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 receiver’s 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 receiver’s 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 user’s 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, it’s 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.
TCP/IP combines the OSI data link and physical layers into one layer.
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.
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 aren’t 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：
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。
TCP/IP将OSI的数据链路层和物理层合成了一层。
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。 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 computer’s 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100米。如果信号传送超过了这一距离，则不能保证网络接口卡能读这一信号。如果存在这样的问题。一个简单的解决办法即使采用中继器。
29. Two reasons to use hubs are to create a central connection point for the wiring media, and to increase the reliability of the network. The reliability of the network is increased by allowing any single cable to fail without disrupting the entire network. This differs from the bus topology where having one cable fail disrupts the entire network. Hubs are considered layer 1 devices because they only regenerate the signal and repeat it out all of their ports.
使用集线器有两个原因，一是对所铺设的媒体设置一个连接点，二是想增加网络的可靠性。网络可靠性的增加是因为它允许任何一个电缆失效而又不影响整个网络。这不同于总线拓扑，因为在总线拓扑中一根电缆失效将影响整个网络。集线器被认为是第一层的部件，因为它仅仅再生信号并重发到它的所有端口上。
30. Bridges filter network traffic by looking only at the MAC address. Therefore , they can rapidly forward traffic representing any network layer protocol. Because bridges look only at MAC addresses, they are not concerned with network layer protocols. Consequently, bridges are concerned only with passing or not passing frames, based on their destination MAC addresses.
网桥根据MAC地址来过滤网络流量。因此，它们能够迅速地将代表着任何网络层协议的流量向前传送。由于网桥只看MAC地址，它们就不涉及网络层的协议。因而，网桥只根据帧的目的MAC地址，关心是通过还是不通过。
31. A switch is a layer 2 device just as a bridge is. In fact, a switch is sometimes called a multi-port bridge, just like a hub is called a multi-port repeater. The difference between the hub and the switch is the same as the difference between a repeater and a bridge: switches make decisions based on MAC address and hubs don’t make decisions at all.交换器是第二层部件，正像网桥一样。事实上，交换器有时被称为多端口网桥，就像集线器被叫做多口中继器一样。集线器和交换器之间的不同与在中继器和网桥之间的不同一样：交换器依据MAC地址做出决策，而集线器根本不做任何决策
32.Because of the decisions that switches make, they make a LAN much more efficient. They do this by “switching” data only out the port to which the proper host is connected. In contrast, a hub sends the data out all its ports so that all the hosts have to see and process (accept or reject) all the data.
由于交换器做出决策，因而它们使LAN高效的多。它们只将数据交换到与合适的主机连接的端口上。相反的，集线器却是将数据发到它全部端口上，以至于所有的主机不得不看到并处理（接收或拒绝）所有的数据。
33. Routers differ from bridges in several respects. First, bridging occurs at layer2, the data link layer, whereas routing occurs at layer 3 ,the network layer of the OSI reference model. Second, bridges use physical or MAC addresses to make data forwarding decisions. Routers use different addressing scheme that occurs at layer 3 to make forwarding decisions. They use network-layer addresses, which are referred to as Internet protocol (IP), or logical, addresses, rather than MAC addresses.
路由器与网桥在许多方面不同。首先，网
桥发生在第二层，即数据链路层，而路由器发生在第三层，即OSI参考模型的网络层。第二，网桥利用物理的或MAC地址对数据送出做出决定。路由器利用不同的寻址体系对数据前进做出决定，该寻址体系发生在第三层。它们利用网络层地址，此地址称为互联协议地址。或逻辑地址，而不是MAC地址.
42.As more and more systems join the Internet, and as more and more forms of information can be converted to digital form, the amount of stuff available to Internet users continues to grow. At some points very soon after the nationwide (and later worldwide) Internet started to grow, people began to treat the net as a community, with its own tradition and customs.随着越来越多的系统加入Internet，同时随着越来越多的信息可以转变成数字形式，Internet用户所能得到的东西也在继续增加。随着国家（后来是国际）Internet的发展，人们很快在某些方面开始将互联网看作是一个社区，有自己的传统和习惯。
34. The Internet is a international collection of computer networks that all understand a standard system of addresses and commands, connected together through backbone systems. It was started in 1969, when the U.S. Department of Defense established a nationwide network to connect a handful of universities and contractors. The original idea was to increase computing capacity that could be shared by users in many locations. uter, it was a simple matter to obtain a copy.
Internet是计算机网络的国际性的集合，这些网络都符合具有地址和命令的标准体系，并经骨干网连在一起。Internet始建于1969年，当时美国国防部为连接一些大学和协议企业而建立了一个全国性网络。最初的想法是要增加计算机能力并可由许多地点的用户共享。
35.People on the ARPNET quickly discovered that the could exchange messages and conduct electronic “conferences” with distant colleagues for purposes that had nothing to duo with the military industrial complex. If somebody else had something interesting stored on his or her comp. Today thousands of networks and millions of computers are connected to the Internet. It is growing so quickly that nobody can say exactly how many users “On the Net”.ARPNET（这种全国网络最初的名称）上的用户很快就发现它们可以与远距离的同事交换消息，并且进行某种目的的电子“会议”，而这些目的与军事工业企业没有任何关系。如果另外一些人在其计算机中存有有趣的东西，得到其拷贝是很容易的事（假定拥有者没有进行保护）。今天，有成千上万的计算机网络和数百万台计算机与Internet相连。Internet发展如此之快以至于没有人能准确的说出网上有多少用户。
36.The Internet is the largest repository of information which can provide very very large network resources. The network resources can be divided into network facilities resources and network information resources. The network facilities resources provide us the ability of remote computation and communication. The network information resources provide us all kinds of information services.
Internet是最大的信息宝库，它可以提供非常巨大的网络资源。这种网络资源可分为网络设备和网络信息资源。网络设备资源使我们能够进行远程计算和通信。网络信息资源向我们提供各种各样的信息服务。
37.The goal of your use of the Internet is exchanging messages or obtaining information. What you need to know is that you can exchange message with other computers on the Internet and use you computer as a remote terminal on distant computers. But the internal details of the link are less important, as long as it works.
使用Internet的目的是交换消息或获得信
息。你只需知道你可以与Internet上的其他计算机交换消息并将你的计算机用作远端计算机的远程终端，而链路的内部细节并不太重要，只要网络能工作就行。
38. The most commonly used network service is electronic mail (E-mail).or simply as mail. Mail permits network users to send textual messages to each other. Computer and networks handle delivering the mail, so the communication mail user do not have to handle details of delivery, and do not have to be present at the same time or place.
