1、Unit 23 NetworkUnit 23 NetworkPassage A Computer Networks(1)Passage B Computer Networks(2)Passage C Network Unit 23 NetworkPassage A Computer Networks(1)The field of communications is experiencing rapid and dramatic change.Traditionally,telecommunications has been the transmission of information ove
2、r long distances by means of telephone,teletype,radio,or television.Data communications has been the exchange of data between computer-related devices.Today,these distinctions have blurred as the two have been converging into networks-a group of computers and other devices connected by communication
3、 facilities-in which information is processed and communicated in the form of sound,data,and images over common media.Unit 23 NetworkWhat does this change mean for society?Just as during the late 1800 s and early 1900 s global societies were shaped by such technologies as ships,electricity,and the i
4、nternal combustion engine,so in the late 1900s and early 2000s they will be shaped by converging computer and communication technologies.The increased capabilities of the networks allow providers to offer a wider variety of communication services(e.g.,video,facsimile,data,and multimedia services)tha
5、n the traditional voice services.How we obtain news and entertainment,conduct business and education,and accomplish scientific and technical research will all be affected.1Unit 23 NetworkNetworks can be analog,in which information is represented by continuous smoothly varying signals,or digital,in w
6、hich information is represented by discrete on/off signals.2Digital signals are a newer technology found in computer systems,local area networks,compact disks,and high definition television(HDTV).Digital signals are discrete on/off signals that can be easily stored,compressed,edited,and manipulated.
7、The telephone industry is in the process of converting to an all-digital network called Integrated Services Digital Network(ISDN)and similar upgrades are proposed for cellular telephone networks.There are three major categories of networks distinguished primarily by geography:wide area networks,metr
8、opolitan area networks,and local-area networks.Unit 23 Network1.Wide area NetworksWide area networks(WANs)provide communications across a country or the entire world.The telephone network is the most pervasive example.It is often viewed as a single entity,even though it is made up of a variety of se
9、parate companies.Whereas the phone system creates a voice connection between your phone and any other phones in the world,the Internet-a world wide network of computer networks-makes it possible for you to connect your computer(equipped with the appropriate hardware and software)to a wide variety of
10、 remote computer.The remote computers may be thousands of miles away,yet you can search libraries of software,access news and other information,conduct business,and meet people with interests or hobbies similar to your own around the globe.Unit 23 NetworkThe Internet isnt the only worldwide computer
11、 network.International companies have private networks to link offices in different countries together electronically to form 24-hour-a-day business operations.Commercial public networks like CompuServe,Prodigy,and America Online are also used by people around the world.As these networks have grown
12、in popularity,they have begun to interconnect,creating a global collection of people and computers that are able to electronically communicate with one another.3Unit 23 Network2.Metropolitan area NetworksMetropolitan area Networks(MANs)provide communications across and among major metropolitan areas
13、 such as New York,Chicago,or Los Angeles.Public-safety agencies such as police and fire departments,and dispatch-oriented companies,like Federal Express,operate and maintain their own private networks.They use MANs to allow their mobile“out-of-office”workers to talk to and pass data back and forth t
14、o one another.Unit 23 NetworkA more recent development is the public voice and data networks.An example you may be familiar with is the cellular telephone network,which is made possible by dividing a geographical area into hexagonal-shaped cells,each with a base station.4 Each cell,which is approxim
15、ately 10 miles(16 kilometers)across,has its own antenna.Like the public telephone network,public MANs eliminate the need for individuals and organizations to build and operate their own networks.Unit 23 Network3.Local area NetworksLocal area Networks(LANs)provide communications within specific build
16、ings or facilities.This type of network could be used,for example,in a company where computers are located in the sales,accounting,production,and purchasing departments.Each computer would handle the applications unique to its department.When communication Unit 23 Networkamong the computers is requi
17、red,the computers pass data to one another over the network.For example,purchasing information must be passed to the accounting department for inclusion in the accounts payable system,or production-scheduling information must be passed to the purchasing department for use by the purchasing and inven
