1、专 业 英 语,Specialized English,Unit 10：,第二部分 通信技术,Introduction to Computer Netwroks,计算机网络介绍,Specialized English on Communication Engineering,NEW WORDS AND PHRASES,peripheral adj. 外围的 n. 外围设备 memory n. 存储器,内存 processor n. 处理器 central processing unit (CPU) 中央处理器 congestion n. 充满，拥挤，拥堵 topology n. 拓扑，布局 m
2、ainframe n. 主机，大型机 bus n. 总线 terminal n. 终端 bidirectional adj. 双向的 token n. 令牌 device n. 设备，装置，器械，部件 access vt. 访问，通路， 入门；accessing,Specialized English on Communication Engineering,During the 1950s, most computers were similar in one respect. They had a main memory, a central processing unit (CPU),
3、and peripherals. main memory ：主存储器 central processing unit ：中央处理器 peripheral：外围设备 in one respect ：在一个方面, 在一点上 在20世纪50年代，大多数的计算机有一点是相似的，即都有一个主存储器，一个中央处理器和一些外围设备。,TEXT,Specialized English on Communication Engineering,Since then a new generation of computing has emerged in which computation and data st
4、orage need not be centralized. computation ：计算 data storage ：数据保存 centralized ：集中的 此后出现了新一代的计算机，其计算和数据存储不必集中。,Specialized English on Communication Engineering,A user may retrieve a program from one place, run it on any of a variety of processors, and send the result to a third location. retrieve ：取回
5、，找回，获得，恢复 run ：运行 processor：处理器 用户可以从一个地方获得程序，在任何一个处理器上运行，并将结果输送到第三方。,Specialized English on Communication Engineering,A system connecting different devices such as PCs, printers, and disk drives is a network. PC： Personal Computer，个人计算机 disk drive ：磁盘驱动器 将不同设备，如个人计算机，打印机和磁盘驱动器连接起来的系统就是一种网络。,Special
6、ized English on Communication Engineering,Typically, each device in a network serves a specific purpose for one or more individuals. For example, a PC may sit on your desk providing access to information or software you need. device ：设备 individual ：个人 通常情况下，网络中的每一个设备为一个或多个用户提供一种特定的服务。例如，计算机可能会在你的案头
7、，提供你需要的信息或软件。,Specialized English on Communication Engineering,A PC may also be devoted to managing a disk drive containing shared files. We call it a file server. devoted to ：专用于 file server：文件服务器 计算机也可以专用于管理共享文件的磁盘驱动器。我们称之为文件服务器。,Specialized English on Communication Engineering,Often a network cov
8、ers a small geographic area and connects devices in a single building or group of buildings. Such a network is a local area network (LAN). geographic area ：地理区域 local area network ：局域网（LAN） 通常一个网络覆盖一个小的地理区域，并且在一个单幢建筑或建筑群中连接设备。这样的网络就是局域网。,Specialized English on Communication Engineering,A network tha
9、t covers a larger area such as a municipality, state, country, or the world is called a wide area network (WAN). municipality ：市，自治市 state ：州 wide area network ：广域网（WAN） 覆盖更大范围的网络，例如一个城市，一个州，一个国家或世界，被称为广域网。,Specialized English on Communication Engineering,Generally speaking, most networks may involv
10、e many people using many PCs, each of which can access any of many printers or servers. With all these people accessing information, their requests inevitably will conflict. access: vt. 访问；accessing inevitably :不可避免的; 无法避免的; 必然(发生)的 conflict :冲突 一般来说，大多数网络可能涉及很多人使用许多计算机，每个人都可以访问许多打印机和服务器。在这么多人存储信息的情
11、况下，不可避免的会发生冲突。,Specialized English on Communication Engineering,Consequently, the devices must be connected in a way that permits an orderly transfer of information for all concerned. concerned :有关的 因此，设备必须按照某种方式连接，该方式使各有关方面有序地传输信息。,Specialized English on Communication Engineering,A good analogy is
12、a street layout in a large city. With only one person driving it matters little where the streets are, which ones are one-way, where the traffic signals are, or how they are synchronized. analogy ：类似, 类推 one-way ：单行道的 synchronized ：同步的 with ：当(有).情况下 一个很好的比喻就是大城市的街道布局。当只有一个人驾驶时，街道在哪，哪些是单行道，交通信号在哪里，或
13、者如何同步，这些都是无关紧要的。,Specialized English on Communication Engineering,But with thousands of the cars on the streets during the morning rush hour, a bad layout will create congestion that caused major delays. rush hour ：高峰时间 layout ：设备布置，布局，规划 congestion ：（交通的） 拥挤； 但是在早晨交通高峰时间，当成千上万的汽车开上大街，一个差劲的布局会造成拥挤，从
