大学毕业论文英文翻译及原文

大学毕业论文英文翻译及原文 大学毕业论文英文翻译 中文译文 计算机网络 每个一个世纪都有一种主导地位的技术。18世纪伴随着工业革命的到来的是大型机械系统的时代;19世纪是蒸汽机的时代，而20世纪的发展历程中，关键的技术是信息收集、处理和分发。在其他的发展方面，我们还可以看到:遍布全球的电话网络的建立起来了，无线电广播和电视的出现了，计算机工业诞生了，并且以超乎想象的速度在成长;另外通信卫星也发射上天了。 技术的快速发展的一个直接结果是，这些领域正在快速地融合，信息收集、传输、存储和处理之间的差别正在迅速地消失。对于具有数百个办公室的大型组织来说，尽管这些办公室分布在广阔的地理区域中，但未来期望的情景是，工作人员只要单击一下按钮，就可以查看最远处分部的状态。随着信息收集、处理和分发能力的不断提高，我们对于更加复杂的信息处理技术的需求也增长得更快。 与其他的工业(比如汽车和航空运输业)相比，计算机工业还是非常年轻，尽管如此，计算机技术却在很短的时间内有了惊人的进展。在计算机诞生之初的20年间，计算机系统是高度集中化的，通常位于一个很大的房间中。该房间通常配有玻璃墙，参观的人透过玻璃墙可以欣赏到里边伟大的电子奇迹。中等规模的公司或者大学可能会有一台或者两台计算机，而大型的研究机构最多也就几十台计算机。要在20年内生产出大量同样功能但是体积比邮票还小的计算机，在当时的人们看来纯属科学幻想。 计算机和通信的结合对于计算机系统的组织方式产生了深远的影响。把一台大型的计算机放在一个单独的房间中，然后用户带着他们的处理任务去房间里上机，这种"计算机中心"的概念现在已经完全过时了。由一台计算机来处理整个组织中所有的计算需求，这种老式的模型已经被新的模型所取代，在新的模型下，由大量独立的，但相互连接起来的计算机来共同完成计算任务。这些 系统称为计算机网络(computer networks)。如何设计这些网络，并且将这些网络组织起来，这就是本书的主题。 在本书中，我们将使用术语"计算机网络"来表示通过同一种技术相互连接起来的一组自主计算机的集合。如果两台计算机能够交换信息，则称这两台计算机相互连接的(interconnected)。两台机器之间的连接不一定通过铜线，光纤，微波，红外线和通信卫星也可以用来建立连接。以后我们将会看到，网络可以有不同的大小，形状和形式。Internet或者万维网(World Wide Wed)都不是计算机网，可能有很多人对此觉得很奇怪。到本书的结尾的时候，你就会明白其中的原因。现在给出一个简单的答案:Internet并不是一个单一的网络，而是一个有许多个网络构成的网络，Web是一个分布式系统，它运行在Internet之上。 在一些文献中，计算机网络和分布式系统(distributed system)这两个概念容易使人混淆。两者的关键差别在于;在一个分布式系统中，一组独立的计算机展现给用户的是一个统一的整体，就好像是一个系统似的。通常，对用户来说，分布式系统只有一个模型或范型。在操作系统之上有一层中间件(middleware)负责实现这个模型。一个著名的分布式系统的例子是万维网(World Wide Web),在万维网中，所有的一切看起来就好像是一个文档(Web页面)一样。 在计算机网络中，这种统一性，模型以及其中的软件都不存在。用户看到的是实际的机器。 计算机网络并没有使这些机器看起来是统一的，或者使它们的行为是统一的。如果这些机器有不同的硬件或者操作系统，那么，这些差异对于用户来说都是完全可见的。如果有一个用户希望在一台远程机器上运行一个程序，那么他必须登陆到远程机器上，然后在那台机器上运行该程序。 实际上分布式系统是建立在网络之上的软件系统。正是因为软件的特性，所以分布式系统具有高度的内敛性和透明性。因此，网络与分布式系统之间的区别更多地在于软件(特别是操作系统)，而不是硬件。 然而，这两个主题之间也有许多重合的地方。例如，分布式系统和计算机网络都需要移动文件。不同之处在于是谁来发起移动操作，是系统还是用户?虽然本书的焦点主要是在于网络，但是讨论到的许多话题在分布式系统中也是很重要的。 1.计算机网络的应用 在开始讨论技术细节在前，首先值得画一点时间来说明为什么人们对于计算机网络很感兴趣，以及计算机网络可以用来做什么事情。毕竟，如果没有人对计算机网络感兴趣的话，那就不会建立这么多的计算机网络了。我们首先讨论针对公司和个人的传统用法，然后再转移到最新的一些发展动向，包括针对移动用户和家庭网络的应用上来。 商业应用 2. 许多公司都具有相当数量的计算机。例如，一家公司可能用一些单独的计算机来监视生产过程， 记录 混凝土 养护记录下载土方回填监理旁站记录免费下载集备记录下载集备记录下载集备记录下载 库存，以及管理工资的发放工作。最初的时候，这些计算机都是独立工作的，但是后来管理部门决定将这些计算机连接起来，以便将有关整个公司的信息关联起俩，并且可以随时访问这些信息。 将这个公司的情形更加范化一点，这里涉及到的问题是资源共享(resource sharing),其目标是，让每一个人都可以访问所有的程序，设备和特殊的数据，并且做到跟这些资源和用户的物理位置无关。一个既显然又非常普通的例子是让一个办公室的所有工作人员共同一台打印机。公司没有必要为每一个工作人员都配备一台个人打印机，而且，一台高性能的网络打印机通常比一大批独立的打印机更加便宜，打印速度更快，而且也容易维护。 然而，比共享物理资源(比如打印机，扫描仪和CD刻录机)更重要的是共享信息。每一个大型的或中等规模的公司和许多小型的公司都越来越依赖于计算机化的信息。大多数公司都有顾客记录，库存信息，收到的账单记录，财务报告，缴税信息以及其他更多的在线信息，如果一家银行的所有计算机都不能工作了，那么这家银行可能坚持不到5分钟。如果一个现代化的生产车间使用了计算机控制的装配线，那么计算机崩溃后也不可能继续工作。现在即使是很小 规模的旅行社，甚至只有三个人规模的律师事务所也与计算机网络有着密切的联系，通过计算机网络，雇员可以即时地访问有关的信息和文档。 对于小公司而言，可能所有的计算机都在一个办公室里，或者位于同一个建筑物内，但是对于大型的公司，计算机和雇员们可能分散在许许多多个办公室中，甚至分散在不同的国家的多个分支机构中，然而，纽约的一个销售员有时需要访问新加坡的产品库存数据库。换句话说，一个用户离他访问的数据库相隔15000公里，但是他仍然要访问这些数据，就好像这些数据存放在本地一样。简而言之，计算机网络的这个目标可以定义为:企图打破"地理位置的束缚(tyranny of geography)"。 安装最简单的形式，你可以把一个公司的信息系统想象成:有一个或者句多个数据库，以及许多需要远程访问这些数据库地雇员们组成的。在这个模型中，数据存储在性能强大的计算机上，称为服务器(server)。通常这些服务器集中在同一个场所。并且由系统管理员对它们进行维护。相反，雇员们的桌子上有一些件的机器，称为客户(client)，通过这些客户，雇员可以访问远程的数据，例如他们可以访问远程的电子 表格 关于规范使用各类表格的通知入职表格免费下载关于主播时间做一个表格详细英语字母大小写表格下载简历表格模板下载 ，(有时候，我们也把客户机器的使用称作"客户"，但是根据上下文环境，你应该可以判断出到底是指机器，还是指 客户和服务器通过网络连接起来，如图1.1所示,请注意，这里机器的用户。) 我们只是用一个简单的椭圆形来表示一个网络，而没有表达其中的任何细节，当我们从抽象意义上来表达一个网络的时候，就使用这种形式。当有需要的时候，我们也会提供更多的细节。 这个结构称为"客户-服务器模型(client-server model)"。这种模型有很多广泛的应用，它也是许多网络应用的基础。当客户和服务器位于同一个建筑物内(比如，属于同一个公司)的时候，这种模型是最适用的;当客户和服务器相隔很远的时候，这种模型也是适用的。 例如，当一个人在家里访问一个Web页面的时候，就使用了这种模型;其中，远程地Web服务器就是模型中的服务器。用户的个人计算机是模型中的客户。在大多数情况下，一台服务器可以处理许多客户的请求。 