it would take to download different sized files at different connection speeds, under the best of circumstances. Many things can interfere with the speed of your file transfer. These can range from excessive line noise on your telephone line and the speed of the web server from which you are downloading files, to the number of other people who are simultaneously trying to access the same file or other files in the same directory.DSLDSL (Digital Subscriber Line) is another high-speed technology that is becoming increasingly popular. DSL lines are always connected to the Internet, so you don't need to dial-up. Typically, data can be transferred at rates up to 1.544 Mbps downstream and about 128 Kbps upstream over ordinary telephone lines. Since a DSL line carries both voice and data, you don't have to install another phone line. You can use your existing line to establish DSLservice, provided service is available in your area and you are within the specified distance from the telephone company's central switching office.DSL service requires a special modem. Prices for equipment, DSL installation and monthly service can vary considerably, so check with your local phone company and Internet service provider. The good news is that prices are coming down as competition heats up.Anatomy of a Web PageA web page is an electronic document written in a computer language called HTML, short for Hypertext Markup Language. Each web page has a unique address, called a URL* or Uniform Resource Locator, which identifies its location on the network.A website has one or more related web pages, depending on how it's designed. Web pages on a site are linked together through a system of hyperlinks* , enabling you to jump between them by clicking on a link. On the Web, you navigate through pages of information according to your interests.Home Sweet Home PageWhen you browse the World Wide Web you'll see the term home page often. Think of a home page as the starting point of a website. Like the table of contents of a book or magazine, the home page usually provides an overview of what you'll find at the website. A site can have one page, many pages or a few long ones, depending on how it's designed. If there isn't a lot of information, the home page may be the only page. But usually you will find at least a few other pages.Web pages vary wildly in design and content, but most use a traditional magazine format. At the top of the page is a masthead* or banner graphic*, then a list of items, such as articles, often with a brief description. The items in the list usually link to other pages on the website, or to other sites. Sometimes these links are highlighted* words in the body of the text, or are arranged in a list, like an index. They can also be a combination* of both. A web page can also have images that link to other content.How can you tell which text are links? Text links appear in a different color from the rest of the text--typically in blue and underlined. When you move yourcursor over a text link or over a graphic link, it will change from an arrow to a hand. The hypertext words often hint* at what you will link to.When you return to a page with a link you've already visited, the hypertext words will often be in a different color, so you know you've already been there. But you can certainly go there again. Don't be surprised though, if the next time you visit a site, the page looks different and the information has changed. The Web is a dynamic* medium. To encourage visitors to return to a site, some web publishers change pages often. That's what makes browsing the Web so excitingA Home (Page) of Your OwnIn the 60s, people asked about your astrological* sign. In the 90s, they want to know your URL. These days, having a web address is almost as important as a street address. Your website is an electronic meeting place for your family, friends and potentially*, millions of people around the world. Building your digital domain can be easier than you may think. Best of all, you may not have to spend a cent. The Web brims with all kinds of free services, from tools to help you build your site, to free graphics, animation and site hosting. All it takes is some time and creativity.Think of your home page as the starting point of your website. Like the table of contents of a book or magazine, the home page is the front door. Your site can have one or more pages, depending on how you design it. If there isn't a lot of information just yet, your site will most likely have only a home page. But the site is sure to grow over time.While web pages vary dramatically* in their design and content, most use a traditional magazine layout. At the top of the page is a banner graphic. Next comes a greeting and a short description of the site. Pictures, text, and links to other websites follow.If the site has more than one page, there's typically a list of items--similar to an index--often with a brief description. The items in the list link to other pages on the website. Sometimes these links are highlighted words in the body of the text. It can also be a combination of both. Additionally, a web page may have images that link to other content.Before you start building your site, do some planning. Think about whom the site is for and what you want to say. Next, gather up the material that you wantto put on the site: write the copy, scan the photos, design or find the graphics. Draw a rough layout on a sheet of paper.While there are no rules you have to follow, there are a few things to keep in mind:∙Start simply. If you are too ambitious at the beginning, you may never get the site off the ground. You can always add to your site.∙Less is better. Most people don't like to read a lot of text online. Break it into small chunks.∙Use restraint. Although you can use wild colors and images for the background of your pages, make sure your visitors will be able to readthe text easily.∙Smaller is better. Most people connect to the Internet with a modem.Since it can take a long time to download large image files, keep the file sizes small.