14、而引起重大延滞。,Specialized English on Communication Engineering,The same is true of computer networks. They must be connected in a way that allows data to travel among many users with little or no delay. 计算机网络也是一样。它们必须以一种能让数据在许多用户间几乎没有延时的传播方式进行连接。,Specialized English on Communication Engineering,We call t
15、he connection strategy the network topology. The best topology depends on the types of devices and user needs. What works well for one group may perform dismally for another. strategy ：策略，方法 network topology ：网络拓扑 我们称这种连接方法为网络拓扑。最好的拓扑结构取决于设备的类型和用户的需求。对某用户组工作地很好的网络可能不适用于另外的用户组。,Specialized English on
16、 Communication Engineering,Fig. 2-11 shows a common bus topology (or simply bus topology) connecting devices such as workstations, mainframes, and file servers. They communicate through a single bus (a collection of parallel lines). workstation：工作站 mainframe：主机，大型机 图2-11表示了一种常用的（简单的）总线结构，该结构连接了如工作站、
17、大型机和文件服务器等设备。它们通过一条总线（一组并行线）进行通信。,Specialized English on Communication Engineering,A common approach gives each device an interface that listens to the bus and examines its data traffic. If an interface determines that data are destined for the device it serves, it reads the data from the bus and tr
18、ansfers it to the device. interface ：接口 be destined for ：预定, 指定 通常的方法是给每个设备提供一个接口，用于监听总线并侦测其数据通信。若某一接口测定到数据的地址指定它所服务的设备，它就从总线上读取数据并传送给该设备。,Specialized English on Communication Engineering,Similarly, if a device wants to transmit data, the interface circuit sense when the bus is empty and then transm
19、it data. This is not unlike waiting on a freeway entrance ramp during rush hour. sense ：感觉，发觉，了解， 明白，检测 wait on ：等待 freeway entrance ramp ：高速公路进路坡道, 进口坡 同样，如果设备要传输数据，接口电路先检测总线何时空闲，然后再传输数据。这就像交通拥堵时，交通工具等候在高速公路的入口坡道上一样。,Specialized English on Communication Engineering,You sense an opening and either q
20、uickly dart to it or muscle your way through, depending on whether youre driving a subcompact or a large truck. dart ：飞奔，突进，飞快的动作 muscle ：强推, 迫使他人让步 subcompact ：微型汽车，超小型汽车 等到通路时，你是快速地飞奔而过还是强行才能挤过去，这取决于你是驾驶着一辆微型汽车还是大卡车。,Specialized English on Communication Engineering,Sometimes, two devices try to tr
21、ansmit simultaneously. Each one detects an absence of traffic and begins transmitting before becoming aware of the other devices transmission. The result is a collision of signals. simultaneously：同时地 collision：碰撞, 冲突 有时，两个设备试图同时进行数据传输。每个设备侦测到通信空闲后，还不清楚其他设备的传输就开始传输数据。其结果就是信号冲突。,Specialized English on
22、 Communication Engineering,As the devices transmit they continue to listen to the bus and detect the noise resulting from the collisions. result from：由.产生 当设备在传输信息时仍继续对总线进行侦听，并检测由冲突产生的噪声。,Specialized English on Communication Engineering,When a device detects a collision it stops transmitting, waits
23、a random period of time, and tries again. This process, called Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) will be discussed later. random：随机的 Carrier Sense：载波监听 Multiple Access：多路访问 当设备检测到信号冲突就停止传输，等待一随机时间后又开始传输数据。这一过程称为载波监听多路访问，将在稍后讨论。,Specialized English on Communication Eng
24、ineering,One popular common bus network is an Ethernet. Its common bus typically is Ethernet cable, which consists of copper, optical fiber, or combinations of both. Ethernet：以太网 一种很流行的总线网络是以太网。其常见的总线通常是以太网电缆，有铜，光纤或两者结合而成。,Specialized English on Communication Engineering,Its design allows terminals,
25、 PCs, disk storage systems, and office machines to communicate. A major advantage of an Ethernet is the ability to add new devices to the network easily. terminal：终端，终端设备 office machine：办公用计算机 其设计允许终端，个人计算机，磁盘存储系统及办公用计算机进行通信。以太网的一个主要优点就是能容易地给网络添加新设备。,Specialized English on Communication Engineering,
26、Another common connecting arrangement is the star topology, shown in Fig.2-12. It uses a central computer that communicates with other devices in the network. star topology：星型拓扑 另外一种常见的连接形式是星型拓扑结构，如图2-12所示。该结构使用了可以和网络中其他设备进行通信的中央计算机。,Specialized English on Communication Engineering,Control is centra
27、lized; if device wants to communicate, it does so only through the central computer. The computer, in turn, routes the data to its destination. route：n. 路线, 路程, 通道；v. 发送 destination：目的地, 目的文件 控制集中化了，如果设备想要通信，它只能通过中央计算机来完成。再由中央计算机依次将数据发送到目的地。,Specialized English on Communication Engineering,Centraliz
