COMMUNICATION CHANNELS
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.
沟通渠道是一个至关重要的元素的每个通信系统。这些渠道实际上把数据从一台计算机到另一个。有两个类别的沟通渠道。一个类别连接发送和接收设备通过提供一个物理连接导线或电缆等。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.. 电话线路你看到两极串在由双绞线, 是由数以百计的铜线。一个双绞线的顶端是一墙杰克, 你可以插你的手机和电脑。电话线被标准的传输介质多年的语音和数据。然而, 他们现在被淘汰的技术更先进、可靠的媒体。同轴电缆、高频传输电缆, 取代了多个连接的电话线和一个固体铜核心。在数量方面的电话连接, 一个同轴电缆有超过80倍的双绞线传输容量。同轴电缆是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.
光纤电缆传输数据的光脉冲, 通过微小的玻璃管。在数量方面的电话连接, 光纤电缆已经超过26000倍的双绞线传输容量。同轴电缆相比, 它们更轻和更可靠的传输数据。他们使用光束传输信息以光的速度而不是的电脉冲, 使他们远远快于铜电缆。光纤电缆正在迅速取代双绞线电缆电话线路。
WIRELESS 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设备。相反, 他们使用空气本身。主要技术用于无线连接红外、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波只能以直线的方式行进。这就要求发送和接收设备必须在清楚彼此的观点没有任何障碍物阻止这一观点。一个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。
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或者掌上电脑到移动电话访问网络从远程位置。大多数的这些网络设备遵循一个标准的称为wi - fi(无线保真度) 。这个无线标准被广泛用于连接电脑到对方和因特网。 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 mountaintops.
微波通信使用高频无线电波。如红外、微波通信提供视距通信因为微波以直线的方式行进。因为海浪不能弯曲曲率的地球, 他们可以传播只在相对较短的距离。因此, 微波是一个很好的媒介建筑物之间发送数据在一个城市或一个大的大学校园。时间的距离, 海浪必须转发通过微波站与微波碗或天线。这些站可以安装在大厦, 高层建筑, 山顶。
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, 使用微波传输数据在短的距离大约33英尺。与传统的微波, 蓝牙不需要视距通信。相反, 它使用无线电波可以通过附近的墙壁和其他非金属壁垒。预计在未来几年中, 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. 卫星通信使用卫星轨道在约22000英里的地球作为微波中继站。许多这些提供的国际通信卫星、国际通信卫星财团旗下114政府和形成一个全球通信系统。卫星旋转在一个精确的点和速度高于地球。他们可以放大和继电器微波信号从一个发射机在地上, 另一个。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。的主要缺陷。
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 to uniquely 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。一个网络24卫星拥有和管理由国防部连续发送位置信息到地球。全球定位系统(GPS)设备使用这些信息来唯一地确定地理位置的装置。可以在一些汽车提供导航支持, 这些系统通常安装到仪表板与监视显示地图和扬声器提供口头的方向。
CONNECTION DEVICES
A 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. 大量的计算机通信发生于电话线然而, 因为电话是最初设计用于语音传输、电话通常发送和接收模拟信号连续电子波。电脑, 相反, 发送和接收数字信号。这些代表的存在或缺少电子脉冲开/关二进制信号我们提到在第六章。转换为数字信号, 模拟信号, 反之亦然, 你需要一个调制解调器。
MODEMS
The 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.
调制解调器是一词简称调制解调器。调制是进程的名称从数字模拟转换。解调是一个过程, 从模拟到数字的转换。现代通信使数字微机在模拟电话线路。这种交流包括语音和数据通信。
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.
调制解调器的速度传输数据变化。这速度, 称为传输速率, 是典型的以比特/秒(bps)。速度越高, 你越快可以发送和接收信息。例如, 将一个图像与图9 - 10或许需要75秒33.6 kbps调制解调器和只有45秒一个56 kbps调制解调器。
There are four basic types of modems:external, internal, PC Card, and wireless. (See Figure 9-12.)
