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To get here to there, data must move through something. A telephone line, cable, or the atmosphere are all transmission media or channels. But before the data can be communicated, it must be converted into a form suitable for communication.
Data communication lines can be connected in two types of configurations: point-to-point and multidrop. A point-to-point line directly connects the sending and the receiving devices, and a multidrop line connects many devices, not just one sending device and one receiving device.
The two ways of connecting microcomputers with each other and with other equipments are through the cable and through the air. There are three basic forms into which data can be converted for communication: electrical pulses or charges, electromagnetic waves, and pulses of light.
Specifically, five kinds of technology are used to transmit data. These are telephone lines (twisted pair), coaxial cable, fiber-optic cable, microwave and satellite.
Telephone Lines [1] Inexpensive, multiple-conductor cable comprised of one or more pairs of 18 to 24 gauge copper strands. The strands are twisted to improve protection against electromagnetic and radio frequency interference. The cable, which may be either shielded or unshielded, is used in low-speed communications, as telephone cable. It is used only in baseband networks because of its narrow bandwidth. Most telephone lines you see strung on poles that consist of cables made up of hundreds of copper wires are twisted pairs. Twisted pairs are susceptible to a variety of types of electrical interference (noise), which limits the practical distance that data can be transmitted without being garbled. Twisted pairs have been used for years for voice and data transmission, however they are now being phased out by more technically advanced and reliable media.
Coaxial Cable Coaxial cable is a type of thickly insulated copper wire that can carry a larger volume of data—about 100 million bits per second, the insulation is composed of a nonconductive material covered by a layer of woven wire mesh and heavy-duty rubber or plastic. In terms of number of telephone connections, a coaxial cable has over 80 times the transmission capacity of twisted pair. Coaxial cables are most often used as the primary communications medium for local connected network in which all computer communication is within a limited geographic area, such as in the same building.
Coaxial cable is also used for undersea telephone lines.
Fiber-Optic Cable [2] A transmission medium composed of a central glass optical fiber cable surrounded by cladding and an outer protective sheath. It transmits digital signals in the form of modulated light from a laser or LED (light-emitting diode). In fiber-optic cable, data is transmitted as pulses of light through tubes of glass. In terms of number of telephone connections, fiber-optic cable has over 20,000 times the transmission capacity of twisted pair. However, it is significantly smaller. Indeed, a fiber-optic tube can be half the diameter of a human hair. Although limited in the distance they can carry information, fiber-optic cables have several advantages. Such cables are immune to electronic interference, which makes them more secure. They are also lighter and less expensive than coaxial cable and are more reliable at transmitting data. They transmit information using beams of light at light speeds instead of pulses of electricity, making them far faster than copper cable. Fiber-optic cable is rapidly replacing twisted-pair telephone lines.
Microwave Instead of using wire or cables, microwave systems can use the atmosphere as the medium through which to transmit signals. Microwaves are high-frequency radio waves that travel in straight lines through the air. Because the waves cannot bend with the curvature of the earth, they can be transmitted only over short distances. Thus, microwave is a good medium for sending data between buildings in a city or on a large college campus. For longer distances, the waves must be relayed by means of "dishes", or antennas. These can be installed on towers, high buildings and mountaintops. Each tower facility receives incoming traffic, boosts the signal strength, and sends the signal to the next station.
Satellites [3] Satellite communications refers to the utilization of geostationary orbiting satellites to relay the transmission received from one earth station to one or more earth stations. They are the outcome of research in the area of communications whose objective is to achieve ever-increasing ranges and capacities with the lowest possible costs. Orbiting about 22,000 miles above the earth, satellites rotate at a precise point and speed above the earth. This makes them appear stationary so they can amplify and relay microwave signals from one transmitter on the ground to another. The primary advantage of satellite communication is the amount of area that can be covered by a single satellite. It also has other features: long communication distance, and the cost of station building is independent of the communication distance, operating in broadcasting mode, easy for multiple access, sustaining heavy traffic, able to transport different types of service, independent sending and receiving, and monitoring. Three satellites placed in particular orbits can cover the entire surface of the earth, with some overlap. Their only drawback is that bad weather can sometimes interrupt the flow of data.