最常使用的网络服务是电子邮件，或简称邮件。电子邮件允许网络用户彼此传送文本消息。邮件的传递有计算机和网络处理，邮件用户不必关心传递的细节，也不必同时在场。
39. Before you can use the Internet, you must choose a way to move data between the Internet an you PC. This link may be a high-speed data communication circuits, a local-area network( LAN), a telephone line or a radio channel. Most likely, you will use a Modem attached to your telephone line to talk to the Internet. Naturally, the quality of your Internet connection and service, like many other things in life, is dictated by the amount of money you are willing to spend.
可使用Internet之前，必须使用一种方法在你的PC机和Internet之间传送数据。这种里那几的链路可以是高速数据通信电路、局域网（LAN）、电话线路或无线信道。最有可能的是，你使用Modem连到电话线上与Internet对话。当然，像生活中许多其他的事物一样，与Internet连接和服务的质量是有你所花钱的数量决定的。
40.Although all these services can well satisfy the needs of the users for information exchange, a definite requirement is needed for the users. Not only should the user know there the resources locate, but also he should know some operating commands concerned. To ease the searching burden of the users, recently some convenient searching tools appear, such as Gopher, WWW and Netscape.
虽然所有这项服务可以很好的满足用户对信息交换的需要，但用户仍旧还需要具有一些特定的先决条件。用户不仅要知道信息资源所处的位置，而且要知道一些有关的操作命令。为了减轻用户寻找信息的负担，近来出现了一些方便的搜索工具，如gopher，www和Netscape
41.World wide web (WWW) is a networked hypertext protocol and user interface. It provides access to multiple services and documents like Gopher does but is more ambitious in its method. A jump to other Internet service can be triggered by a mouse click on a “hot-linked” word, image, or icon on the Web page.
全球网(WWW)是一种网络的超文本协议和用户界面。像Gopher一样，它提供多种服务和文件接入方法，但其方法更加有前途。向Internet其他服务的跳转可在网页上由鼠标器点击“热链接”的字、图像或小相框来启动。
43. Although telecommunication is the rationale for most of the current interest in fiber optics, this was not the case during the early days of the technology. The researchers who produced the first clad glass optical fibers in the early 1950s were not thinking of using them for communications; they wanted to make imaging bundles for endoscopy. Fiber optics was already a well-established commercial technology when the famous paper by Kao and Hockham, suggesting the use of low-loss optical fibers for communication, appeared in 1966. 虽然现在人民对纤维光学的兴趣主要在于通信，但早期发展纤维光学的目的并不在此。50年代初研究人员制造出第一根具有包层玻璃光纤时，并不想用于通信而是想用它们传送内窥镜需要的成像光束。1966年Kao和Hockham发表了那篇著名的论文，建议将低损耗光纤用于通信，此时纤维光学已发展为一项很实用的技术了。 44.The first low-loss (20dB/km) silica fiber was described in a publication which appeared in October of 1970. The date of this publication is sometimes cited as the beginning of the era of fiber communication. Although this development did receive considerable attention in the research community at the time, it was far from inevitable that a major industry would evolve. The 20dB/km loss figure was still too high for long-hall telecommunication systems. The fibers were fragile, and a way to protect them would have to be found. There were no suitable light sources. Researchers did not know whether field termination and splicing of optical cables would ever be practical. Finally, there were serious doubts as to whether these components could ever be produced economically enough for the technology to play a major role in the marketplace.1970年10月，第一根低损耗（20 dB/km）石英光纤问世了。有时将这以日期作为光纤通信时代的开端。虽然这一成果当时在研究领域确实引起了极大的关注，但因此就认为一项产业会发展起来尚为时太早: 每千米20dB的损耗对于长途通信系统仍然是太大了；光纤易断裂，必须寻找保护方法；没有合适的光源。研究人员不知道光缆终结和拼接是否会发展到实用阶段，最后，这些部件是否能够足够经济的生产出来，从而使之在市场上占用重要的地位，它们更是存有严重的疑虑。
45. During the middle and late 1970s, the
rate of progress toward marketable products accelerated as the emphasis shifted from research to engineering. Fibers with losses approaching the Rayleigh limit,were produced in the laboratory. Microbend loss problems were overcome through the use of improved fiber coatings and cabling techniques. Light sources and improved photodetectors which operated near 1.3um were developed to take advantage of the low fiber loss and dispersion in this “longer wavelength region”. 70年代中后期，由于发展重点研究领域转入工程，因而加速发展了适应市场的产品。在实验室研制的光纤衰减值接近瑞利极限值。通过改进光纤外涂层方法和成缆技术，克服了微弯损耗。开发了工作于1.3um波长附近的光源和改进的光检测器，从而可以利用光纤在长波长区的低损耗和低色散特性。
46. Data rates for installed fiber optic system have recently move into gigabit per second range. Such system use the spectrally pure distributed-feedback lasers to minimize fiber dispersion effects. Fibers designed for low dispersion at 1.55UM wavelength, which corresponds to minimum fiber loss, are now commonly used in long distance transmission. The use of wavelength multiplexing to further increase the fiber information capacity is becoming more widespread.最近敷设的光纤系统的数据速率已移至每秒吉比特范围。这种系统采用光谱纯的分布反馈激光器，将光纤色散效应减至最小。在1.55um波长上设计的低色散光纤，相应地具有低损耗特性，目前广泛用于长途通信。为进一步增加光纤的信息容量，逐渐广泛采用波分复用的方法。
47.The potential of fiber optics in other areas is only beginning to be realized. Fiber optic networks for computer systems and offices are becoming more prominent. In the telephone system, the use of fiber optics for interconnecting central offices within a metropolitan area and for lower levels in the switching hierarchy is still increasing rapidly. Fiber links to the home have been used in demonstration projects. Many observers believe that national telephone systems will eventually be upgraded to handle video bandwidths by using fiber optics.人们对于光纤在其他领域的潜力刚刚开始认识。用于计算机系统和办公室的光纤网络逐渐变的更加重要。在电话系统中，光纤在主要城市地区中心交换局间互联和低级交换中的使用继续迅速增加。入户光缆已经有了示范工程。许多观察家相信，全国电话系统将使用光纤传输视频宽带信号而逐渐升级。
51.In 1971, the computer industry entered a new era. Microprocessors and minicomputers are now used for controlling many complicated features and functions with less power and size than was previously possible. Large-Scale Integrated (LSI) circuit technology reduced the size of mobile transceivers so that they easily fit the standard automobile. These achievements were a few of the requirements for developing advanced mobile phone systems and encouraging engineers to pursue this direction.1971年，计算机工业进入了一个新纪元。
现在许多复杂的特性和功能都使用微处理机和小型计算机进行控制，这比以前的控制方法减少了功率，缩小了体积。大规模集成电路技术减少了移动收发两用机的体积，而容易安装在标准汽车中。这些成果满足了发展先进移动电话系统的一些要求，促使工程人员进一步的向这个方向
努力。
48. One of many reasons for developing a
cellular mobile telephone system and deploying it in many cities is the operational limitations of conventional mobile telephone systems: limited service capability, poor service performance, and inefficient frequency spectrum utilization.开发蜂窝式移动电话系统并将其在许多城市中推广应用的原因之一是传统的移动电话系统存在容量有限、性能差、频普利用率低的缺点。