∙Have the rights. Don't put any material on your site unless you are sure you can do it legally. Read Learn the Net's copyright article for moreabout this.Stake Your ClaimNow it's time to roll up your sleeves and start building. Learn the Net Communities provides tools to help you build your site, free web hosting, and a community of other homesteaders.Your Internet service provider may include free web hosting services with an account, one alternative to consider.Decoding Error MessagesAs you surf the Net, you will undoubtedly find that at times you can't access certain websites. Why, you make wonder? Error messages attempt to explain the reason. Unfortunately, these cryptic* messages baffle* most people.We've deciphered* the most common ones you may encounter.400 - Bad RequestProblem: There's something wrong with the address you entered. You may not be authorized* to access the web page, or maybe it no longer exists.Solution: Check the address carefully, especially if the address is long. Make sure that the slashes are correct (they should be forward slashes) and that all the names are properly spelled. Web addresses are case sensitive, socheck that the names are capitalized in your entry as they are in the original reference to the website.401 - UnauthorizedProblem: You can't access a website, because you're not on the guest list, your password is invalid or you have entered your password incorrectly.Solution: If you think you have authorization, try typing your password again. Remember that passwords are case sensitive.403 - ForbiddenProblem: Essentially the same as a 401.Solution: Try entering your password again or move on to another site.404 - Not FoundProblem: Either the web page no longer exists on the server or it is nowhere to be found.Solution: Check the address carefully and try entering it again. You might also see if the site has a search engine and if so, use it to hunt for the document. (It's not uncommon for pages to change their addresses when a website is redesigned.) To get to the home page of the site, delete everything after the domain name and hit the Enter or Return key.503 - Service unavailableProblem: Your Internet service provider (ISP) or your company's Internet connection may be down.Solution: Take a stretch, wait a few minutes and try again. If you still have no luck, phone your ISP or system administrator.Bad file requestProblem: Your web browser may not be able to decipher the online form you want to access. There may also be a technical error in the form.Solution: Consider sending a message to the site's webmaster, providing any technical information you can, such as the browser and version you use.Connection refused by hostProblem: You don't have permission to access the page or your password is incorrect.Solution: Try typing your password again if you think you should have access.Failed DNS lookupProblem: DNS stands for the Domain Name System, which is the system that looks up the name of a website, finds a corresponding number (similar to a phone number), then directs your request to the appropriate web server on theInternet. When the lookup fails, the host server can't be located.Solution: Try clicking on the Reload or Refresh button on your browser toolbar. If this doesn't work, check the address and enter it again. If all else fails, try again later.File contains no dataProblem: The site has no web pages on it.Solution: Check the address and enter it again. If you get the same error message, try again later.Host unavailableProblem: The web server is down.Solution: Try clicking on the Reload or Refresh button. If this doesn't work, try again later.Host unknownProblem: The web server is down, the site may have moved, or you've been disconnected from the Net.Solution: Try clicking on the Reload or Refresh button and check to see that you are still online. If this fails, try using a search engine to find the site. It may have a new address.Network connection refused by the serverProblem: The web server is busy.Solution: Try again in a while.Unable to locate hostProblem: The web server is down or you've been disconnected from the Net.Solution: Try clicking on the Reload or Refresh button and check to see that you are still online.Unable to locate serverProblem: The web server is out-of-business or you may have entered the address incorrectly.Solution: Check the address and try typing it again.Web BrowsersA web browser is the software program you use to access the World Wide Web, the graphical portion of the Internet. The first browser, called NCSA Mosaic, was developed at the National Center for Supercomputing Applications in the early '90s. The easy-to-use point-and-click interface*helped popularize the Web, although few then could imagine the explosive growth that would soon occur.Although many different browsers are available, Microsoft Internet Explorer* and Netscape Navigator* are the two most popular ones. Netscape and Microsoft have put so much money into their browsers that the competition can't keep up. The pitched battle* between the two companies to dominate* the market has lead to continual improvements to the software. Version 4.0 and later releases of either browser are excellent choices. (By the way, both are based on NCSA Mosaic.) You can download Explorer and Navigator for free from each company's website. If you have one browser already, you can test out the other. Also note that there are slight differences between the Windows and MacIntosh* versions.You can surf to your heart's content, but it's easy to get lost in this electronic web. That's where your browser can really help. Browsers come loaded with all sorts of handy features. Fortunately, you can learn the basics in just a few minutes, then take the time to explore the advanced functions.Both Explorer and Navigator have more similarities than differences, so we'll primarily cover those. For the most up-to-date information about the browsers, and a complete tutorial, check the online handbook under the Help menu or go to the websites of the respective* software companies.Browser AnatomyWhen you first launch your web browser, usually by double-clicking on the icon on your desktop, a predefined web page, your home page, will appear. With Netscape Navigator for instance, you will be taken to Netscape's NetCenter.The Toolbar （工具栏）The row of buttons at the top of your web browser, known as the toolbar, helps you travel through the web of possibilities, even keeping track ofwhere you've been. Since the toolbars for Navigator and Explorer differ slightly, we'll first describe what the buttons in common do:o The Back button returns you the previous page you've visited.o Use the Forward button to return to the page you just came from.o Home takes you to whichever home page you've chosen. (If you haven't selected one, it will return you to the default home page,usually the Microsoft or Netscape website.)。