有四种基本类型的调制解调器:外部、内部、PC 卡和无线。
● The external modem stands apart from the computer and typically is connected
by a cable to the computer's serial port. Another cable connects the modem to the telephone wall jack. 外部调制解调器站除了电脑, 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墙上电话插孔
● 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。电话电缆连接调制解调器墙上电话插孔
● 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. PC 卡调制解调器是一种信用卡片尺寸扩展板, 插入到手提电脑。电话电缆连接调制解调器墙上电话插孔
● A wireless modem may be internal, external,a PC Card.Unlike the other
modems, it does not use cables. Instead,wireless modems send and receive through the air.
一个无线调制解调器可能是内部的、外部的、一个PC 卡片。与其他调制解调器, 它不使用电缆。相反, 无线调制解调器发送和接收通过空气
Standard telephone lines and conventional modems provide what is called a dial-up service. 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. 标准电话线路和传统的调制解调器提供所谓的拨号服务。虽然仍然是最受欢迎
的类型的连接服务, 拨号服务相当差, 很多用户发现它不足以满足其通信需求。
For years, large corporations have been leasing special high-speed lines from telephone companies. These lines-known as T1, T2, T3, and T4 lines-support all digital communications, do not require conventional modems, and provide very high capacity. Unfortunately, this type of connection is very expensive.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.
多年以来, 大型公司已经租赁特殊的高速线路从电话公司。这些线已知为T1、T2、T3、T4线支持所有数字通信, 不需要传统的调制解调器, 并提供非常高的能力。不幸的是, 这种类型的连接是非常昂贵的。例如,T1线路提供的速度为1.5 mbps(超过26倍常规调制解调器) 和成本数千美元。
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. These include DSL, cable modems, satellite,and cellular. For a comparison of typical user connection costs and speeds,see figure 9-13
而特殊的高速线太过昂贵, 大多数个人, 有负担得起的连接, 提供能力显著高于比标准的拨号服务。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, 见图9 - 13
● 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) 是一种最广泛使用的类型的DSL 。这个技术是广泛使用在大多数地区。
● 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 companies 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提供高速互联网连接速度T1或DSL 连接, 以较低的成本。虽然电缆连接达到90%的家庭在美国, 所有的电缆公司不支持电缆调制解调器。行业观察人士, 然而。100%的可用性预测在未来几年。
● 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/空气连接服务使用卫星和空中下载(下行), 或将数据发送给用户速率比拨号连接快7倍。而年长的卫星服务不能上传(上行) 或将数据发送给卫星, 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/空气连接可用的几乎任何地方, 卫星接收磁盘可以针对南方的天空。
● 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可以适当装备从网上下载数据在400至700 kbps。上传数据, 然而, 是一个非常慢的50 kbps。目前, 可用性的服务仅限于特定的大都市地区。在未来, 然而, 这种服务将成为更广泛和运行时速与DSL 。
DATA TRANSMISSION
Several factors affect how data is transmitted. These factors include bandwidth and protocols.
几个因素影响数据传输。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, 这意味着多少信息可以移动整个通信信道在给定的时间。例如, 传输文本文档, 一个缓慢的带宽将是可以接受的。然而, 有效地传输视频和音频, 一个更广泛的带宽是必需的。有三个类别的带宽。
● V oiceband, 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, 是用于标准的电话沟通。微型计算机与标准调制解调器和拨号服务使用这个带宽。而有效的传输文本文档, 它太慢了许多类型的传输, 包括高质量的音频和视频。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 dis-tances. Unlike voiceband and broadband, medium band is not typically used by individuals.
介质带的带宽用于特殊租用线路连接小型机、大型机以及传输数据长dis 自然界。与音频带宽和宽带、中带通常不是由个人使用。
● 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 today's communication needs, including transmitting high-quality audio and video. Speeds are typically 1.5 mbps, although much higher speeds are possible.
