（中职）计算机专业英语unit 5教学课件

1、YCF正版可修改PPT（中职）计算机专业英语unit 5教学课件 Unit 5 Computer Network 5. 1 Computer Network Basics 5. 2 Computer Network SecuritywordPhrasesGrammarReference Translation A computer network is composed of multiple computers connected together using a telecommunication system for the purpose of sharing data, resour

2、ces and communication. For instance, a home computer network may consist of two or more computers that share files and a printer using the network. The size and scalability of any computer network are determined by the hardware used as well as which protocols are being implemented. 5. 1 . 1 Definiti

3、onExperts in the field of networking debate whether two computers that are connected together using some form of communications medium constitute a network. Therefore, some works state that a network requires three connected computers. One source states that a computer network is “a network of data

4、processing nodes that are interconnected for the purpose of data communication”. The term “network” is defined in the same document as “an interconnection of three or more communicating entities. ”A computer connected to a non-computing device may also represent a computer network, although this art

5、icle does not address this configuration. 5. 1 Computer Network Basics下一页返回This article uses the definition which requires two or more computers to be connected together to form a network. The same basic functions are generally present in this case as with larger numbers of connected computers. In o

6、rder for a network to function, it must meet three basic requirements. It must provide connections, communication and services. Connections refer to the hardware, and communication is the way in which the devices talk to each other, and services are the things which are shared with the rest of the n

7、etwork. 5. 1. 2 Types of Computer Networks1. By Network LayerComputer networks may be classified according to the network layer at which they operate according to some basic reference models that are considered to be standards in the industry such as the seven-layer OSI Reference Model and the five-

8、layer TCP/IP model. 15. 1 Computer Network Basics上一页下一页返回2. By ScaleComputer networks may be classified according to the scale or extent of reach of the network, for example, as a personal area network ( PAN ) , local area network ( LAN ) , campus area network ( CAN ) , metropolitan area network ( M

9、AN ) , or wide area network ( WAN ) . 3. By Connection MethodComputer networks may be classified according to the technology that is used to connect the individual devices in the network such as HomePNA, power line communication, Ethernet, or wireless LAN. 4. By Functional RelationshipComputer netwo

10、rks may be classified according to the functional relationships which exist between the elements of the network, for example active networking, client-server and peer-to- peer ( workgroup ) architectures. 5. 1 Computer Network Basics上一页下一页返回5. By Network TopologyComputer networks may be classified a

11、ccording to the network topology upon which the network is based, such as bus network, star network, ring network, mesh network, star-bus network, tree or hierarchical topology network, etc. Topology can be arranged in a geometric arrangement. Network topologies are logical layouts of the network. T

12、he term “logical” used here marks a great significance. That means network topologies depend not on the “physical” layout of the network. Even if computers on a network are placed in a linear format, but if they are connected via a hub they are forming a star topology, not the bus topology. And here

13、 is the important factor where networks differ, visually and operationally. 5. 1 Computer Network Basics上一页下一页返回6. By Services ProvidedComputer networks may be classified according to the services which they provide, such as storage area networks, server farms, process control networks, value-added

14、network, wireless community network, etc. 7. By ProtocolComputer networks may be classified according to the communication protocol that is being used on the network. 5. 1. 3 Types of NetworksBelow is a list of the most common types of computer networks in order of scale. 1. Personal Area Network (

15、PAN ) A personal area network is a computer network used for communication among computer devices ( including telephones and personal digital assistants ) close to one person. The devices may or may not belong to the person in question. 5. 1 Computer Network Basics上一页下一页返回The reach of a PAN is typic

16、ally a few meters. PANs can be used for communication among the personal devices themselves ( intrapersonal communication ) , or for connecting to a higher level network and the Internet ( an uplink ) . Personal area networks may be wired with computer buses such as USB and FireWire. A wireless pers

17、onal area network ( WPAN ) can also be made possible with network technologies such as IrDA and Bluetooth. 2. Local Area Network ( LAN ) Local area network is a network covering a small geographic area, like a home, office, or building. Current LANs are most likely to be based on Ethernet technology

18、. The defining characteristics of LANs, in contrast to WANs ( wide area networks ) , include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. 5. 1 Computer Network Basics上一页下一页返回3. Campus Area Network ( CAN ) Campus area network

19、is a network that connects two or more LANs but that is limited to a specific ( possibly private ) geographical area such as a college campus, industrial complex, or a military base. A CAN may be considered a type of MAN ( metropolitan area network ) , but is generally limited to an area that is sma

20、ller than a typical MAN. 4. Metropolitan Area Network ( MAN ) MAN is a network that connects two or more LANs or CANs together but does not extend beyond the boundaries of the immediate town, city, or metropolitan area. Multiple routers , switches and hubs are connected to create a MAN. 5. Wide Area