附件1：外文资料翻译译文大容量存储器由于计算机主存储器的易失性和容量的限制, 大多数的计算机都有附加的称为大容量存储系统的存储设备, 包括有磁盘、CD 和磁带。

相对于主存储器，大的容量储存系统的优点是易失性小，容量大，低成本, 并且在许多情况下, 为了归档的需要可以把储存介质从计算机上移开。

术语联机和脱机通常分别用于描述连接于和没有连接于计算机的设备。

联机意味着，设备或信息已经与计算机连接，计算机不需要人的干预，脱机意味着设备或信息与机器相连前需要人的干预，或许需要将这个设备接通电源，或许包含有该信息的介质需要插到某机械装置里。

大量储存器系统的主要缺点是他们典型地需要机械的运动因此需要较多的时间，因为主存储器的所有工作都由电子器件实现。

1. 磁盘今天，我们使用得最多的一种大量存储器是磁盘，在那里有薄的可以旋转的盘片，盘片上有磁介质以储存数据。

盘片的上面和（或）下面安装有读/写磁头，当盘片旋转时，每个磁头都遍历一圈，它被叫作磁道，围绕着磁盘的上下两个表面。

通过重新定位的读/写磁头，不同的同心圆磁道可以被访问。

通常，一个磁盘存储系统由若干个安装在同一根轴上的盘片组成，盘片之间有足够的距离，使得磁头可以在盘片之间滑动。

在一个磁盘中，所有的磁头是一起移动的。

因此，当磁头移动到新的位置时，新的一组磁道可以存取了。

每一组磁道称为一个柱面。

因为一个磁道能包含的信息可能比我们一次操作所需要得多，所以每个磁道划分成若干个弧区，称为扇区，记录在每个扇区上的信息是连续的二进制位串。

传统的磁盘上每个磁道分为同样数目的扇区，而每个扇区也包含同样数目的二进制位。

（所以，盘片中心的储存的二进制位的密度要比靠近盘片边缘的大）。

因此，一个磁盘存储器系统有许多个别的磁区, 每个扇区都可以作为独立的二进制位串存取，盘片表面上的磁道数目和每个磁道上的扇区数目对于不同的磁盘系统可能都不相同。

磁区大小一般是不超过几个KB; 512 个字节或1024 个字节。

本科毕业设计(论文) 外文翻译（附外文原文）系 ( 院 )：信息科学与工程学院课题名称：学生信息管理系统专业(方向)：计算机科学与技术（应用）7.1 Enter ActionMappingsThe Model 2 architecture (see chapter 1) encourages us to use servlets and Java- Server Pages in the same application. Under Model 2, we start by calling a servlet.The servlet handles the business logic and directs control to the appropriate pageto complete the response.The web application deployment descriptor (web.xml) lets us map a URL patternto a servlet. This can be a general pattern, like *.do, or a specific path, like saveRecord.do.Some applications implement Model 2 by mapping a servlet to each business operation. This approach works, but many applications involve dozens or hundredsof business operations. Since servlets are multithreaded, instantiating so manyservlets is not the best use of server resources. Servlets are designed to handle anynumber of parallel requests. There is no performance benefit in simply creatingmore and more servlets.The servlet’s primary job is to interact with the container and HTTP. Handlinga business operation is something that a servlet could delegate to another component. Struts does this by having the ActionServlet delegate the business operationto an object. Using a servlet to receive a request and route it to a handler is knownas the Front Controller pattern [Go3].Of course, simply delegating the business operation to another componentdoes not solve the problem of mapping URIs [W3C, URI] to business operations.Our only way of communicating with a web browser is through HTTP requests and URIs. Arranging for a URI to trigger a business operation is an essential part of developing a web application.Meanwhile, in practice many business operations are handled in similar ways.Since Java is multithreaded, we could get better use of our server resources if wecould use the same Action object to handle similar operations. But for this towork, we might need to pass the object a set of configuration parameters to usewith each operation.So what’s the bottom line? To implement Model 2 in an efficient and flexibleway, we need to:Enter ActionMappings 195♉ Route requests for our business operations to a single servlet♉ Determine which business operation is related to the request♉ Load a multithreaded helper object to handle the business operation♉ Pass the helper object the specifics of each request along with any configuration detail used by this operationThis is where ActionMappings come in.7.1.1 The ActionMapping beanAn ActionMapping (org.apache.struts.action.ActionMapping) describes howthe framework handles each discrete business operation (or action). In Struts,each ActionMapping is associated with a specific URI through its path property. When a request comes in, the ActionServlet uses the path property to select the corresponding ActionMapping. The set of ActionMapping objects is kept in an ActionMappings collection (org.apache.struts.action.ActionMappings). Originally, the ActionMapping object was used to extend the Action objectrather than the Action class. When used with an Action, a mapping gives a specific Action object additional responsibilities and new functionality. So, it was essentiallyan Action decorator [Go4]. Along the way, the ActionMapping evolved into anobject in its own right and can be used with or without an Action.DEFINITION The intent of the decorator pattern is to attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassingfor extending functionality [Go4].The ActionMappings are usually created through the Struts configuration file.For more about this file, see chapter 4.7.1.2 The ActionMappings catalogThe ActionMappings catalog the business logic available to a Struts application.When a request comes in, the servlet finds its entry in the ActionMappings catalogand pulls the corresponding bean.The ActionServlet uses the ActionMapping bean to decide what to do next. Itmay need to forward control off to another resource. Or it may need to populateand validate an ActionForm bean. At some point, it may have to pass control to an Action object, and when the Action returns, it may have to look up an Action-Forward associated with this mapping.196 CHAPTER 7Designing with ActionMappingsThe ActionMapping works like a routing slip for the servlet. Depending onhow the mapping is filled out, the request could go just about anywhere.The ActionMappings represent the core design of a Struts application. If youwant to figure out how a Struts application works, start with the ActionMappings. Ifyou want to figure out how to write a new Struts application, start with the Action- Mappings. The mappings are at the absolute center of every Struts application.In this chapter, we take a close look at the ActionMapping properties andexplore how they help you design the flow of a Struts application.1.0 vs 1.1 In Struts 1.1, ActionMapping subclasses ActionConfig (org.apache. struts.config.ActionConfig) and adds API methods required forbackward compatibility. ActionMapping is not deprecated, and how thehierarchy will be handled in future releases has not been determined.For now, we refer to the ActionMapping class, but you should note thatin Struts 1.1 all of the action properties are actually defined by the ActionConfigsuper class. The ActionMapping class otherwise works thesame way in both versions.7.2 ActionMapping propertiesTable 7.1 describes the base ActionMapping properties. As with other configuration components, developers may extend ActionMapping to provide additionalproperties.Table 7.1 The base ActionMapping propertiesProperty Descriptionpath The URI path from