通信英语（翻译短文）.doc

1.If we consider binary transmission, the complete information about a particular message will always be obtained by simply detecting the presence or absence of the pulse. By comparison, most other forms of transmission systems convey the message information using the shape, or level of the transmitted signal; parameters that are most easily affected by the noise and attenuation introduced by the transmission path. Consequently there is an inherent advantage for overcoming noisy environments by choosing digital transmission.研究二进制信号的传输可见，只要简单的区判别脉冲的有和无，完美就获得了一条消息的全部信息。相比之下，许多其他形式的传输系统是利用被传信号的波形或电平的高低来传送信息的，而这些参数又极易受到传输途径中的噪声和衰耗的影响。因此选择数字传输系统在克服噪声环境的影响方面有其固有的优势。2.The reader may ask, how does the demultiplexer know which group of 8-digits relates to channel 1,2, and so on? Clearly this is important! The problem is easily overcome by specifying a frame format, where at the start of each frame a unique sequence of pulses called the frame code, or synchronization word, is placed so at to identify the start of the frame. A circuit of the demultiplexer is arranged to detect the synchronization word, and thereby it knows that the next group of 8-digits corresponds to channel 1.读者也许会问，分路设备怎么会知道哪一组8位码对应于第1路、第2路及其他各路呢？显然这是很重要的。这个问题是很容易解决的。我们只要制定一个帧格式，即在每一帧的开始放置一个被称作帧码或同步字的独特码序列以标志每帧的起始，而用分路设备的一个电路去检测同步字，从而就知道下一个8位码组对应于话路1。3.Noise can be introduced into transmission patch in many different ways : perhaps via a nearby lightning strike, the sparking of a car ignition system, or the thermal low-level noise within the communication equipment itself. It is the relationship of the true signal to the noise signal, known as the signal-to-noise ratio, which is of the most interest to the communication engineer. 噪声可以以多种不同方式进入传输信道，比如说因为附近的闪电、汽车点火系统的打火或因通信设备本身低电平的热噪声所致。正是这种被称为信噪比的东西，即真实信号与噪声的关系引起了通信工程师的极大的兴趣。4.Basically, if the signal is very large compared to the noise level, then a perfect message can take place; however, this is not always the case. For example, the signal received from a satellite, located in far outer space, is very weak and is at a level only slightly above that of the noise. Alternative examples may be found within terrestrial systems where, although the message signal is strong, so is the noise power.从本质上讲，如果信号比噪声电平大得多，则信息的传输是完美的。但是，实际情况并不总是这样，例如，从位于遥远太空中的卫星接收到的信号极其微弱，其电平仅比噪声稍高一点。地面系统则是另一类例子，尽管信号很强，噪声也很强。5.So far in this discussion we have assumed that each voice channel has a separate coder, the unit that converts sampled amplitude values to a set of pulses; and decoder, the unit that performs the reverse operation. This need not be so, and systems are in operation where a single codec (i.e., coder and its associated decoder) is shared between 24, 30, or even 120 separate channels. 到目前为止，在这个讨论中，我们一直假定每个话路各有一个编码器和解码器。前者是将幅度采样值变换成脉冲，而后者则施行相反的变换，这种设置并非必须。在实际运行的PCM系统中，一个编、译码器为24路、30路，甚至120路所共用（注：在当代的PCM设备中，编、译码器系分路设备，即每个话路各有一套。）6. A high-sped electronic switch is used to present the analog information signal of each channel, taken in turn, to the codec. The codec is then arranged to sequentially sample the amplitude value, and code this value into the 8-digit sequence. Thus the output to the codec may be seen as a sequence of 8 pulses relating to channel 1, then channel 2, and so on. This unit is called a time division multiplexer .一个高速的电子开关被用来将每一话路的模拟信号依次的送往编、译码器。然后编、译码器再顺序采样幅值并把这个幅值编成8位码序列。这样，编解码器输出的8位码序列就分别对应于话路1、话路2，等等。这种设备称为时分复用（TDM）,如图1-2所示。由于8位码的码字序列按时间顺序插接在一起，所以上面所用的复用原则称为码字插接。