Words
| antenna | n. | 天线 |
| axis | n. | 轴心，中轴 |
| baseband | n. | 基带 |
| boost | v. | 增加，放大 |
| cable | n. | 电缆 |
| charge | n. | 电荷 |
| cladding | n. | 金属包层，外罩 |
| coaxial | adj. | 同轴的，共轴的 |
| configuration | n. | 配置，结构，布局，格局 |
| curvature | n. | 弯曲，曲度，弧度 |
| electromagnetic | adj. | 电磁的 |
| fiber-optic | n. | 光纤 |
| garble | v. | 精选，筛去……的杂质，误解 |
| gauge | n. | 规格，标准 |
| geostationary | adj. | 与地球相对位置保持不动的 |
| interference | n. | 干扰，干涉 |
| immune | adj. | 免疫的，免除的，不受影响的 |
| multidrop | n. | 多点 |
| multiple-conductor | n. | 多重导体 |
| outcome | n. | 结果，产量，出口 |
| pole | n. | 极（点），杆，电极 |
| pulse | n. | 脉冲 |
| relay | v. | 中继，转播 |
| sheath | n. | 外皮，外层覆盖物 |
| shield | n. | 屏，保护，防护，屏蔽 |
| strand | n. | 绳股，绞合线 |
| susceptible | adj. | 灵敏的，敏感的 |
| utilization | n. | 使用，利用 |
| woven | adj. | 纺织的，编织的 |
Phrases
| coaxial cable | 同轴电缆 |
| fiber-optic cable | 光缆 |
| geostationary orbiting satellite | 轨道通信卫星 |
| phase out | 逐步停止采用，逐步退出 |
| twisted-pair | 双绞线 |
| woven wire | 钢丝网，铁丝网 |
Abbreviations
LED (Light-Emitting Diode) 发光二极管
Notes
［1］ 例句：Inexpensive, multiple-conductor cable comprised of one or more pairs of 18 to 24 gauge copper strands.
分析：这句话实际上是一个名词解释，过去分词短语comprised of…作定语，修饰cable。
译文：由一对或多对18到24规格的铜线组成的并不昂贵的多重导体电缆。
［2］ 例句：A transmission medium composed of a central glass optical fiber cable surrounded by cladding and an outer protective sheath.
分析：这句话也是一个名词解释，过去分词短语composed of…作定语，修饰medium，同样过去分词短语surrounded by…作定语，修饰cable。
译文：由金属包裹层和外层保护层包裹的、中央玻璃光学纤维绳组成的一种传输介质。
［3］ 例句：Satellite communications refers to the utilization of geostationary orbiting satellites to relay the transmission received from one earth station to one or more earth stations.
分析：本句为简单句，句中refer to意思是“指的是”。例如：Loosely, the term "job" is sometimes used to refer to a representation of a job. 习惯上说，“作业”这个术语通常指的是作业的一种表述。
译文：卫星通信指的是利用轨道通信卫星把地面站发来的信息传送到另外一个或多个地面站。
Exercises
Ⅰ. Put "true" or "false" in the brackets for the following statements according to the passage.
1. ( ) Air can be used to transmit data.
2. ( ) Point-to-point and multidrop are two types of configuration for communications lines connecting.
3. ( ) Twisted pairs are susceptible to noise, which limits the practical distance they can transmit.
4. ( ) Coaxial cable and twisted pairs are all high-frequency transmission cables.
5. ( ) Optic-fiber cables can transmit both electrical and light signals.
6. ( ) Optic-fiber cables are more secure because they are not susceptible to electrical interference.
7. ( ) Microwave can transmit data over long distances.
8. ( ) The primary advantage of satellite communication is the long distance a single satellite can cover.
9. ( ) Three satellites placed in random orbits can cover the entire surface of the earth.