the request used to select this mapping. (API command) forward The context-relative path of the resource that should serve this request via a forward.Exactly one of the forward, include, or type properties must be specified.orinclude The context-relative path of the resource that should serve this request via aninclude. Exactly one of the forward, include, or type properties must be specified.ortype Optionally specifies a subclass oforg.apache.struts.action.ActionMappingthat should be used when instantiating this mapping.className The fully qualified name of the Action class used by this mapping. SinceStruts 1.1ActionMapping properties 197In the sections that follow, we take a look at each of these properties.7.2.1 The path propertyThe ActionMapping URI, or path, will look to the user like just another file onthe web server. But it does not represent a file. It is a virtual reference to our ActionMapping.Because it is exposed to other systems, the path is not really a logical name, likethose we use with ActionForward. The path can include slashes and an extension—as if it referred to a file system—but they are all just part of a single name.The ActionMappings themselves are a “flat” namespace with no type of internalhierarchy whatsoever. They just happen to use the same characters that we areused to seeing in hierarchical file systems.name The name of the form bean, if any, associated with this action. This is not the classname. It is the logical name used in the form bean configuration.roles The list of security roles that may access this mapping.scope The identifier of the scope (request or session) within which the form bean, if any,associated with this mapping will be created.validate Set to true if the validate method of the form bean (if any) associated with thismapping should be called.input Context-relative path of the input form to which control should be returned ifa validationerror is encountered. This can be any URI: HTML, JSP, VM, or another Action- Mapping.parameter General-purpose configuration parameter that can be used to pass extra informationto the Action selected by this ActionMapping.attribute Name of the request-scope or session-scope attribute under which our form bean isaccessed, if it is other than the bean's specified name.prefix Prefix used to match request parameter names to form bean property names, if any.suffix Suffix used to match request parameter names when populating the properties ofour ActionForm bean, if any.unknown Can be set to true if this mapping should be configured as the default for this application(to handle all requests not handled by another mapping). Only one mappingcan be defined as the default unknown mapping within an application.forwards(s) Block of ActionForwards for this mapping to use, if any.exception(s) Block of ExceptionHandlers for this mapping to use, if any.Table 7.1 The base ActionMapping properties (continued)Property DescriptionSinceStruts 1.1SinceStruts 1.1198 CHAPTER 7Designing with ActionMappingsOf course, it can still be useful to treat your ActionMappings as if they werepart of a hierarchy and group related commands under the same "folder." Theonly restriction is that the names must match whatever pattern is used in the application’s deployment description (web.xml) for the ActionServlet. This is usuallyeither /do/* or *.do, but any similar pattern can be used.If you are working in a team environment, different team members can begiven different ActionMapping namespaces to use. Some people may be workingwith the /customer ActionMappings, others may be working with the /vendor ActionMappings. This may also relate to the Java package hierarchy the team isusing. Since the ActionMapping URIs are logical constructs, they can be organizedin any way that suits your project.With Struts 1.1, these types of namespaces can be promoted to applicationmodules. Each team can work independently on its own module, with its own setof configuration files and presentation pages. Configuring your application to use multiple modules is covered in chapter 4.DEFINITION The web runs on URIs, and most URIs map to physical files. If you want to change the resource, you change the corresponding file. Some URIs, likeStruts actions, are virtual references. They do not have a correspondingfile but are handled by a programming component. To change the resource,we change how the component is programmed. But since thepath is a URI and interacts with other systems outside our control, thepath is not a true logical reference—the name of an ActionForward, forinstance. We can change the name of an ActionForward without consultingother systems. It’s an internal, logical reference. If we change thepath to an ActionMapping, we might need to update other systems thatrefer to the ActionMapping through its public URI.7.2.2 The forward propertyWhen the forward property is specified, the servlet will not pass the request to an Action class but will make a call to RequestDispatcher.forward. Since the operationdoes not use an Action class, it can be used to integrate Struts with otherresources and to prototype systems. The forward, include, and type propertiesare mutually exclusive. (See chapter 6 for more information.)7.2.3 The include propertyWhen the include property is specified, the servlet will not pass the request to an Action class but will make a call to RequestDispatcher.include. The operationActionMapping properties 199does not use an Action class and can be used to integrate Struts with other components. The forward, include, and type properties are mutually exclusive. (Seechapter 6 for more information.)7.2.4 The type propertyMost mappings will specify an Action class type rather than a forward or include.An Action class may be used by more than one mapping. The mappings may specifyform beans, parameters, forwards, or exceptions. The forward, include, andtype properties are mutually exclusive.7.2.5 The className propertyWhen specified, className is the fully qualified Java classname of the ActionMapping subclass that should be used for this object. This allows you to use your own ActionMapping subclass with specialized methods and properties. See alsosection 7.4.7.2.6 The name propertyThis property specifies the logical name for the form bean, as given in the formbean segment of the Struts configuration file. By default, this is also the name tobe used when placing the form bean in the request or session