1、Unit15 Network Unit15 Network PassageA Computer Networks(1)PassageB Computer Networks(2)PassageC NetworkUnit15 Network Passage A Computer Networks(1)The field of communications is experiencing rapid and dramatic change.Traditionally,telecommunications has been the transmission of information over lo
2、ng 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 fac
3、ilities-in which information is processed and communicated in the form of sound,data,and images over common media.Unit15 Network What 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 inter
4、nal 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)than th
5、e traditional voice services.How we obtain news and entertainment,conduct business and education,and accomplish scientific and technical research will all be affected.1Unit15 Network Networks can be analog,in which information is represented by continuous smoothly varying signals,or digital,in which
6、 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.The
7、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.Unit15 Network There are three major categories of networks distinguished primarily by geography:wide area ne
8、tworks,metropolitan area networks,and local-area networks.Unit15 Network 1.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 va
9、riety of separate companies.Unit15 Network 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 sof
10、tware)to a wide variety of 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.Unit15 Network The Internet isnt t
11、he only worldwide computer 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
12、these networks have grown in popularity,they have begun to interconnect,creating a global collection of people and computers that are able to electronically communicate with one another.3Unit15 Network 2.Metropolitan area NetworksMetropolitan area Networks(MANs)provide communications across and amon
13、g major metropolitan areas 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
14、pass data back and forth to one another.Unit15 Network A 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 E
15、ach cell,which is approximately 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.Unit15 Network 3.Local area NetworksLocal area Networks(LANs)provide communicat
16、ions within specific buildings 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 among the compute
17、rs is required,the computers pass data to one another over the network.Unit15 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 b
18、y the purchasing and inventory 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.Unit15 Network(1)Ring Networks.Computers on a ring network are hooked together one
19、 after the other to form 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 pass
20、ing,in which one token at a time passes from computer to computer and carries messages around the network.Unit15 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 netw
21、ork is called Ethernet.Bus 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 collisi
22、on occurs,and after detecting 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).Unit15 Network The three major categories of networks are not mutually exclusive.A m
23、obile worker using a portable 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,swi
24、tched to the telephone network and carried across the country to the coworkers office,then transferred via gateway into the office LAN.Unit15 Network Notes1 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
25、闻和娱乐、处理事务和进行教育以及完成科学研究的方式将全部受到影响。how引导的是主语从句。主语从句中的谓语是三个并列成分。Unit15 Network 2 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。
26、主句有并列表语analog和or digital。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。Unit15 Network 3 As these networks have grown in popularity,they have begun to interconnect,creating a global collection of people and compu
27、ters that are able to electronically 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。Unit15 Network 4 An example you may be familiar with is the cellular telephone net
28、work,which is made possible 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。Unit15 Network 5 On a bus
29、 network computers are simply 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。Unit15 Network Exercises1.Please translate the following phra
30、ses into Chinese.(1)Integrated Services Digital Network(ISDN)(2)commercial public network(3)communication among the computers(4)toke passing(5)transmission of information(6)collision detection(CD)Unit15 Network 2.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 (
31、6)网关Unit15 Network 3.Match 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 networkUnit15 Network a.A type of network in which information is represented by con
32、tinuous smoothly varying 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 spec
33、ific buildings or facilities.Unit15 Network e.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
34、 devices connected by communication facilities.Unit15 Network 4.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
35、 of metropolitan-area networks 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.()Unit15 Network 5.Answer the questions.(1)Are the three major catego
36、ries of networks mutually exclusive?Why?(2)What kind of the computer network is the Internet?If you have used it,do you think it is convenient?Unit15 Network Passage B Computer Networks(2)Communication networks enable users to transfer information in the form of voice,video,electronic mail or E-mail
37、,and computer files.Users 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.Unit15 Network Telephone NetworksNote how the transmission lines
38、are allocated to the phone 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
39、operators can switch the 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.Unit15 Network Figure 23.1 illustrates the telephone network around 1988.On major development at this s
40、tage is that the transmission 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
41、 interface converts the digital signal that travels between the switches into an analog signal before sending it from the switch to the telephone.Unit15 Network Figure 15.1 Telephone network around 1988.The transmissions are analog(A)or digital(D).The switches are electronic and exchange control inf
42、ormation by using a data network called common channel signaling(CCS)Unit15 Network The 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.Figure15.1also shows another maj
43、or development-common 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 funct
44、ions of call control from 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.Unit15 Network In current telephone networks,the bit streams in the trunks(lines connecting switches)an
45、d access links(lines connecting 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
46、.1,are called DS-1,DS-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 Europ
47、e are different.The most common channels are DS-1 and DS-3.Unit15 Network Unit15 Network This 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 re
48、quired.Unit15 Network Observe in the 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 sign
49、als are generated using clocks that are not perfectly synchronized.Unit15 Network The last major innovation in telephony is the integration of voice and data signals through the introduction of the Integrated Services Digital Network(ISDN),illustrated in Figure 15.2.The ISDN basic access offered to
50、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,channel at 64 kbps.One B channel can carry either a circuit-switched connection,a packet-switched transmission services(described below),or a permanent digital
