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Communication channels are an essential element of every communication system. These channels actually carry the data from one computer to another. There are two categories of communication channels. One category connects sending and receiving devices by providing a physical connection such as a wire or cable. The other category is wireless. 通信管道对于每一个通信系统来说是一个至关重要的元素。这些管道实际上把数据从一台计算机传输到另一台。有两个类别的通信管道。一类是有线，另一类是无线。PHYSICAL CONNECTIONS Physical connections use a solid medium to connect sending and receiving devices. These connections include telephone lines (twisted pair), coaxial cable,and fiber-optic cable. 物理连接使用固体介质连接、发送和接收设备。这些连接包括电话线路(双绞线)、同轴电缆和光纤电缆。 Telephone lines you see strung on poles consist of twisted-pair cable,which is made up of hundreds of copper wires. A single twisted pair culminates in a wall jack into which you can plug your phone and computer. (See Figure 9-3.) Telephone lines have been the standard transmission medium for years for both voice and data. However, they are now being phased out by more technically advanced and reliable media.Coaxial cable, a high-frequency transmission cable, replaces the multiple wires of telephone lines with a single solid-copper core. (See Figure 9-4.) In terms of the number of telephone connections, a coaxial cable has over 80 times the transmission capacity of twisted pair.Coaxial cable is used to deliver television signals as well as to connect computers in a network. 你所看到的电话线，是由上百根铜线缠绕而成的双绞线。双绞线的一个端口是固定电话,当然也可以插在手机和电脑上。电话线作为一个标准传输媒介，多年一来一直在负责语音和数据的传输。然而,他们现在已经被更先进、可靠的媒体所淘汰了。同轴电缆、高频传输电缆,取代了多个连接的电话线。在传输能力方面,一个同轴电缆是双绞线传输容量的80倍。同轴电缆先主要用于提供电视信号以及连接网络的电脑. . Fiber-optic cable transmits data as pulses of light through tiny tubes of glass. (See Figure 9-5.) In terms of the number of telephone connections,fiber-optic cable has over 26,000 times the transmission capacity of twisted-pair cable. Compared to coaxial cable, they are lighter and 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 cable telephone lines.光纤电缆传输数据是由光脉冲在细小的玻璃导管内进行的。(参见图9 - 5)。在传输能力上，光纤电缆的传输速度已经超过双绞线26000倍。与同轴电缆相比,光纤更轻，传输更快。光纤使用光束传输信息，而不是的电脉冲,这使他们传输速度远远快于铜电缆。光纤电缆正在迅速取代电缆电话线路。 244WIRELESS CONNECTIONS Wireless connections do not use a solid substance to connect sending and receiving devices.Rather, they use the air itself. Primary technologies used for wireless connections are infrared, broadcast radio, microwave, and satellite. 无线连接不使用固体物质连接发送和接收数据。相反,他们使用空气本身传播。目前此项技术主要用于红外连接、广播无线电、微波和卫星。 Infrared uses infrared light waves to communicate over short distances.It is sometimes referred to as line-of-sight communication because the light waves can only travel in a straight line. This requires that sending and receiving devices must be in clear view of one another without any obstructions blocking that view.One of the most common applications is to transfer data and information from a portable device such as a notebook computer or PDA to a desktop computer. 红外技术使用红外光波在短距离通信。它有时被称为视距通信，因为光只能沿直线传播。这就要求发送和接收设备必须没有任何障碍物阻止。最常见的应用是用来传送数据和信息从一个便携式设备，如笔记本电脑或掌上电脑桌面计算机上传输数据。 Broadcast radio uses radio signals to communicate with wireless devices.For example, cellular telephones and many Web-enabled devices use broadcast radio to place telephone calls and/or to connect to the Internet. Some end users connect their notebook or handheld computers to a cellular telephone to access the Web from remote locations. Most of these Web-enabled devices follow a standard known as Wi-FI (wireless fidelity). This wireless standard is widely used to connect computers to each other and to the Internet. 广播电台使用无线电信号与无线设备。例如,移动电话和许多网络设备使用广播电台将电话和/或连接到互联网。一些终端用户连接他们的笔记本通过无线信号连接上网。大多数的这些网络设备遵循一个标准，称为wi - fi。这个无线标准被广泛用于局域网和因特网。 