10. ( ) All the five kinds of communications channels have their own advantages and disadvantages, so they can not replace with each other.
Ⅱ. Fill in the blanks according to the passage.
1. The two ways of connecting microcomputers with each other and with other equipments are through theand through the.
2. Data communications lines can be connected in two types of configurations:and.
3. Twisted pairs areto a variety of types of electrical interference (noise), which limits the practical distance that data can be transmitted without being.
4. transmits digital signals in the form of modulated light from a laser or LED
5. systems can use the atmosphere as the medium through which to transmit signals.
6. Microwaves cannot bend with theof the earth, they can be transmitted only over short distances.
7. Satellite communications refers to theof geostationary orbiting satellites tothe transmission received from one earth station to one or more earth stations.
8. Three satellites placed in particular orbits can cover the entire surface of the earth, with some.
9. Fiber-optic cables areto electronic interference, which makes them more secure.
10. In order to transmit longer distance, microwave must be relayed by means of.
Ⅲ. Translate the following words and expressions into Chinese.
1. communications channels
2. multiple-conductor
3. facility
4. stationary
5. transmission capacity
6. electromagnetic waves
7. pulses of light
8. light-emitting diode
9. a solid wire conductor
10. electrical interference
Computer networking devices are units that mediate data in a computer network. Units which are the last receiver or generate data are called hosts or data terminal equipment.
List of computer networking devices
Common basic network devices:
● Gateway: Device sitting at a network node for interfacing with another network that uses different protocols. Works on OSI layers 4 to 7.
● Router: A specialized network device that determines the next network point to which to forward a data packet toward its destination. Unlike a gateway, it cannot interface different protocols. Works on OSI layer 3.
● Bridge: A device that connects multiple network segments along the data link layer. Works on OSI layer 2.
● Switch: A device that allocates traffic from one network segment to certain lines (intended destination(s)), which connect the segment to another network segment. So unlike a hub a switch splits the network traffic and sends it to different destinations rather than to all systems on the network. Works on OSI layer 2.
● Hub: Connects multiple Ethernet segments together making them act as a single segment. When using a hub, every attached device shares the same broadcast domain and the same collision domain. Therefore, only one computer connected to the hub is able to transmit at a time. Depending on the network topology, the hub provides a basic level 1 OSI model connection among the network objects (workstations, servers, etc). It provides bandwidth that is shared among all the objects, compared to switches, which provide a dedicated connection between individual nodes. Works on OSI layer 1.
● Repeater: Device to amplify or regenerate digital signals received while setting them from one part of a network into another. Works on OSI layer 1.
Hardware or software components that typically sit on the connection point of different networks, e. g. between an internal network and an external network:
● Proxy: computer network service which allows clients to make indirect network connections to other network services.
● Firewall: a piece of hardware or software put on the network to prevent some communications forbidden by the network policy.
● Network Address Translator: network service provide as hardware or software that converts internal to external network addresses and vice versa.
Other hardware for establishing networks or dial-up connections:
● Multiplexer: Device that combines several electrical signals into a single signal.
● Network Card: A piece of computer hardware to allow the attached computer to communicate by network.
● Modem: Device that modulates an analog "carrier" signal (such as sound), to encode digital information, and that also demodulates such a carrier signal to decode the transmitted information, as a computer communicating with another computer over the telephone network.
● ISDN terminal adapter (TA): A specialized gateway for ISDN.
● Line Driver: A device to increase transmission distance by amplifying the signal. Base-band networks only.
