英语科普文选：计算机与因特网插件指南

melodyhata200

Life on-line can be a much richer experience when you aren't restricted to just written words and still pictures. Even if you're new to the Net, you've probably heard aobut multimedia on-line--listening to audio, watching animations and videos, even playing in three-dimensional space. Sound and movement make information come alive.　　To experience it, you'll need special pieces of software called plug-ins. The term "plug-in" refers to a small, add-on piece of software which extends the capabilities of your web browser, like Netscape Navigator or Microsoft Explorer, turning your computer into a radio or TV.
　　When you arrive at a web page which contains a file requiring a plug-in which you don't have, you will usually receive a message asking if you want to get it by downloading it and installing it into your computer. Most of the time, the installation will be automatic.
　　Occasionally, you'll run into a downloaded file which needs to be decompressed or un-zipped before installation. Once installed, plug-ins run automatically, without you having to do anything.
　　Many multimedia controls still need to be obtained from the developer but are installed automatically.
　　Shockwave is a good example of this. All you need to do is go to the Macromedia site and click on the link to install the ActiveX control. The rest happens automatically. The next time you go to a "Shocked" website, the Shockwave control loads and plays the movie.
　　Most plug-ins and controls can be downloaded for free on the Internet, although not all will work with every system. Some of them, for instance, only work with windows95.

　　当你不再仅仅限于文字和静止图片时，网上生活会丰富多彩得多。即使是个因特网新手，你也许听说过在线式多媒体——听音乐、看动画和录像，甚至玩三维游戏。声音和活动画面使信息变得活灵活现。

　　要体验这些，你需要叫作“插件”的特殊软件。术语“插件”指的是一个附加的小软件，他能够扩展你的网络浏览器（比如网景的导航者或微软的探索者）的功能——把你的计算机变成一台收音机或电视机。
　　当你进入的网页含有需要使用插件程序的文件，而你却又没有这个程序时，通常你会收到一个信息，询问你是否需要下载该程序并安装到你的电脑里。多数情况下安装是自动运行的。
　　有时候，你会碰到一个需要在安装前先进行解压缩的下载文件。一旦安装完毕，插件就会自动运行，无需你做任何事。
　　许多多媒体的控制程序仍需从开发商处索取，不过都是自动安装的。
　　Shockwave 就是这方面一个很好的例子。你所需要做的就是到Macromedia网站上点击链接来安装ActiveX控制程序，剩下的事自动就会完成。下次再到一个需要使Shockwave 的网站时，Shockwave控制程序就会自动装载并播放电影。
　　大多数插件和控制程序可以从因特网上免费下载，尽管并非所有这些程序都会在每一个系统上正常运行。例如有些就只适用于视窗95。

melodyhata200

There are two categories of networks: local area networks (LANs) and wide area networks (WANs). Here I'd like to mainly talk about LAN.
　　A LAN is a system of computers and associated peripherals such as Printers that are physically connected by cable within a limited geographical area-typically in an office building or on a college campus. The topology of the network may be ring, star or bus. LANs use fiber optics or coaxial cable to connect computers, and each computer must have special communications software installed oil its hard disk. Software has been developed that allows computers with different platforms to coexist and exchange data on the same LAN.
A File Server
　　Many networks are managed by a computer called a file server.A file server has a large－capacity hard disk and special software that manages access to flies on the network. It controls how data and database are shared among users on the network and how users access master copies of data and application software on the centralized hard disk. It is the file server's job to make sure that users don't accidentally try to update a file at the same time and scramble the data. The file server may also manage the access to an expensive piece of hardware such as a laser printer.
　　When a file server is used on a LAN, large databases are stored on the server and users may store all of their work flies there as well.This operation is analogous to someone's manually collecting all the data each day placing them in a file drawer, and then redistributing these data to the workers as needed.
The LAN Operating System--Net Ware
　　Novell's Net ware has become the strongest competitor to IBM's own network operating environment and predates IBM in its support for LANs. In fact,IBM now markets Net ware for both the PC Network and for Token Ring in addition to its own software.The key to Net ware is the file server software and directories on the server are mapped to network drives, thus operating like virtual diskettes from a user's perspective. The server software is a multitasking operating system with a file structure consistent with DOS 2.x and DOS 3.x. The Net ware shell in each workstation intercepts 21H DOS calls and redirects appropriate calls to the server.
　　Perhaps the primary advantage of Net ware is that it runs on a wide variety of networking hardware.Regardless of whether you choose an IBM network, Corvus OMNINET, AT＆T Star LAN, etc., you can obtain Net ware as the operating environment. In fact, if you have one of each the user will not be able to tell the difference(unless there are some distinct performance differentials), and they can be interconnected through Novell's system of bridges and gateways.　　