宽带是用于高容量的传输带宽。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。它能够有效的会议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,尽管更高的速度是可能的。
PROTOCOLS
For 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 www.computing2008.com, 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 www.computing2008.com,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 ARCHITECTURE
Network 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。
CONFIGURATIONS
A network can be arranged or configured in several different ways. This arrangement is called the network's 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 be used to support a time-sharing system. That is,several users can share resources (time) on a central computer. The star is a common topology for linking microcomputers to a mainframe that allows access to an organization's database.
一个特定的优势是, 它星型拓朴结构可以用于支持分时系统。那是, 几个用户可以共享资源(时间) 在一个中央计算机。星是一种常见的拓扑结构为连接微机到主机, 允许访问一个组织的数据库。
In a bus network each device in the network handles its own communications control. There is no host computer. All communications travel along a common connecting cable called a bus or backbone. (See Figure 9-19.) As the information
formation is intended for it.
在总线网络每个设备在网络中处理它自己的通信控制。没有主机。所有通信沿着一个共同的连接电缆称为总线或骨干。随着信息形成适用于它。
The bus network is typically used when only a few microcomputers are to be linked together.This arrangement is common for sharing data stored on different microcomputers. Because a star network typically provides a more direct path to shared resources,it is more efficient than a bus network for sharing these resources.However, a bus network is easy to install and is less expensive.
总线网络通常用在当只有很少的微机都联系到一起。这样的安排是常见的共享数据存储在不同的微型计算机。因为一个星形网络通常提供了一个更直接的路径共享资源, 它是更有效的比一个总线网络共享这些资源。然而, 一个总线网络易于安装和不昂贵的。
In a ring network, each device is connected to two other devices, forming a ring.(See Figure 9-20.) There is no central file server or computer. Messages are passed around the ring until they reach the correct destination. With microcomputers, the ring arrangement is the least frequently used of the four networks. However, it often is used to link mainframes, especially over wide geographical areas. These mainframes tend to operate fairly autonomously. They perform most or all of their own processing and only occasionally share data and programs with other mainframes.
在一个环网络, 每个设备被连接到其他两个设备, 形成一个环。没有中央文件服务器或计算机。消息被传递在环直到他们到达正确的目的地。用微型计算机, 戒指的安排是最常用的四种网络。然而, 它通常是用来连接主机, 尤其是在广泛的地理区域。这些大型机往往相当独立地运作。他们表演的大多数或所有他们自己处理, 只有偶尔分享数据和程序与其他主机。
A ring network is useful in a decentralized organization because it makes possible a distributed data processing system. That is,computers can perform processing tasks at their own dispersed locations,However, they also can share programs, data,and other resources with each other.
一个环网络是有用的在一个分散的组织, 因为它使可能的分布式数据处理系统。那是, 电脑可以执行处理任务在自己的分散的位置, 然而, 他们也可以共享程序、数据和其他资源, 与对方。
The hierarchical network consists of several computers linked to a central host computer, just like a star network. However, these other computers are also hosts to
other, smaller computers or to peripheral devices. (See Figure 9-21.)
层次网络由多个计算机连接到一个中央主机电脑, 就像一个星形网络。然而, 这些其他的电脑也主机, 其它的小电脑或外围设备。
Thus, the host at the top of the hierarchy could be a mainframe. The computers below the mainframe could be minicomputers, and those below, microcomputers. The hierarchical network allows various computers to share databases, processing power, and different output devices.
因此, 主持人最高等级的可以是一个主机。下面的电脑主机可以小型机, 下面, 微型计算机。层次网络允许各种电脑共享数据库、处理能力和不同输出设备。
A hierarchical network is useful in centralized organizations. For example,different departments within an organization may have individual microcomputers connected to departmental minicomputers. The minicomputers in turn may be connected to the corporation's mainframe, which contains data and programs accessible to all.
一个层次网络是有用的在集中组织。例如, 不同的部门在一个组织中可能存在个人微型计算机连接到部门的小型计算机。小型机的反过来又可以连接到公司的主机, 它包含数据和程序都可接受。
For a summary of the network configurations, see Figure 9-22.
对于一个总结的网络配置, 见图9-22。
COMMUNICATION CHANNELS
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.
