Text 11Introduction to Data Communication

The need to communicate is part of2mans inherent being.Since the beginning of time man has communicated3using different techniques and methods.Circumstances and available technology have dictated the method and means of communications.

Data communications concerns itself with the transmission(sending and receiving)of information between two parties.In modern times, this means sending information between machines, which are connected together by physical wires or radio links.

There are FOUR basic elements involved in communication.

The TRANSMITTER initiates the communication.

The CHANNEL is the mechanism by which the communication is conveyed to the receiver.

The RECEIVER receives the communication.

The MESSAGE is the information content that is transferred between the sender and receiver via the medium.

In real world terms, this can be illustrated by a simple telephone conversation between two people.The person who initiates the call by lifting the telephone handset and dialing a number is the TRANSMITTER, whilst the person who answers the ringing telephone is the RECEIVER.The CHANNEL is the Public Telephone Switched Network, and the MESSAGE is the topic of conversation[speech]that was the reason for the call being made.

The message source is the transmitter, and the destination is the receiver.A channel whose direction of transmission is unchanging4is referred to as a simplex channel.For example, a radio station is5a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back.A half-duplex channel is a single physical channel in which the direction may be reversed.Messages may flow in two directions, but never at the same time, in a half-duplex system.In a telephone call, one party speaks while the other listens.After a pause, the other party speaks and the first party listens.Speaking simultaneously results in garbled sound that cannot be understood.A full-duplex channel allows simultaneous message exchange in both directions.It really6consists of two simplex channels, a forward channel and a reverse channel, linking the same points.Figure 1 illustrates the three channel types.

Consider7Morse Code, a two-state data communication system that functions very similarly to today’s computerized data communication systems.Developed by Samuel E.B.Morse in the 19th century, Morse code uses electrical current to transmit a series of dashes and dots that represent letters of the alphabet, numbers, a comma and a period.A basic Morse Code transaction works as follows: A“message”is given to an operator who translates that message into dots and dashes(TRANSMITTER), then the transmitting operator uses the telegraph key to send an electrical signal to the receiving operator at the desired location to indicate that a message is about to come through.The receiving operator(RECEIVER)sends back an acknowledgment that he is ready, and the transmitting operator then sends the message which the receiving operator takes down.When the message is completely transmitted, the transmitting operator signals to the receiving operator that he is done, and the transmission line is closed.The receiving operator then translates the code back into the original message, and delivers it to the designated recipient.

Clearly, in a system of this type, accuracy is extremely important.As only two characters—dot or dash—are used to create a code for an entire language system, the transmitting and receiving operators must be extremely accurate.This system can only work if both sides of the data communication system know the code and can8encrypt and decode messages.It is also essential that the transmitter not send faster than the receiver can take down the information.Even using expert operators, static on the line could obscure the signals making a dash sound like a dot and thereby corrupting the message.Thus, it becomes obvious that the most important aspect of designing a data communication system is ensuring not only that RECEIVER can receive and understand the data transmitted by TRANSMITTER, but also that the data remain uncorrupted during transmission.

These are the very9same concerns faced by computerized data communication system designers.Indeed Morse code is often though of as the forerunner of the computer’s binary communication system.The binary system uses the numbers 0 and 1 as the symbols for transmission of data.Using position notation, any value is represented by a10weighted series of 1s and 0s.Thus the decimal number“33”would be represented by“100001”(1×25＋0×24＋0×23＋0×22＋0×21＋1×20).

The above is a simple example of binary coding, but the same system is used by computers to transmit complex text messages, complex graphics, and streaming video.In order to accurately transmit many different types of data, numerous interfaces and protocols have been created.In such a system, the distance over which data moves within a computer may vary from a few thousandths of an inch, as is the case within a single chip, to as much as several feet along the backplane of the main circuit board.Frequently, however, data must be sent beyond the local circuitry that constitutes a computer.In many cases, the distances involved may be enormous.Unfortunately, as the distance between the source of a message and its destination increases, accurate transmission becomes increasingly difficult.This results from the electrical distortion of signals traveling through long distance, and from noise added to the signal as it propagates through a transmission medium.It is the relationship of the true signal to the noise signal, known as11the signal to noise ratio, which is the most interest to the communication engineer.