局域网
　　网络有两类：局域网（LANs）和广域网（WAN）。在这里主要谈一下局域网。
　　局域网就是在一个有限的地域范围内，典型的如在办公大楼或校园中，将计算机及相关外设（如打印机）用电缆进行物理连结而组成的一个系统。网络的拓扑结构可以是环形的、星形的或总线形的。在局域网中，可用光纤或同轴电缆连结多台计算机，且每台计算机的硬盘上必须装有专门的通信软件。允许具有不同平台的计算机共存于同一局域网中，且能进行数据交换的软件早已开发出来了。
文件服务器
　　许多网络由一称为文件服务器的计算机来管理。文件服务器具有大容量的硬盘和一些用来管理对网上文件访问的专门软件。它控制着网上用户如何共享数据和数据库，以及用户如何存取中央硬盘上的数据和应用软件的主拷备。确保两个用户不能偶然地在同一时刻对同一文件进行更新或争夺数据也是文件服务器的工作。文件服务器还可以管理对昂贵硬件（如激光打印机）的访问。
　　当文件服务器用于局域网时，大型数据库就被存储在服务器上，而且用户也可以将其所有的工作文件存放在上面。这就像是一个人每天手工收集所有的数据，放到一文件抽屉中，然后等需要时，再将这些数据发到工作人员手中。
局域网络操作系统——Net ware
　　Novell公司的Net ware操作系统已经成为IBM公司特有的网络操作环境最强有力的竞争对手，它在对局域网络的支持方面也早于IBM公司。事实上，除了自己的一部分软件外，IBM公司也为所有的PC网和令牌环网购买了Net ware操作系统。Net ware 操作系统的最主要部分是文件服务器软件，服务器上的目录被映射到网络驱动器，这样在用户看来，它运行起来就如同是虚拟磁盘。它的服务器软件是一个多任务的操作系统，文件结构也与
　　DOS 2.x和DOS 3.x的文件结构一致。工作站上的 Net ware外壳程序拦截了DOS的21H中断，并重新走向为对服务器的调用。
　　Net ware操作系统的主要优点是能够运行在多种网络硬件上。不论是选择IBM网络，Corvus公司的OMNINET网络，还是AT＆T公司的星形局域网络等，都可获得Net ware的操作环境。

melodyhata200

ARPANET started by the US Dept. of Defense for research.It is the original basis for what now forms the Internet.It was opened to non-military users later in the 1970s.　　The first international connections to ARPANET are established.
　　The TCP/IP Protocol established,and the "Internet" is formed as a connected set of networks using TCP/IP.
　　DNS(Domain Name Server) introduced to the Internet, with then consisted of about 1000 hosts.
　　World Wide Web,invented by Tim Berners-Lee who saw the need for a global information exhcange that would allow physicists to collaborate on research.The Web started as a text-only interface,but NCSA Mosaic later presented a graphical interface for it.   
　　Web traffic over the Internet increased by 300,000%.
　　Netscape 1.0 was written as an alternative browser to NCSA Mosaic.
　　December JavaScript development announced by Netscape.
　　Now Internet is still developing faster and faster...
　　美国国防部建立了阿帕网，作研究用。这就是因特网的前身。阿帕网到七十年代开放给非军事界。
　　阿帕网第一次建立了国际联系。
　　TCP/IP协议建立，由TCP/IP协议连接起来的因特网网络形成了。
　　因特网引进域名服务器，当时整个互联网有一千部主机。
　　提姆.伯纳斯-李觉得有必要让世界上的物理家相互交换资料，进行科研合作，他发明了国际互联网。当时的国际互联网中有的只是文字，但后来，NCSA Mosaic推出了适合国际互联网使用的图形界面。
　　国际互联网流量上升了百分之三十万。
　　继浏览器NCSA Mosaic后，网景1.0推出。
　　12月网景公布，它正在进行JavaScript的开发。
　　Internet 现在仍以越来越快的速度高速发展着。