49.A major problem facing the radio communication industry is the limitation of the available radio frequency spectrum. In setting allocation policy, the Federal Communications Commission (FCC) seeks systems which need minimal bandwidth but provide high usage and consumer satisfaction.无线通信领域面临的一个主要问题是可使用的无线电频普有限。在确定分配时，美国联邦通信委员会寻求的是只需要最小的带宽却能提供高使用率并使用户满意的系统。
50.The ideal mobile telephone system
would operate within a limited assigned frequency band and would serve an almost unlimited number of users in unlimited areas. Three major approaches to achieve the ideal are:
理想的移动电话系统将在有限的给定频
段上工作，但却可以向任意多的地区中几乎是任意多的用户提供服务。实现这种理想系统的方法主要有三种：
1.Single-sideband (SSB) , which divides the allocated frequency band into maximum numbers of channels;
2.Cellular, which reuses the allocated frequency band in different geographic locations;
3.Spread spectrum, frequency-hopped, which generates many codes over a wide frequency band. 1.单边带，可将给定的频段分为最多的信道。 2.蜂窝式，可使给定的频段在不同的地理位置上重复使用。
3.扩展频普与跳频，能在宽频带上产生许多代码。
52. Why 800 MHZ? The FCC’s decision to
choose 800 MHZ was made because of severe spectrum limitations at lower frequency bands. FM broadcasting services operate in the vicinity of 100 MHZ. The television broadcasting service starts at 41 MHZ and extends up to 960 MHZ. Air-to-ground systems use 118 to 136MHZ;military aircraft use 255 to 400 MHZ. The maritime mobile service is located in the vicinity of 160 MHZ. Also fixedstation services are allocated portions of the 30 to 100 MHZ band. Therefore, it was hard for the FCC to allocate a spectrum in the lower portions of the 30 to 400 MHZ band since the services of this band had become so crowded. On the other hand, mobile radio transmission cannot be applied at 10 GHZ or above because severe propagation path loss, multipath fading, and rain activity make the medium improper for mobile communications.
为什么选择800MHZ作为蜂窝式电话通
信的频率？美国联邦通信委员会选择800MHZ是因为低频段的频普非常有限。跳频广播工作在100MHZ附近，电视广播工作在41-960MHZ.空对地系统使用118-136MHZ,军用飞机使用225-400MHZ,海事移动服务在160MHZ附近，地面固定无线电台使用30-100MHZ.30-400MHZ低频段这样拥挤使得联邦通信委员会很难将移动电话频率安排在此波段。另一方面，移动无线传输不能使用10GHZ以上的波段，因为严重的传输路径损耗、多径衰落和降雨，使该波段的空间媒质不适宜移动通信。
53. A basic cellular system consists of three
parts: a mobile unit, a cell site, and a mobile telephone switching office (MTSO). A mobile telephone unit consists a control unit, a transceiver, and an antenna system. The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets,antennas,a power plant, and data terminals. The switching office, the central coordinating element for all cell sites, consists of the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing and handles billing activities. The radio and high-speed data links connect the three subsystem. Each mobile unit can only use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to many mobile units. The cellular switch, which can be either analog or digital, switches calls to connect mobile subscriber to other mobile subscribers and to the nationwide telephone network.
一个蜂窝式系统基本上由三部分组成：移
动机、无线基站和移动电话交换局。移动电话机包括控制单元、收发信号机和天线系统。无线基地站在移动电话交换局和移动机之间提供接口，它有控制单元、无线机柜、天线、电源装置、和数据终端。它是所有无线基地站的协调中心，包括蜂窝处理机和蜂窝交换机。它与电话公司的地区交换局接口，控制呼叫处理并进行计费业务。无线线路和高速数据线连接以上三个部分。每个移动机每次只占用通信线路的一个频道。频道不是固定的，可以是服务区分配的全部频道中的任何一个。每一个无线基站具有多频道能力，可同时联通许多移动机。蜂窝交换机既可以是模拟的也可以是数字的，它对移动用户之间和移动用户与全国电话网之间的呼叫进行交换连接。
54. The success of mobile systems across the world is a sign that communication is moving towards a more personalized, convenient system. People who have to use a mobile phone on business soon begin to realize that the ability to phone any time, any place in one’s personal life rapidly becomes a necessity, not a convenience.
世界范围移动通信的成功标志着通信正在向着更加个人化，更加方便的通信系统迈进。在公务中需使用移动电话的人们很快就认识到在个人生活中随时随地打电话不仅仅带来了方便，而且是一种必须。
55. The fixed telephone service is global and the interconnection varies from coaxial cable to optical fiber and satellite. The national standards are different, but with common interfaces and interface conversion, interconnection can take place. For mobile the problem is far more complex, with the need to roam creating a need for complex networks and systems. Thus in mobile the question of standards is far more crucial to success than fixed systems.
固定电话业务是全球性的，系统的相互采用不同的方式由同轴电缆到光纤和卫星。虽然各国的标准不同，但却采用共同的接口和接口转换装置，使得相互连接可以进行。对移动通信来说，问题要更加的复杂，这是由于需要漫游业务就要有更复杂的网络系统。这样比起固定通信系统，移动通信的标准问题是取得成功更为关键的因素。
56. The GSM system is based on a cellular communications principle which was first proposed as a concept in the 1940s by Bell System engineers in the US. The idea came out of the need to increase network capacity and got round the fact that broadcast mobile networks, operating in densely populated areas, could be jammed by a very small number of simultaneous calls. The power of the cellular system was that it allowed frequency reuse.GSM系统基于蜂窝通信系统原则。这一概念是由美国贝尔系统工程师在40年代首先提出来的。这一思想出自于增加网络容量的需要以及解决网络堵塞的问题。在人口稠密地区运行的广播式移动网络系统会由于很少的几个用户同时呼叫而引起堵塞。蜂窝系统的威力在于允许频率的再利用。
57.The cellular concept is defined by two features, frequency reuse and cell splitting. Frequency reuse comes into play by using radio channels on the same frequency in coverage areas that are far enough apart not to cause co-channel interference. This allows handling of simultaneous calls that exceed the theoretical spectral capacity. Cell splitting is necessary when the traffic demand on a cell has reached the maximum and the cell is then divided into a microcellular system.蜂窝的概念有两个特点确定，即频率重复利用和蜂窝裂变。在覆盖区域相隔足够远而不至于引起公共信道干扰时，通过使用同一频率的无线信道，频率再利用才起作用。这样可以处理出现并超过了理论频普容量的同时呼叫。当小区的业务需求增到最大时，就要进行蜂窝裂变，蜂窝再被分成微蜂窝系统。
58. The cell coverage area is controlled by a base station which is itself made up of two elements. The first element is the transmission system which communicates out to the mobile and also receives information from it to set up and maintain call when actually in operation. The base station transceiver (BST) is controlled by the base station controller (BSC), which communicates with the mobile switching center (MSC)---the essential link to the local public switched telephone network (PSTN), and to the subscriber data which is stored in registers within the system.
