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1、外文翻译毕业设计题目：基于PSTN网的远程分布式智能传感器阵列学生：黄淮滨 1002311指导老师：刘大茂、魏云龙原文1： The Public Telecommunications NetworkBy far the largest circuit-switched network is the public telecommunications networkthe telephone network. This is actually a collection of national network interconnected to form an international serv
2、ice. 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.The public telecommunications network, as with any communications network, can be described using four gener
3、ic architectural components.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 b
4、etween nodes, referred to as trunks. Most of the subscribers on the network are telephones. The telephone contains a transmitter and receiver for converting back and forth between analog voice and analog electrical signals. With the introduction of the digital data system, some subscribers that tran
5、smit digital signals have been incorporated into the network.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.A two-wire connect
6、ion is inherently half-duplex; that is, it can carry voice in one direction at a time only. Similarly, it can carry digital signals in one direction at a time only. For full-duplex DDS connections, two twisted pair links are generally used.Each subscriber connects via local loop to a switching cente
7、r, known as an end office, Typically, an end office will support many thousands of subscribers in a localized area. There are over 19000 end offices in the United States, so it is clearly impractical for each end office to have a direct link to each other end office; this would require on the order
8、of 200000000 links. Rather, intermediate switching nodes are used. Designers have found it convenient to organize these nodes into a hierarchy or tree topology(Fig.14-1), consisting of five classes of switching centers or nodes.Class 1: regional center.Class 2: sectional center.Class 3: primary cent
9、er.Class 4: toll center.Class 5: end office.10 Regional centers67 Sectional centers230 Primary centers1300 Toll centers19000 End officers 150000000SubscribersFig.14-1 U.S. Public Circuit-switched Network OrganizationSubscribers connect directly to an end office, which must perform the same functions
10、 as a PBX. The remaining centers simply serve the function of concentration traffic so as to reduce transmission facility requirements.The switching centers are linked together by trunks. These trunks are designed to carry multiple voice-frequency circuits using either FDM or synchronous TDM.Two add
11、itional elements are needed to complete the picture. In addition to the five classes of switching centers listed above, the network is augmented with additional switching nodes called tandem switches. These are used to interconnect adjacent end office. Finally, PBX facilities connect to the network
12、not via local loop but via trunk. Since the PBX services multiple subscribers, a multiplexed link is needed to the end office. Generally, the link has lower capacity than the total number of PBX subscribers, reflecting the fact that, at any given time, only a fraction of the subscribers will be enga
13、ged in external calls.The structure depicted in Fig.14-1 is referred to as having a tree topology. Actually, it is a set of 10 trees, each rooted in a regional center. The 10 regional centers are meshed together to provide full connectivity. If this were the extent of architecture, routing would be
14、quite simple. Consider a request from one subscriber to establish a connection with another. The following rules would apply:(1) If both subscribers attach to the same end office, that end office makes the connection.(2) If (1) fails, the subscribers attach to different end offices. If those end off
15、ices. If those end offices attach to the same toll center, a connection is established between end office via the toll center.And so on. The search continues up the hierarchy until a common node is reached. If the two subscribers are under the aegis of different regional centers, the circuit will in
16、volve a trunk between regional centers, for a total of nine trunks in the path between the subscribers.This architecture has several drawbacks. First, a tremendous amount of traffic must be carried at the upper levels of the hierarchy. Second, the loss or saturation of a single switching center deco
17、uples the network into isolated subnetworks. Finally, signal quality degrades as the number of switches and trunks increases.To compensate for these problems, a large number of high-usage trunks augment the basic architecture. High-usage trunks are used for direct connection between switching center
18、s with high volumes of internode traffic. Traffic is always routed through the lowest available level of the network. Fig.14-2 shows the basic order of selection for alternate routes. The high-usage trunks are depicted as dashed lines, and the backbone hierarchical network as shown with solid lines.