context. Use theattribute property of this class to specify a different attribute key.7.2.7 The roles propertyThis property is a comma-delimited list of the security role names that are allowed access to this ActionMapping object. By default, the same system that is used with standard container-based security is applied to the list of roles given here. Thismeans you can use action-based security in lieu of specifying URL patterns in the deployment descriptor, or you can use both together.The security check is handled by the processRoles method of the Request- Processor (org.apache.struts.action.RequestProcessor). By subclassing RequestProcessor, you can also use the roles property with application-based security. See chapter 9 for more about subclassing RequestProcessor.7.2.8 The scope propertyThe ActionForm bean can be stored in the current request or in the session scope (where it will be available to additional requests). While most developers userequest scope for the ActionForm, the framework default is session scope. Tomake request the default, see section 7.4.SinceStruts 1.1SinceStruts 1.1200 CHAPTER 7Designing with ActionMappings7.2.9 The validate propertyAn important step in the lifecycle of an ActionForm is to validate its data before offering it to the business layer. When the validate property for a mapping is true, the ActionServlet will call the ActionForm’s validate method. If validate returns false, the request is forwarded to the resource given by the input property.Often, developers will create a pair of mappings for each data entry form. Onemapping will have validate set to false, so you can create an empty form. Theother has validate set to true and is used to submit the completed form.NOTE Whether or not the ActionForm validate method is called does not relateto the ActionServlet’s validating property. That switch controlshow the Struts configuration file is processed.7.2.10 The input propertyWhen validate is set to true, it is important that a valid path for input be provided. This is where control will pass should the ActionForm validate methodreturn false. Often, this is the address for a presentation page. Sometimes it willbe another Action path (with validate set to false) that is required to generatedata objects needed by the page.NOTE The input path often leads back to the page that submitted the request.While it seems natural for the framework to return the request to whereit originated, this is not a simple task in a web application. A request is oftenpassed from component to component before a response is sent backto the browser. The browser only knows the path it used to retrieve theinput page, which may or may not also be the correct path to use for theinput property. While it may be possible to try and generate a default inputpage based on the HTTP referrer attribute, the Struts designersdeemed that approach unreliable.inputForwardIn Struts 1.0, the ActionMapping input property is always a literal URI. InStruts 1.1, it may optionally be the name of an ActionForward instead. The ActionForward is retrieved and its path property is used as the input property.This can be a global or local ActionForward.To use ActionForwards here instead of literal paths, set the inputForwardattribute on the <controller> element for this module to true:SinceStruts 1.1ActionMapping properties 201<controller inputForward="true">For more about configuring Struts, see chapter 4. For more about ActionForwards,see chapter 6.7.2.11 The parameter propertyThe generic parameter property allows Actions to be configured at runtime. Severalof the standard Struts Actions make use of this property, and the standardScaffold Actions often use it, too. The parameter property may contain a URI, the name of a method, the name of a class, or any other bit of information an Actionmay need at runtime. This flexibility allows some Actions to do double and tripleduty, slashing the number of distinct Action classes an application needs on hand.Within an Action class, the parameter property is retrieved from the mappingpassed to perform:parameter = mapping.getParameter();Multiple parametersWhile multiple parameters are not supported by the standard ActionMappingsclass, there are some easy ways to implement this, including using HttpUtils, a StringTokenizer, or a Properties file (java.util.Properties).HttpUtils. Although deprecated as of the Servlet API 2.3 specification, theHttpUtils package (javax.servlet.http.HttpUtils) provides a static method that parses any string as if it were a query string and returns a Hashtable(java.util.Hashtable):Hashtable parameters = parseQueryString(parameter);The parameter property for your mapping then becomes just another query string, because you might use it elsewhere in the Struts configuration. stringTokenizer. Another simple approach is to delimit the parameters using the token of your choice—such as a comma, colon, or semicolon—and use the StringTokenizer to read them back:StringTokenizer incoming =new StringTokenizer(mapping.getParameter(),";");int i = 0;String[] parameters = new String[incoming.countTokens()]; while (incoming.hasMoreTokens()) {parameters[i++] = incoming.nextToken().trim();}202 CHAPTER 7Designing with ActionMappingsProperties file. While slightly more complicated than the others, another popular approach to providing multiple parameters to an ActionMapping is with a standard Properties files (java.util.Properties). Depending on your needs, the Properties file could be stored in an absolute location in your file system or anywhere on your application’s CLASSPATH.The Commons Scaffold package [ASF, Commons] provides a ResourceUtils package (mons.scaffold.util.ResourceUtils) with methods forloading a Properties file from an absolute location or from your application’s CLASSPATH.7.2.12 The attribute propertyFrom time to time, you may need to store two copies of the same ActionForm inthe same context at the same time. This most often happens when ActionFormsare being stored in the session context as part of a workflow. To keep their names from conflicting, you can use the attribute property to give one ActionForm bean a different name.An alternative approach is to define another ActionForm bean in the configuration, using the same type but under a different name.7.2.13 The prefix and suffix propertiesLike attribute, the prefix and suffix properties can be used to help avoid naming conflicts in