Microwave communication uses high-frequency radio waves. Like infrared, microwave communication provides line-of-sight communication because microwaves travel in a straight line. Because the waves cannot bend with the curvature of the earth, they can be transmitted only over relatively 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 microwave stations with microwave dishes or antennas. (See Figure 9-6.) These stations can be installed on towers. high buildings. and mountaintoos. 微波通信使用高频无线电波。如红外、微波通信只能提供短距通信因为微波以直线的方式行进。因 此,微波是非常适合用来在大城市或者校园内作为数据传输的媒介。微波发射台可以安装在塔或者高的建筑之上。 Bluetooth is a short-range wireless communication standard that uses microwaves to transmit data over short distances of up to approximately 33 feet. Unlike traditional microwaves,Bluetooth does not require line-of-sight communication. Rather, it uses radio waves that can pass through nearby walls and other nonmetal barriers. It is anticipated that within the next few years, this technology will be widely used to connect a variety of different communication devices. 蓝牙是一种短距离的无线通信标准,通信距离大约33英尺。与传统的微波相比,蓝牙不需要视距通信。相反,它使用无线电波可以穿透附近的墙壁和其他非金属壁垒。在未来几年中,这种技术将被广泛用于连接各种不同的通信设备。 Satellite communication uses satellites orbiting about 22,000 miles above the earth as microwave relay stations. (See Figure 9-7.) Many of these are offered by Intelsat, the International Telecommunications Satellite Consortium, which is owned by 114 governments and forms a worldwide communication system. Satellites rotate at a precise point and speed above the earth. They can amplify and relay microwave signals from one transmitter on the ground to another. Satellites can be used to send and receive large volumes of data. Uplink is a term relating to sending data to a satellite.Downlink refers to receiving data from a satellite. The major drawback to. 卫星通信使用在离地约22000英里的轨道卫星作为微波中继站。许多这些提供的国际通信卫星、国际通信卫星的财团旗下有和114政府相联合，形成一个全球通信系统。卫星旋转在定位精确，速度高于地球。他们可以通过地面发射机放大微波信号转发到另一个地面站。卫星可以用来发送和接收大量的数据。上行链路是一个术语，涉及将数据发送到卫星。下行是指从卫星接收数据。One of the most interesting applications of satellite communications is for global positioning. A network of 24 satellites owned and managed by the Defense Department continuously sends location information to earth. Global positioning system (GPS) devices use that information touniquely determine the geographical location of the device. Available in some automobiles to provide navigational support, these systems are often mounted into the dash with a monitor to display maps and speakers to provide spoken directions. (See Figure 9-8.) 其中最有趣的应用卫星通信是为全球定位。一个拥有24个卫星的网络，国防部管理。全球定位系统(GPS)设备的独特功能决定了它可以为汽车提供导航支持,这些系统通常安装到汽车上，通过仪表和声音为车主提供导航。 245A great deal of computer communication takes place over telephone lines. However, because the telephone was originally designed for voice transmission, telephones typically send and receive analog signals, which are continuous electronic waves. Computers, in contrast, send and receive digital signals. (See Figure 9-10.) These represent the presence or absence of an electronic pulse-the on/off binary signals we mentioned in Chapter 6. To convert the digital signals to analog signals and vice versa, you need a modem.大量的计算机通信通过电话线传播。然而,由于电话最初是设计用来传输信息，所以只发送和接收模拟信号。相反,不发送和接收数字信号。(参见 图9)。所以想用电话线来上网，你需要一个调制解调器MODEMSThe word modem is short for modulator-demodulator. Modulation is the name of the process of converting from digital to analog. Demodulation is the process of converting from analog to digital. The modem enables digital microcomputers to communicate across analog telephone lines. This communication includes both voice and data communications.调制解调器简称猫。调制指数字信号转换转换为模拟信号。解调指从模拟信号转化为数字信号，现代通信使数字计算机能模拟电话线路。从而实现语音交流和数据通信。 The speed with which modems transmit data varies. This speed,called transfer rate, is typically measured in bits per second (bps).(See Figure 9-11.) The higher the speed, the faster you can send and receive information. For example, transferring an image like Figure 9-10 might take 75 seconds with a 33.6 kbps modem and only 45 seconds with a 56 kbps modem.