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本课件对应2.3 Main Factors Affecting Data Transmission（影响数据传输的因素）一节，分析传输速率、串行与并行传输、数据流向与传输模式等关键因素，并解释基带与宽带、同步与异步的差异。课文配中英文对照译文、生词短语表与课后练习，可用于课堂讲授与课前预习。
| 单词 | 词性 | 中文释义 |
|---|---|---|
| assume | v. | 假定，设想，采取，呈现 |
| asynchronous | adj. | 异步的 |
| broadband | n. | 宽频带，宽波段 |
| bulletin | n. | 公报，通报，告示 |
| duplex | adj. | 双工的，双向的Word |
| 单词 | 词性 | 中文释义 |
|---|---|---|
| implement | v. | 实现，完成 |
| interface | n. | 界面 |
| parallel | adj. | 平行的，并联的 |
| protocol | n. | 协议 |
| serial | adj. | 连续的，串联的 |
| simplex | adj. | 单工的，单向的 |
| voiceband | n. | 话音频带 |
| 短语 | 中文释义 |
|---|---|
| dial into | 拨入 |
| open system protocol | 开放系统协议 |
| AbbreviationsOSI（Open Systems Interconnection） | 开放系统互连 |
| POS（Point-Of-Sale） | 零售点 |
| SNA（Systems Network Architecture） | 系统网络结构 |
| TCP/IP（Transmission Control Protocol and Internet Protocol） | 传输控制协议和因特网协议 |
There are several factors that affect data transmission. They include speed or bandwidth,serial or parallel transmission,direction of dataflow,modes of transmission data,andprotocols.有几个影响数据传送的因素，包括速度或带宽、串行或并行传送、数据流的方向、数据传送的方式及协议。
Bandwidth The different communications channels have different data transmission speeds. This bit-per-second transmission capability of a channel is called its bandwidth. Bandwidth maybe of three types:voiceband,medium band and broadband. Voicebandis the bandwidth of a standard telephone line and used often for microcomputer transmission,thebpsis300~9600.带宽不同的通信信道有不同的数据传送速度。信道每秒钟能传送二进制位的能力被称为带宽。带宽可以是三种类型：话音频带，中速带宽和宽带。话音频带是标准电话线的带宽，通常被用于微型机的传送，其每秒钟传送的二进制位数是300到9 600。
Mediumbandis the bandwidth of special liased lines used mainly with minicomputers and mainframe computers,thebpsis56,000~264million. Broadband is the bandwidth that includes microwave,satellite,coaxialcable,and fiber-optic channels. Itisusedfor very high-speed computers whose processors communicate directly with eachother. Itisinthe range of 56,000~30billionbps.中带带宽主要是由小型机和大型机使用的特殊连接线的带宽，其每秒钟传送的位数是56 000到2.64亿。宽带包括微波、卫星、同轴电缆和光缆的信道的带宽。它主要用于处理器之间直接进行通信的高速计算机中，其每秒钟传送的位数在56 000到300亿位。
Serialor Parallel Transmission Datatravels in two ways:serially and in parallel. Inserialdata transmission,bitsflowina serial or continuous stream,like cars crossing a one-lane bridge. Eachbit travels on its own communications line.串行或并行传输数据以串行和并行两种方式传送。在串行数据传输中，数据位的流动是一串或连续的流量，就像汽车通过只有一个车道的桥。每一位在自己的通信线路上流动。
[1]Serial transmission is the way most data is sent over telephones lines. Thus,the plug-in board making up the serial connector in a microcomputer 'smodemis usually called a serial port. More technical names for the serial port are RS-232 Cconnector and asynchronous communication sport.分析：most data is sent over telephones lines是定语从句，修饰theway。串行传输是绝大多数数据通过电话线传输的方式。因此，构成微机调制解调器的串行连接器的插入板通常被称为串行口。串行口的技术名称是RS-232C连接器和异步通信端口。
With parallel data transmission,bits flow through separate lines simultaneously. Inotherwords,they resemble cars moving together at the same speed on a multilane freeway. Parallel transmission is typically limited to communications over short distances and is not used over telephone lines. Itis,however,a standard methods of sending data from a computer's CPUtoa printer.采用并行数据传输时，数据位是通过分离的线路同时流动的。换句话说，它们类似于汽车一起以同样的速度通过多道快车道。并行传送主要局限于短距离的传送，并不通过电话线。无论如何，它是从计算机的CPU到打印机传送数据的标准方式。
Direction of Data Transmission There are three directions or modes of dataflow in a data communications system:simplex communication,half-duplex communication,and full-duplex communication. Simplex communication resembles the movement of carson a one-way street. Datatravels in one direction only. Itisnot frequently used in data communication systems today.数据传输的方向在数据通信系统中有三种数据流的方向或方式：单工通信、半双工通信和全双工通信。单工通信类似于汽车在单向的街上行驶，数据仅以一个方向传送。它并不经常用于现在的数据通信系统中。
One instance in which it is used maybe in point-of sale(POS)terminals in which data is being entered only. In half-duplex communication,data flows in both directions,but not simultaneously. Thatis,data flows in only one direction at anyone time. This resembles traffic on a one-lane bridge.使用单工通信的一个例子是只输入数据的零售点终端。在半双工通信中，数据以两个方向流动，但不是同时。也就是说，数据在任何时候都只以一个方向流动，这类似于只有一个车道的桥上的交通。
Half-duplex is very common and is frequently used for linking microcomputers by telephone lines to other microcomputers,minicomputers,and mainframes. Thus,when you dial into an electronic bulletin board through your microcomputer,you may well be using half-duplex communication. In full-duplex communication,data is transmitted back and for that the same time,like traffic on a two-way street.半双工很普遍，经常应用于通过电话线连接的微型机之间、微型机和小型机及大型机之间的通信中。因此，当你通过微机拨入到电子公告牌，你也许会使用半双工通信。在全双工通信中，数据同时来回地传送，像双向街道上的交通。
Itisclearly the fastest and most efficient form of two-way communication. However,it requires special equipment and is used primarily for mainframe communications. Anexampleis the weekly sales figures that a supermarket or regional office sends to its corporate headquarters in another place.很显然全双工通信是双向通信中最快最有效的形式。但是，全双工通信需要特殊的设备并且主要用于大型机的通信。其应用的一个例子是超市或区域办公室每周将其销售的数据发送给在另一个地方的公司总部。
数据传送方式数据能以异步或同步方式传送。在异步传送方式下，一次只能发送和接收一个字节，是微型机经常使用的方法。异步传送也常用于速度慢的终端设备。其优点是发送方合适时就可以传送数据。缺点是数据传送速率相对较慢。Modesof Transmitting Data Datamaybe sent by asynchronous or synchronous transmission. In asynchronous transmission,the method frequently used with microcomputers,data is sent and received one by teatime. Asynchronous transmission is often used for terminals with slow speeds. Its advantage is that the data can be transmitted whenever convenient for the sender. Its disadvantage is a relatively slow rate of data transfer.