沟通渠道是一个至关重要的元素的每个通信系统。这些渠道实际上把数据从一台计算机到另一个。有两个类别的沟通渠道。一个类别连接发送和接收设备通过提供一个物理连接导线或电缆等。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.. 电话线路你看到两极串在由双绞线, 是由数以百计的铜线。一个双绞线的顶端是一墙杰克, 你可以插你的手机和电脑。电话线被标准的传输介质多年的语音和数据。然而, 他们现在被淘汰的技术更先进、可靠的媒体。同轴电缆、高频传输电缆, 取代了多个连接的电话线和一个固体铜核心。在数量方面的电话连接, 一个同轴电缆有超过80倍的双绞线传输容量。同轴电缆是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.
光纤电缆传输数据的光脉冲, 通过微小的玻璃管。在数量方面的电话连接, 光纤电缆已经超过26000倍的双绞线传输容量。同轴电缆相比, 它们更轻和更可靠的传输数据。他们使用光束传输信息以光的速度而不是的电脉冲, 使他们远远快于铜电缆。光纤电缆正在迅速取代双绞线电缆电话线路。
WIRELESS 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设备。相反, 他们使用空气本身。主要技术用于无线连接红外、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波只能以直线的方式行进。这就要求发送和接收设备必须在清楚彼此的观点没有任何障碍物阻止这一观点。一个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。
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或者掌上电脑到移动电话访问网络从远程位置。大多数的这些网络设备遵循一个标准的称为wi - fi(无线保真度) 。这个无线标准被广泛用于连接电脑到对方和因特网。 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 mountaintops.
微波通信使用高频无线电波。如红外、微波通信提供视距通信因为微波以直线的方式行进。因为海浪不能弯曲曲率的地球, 他们可以传播只在相对较短的距离。因此, 微波是一个很好的媒介建筑物之间发送数据在一个城市或一个大的大学校园。时间的距离, 海浪必须转发通过微波站与微波碗或天线。这些站可以安装在大厦, 高层建筑, 山顶。
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, 使用微波传输数据在短的距离大约33英尺。与传统的微波, 蓝牙不需要视距通信。相反, 它使用无线电波可以通过附近的墙壁和其他非金属壁垒。预计在未来几年中, 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. 卫星通信使用卫星轨道在约22000英里的地球作为微波中继站。许多这些提供的国际通信卫星、国际通信卫星财团旗下114政府和形成一个全球通信系统。卫星旋转在一个精确的点和速度高于地球。他们可以放大和继电器微波信号从一个发射机在地上, 另一个。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。的主要缺陷。
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 to uniquely 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。一个网络24卫星拥有和管理由国防部连续发送位置信息到地球。全球定位系统(GPS)设备使用这些信息来唯一地确定地理位置的装置。可以在一些汽车提供导航支持, 这些系统通常安装到仪表板与监视显示地图和扬声器提供口头的方向。
CONNECTION DEVICES
A 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. 大量的计算机通信发生于电话线然而, 因为电话是最初设计用于语音传输、电话通常发送和接收模拟信号连续电子波。电脑, 相反, 发送和接收数字信号。这些代表的存在或缺少电子脉冲开/关二进制信号我们提到在第六章。转换为数字信号, 模拟信号, 反之亦然, 你需要一个调制解调器。
MODEMS
The 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.
调制解调器是一词简称调制解调器。调制是进程的名称从数字模拟转换。解调是一个过程, 从模拟到数字的转换。现代通信使数字微机在模拟电话线路。这种交流包括语音和数据通信。
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.
调制解调器的速度传输数据变化。这速度, 称为传输速率, 是典型的以比特/秒(bps)。速度越高, 你越快可以发送和接收信息。例如, 将一个图像与图9 - 10或许需要75秒33.6 kbps调制解调器和只有45秒一个56 kbps调制解调器。
There are four basic types of modems:external, internal, PC Card, and wireless. (See Figure 9-12.)