如果更加仔细的看一看客户-服务器模型，我们就可以看到，该模型涉及到了两个进程，一个位于客户机器上的，另一个位于服务器上。其中的通信形式 是这样的:客户进程通过网络将一个消息发送给服务器进程。然后客户进程等待应答消息。当服务器进程获得了请求消息之后，它执行所请求的工作，或者查到客户所要的数据，然后送回一个应答消息。 3.显示了这些信息。 建立计算机网络的第二个目标是为了满足人的需要，而不是信息或者计算机的要求。计算机网络可以为雇员们提供一个功能强大的通信媒介(communication medium)。现在，几乎每一家公司，只要有两台以上的计算机， mail)系统，雇员们利用该系统来进就会有一个电子邮件(electronic mail E-行大量的日常的通信。实际上，这样带来的一个问题是，每个人每天必须处理大量的E-mail，而且，其中有一些E-mail是没有意义的。因为公司的老板发现，他们只需点一下按钮就可以将同样的E-mail消息(通常并没有多少实际内容)发送给所有部下。 但是E-mail并不是计算机网络带来的唯一的一种新型的通信形式。有了网络之后，两个或者多个相距甚远的人可以联合起来写一份报告，这是非常容易做到的。当一个工作人员修改了一份在线文档的时候，其他的人立即就可以看到文档被改动了，根本不用为一封信而等上几天。这样的速度使得那些分散在各地的工作组协同工作非常容易，而这在以前是根本不可能的。 另一种计算机辅助通信的形式是视频会议。利用这项技术，即使雇员们处于相距甚远的不同地点上，他们也可以开会，相互之间看得到，听得到，甚至还可以在一个共享的虚拟黑板上写写画画。视频会议是一个功能强大的工具。它消除了以前的差旅开销和路途上所需的时间。有时候你可能会听到这样的说法，通信和运输在进行一场竞争，不管哪一方赢了，都会淘汰掉另一方。 第三个目标是，越来越多的公司与其它公司进行电子商务活动，特别是跟供应商和客户公司之间的商务活动。例如，汽车，飞机和计算机的制造商需要从各种不同的供应商购买子系统，然后再将这些部件装配起来。这些制造商利用计算机网络，就可以根据需要以电子的方式下订单。这种实时(即根据需要)下订单的能力大大降低了对于大库存量的需求，并且也提高了效率。 第四个目标正在变得越来越重要，那就是通过Internet与客户做各种交易。 航空公司，书店和音乐零售商发现，许多顾客喜欢在家里购买他们的商品或者 服务。因此，许多公司提供了在线查询商品和服务的功能，并且允许在线下订 单。计算机网络的这种用途有望在将来得到进一步的发展。这就是电子商务 commerce)。 (electronic commerce- 访问远程信息可以有多种方式。浏览Web页面可以是为了获取远程信息， 也可以纯粹为了娱乐。获取到的信息多种多样，包括艺术，商务，烹饪，政府， 健康，历史，爱好，娱乐，科学，运动，旅游等等。通过Internet进行娱乐的 方式有很多是可以提上台面的，然而，还有一些娱乐方式最好不提为妙。 许多报纸已经变成了在线形式了，并且可以针对个人进行定制。例如，你 可以告诉一份报纸，你希望阅读有关腐败政客，火灾，名人丑闻，以及流行疾 病方面的信息，而不需要足球信息。有的时候甚至还可能在你睡觉的时候，这 些被选出来的文章自动下载到你的硬盘上，或者在早餐前输出到打印机上。这 种趋势继续发展的结果是，一大批12岁报童们将会失业，但是报社喜欢这样， 因为报纸发行是整个产品链中最薄弱的环节。 出处:外文资料原文 Computer network Each of the past three centuries has been dominated by asingle technology.The 18th century was the era of the great mechanical systems accompanying the Industrial Revolution.The 19th century was the age of the steam engine.During the 20th century,the key technology was information gathering,processing,and distribution.Among other developments,we saw the installation of worldwide telephone networks,the invention of radio and television,the birth and unprecedented growth of the computer industry,and the launching of communication satellites. As aresult of rapid technological progress,these areas are rapidly converging and the differences between collecting,transporting,storing,and processing information are quickly disappearing.Organizations with hundreds of offices spread over awide geographical area routinely expect to be able to examine the current status of even their most remote outpost at the push of abutton.As our ability to gather,process,and distri bute information grows,the demand for ever more sophisticated information processing grows even faster[15]. Although the computer industry is still young compared to other industries(e.g.,automobiles and air transportation),computers have made spectacular progress in ashort time.During the first two decades of their existence,computer systems were highly centralized,usually within asingle large room.Not infrequently,this room had glass walls,through which visitors could gawk at the great electronic wonder inside.A medium-sized company or university might have had one or two computers,while large institutions had at most afew dozen.The idea that within twenty years equally powerful computers smaller than postage stamps would be mass produced by the millions was pure science fiction. The merging of computers and communications has had aprofound influence on the way computer systems are organized.The concept of the''computer center''as aroom with alarge computer to which users bring their work for processing