converter
转炉
bulletin
公告，报告
demodulation 解调，检波
asynchronous
不同时的，异步的
tremendous
极大的，巨大的
implementation
执行
hassle
激战，争论
bitmap
位图
6.5 Internet Service
From the user’s point of view， a TCP/IP internet appears to be a set of application programs that use the network to carry out useful communication tasks. We use the term 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 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 application programs need to view the internet as a network and need to understand some of the technology.
The most popular and widespread Internet application services include：
（1） WWW （word wide web）
WWW is a large network of Internet servers providing hypertext and other services to terminals running client applications such as a browser.
WWW enables users to search， access， and download information from a worldwide series of networked servers where information is dynamically interlinked. A Web client passes a user’s request for information to a server， usually by way of a Web browser. The server and client communicate through a transfer protocol， usually the Hyper Text Transfer Protocol （HTTP）. The server then accesses a Web page using a Uniform Resource Locator （URL）. Search engines are available to simplify access by enabling users to enter search criteria on a topic and have several URLs returned for Web pages that pertain to the desired information.
Browser is a GUI-based hypertext client application， such as Internet explorer （IE）， Mosaic， and Netscape Navigator， used to access hypertext documents and other services located on innumerable remote servers throughout the WWW and Internet.
（2） Electronic mail
Electronic mail allows a user to compose memos and send them to individuals or groups. Another part of the mail application allows users to read memos that they have received. Electronic mail has been so successful that many Internet users depend on it for normal business correspondence. Although many electronic mail systems exist， using TCP/IP makes mail delivery more reliable because it does not rely on intermediate computers to relay mail messages. A TCP/1P mail delivery system operates by having the sender’s machine contact the receiver’s machine directly. Thus， the sender knows that once the message leaves the local machine， it has been successful received at the destination site.
There are two kinds of E-mail protocol used in the Internet. One is Simple Mail Transfer Protocol （SMTP） which accepts incoming connections and copies messages from them into the appropriate mailboxes. Another is Post Office Protocol-3 （POP3） which fetches E-mail from the remote mailbox and stores it on the user’s local machine to be read later.
（3） File Transfer Protocol （FTP）
FTP is an application protocol， part of the TCP/IP protocol stack， used for transferring files between network nodes. Although users sometimes transfer files using electronic mail， mail is designed primarily for short text messages. The TCP/IP protocols include a file transfer application program that allows users to send or receive arbitrarily large files of programs or data. For example， using the file transfer program， one can copy from one machine to another a large database containing satellite images， a program written in Pascal or C ++ ， or even an English dictionary. The system provides a way to check for authorized users， or even to prevent all access. Like mail， file transfer across a TCP/IP internet is reliable because the two machines involved communicate directly， without relying on intermediate machines to make copies of the file along the way.
（4） Telecommunication network （Telnet）
Telnet is a standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is used for remote terminal connection， enabling users to log in to remote systems and use resources as if they were connected to a local system.
Remote login allows a user sitting at one computer to connect to a remote machine and establish an interactive login session. The remote login makes it appear that a window on the user’s screen connects directly to the remote machine by sending each keystroke from the user’s keyboard to the remote machine and displaying each character the remote computer prints in the user’s window. When the remote login session terminates， the application returns the user to the local system.
【Vocabulary】
interoperability
n. 互用性，协同工作能力
destination
n. 目的地，目标文件
exhibit
vt. 展出，陈列 n. 展览，展览品
widespread
adj. 分布广泛的，普遍的
dynamical
adj. 动力学的，有力量的
interlink
vt. 连接
criteria
n. 标准
protocol
n. 草案，协议
innumerable
adj. 无数的，数不清的
satellite
n. 人造卫星
mailbox
n. 邮筒，邮箱
correspondence
n. 通信，信件，相应
intermediate
adj. 中间的，媒介的
memo
n. 备忘录
remote
adj. 遥远的，偏僻的，细微的
login
n. 注册，登录
delivery
n. 发送，传输，送货
establish
vt. 建立，设立，安置
【参考译文】
因特网服务
从用户观点来看，TCP/IP协议下的互连网有一条通过网络实现多种通信任务的应用程序。术语互操作性用来表明各计算系统解决计算问题时相互作用的能力。因特网应用程序展示了高度的互操作性。大多数访问因特网的用户只是运行应用程序而不了解TCP/IP技术、互连网基本结构，甚至不了解数据到达目的站的路径；他们依靠应用程序和基本网络软件来处理这些细节。只有编写网络应用程序的程序员，才需要把互联网视为一个网络并了解某些技术。
应用最广的因特网应用服务包括如下诸方面。
（1）万维网
万维网是一种提供因特网服务的大型网络，它为运行客户应用程序，如浏览器软件的终端提供超文本和其他服务。
万维网允许用户从动态链接信息的全球服务器系列中搜索、访问和下载信息。Web客户通常通过Web浏览器，向服务器传送用户需求信息的请求。服务器与客户通过传输协议，通常是文本传输协议（HTTP）进行通信。然后，服务器用统一资源地址（URL）访问网页，搜索引擎可用来简化访问，允许用户输入有关专题的搜索条件，从而使若干URL返回有关所需信息的网页。
浏览器是一种基于图形用户界面的超文本客户程序，如 IE，Mosaic 和Netscape公司的Navigator等。通过浏览器可以对万维网和因特网上无数个远程服务器上的超文本文件和其他服务进行访问。
（2）电子邮件
