第九次课讲义
2006年10月2日星期一
章节:
第三章 计算机网络
4.1 网络原理
4.2 综合业务数字网
教学任务:
通过本课学习掌握网络原理和综合业务数字网的原理及应用。
重点及难点:
重点掌握ISDN的使用及特点。
难点是理解网络的基本组成。
教学内容提要:
生词及表达方式
antenna 天线
enterprise 事业,计划,干事业,企业
metropolitan 主要都市的,首都的,大城市
parallel 平行面,平行线,平行,平行的
continent 陆地,大陆
collision 冲突,碰撞
troubleshoot 充当故障检修员
suit for 适合于……
controller 控制器，管理员
grid 栅格，格子
dimensional 空间的
hypercube 超立方体
hyper tree 超级树
toroidal 环形的，超环面的，中国螺旋管
hyperlink n.[计]超链接 pundit n. 空谈家, 权威性的评论者
glibly adv.流利地, 流畅地 conceptualize v.使有概念
legitimization n.合法化, 认为正当 institutional adj.制度上的,机构上的
fledging n.刚会飞的幼鸟,无经验的人 follow-on 继续
slice vt.把...切成薄片; 把...分成部分 break down 破坏；拆散细分；分类
disabled n.禁止使用的 quad n. 四元组
mainframe n.[计]主机, 大型机 vulnerable adj.易受攻击的, gigabyte n.十亿字节(giga-为字首,“十亿”) supercomputer n.[计] 超型计算机
reminiscent adj. 怀旧的；回忆的 Domain Name 域名
host computer (host) 主机 instant messaging 即时信息服务
packet-switching 分组交换技术 client/server 客户机/服务器
circuit switching 线路转接 bit n.[计]位, 比特
Router [计] 路由器 routing algorithm 路径算法
Ethernet n.以太网 token ring networks 令牌环形网
Telnet 远程登录 Finger 查找因特网用户的程序
Ping 测试IP地址的程序 Tracert 检查路由器程序
UNIX 美国AT&T公司的操作系统, UNIX操作系统 let alone 更不用说…
substrate n. 衬底，基底 middleware n.中间设备,中间件
authentication n.证明, 鉴定 repository n. 储存库，资料档案库
backbone n.主干,干线 pipeline n. 流水线，管线
Mbps n. 兆比特每秒 Gbps n. 千兆比特每秒
redundancy n.冗余 built-in adj.内置的, 嵌入的n.内置
tier n.一层，一排 curb n.路边
mom and pop shop n. 夫妻店 dial-up 拨号上网
narrowband n. 窄带 broadband n. 宽带
throughput n. 吞吐量 objectionable adj.该反对的,不能采用的
modem n.调制解调器 delay n.延迟
TCP (Transfer Control Protocol) 传输控制协议
IP(Internet Protocol) 网际协议
FTP (File Transfer Protocol) 文件传输协议
POP (Post Office Protocol)邮局协议
HTTP(Hypertext Transfer Protocol)超文本传输协议
IP address IP地址
URL (Uniform Resource Locator) 统一资源定位
DNS (domain name system) 域名系统
SSL (Security Socket Layer) 加密套接字协议层
RAM (random access memory) 随机存储器
ARPANET (Advanced Research Projects Agency Network) 美国国防部高级研究计划局建立的计算机
计算机英语相关知识及语法
网络原理
1. 网络的基本组成
l 至少两台计算机
l 网络接口
l 连接媒介
l 网络操作系统
2. 虚拟专用网
l 虚拟专用网（VPN）被定义为通过一个公用网络（通常是因特网）建立一个临时的、安全的连接，是一条穿过混乱的公用网络的安全、稳定的隧道。虚拟专用网是对企业内部网的扩展。
l 虚拟专用网可以帮助远程用户、公司分支机构、商业伙伴及供应商同公司的内部网建立可信的安全连接，并保证数据的安全传输。通过将数据流转移到低成本的压网络上，一个企业的虚拟专用网解决方案将大幅度地减少用户花费在城域网和远程网络连接上的费用。同时，这将简化网络的设计和管理，加速连接新的用户和网站。另外，虚拟专用网还可以保护现有的网络投资。随着用户的商业服务不断发展，企业的虚拟专用网解决方案可以使用户将精力集中到自己的生意上，而不是网络上。虚拟专用网可用于不断增长的移动用户的全球因特网接入，以实现安全连接；可用于实现企业网站之间安全通信的虚拟专用线路，用于经济有效地连接到商业伙伴和用户的安全外联网虚拟专用网。
l 虚拟专用网至少应能提供如下功能：
1) 加密数据，以保证通过公网传输的信息即使被他人截获也不会泄露。
2) 信息认证和身份认证，保证信息的完整性、合法性，并能鉴别用户的身份。
3) 提供访问控制，不同的用户有不同的访问权限。
综合业务数字网（ISDN: Integrated Services Digital Network）
综合业务数字网（ISDN）是一种数字化电话连接系统。几十年来，电话通信使用的一直是模拟连接方式，而 ISDN 是第一部定义数字化通信的协议，该协议支持标准线路上的语音、数据、视频、图形等的高速传输服务。ISDN 的承载信道（B 信道）负责同时传送各种媒体，占用带宽为 64 kb/s （有些交换机将带宽限制为 56 kb/s）。数据信道（D 信道）主要负责处理信令，传输速率从 16 kb/s 到 64 kb/s 不定，这主要取决于服务类型。ISDN 不仅限于公共电话网络，其传输还可以通过分组交换网络、电报网或有线电视网络等完成。ISDN 有两种基本服务类型：
- 基本速率接口（BRI：Basic Rate Interface）：由两个 64 kb/s 的 B 信道和一个 16 kb/s 的 D 信道构成，总速率为 144 kb/s。该服务主要适用于个人计算机用户。Telco 提供的 U 接口的 BRI 支持双线、传输速率为 160 kb/s 的数字连接。通过回波消除操作降低噪音影响。各种数据编码方式（北美使用 2B1Q，欧洲国家使用 4B3T）可以为单线本地环路提供更高的数据传输率。