Words
| amplify | v. | 放大，增强，扩大 |
| bandwidth | n. | ［电信］带宽，频带宽度 |
| bridge | n. | 桥 |
| convert | v. | 使转变，转换……，使……改变信仰 |
| dedicated | adj. | 专注的，献身的 |
| demodulate | v. | ［讯］使解调，使检波 |
| destination | n. | 目的地，终点，［计］目的文件，目的单元格 |
| determine | v. | 决定，确定，测定，使下定决心 |
| firewall | n. | 防火墙 |
| forbidden | adj. | 禁止的，严禁的 |
| hub | n. | 网络集线器，网络中心 |
| hybrid | n. | 混合物 |
| adj. | 混合的 | |
| interface | n. | ［地质］分界面，接触面，［物、化］界面 |
| internal | adj. | 内在的，国内的 |
| interwork | v. | 互相作用，互通 |
| mediate | v. | 仲裁，调停，作为引起……的媒介 |
| modem | n. | 调制解调器 |
| multiplexer | n. | 多路（复用）器 |
| packet | n. | 小包裹，小捆，信息包 |
| proxy | n. | 代理人，代理服务器，即proxy服务器 |
| regenerate | v. | 使新生，重建，再生 |
| adj. | 新生的，更新的 | |
| router | n. | ［计］路由器 |
| split | v. | 劈开，（使）裂开，分裂，分离 |
| n. | 裂开，裂口，裂痕 | |
| switch | n. | 交换机，交换器，开关，电闸，转换 |
| traffic | n. | 交通，运输，交通量，通信量 |
| v. | 交易，买卖 |
Phrases
| broadcast domain | 广播域 |
| collision domain | 碰撞域 |
| data terminal equipment | 数据终端设备 |
| ISDN terminal adapter | 综合业务数字网终端适配器 |
| Line Driver | 线路驱动器 |
| Network Address Translator | 网络地址转换器 |
| Network Card | 网卡 |
| Network Device Connectivity | 网络设备连接 |
为了传递数据，必须有一些介质，电话线、电缆或空气都是传输介质，即信道。但是，数据在传送前必须先被转化成适于通信的形式。
数据通信线路可以以两种配置方式连接：点到点和多点线路连接。点到点线路直接连接发送设备和接收设备，多点线路连接多个设备而不仅一个发送设备和一个接收设备。
微型机之间以及与其他设备的连接一般通过电缆和通过空气两种方法。数据可以被转化为用于通信的三种基本形式：电脉冲或电荷、电磁波和光波。
具体来说，有五种传送数据的方式。它们是电话线（双绞线）、同轴电缆、光缆、微波和卫星。
电话线 即由一对或多对18到24规格的铜线组成的并不昂贵的多重导体电缆。铜线绞合着以提高防止电磁和无线电频率干扰的能力。这些电缆，可以是屏蔽的也可以是非屏蔽的，作为电话线用于低速通信中。由于带宽窄，因此仅被用于基带网络中。架在电线杆上的绝大多数电话线是双绞线，它们是由数以百计的铜线组成的电缆。双绞线对于各类电的干扰很敏感，因此它传送的数据很容易被窜改。双绞线被用于传递声音和数据已有好多年了，然而它们正被技术更先进和可靠的介质逐步代替。
同轴电缆 同轴电缆是一种有厚绝缘层的铜线，可以携带大量的数据——大约每秒100万位。这个绝缘层由覆盖有一层金属筛网的绝缘材料和厚的橡胶或塑料组成。根据电话连接的数目，同轴电缆的传输容量可达双绞线的80多倍。同轴电缆是局域网最常用的且很主要的通信介质，而局域网中所有计算机间的通信是局限在有限的地理区域（比如一座大楼内）。