melodyhata200

The Internet is a giant network of computers located all over the world that communicate with each other.　　The Internet is an international collection of computer networks` that all understand a standard system of addresses and commands, connected together through backbone systems. It was started in 1969, when the U.S. Department of Defence established a nationwide network to connect a handful of universities and contractors. The original idea was to increase computing capacity that could be shared by users in many locations and to find out what it would take for computer networks to survive a nuclear war or other disaster by providing multiple path between users. People on the ARPNET (as this nationwide network was originally called) quickly discovered that they could exchange messages and conduct electronic "conferences" with distant colleagues for purposes that had nothing to do with the military industrial complex. If somebody else had something interesting stored on their computer, it was a simple matter to obtain a copy (assuming the owner did not protect it).
　　Over the years, additional networks joined which added access to more and more computers. The first international connections, to Norway and England, were added in 1973. Today thousands of networks and millions of computers are connected to the Internet. It is growing so quickly that nobody can say exactly how many users "On the Net".
　　The Internet is the largest repository of information which can provide very very large network resources. The network resources can be divided into network facilities resources and network information resources. The network facilities resources provide us the ability of remote computation and communication. The network information resources provides us all kinds of information services, such as science, education, business, history, law, art, and entertainment, etc.
　　The goal of your use of the Internet is exchanging messages or obtaining information. What you need to know is that you can exchange message with other computers on the Internet and use your computer as a remote terminal on distant computers. But the internal details of the link are less important, as long as it works. If you connect computers together on a network, each computer must have a unique address, which could be either a word or a number. For example, the address of Sam's computer could be Sam, or a number.
　　The Internet is a huge interconnected system, but it uses just a handful of method to move data around. Until the recent explosion of public interest in the Internet, the vast majority of the computers on the Net use the Unix operating system. As a result, the standard Unix commands for certain Internet services have entered the online community's languages as both nouns and verbs to describe the services themselves. Some of the services that the Internet can provide are: Mail, Remote use of another computer (Telnet), File Transfer (FTP), News, and Live conversation.
　　The most commonly used network service is electronic mail (E-mail), or simply as mail. Mail permits network users to send textual messages to each other. Computers and networks handle delivering the mail, so that communicating mail users do not have to handle details of delivery, and do not have to be present at the same time or place.
　　The simplest way to access a file on another host is to copy it across the network to your local host. FTP can do this.
　　Presently, a user with an account on any Internet machine can establish a live connection to any other machine on the Net from the terminal in his own office or laboratory. It is only necessary to use the Unix command that sets up a remote terminal connection (Telnet), followed by the address of the distant machine.