- 主要速率接口（PRI：Primary Rate Interface）：能够满足用户的更高要求。典型的 PRI 由 23 个 B 信道和一个 64kb/s 的 D 信道构成，总速率为 1536 kb/s。在欧洲，PRI 由30个 B 信道和一个 64kb/s 的 D 信道构成，总速率为 1984kb/s。通过 NFAS（Non-Facility Associated Signaling），PRI 也支持具有一个 64 kb/s D 信道的多 PRI 线路。
CCITT（现在为 ITU-T）研究组于1984年首次推出了一组 ISDN 推荐协议。在此之前，各个地区都有各自的 ISDN 版本。通过代码设置机制可以启用国家专用信息单元（nation-specific information elements）以支持各个地区使用数据结构内自己的信息单元。其中通用的一种国家 ISDN 专用单元为 National ISDN，由 Bellcore 提出，适用于美国。National ISDN 中包含四种专用网络信息类型，它不包含任何单个八位字节信息单元。与其它信息单元相比，National ISDN 中增加了 SEGMENT、FACILITY 和 REGISTER 信息类型以及分段信息和扩展功效信息单元，此外也改变了一些字段值意思，并增加了一些新字段值。
受带宽和服务的限制，传统的 ISDN 被称之为窄带 ISDN，与如今的宽带 ISDN（BISDN）相对。
难点解析
1. Computer networks link computer by communication lines and software protocols, allowing data to be exchanged rapidly and reliably
计算机网络通过通讯线路和软件协议把计算机连接起来，允许数据快速可靠地进行交换
2. Traditionally, networks have been split between wide area networks( WANs ) and local area networks ( LANs)
传统上把网络划分成广域网（WAN）和局域网（LAN）
3. A WAN is a network connected over long-distance telephone lines, and a LAN is a localized network usually in one building or group of buildings close together
WAN是通过长途电话线连接的网络，LAN通常是在一个建筑物内或邻近的一群建筑物间的网络
4. The distinction, however, is becoming blurred
不过，其界限正变得模糊起来
5. It is now possible to connect up LANs remotely over ttelephone links so that they look as a single LAN
现在，可以通过电话线把远距离的LAN连接起来，使其看上去像一个LAN
6. Originally, networks were used to provide terminal access to another computer and to transfer files between computer
最初，网络用来提供计算机终端访问和计算机之间的文件传输
7. Today, networks carry e-mail, provide access to public databases and bulletin boards, and are beginning to be used for distributed systems
如今，网络提供传递邮件，提供访问公共数据库和电子公告板的功能，并已经用语分布式系统
8. Networks also allow users in one locality to share expensive resources, such as printers and disk-systems
网络还允许多个用户异地共享贵重资源，如打印机和磁盘系统
9. Distributed computer systems are built using networked computers that cooperate to perform tasks
使用连网计算机协同完成任务，就构成了分布式计算机系统
10. In this environment each part of the networked system does what it is best at
在这种环境中，网络系统的每一部分都去做它最擅长的事
In packet-switched networks, messages are first broken down into packets. Appended to each packet are digital codes that indicate a source address (the origination point) and a destination address, as well as sequencing information and error control information for the packet. Rather than being sent directly to the destination address, in a packet network, the packets travel from computer to computer until they reach their destination. These computers are called routers. Routers are special purpose computers that interconnect the thousands of different computer networks that make up the Internet and route packets along to their ultimate destination as they travel4. To ensure that packets take the best available path toward their destination, the routers use computer programs called routing algorithms.