21、 Network ( WAN ) WAN is a data communication network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. 2WAN technologies generally function at the lower three layers of the OSI reference model: the phy

22、sical layer, the data link layer, and the network layer.5. 1 Computer Network Basics上一页下一页返回6. InternetworkInternetwork is two or more networks or network segments connected using devices that operate at layer 3 ( the “network” layer ) of the OSI Basic Reference Model, such as a router. Any intercon

23、nection among or between public, private, commercial, industrial, or governmental networks may also be defined as an Internetwork. ( 1 ) Internet. It is a specific Internetwork, consisting of a worldwide interconnection of governmental, academic , public, and private networks based upon the Advanced

24、 Research Projects Agency Network ( ARPANET ) developed by ARPA of the U. S. Department of Defense3also home to the World Wide Web ( WWW ) and referred to as the “Internet” with a capital “ I” to distinguish it from other generic Internetworks. 5. 1 Computer Network Basics上一页下一页返回( 2 ) Extranet and

25、intranet. Extranet is a network or Internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities. For example, a companys customers may be provided a

26、ccess to some part of its intranet thusly creating an extranet while at the same time the customers may not be considered “trusted” from a security standpoint. 4 Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot

27、 consist of a single LAN, because an extranet must have at least one connection with an outside network. 5Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet withou

28、t proper authorization. The Internet itself is not considered to be a part of the intranet or extranet, although the Internet may serve as a portal for access to portions of an extranet. 5. 1 Computer Network Basics上一页返回 In recent years, Internet changes our life a lot. We use E-mail and Internet ph

29、one to talk with our friends, and we get up-to-date information through web and we do shopping in the cyber market. Internet has many advantages over traditional communication channels, e. g. its cost effective, and it delivers information fast and it is not restricted by time and place. The more pe

30、ople use Internet, the more concerns about Internet security. 6In person-to-person community, security is based on physical cues. To name but a few, we use our signature to authenticate ourselves；we seal letters to prevent others inspection and modification；we receive receipt with the shops chop to

31、make sure we paid；we get information from a reliable source. But in the Internet society, no such physical cue is available. There are two areas that we concern about in Internet communication. The first one is secrecy-how we can ensure no one reads the data during its transmission. The second one i

32、s authentication-how we can be sure of the identity of someone ( a person or a computer ) claiming “who it is. ”5. 2 Computer Network Security下一页返回5. 2. 1 CryptographyEncryption can be used to protect data in transit as well as data in storage. Some vendors provide hardware encryption devices that c

33、an be used to encrypt data. There are also software encryption packages which are available either commercially or as free software. Encryption can be defined as the process of tasking information that exists in some readable form ( plain text ) and converting it into a form ( cipher text ) so that

34、it cannot be understood by others. If the receiver of the encrypted data wants to read the original data, the receiver must convert it back to the original through a process called decryption. Decryption is the inverse of the encryption process. In order to perform the decryption, the receiver must

35、be in possession of a special piece of data called the key. The two main competing cryptography schemes are known as the secret-key ( symmetric ) system and the public-key ( asymmetric ) system. The secret-key system uses a single, wholly secret sequence both to encrypt and to decrypt messages. 5. 2

36、 Computer Network Security上一页下一页返回The public key system uses a pair of mathematically related sequences, one for encryption and the other for decryption. Because secret-key systems use only one key, care must be taken that the key is communicated only to authorized parties. This means that it must b

37、e sent either in person or through a set of protocols. In contrast, users of public-key systems can distribute their public keys to allow others to encrypt messages that can then be read only with the private key. Therefore, they do not require the complicated key exchange protocols needed by secret

38、-key systems. Public-key systems also allow electric signatures and data integrity checks. One disadvantage of public-key systems is that they are slower than secret-key systems. 7 Some applications try to combine public-key and secret-key cryptography to achieve security and performance. Many encry

39、ption packages that are basically secret-key systems use public-key encryption to communicate the secret session keys. The secret-key systems then encrypt the data. 5. 2 Computer Network Security上一页下一页返回In both public-key and secret-key systems there is a problem of keeping the secret/private keys s

40、afe. Some systems have tried encoding keys on smart cards, but these must be kept physically secure and each computer must be equipped with a card reader. 5. 2. 2 Encryption on the InternetModern Web browsers use the Secure Sockets Layer ( SSL ) protocol for secure transactions like E-commerce purch

41、ases and E-bank. SSL works by using a public key for encryption and a different private key for decryption. Because SSL encryption depends so heavily on keys, one normally measures the effectiveness or strength of SSL encryption in terms of key length-number of bits in the key. The early implementat