NEW WORDS AND PHRASES

inherent[in´hiərənt]adj.固有的，内在的，与生俱来的

circumstance[´sə:kəmstəns]n.环境，详情，境况

dictate[dik´teit]v.口述，口授，指令，指示，命令 n.指示（指理智，变心）

illustrate[´iləstreit]vt.举例说明，图解，阐明 vi.举例

initiate[i´niʃieit]vt.开始，发动，传授 v.开始，发起

switch[switʃ]n.开关，电闸，转换 vt.转换，转变

simultaneously[siməl´teiniəsly;(US)saim-]adv.同时地

medium[´mi:djəm]n.媒体，方法，媒介 adj.中间的，中等的，半生熟的

mechanism[´mekənizəm]n.机械装置，机构，机制

receiver[ri´si:və(r)]n.接受者，接收器，收信机

transmitter[trænz´mitə]n.转送者，传导物，发报机，发射机

simplex[´simpleks]adj.单工的

full-duplex 全双工通信

half-duplex 半双工通信

Morse Code 莫尔斯电码

computerize[kəm´pju:təraiz]vt.用计算机处理，使计算机化

designate[´dezigneit]vt.指明，指出，任命 v.指定，指派

uncorrupt[´ʌnkə´rʌpt]adj.未堕落的，不腐败的，（文本等）无讹误的，纯正的

obscure[əb´skjuə]adj.暗的，朦胧的 vt.使暗，使不明显

dash[dæʃ]n.少量（搀加物），冲撞 vi.猛掷，冲撞 vt.泼溅，使猛撞

dot[dɔt]n.点，圆点，园点 vt.在……上打点

binary[´bainəri]adj.二进位的，二元的

notation[nəu´teiʃən]n.符号

decimal[´desiməl]adj.十进的，小数的，以十为基础的，十进制 n.小数

backplane 底板

signal to noise ratio 信噪比

propagate[prɔpə´geit]v.动植物，繁殖，（声波，电磁辐射等）传播

destination[desti´neiʃən]n.目的地，[计]目的文件，目的单元格

circuitry[´sə:kitri]n.电路，线路

protocol[´prəutəkɔl]n.草案，协议

distortion[dis´tɔ:ʃən]n.扭曲，变形，曲解，失真

forerunner[´fɔ::rʌnə]n.先驱（者），传令官，预兆

garble[´ga:bl]vt.断章取义，混淆

comma[´kɔmə]n.逗点，逗号

accuracy[´ækjurəsi]n.精确性，正确度

extremely[iks´tri:mli]adv.极端地，非常地

NOTES

1.Introduction to Data Communication本文是对数据通信的简单介绍，包括通信系统的组成、通信的方式和二进制传输的原理。

2.mans inherent being 人类固有的。

3.using different techniques and methods中的“using”是现在分词做状语，表示使用不同的技术和方式来通信。

4.is referred to固定词组，译为“被称为”。

5.a simplex channel指的是单工通道。数据通信的三种方式：单工通信、半双工通信和全双工通信。见下文中的half duplex 和full duplex。

6.consists of 固定词组，译为“由……组成”。

7.Morse Code莫尔斯电报码。是利用“点”和“划”完成的数据编码，一种近似于二进制传输的早期通信的应用。

8.encrypt and decode messages消息的加密和解码。

9.same concerns faced by computerized data communication system designers 句中的concern 翻译为“关心的问题”，faced 是过去分词做“concern”的定语，指的是计算机数据通信系统同样要面临和关注的问题。

10.weighted series of 1s and 0s句中的weighted指的是权值，翻译为“一系列带有权值的1和0”。

11.the signal to noise ratio信噪比。

EXERCISES

一、请将下列词组译成英文

1.发射机　　　　　　　　　　　　 2.接收机

3.数据通信　　　　　　　　　　　 4.电话交谈

5.电话交换网　　　　　　　　　　 6.单工

7.半双工　　　　　　　　　　　　 8.全双工

9.传输信道　　　　　　　　　　　 10.编码和译码

二、请将下列词组译成中文

1.computerized data communication systems

2.electrical signal

3.encrypt and decode

4.binary communication system

5.decimal

6.signal to noise ratio

7.dot or dash

8.position notation

9.streaming video

10.take down the information

三、选择题

1.In real world terms, this can be illustrated by a simple telephone conversation between two people.The person who initiates the call by lifting the telephone handset and dialing a number is the______.

A.TRANSMITTER　　　　　　　　　　B.RECEIVER

C.MESSAGE　　　　　　　　　　　　D.CHANNEL

2.______is a single physical channel in which the direction may be reversed.Messages may flow in two directions, but never at the same time.

A.A half-duplex channel　　　　　　　　 B.A simplex channel

C.A full-duplex channel　　　　　　　　 D.Cable

3.______allows simultaneous message exchange in both directions.It really consists of two simplex channels, a forward channel and a reverse channel, linking the same points.

A.A half-duplex channel　　　　　　　　 B.A simplex channel

C.A full-duplex channel　　　　　　　　 D.Cable

4.Morse code uses______to transmit a series of dashes and dots that represent letters of the alphabet, numbers, a comma and a period.

A.sound signals　　　　　　　　　　　 B.smoke signals

C.light signals　　　　　　　　　　　　D.electrical current

5.It is the relationship of the true signal to the noise signal, known as______, which is the most interest to the communication engineer.

A.bits rate　　　　　　　　　　　　　 B.noise power

C.the signal to noise ratio　　　　　　　　D.signal power

四、请将下列句子译成中文

1.This results from the electrical distortion of signals traveling through long distance, and from noise added to the signal as it propagates through a transmission medium.