调制解调器的传输数据的速度称为传输速率,以比特/秒(bps)为单位。速度越高,你可以越快地发送和接收信息。 There are four basic types of modems:external, internal, PC Card, and wireless. 调制解调器分为四种：外置、集成、芯片、和无线The external modem stands apart from the computer and typically is connected by a cable to the computers serial port. Another cable connects the modem to the telephone wall jack.外部调制解调器除了电脑,通常由电缆连接到计算机的串行端口。另一个电缆连接调制解调器固定电话 The internal modem consists of a plug-in circuit board inside the system unit. A telephone cable connects the modem to the telephone wall jack.、 内部调制解调器包含插件电路板系统内的单位。电话电缆连接墙上电话端口。The PC Card modem is a credit card-size expansion board that is inserted into portable computers. A telephone cable connects the modem to the telephone wall jack. 卡片调制解调器是一种信用卡大小的卡片,插入到手提电脑。电话端连接到固定电话端口247A wireless modem may be internal, external,a PC Card.一个无线调制解调器可能是内置的、外置的的、或者一个电脑芯片。Unlike the other modems, it does not use cables. Instead,wireless modems send and receive through the air.与其他调制解调器不同,它不使用电缆。相反,无线调制解调器通过无线电波发送和接收。Standard telephone lines and conventional modems provide what is called a dial-up service. 标准电话线路和传统的调制解调器提供一般的的拨号服务。Although still the most popular type of connection service,dial-up service is quite slow, and many users find it inadequate to meet their communication needs.虽然仍然是最受欢迎的连接服务,但是拨号服务相当慢,很多用户发现它不足以满足需求。For years, large corporations have been leasing special high-speed lines from telephone companies. 多年以来,大型公司已经从电信公司租赁使用特殊的高速线路。These lines-known as T1, T2, T3, and T4 lines-support all digital communications, do not require conventional modems, and provide very high capacity. 这些线已知为T1、T2、T3、T4，这些线路支持所有数字通信,不需要传统的调制解调器,并提供非常高的传输能力Unfortunately, this type of connection is very expensive.不幸的是,这类线路的连接费用非常昂贵For example,T1 lines provide a speed of 1.5 mbps (over 26 times as fast as a conventional modem) and cost several thousand dollars.例如,T1线路提供的速度为1.5 mbps(速度超过26倍常规调制解调器)，但成本却是数千美元。While the special high-speed lines are too costly for most individuals,there are affordable connections that provide significantly higher capacity than standard dial-up service. 特殊的高速线路对于大多数人来说，太过昂贵。但如果能够负担这笔费用,高速线路显然能比普通线路提供更快捷的服务。These include DSL, cable modems, satellite,and cellular. For a comparison of typical user connection costs and speeds,see figure 9-13这些包括DSL、电缆调制解调器、卫星,和细胞。对于一个比较典型的用户连接的成本和速度。Digital subscriber line (DSL) uses existing telephone lines to provide high-speed connections. ADSL (asymmetric digital subscriber line) is one of the most widely used types of DSL. This technology is widely available in most areas.数字用户线(DSL)使用现有电话线提供高速连接。ADSL(不对称数字用户线)是一种使用最广泛的DSL。此项技术仍使用在大多数地区。Cable modems use existing television cables to provide high-speed connections as fast as a T1 or DSL connection, at a lower cost. Although cable connections reach 90 percent of the homes in America, all cable comnanies do not support cable modems. Industry observers, however. Predict 100 percent availability within the next few years. 电缆调制解调器使用现有的电视电缆，以较低的成本提供高速互联网连接速度T1或DSL连接,。虽然电缆在美国家庭覆盖率达到90%。行业认识预测，几年内覆盖率就可达到100%248 Satellite/air connection services use satellites and the air to download(downlink) or send data to users at a rate seven times faster than dial-up connections. While older satellite services could not upload (uplink)or send data to satellites and had to rely on slow dial-up connections,newer two-way satellite connections are now available. While slower than DSL and cable modems, satellite/air connections are available almost anywhere that a satellite-receiving disk can be aimed at the southern skies. 卫星/无线连接服务使用卫星和无线下载(下行),或将数据发送给用户速率比拨号连接快7倍。而老旧的卫星服务不能上传(上行)或将数据发送给卫星,所以不得不依靠缓慢的拨号连接,更新的双向卫星连接现在可用。