　　Before you can use the Internet, you must choose a way to move data between the Internet and your PC. This link may be a high-speed data communication circuit, a local area network (LAN), a telephone live or a radio channel. Most likely, you will use a Modem attached to your telephone line to talk to the Internet. Naturally, the quality of your Internet connection and service, like many other things in life, is dictated by the amount of money that you are willing to spend.
　　Although all these services can well satisfy the needs of the users for information exchange, a definite requirement is needed for the users. Not only should the users know where the resources locates, but also he should know some operating commands concerned to ease the searching burden of the users, recently some convenient searching tools appears, such as Gopher, WWW and Netscape.
　　World wide web (www) is a networked hypertext protocol and user interface. It provides access to multiple services and documents like Gopher does but is more ambitious in its method. A jump to other Internet service can be triggered by a mouse click on a "hotlinked" word, image, or icon on the Web page.
　　As more and more systems join the Internet, and as more and more forms of information can be converted to digital form, the amount of stuff available to Internet users continues to grow. At some points very soon after the nationwide (and later worldwide) Internet started to grow, people began to treat the Net as a community, with its own tradition and customs. For example, somebody would ask a question in a conference, and a complete stranger would send back an answer: after the same question were repeated several time by people who hadn't seen the original answers, somebody else gathered list of  "frequently asked questions" and placed it where newcomers could find it.
　　So we can say that the Internet is your PC's window to the rest of the world.
　　Internet是由位于世界各地相互通信的计算机连接而成的巨大的计算机网络。
　　Internet是计算机网络的国际性的集合，这些网络都符合具有地址和命令的标准体系，并经骨干网连在一起。Internet始建于1969年，当时美国国防部为连接少数几所大学和协议企业而建立了一个全国性网络。最初的想法是要增加计算机能力并可由许多地点的用户共享，并且通过提供用户间多条路径来找到哪一种计算机网络能够在核战或其他灾难中幸存。ARPNET（这种全国网络最初的名称）上的用户很快就发现他们可以与远距离的同事交换消息，并且进行某种目的的电子“会议”，而这些目的与军事工业企业没有任何关系。如果另外一些人在其计算机中存有有趣的东西，得到其拷贝是很容易的事（假定拥有者没有进行保护）。
　　几年间，新的网络接入使越来越多的计算机加入进来。在1973年进行了第一次与挪威和英国的国际连接。今天，有成千上万的计算机网络和数百万台计算机与Internet相连。Internet发展如此之快以至于没有人能准确地说出网上有多少用户。
　　Internet是最大的信息宝库，它可以提供非常巨大的网络资源。这种网络资源可分为网络设备资源和网络信息资源。网络设备资源使我们能够进行远程计算和通信。网络信息资源向我们提供各种各样的信息服务，如科学、教育、商务、历史、法律、艺术和娱乐等等。
　　使用Internet的目的是交换消息或获得信息。你只须知道你可以与Internet上的其他计算机交换消息并将你的计算机用作远端计算机的远程终端，而链路的内部细节并不太重要，只要网络能工作就行。若将多台计算机连接到网络上，每台计算机须有惟一的地址，地址可以是一个字或一个数字。例如Sam的计算机地址可以是Sam或一个数字。
　　虽然Internet是一个巨大的互联系统，但它仅使用一点简单办法就将数据传来传去。近来公众对Internet产生了极大的兴趣，在这之前，绝大多数网上计算机都使用Unix操作系统。结果，用于某些Internet业务的标准Unix命令已经进入联机团体语言当作名词和动词来描述业务本身。Internet可以提供的一些服务为：电子邮件、远程使用其他计算机、文件传送、电子新闻和实况对话。
　　最常使用的网络服务是电子邮件，或简称邮件。电子邮件允许网络用户彼此传送文本消息。邮件的传递由计算机和网络处理，邮件用户不必关心传递的细节，也不必同时在场。
　　从其他主机中获得文件的最简单的方式是通过网络将其拷贝到你的计算机上。文件传送（FTP）可完成这项工作。
　　目前，用户在Internet建立账户后，就可从其办公室和实验室的终端上与网上其他计算机建立实时连接。只需使用Unix命令Telnet来建立远程终端连接，命令后跟上远端计算机的地址即可。
　　在使用Internet之前，必须使用一种方法在你的PC机和Internet之间传送数据。这种连接的链路可以是高速数据通信电路、局域网（LAN）、电话线路或无线信道。最有可能的是，你使用Modem连到电话线上与Internet对话。当然，像生活中许多其他的事物一样，与Internet连接和服务的质量是由你所花钱的数量决定的。
　　虽然所有这些服务可以很好地满足用户对信息交换的需要，但用户仍旧还需要具有一些特定的先决条件。用户不仅要知道信息资源所处的位置，而且要知道一些有关的操作命令。为了减轻用户寻找信息的负担，近来出现了一些方便的搜索工具，如Gopher，WWW和Netscape。
　　全球网（WWW）是一种网络的超文本协议和用户界面。像Gopher一样，它提供多种服务和文件接入方法，但其方法更加有前途。向Internet其他服务的跳转可在“网”页上由鼠标器点击“热链接”的字、图像或按钮来启动。
　　随着越来越多的系统加入Internet，同时随着越来越多的信息可以转变成数字形式，Internet用户所能得到的东西也在继续增加。随着国家（后来是国际）Internet的发展，很快在某些方面人们开始将互联网看作是一个社区，有自己的传统和习惯。例如某些人会在会议上提出一个问题，一个完全陌生的人会传送一个答案；由于一些人没有看到最初的答案而多次重复这一问题，这时另外一些人会搜集一系列“经常提到的问题”并将其放置在新来者能找到的地方。
　　所以我们可以说，Internet是你的PC机通向世界其他地方的窗口。