2.The binary system uses the numbers 0 and 1 as the symbols for transmission of data.Using position notation, any value is represented by a weighted series of 1s and 0s.

3.Clearly, in a system of this type, accuracy is extremely important.As only two characters—dot or dash—are used to create a code for an entire language system, the transmitting and receiving operators must be extremely accurate.

4.When the message is completely transmitted, the transmitting operator signals to the receiving operator that he is done, and the transmission line is closed.

5.Data communications concerns itself with the transmission(sending and receiving)of information between two parties.

参考译文

数据通信介绍

通信是人类固有需求的一部分，人类一开始就使用不同的技术和方式进行交流，而环境和可利用的技术也限定了人类通信的方法和方式。

数据通信涉及在通信双方传递信息（发送和接收）。在当代，通信设备之间发送信息的连接主要是靠物理线路和无线链路。

在通信中，主要包括以下4个基本成分：

发射机用于发起通信；

信道是将信息传递到接收机的媒介；

接收机用于接收信息；

消息指通过媒介在发射机和接收机之间传递的信息的内容。

现实生活中，可以用一个简单的两个人电话交谈的例子来说明。拿起电话机拨号进行初始化呼叫的那个人就是发射机，而应答响铃电话的那个人就是接收机。传输的通道就是公共电话交换网络。消息就是这次呼叫所进行交流的主题。

消息源是发射端，目的地是接收端，传输的信道方向不变的通信方式被称为单工通信。例如，无线电台就是一个典型的单工通信，因为电台发出的信号总是传向听众，而从来不会反方向传输。半双工通信的信道是一个单独的物理通道，消息可以在两个方向传输，但不能同时进行。在一个半双工的系统中，例如电话呼叫，一方说话的时候，另外的一方只能听，暂停后，反过来这方说话的时候另外一方也只能听。如果同时说话会导致声音相互干扰而没办法相互理解意思。全双工通信允许消息在两个方向上同时传输，实际上，全双工包含了两个单工通道，一个前向通道和一个反向通道来用于连接通信点。图1可以说明这三种信道的类型。

考虑莫尔斯电报码，一个两种状态的通信系统，它的功能和我们今天的计算机处理的数据通信系统非常相似。这个系统是Samuel E.B.Morse在19世纪研制开发的。莫尔斯电报码使用电流来发送一系列的点和划来代表字母表中的字母、数字、逗号和周期。莫尔斯电报码的处理流程如下：“消息”交给操作员后，由操作员将消息翻译成点和划，发射端的操作员利用电报开关发送一个电信号给指定位置接收端的操作员，告诉其将有消息传输。接收端的操作员就回送一个确认信号表示已处于就绪状态，然后发射端的操作员将消息发送出来，再由接收端的操作员记下内容。当消息发送结束后，发射端的操作员将发送信号告诉接收端发送结束，传输的线路将被关闭。接收端的操作员将消息编码还原成原始信息后，交给指定的收信人。

很清楚，在这样一个系统中，准确率是特别重要的，只有两个字符：点和划用来产生一个完整语言系统的编码，发送和接收的操作员必须特别地精确。这个数据通信系统中，通信的双方只有知道编码内容，并能对消息进行解密和解码后，系统才能正常工作。还有一个非常关键的是发射端发射的速度不能过快，否则会导致接收端无法记下消息。即使是专业的操作员，在传输线上的静电噪声也会干扰信号，导致一个“划”听起来像一个“点”而使消息混淆。因此，一个非常重要的方面就是：设计一个数据通信系统，不仅要确保接收端能收到和理解发送端发出的消息，而且在数据传输的过程中不能由于受到干扰而出错。

这个和由计算机控制的数据通信系统的设计者关心的问题是相同的。事实上，莫尔斯电报码经常被认为是计算机二进制通信系统的先驱。二进制通信系统使用数字0和1来作为传输数据的符号。使用位置标记，任何值都可以用一系列带有权值的1和0来表示。因此十进制数字“33”可以表示为“100001”。

上面是一个简单的二进制的编码，计算机使用相同的系统来发送复杂的文本消息、图片和视频流。为了能准确地发送不同类型的数据，许多通信的接口和协议就产生了。在这样一个系统中，计算机内部数据传输的距离可以是 1 英寸的千分之一，例如在一个单独的芯片中，也可以是主电路板内的几英尺长的距离。另外最多的还是数据经常要从构建计算机的本身电路中发送出。在很多场合，这个距离是非常大的。不幸的是，随着源端到目的端距离的增长，完成准确的传输就变得困难了，原因是电信号经过长距离的传输后会失真。同时，信号在传输的过程中在传输媒介中会加进来噪声。这就是信号与噪声关系中的信噪比的问题，也是通信工程师最感兴趣的问题。