your application. When specified, these switches enable aprefix or suffix for the property name, forming an alias when it is populatedfrom the request.If the prefix this was specified, thenthisName=McClanahanbecomes equivalent toname=McClanahanfor the purpose of populating the ActionForm. Either or both parameters would call getName("McClanahan");This does not affect how the properties are written by the tag extensions. It affects how the autopopulation mechanism perceives them in the request.Nested components 2037.2.14 The unknown ActionMappingWhile surfing the Web, most of us have encountered the dreaded 404— page not found message. Most web servers provide some special features for processing requests for unknown pages, so webmasters can steer users in the right direction. Struts offers a similar service for ActionMapping 404s—the unknown ActionMapping. In the Struts configuration file, you can specify one ActionMapping toreceive any requests for an ActionMapping that would not otherwise be matched:<actionname="/debug"forward="/pages/debug.jsp"/>When this option is not set, a request for an ActionMapping that cannot bematched throws400 Invalid path /notHere was requestedNote that by a request for an ActionMapping, we mean a URI that matches the prefix or suffix specified for the servlet (usually /do/* or *.do). Requests for other URI patterns, good or bad, will be handled by other servlets or by the container:/do/notHere (goes to the unknown ActionMapping)/notHere.txt (goes to the container)7.3 Nested componentsThe ActionMapping properties are helpful when it comes to getting an Action torun a business operation. But they tell only part of the story. There is still much todo when the Action returns.An Action may have more than one outcome. We may need to register several ActionForwards so that the Action can take its pick.7.3.1 Local forwardsIn the normal course, an ActionMapping is used to select an Action object to handle the request. The Action returns an ActionForward that indicates which pageshould complete the response.The reason we use ActionForwards is that, in practice, presentation pages areeither often reused or often changed, or both. In either case, it is good practice to encapsulate the page’s location behind a logical name, like “success” or “failure.”The ActionForward object lets us assign a logical name to any given URI.204 CHAPTER 7Designing with ActionMappingsOf course, logical concepts like success or failure are often relative. What represents success to one Action may represent failure to another. Each Action-Mapping can have its own set of local ActionForwards. When the Action asks for a forward (by name), the local set is checked before trying the global forwards. See chapter 6 for more about ActionForwards.Local forwards are usually specified in the Struts configuration file. See chapter4 for details.7.3.2 Local exceptionsMost often, an application’s exception handlers (org.apache.struts.action. ExceptionHandler) can be declared globally. However, if a given ActionMapping needs to handle an exception differently, it can have its own set of local exception handlers that are checked before the global set.Local exceptions are usually specified in the Struts configuration file. Seechapter 4 for details.7.4 Rolling your own ActionMappingWhile ActionMapping provides an impressive array of properties, developers may also provide their own subclass with additional properties or methods. InStruts 1.0, this is configured in the deployment descriptor (web.xml) for the ActionServlet:<init-param><param-name>mapping</param-name><param-value>app.MyActionMapping</param-value></init-param>In Struts 1.1, this is configured in the Struts configuration file as an attribute to the <action-mappings> element:<action-mappings type="app.MyActionMapping">Individual mappings may also be set to use another type through the className attribute:<action className="app.MyActionMapping">For more about configuring Struts, see chapter 4.SinceStruts 1.1Summary 205The framework provides two base ActionMapping classes, shown in table 7.2. They can be selected as the default or used as a base for your own subclasses.The framework default is SessionActionMapping, so scope defaults to session. Subclasses that provide new properties may set them in the Struts configuration using a standard mechanism:<set-property property="myProperty" value="myValue" /> Using this standard mechanism helps developers avoid subclassing the Action- Servlet just to recognize the new properties when it digests the configuration file. This is actually a feature of the Digester that Struts simply inherits.7.5 SummarySun’s Model 2 architecture teaches that servlets and JavaServer Pages should be used together in the same application. The servlets can handle flow control and data acquisition, and the JavaServer Pages can handle the HTML.Struts takes this one step further and delegates much of the flow control anddata acquisition to Action objects. The application then needs only a single servletto act as a traffic cop. All the real work is parceled out to the Actions and theStruts configuration objects.Like servlets, Actions are efficient, multithreaded singletons. A single Actionobject can be handling any number of requests at the same time, optimizing your server’s resources.To get the most use out of your Actions, the ActionMapping object is used as a decorator for the Action object. It gives the Action a URI, or several URIs, and away to pass different configuration settings to an Action depending on which URIis called.In this chapter, we took a close look at the ActionMapping properties andexplained each property’s role in the scheme of things. We also looked at extendingthe standard ActionMapping object with custom properties—just in case yourscheme needs even more things.Table 7.2 The default ActionMapping classesActionMapping Descriptionorg.apache.struts.action.SessionActionMapping Defaults the scope property to sessionorg.apache.struts.action.RequestActionMapping Defaults the scope property to request206 CHAPTER 7Designing with ActionMappingsIn chapter 8, the real fun begins. The configuration objects covered so far aremainly a support system. They help the controller match an incoming requestwith a server-side operation. Now that we have the supporting players, let’s meet the Struts diva: the Action object.7.1 进入ActionMappingModel 2 架构(第1章)鼓励在同一个应用中使用servlet和JSP页面。