8.When the transmitter wishes to send data, it first places the line in a space level (i.e., the complement of a mark ) for one element period. This element is called the start bit and has a duration of T seconds. The transmitter then sends the character, 1 bit at a time, by placing each successive bit on the line for a duration of T seconds, until all bits have been transmitted. Then a single parity bit is calculated by the transmitter and sent after the data bits. Finally, the transmitter sends a stop bit at a mark level (i.e., the same level at the idle state) for one or two bit periods. 当发送器想要发送数据时，它首先将线路置成空号电平（即信号的反码），此电平持续一个单元（码元）的间隔时间。此（空号）单元称为起始位，持续时间为T秒。然后发送器发送字符，一次一个比特的将相继的码元送上线路。每个码元持续T秒，直到所有码元发完为止。此后发送器计算得出一个奇偶校验位并将它在数据码元之后发出。最后，发送器送出一个停止位，其电平为信号电平（于空闲状态电平相同），时长为1个或2个比特宽度。7.An asynchronous serial data link is said to be character-oriented, as information is transmitted in the form of groups of bits called characters. These characters are invariable units comprising 7 or 8 bits of “information” plus 2 to 4 control bits and frequently correspond to ASCII-encoded characters. Initially ,when no information is being transmitted, the line is in an idle state. Traditionally, the idle state is referred to as the mark level. By convention this corresponds to a logical 1 level.异步串行数据链路被称为面向字符的，因为信息是以被称作字符的比特组的形式传送的。这些字符是一些故此能够的单元，每个单元都包含7或8个信息比特加上2-4个控制比特，并通常与ASCII码的字符一致。当传输开始，无信息发送时，线路处于空闲状态，而空闲状态习惯上被称为信号电平。通常它对应于逻辑1电平。9.At the receiving end of an asynchronous serial data link, the receiver continually monitors the link looking for a start bit. Once the start bit has been detected, the receiver waits until the end of the start bit and then samples the next N bits at their centers, using a clock generated locally by the receiver. As each incoming bit is sampled, it is used to construct a new character. 在异步串行数据链路系统的接收端，接收器持续监视着线路。搜索着起始位。一旦检测到起始位并等到它结束，接收器就对随后的N个码元抽样，抽样点选择这些码元的中心处。抽样所用的时钟是由接收器本地产生的。10.The most critical aspect of the system is the receiver timing. The falling edge of the start bit triggers the receivers local clock, which samples each incoming bit at its nominal center. Suppose the receiver clock waits T/2 seconds from the falling edge of the start bit and samples the incoming data every T seconds thereafter until the stop bit has been sampled. Fig .2-2 shows this situation. As the receivers clock is not synchronized with the transmitter clock, the sampling is not exact.对系统来说，最关键的问题是接收器的定时。接收器的本地时钟由起始位的下降沿启动，然后在码元的标称中心处对每个输入比特进行抽样。接收器的时钟自起始位的下降沿开始等待T/2，而后每隔T秒抽样输入数据，直至抽样到停止位。图2-2标明的这种情况。如果接收器时钟与发送器时钟不同步，抽样则是不准确的。11.The most obvious disadvantage of asynchronous data transmission is the need for a start, parity, and stop bit for each transmitted character. If 7 bit characters are used, the overall efficiency is only 70%. A less obvious disadvantage is due to the character-oriented nature of the data link. Whenever the data link connects a CRT terminal to a computer, few problems arise, as the terminal is itself character oriented.对于每一个传送的字符，异步数据传输都需要起始位、奇偶校验位和停止位，这是它最明显的缺点。如果采用7比特字符，则总效率仅为70。一个不太明显的缺点是由于数据链路面向字符的特性造成的。在数据链路中无论何时将CRT终端连接到计算机上都不会出现什么问题，因为终端本身也是面向字符的。12.By far the most popular serial interface between a computer and its CRT terminal is the asynchronous serial interface. This interface is so called because the transmitted data and the received data are not synchronized over any extended period and therefore no special means of synchronizing the clocks at the transmitter and receiver is necessary. In fact ,the asynchronous serial data link is a very old form of data transmission system and has its origin in the era of teleprinter.