28、ation provides a focal point for responsibility, an advantage of the star topology. focal point：焦点 responsibility：责任, 职责 集中化提供了责任焦点，这正是星型拓扑结构的优点。,Specialized English on Communication Engineering,The bus topology, however, has some advantages over a star topology. The lack of central control makes ad
29、ding new devices easy because no device needs to be aware of others. central control ：中央控制器 而总线结构和星型结构相比具有一些优势。由于不存在中央控制器，不需要考虑其他设备，这就很容易添加新设备。,Specialized English on Communication Engineering,In addition, the failure or removal of a device in a bus network does not cause the network to fail. In a s
30、tar topology, the failure of the central computer brings down the entire network. failure： n. 故障；fail： v. 彻底中止运转 removal ：移动，移除 bring down ：使崩溃 此外，总线网络中某一设备的故障或移除并不会引起整个网络彻底停止运转。而在星型结构中，中央计算机的故障会导致整个网络的瘫痪。,Specialized English on Communication Engineering,Star topologies often involve a single mainfr
31、ame computer that services many terminals and secondary devices. mainframe computer ：大型计算机 星型拓扑结构往往包含一个大型计算机，该计算机可以服务于很大终端和次级设备。,Specialized English on Communication Engineering,With appropriate terminal emulation software, PCs can communicate with the mainframe. Data transfers between terminals or
32、between terminals and storage devices occur only through the main computer. emulation software：仿真软件 mainframe ：主机 通过适当的终端仿真软件，个人计算机就能够与主机通信。终端之间或终端与存储设备之间的数据传输只能通过主机完成。,Specialized English on Communication Engineering,In a ring topology shown in Fig.2-13, devices are connected circularly. Each one c
33、an communicate directly with either or both of its neighbors but nobody else. ring topology ：环形拓扑 在图2-13所示的环形拓扑中，设备连接成环型。每个设备都可以直接和相邻的一方或两方，而不能和其他设备进行通信。,Specialized English on Communication Engineering,If it wants to communicate with a device farther away, it sends a message that passes through eac
34、h device in between. 如果它想和距离更远的设备通信，它发送的信息就必须通过两者之间每一个设备。,Specialized English on Communication Engineering,A ring network may be either unidirectional or bidirectional. Unidirectional means that all transmissions travel in the same direction. Thus, each device can communicate with only one neighbor.
35、 unidirectional：单向的, 单向性的 bidirectional：双向的 环型网络可以是单向或双向的。单向意味着所有传输在同一方向。因此，每一设备只能和一个相邻设备通信。,Specialized English on Communication Engineering,Bidirectional means that data transmissions travel in either direction, that is, a device can communicate with both neighbors. 双向意味着数据可以在两个方向上传输，也就是设备可以和相邻的两个
36、设备通信。,Specialized English on Communication Engineering,Ring topologies such as IBMs token ring network often connect PCs in a single office or department. token ring network ：令牌环网 环型拓扑，如IBM令牌环网，往往将一个办公室或一个部门的计算机连接在一起。,Specialized English on Communication Engineering,Applications from one PC thus can
37、 access data stored on others without requiring a mainframe to coordinate communication. applications ：应用软件 因此一台计算机上的应用软件可以访问存储在其他计算机上的数据，而不需要请求主机来协调通信。,Specialized English on Communication Engineering,Instead, communications are coordinated by passing a token among all the stations in the ring. A s
38、tation can send something only when it receives the token. 相反，通信的协调是通过在环中所有站点中传递令牌。一个站点只有在它接受到令牌时才能发送数据。,Specialized English on Communication Engineering,A disadvantage of the ring topology is that when one station sends to another, all stations in between are involved. 环型拓扑的一个缺点体现在当一个站点向另一个站点发送数据时，
39、两者之间所有的站点都参与了。,Specialized English on Communication Engineering,More time is spent relaying messages meant for others than in, for example, a bus topology. Moreover, the failure of one station causes a break in the ring that affects communications among all the stations. 和其他拓扑，例如总线拓扑相比，环型拓扑花费了更多的时间用于传递信息。此外，一个站点的故障就会在环中引起中断，这会影响到所有站点间的通信。,Specialized English on Communication Engineering,Many computer networks are combinations of various topologies. Fig. 2-14 shows a possible combination. combination ：结合，联合，组合 许多计算机网络是不同拓扑的组合。图 2-14显示了一种可能的组合形式。,小 结,被动语态及其译法Introduction to Computer Netwroks,
