外文原文：
Internet Introduction And Management
Internet communication has become a fundamental part of life. The World Wid Web conditions information about such diverse subjects as atmospheric conditions, crop production, stock prices, and airline traffic. Groups establish electronic mailing lists shey can share information of common interest. Professional colleagues exchange business correspondence electronically, and relatives exchange personal greetings.
Unfortunately, most network technologies are designed for a specific purpose. Each enterprise chooses hardware technology appropriate for specific communication needs and budget. More important, it is impossible to engineer a universal network from a single network technology because no single network suffices for all uses. Som groups need high-speed networks to connect computers in a single building. Low-cos technologies that fill the need cannot span large geographic distances. Other groups settle for a slower speed network that connects machines thousands of miles apart.
For over two decades, a new technology has evolved that makes it possible to in terconnect many disparate physical networks and make them function as a coordinate unit. The technology, called internetworking, accommodates multiple, diverse underlying hardware technologies by providing a way to interconnect heterogeneous networks
and a set of communication conventions that makes them interoperate. The internet technology hides the details of network hardware, and permits computers to communicate independent of their physical network connections.
To appreciate internet technology, think of how it has changed business. In addition to high-speed communication among employees in the office environment, networking technologies provide instant feedback among the production side of the business, sales and marketing, and customers. As a result, the speed with which business can plan, implement, assess, and retool has increased; the change is dramatic. The term server applies to any program that offers a service that can be reached a network . A server accepts a request over the network, performs its service, and returns the result to the requester. For the simplest services, each request arrives in a IP datagram and the server returns a response in another datagram.
An executing program becomes a client when it sends a request to a server and waits for a response. Because the client-server model is a convenient and natural extension of interprocess communication on a single machine, it is easy to build programs that use the model to interact.
Servers can perform simple or complex tasks. For example, a time-of-day server merely returns the current time whenever a client sends the server a packet. A web server receives requests from a browser to fetch a copy of a Web page; the server obtains a copy of the file for the page and returns it to the browser.
Usually, servers are implemented as application programst. The advantage of
implementing servers as application programs is that they can execute on any computing system that supports TCP/IP communication. Thus, the server for a particular service can execute on a timesharing system along with other programs, or it can execute on a personal computer. Multiple servers can offer the same service, and can execute on the In fact, managers commonly replicate copies of same machine or on multiple machines. a given server onto physically independent machines to increase reliability or improve performance. If a computer's primary purpose is support of a particular server program, the term \ may be applied to the computer as well as to the server program. Thus, one hears statements such as \
Other groups around the world continue to connect to the Internet as well. The most significant change comes not from added network connections, however, but from additional traffic. As new users connect to the Internet and new applications appear, traffic patterns change. When physicists, chemists, and biologists began to use the Internet, they exchanged files of data collected from experiments. Files of scientific data were large compared to electronic mail messages. As the Internet became popular and users began to browse information using services like the World Wide Web, traffic patterns increased again.
To accommodate growth in traffic,the capacity of the NSFNET backbone was increased three times.The final version, known as ANSNET after the company that supplied it, had a capacity approximately 840 times larger than the original.Since 1995,companies known as Internet Service Providers (ISPs) have each built their own backbone network, many of which have significantly more capacity than the last government-funded backbone. At the current time, it is difficult to foresee an end to the need for more capacity.
Growth in demands for networking is not unexpected. The computer industry has enjoyed a continual demand for increased processing power and larger data storage for many years. Users have only begun to understand how to use networks. In the future we can expect continual increases in the demand for communications. Soon, for example, TCP/IP technologies will be used for telephone and video services.as well as data be needed to accom modate the growth services. Thus, higher-capacity communication technologies will be needed to accommodate the growth.
Properties Of The Internet
The notion of universal service is important, but it alone does not capture all the ideas we have in mind for a unified internet because there can be many implementations of universal services. In our design, we want to hide the underlying internet architecture from the user. That is, we do not want to require users or application programs to understand the details of hardware interconnections to use the internet. We also do not want to mandate a network interconnection topology. In particular, adding a new network to the internet should not mean connecting to a centralized switching point, nor should it mean adding direct physical connections between the new network and all existing networks. We want to be able to send data across intermediate networks even though they are not directly connected to the
source or destination computers. We want all computers in the internet to share a universal set of machine identifiers (which can be thought of as names or addresses). Our notion of a unified internet also includes the idea of network independence in the user interface. That is, we want the set of operations used to establish cornrnunication or to transfer data to remain independent of the underlying network technologies and the destination computer. Certainly, a user should not have to understand the network interconnection topology when creating or using application programs that communicate. Internet Architecture
We have seen how computers connect to individual networks. The question arises, \ are networks interconnected to form an internetwork?\ The answer has two parts. Physically, two networks can only be connected by a computer that attaches to both of them. A physical attachment does not provide the interconnection we have in mind, however, because such a connection does not guarantee that the computer will cooperate with other machines that wish to communicate. To have a viable internet, we need special computers that are willing to transfer packets from one network to another. Computers that interconnect two networks and pass packets from one to the other are called internet gateways or internet routersf.
. From the user's point of view, the Internet appears to consist of a set of application programs that use the underlying network to carry out useful communication tasks. We use the tern1 interoperability to refer to the ability of diverse computing systems to cooperate in solving computational problems. Internet application programs exhibit a high degree of interoperability. Most users that access the Internet do so merely by running application programs without understanding the types of computers being accessed, the TCP/IP technology, the structure of the underlying internet, or even the path the data travels to its destination; they rely on the application programs and the underlying network software to handle such details. Only programmers who write network applica tion programs need to view a TCP/IP internet as a network and need to understand some of the technology. The most popular and widespread Internet application services include:
? World Wide Web. The Web allows users to view documents that contain text
and graphics, and to follow hypermedia links from one document to another. The Web grew to become the largest source of traffic on the global Internet between 1994 and 1995, and continues to dominate. Some service providers estimate that the Web now accounts for 80% of their Internet traffic.
? Electronic mail (e-mail). Electronic mail allows a user to compose a memo
and send a copy to individuals or groups. Another part of the mail application allows users to read memos that they have received. A recent innovation allows users to include \ with a mail message that consist of arbitrary files. Electronic mail has been so successful that many Internet users depend on it for most correspondence. One reason for the popularity of Internet e-mail arises from a careful design: the protocol makes delivery reliable. Not only does the mail system on the sender's computer
计算机科学与技术专业外文翻译--D游程编码的设计及性能研究