在计算机及其显示器之间最为常见的串行接口是异步串行接口。这个接口之所以如此称呼，是因为无论在多长的时间区间里发送的数据和接收的数据是不同步的，因而没有必要采用特殊的手段使发送器和接收器的时钟同步。实际上，异步串行数据链路是一种古老的数据传输方式，它起源于电传打字机的时代。13 The early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980s saw tremendous increases in the numbers and sizes of networks. As companies realized they could save money and gain productivity by using networking technology, they added networks and expanded existing networks almost as rapidly as new network technologies and products could be introduced.局域网、城域网和广域网的早期发展，在许多方面是混乱无序的。80年代初期，网络的数量和规模发展极快。由于公司意识到，利用联网技术，它们可以省钱和提高生产率，因而只要能引入新的网络技术和产品，它们就要扩充现有的网络和添加网络。21. The purpose of the internet layer is to send source packets from any netwok on the internetwork and have them arrive at the destination independent of the path an networks they took to get there. The specific protocol that governs this layer is called the Internet Protocol (IP). Best path determination and packet switching occur at this layer. Think of it in terms os the postal system. When you mail a letter, you do not know how it gets there (there are various possible routes), but you do care that it arrives.互联层的作用是由互联网络上任意网络发出源数据包并将他们送至信宿，而与到信宿所走的路由和网络无关。主宰这一层的特定协议被称为互联协议（IP）。最佳路由确定和分组交换都发生在这一层。可联想一些邮政系统，如果想寄一封信，你并不知道它是怎样到达对方的（可能会有多条路由），但你会关心它是否到达。14. Proprietary systems are privately developed, owned, and controlled. In the computer industry, proprietary is the opposite of open. Proprietary means that one or a small group of companies controls all usage and evolution of the technology. Open means that free usage of the technology is available to the public.专用系统是指那些被私人开发、拥有和控制的网络。在计算机行业，专用是开放的反义词。专用意味着一个公司或一小群公司完全控制着技术的适用和发展。而开放却意味着技术可为整个公众所利用。15. The OSI reference model allows you to view the network functions that occur at each layer. More importantly, the OSI reference model is a framework you can use to understand how information travels throughout a network. In addition, the OSI reference model can be used to visualize how information, or data packets, travels from application programs, through a network medium, to other application programs that are located in another computer on a network, even if the sender and the receiver have different types of network media.OSI参考模型使人看到在每一层上的网络功能。更为重要的是，OSI参考模型是这样一种结构，它使人理解信息如何通过网络传送。此外，OSI参考模型可被用来想象信息，或数据包是如何有应用程序通过网络截至被传送到位于网络傻瓜等另一个计算机上的，即使发送机和接收机具有不同类型的网络介质亦是如此。16. The application layer is the OSI layer that is closest to the user. It provides network services, such as file access and printing, to the users applications. It differs form the other layers in that is does not provide services to any other OSI layer, but rather, only to applications outside the OSI model. The application layer establishes the availability of intended communication partners. It also synchronizes and establishes an agreement on procedures for error recovery and control of data integrity.应用层是OSI参考模型中离用户最近的层。它为用户的应用提供了网络服务，诸如文件接入和打印等。与其它层的区别在于，它不向OSI任何其他层提供服务，相反的，它仅向OSI参考模型之外提供服务。应用层使对多方通信成为可能。它还对纠错和数据整合的过程进行同步并达成协议。17. The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. Specifically, such issue as how reliable transport between two hosts is accomplished in the concern of the transport layer. In providing communication service, the transport layer establishes, maintains, and properly terminates connection-oriented circuits. In providing reliable service, transport error detection-and recovery and information flow control are used.