虽然低于DSL和电缆调制解调器、卫星/无线连接几乎可以在任何地方使用 Cellular services offer an alternative for mobile devices and laptops. Using 3G cellular networks, devices such as cell phones and appropriately equipped laptop computers can download data from the Internet at 400 to 700 kbps. Uploading data, however, is a much slower 50 kbps. At present,the availability of this service is limited to specific metropolitan areas. In the future, however, this service is expected to be more widespread and to operate at speeds comparable to DSL. 蜂窝服务替代移动设备和笔记本电脑。使用3 g蜂窝网络,设备,如手机和笔记本电脑的下载速度可以维持在400至700 kbps。但是上传数据是非常慢，只有50 kbps。目前，此服务仅限于特定的大都市地区开放。在未来,然而,这种服务相对于DSL，应用将更快，更广泛。DATA TRANSMISSION Several factors affect how data is transmitted. These factors include bandwidth and protocols. 有几个因素影响数据传输速度。这些因素包括带宽和协议。BANDWIDTH Bandwidth is a measurement of the width or capacity of the communication channel. Effectively, it means how much information can move across the communication channel in a given amount of time. For example, to transmit text documents, a slow bandwidth would be acceptable. However, to effectively transmit video and audio, a wider bandwidth is required. There are three categories of bandwidth. 带宽是一个测量的宽度或容量的通信通道。实际上,带宽意味着可以在固定时间内传输多少数据量。传输文本文档,小带宽可以接受。但是,为了流畅的传输视频和音频,则需要比较高的带宽。目前，有三个类别的带宽。 Voiceband, also known as low bandwidth, is used for standard telephone communication. Microcomputers with standard modems and dial-up service use this bandwidth. While effective for transmitting text documents, it is too slow for many types of transmission, including high-quality audio and video. Typical speeds are 56 to 96 kbps. 音频带宽,也称为低带宽,是用于电话沟通的标准形式。微型计算机与标准调制解调器和拨号服务使用这个带宽。能有效的传输文档，但是太慢了，不能满足,包括高质量的音频和视频传输。典型的速度是56至96 kbps。 Medium band is the bandwidth used in special leased lines to connect minicomputers and mainframes as well as to transmit data over long distances. Unlike voice band and broadband, medium band is not typically used by individuals.“中型带宽用于特殊租用线路，连接小型机、大型机以及长距离传输数据。不像声音带和宽带、中型带宽通常不是私人使用的。 Broadband is the bandwidth used for high-capacity transmissions. Microcomputers with DSL, cable, and satellite connections as well as other more specialized high-speed devices use this bandwidth. It is capable of effectively meeting most of todays communication needs, including transmitting high-quality audio and video. Speeds are typically 1.5 mbps, although much higher speeds are possible.“广播宽带是用于大容量传输的带宽。微型计算机与DSL,有线电视和卫星连接以及其他更专门的高速设备使用这个带宽。它能够有效满足今天大多数的通信需求,包括传输高质量的音频和视频。速度通常是1.5 mbps,当然也可以更快PROTOCOLSFor data transmission to be successful, sending and receiving devices must follow a set of communication rules for the exchange of information. These rules for exchanging data between computers are known as protocols.对于数据传输成功,发送和接收设备必须遵循一套通信规则。这些规则之间交换数据的计算机被称为协议。The standard protocol for the Internet is TCP/IP (transmission control protocol/Internet protocol). The essential features of this protocol involve (1) identifying sending and receiving devices and (2) reformatting information for transmission across the Internet.互联网的标准协议是TCP / IP(传输控制协议/互联网协议)。本议定书的基本特点包括:(1)确定发送和接收设备和(2)重新格式化信息在互联网上传播。Identification: Every computer on the Internet has a unique numeric address called an IP address (Internet protocol address). Similar to the way a postal service uses addresses to deliver mail, the Internet uses IP addresses to deliver e-mail and to locate Web sites. Because these numeric addresses are difficult for people to remember and use, a system was developed to automatically convert text-based addresses to numeric IP addresses. This system uses a domain name server (DNS) that converts text-based addresses to IP addresses. For example, whenever you enter a URL, say , a DNS converts this to an IP address before a connection can be made. (See Figure 9-14.)标识:每台电脑在互联网上有一个独特的数字地址称为一个IP地址(互联网协议地址)。类似于一个邮政服务使用地址送邮件,互联网使用IP地址提供电子邮件 和网站定位。因为这些数字地址是困难的对于人们记住和使用,系统的开发目的自动转换文本地址到IP地址。该系统使用一个域名服务器(DNS),将基于文本 的地址的IP地址。例如,当你输入一个URL ,一个域名,说到IP地址转换这个连接可以之前。(参见图第9 - 14个)。