小区的覆盖面由一个基站控制，基站本身由两个单元组成。第一个单元是传输系统，它在实际运行中与移动台进行通信以建立并保持通话。第二是基站收发机（BTS）,由基站控制器（BSC）控制,而BSC与移动交换中心（MSC）进行通信。这条通信链路，对于本地公共交换电话网（PSTN）的链接以及连接存储在系统寄存器中的用户数据来说，都十分的重要。
59. The GSM system operates in a burst transmission mode with 124 radio channels in the 900MHZ band, and these bursts can carry different types of information. The first type of information is speech, which is coded at 6.5 kbit/s or 13 kbit/s. The second type is data, which can be sent at 3.6 kbit/s, 6 kbit/s or 12.6 kbit/s. These two forms of transmission are the useful parts of the transmission, but have to be supported by overhead information which is sent in control channels (CCH).
GSM系统以突发脉冲序列传输方式工作。在900MHZ频带有124个无线信道，这些突发脉冲序列可以传送不同的信息。第一类信息是话音，编码速率为6.5 kbit/s 或 13 kbit/s。第二类信息是数据，可以3.6 kbit/s, 6 kbit/s或12.6 kbit/s的速率传输。这两类信息是传输信息的有用部分，但是传输还必须得到额外开销信息的支持，开销信息通过控制信道传输。
60.The use of digital radio transmission and the advanced handover algorithms between radio cells in GSM network allows for significantly better frequency usage than in analogue cellular systems, thus increasing the number of subscribers that can be served. Since GSM provides common standard, cellular subscribers will also be able to use their telephones over the entire GSM service area. Roaming is fully automatic between and within all countries covered by GSM system. In addition to international roaming , GSM provides new services, such as high-speed data communication, facsimile and short message service .The GSM technical specifications are designed to work in concert with other standards, e.g. ISDN. Interworking between the standard is in this way assured . In the long term perspective cellular systems, using a digital technology, will become the universal method of telecommunication.
GSM网络使用数字无线传输和先进的无线越区切换算法，可以得到比模拟蜂窝系统好的多的频率利用，因而增加了服务的用户数。由于GSM可提供共同的标准，蜂窝用户就可以在整个GSM服务区使用其电话。在GSM覆盖的国内或国际区域内，其漫游是完全自动的。除了国际漫游外，GSM还可提供新型用户服务，如高速数据通信、传真和短电文业务。GSM技术规范可与其它的通信标准保持一致，如ISDN.这样可保证标准间的互通。展望未来，利用数字技术的蜂窝系统将成为通信的通用方式。
61. In circuit switching, a total path of 63.Packet switching achieves the benefits connected lines is set up from the origin to the discussed so far and offers added features. It destination at the time the call is made, and the path provides the full advantage of the dynamic remains allocated to the source-destination pair allocation of the bandwidth, even when messages (whether used or not) until it is released by the are long. Indeed, with packet switching, many communicating parties. The switches, called circuit packets of the same message may be in switches (or office exchange in telephone jargon), transmission simultaneously over consecutive link have no capability of storing or manipulating user’s of a path from source to destination, thus achieving data on their way to the destination. The circuit is a “pipelining” effect and reducing considerably the set up by a special signaling message that finds its overall transmission delay of the message as way through the network, seizing channels in the compared to message switching .It tends to require path as it proceeds. Once the path is established, a smaller storage allocation at the intermediate return signal informs the source to begin switches. It also has better error characteristics and transmission.在电路交换中，当呼叫发生时，由呼leads to more efficient error recovery procedures, as 叫源点到终点之间要建立整个通路的连线，而且it deals with smaller entities. Needless to say, 在通信双方释放该电路之前，此通路一直保持分packet switching presents design problems of its 配给这对源点－终点（不管通路是否使用）。被own, such as the need to reorder packets of a given 称为电路交换机（或以电话行业用语称为局交message that may arrive at the destination node out 换）的交换设备没有存储或控制用户送往终点路of sequence. 由数据的能力。电路由特殊的信令建立，该信令分组交换除具有以上讨论的优点外，还具通过网络选择路由，并在其进程中确定信道。一有一些特点。它提供动态分配的全部优势，甚至旦路由建立，一个返回信号就通知呼叫源开始传当报文很长时依然如此。由于有分组交换，一个输（数据）。 报文的多个分组确定可以通过源点到终点的通
62. In message switching, the transmission 路中的多条链路同时传送，因而达到“管道传送”unit is a well-defined block of data called a message. 的效应。与报文交换相比，它大大的减少了报文In addition to the text to be transmitted, a message 整体的传送时延。在中间交换设备中，这种方式comprises a header and a checksum. The header 只需要较小的存储分配区域。分组交换的误码特contains information regarding the source and 性较好，由于它只涉及很短的长度，因而导致了destination addresses as well as other control 高效的纠错方式。当然，分组交换也有它设计上information; the checksum is used for error control 的麻烦，例如当报文无序的到达目的节点时，需purposes. The switching element is a computer 要重新对核对报文进行分组排序。 referred to as a message processor, with processing . ATM (Asynchronous Transfer Mode) is and storage capabilities. Messages travel both a multiplexing and switching technique. It was independently and asynchronously, finding their initially intended to handle high bit rates , but it has own way from source to destination. First the in fact proved to be a universal technique for message is transmitted from the host to the message transporting and switching any type of digitized processor to which it is attached. Once the message information at a wide variety of bit rates. is entirely received, the message processor ATM（异步转移模式）既是复用技术又是examines its header, and accordingly decides on the 交换技术。最初，人们是想用ATM来处理高比next outgoing channel on which to transmit it. If 特率的数字信号，事实确证明它是一种通用的技this selected channel is busy, the message waits in a 术，可以用来传输和交换任何类型并具有各种比queue until the channel becomes free, at which time 特率的数字化信息。 transmission begins.在报文交换中，传输单元是一65.ATM transfers information in short 个被精心定义的数据块，该数据块被称为报文。packets call “cells” with a fixed length of 48 bytes 除了要发送的内容外，报文还包括有报头和校验plus five header bytes, irrespective of the 项。报头含有源地址和目的地址的信息，以及其underlying type of transmission. Cell routing is 它的控制信息，而校验项用于误码控制。交换单based on the principle of logical channels with dual 元是一台被称为报文处理器的计算机，它具有处identification: the cell header contains the identifier 理和存储的能力。报文并异步的传输，在源of the basic connection to which the cell belongs－点与终点间选择自己的传送路由。首先报文由主called a virtual circuit (VC) and the identifier of the 机送往与之相连的报文处理机。一旦报文被完全group of VCs to which the connection belongs－收到，报文处理机就检查它其报头，并相应的决called a virtual path (VP). 定该报文传送的下一个输出信道。 无论传输的信息是什么，ATM都以称作
63. A variation of message is broken up “信元”的短的分组采传送信息。信元是由固定
into several pieces of a given maximum length, 的48字节加上5个字节的信头组成。信元寻找called packets. As with message switching, each 路由是基于带有双重识别的逻辑信道原理：信元packet contains a header and a checksum. Packets 头包含了信元所属的基本连接识别符，这种基本are transmitted independently in a 连接称作虚电路（VC）, 另一种是连接所属的store-and-forware manner. VC组识别符，称作虚路径（VP）.