计算机外⽂翻译（完整）毕业设计(论⽂)外⽂资料翻译专业：计算机科学与技术姓名：王成明学号：06120186外⽂出处：The History of the Internet附件： 1.外⽂原⽂ 2.外⽂资料翻译译⽂；附件1：外⽂原⽂The History of the InternetThe Beginning - ARPAnetThe Internet started as a project by the US government. The object of the project was to create a means of communications between long distance points, in the event of a nation wide emergency or, more specifically, nuclear war. The project was called ARPAnet, and it is what the Internet started as. Funded specifically for military communication, the engineers responsible for ARPANet had no idea of the possibilities of an "Internet."By definition, an 'Internet' is four or more computers connected by a network.ARPAnet achieved its network by using a protocol called TCP/IP. The basics around this protocol was that if information sent over a network failed to get through on one route, it would find another route to work with, as well as establishing a means for one computer to "talk" to another computer, regardless of whether it was a PC or a Macintosh.By the 80's ARPAnet, just years away from becoming the more well known Internet, had 200 computers. The Defense Department, satisfied with ARPAnets results, decided to fully adopt it into service, and connected many military computers and resources into the network. ARPAnet then had 562 computers on its network. By the year 1984, it had over 1000 computers on its network.In 1986 ARPAnet (supposedly) shut down, but only the organization shut down, and the existing networks still existed between the more than 1000 computers. It shut down due to a failied link up with NSF, who wanted to connect its 5 countywide super computers into ARPAnet.With the funding of NSF, new high speed lines were successfully installed at line speeds of 56k (a normal modem nowadays) through telephone lines in 1988. By that time, there were 28,174 computers on the (by then decided) Internet. In 1989 there were 80,000 computers on it. By 1989, there were290,000.Another network was built to support the incredible number of people joining. It was constructed in 1992.Today - The InternetToday, the Internet has become one of the most important technological advancements in the history of humanity. Everyone wants to get 'on line' to experience the wealth of information of the Internet. Millions of people now use the Internet, and it's predicted that by the year 2003 every single person on the planet will have Internet access. The Internet has truly become a way of life in our time and era, and is evolving so quickly its hard to determine where it will go next, as computer and network technology improve every day.HOW IT WORKS:It's a standard thing. People using the Internet. Shopping, playing games,conversing in virtual Internet environments.The Internet is not a 'thing' itself. The Internet cannot just "crash." It functions the same way as the telephone system, only there is no Internet company that runs the Internet.The Internet is a collection of millioins of computers that are all connected to each other, or have the means to connect to each other. The Internet is just like an office network, only it has millions of computers connected to it.The main thing about how the Internet works is communication. How does a computer in Houston know how to access data on a computer in Tokyo to view a webpage?Internet communication, communication among computers connected to the Internet, is based on a language. This language is called TCP/IP. TCP/IP establishes a language for a computer to access and transmit data over the Internet system.But TCP/IP assumes that there is a physical connecetion between onecomputer and another. This is not usually the case. There would have to be a network wire that went to every computer connected to the Internet, but that would make the Internet impossible to access.The physical connection that is requireed is established by way of modems,phonelines, and other modem cable connections (like cable modems or DSL). Modems on computers read and transmit data over established lines,which could be phonelines or data lines. The actual hard core connections are established among computers called routers.A router is a computer that serves as a traffic controller for information.To explain this better, let's look at how a standard computer might viewa webpage.1. The user's computer dials into an Internet Service Provider (ISP). The ISP might in turn be connected to another ISP, or a straight connection into the Internet backbone.2. The user launches a web browser like Netscape or Internet Explorer and types in an internet location to go to.3. Here's where the tricky part comes in. First, the computer sends data about it's data request to a router. A router is a very high speed powerful computer running special software. The collection of routers in the world make what is called a "backbone," on which all the data on the Internet is transferred. The backbone presently operates at a speed of several gigabytes per-second. Such a speed compared to a normal modem is like comparing the heat of the sun to the heat of an ice-cube.Routers handle data that is going back and forth. A router puts small chunks of data into packages called packets, which function similarly to envelopes. So, when the request for the webpage goes through, it uses TCP/IP protocols to tell the router what to do with the data, where it's going, and overall where the user wants to go.4. The router sends these packets to other routers, eventually leadingto the target computer. It's like whisper down the lane (only the information remains intact).5. When the information reaches the target web server, the webserver then begins to send the web page back. A webserver is the computer where the webpage is stored that is running a program that handles requests for the webpage and sends the webpage to whoever wants to see it.6. The webpage is put in packets, sent through routers, and arrive at the users computer where the user can view the webpage once it is assembled.The packets which contain the data also contain special information that lets routers and other computers know how to reassemble the data in the right order.With millions of web pages, and millions of users, using the Internet is not always easy for a beginning user, especially for someone who is not entirely comfortale with using computers. Below you can find tips tricks and help on how to use main services of the Internet.Before you access webpages, you must have a web browser to actually be able to view the webpages. Most Internet Access Providers provide you with a web browser in the software they usually give to customers; you. The fact that you are viewing this page means that you have a web browser. The top two use browsers are Netscape Communicator and Microsoft Internet Explorer. Netscape can be found at /doc/bedc387343323968011c9268.html and MSIE can be found at /doc/bedc387343323968011c9268.html /ie.The fact that you're reading this right now means that you have a web browser.Next you must be familiar with actually using webpages. A webpage is a collection of hyperlinks, images, text, forms, menus, and multimedia. To "navigate" a webpage, simply click the links it provides or follow it's own instructions (like if it has a form you need to use, it will probably instruct you how to use it). Basically, everything about a webpage is made to be self-explanetory. That is the nature of a webpage, to be easily navigatable."Oh no! a 404 error! 