Reformatting: Information sent or transmitted across the Internet usually travels through numerous interconnected networks. Before the message is sent, it is reformatted or broken down into small parts called packets. Each packet is then sent separately over the Internet, possibly traveling different routes to one common destination. At the receiving end, the packets are reassembled into the correct order.重新格式化:信息发送或通过互联网通常穿过众多的互联网络。在消息被发送,这是重新格式化或分解成小的部分称为数据包。每个包然后在互联网上单独发送,可能旅行不同的路线到一个共同的目标。在接收端,包成正确的顺序重新组装。250PROTOCOLSFor data transmission to be successful, sending and receiving devices must follow a set of communication rules for the exchange of information. These rules for exchanging data between computers are known as protocols.对于数据传输成功,发送和接收设备必须遵循一套通信规则的信息交流。这些规则之间交换数据的计算机被称为协议。The standard protocol for the Internet is TCP/IP (transmission control protocol/Internet protocol). The essential features of this protocol involve (1) identifying sending and receiving devices and (2) reformatting information for transmission across the Internet.互联网的标准协议是TCP / IP(传输控制协议/互联网协议)。本议定书的基本特点包括:(1)确定发送和接收设备和(2)重新格式化信息在互联网上传播。Identification: Every computer on the Internet has a unique numeric address called an IP address (Internet protocol address). Similar to the way a postal service uses addresses to deliver mail, the Internet uses IP addresses to deliver e-mail and to locate Web sites. Because these numeric addresses are difficult for people to remember and use, a system was developed to automatically convert text-based addresses to numeric IP addresses. This system uses a domain name server (DNS) that converts text-based addresses to IP addresses. For example, whenever you enter a URL, say , a DNS converts this to an IP address before a connection can be made. (See Figure 9-14.)标识:每台电脑在互联网上有一个独特的数字地址称为一个IP地址(互联网协议地址)。类似于一个邮政服务使用地址送邮件,互联网使用IP地址提供电子邮件和网站定位。因为这些数字地址是困难的对于人们记住和使用,系统的开发目的自动转换文本地址到IP地址。该系统使用一个域名服务器(DNS),将基于文本的地址的IP地址。例如,当你输入一个URL ,一个域名,说到IP地址转换这个连接可以之前。Reformatting: Information sent or transmitted across the Internet usually travels through numerous interconnected networks. Before the message is sent, it is reformatted or broken down into small parts called packets. Each packet is then sent separately over the Internet, possibly traveling different routes to one common destination. At the receiving end, the packets are reassembled into the correct order.重新格式化:信息发送或通过互联网通常穿过众多的互联网络。在消息被发送,这是重新格式化或分解成小的部分称为数据包。每个包然后在互联网上单独发送,可能旅行不同的路线到一个共同的目标。在接收端,包成正确的顺序重新组装。NETWORK ARCHITECTURENetwork architecture describes how a network is arranged and how resources are coordinated and shared.It encompasses a variety of different network specifics, including network configurations and strategies.Network configurations describe the physical arrangement of the network. Network strategies define how information and resources are shared.网络体系结构描述了一个网络安排和资源协调和共享。它包含了各种不同的网络细节,包括网络配置和策略。网络配置描述物理安排网络。网络策略定义信息和资源共享。CONFIGURATIONSA network can be arranged or configured in several different ways. This arrangement is called the networks topology.The four principal network topologies are star, bus, ring, and hierarchical.一个网络可以安排或配置在几种不同的方式。这样的安排被称为网络的拓扑结构。四项主要的网络拓扑是明星,巴士,戒指,和层次。In a star network, a number of small computers or peripheral devices are linked to a central unit.(See Figure 9-18.) The central unit is the network hub and is typically a host computer or file server.在一个星形网络,一些小型计算机或外围设备被连接到一个中央单位。中央单位是网络中心,通常是一个主机或文件服务器。All communications pass through this central unit. Control is maintained by polling. That is, each connecting device is asked (polled) whether it has a message to send. Each device is then in turn allowed to send its message所有通信通过这个中央单位。控制是由轮询。即,每个连接装置是问(“调查”)是否有一个消息发送。每个装置,然后反过来允许发送的信息One particular advantage of the star topology is that it can
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