传输层试图提供数据传送服务而将较高层在传输实现细节方面屏蔽。尤其应指出，在两个主机之间数据传送是否可靠之类的问题是由传输层来考虑实现的。在提供通信服务时，传输层建立、维持和终结面向连接的电路。在提供可靠服务方面，传输层采用了传输误码检测和恢复。以及信息流控制技术。18. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications.物理层定义了在端系统之间为了建立、维持和撤销物理链路所需的电气的、机械的、过程的和功能的特性。这些特性，例如电压电平、电压改变的定时、物理数据的速率、最大传输距离、物理连接器件以及其他类似的特征，都由物理层的说明来定义。20. The transport layer typically deals with the issues of the reliability, flow control, and retransmission. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well flowing, network communications. TCP is a connection-oriented protocol. It supports dialogues between source and destination while packaging application layer information into units called segments. Connection-oriented does not mean that a physical circuit exists between the communication computers (that would be circuit switching). It does mean that a logical connection must be set up before data can be sent.典型地，传输层涉及可靠性、流量控制和重传问题。其协议之一，传输控制协议（TCP）提供了完美的和灵活的方法以产生可靠、通畅的网络通信。TCP是一个面向连接的协议。它将应用层的信息打包，成为一种被人们称为字段的单元，以此支持在信源和信宿间的对话。面向连接并不意味着在通信的计算机之间存在着物理的电路。它意味着在两个主机之间必须来回传送第四层的字段，以便在可传送数据之前建立逻辑连接。22.The name of this layer is broad and somewhat confusing. It also is called the host-to-network layer. Sometimes, its shown at tow layers, as in the OSI model. The network access layer is concerned with all the issues that an IP packet requires to actually cross a physical link from one device to a directly connected one. It includes the LAN and WAN technology details, and all the details in the OSI physical and data link layers这一层的名字是宽泛的，在某种程度上令人有点迷惑不解。它还被人们称为主机到网络层。有时它就像OSI模型中的那样，被用两层表示。网络接入层涉及到一个IP分组从一个部件实际穿过一条物理链路而达到直接相连的一个部件的所有问题。它包括局域网和广域网的技术细节，以及在OSI模型中物理层和数据链路层的所有细节。23. If you compare the OSI model and the TCP/IP model, you can notice that they have similarities and differences . TCP/IP combines presentation and session layer functions into its application layer.l TCP/IP combines the OSI data link and physical layers into one layer.l TCP/IP appears simpler because it has fewer layers; however, this is a misconception. The OSI reference model, with its less complex and multiple layers, is simpler to develop and troubleshoot.l TCP/IP protocols are the standards around which the Internet was developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, networks typically arent built with the OSI protocols, although the OSI model is used as a guide.如果将OSI模型与TCP/IP模型作一比较，你会注意到，他们既有一些不同：l TCP/IP将表示层和会话层的功能全部并入到它网的应用层中。l TCP/IP将OSI的数据链路层和物理层合成了一层。l TCP/IP显得更为简单，因为其层数少，但是这只是一种错觉。OSI参考模型具有多层但却不复杂，因而在开发和故障诊断方面更为简单。l 由于TCP/IP协议是当时开发建设互联网所基于的标准，因而TCP/IP模型由于其协议获得了广泛的信任。与之相反，尽管OSI模型被用来引导入门，但以OSI协议所建设的网络却不典型。24. LANs are high-speed, low-error data networks that cover a relatively small geographic area (up to a few thousand meters). LANs connect workstations, peripherals, and other devices in a single building or other geographically limited area. Topology defines the structure of the network. The topology definition contains two parts: the physical topology, which is the actual layout of the wire (media), and the logical topology, which defines how the media is accessed by the hosts.我们知道，局域网是一个覆盖在地理上相对面积不大（不超过几千米）的高速的、低误码的数据网络。局域网连接工作站、外部设备和其他设施，这些设施分布在一座大楼，或其他有限面积的地段。拓扑定义网络的结构。拓扑的定义含有两个部分：物理拓扑，它是线路（介质）的实际安排；逻辑拓扑，它定义了媒体如何接入主机。