'Cannot find web page?'" is a common remark made by new web-users.Sometimes websites have errors. But an error on a website is not the user's fault, of course.A 404 error means that the page you tried to go to does not exist. This could be because the site is still being constructed and the page hasn't been created yet, or because the site author made a typo in the page. There's nothing much to do about a 404 error except for e-mailing the site administrator (of the page you wanted to go to) an telling him/her about the error.A Javascript error is the result of a programming error in the Javascript code of a website. Not all websites utilize Javascript, but many do. Javascript is different from Java, and most browsers now support Javascript. If you are using an old version of a web browser (Netscape 3.0 for example), you might get Javascript errors because sites utilize Javascript versions that your browser does not support. So, you can try getting a newer version of your web browser.E-mail stands for Electronic Mail, and that's what it is. E-mail enables people to send letters, and even files and pictures to each other.To use e-mail, you must have an e-mail client, which is just like a personal post office, since it retrieves and stores e-mail. Secondly, you must have an e-mail account. Most Internet Service Providers provide free e-mail account(s) for free. Some services offer free e-mail, like Hotmail, and Geocities.After configuring your e-mail client with your POP3 and SMTP server address (your e-mail provider will give you that information), you are ready to receive mail.An attachment is a file sent in a letter. If someone sends you an attachment and you don't know who it is, don't run the file, ever. It could be a virus or some other kind of nasty programs. You can't get a virus justby reading e-mail, you'll have to physically execute some form of program for a virus to strike.A signature is a feature of many e-mail programs. A signature is added to the end of every e-mail you send out. You can put a text graphic, your business information, anything you want.Imagine that a computer on the Internet is an island in the sea. The sea is filled with millions of islands. This is the Internet. Imagine an island communicates with other island by sending ships to other islands and receiving ships. The island has ports to accept and send out ships.A computer on the Internet has access nodes called ports. A port is just a symbolic object that allows the computer to operate on a network (or the Internet). This method is similar to the island/ocean symbolism above.Telnet refers to accessing ports on a server directly with a text connection. Almost every kind of Internet function, like accessing web pages,"chatting," and e-mailing is done over a Telnet connection.Telnetting requires a Telnet client. A telnet program comes with the Windows system, so Windows users can access telnet by typing in "telnet" (without the "'s) in the run dialog. Linux has it built into the command line; telnet. A popular telnet program for Macintosh is NCSA telnet.Any server software (web page daemon, chat daemon) can be accessed via telnet, although they are not usually meant to be accessed in such a manner. For instance, it is possible to connect directly to a mail server and check your mail by interfacing with the e-mail server software, but it's easier to use an e-mail client (of course).There are millions of WebPages that come from all over the world, yet how will you know what the address of a page you want is?Search engines save the day. A search engine is a very large website that allows you to search it's own database of websites. For instance, if you wanted to find a website on dogs, you'd search for "dog" or "dogs" or "dog information." Here are a few search-engines.1. Altavista (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed2. Yahoo (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed Collection3. Excite (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed4. Lycos (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed5. Metasearch (/doc/bedc387343323968011c9268.html ) - Multiple searchA web spider is a program used by search engines that goes from page to page, following any link it can possibly find. This means that a search engine can literally map out as much of the Internet as it's own time and speed allows for.An indexed collection uses hand-added links. For instance, on Yahoo's site. You can click on Computers & the Internet. Then you can click on Hardware. Then you can click on Modems, etc., and along the way through sections, there are sites available which relate to what section you're in.Metasearch searches many search engines at the same time, finding the top choices from about 10 search engines, making searching a lot more effective.Once you are able to use search engines, you can effectively find the pages you want.With the arrival of networking and multi user systems, security has always been on the mind of system developers and system operators. Since the dawn of AT&T and its phone network, hackers have been known by many, hackers who find ways all the time of breaking into systems. It used to not be that big of a problem, since networking was limited to big corporate companies or government computers who could afford the necessary computer security.The biggest problem now-a-days is personal information. Why should you be careful while making purchases via a website? Let's look at how the internet works, quickly.The user is transferring credit card information to a webpage. Looks safe, right? Not necessarily. As the user submits the information, it is being streamed through a series of computers that make up the Internet backbone.The information is in little chunks, in packages called packets. Here's the problem: While the information is being transferred through this big backbone, what is preventing a "hacker" from intercepting this data stream at one of the backbone points?Big-brother is not watching you if you access a web site, but users should be aware of potential threats while transmitting private information. There are methods of enforcing security, like password protection, an most importantly, encryption.Encryption means scrambling data into a code that can only be unscrambled on the "other end." Browser's like Netscape Communicator and Internet Explorer feature encryption support for making on-line transfers. Some encryptions work better than others. The most advanced encryption system is called DES (Data Encryption Standard), and it was adopted by the US Defense Department because it was deemed so difficult to 'crack' that they considered it a security risk if it would fall into another countries hands.A DES uses a single key of information to unlock an entire document. The problem is, there are 75 trillion possible keys to use, so it is a highly difficult system to break. One document was cracked and decoded, but it was a combined effort of14,000 computers networked over the Internet that took a while to do it, so most hackers don't have that many resources available.附件2：外⽂资料翻译译⽂Internet的历史起源——ARPAnetInternet是被美国政府作为⼀项⼯程进⾏开发的。