分组交换是报文交换的一种变形。在分组66.ATM is related to both circuit and packet
交换中，报文被以指定的最大长度分成若干个被modes. Because of the simplicity of the protocol 称为分组的段节。与报文交换一样，每个分组都used, the transfer of cells to the network nodes can 含有一个报头和校验项。分组以存储转发方式独be handled entirely by hardware, which leads to 立传送。 very short transit time and high usage of
61.With circuit switching, there is always an transmission paths, even at bit rates of several initial connection cost incurred in setting up the hundred megabits a second. On the other hand, circuit .It is cost-effective only in those situations ATM retains all the flexibility of the packet mode, where once the circuit is set up there s a guaranteed enabling only required information to be conveyed, steady flow of information transfer to amortize the offering a simple, unique multiplexing method initial cost. This is certainly the case with voice irrespective of the bit rates of the different communication in the traditional way, and indeed information flows, and allowing these bit rates to be circuit switching is the technique used in the varied. telephone system. Communication among ATM既与电路方式有关又与分组方式有computers, however, is characterized as bursty. 关。由于使用简单的协议，信元至网络节点的转Burstiness is a result of the high degree of 移可完全有硬件处理完成，这就缩短了传送的时randomness encountered in the message-generation 间，提高了传输路径的速率，使比特率甚至可以process and the message size, and of the low delay 达到每秒几百兆比特。另一方面，ATM保留了分constraint required by the user. The users and 组方式的所有灵活性：只传送所需要的信息，提devices require the communication resources 供简单、独特的复用方法而不管不同信息流的比relatively infrequently; but when they do, they 特率，并且允许比特率变化。 require a relatively rapid response. 在电路交换的67.ATM dated from the beginning of the 情况下，建立电路总要对开始的接续付出代价。1980s: at the time, people were trying to find the 只有在这种情况下，即一旦电路建立后，信息的most suitable technique for switching high bit rate 传送确保持续、源源不断，以便摊分初始花费，channel at more than 100 Mbit/s. In1988, the ITU 才能提高价格的效率比。传统方式的语音通信就approved recommendation I. 121 which ratified the 属于这种情况，因而电路交换确实是电话系统中choice of ATM as the target transfer mode for 使用的技术。但是，计算机通信具有突发特性。broadband networks for all types of information, 突发性是报文产生过程和报文长度高度的随机including low bit rate information such as voice. In 性所造成的，也是用户对时延要求很短造成的结1991 several operators and manufacturers founded 果。用户和设备不怎么经常的用到通信资源，但the ATM Forum to expedite standardization. The 是当它们用到时，它们就要求相当迅速的反应。 ATM Forum now has more than 600 members and
62.Therefore, for bursty users (which can has a significant influence on ATM standards and also be characterized by high peak-to-average date specifications. The first ATM produces appeared rate requirements), store-and-forward transmission on the market in 1992: they were for local area techniques offer a more cost-effective solution, networks and were designed to solve the problems since a message occupies a particular of sharing the same bearer circuit between communications link only for the duration of its computer terminals as they continue to increase in transmission on that link; the rest of the time it is numbers and power. stored at some intermediate message switch and the ATM开始于80年代初，那时人们试图找link is available for other transmissions. Thus the 一种更合适的技术用于交换超过100Mbit/S的高main advantage of store-and-forward transmission 比特率信道。1988年，ITU批准了I.121建议，over circuit switching is that the communication 该建议选择ATM作为用于各种类型信息宽带网bandwidth is dynamically allocated, and the 络的目标传送模式，其中包括如话音的低比特率allocation is done on the fine basis of a particular 信息。1991年一些运营公司和厂商建立了ATMlink in the network and a particular message(for a 论坛以加速ATM标准化工作。现在ATM论坛的particular source-destination pair).因此，对突发性成员已经超过600个，它对ATM的标准化和规用户（峰值速率于平均速率很高为该用户的特范化有着重大影响。第一批ATM产品1992年面征），存储转发传输技术提供了一个更低价高效世，主要用于局域网，其设计主要是用来解决计的解决办法，因为只有在报文传送的时间里，报算机终端间随计算机数量和功能不断增加所带文才占据一条特定的链路。在其它时间，报文是来的共享同一承载电路的问题。 被存储在某个中间交换机中，因而此时的链路可 用于其他传输。这样，与电路交换相比，存储转 发方式的主要优点是通信带宽的动态分配，而且 这种分配是以网络中的特定链路和特定报文（对 一个特定的源点－终点对来说）为基础的。 68.An ATM network can be considered, in a
first approximation, as being three overlaid functional levels: a services and applications level, an ATM network level and a transmission level. The applications provide an end-to-end service. They use the logical connections of ATM network level which in turn multiplexes and logically routes the information flow as ATM cells go through the transmission links shared by logical connections called virtual connections. The transmission level provides these physical links and handles the actual physical transport of the cells.
ATM网络可以近似的看做是有三个覆盖功能层组成：业务和应用层，ATM网络层和传输层。应用层提供端到端的业务。应用层使用ATM网络层的逻辑连接，当信元通过由逻辑连接（称作虚连接）共享的传输链路时，ATM网络层依次对信息流复用并寻找信息流的逻辑路由。传输层提供物理链路并处理信元的实际物理传输。
69.An ATM network can transport and switch voice, data and video which, seen from the access, use traditional digital interfaces with the same quality of service. This means that a physical connection between any two terminals can be replaced with an equivalent logical connection which is multiplexed with other in a common transmission link. The resource is shared dynamically between all the connections.