有四种基本类型的调制解调器:外部、内部、PC 卡和无线。
● The external modem stands apart from the computer and typically is connected
by a cable to the computer's serial port. Another cable connects the modem to the telephone wall jack. 外部调制解调器站除了电脑, 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墙上电话插孔
● 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。电话电缆连接调制解调器墙上电话插孔
● 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. PC 卡调制解调器是一种信用卡片尺寸扩展板, 插入到手提电脑。电话电缆连接调制解调器墙上电话插孔
● A wireless modem may be internal, external,a PC Card.Unlike the other
modems, it does not use cables. Instead,wireless modems send and receive through the air.
一个无线调制解调器可能是内部的、外部的、一个PC 卡片。与其他调制解调器, 它不使用电缆。相反, 无线调制解调器发送和接收通过空气
Standard telephone lines and conventional modems provide what is called a dial-up service. 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. 标准电话线路和传统的调制解调器提供所谓的拨号服务。虽然仍然是最受欢迎
的类型的连接服务, 拨号服务相当差, 很多用户发现它不足以满足其通信需求。
For years, large corporations have been leasing special high-speed lines from telephone companies. These lines-known as T1, T2, T3, and T4 lines-support all digital communications, do not require conventional modems, and provide very high capacity. Unfortunately, this type of connection is very expensive.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.
多年以来, 大型公司已经租赁特殊的高速线路从电话公司。这些线已知为T1、T2、T3、T4线支持所有数字通信, 不需要传统的调制解调器, 并提供非常高的能力。不幸的是, 这种类型的连接是非常昂贵的。例如,T1线路提供的速度为1.5 mbps(超过26倍常规调制解调器) 和成本数千美元。
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. These include DSL, cable modems, satellite,and cellular. For a comparison of typical user connection costs and speeds,see figure 9-13
而特殊的高速线太过昂贵, 大多数个人, 有负担得起的连接, 提供能力显著高于比标准的拨号服务。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, 见图9 - 13
● 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) 是一种最广泛使用的类型的DSL 。这个技术是广泛使用在大多数地区。
● 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 companies 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提供高速互联网连接速度T1或DSL 连接, 以较低的成本。虽然电缆连接达到90%的家庭在美国, 所有的电缆公司不支持电缆调制解调器。行业观察人士, 然而。100%的可用性预测在未来几年。
● 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/空气连接服务使用卫星和空中下载(下行), 或将数据发送给用户速率比拨号连接快7倍。而年长的卫星服务不能上传(上行) 或将数据发送给卫星, 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/空气连接可用的几乎任何地方, 卫星接收磁盘可以针对南方的天空。
● 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可以适当装备从网上下载数据在400至700 kbps。上传数据, 然而, 是一个非常慢的50 kbps。目前, 可用性的服务仅限于特定的大都市地区。在未来, 然而, 这种服务将成为更广泛和运行时速与DSL 。
DATA TRANSMISSION
Several factors affect how data is transmitted. These factors include bandwidth and protocols.
几个因素影响数据传输。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, 这意味着多少信息可以移动整个通信信道在给定的时间。例如, 传输文本文档, 一个缓慢的带宽将是可以接受的。然而, 有效地传输视频和音频, 一个更广泛的带宽是必需的。有三个类别的带宽。
● V oiceband, 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, 是用于标准的电话沟通。微型计算机与标准调制解调器和拨号服务使用这个带宽。而有效的传输文本文档, 它太慢了许多类型的传输, 包括高质量的音频和视频。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 dis-tances. Unlike voiceband and broadband, medium band is not typically used by individuals.
介质带的带宽用于特殊租用线路连接小型机、大型机以及传输数据长dis 自然界。与音频带宽和宽带、中带通常不是由个人使用。
● 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 today's communication needs, including transmitting high-quality audio and video. Speeds are typically 1.5 mbps, although much higher speeds are possible.