melodyhata200

Unencrypted messages can be hijacked in transit and read or altered．If the mail is not digitally signed，you can't be sure where it came from．　　未加密的信息可能在传输中被截获、偷看或窜改。如果邮件不是数字签名的，你就不能肯定邮件是从哪里来的。

　　There are many options for securing e-mail，all with a few strengths and probably more weaknesses．
　　确保电子邮件的安全有多种选择，它们都有些长处，但有可能存在更多弱点。
　　Let's take care of the easy decisions．Secure/Multipurpose Internet Mail Extensions(S/MIME)should be the message encryption and digital signature format because it's the accepted standard and is built into leading e-mail clients such as Microsoft Outlook 98/2000 and Lotus Notes R5．Yet a standard such as S/MIME only takes you so far．Each vendor has implemented its own interpretation of S/MIME，which makes interoperability problematic．This drawback is exacerbated by the emergence of S/MIME Version 3 in the newest e-mail clients，which again could create interoperability issues．
　　让我们先关注一下容易做的决定，安全/多用途因特网邮件扩展(S/MIME)应该是信息加密和数字签名的格式，因为它是已被认可的标准，被做进了主要的电子邮件客户端软件中，如微软的Outlook 98/2000和莲花公司的Notes R5。迄今为止，你只能用S/MIME一类的标准。每家供应商都有自己对S/MIME的解释，这就引出 了互用性问题，最新的电子邮件客户端软件中S/MIME三版的出现，加重了这个缺陷，它再次可能带来互用性问题。
　　The path of least resistance is to get an e-mail security gateway， which is analogous to a firewall for e-mail．Every message going in or out pases through the gateway，allowing security policies to be enforced (where and when messages can be sent)，virus checking to be performed，and messages to be signed and encrypted． One drawback of the gateway approach is that it doesn't provide user-based security．For example，the gateway encrypts outbound messages so recipients can verify they came from your company，but recipients can't prove from whom they came．
　　阻力最小的道路就是采用电子邮件安全网关，它相当于电子邮件的防火墙。进出的每一条信息都要经过网关，网关可以实施安全政策(信息在何 时向何地发送)、执行病毒检查并给信息签名和加密。这种网关方法的一个缺陷就是它不 能提供基于用户的安全性。例如，网关对向外发的信息进行加密，因而接收方能验证它 们来自你的公司，但接收方不能证明它们来自哪个人。
　　Client-based methods use your private key to sign messages(proving it came from you)，which is a more granular level of security，but they have weaknesses as well．They need to be configured on each desktop，which includes issuing a digital certificate to each user (for encryption and digital signature)，and ensuring that a proper security profile is configured within the e-mail client．
　　基于客户端的方法采用你私人密钥来签署信息(证明它出自于你)，这是更细化的安全等级，但它们也有弱点。它们需要配置到每个桌面系统，包括向每个用户发数字证书(用于加密和数字签名)，并确保在每个电子邮件客户端都配置了合适的安全配置文件。
　　There are also a number of Web-based secure mail services that keep all messages within their environment at all times to ensure security．You use a secure site on the Internet to compose a message．Once you hit“Send”，the site encrypts and stores the message on its site，and sends the recipient an e-mail notification that a secure message is waiting．The recipient links to the site， provides a shared secret for authentication，and accesses the message via Secure Sockets Layer． Unfortunately，this method does not work with existing enterprise e-mail systems．
　　也有多种基于Web的安全邮件服务，这些服务在任何时候把所有信息都保持在它们的环境中，以确保安全性。你利用因特网上一个安全网站来 编写信息，一旦你点击了“发送”，网站就进行加密和把信息保存在该网站中，并向接收方发一份电子邮件通知，告诉他有一份安全的信息等他去接收。接收方链接到该网站，提供用于认证的共享秘密，通过安全入口层(SSL)访问该信息。可惜，此方法不能与现有的企业电子邮件系统一起工作。
　　The stickiest issue is building a directory of digital certificates．This directory holds the certificates needed to encrypt messages to a recipient．Internally，building the directory may not be a big deal because all certificates for a company can be published in a central Lightweight Directory Access Protocol server，but externally this causes many problems．You will need to establish an agreement with a recipient's organization to ensure access to the right digital certificates．This process， however， creates more user training issues and adds complexity to e-mail communications．
　　最困难的问题是建立数字证书目录。此目录保存着向一名接收人发的信息进行加密所需的证书。从内部讲，建目录可能不是件大事，因为一家公司的所有证书可以由中央简化目录访问协议服务器颁发，但从外部讲，这会引起很多问题。你需要与收件人所在组织达成协议，以确保访问正确的数字证书。然而，这个过程会造成更多的用户培训问题以及增加电子邮件通信的复杂性。
　　Although there is technology available for secure e-mail， widespread deployment is still problematic． However，as more companies and regular e-mail users see the need to secure their messages，the use of digital certificates will one day become a transparent part of your everyday activities．
　　虽然已有技术可用于安全的电子邮件，但广泛部署仍是个问题。然而，随着更多的公司和普通电子邮件用户看到了确保其信息安全的需要，终 有一天使用数字证书会变得透明，成为你日常生活的一部分。

melodyhata200

Internet use appears to cause a decline in psychological well-being, according to research at Carnegie Mellon University.

Even people who spent just a few hours a week on the Internet experienced more depression and loneliness than those who logged on less frequently, the two-year study showed. And it wasn't that people who were already feeling bad spent more time on the Internet, but that using the Net actually appeared to cause the bad feelings.

Researchers are puzzling over the results, which were completely contrary to their expectations. They expected that the Net would prove socially healthier than television, since the Net allows users to choose their information and to communicate with others.

The fact that Internet use reduces time available for family and friends may account for the drop in well-being, researchers hypothesized. Faceless, bodiless "virtual" communication may be less psychologically satisfying than actual conversation, and the relationships formed through it may be shallower. Another possibility is that exposure to the wider world via the Net makes users less satisfied with their lives.

"But it's important to remember this is not about the technology, per se; it's about how it is used," says psychologist Christine Riley of Intel, one of the study's sponsors. "It really points to the need for considering social factors in terms of how you design applications and services for technology."  