同步传送一次发送几个字节或数据块，用于大量信息的传送。为了实现数据传送，发送和接收的字节块必须以精确的时间间隔出现。因此，系统需要一个同步时钟。其优点是可以很快地传送数据，缺点是需要设备费用。Synchronous transmission is used to transfer great quantities of information by sending several bytesorablockata time. For the data transmission to occur,the sending and receiving of the blocks of bytes must occur at carefully timed intervals. Thus,the system requires a synchronized clock. Its advantage is that data can be sent very quickly. Its disadvantage is the cost of the required equipment.
协议为了成功的传送数据，发送方和接收方必须遵循用于信息交换的一套通信规则。这些用于计算机之间交换数据的规则被称为是线路协议。为了和其他微机连接，像Crosstalk这样的通信软件包就帮助定义协议，诸如速度和方式。TCP/IP（传输控制协议和因特网协议）是Internet上的用于通信的两个标准协议。Protocols For data transmission to be successful,sender and receiver must follows a set of communication rules for the exchange of information. These rules for exchanging data between computers are known as the line protocol. A communication software package like Crosstalk helps define the protocol,such as speeds and modes,for connecting with another microcomputer. TCP/IP(Transmission Control Protocol and Internet Protocol)are the two standard protocols for communications on the Internet.
TCP/IPisthe"language"ofthe Internet. Itisanet working technology developed by the United States Government Defense Advanced Research Project Agency(DARPA)inthe 1970 s. Itismostcomm only employed to provide access to the Internet but can be and is used by many people to create a LANthatmayor may not connect to the Internet. TCP/IP是Internet的“语言”，它是由美国政府防御高级研究项目机构在20世纪70年代开发的网络技术。它最常用于提供访问Internet，但也被许多人用于建立与Internet相联或不联的局域网。
InmanyaspectsTCP/IPisaclient/server-typeLAN,but many manufacturers of TCP/IPsoftware have applications that allow the"clients"to serve files or even applications. TCP/IPistrulyan open systems protocol. This means that no one manufacturer creates the product—any computer running TCP/IPsoftware can connect to anyone else who has TCP/IPsoftware(provided the user has an account and security permissions),regardless of who made the particular version of software.在许多方面，TCP/IP是客户服务器类型的局域网，但许多TCP/IP软件的生产商具有允许“客户机”提供文件甚至应用的应用软件。TCP/IP是一个真正的开放系统协议。这就意味着没有一个生产商生产这个产品——任何运行TCP/IP软件的计算机都可以和另一个具有TCP/IP软件的任一机器相联（假设该用户具有账号和安全许可），而不管是谁制定的这个软件版本。
When different types of microcomputers are connected in a network,the protocols can become very complex. Obviously,for the connections to work,these network protocols must adhere to certain standards. The first commercially available set of standards was IBM's Systems Network Architecture(SNA).This works for IBM'sown equipment,but other machines won't necessarily communicate with them.当不同类型的微机在网络中相联时，协议变得非常复杂。显然，为了实现通信，这些网络协议必须坚持一定的标准。第一套商业可用标准是IBM公司的系统网络结构，该标准只用于IBM自己的设备，不过，其他的机器不需要与它们交流。
The International Standards Organization has defined a set of communications protocols called the Open Systems Interconnection(OSI).The purpose of the OSImodelisto identify functions provided by any network.[2]It separates each network's functions into seven"layers"ofprotocols,or communication rules. When two network systems communicate,their corresponding layers may exchange data.[3]This assumes that the microcomputers and other equipment on each network have implemented the same functions and interfaces.分析：短语separate…into…意思是“把……分成……”。分析：句中that引导的是宾语从句，作assumes的宾语。国际标准化组织已经制定了一套被称为开放系统互联的通信协议。开放系统互联模式的目的是判别由任一网络提供的功能。它把每个网络的功能分成七层协议，即七层通信规则。当两个网络系统通信时，它们对应层可以交换数据。这种情况假设每一个网络中的微机和其他设备实现了同样的功能和同样的接口。
习题2.3 一 判断正误：根据课文内容判断下列说法正误。
Put“true”or“false”in the brackets for the following statements according to the passage.
习题2.3 二 练习：根据课文内容完成下列练习。
Fillinthe blanks according to the passage.
习题2.3 三 翻译：将下列单词和短语译成中文。
Translate the following words and expressions into Chinese.
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