18、tory systems.Local-area networks have many different topologies,or ways in which computer can be connected to the network.Today,the two most common topologies are called ring and bus networks.Unit 23 Network(1)Ring Networks.Computers on a ring network are hooked together one after the other to form
19、one continuous ring.To access each computer,a piece of software called a token is passed around the ring.A token is the electronic equivalent of an envelope.It contains a destination address and a fixed amount of information.The most popular token method is called token passing,in which one token at
20、 a time passes from computer to computer and carries messages around the network.Unit 23 Network(2)Bus Networks.On a bus network computers are simply connected by a cable routed along a path in the vicinity of each device that must be connected to it.The most popular bus network is called Ethernet.B
21、us networks allow multiple messages to be sent simultaneously.This is often accomplished by“listening”to the network to see if any computer is transmitting.If the network is free,the computer can send its data.If two computers accidentally send data at the same time,a collision occurs,and after dete
22、cting the collision,the computers wait a random amount of time to transmit again.This access control method goes by the name of Carrier Sense Multiple Access with Collision Detection(CSMA/CD).Unit 23 NetworkThe three major categories of networks are not mutually exclusive.A mobile worker using a por
23、table computer can send a message to a coworker in a cross the country office because the networks can be linked through gateways-devices used to connect dissimilar networks.For example,the mobile workers message might first be transmitted to a local cellular base station,switched to the telephone n
24、etwork and carried across the country to the coworkers office,then transferred via gateway into the office LAN.Unit 23 NetworkNotes1 How we obtain news and entertainment,conduct business and education,and accomplish scientific and technical research will all be affected.我们获得新闻和娱乐、处理事务和进行教育以及完成科学研究的方
25、式将全部受到影响。how引导的是主语从句。主语从句中的谓语是三个并列成分。Unit 23 Network2 Networks can be analog,in which information is represented by continuous smoothly varying signals,or digital,in which information is represented by discrete on/off signals.网络可以是模拟的,其中信息以连续平稳变化的信号表示;也可以是数字的,其中信息以离散的有/无信号表示。主句有并列表语analog和or digital
26、。analog和or digital分别有一个非限定性定语从句以in which引导,在这里which显然指的是network,所以可知analog和digital是analog network和digital network的省略,为避免重复,句中去掉了network。Unit 23 Network3 As these networks have grown in popularity,they have begun to interconnect,creating a global collection of people and computers that are able to el
27、ectronically communicate with one another.由于这些网络渐渐普及,它们已开始互联,创建了一种全球性的人机集合,使他们彼此间可以进行电子化通信。as引导的是表示原因的状语从句。主句中的主语they指的是前面提到的these networks。creating引导的是分词短语作为状语,表示伴随发生的情况。that引导的定语从句修饰computers。Unit 23 Network4 An example you may be familiar with is the cellular telephone network,which is made possi
28、ble by dividing a geographical area into hexagonal-shaped cells,each with a base station.你可能熟知的一个例子是蜂窝式电话网,其构成通常是把一块地理区域划分为若干六边形的单元,每个单元具有一个基站。you may be familiar with是example的定语从句。which引导的定语从句修饰cellular telephone network,是非限定性定语从句。each是each cell的省略语。Unit 23 Network5 On a bus network computers are s
29、imply connected by a cable routed along a path in the vicinity of each device that must be connected to it.在总线形网络上,计算机由一根电缆简单地连接,电缆的路由是按附近每个必须连接到电缆上的设备的路径而定的。routed引导的分词短语是cable的定语。最后的it指的是cable。that引导的定语从句修饰device。Unit 23 NetworkExercises1.Please translate the following phrases into Chinese.(1)Inte
30、grated Services Digital Network(ISDN)(2)commercial public network(3)communication among the computers(4)toke passing(5)transmission of information(6)collision detection(CD)Unit 23 Network2.Please translate the following phrases into English.(1)广域网 (2)城域网(3)局域网 (4)环形网(5)总线形网 (6)网关Unit 23 Network3.Mat
31、ch the following key terms to the appropriate definition.(1)Telecommunications(2)Data communications(3)Network (4)Analog(5)Digital(6)Wide-area network(7)Metropolitan-area network(8)Local-area networkUnit 23 Networka.A type of network in which information is represented by continuous smoothly varying
32、 signals.b.The transmission of information over long distances by means of telephone,teletype,radio,or television.c.A network that provides communications across and among major areas such as New York,Chicago,or Los Angeles.d.A network that provides communications within specific buildings or facili
33、ties.Unit 23 Networke.The transfer of data or information between computer related devices.f.A network in which information is represented by discrete on/off signals.g.A network that provides communications across a country or the entire world.h.A group of computers and other devices connected by co