在分组交换的网络中，信息首先被分解为许多信息包。每个信息包附加数字代码用于指示其源地址(开始点)和目的地地址、以及顺序信息和错误控制信息。在分组网络中，信息包不是直接被送到目的地地址,而是在计算机与计算机之间旅行直到它们到达目的地。这些计算机叫做路由器。路由器是一种特殊用途的计算机，它将组成因特网的成千上万个不同计算机网络互相联接起来，并在信息包旅行时将它们的向终极目的地发送。路由器使用一种叫做路由算法的计算机程序，以确保信息包取通向它们目的地的最佳可用路径。
Packet switching does not require a dedicated circuit but can make use of any spare capacity that is available on any of several hundred circuits. Packet switching makes nearly full use of almost all available communication lines and capacity. Moreover, if some lines are disabled or too busy, the packets can be sent on any available line that eventually leads to the destination point.
分组交换不需要一个专用线路，但是可以利用几百条线路中任何可用的空闲能力。分组交换几乎充分利用了所有可用的通信线路和能力。而且，如果一些线路不通或太忙的话,信息包能在任何可用的、最终通向目的地的线路上传送。
TCP/IP. While packet switching was an enormous advance in communications capacity, there was no universally agreed upon method for breaking up digital messages into packets, routing them to the proper address, and then reassembling them into a coherent message. This was like having a system for producing stamps, but no postal system (a series of post offices and a set of addresses).
TCP/IP。尽管分组交换是通信能力的一个巨大进步,但对于将数字信息分解为信息包、将它们传输到适当的地址, 然后重新组装为原来的信息，还没有一种公认的方法。这就像有了一个生产邮票的系统,而没有邮政系统(一系列的邮局和一套住址)一样。
TCP/IP answered the problem of what to do with packets on the Internet and how to handle them. TCP refers to the Transmission Control Protocol (TCP). IP refers to the Internet Protocol (IP). A protocol is a set of rules for formatting, ordering, compressing, and error-checking messages. It may also specify the speed of transmission and means by which devices on the network will indicate they have stopped sending and/or receiving messages. Protocols can be implemented in either hardware or software. TCP/IP is implemented in Web software called server software (described below). TCP is the agreed upon protocol for transmitting data packets over the Web. TCP establishes the connections among sending and receiving Web computers, handles the assembly of packets at the point of transmission, and their reassembly at the receiving end.
TCP/IP回答了在因特网上用信息包做什么和如何处理信息包的问题。TCP指传输控制协议，IP表示网际协议。协议是一组用于信息的格式化、次序化、压缩和检查错误的规则。它也可以限定传输速度和网上设备显示它们已停止发送及(或)接收信息的方法。协议既可以通过硬件也可通过软件来实现。TCP/IP(传输控制协议/网际协议)通过被称为服务器软件的网络软件来实现(在下文描述)。TCP(传输控制协议)是用来在网上传输数据的公认协议。TCP(传输控制协议)在发送和接收网络计算机之间建立连接,处理信息包在传输点的组装和在接收端的重新组装。
TCP/IP is independent from any local network technology and can adapt to changes in the local level. The Internet Layer is responsible for addressing, packaging, and routing messages on the Internet. The Transport Layer is responsible for providing communication with the application by acknowledging and sequencing the packets to and from the application. The Application Layer provides a wide variety of applications with the ability to access the services of the lower layers. Some of the best known applications are Hyper Text Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP), all of which we will discuss later in this chapter.