is now totally obsolete.The old model of asingle computer serving all of the organization's computational needs has been replaced by one in which alarge number of separate but interconnected computers do the job.These systems are called computer networks.The design and organization of these networks are the subjects of this book. Throughout the book we will use the term''computer network''to mean acollection of autonomous computers interconnected by asingle technology.Two computers are said to be interconnected if they are able to exchange information.The connection need not be via acopper wire;fiber optics,microwaves,infrared,and communication satellites can also be used.Networks come in many sizes,shapes and forms,as we will see later.Although it may sound strange to some people,neither the Internet nor the World Wide Web is acomputer network.By the end the Internet of this book,it should be clear why.The quick answer is: is not asingle network but anetwork of networks and the Web is adistributed system that runs on top of the Internet. There is considerable confusion in the literature between acomputer network and adistributed system.The key distinction is that in adistributed system,a collection of independent computers appears to its users as asingle coherent system.Usually,it has asingle model or paradigm that it presents to the users.Often alayer of software on top of the operating system,called middleware,is responsible for implementing this model.A well-known example of adistributed system is the World Wide Web,in which everything looks like adocument(Web page). In acomputer network,this coherence,model,and software are absent.Users are exposed to the actual machines,without any attempt by the system to make the machines look and act in acoherent way.If the machines have different hardware and different operating systems,that is fully visible to the users. In effect,a distributed system is asoftware system built on top of anetwork.The software gives it ahigh degree of cohesiveness and transparency.Thus,the distinction between anetwork and adistributed system lies with the software(especially the operating system),rather than with the hardware. Nevertheless,there is considerable overlap between the two subjects.For example,both distributed systems and computer networks need to move files around.The difference lies in who invokes the movement,the system or the user.Although this book primarily focuses on networks,many of the topics are also important in distributed systems. 1.Uses of Computer Networks Before we start to examine the tec hnical issues in detail,it is worth devoting some time to pointing out why people are interested in computer networks and what they can be used for.After all,if nobody were interested in computer networks,few of them would be built.We will start with traditional uses at companies and for individuals and then move on to recent developments regarding mobile users and home networking. 