42、ions of SSL in Web browsers, first Netscape 3 and then Microsoft Internet Explorer 3，used a 40-bit SSL encryption standard. Unfortunately, 40-bit encryption proved too easy to decipher or crack in practice. To decipher an SSL communication, one simply needs to generate the correct decoding key. 5. 2

43、 Computer Network Security上一页下一页返回In cryptography, a common deciphering technique is brute-force decryption；essentially, using a computer to exhaustively calculate and try every possible key one by one. 2-bit encryption, for example, involves four possible key values: 00, 01, 10, and 11. 3-bit encry

44、ption involves eight possible values, and so on. Mathematically speaking, 2n possible values exist for an n-bit key. Compared with 40-bit encryption, 128-bit encryption offers 88 additional bits of key length. This translates to 288 240 possible values required for a brute force crack. Based on the

45、past history of improvements in computer performance, security experts expect that 128-bit encryption will work well on the Internet for at least the next ten years. 5. 2. 3 Digital Certificate and AuthenticationConsider this scenario: When A sends a message to B, A gets Bs public key from the Inter

46、net but how can A know the public key obtained actually belongs to B? Digital certificate emerges to solve this problem. 5. 2 Computer Network Security上一页下一页返回Digital certificate is an identity card counterpart in the computer society. When a person wants to get a digital certificate, he generates h

47、is own key pair, gives the public key as well as some proof of his identification to the Certificate Authority ( CA ) . CA will check the persons identification to assure the identity of the applicant. If the applicant is really the one “who claims to be, ”CA will issue a digital certificate, with t

48、he applicants name, E-mail address and the applicants public key, which is also signed digitally with the CAs private key. 8 When A wants to send B a message, instead of getting Bs public key, A now has to get Bs digital certificate. A first checks the certificate authoritys signature with the CAs p

49、ublic key to make sure its a trustworthy certificate. Then A obtains Bs public key from the certificate, and uses it to encrypt messages and sends to B. Authentication is an important part of everyday life. The lack of strong authentication has inhibited the development of electronic commerce. It is

50、 still necessary for contracts, legal documents and official letters to be produced on paper. 5. 2 Computer Network Security上一页下一页返回Strong authentication is then a key requirement if the Internet is to be used for electronic commerce. Strong authentication is generally based on modern equivalents of

51、 the one time pad. For example, tokens are used in place of one-time pads and are stored on smart cards or disks. 5. 2. 4 Digital SignatureFor ages, special seals or handwritten signatures on documents have served as proof of authorship of, or agreement with, the contents of a document. Several attr

52、ibutes make the use of handwritten signatures compelling. These include the following. ( 1 ) A signature is not forgeable and serves as proof that the signer deliberately signed the document. ( 2 ) A signature is authentic and convinces the recipient that the signer deliberately signed the document.

53、 ( 3 ) A signature is not reusable. It s a part of the document, and an unscrupulous person cannot transfer it to a different document. ( 4 ) Once signed, a document is unalterable. 5. 2 Computer Network Security上一页下一页返回( 5 ) A signature cannot be repudiated. Since the signature and the document are

54、 physical objects, the signer cannot later claim that he or she didnt sign it. It would be nice to have non-forgeable signatures on computer documents, but there are problems with this concept. First, bit streams are easy to copy；the mere presence of such a signature means nothing. Even if a persons

55、 signature was made difficult to forge-if it was accomplished by a graphical image of a fingerprint, for example-with todays cut-and- paste software it is all too easy to move a valid signature from one document to another. Second, documents are easy to modify after they are signed, without leaving

56、any evidence of modification. A digital signature is an electronic signature that can be used to authenticate the identity of the sender of a message or the signer of a document, and possibly to ensure that the original content of the message or document that has been sent is unchanged. A digital si

57、gnature is a string of bits attached to an electronic document, which could be a word processing file or an E-mail message. 5. 2 Computer Network Security上一页下一页返回This bit string is generated by the signer, and it is based on both the documents data and the persons secret password. Digital signatures

58、 are easily transportable, cannot be imitated by someone else, and can be automatically time-stamped. Someone who receives the document can prove that the signer actually signed the document. If the document is altered, the signer can also prove that he did not sign the altered document. A digital s

59、ignature can be used with any kind of message, whether it is encrypted or not, simply so that the receiver can be sure of the senders identity and that the message arrived intact. Public-key cryptography can be used for digital signatures, public-key cryptography uses special encryption algorithms w

60、ith two different keys: a public-key that everyone knows, and a private key that only the person knows. Public-key algorithms encrypt the contents of an electronic document using both keys. The resulting file is an amalgam of both the public and private keys and the original documents contents. 5. 2