ATM网络能够传输和交换话音、数据和视频业务，从接入的角度看这些业务使用传统的数字接口并具有同样的服务质量。这就意味着任何两个终端间的物理连接都可由等效的逻辑连接代替，逻辑连接可在公用的传输链路中与其他的逻辑连接复用。资源可在所有的连接中动态共享。
70.The ATM technique completely separates the applications and services transported over a network from the transmission resources used. The ability to construct virtual networks means that the physical network can be shared by many users dynamically and in real time, thereby achieving cost-effective use of infrastructure, for high bit rate services too. Investments at all levels are also future-proofed, because of the different applications can be reallocated in time over the same network infrastructure as requirements arise. ATM offers a unique way of coordinating different networks carrying different services into a single physical network.
ATM技术将网络传输的应用和业务与所使用的传输资源完全分开。构成虚网络的能力意味着物理网络可以由许多用户动态时实的共享，因而使网络结构得到高价高效的使用，对高比特率业务也一样。对所有网络层的投资都是适应未来需要的，因为不同的应用在出现新的需求时可及时在同一网络结构中进行重新分配。ATM提供一种独特的方式将传输不同业务的网络协调成单一的物理网络。
71. ATM, much more than any other telecommunications technique , is able to meet the current and the future requirements of both operators and users. Compared with other techniques that may compete in certain applications, ATM is special ( mainly due to its universal nature, both in terms of bit rate and type of information transferred）. ATM offers a switching function for all bit rates and this is particularly suitable for high and variable bit rates.
ATM，远比任何其它电信技术更能满足运营公司和用户对当前和未来业务的需求。与其它有可能在某些应用领域于ATM竞争的技术相比，ATM（主要由于其通用性，无论是比特率还是传输信息的类型）都具有特殊的优点。ATM对所有比特率的信号都可提供交换功能，这一点特别适合于高比特率和可变化比特率信号。
72. By far the largest circuit-switched network is the public telecommunications network－the telephone network. This is actually a collection of national networks interconnected to form an international service. Although originally designed and implemented to service analog telephone subscribers, it handles substantial data traffic via modem, and is gradually being converted to a digital network.迄今最大的电路交换王是公共电信网－电话网，实际上是由一些国家网相互连接而形成的国际业务。虽然原来的设计是为模拟电话用户服务的，但是它通过利用调制解调器处理大量的数据业务，而正在逐渐变为数字网。
73.The public telecommunications network, as with any communications network, can be described using four generic architectural components.
如同任何通信网一样，公用电信王可以用四个一般的结构组件来描述。
Stations: generally denoted as subscribers, these are the devices that attach to the network.
Interfaces: the interface between the stations and the network, referred to in the phone system as the local loop.
Nodes: the switching centers in the network. Links: the branches between nodes, referred to as trunks.
站：一般只用户，这些是接入网络的装置。 接口：站和网络间的接口，在电话系统中称为本地环路。
节点：网络交换的中心
链路：节点之间的支路，称为中继线。 74.The local loop is a pair of wires, generally twisted pair, that connects a subscriber to one of the nodes in the network. The local loop generally covers a distance of a few kilometers to a few tens of kilometers at most.
本地环路是将用户连接到网络中的一个节点的一对导线，通常是双绞线。一般本地环路的覆盖范围从几公里到几十公里。
75.A two-wire connection is inherently 85. The integrated services digital network
half-duplex; that is, it can carry voice in one (ISDN) is the road map to the future for all forms of A telex network transports telex information, i.e. direction at a time only. Similarly, it can carry digital communications. Since more than 90% of all messages of characters, transported at very low digital signals in on direction at a time only. For communication systems will be digital by the year speed. The characters are coded based on a specific full-duplex DDS connections, two twisted pair links 2000, consequently ISDN standards and system 5-bit code.
are generally used.两线连接本质上是半双工的，architecture will largely dictate the shape and form POTS (Plain Old Telephone Service) is 即它一次只可传送一个方向的话音。同样，它一of global communications in the 21st century.对各transported via the Public Switched Telephone 次仅能传送一个方向上的数字信号。对于全双工种形式的数字通信来说，综合业务数字网（ISDN）Network (PSTN). This ubiquitous network offers DDS连接，一般采用两对双绞线链路。 是未来发展方向。因为到2000年，90％以上的the customers classical two-way voice conversation.
76.Each subscriber connects via local loop to a 通信系统将会实现数字化，因而ISDN的标准和Computer data are transported in the public switching center, known as an end office. Typically, 系统结构将会充分的确定21世纪的全球通信方domain either by a packet switched data network an end office will support many thousands of 式。 based on X.25 protocols, or in a very limited subscribers in a localized area. There are over 19 86.ISDN’s enormous importance is only number of countries by a circuit switched data 000 end offices in the United States, so it is clearly gradually being understood. Years of work have network based on X.21 protocols.
impractical for each end office to have a direct link been carried out by the International Telegraph and Television signals can be transported in three to each other end office; this would require on the Telephone Consultative Committee (CCITT) and ways: broadcast via radio waves using ground order of 2*108 links. Rather, intermediate switching the International Organization for Standardization antenna, by the coaxial tree network of community nodes are used.每个用户通过本地环路接到一个(ISO). As it evolved, ISDN has been put into the antenna TV network or recently via a satellite, using 称为端局的交换中心。通常，一个端局支持一个spotlight. This new prominence does not mean that the so-called direct broadcast system.
局部地区的数千个用户。在美国有19000多个端the idea is new. In fact, ISDN was conceived a In the private domain, computer data are mainly 局，所以要想每个端局都有到其它每一个端局的quarter of a century ago and is only now beginning transported by LANs (Local Area Network). The 直接链路显然是不实际的，这将需要2*108数量to reach maturity. The digital service offered the most famous ones are Ethernet, token bus and token 级的链路。因此，应采用中间交换节点 PTTs and other public communication carriers three ring (IEEE802 series).