34、mmunication facilities.Unit 23 Network4.True/False.(1)The distinction between telecommunications and data communications has become much clearer with the advent of computer networks.()(2)Analog signals for communication were developed before computers were invented.()(3)Users of metropolitan-area ne
35、tworks include police and fire departments.()(4)Metropolitan-area networks are designed to provide communications within specific buildings.()(5)The three major categories of networks are mutually exclusive.()Unit 23 Network5.Answer the questions.(1)Are the three major categories of networks mutuall
36、y exclusive?Why?(2)What kind of the computer network is the Internet?If you have used it,do you think it is convenient?Unit 23 NetworkPassage B Computer Networks(2)Communication networks enable users to transfer information in the form of voice,video,electronic mail or E-mail,and computer files.User
37、s request the communication service they need by means of simple procedures using a telephone handset cellular phone,set-top TV box,or through applications running on a host computer such as a PC or workstation.Unit 23 NetworkTelephone NetworksNote how the transmission lines are allocated to the pho
38、ne conversation.This is accomplished by circuit switching,where“circuit”refers to the capability of transmitting one telephone conversation along one link.1 To set up a call,a set of circuits has to be connected,joining the two telephone sets.By modifying the connections,the operators can switch the
39、 circuits.Circuits switching occurs at the beginning of a new telephone call.Operators were later replaced by mechanical switches and,100 years later,by electronic switches.Unit 23 NetworkFigure 23.1 illustrates the telephone network around 1988.On major development at this stage is that the transmi
40、ssion of the voice signals between switches is digital,as indicated by the letter D,instead of analog.An electronic interface in the switch converts the analog signal traveling on the link from the telephone set to the switch into a digital signal,called a bit stream.The same interface converts the
41、digital signal that travels between the switches into an analog signal before sending it from the switch to the telephone.Unit 23 NetworkFigure 23.1 Telephone network around 1988.The transmissions are analog(A)or digital(D).The switches are electronic and exchange control information by using a data
42、 network called common channel signaling(CCS)Unit 23 NetworkThe switch themselves are computers,which makes them very flexible.This flexibility allows the telephone company to modify connections by sending specific instructions to the computer.Figure 23.1 also shows another major development-common
43、channel signaling(CCS).CCS is a data communication networks that the switches use to exchange control information.This“conversation”between switches serves the same function as the conversation that took place between operators in the manual network.Thus CCS separates the functions of call control f
44、rom transfer of voice.Combined with the flexible computerized switches,this separation of function facilitates new services such as call waiting,call forwarding,and call back.Unit 23 NetworkIn current telephone networks,the bit streams in the trunks(lines connecting switches)and access links(lines c
45、onnecting subscriber telephones to the switch)are organized in the digital signal(DS)hierarchy.The links themselves-the“hardware”-are called digital carrier systems.Trunk capacity is divided into a hierarchy of logical channels.In North America these channels,listed in Table 23.1,are called DS-1,DS-
46、4 and have rates ranging from 1.544 to 274.176 Mbps(megabits per second).The basic unit is set by the DS-0 channel,which carries 64 kbps(kilobits per second)and accommodates one voice circuit.Larger-capacity channels multiplex several voice channels.The rates in Japan and Europe are different.The mo
47、st common channels are DS-1 and DS-3.Unit 23 NetworkUnit 23 NetworkThis is the hierarchy of digital signals that telephone networks uses.Note that the bit rate of a DS-1 signal is greater than 24 times the rate of a voice signal(64 kbps)because of the additional framing bits required.Observe in the
48、table,that the rates are not multiples of one another:the DS-1 signal carries 24 DS-0 channels,but its rates are more than 24 times 64 kbps.The additional bits are used to accommodate DS-0 channels with rates that deviate from the nominal 64 kbps because the signals are generated using clocks that a
49、re not perfectly synchronized.Unit 23 NetworkFigure 23.2 Integrated Services Digital Network(ISDN).The basic access provided two bidirectional 64-kbps links and one 16-kbps link.These links can be used to transmit voice or data Unit 23 NetworkThe last major innovation in telephony is the integration
50、 of voice and data signals through the introduction of the Integrated Services Digital Network(ISDN),illustrated in Figure 23.2.The ISDN basic access offered to a customer consists of two B channels and one D channel(both B and D channels are digital).Each B channel is a bidirectional,or full-duplex