TCP/IP(传输控制协议/网际协议)独立于任何局部网络技术，并能适应在局部水平上的改变。因特网层负责信息的寻址、封装及其在因特网上的路线排定。通过对信息包去与来应用层的确认和排序，传送层负责为应用层提供通信。应用层为许多应用提供访问较低层服务的能力。一些众所周知的应用是超文本传输协议(HTTP)、文件传输协议(FTP)和简单邮件传输协议(SMTP),稍后我们将本章中讨论
IP Addresses. TCP handles the packetizing and routing of Internet messages. IP provides the Internet’s addressing scheme. Every computer connected to the Internet must be assigned an address—otherwise it cannot send or receive TCP packets. For instance, when you sign onto the Internet using a dial-up telephone modem, your computer is assigned a temporary address by your Internet Service Provider
IP地址。TCP(传输控制协议)处理因特网信息的分组化和传输路线排定。IP(网际协议)提供因特网的地址分配方案。每部连接到因特网的计算机必须分配一个地址，否则它不能够发送或接受TCP(传输控制协议)信息包。举例来说，当你在使用调制解调器拨号上网时，你的计算机由因特网服务提供商分配一个临时地址。
Internet addresses, known as IP addresses, are 32-bit numbers that appear as a series of four separate numbers marked off by periods, such as 201.61.186.227. Each of the four numbers can range from 0–255. This “dotted quad” addressing scheme contains up to 4 billion addresses (232). The leftmost number typically indicates the network address of the computer, while remaining numbers help to identify the specific computer within the group that is sending (or receiving ) a message.
因特网地址,即众所周知的IP地址，是一个32位的数字，它以一串由园点隔开的四个数字出现,例如 201.61.186.227。四个数字中的每一个都在0-255范围内。这个“点分四元组”地址分配方案包含多达到40亿个地址(232)。最左边的那个数字指示计算机的网络地址，而其余的数字帮助识别正在发送(或接收)信息的团体里面一台特定的计算机。
SMTP, POP, and IMAP: Sending E-mail. E-mail is one of the oldest, most important, and frequently used Internet services. STMP (Simple Mail Transfer Protocol) is the Internet protocol used to send mail to a server. POP (Post Office Protocol) is used by the client to retrieve mail from an Internet server. You can see how your browser handles SMTP and POP by looking in your browser’s Preferences or Tools section, where the mail settings are defined.
SMTP、POP和IMAP:发送电子邮件。电子邮件是最老、最重要和经常使用的因特网服务之一。STMP(简单邮件传输协议)是用于将将邮件发送到服务器的因特网协议。客户机利用POP(邮局协议)从因特网服务器上检索邮件。你可以通过察看浏览器上“收藏”和“工具”栏看见你的浏览器如何处理SMTP和POP协议,邮件设定在这两栏定义。
You can set POP to retrieve e-mail messages from the server and then delete the messages on the server, or retain them on the server. IMAP (Internet Message Access Protocol) is a more current e-mail protocol supported by many servers and all browsers. IMAP allows users to search, organize, and filter their mail prior to downloading it from the server.
你可以设定用POP协议取回来自服务器的电子邮件信息然后删除服务器上的信息,或在服务器上保留它们。IMAP(因特网消息访问协议)是一个由许多服务器和所有的浏览器支持的更通用的电子邮件协议。IMAP允许使用者在从服务器下载邮件之前搜寻、组织和过滤邮件
FTP: Transferring Files. FTP (File Transfer Protocol) is one of the original Internet services. It is a part of the TCP/IP protocol and permits users to transfer files from the server to their client machine, and vice versa. The files can be documents, programs, or large database files. FTP is the fastest and most convenient way to transfer files larger than 1 megabyte, which many mail servers will not accept.