19、1234512345Regional centerSectional centerPrimary centerToll centerEnd officeSubscribersFig.14-2 Public Circuit-switched Network Routing (North America)Consider again two subscribers attached to different end offices. The calling subscribers end office will determine if a direct trunk exists to the c
20、alled end office. If not, or if that trunk is fully loaded, the call is routed up to a toll center. The toll center will try to find an available direct route to a center in the immediate hierarchy of the called end office. Failing that, the call is switched up a primary center, and so on. This proc
21、ess is dynamic and depends on the availability of high-usage trunks at the time the call is placed. Thus calls between two subscribers might follow different routes at different times. The routing algorithm is driven by seven-orten-digit telephone number, which uniquely identifies a subscriber and c
22、enters in its direct hierarchy.The tandem switches mentioned above provide additional linkages between end offices that closely located. Tandem switches are part of what is referred to as an exchange area. Typically, all calls within an exchange area are local calls(no longdistance charges).翻译1： 公用电
23、信网迄今最大的电路交换网是公用电信网电话网，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，而正在逐渐被变为数字网。如同任何通信网一样，公用电信网可以用四个一般的结构组件来描述。站一般指用户，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。节点网络中的交换中心。链路节点间的支路，称为中继线。网络中的大多数用户是电话机。电话机包括一个送话器和一个受话器，用来模拟话音（声波）和模拟电信号（声频）之间进行变换。随着数字数据系统（DDS）的引入，有些传输数字信号的用户已经并入该网。本地环路是将用户连到
24、网络中的一个节点的一对导线，通常是双绞线上。一般本地环路的覆盖范围从几公里到几十公里。两线连接本质上是半双工的，即它一次只可传送一个方向的话音。同样，它一次仅能传送一个方向上的数字信号。对于全双工DDS连接，一般采用两对双绞线链路。每个用户通过本地环路接到一个称为端局的交换中心。通常，一个端局支持一个局部地区的数千个用户。在美国有19000多个端局，所以要想每个端局有到其他每一个端局的直接链路显然是不实际的，这将需要有200000000数量级的链路。因此，应采用中间交换节点，设计人员发现可很方便地把这些节点组织成由五级交换中心或节点构成的层次结构或树状拓扑。（见图1）第1级：大区中心第2级：地
25、区中心第3级：初级中心第4级：长途中心第5级：端局用户直接连到端局，端局必须执行与PBX相同的功能。其余的中心仅提供集中通信业务量的功能，以减少对传输设备的需求。交换中心由中继线连接在一起，这些中继线的设计或是采用FDM或是采用同步TDM来传送多条音频线路。还需要两个附加的组成部分来完成这个结构图。除了上面列出的五级交换中心外，网络用另外的称之为汇接局的交换节点加以扩展。这是用于连接邻近端局的。最后，PBX设备不是通过本地环路而是通过中继线接到网中。由于PBX服务多个用户，所以需要一条复用链路连到端局。一般说来这条链路上的容量较PBX用户总数为低，这反映了在任一给定时间只有部分用户进行外线呼叫
26、这一事实。图1所画的结构称为树状拓扑。实际上，这一共是10棵树，每一棵扎根于大区中心。这10个大区中心以网状形式连接在一起（用45条全双工链路）来提供全连通。如果结构的规模仅到此为止，则路由选择就相当简单了。设有一用户要求与另一用户建立连接，则可以采用以下规则：（1） 如果这两上用户接到同一端局，是由该端局建立连接；（2）如果不是（1）的情况而是用户接到不同的端局，若那些端局接到同一个长途中心，则端局间的连接由该长途中心建立。如此继续下去。搜索沿层次而上直至到达公共节点。如果两用户是在不同的大区中心的覆盖之下，电路将包括两个大区中心间的一条中继线，用户间的路径总共可有9条中继线。这种结构存在几
27、个缺点。首先，在较高的层次间要传送大的通信量。其次，单个交换中心的故障或饱和会将该网络拆成孤立的子网。最后，当交换的次数和中继的段数量增加时，（对于模拟传输）信号质量要降低。为弥补这些问题，要在基本结构中加入大量的高效中继线。高效中继线用于直接连接有大量的节点间通信业务量的各交换中心。通信量常常是通过网络中可用的最低层次传送的。图2显示了选择迂回路由的基本次序。图中高效中继线用虚线画出，主干层次网用实线表示。另外要考虑接到不同端局的两上用户。主叫用户的端局要确定是否有一条直达中继线连到被叫端局。如果没有，或者那条中继线已经满载，则把该呼叫转到长途中心。长途中心将试图寻找一条可用的直达路由到被叫
28、端局的中继层次的中心。如果还不行，该呼叫又转到初级中心，等等。这一过程是动态的，并且取决于呼叫时高效中继线的可用性。因而两用户间的呼叫在不同时间可以有不同的路由。路由选择算法由7位或10位数字的电话号码启动，电话号码惟一地标识了用户及其直接层次的中心。上面提到的汇接局在位置靠近的端局之间提供附加的链接。汇接局是所谓的交换区的一部分。通常，在一个交换区之内的所有呼叫都属本地呼叫（不计长途费用）。原文2： Data transmission with analog carriers and modemData transmission involves the transmittal of dig