同轴电缆也可用作海底电话线。
光缆 它是由金属包裹层和外层保护层包着的、中央玻璃光学纤维绳组成的一种传输介质。光缆以调制的从激光或光发射二板管中发射的光的形式来传递数字信号。在光缆中，当光脉冲通过玻璃管时数据就被传送。由于可连接的电话很多，光缆的传输容量可达双绞线的2万多倍，然而它的体积却非常小。实际上，光缆管的直径仅仅是人的头发丝的一半。虽然传送信息的距离有限，但它具有一些优点：不受电子的干扰，这就使得它们更安全；它们比同轴电缆轻、便宜，而且传输数据更可靠；光缆用光束以光的速度而不是电脉冲的速度传输数据，这就使得通过光缆传输比通过铜缆传输快得多。因此光缆正迅速地取代双绞线电话线。
微波 它不使用电线或电缆，而使用大气作为传输介质传送信号。微波是以直线穿过空气的高频率的无线电波。由于微波不能随地球表面的弯曲度弯曲，所以只能传送很短的距离。这样，对于一个城市中或一个大学校园内的两个大楼之间，微波是很好的传送数据的介质。对于较长距离，微波必须通过“盘子”（即天线）放大。这些天线可以安装在塔上、高楼上以及山顶上。它们接收信号，增强信号的强度，然后将信号发送给下一站。
卫星 卫星通信指的是利用轨道通信卫星把地面站发来的信息传送到另外一个或多个地面站。卫星通信是通信领域的研究成果，其目的就是以尽可能少的代价获得不断增加的信息范围和容量。卫星在距地球表面22000英里的轨道上在精确的位置以精确的速度旋转着。这就使得它们看起来是静止的，从而可以将一个地面站转发器发送来的信息放大并传送到下一站。通信卫星的主要优点是一个卫星可以覆盖的面积很大。它还具有其他的特点：远距离通信，转发站的费用独立于通信距离，以广播方式运作，易于多路访问，承受大量的信息，能够传送不同类型的服务，独立地发送、接收和监控。在特定轨道放置三颗卫星就可覆盖整个地球表面，而且还有些重叠。其惟一缺点是恶劣的天气有时会影响数据的传送。
计算机网络设备在计算机网络中负责传输数据。最终接收或产生数据的设备称为主机或数据终端设备。
计算机网络设备列表
常见的基本网络设备：
● 网关：在网络节点与另一个使用不同协议的网络进行连接的装置。工作在OSI模型的第4至第7层。
● 路由器：一个专门的网络设备，决定传输数据包到目的地的下一个网络点。不同于网关，它不能连接不同的协议。工作在OSI模型的第3层。
● 网桥：连接数据链路层沿线的多个网段的装置。工作在OSI模型的第2层。
● 交换机：一个分配流量从一个网段到某些线路（预期的目的地）的设备，这些线路将网段连接到另一个网段。因此，不同于集线器，一个交换机分解网络流量并传送到不同的目的地，而不是传送到网络上的所有系统。工作在OSI模型的第2层。
● 集线器：把多个以太网段连接在一起，使它们作为一个单一的网段。当使用集线器时，每个连接的设备共享相同的广播域和相同的碰撞域。因此，某个时间只有一台连接到集线器的计算机能够传输。利用网络的拓扑结构，集线器在网络对象（工作站、服务器等）之间提供了一个基本的OSI模型第1层的连接。它提供的带宽被所有对象共享，与交换机不同，集线器提供各节点之间的专用连接。工作在OSI模型的第1层。
● 中继器：该设备在把数字信号从网络的一部分传到另一部分时，放大或再生接收的数字信号。工作在OSI模型的第1层。
在不同网络的连接节点（例如在内部网络和外部网络之间）上的硬件或软件组件：
● 代理：让客户能够间接连接到其他网络服务的计算机网络服务。
● 防火墙：放在网络上的硬件或软件，以阻止网络政策所禁止的一些通信。
● 网络地址转换：硬件或软件提供的将内部网络地址转换为外部网络地址（反之亦然）的网络服务。
建立网络或拨号连接的其他硬件：
● 复用器：将多路电信号结合成为一个单路电信号的设备。
● 网卡：使连接的计算机能够通过网络进行通信的一块计算机硬件。
● 调制解调器：该设备将模拟“载波”信号（如声音）调制为已编码的数字信息，而且还可以解调这种载波信号对传输信息的解码，以使一台计算机通过电话网与另一台计算机通信。
● 综合业务数字网终端适配器（TA）：专门用于ISDN的网关。
● 线路驱动器：它通过放大信号增加传输距离，且仅用于基带网络。