根据卡内基·麦伦大学的研究，使用因特网可能会导致心理健康程度下降。

两年的研究表明，上网次数多的人与较少的人相比，即使是一周仅上网几小时也会经常地感觉到沮丧和孤独。这不是说已经有不良感觉的人在网上花费了更多的时间，而是说使用因特网似乎确实诱发了人们的不良感觉。

研究人员对这些结果困惑不解，因为这与他们的预料截然相反。他们预测，和看电视相比，从社交角度来说，上网可能更健康一些因为网络允许使用者选择自己需要的信息并且和别人进行交流。

研究者推测说，实际情况是上网使网民减少了和家人及朋友共度的时光，这也许可以解释他们心理健康状况下降的原因。和面对面的交谈相比，这种见不着面、看不见人的“虚”的交流可能会使人从心理上缺乏满足感。人们通过这种交流结下的友谊也不会太深。还有一种可能是，网民通过因特网所了解到的广阔世界使他们对自己的生活不那么满意了。

“然而，重要的是不要忘记这与技术本身是无关的，问题在于如何使用因特网。”这项研究的发起人之一、心理学家、英特尔公司的克里斯廷·赖利说，“这的确表明在考虑从技术上如何设计应用和服务时有必要把社会因素考虑进去。

melodyhata200

What Does the Multimedia Actually Mean?
　　Literally, multimedia means TWO or more media. If the publisher of this book Wanted to join the Current hype about multimedia, he could advertise the book as using multimedia technology. After all, it contains two media：text and graphics(the figures). Nevertheless, when most pepole refer to multimedia, they generally mean the combination of two or more continuous media, that is, media have to be played during some well-defined timed times interval, usually With the users interaction. In practice, the two media are normally audio and video, that is, sound plus moving pictures.
　　In the multimedia environment, we have graphics and text at the same time, we can also add the photograph, animation, good-quality sound, and full motion video. All of the technologies make computers more interesting and much easier to use. For example, a multimedia program can play a segment of movie：a cat was playing a reel of thread, with "mewing…" The tableau can a be immediately captured and pasted to a text at the side of a paragraph of words.

CD－ROM
  CD－ROM is an optical technology that is used to make compact disks for CD stereo systems；it has now been adapted to store data for computers. CD disk is an optical disc, on which data is recorded as a set of dips in the surface. When a laser beam shines across the dips, the reflected distortion represents the data. Optical disc has a 600 megabyte capacity and are well suited for the computer applications needing the vast storage. The playback effect of optical discs for music is very perfect, no hiss and crackle as with other photo-records. A computer system must have a CD drive that is designed to read the disks and translate the data into a form it can process. CD disks can store huge volumes of data and are very useful for reference material an encyclopedia, for example. However, as the name(read-only memory)implies, CD disks cannot be written on or changed in any way.

Musical Instrument Digital Interface
　　Musical Instrument Digital Interface, or MIDI, is a communication interface provided on virtually every synthesizer made today. It provides a standard that allows Products by different companies to communicate with each other. MIDI's original purpose as to allow a musician to control several synthesizers from one keyboard rather than connecting a number of keyboard-equipped synthesizers, to produce the multilayered sound. The MIDI standard provides for 16 channels. You can assign each channel to a specific synthesizer or synthesizer voice. If you record a sequence and assign it to channel 5, for example, only an instrument set to listen on channel 5 plays back the sequence, this feature enables MIDI networks to sound like orchestras.

Audio
　　An audio(sound) wave is a one-dimensional acoustic(pressure) wave. When an acoustic wave enters the ear, the eardrum vibrates, causing the tiny bones of the inner ear to vibrate along with it, and sending nerve pulses to the brain. These pluses are perceived as sound by the listener. Audio waves can be converted to digital form by an ADC (Analog Digital Converter). An ADC takes an electrical voltage as input and generates a binary number as output. Music, of course, is just a special case of general audio, but an important one. The representation, processing, storage, and transmission of such audio signals are a major part of the study of multimedia systems.

Virtual Reality－Enter a Fancy Space
　　In the science fiction "Neuromancer", William Gibson describes a space, Cyberspace, controlled by a computer. Once his brain was linked with the computer, a man would undergo all experiences in the space. His various senses in the realistic world would be replaced with a series of new electric stimuli. The Cyberspace is regarded as a goal of future virtual reality.
　　In fact, virtual reality is exploiting software and hardware of the computer to generate a simulation of an environment, such as an easeful classroom, a breathtaking action and so on. In such a simulated environment, one would have an immersed sense. For example, in a Virtual space, Students can "dissect" a human body, "visit" ancient battlefields, or "talk" with Shakespeare, …