FTP:传输文件。FTP(文件传输协议)是最初的因特网服务之一。它是TCP/IP协议的一部份，它允许用户把文件从服务器传送到客户机，反之亦然。文件可以是文档、程序、或大型数据库文件。FTP是传输大于1M文件最快速和最方便的方法，而许多邮件服务器将不接受大于1M的文件
The Internet can be viewed conceptually as having four layers: the network technology substrate, transport services and representation standards, middleware services, and applications. The network technology substrate is composed of telecommunications networks and protocols. The transport layer houses the TCP/IP protocol. The applications layer contains client applications such as the World Wide Web, e-mail, and audio or video playback. Middleware is the glue that ties the applications to the communications networks, and includes such services as security, authentication, addresses, and storage repositories.
因特网能被概念性地分为四层：网络技术基体、传送服务和表示标准、中间件服务和应用。网络技术基体由电信网络和协议组成。传送层收容TCP/IP协议。应用层包含客户机的应用，例如万维网、电子邮件和声频或视频回放。中间件是将应用与通信网络连接起来的粘合剂,并且包括如安全、验证、地址和存储等服务。
Figure 8-3 illustrates the main physical elements of today’s Internet. The Internet’s backbone is formed by Network Service Providers (NSPs), which own and control the major networks. The backbone has been likened to a giant pipeline that transports data around the world in milliseconds. In the United States, the backbone is composed entirely of fiber-optic cable (described more fully below), with bandwidths ranging from 155 Mbps to 2.5 Gbps. Bandwidth measures how much data can be transferred over a communications medium within a fixed period of time, and is usually expressed in bits per second (bps), kilobits (thousands of bits) per second (Kbps), megabits (millions of bits) per second (Mbps), or gigabits (billions of bits) per second (Gbps).
图8-3说明了今天因特网的主要物理组成。因特网主干由拥有且控制主要网络的网络服务提供商(NSPs)形成。主干被比作一个能在几毫秒内将数据传遍全球的巨大通道。在美国，主干完全由光缆(在文下更完整地描述)组成，其带宽从155兆比特/秒到2.5吉比特/秒。带宽衡量在固定的时间内、通过某种通信媒体能传输多少数据，通常以每秒位(bps)、每秒千位 (Kbps)、每秒兆位(Mbps)或每秒千兆位(Gbps)来表示。
There are two types of ISP service: narrowband and broadband. Narrowband service is the traditional telephone modem connection now operating at 56.6 Kbps (although the actual throughput hovers around 30 Kbps due to line noise that causes extensive resending of packets). This is the most common form of connection worldwide. Broadband service is based on DSL, cable modem, telephone (T1 and T3 lines), and satellite technologies. Broadband—in the context of Internet service —refers to any communication technology that permits clients to play streaming audio and video files at acceptable speeds—generally anything above 100 Kbps.
因特网服务提供商提供两种服务:窄带和宽带。窄带服务是目前以56.6 Kbps运行的传统电话调制解调器连接(虽然由于线路噪音造成大量的信息包重新发送,真正的吞吐量徘徊在30Kbps附近)。这是全世界最普通的联结形式。宽频服务以数字用户专线、电缆调制解调器，电话(T1和T3线)和人造卫星技术为基础。在因特网服务的环境中，宽频指的是任何允许客户机以可接受的速度运行流型声频和视频文件的通信技术，通常可接受的速度指在100 Kbps以上。
Cable modem refers to a cable television technology that piggybacks digital access to the Internet on top of the analog video cable providing television signals to a home3. Cable modem services ranges from 350 Kbps up to 1 Mbps. Cable service may degrade if many people in a neighborhood log on and demand high-speed service all at once.
电缆调制解调器是一种在向家庭提供电视信号的视频电缆上搭载对因特网进行数字式访问的有线电视技术。电缆调制解调器服务范围从350 Kbps到1Mbps。如果临近有许多人同时登录并需要高速服务，缆传服务速度可能降低。
复习思考题、作业、参考书目
说明ISDN的工作原理
I、Reading each of the following statements carefully and then decide whether it is true or false
1.The network includes the cables connecting them and all supporting hardware in between such as bridges, routers and switches. Besides the network operating system in the client and server machines.
2.A local area network(LAN),which is restricted by distances of up to 60 miles.
3.A wide area network(WAN) connects large numbers of computer over long-distance communications links.
II、Translate the following phrases from English into Chinese vice versa.
(1)Routers and switches
(2)Private communications
(3)resource sharing
(4)network topology
(5)acrylic hyper graph
(6)physical topology
(7)数据交换
(8)传输信息
课后小节:
注意前后课程连接
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