2.Business Applications Many companies have asubstantial number of computers.For example,a company may have separate computers to monitor production,keep track of inventories,and do the payroll.Initially,each of these computers may have worked in isolation from the others,but at some point,management may have decided to connect them to be able to extract and correlate entire company. information about the Put in slightly more general form,the issue here is resource sharing,and the goal is to make all programs,equipment,and especially data available to anyone on the network without regard to the physical location of the resource and the user.An obvious and widespread example is having agroup of office workers share acommon printer.None of the individuals really needs aprivate printer,and ahigh-volume networked printer is often cheaper,faster,and easier to maintain than alarge collection of individual printers. However,probably even more important than sharing physical resources such as printers,scanners,and CD burners,is sharing information.Every large and medium-sized company and many small companies are vitally dependent on computerized information.Most companies have customer records,inventories,accounts receivable,financial statements,tax information,and much more online.If all of its computers went down,a bank could not last more than five minutes.A modern manufacturing plant,with acomputer-controlled assembly line,would not last even that long.Even asmall travel agency or three-person law firm is now highly dependent on computer networks for allowing employees to access relevant info rmation and documents instantly. For smaller companies,all the computers are likely to be in asingle office or perhaps asingle building,but for larger ones,the computers and employees may be scattered over dozens of offices and plants in many countries.Nevertheless,a sales person in New York might sometimes need access to aproduct inventory database in Singapore.In other words,the mere fact that auser happens to be 15,000 km away from his data should not prevent him from using the data as though they were local.This goal may be summarized by saying that it is an attempt to end the''tyranny of geography.'' In the simplest of terms,one can imagine acompany's information system as consisting of one or more databases and some number of employees who need to access them remotely.In this model,the data are stored on powerful computers called servers.Often these are centrally housed and maintained by asystem administrator.In contrast,the employees have simpler machines,called clients,on their desks,with which they access remote data,for example,to include in spreadsheets they are constructing.(Sometimes we will refer to the human user of the client machine as the''client,''but it should be clear from the context whether we mean the computer or its user.)The client and server machines are connected by anetwork.Note that we have shown the network as asimple oval,without any detail.We will use this form when we mean anetwork in the abstract sense.When more detail is required,it will be provided. This whole arrangement is called the client-server model.It is widely used and forms the basis of much network usage.It is applicable when the client and server are both in the same building(e.g.,belong to the same company),but also when they are far apart.For example,when aperson at home accesses apage on the World Wide Web,the same model is employed,with the remote Web server being the server and the user's personal computer being the client.Under most conditions,one server can handle alarge number of clients. A second goal of setting up acomputer network has to do with people rather than information or even computers.A computer network can provide apowerful communication medium among employees.Virtually every company that has two or more computers now has e- mail(electronic mail),which employees generally