“多媒体”到底意味着什么？
　　从字面上看，多媒体是指两种或两种以上媒体。如果本书的出版者想加入当前多媒体广告的行列，那么，他就可以声称这本书已经使用了多媒体技术。因为，毕竟，它包含两种媒体，文本和图形（插图）。然而，当大多数人谈及多媒体时，他们通常是指混合了两种或两种以上连续的媒体，即，媒体可以在一段规定的时间之内连续播放，并且通常伴随着与用户的交互。实际上，这两种媒体通常指音频和视频，即声音加移动的图片。
　　在多媒体环境中，我们可以同时拥有图形和文本，也可以增加图片、动画、高质量的音响和全动录像，这些技术使得计算机使用起来更有趣也更容易。例如，一个多媒体程序可以播放一段电影：一只小猫正在玩线团，发出“喵喵”的声音……，这个画面可以立即剪贴到一段文字旁边。

CD-ROM
　　CD－ROM原是一种用于制作CD立体声系统的小型圆盘的光学技术，现在它已经被用来为计算机储藏数据。CD盘是一种光盘，其数据以一组坑的形式记录在光盘表面上，当激光束扫过这些坑的时候，返回的失真程度就代表了这些数据。光盘有600兆字节的容量，非常适合需要大存储量的计算机应用。音乐光盘播放音乐的效果非常完美，没有像用其他唱片那样的嘶嘶声和噼啪声。计算机系统必须配备一个CD驱动器用来读取光盘并将数据转换到计算机能够处理的形式。CD盘可以储存大量数据，这对于文献资料的存储非常有用，如存储一部百科全书。然而，正如其名字（只读存储器）所指出的那样，CD盘不能写，也不能用任何方式改变其内容。

音乐设备数字接口
　　音乐设备数字接口，或称MIDI，实际上是为目前制造的各种合成器提供的一种通信接口。它提供的标准允许不同厂家生产的音乐产品可以进行通信。MIDI原本的目的并不是为了连接许多装备了键盘的合成器来产生多层次的声音,而是为了使音乐家能够用一个键盘控制几个合成器进行演奏。MIDI标准提供16个声道，你可以为每一个声道设置一个特别的合成器或合成器声音。例如，你记录了一个音乐序列并将它设置到第五声道上，并且只有一种乐器在第五声道上回放这个音序，其效果使MIDI网听起来很像乐队。

声波
　　声波是一维的声学（气压）波，当声波进入人的耳朵时，耳膜振动，引起耳朵内部的一些小骨头振动，并向大脑发出神经脉冲，这些脉冲被听者作为声音听到了。声波可以通过ADC（模拟数字转换器）转换到数字形式，ADC采用电压作为输入并产生二进制数作为输出。当然，音乐是声波的一种特殊情况，但却是重要的一种。这些音频信号的表达、处理、存储和转换是多媒体系统研究的主要部分。

虚拟现实——进入想像的空间
　　William Gibson在他的科幻小说《Neuromancer》中描述了一个计算机控制空间Cyberspace，一个人只要将自己的大脑与计算机连接起来，进入Cyberspace，就可以体验在该空间的一切遭遇。一套全新的电子刺激将代替人在真实世界的各种感觉。这个Cyberspace被认为是未来虚拟现实的目标。
   实际上，虚拟现实就是利用计算机的软件和硬件去产生一种境界的仿真，如仿真一个学习场所，一次惊险的行为等等，人在这个仿真的环境中，有一种“身临其境”的感觉。如，在虚拟的空间中，学生们可以“解剖”人体，“参观”古战场，与莎士比亚“对话”等等。

melodyhata200

What can Internet do for us? Internet has four main applications as follows:
　　l. E－mail. With Internet, people can compose, send, and receive electronic mails. Many people get dozens of messages a day and consider it as their primary way of interacting with the outside world, far outdistancing the telephone and snail mail.
　　2. News. Newsgroups are specialzied forums in which users with a common interest can  exchagne messages. Thousands of newsgroups exist on Internet,on technical and nontechnical topics,including computers, science, ecreation, and politics.
　　3. Remote login. Using the Telnet,Rlogin, or other programs, the  user anywhere on  Internet can log into any other machine  on  which  he  has  an  account.
　　4. File transfer. Using the FTP program, it is possible to copy files from one machine on Internet to another. Vast numbers of articles, databases, and  other information are available in this way.

The Network Computer
　　The network computer, also known as the Internet toaster,Internet appliance or Internet device,is the low cost,no　maintenance desktop device. It allows users to effortlessly connect to　Internet and network resources. From there, they can share any　resource and perform all computing tasks that they currently do on　their PCs.
　　The network computer offers simplicity. Stripped of the　hardware and software that　complicate the PC life and only capable of network access and display, the network computer relies on the network for virtually all software, services, processing, data, and resources. It eliminates the continuous cycle of desktop hardware and software upgrades, pushing that burden instead on to the network.
　　Need the latest word processor or spreadsheet? Run it off the server, Want to save your work?  Just send it off to the network, where it will be stored secured, and backed up.