宽带是用于高容量的传输带宽。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。它能够有效的会议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,尽管更高的速度是可能的。
PROTOCOLS
For 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 www.computing2008.com, 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 www.computing2008.com,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 ARCHITECTURE
Network 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。
CONFIGURATIONS
A network can be arranged or configured in several different ways. This arrangement is called the network's 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 be used to support a time-sharing system. That is,several users can share resources (time) on a central computer. The star is a common topology for linking microcomputers to a mainframe that allows access to an organization's database.
一个特定的优势是, 它星型拓朴结构可以用于支持分时系统。那是, 几个用户可以共享资源(时间) 在一个中央计算机。星是一种常见的拓扑结构为连接微机到主机, 允许访问一个组织的数据库。
In a bus network each device in the network handles its own communications control. There is no host computer. All communications travel along a common connecting cable called a bus or backbone. (See Figure 9-19.) As the information
formation is intended for it.
在总线网络每个设备在网络中处理它自己的通信控制。没有主机。所有通信沿着一个共同的连接电缆称为总线或骨干。随着信息形成适用于它。
The bus network is typically used when only a few microcomputers are to be linked together.This arrangement is common for sharing data stored on different microcomputers. Because a star network typically provides a more direct path to shared resources,it is more efficient than a bus network for sharing these resources.However, a bus network is easy to install and is less expensive.
总线网络通常用在当只有很少的微机都联系到一起。这样的安排是常见的共享数据存储在不同的微型计算机。因为一个星形网络通常提供了一个更直接的路径共享资源, 它是更有效的比一个总线网络共享这些资源。然而, 一个总线网络易于安装和不昂贵的。
In a ring network, each device is connected to two other devices, forming a ring.(See Figure 9-20.) There is no central file server or computer. Messages are passed around the ring until they reach the correct destination. With microcomputers, the ring arrangement is the least frequently used of the four networks. However, it often is used to link mainframes, especially over wide geographical areas. These mainframes tend to operate fairly autonomously. They perform most or all of their own processing and only occasionally share data and programs with other mainframes.
在一个环网络, 每个设备被连接到其他两个设备, 形成一个环。没有中央文件服务器或计算机。消息被传递在环直到他们到达正确的目的地。用微型计算机, 戒指的安排是最常用的四种网络。然而, 它通常是用来连接主机, 尤其是在广泛的地理区域。这些大型机往往相当独立地运作。他们表演的大多数或所有他们自己处理, 只有偶尔分享数据和程序与其他主机。
A ring network is useful in a decentralized organization because it makes possible a distributed data processing system. That is,computers can perform processing tasks at their own dispersed locations,However, they also can share programs, data,and other resources with each other.
一个环网络是有用的在一个分散的组织, 因为它使可能的分布式数据处理系统。那是, 电脑可以执行处理任务在自己的分散的位置, 然而, 他们也可以共享程序、数据和其他资源, 与对方。
The hierarchical network consists of several computers linked to a central host computer, just like a star network. However, these other computers are also hosts to
other, smaller computers or to peripheral devices. (See Figure 9-21.)
层次网络由多个计算机连接到一个中央主机电脑, 就像一个星形网络。然而, 这些其他的电脑也主机, 其它的小电脑或外围设备。
Thus, the host at the top of the hierarchy could be a mainframe. The computers below the mainframe could be minicomputers, and those below, microcomputers. The hierarchical network allows various computers to share databases, processing power, and different output devices.
因此, 主持人最高等级的可以是一个主机。下面的电脑主机可以小型机, 下面, 微型计算机。层次网络允许各种电脑共享数据库、处理能力和不同输出设备。
A hierarchical network is useful in centralized organizations. For example,different departments within an organization may have individual microcomputers connected to departmental minicomputers. The minicomputers in turn may be connected to the corporation's mainframe, which contains data and programs accessible to all.
一个层次网络是有用的在集中组织。例如, 不同的部门在一个组织中可能存在个人微型计算机连接到部门的小型计算机。小型机的反过来又可以连接到公司的主机, 它包含数据和程序都可接受。
For a summary of the network configurations, see Figure 9-22.
对于一个总结的网络配置, 见图9-22。