29、ital information from one DTE another. Rather than construct special lines for this purpose, it is more convenient to use the public telephone network (PTN). Unfortunately, the vast majority of the PTN was installed long before the advent of large-scale data communications and was designed for conve
30、ying information that is analog in nature. In addition, the analog information is restricted to the frequency range from 300 to 3000 Hz. The primary function of the data set (modem ) at the transmit end is to convert digital pulses to analog signal suitable for transmission on the PTN and ,at the re
31、ceive end, to convert the analog signals back to digital information.ModemThe term modem is a composite word that refers to the two functional entities that make up the device: a signal modulator and a signal demodulator. The relationship of the two parts is shown in Figure 4-puterModulatorDemodulat
32、orcomputerModulatorDemodulatorFigure 4-1 Modem conceptFigure 4-1 shows the relationship of modems to a communication link. The two PCs at the ends are the DTEs; the modems are the DCEs. The DTE creates a digital signal and relays it to the modem via an interface (like the EIA-232, as discussed befor
33、e). The demodulated signal is received by the demodulation function of the second modem. The modulator takes the ASK, FSK, PSK, or QAM signal and decodes it into whatever format its computer can accept. It then relays the resulting digital signal to the receiving computer via an interface. Each DCE
34、must be compatible with both its own DTE and with other DCEs. A modem must use the same type of encoding (such as NRA-L), the same voltage levels to mean the same things, and the same timing conventions as its DTE. A modem must also be able to communication with other modems.Bit rate and baud rateTw
35、o terms used frequently in data communication are bit rate and baud rate. Bit rate is the number of bits transmitted during one second. Baud rate refers to the number of signal units per second that are required to represent those bits. In discussions of computer efficiency the bit rate is the more
36、importantwe want to know how long it takes to process each piece of information. In data transmission, however, we are more concerned with how efficiently we can move that data from place to place, whether in pieces or blocks. The fewer signal units required, the more efficient the system and the le
37、ss bandwidth required to transmit more bits; so we are more concerned with baud rate, The baud rate determines the bandwidth required to send the signal.Bit rate equals the baud rate times the number of bits represented by each signal unit. The baud rate equals the bit rate divided by the number of
38、bits represented by each signal shift. Bit rate is always greater than or equal to the baud rate.翻译2： 使用模拟载的数据传输和调制解调器数据传输涉及数字信息从一个数据终端设备（DTE）传向另一个数据终端设备。这种传输与其使用专门为这一目的构建的线路，不如使用公用电话网(PTN)更为合适。不幸的是，在大规模数据通信出现很久之前，就已经安装了十分庞大的公用电话网，用于传输现实存在的模拟信息。另外，这种模拟信息的频率范围被限制在300至3000赫兹。数传机（调制解调器）的主要功能是在发送端把数字脉冲转
39、换成适合在公用电话网上传输的模拟信号，在接收端把模拟信号再转换成数字信息。调制解调器术语调制解调器是一个组合词，它代表构成该设备的两个功能实体：信号调制器和信号解调器。这两者之间的关系如图41所示。图41给出了调制解调器与通信链路之间的关系。两端的PC机是数据终端设备（DTE）；调制解调器是数据电路端接设备（DCE）。DTE产生一个数字信号并通过接口（如前面讨论过的EIA-232）把它传给调制解调器。被调制的信号为第二个调制解调器所接收。解调器得到ASK,FSK,PSK或QAM信号并将其解码为该计算机所能接收的技术。然后再把最后的数字信号经接口传送给接收方的计算机。每个DCE必须与其自己的DT