use for agreat deal of daily communication.In fact,a common gripe around the water cooler is how much e-mail everyone has to deal with,much of it meaningless because bosses have discovered that they can send the same(often content-free)message to all their subordinates at the push of abutton. But e-mail is not the only form of improved communication made possible by computer networks.With anetwork,it is easy for two or more people who work far apart to write areport together.When one worker makes achange to an online document,the others can see the change immediately,instead of waiting several days for aletter.Such aspeedup makes cooperation among far-flung groups of people easy where it previously had been impossible. Yet another form of computer-assisted communication is videoconferencing.Using this technology,employees at distant locations can hold ameeting,seeing and hearing each other and even writing on ashared virtual blackboard.Videoconferencing is apowerful tool for eliminating the cost and time previously devoted to travel.It is sometimes said that communication and transportation are having arace,and whichever wins will make the other obsolete. A third goal for increasingly many companies is doing business electronically with other companies,especially suppliers and customers.For example,manufacturers of automobiles,aircraft,and computers,among others,buy subsystems from avariety of suppliers and then assemble the parts.Using computer networks,manufacturers can place orders electronically as needed.Being able to place orders in real time(i.e.,as needed)reduces the need for large inventories and enhances efficiency. A fourth goa lthat is starting to become more important is doing business with consumers over the Internet.Airlines,bookstores,and music vendors have discovered that many customers like the convenience of shopping from home.Consequently,many companies provide catalogs of their goods and services online and take orders on-line.This sector is expected to grow quickly in the future.It is called e-commerce(electronic commerce). 3.Home Applications Access to remote information comes in many forms.It can be surfing the World Wide Web for information or just for fun.Information available includes the arts,business,cooking,government,health,history,hobbies,recreation,science,sports,travel,and many others.Fun comes in too many ways to mention,plus some ways that are better left unmentioned. Access to remote information comes in many forms.It can be surfing the World Wide Web for information or just for fun.Information available includes the arts,business,cooking,government,health,history,hobbies,re creation,science,sports,travel,and many others.Fun comes in too many ways to mention,plus some ways that are better left unmentioned./Oc*Many newspapers have gone on-line and can be personalized.For example,it is sometimes possible to tell anewspaper that you want everything about corrupt politicians,big fires,scandals involving celebrities,and epidemics,but no football,thank you.Sometimes it is even possible to have the selected articles downloaded to your hard disk while you sleep or printed on your printer just before breakfast.As this trend continues,it will cause massive unemployment among 12-year-old paperboys,but newspapers like it because distribution has always been the weakest link in the whole production chain.? 特别声明: 1:资料来源于互联网，版权归属原作者 2:资料内容属于网络意见，与本账号立场无关 3:如有侵权，请告知，立即删除。