Firewall: a Measure of Network Security
　　With the popularity of Internet,it is increasingly difficult to guarantee the network security. Sometimes the network  was breached because of not only hackers' visit but also authorized users' care out of ordinary. Firewall just is an effective measure of network security. It will ensure that all communications conform to your Security policy.
　　Once two lads broke into the university network,created a bulletin board,loaded popular commercial software on it and invited users across Internet to download the packages. Thanks to a firewall, network administrators found the system breached. They checked the audit and found the students' IP addresses, names and location. The police arrested the two hackers.

E－mail
　　One important application for Internet is communication between computer users by the electronic mail, or E－mail. E－mail allows users to electronically transmit and receive messages, text, or data. E-mail functions futher like a mailbox: the user can send messages whether or not the intended receiver is currently on the network and the message is stored, along with a signal for the receiver that indicates that there is a message waiting.
　　E-mail has replaced the telephone for many messages. Users can respond when it is convenient,without being interrupted,  and can get their message either on-screen or in the printed form.

Electronic Bulletin Boards
Another interesting application for Internet is the electronic bulletin board which is also called bulletin board service, BBS for short.It allows users to post and retrieve messages that are not directed to a specific user, much like announcements are posted on an office bulletin board. BBS has been used for everything from dating service and want ads to highly specialized applications such as the exchange of research data in a narrow scientific field.

因特网能为我们做什么？因特网有如下四个主要的应用：
　　1、电子函件。在因特网上，人们可以创作、发送和接收电子函件。许多人每天收到成打的电子函件信息，这大大超过电话和蜗牛式的普通邮件，他们把它看作与外界交流的主要方式。
　　2、新闻。新闻组是供具有共同爱好的用户交流信息的限定范围的论坛。在因特网上，有上千个技术性和非技术性的新闻组，涉及计算机、科学、娱乐和政治等专题。
　　3、远程登录。在因特同任何地方的用户都可以使用Telnet、Rlogin或其他的程序登录到他具有帐号的其他计算机上。
　　4、文件传输。在因特网上，使用FTP程序可以从一台计算机向另一台计算机拷贝文件。大量的文章、数据库及其他信息都可以通过这种方式传输。

网络计算机
　　网络计算机，也叫“因特网烤炉”、因特网装置、因特网设备，是一种价格低廉、无需维护的台式装置。它可以让用户不费力气就能接到因特网和网络资源上。从这种台式装置上，他们就能分享到任何资源，完成所有目前需要在PC机上进行的计算任务。
　　网络计算机提供了简洁性，去掉了使PC应用复杂化的软硬件，只留下网络访问与显示功能，实际上，网络计算机在软件、服务、处理、数据和资源上全部都依赖网络。这就消除了周而复始的台式软硬件升级，而是把这个负担放到了网络上。
　　需要最新的字处理软件或电子表格软件？将它从服务器上下载来运行即可。需要把你做的工作保存起来吗？只需把它送上网络,在那它被安全地存储并作好备份。

防火墙：一项网络安全措施
　　随着因特网的普及，网络的安全性越来越难以保证，网络遭破坏的事件时有发生。这既有“黑客”们的“光顾”，也有合法授权用户的“特别关照”。而防火墙就是保证网络安全的一种有效措施。它可以保证所有的通信符合安全政策。
　　曾有两位年轻人非法侵入了大学网络，创建了一个公告牌，并在上面装入了流行的商业软件，还邀请因特网上的用户下载这些软件包。幸好有了防火墙，网络管理员发现系统遭到了破坏。他们检查了审计记录，找到了这两名学生的IP地址、姓名及地点。警察就逮捕了这两名“黑客”。

电子函件
　　因特网的一个重要应用是计算机用户间利用电子函件相互通信。电子函件使用户可以用电子手段传递和接收消息、文本和数据。电子函件的功能非常类似于邮箱：无论接收者是否在网络上，用户都可以向他发送消息，该消息被保存起来，同时向接收者发送一个信号表明有一个消息在等待处理。
电子函件传送消息已经代替了电话。用户可以在方便的时候再去对它作出反应，而不会被打扰，并可以在屏幕上显示消息或把它打印出来。

电子公告板
　　因特网的另一个有趣的应用是电子公告板，又称公告服务，简称BBS。它允许用户的投递或回收消息，这些消息不是针对某一个具体的用户发送的，它就像在办公室的公告牌上发布通知一样。BBS被广泛应用在从日常服务、招聘广告到非常专业的领域中，例如在一个非常狭窄的科学领域中交换调查数据。