25. Host devices are not part of any layer. They have a physical connection to the network media by having a network interface card (NIC) and the functions of the other OSI layers are performed in software inside the hose. This means that they operate at all seven layers of the OSI model. Host devices perform the entire process of the encapsulation and de-encapsulation to do their job of sending e-mails, printing reports, scanning pictures, or accessing databases. For those who are familiar with the inner workings of PCs, think of the PC itself as a tiny network that connects the bus and the expansion slots to the CPU, the RAM, and the ROM.主机设备并不属于某一层。他们到网络介质的物理连接有网络接口卡（NIC）来完成，而OSI其他层的功能由主机中的软件来完成。这就意味这他们运行于OSI模型的整个7层中。主机设备实现着打包和拆包的全过程，以完成诸如发送电子邮件、打印报告、扫描图片或接入数据库等工作。对熟悉个人计算机内部工作原理的人来说，可将PC机本身看成一个小型的网络，它将总线和扩展口连接到CPU，RAM和ROM.26. In terms of physical appearance, a NIC is a printed circuit board, that fits into the expansion slot of a bus on a computers motherboard or peripheral device. It is also called a network adapter. Its function is to adapt the host device to the network medium.就物理外形来看，网络接口卡是一个印刷电路板，它可插入计算机主板或外设的总线扩展槽里。它亦被称为网络适配器，其功能是使主机适应网络介质。The NIC is the basic hardware component of network communications. It translates the parallel signal produced by the computer into the serial format that is sent over the network cable. The 1s and 0s of binary communication are turned into electrical impulse, pulses of light , radio waves , or whatever signaling scheme he network media uses. 网络接口卡是网络通信的基本硬件。它将由计算机产生的并行信号转换成串行格式，此串行信号才通过网络电缆发送。二进制通信的1和0被转换为电脉冲、光脉冲、无线电波或网络介质使用的不管何种信号体系。27. The symbols for media vary. For example, the Ethernet symbol is typically a straight line with perpendicular lines projecting from it ; the Token Ring network symbol is a circle with hosts attached to it ; and for FDDI, the symbol is two concentric circles with attached devices.介质的符号有多种。例如，以太网的典型符号是一条直线，并由它射出许多垂直线。令牌环网的符号是一个圆圈，许多主机与之相连。对FDDI，符号是两个同心圆并连有许多部件。The basic functions of media are to carry a flow of information, in the form of bits, through a LAN. Other than wireless LANs (that use the atmosphere, or space, as the medium) and the new PANs (personal-area networks, which used the human body as a networking medium!), networking media confine network signals to wire, cable, or fiber. Networking media are considered Layer 1 components of LANs.介质的基本功能以比特的形式通过局域网传送信息流。与无线局域网（它采用大气或空间作为媒体）和新的人体网（个人网络它利用人体作为联网介质）不同，网络介质将网络信号局限于导线、电缆或光纤上。联网介质被认为是局域网的第一层部件。28. Like networking media, repeaters are networking devices that exist at layer 1, the physical layer, of the OSI reference model. To begin understanding how a repeater works , it is important to understand first that as data leaves a source and goes out over the network, it is transformed into either electrical or light pulses that pass along the networking media. These pulses are referred to as signals. When signals first leave a transmitting station, they are clean and easily recognizable. However, the longer the cable length , the weaker and more deteriorated the signals become as they pass along the networking media. For example, specifications for Category 5 twisted-pair Ethernet cable establish the maximum distance that signals can travel along a network as 100 meters. If a signal travels beyond that distance, there is no guarantee that a NIC can read the signal. A repeater can provide a simple solution if this problem exists.和联网介质一样，中继器也是OSI参考模型第一层，即物理层的联网部件。要懂得中继器如何工作，重要的是首先要得知，当数据离开信号源并且通过网络传输时，它被转换成要么电脉冲，要么光脉冲再通过联网介质传送。这些脉冲被称为信号。当信号最初离开一个发送站，它们是清晰的、容易识别的。但是当它们通过联网介质传送的时候，电缆的长度越长，信号就越弱，信号质量就越差。例如，五类双绞以太网电缆的技术标准设定的最大距离是信号能够经网络传输10