77.Subscribers connect directly to an end office, very appealing characteristics: enhanced quality, 电传网络传送电传信息，即字符消息，它以很which must perform the same functions as a PBX. economy, and flexibility. 低的速率传送。字符编码的基础是特定5比。 The remaining centers simply serve the function of ISDN的极其重要性只是逐渐被认识到的。国POTS(旧式的电话业务)经由公众电话交换网concentrating traffic so as to reduce transmission 际电报电话咨询委员会（CCITT）和国际标准化（PSTN）.这种无处不在的网络向顾客提供传统facility requirements.用户直接连接到端局，端局组织（ISO）为此进行了数年的研究工作。随着的双向话音的交流。 必须执行PBX的相同功能。其余的中心仅提供集工作的进展，ISDN开始成为注目的中心。其新计算机数据在公共域中在以X.25协议为基础中通信业务量的功能，以减少对传输设备的需的显著地位并不意味着其思想是新的。事实上，的分组交换数据网（PSDN）中传送，而在有限求。 25年以前就有了ISDN的构想，只是到现在才开的数个国家，它是在X.21协议为基础的电路交
78.The switching centers are linked together by 始走向成熟。PTT和其它公共通信利用的数字业换数据网中传送的。
trunks. These trunks are designed to carry multiple 务具有下列三个非常吸引人的特点：高质量、经电视信号可以三种方式传送：利用地面天线，voice-frequency circuits using either FDM or 济性、和灵活性。 通过无线电波；由公共天线TV（CATV）的通州synchronous TDM.交换中心由中继线连接在一87.The first characteristic is achieved when a 电缆树型网络传送；近来，人们又利用称为直接起。这些中继线的设计采用FDM或同步TDM来signal is a digitally encoded to overcome 广播系统（DBS）经卫星进行传送。 传送多条音频线路。 degradation due to the transmission medium. As 计算机数据在私用域主要由局域网（LAN）传
79.PBX facilities connect to the network not regards economy, early studies suggested that 送。最著名的一些局域网是以太网、令牌总线网via local loop but via trunk. Since the PBX services digital services offered significant advantages since 和令牌环状网（IEEE802系列）。 multiple subscribers, a multiplexed link is needed to a substantial quantity of interface equipment in the 91.Each of these networks was specially the end office. Generally, this link has lower telephone exchange could be eliminated. Finally, designed for that specific service and is often not at capacity than the total number of PBX subscribers, digital service also provides flexibility in the all applicable to transport another service. For reflecting the fact that, at any given time, only a transmission of voice, data, video, and other instance, the original CATV networks did not allow fraction of the subscribers will be engaged in services with equal ease. 为克服由于传输和媒介the transportation of POTS; or the PSTN does not external calls. 引起的失真而对信号进行数字编码，表现出了第transport TV signals; or the transfer of voice over
还需要两个附加的组成部分来完成这个结构一个特点。至于经济性，早期的研究表明由于数an X.25 network is very problematic because of too 图。除了上面列出的五级交换中心外，网络用另字业务可能避免在电话交换中使用大量的接口large end-to-end delay and jitter on this delay. 外的称之为汇接局的交换节点加以扩展。这是用设备，因而具有重要的经济优势。最后，数字业这些网中的每个网都是为该特定业务特别设于连接邻近的端局的。最后，PBX设备不是通过务在同样方便的传输声音、数据、视频、和其他计的，通常不适于传送另一种业务。例如，原来本地环路而是通过中继线接到网中。由于PBX业务时也表现出了灵活性。 的CATV网不允许POST业务的传送；PSTN也
88.The term ISDN was first defined within the 不能传送TV信号；服务多个用户，所以需要一条复用链路连到端在X.25网上传送话音也很成
局。一般说来这条链路上的容量较PBX用户总数CCITT in 1972. It referred to a network providing 问题，因为端到端的时延太长以及在时延上有抖
digital connectivity for a range of 为低，这反映了在任一给定的时间只有部分用户end-to-end services. It also used the idea of a totally open 动。
进行外线呼叫这一事实。 Only in limited and special cases can service network with universally available and standardized
80.Consider a request from one subscriber to “ports” that can plug in every-where. The concept types other than the one the network was originally establish a connection with another. The following has since then evolved from the existing telephone designed for be transported over it.仅在有限和特rules would apply:设有一用户要与另一用户建立networks of countries undergoing changes due to 殊的情况下，非原设计的业务类型才可在原设计
the introduction of totally new digital technology in 业务的网上传送。 连接，则可以采用以下规则：
switching and transmission equipment. 1. If both subscribers attach to the same end office, the 92.An important consequence of this service Similarly, the introduction of voice encoding
that end office makes the connection. techniques using pulse code modulation contribute specialization is the existence of a large number of 2. If (1) fails, the subscribers attach to different end to the concept’s evolution. Initially, voice encoding often world-wide independent networks, each offices .If those end offices attach to the same toll was base upon a kbit/s transmission rates. Today requiring its own design phase, manufacturing and center, a connection is established between end two kbit/s, or 2B-channel coupled with a basic maintenance. In addition, the dimensioning of each
access signaling rate of 16kbit/s, that is, the network must be done for every individual service office via the toll center.
constitute the essential ISDN １．如果这两个用户接到同一端局，则由该端局D-channel, transmission rate of 144kbit/s (2B+D). Currently type. Even if resources are freely available in one
建立连接。 PSTN, available in almost 200 countries and network, they cannot be used by another service ２．如果不是1的情况而是用户接到不同的端territories around the world, offers universal voice type. For example, the peak hours in the telephone 局，若那些端局接到同一个长途中心，则端局间access to almost 750 million telephones around the network are between 9 a.m. and 5 p.m., whereas the
world. ISDN is the network which will peak hours in the CATV network are during 的连接由该长途中心建立。
81.And so on. The search continues up the progressively provide a similar universal access for evening. Since resource pooling is impossible, each
only voice, facsimile, video, text, and other network must be dimensioned for its worst case hierarchy until a common node is reached. If the not services as well
two subscribers are under the aegis of different traffic conditions which is the peak hour traffic. regional centers, the circuit will involve a trunk 这种业务专用化的一个严重后果是存在着大.ISDN这一术语由CCITT在1972年第一次被
between regional centers, for a total of nine trunks 定义。它指的是一个网络，他为一系列业务提供量的、常常是世界范围的性网络，每个网络in the path between the subscribers.如此继续下去。端对端数字连接。它也指一个完全开放的网络，都需要自己的世界流程、生产和维护。此外对每搜索沿层次而上直至到达公共节点。如果两个用并具有普遍使用的标准化接口，而这个接口可以种业务，每个网必须自成规模。即使一个网户是在不同的大区中心的覆盖之下，电路将包括在任何的地方插接。从那时起，由于在交换和传输设备上引入全新的数字技术，一些国家对现有的资源空着不用，也不能被另一种业务类型所使两个大区中心间的一条中继线，用户间的路径总电话网络进行改造，使这一概念有了进一步发用。例如，电话网中的峰值繁忙时间为上午9时共可有9条中继线。 展。同样使用脉码调制的话音编码技术也对这一到下午5时之间，而在CATV网中的繁忙时间是
83.To compensate for these problems, a large 概念的发展起了作用。最初，话音编码以kbit/S在傍晚。由于不能资源共享，每个网络必须针对