40、E和对方的DCE兼容。调制解调器必须使用相同类型的编码（例如NRZ-L），相同的电压电平所表示相同的值和与DTE相同的定时规则。一个调制解调器也必须能与其他调制解调器通信。位速率与波特率数据通信中两种常用的术语是位速率与波特率。位速率指一秒钟内传送的位数。波特率是指这些位所要求的每秒钟传送的信号单元数。在讨论计算机的效率时，位速率是很重要的因为我们希望知道在处理每段信息时计算机花多少时间。但是，在数据传输中，我们更关心的是从一处向另一处传送的数据的效率如何，不管是一段还是一块数据。信号单元越少，系统的效率越高，并且传送更多位所需要的带宽越窄；所以我们更关心波特率。波特率决定了发送信号所要求的带
41、宽。位速率等于波特率乘上每个信号单元所代表的位数。波特率等于位速率除以每个信号所代表的位数。位速率总是大于或等于波特率。原文3：Telephone SystemOf all the inventions in the past century, the telephone certainly has had one of the most profound effects on our lives. The ability to call almost anywhere in the world by specifying (dialing) a few numbers is absolute
42、ly incredible. But the telephone is more than just voice communications among friends and relatives; it is becoming indispensable to business using it for computer communications.The telephone works by converting sound into electrical energy. What we perceive as sound is caused by small fluctuations
43、 in air pressure (sound waves).The waves travel through the air, causing some objects to vibrate. The same principle causes old windows or light fixtures to rattle during a thunderstorm. It also allows us to hear: The waves cause the eardrum to vibrate and send signals to the brain.The telephone mou
44、thpiece consists of a chamber filled with carbon granules. Two electrical contacts are connected to the chamber. When you speak to the mouthpiece, the sound waves cause a diaphragm covering the chamber to vibrate. As it vibrates it exerts varying pressure on the carbon granules. Higher pressure caus
45、es them to be compacted more closely, which in turn, causes them to be a better conductor of electricity. Less pressure has the opposite effect. The net result is that varying amounts of electricity caused by sound are conducted. On the receiving end, the electricity activates a voice coil, causing
46、an attached speaker to vibrate. The vibrating speaker causes changes in air pressure, which we interpret as sound.To place a phone call, the caller enters a sequence of digits by dialing or touching buttons. Each digit wends a code to a local exchange office, which interprets the sequence and determ
47、ines the destination. If there is an available route to the destination and the phone is not busy, two signals are sent. The first goes to the destination and causes the phone to ring. The second goes to the source and alerts the caller that the phone is now ringing.The code for each digit depends o
48、n whether you have tone or pulse dialing. With tone dialing, each digit sends a tone consisting of a unique pair of frequencies. With pulse dialing, each digit generates from 1 to 10 pulses. Each pulse actually corresponds to opening and closing a circuit, similar to depressing the hook. In fact, if your fingers are fast enough, you can actually dial a number by rapidly depressing the hook the proper number of times for each digit.The way in which phone calls are routed is an amazing fact of engineering. Remember, we are discussing a network connecting many millions of users. The first