今天，两个kbit/s(2B信道)其最坏的流量条件，即最忙时间的流量来设计规number of high-usage trunks augment the basic 的传输速率为基础。
architecture. High-usage trunks are used for direct 与一个16kbit/s(D信道)基本通路信令速率构成模。
的144kbit/s(2B+D)的主要传输速率。目前，connection between switching centers with high ISDN93.A first step, albeit a limited one towards a 几乎近200个国家和地区的PSTN向全世界7亿
volumes of internode traffic. Traffic is always 5千万部电话提供广泛的话音通路。ISDN这一网single universal network, is the introduction of routed through the lowest available level of the 络将会逐步提供不仅像话音这样普遍的通路，而N-ISDN in which voice and data are transported network. 为弥补这些问题，要在基本结构中加入且包括数据、传真、视频、文本及其他业务。 over a single medium. This network cannot 大量的高效中继线。高效中继线用于直接连接有.In the industrialized countries, the transport TV signals due to its limited bandwidth 大量的节点间通信业务量的各交换中心。通信量evolutionary nature of the ISDN concept has been a capabilities, so a special TV network is still 常常是通过网络中可用的最低层次传送的。 matter of economic practicality considering the vast required. Even in N-ISDN the integration of
84.Consider again two subscribers attached to telephone infrastructure already in place. The narrowband services such as data and voice can be different end offices. The calling subscriber’s end primary emphasis, however, has been the considered as being rather limited: the user access office will determine if a direct trunk exists to the enhancement of services rather than a simple to the network is fully integrated, either by a basic called end office. If not, or if that trunk is fully replacement of existing equipment. This access or primary rate interface. However, inside loaded, the call is routed up to a toll center. The toll evolutionary approach has also merit for most of the the network there will still exist for some time a center will try to find an available direct route to a developing countries, although in some instances packet switched and a circuit switched network as center in the immediate hierarchy of the called end where analogue facilities do not exist presently or two overlay networks incapable of transporting office. Failing that, the call is switched up to a where they are very limited, the direct installation other traffic types and each dimensioned either for primary center, and so on. This process is dynamic of digital facilities could allow a direct leapfrog to voice or X.25 data. and depends on the availability of high-usage trunks ISDN capabilities.在一些工业化国家，ISDN发展迈向单一的通用网的第一步（尽管是很有限at the time the call is placed. Thus calls between 的本质一直是一个经济实用性问题，因为要考虑的一步），就引入N-ISDN（窄带ISDN）.在该网two subscribers might follow different routes at 到现有的庞大电话设施。然而，主要方面一直是中，话音和数据通过同一种媒介来传送。由于它different times.另外考虑接到不同端局的两个用提高业务能力而不是简单的取代现有的设备。这有限的带宽，这个网络不能传送TV信号，因而户。主叫用户的端局要确定是否有一条直达中继一发展方式对多数发展中国家大有好处。因为在仍需要一个专用的TV网络。甚至在N-ISDN中，线连接到被叫端局。如果没有，或者那条中继线一些国家，目前的模拟设备不存在或很有限，所就像数据和话音这样的窄带业务的综合也被认已经满载，则把该呼叫转到长途中心。长途中心以直接安装数字设备可以直接跨越到ISDN的通为是相当有限的：用户对网络的接入是完全综合将试图寻找一条可用的直达路由到被叫端局的信能力。 的，采用基本接入或基本速率接口接入。但是在中继层次中。如果还不行，该呼叫又转到初级中90. Today’s telecommunication networks are 网络内部，仍将在一段时间里存在着两个包层网心，等等。这一过程是动态的，并且取决于呼叫characterized by specialization. This means that for 络：分组交换网络和电路交换网，它们不能传送时高效中继线的可用性。因而两用户间的呼叫在every individual telecommunication service at least 其他类型的电信业务，而且对话音或X.25数据不同实际可以有不同的路由。 one network exists that transports this service. A 自成规模。
few examples of existing public networks are described below: 当前电信网络的特征是专用化。即每一种电 信业务，至少有一个传送该业务的网络存在。下 面列举了当今几个公用网的例子：
94.Another important consequence of this service specialization is the inability of the network to benefit highly from the progress made in technology and coding algorithms. For instance, current digital N-ISDN switches are designed for kbit/s voice channels. However , with the current progress in speech coding and chip technology, bit rates of 32 kbit/s, 13kbit/s and even lower will be used in the future. The existing switches and transmission systems are not directly suited and thus need an adaptation, or will not efficiently use their internal resources for these lower speed bit rates.
这种业务专用化的另一个重要的后果，是网络不能充分得益于技术和编码算法的进步。例如，当今的数字N-ISDN交换是针对kbit/s话音信道而设计的。但是，随着当前语音编码和芯片技术的进步，比特速率为32 kbit/s（ADPCM）、13 kbit/s（移动网用），甚至更低速率的信号将在未来使用。由于现有的交换和传输系统不能直接失配这些信号，因而需要适配器，否则对于这些低速比特率将不能有效利用其内部资源。
95.When designing the future B-ISDN network, one must take into account all possible existing and future services. Suppose a network is capable of transporting a specific service, e.g. a circuit switched service with a channel rate of 70 Mbit/s. Suppose also that it is specifically designed to transport this bit rate. Some years later a new teleservice of , for example, 40 Mbit/s appears on the scene. This would mean that the network designed of that service (i.e. 70 Mbit/s)will be capable of transporting the new teleservice, but with a large inefficiency: only 40 out of the 70 Mbit/s available will be used. This example is not unrealistic. It is very likely that in the future new services will emerge which have not yet been identified, and of which the requirements are unknown today.
当设计未来的B-ISDN网络时，人们必须考虑到所有可能现有的业务和未来业务。假定某网络能够传送某一种特定业务，如一种信道速率为70Mbit/s的电路交换业务，同时,我们假定它是被专门设计来传送这种速率。经若干年后，出现了一种新的电信业务，例如40Mbit/s业务。这意味着为那种业务（即70Mbit/s）设计的网络将可传送这种业务，但效率大大降低了：在70Mbit/s中仅有40Mbit/s将被有效利用。这个例子并非杜撰。十分有可能的是:在未来，将出现一些新的业务，只是这些业务至今还未开发，其要求在今天还不清楚罢了。
96.Advances in audio, video and speech coding and compression algorithms and progress in Very Large Systems Integration (VLSI) technology influence the bit rate generated by a certain service and thus change the service requirements for the network. In the future, new services with unknown requirements will appear. For the time being it is yet unclear, e.g. what the requirements in terms of bit rate for HDTV will be. A specialized network has great difficulties in adapting to changing or new service requirements.在声音、图像、语音编码和压缩算法以及在特大系统综合（VLSI）技术方面的进步影响着一定业务的比特率，因而改变着对网络的业务要求。在将来，未知需求的新业务将会出现。例如，HDTV(高清晰度TV)将在比特率上有何要求当前仍不清楚。一个专用化网络在适应变化和新业务要求方面具有极大困难。
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