計(jì)算機(jī)專業(yè)畢業(yè)設(shè)計(jì)外文翻譯--互聯(lián)網(wǎng)絡(luò)分層模型

1、<p>　　附錄A 外文翻譯-原文部分</p><p>　　Internetworking and Layered Models</p><p>　　The Internet today is a widespread information infrastructure, but it is inherently an insecure channel for sending

2、messages. When a message (or packet) is sent from one Website to another, the data contained in the message are routed through a number of intermediate sites before reaching its destination. The Internet was designed to

3、accommodate heterogeneous platforms so that people who are using different computers and operating systems can communicate. The history of the Internet is complex a</p><p>　　Early arpanet researchers accompl

4、ished the initial demonstrations of packetswitching technology. In the late 1970s, the growth of the Internet was recognised and subsequently a growth in the size of the interested research community was accompanied by a

5、n increased need for a coordination mechanism. The Defense Advanced Research Projects Agency (DARPA) then formed an International Cooperation Board (ICB) to coordinate activities with some European countries centered on

6、packet satellite research, </p><p>　　review body. </p><p>　　Since the early 1980s, the Internet has grown beyond its primarily research roots, to include both a broad user community and increase

7、d commercial activity. This growth in the commercial sector brought increasing concern regarding the standards process. Increased attention was paid to making progress, eventually leading to the formation of the Internet

8、 Society in 1991. In 1992, the Internet Activities Board was reorganised and renamed the Internet Architecture board (IAB) operating under the aus</p><p>　　1.1 Networking Technology</p><p>　　Dat

9、a signals are transmitted from one device to another using one or more types of transmission media, including twisted-pair cable, coaxial cable and fibre-opticable. A message to be transmitted is the basic unit of networ

10、k communications. A message may consist of one or more cells, frames or packets which are the elemental units for network communications. Networking technology includes everything from local area networks (LANs) in a lim

11、ited geographic area such as a single building, departmen</p><p>　　1.1.1 Local Area Networks (LANs)</p><p>　　A local area network (LAN) is a communication system that allows a number of independ

12、ent devices to communicate directly with each other in a limited geographic area such as a single office building, a warehouse or a campus. LANs are standardised by three architectural structures: Ethernet, token ring an

13、d fibre distributed data interface (FDDI).</p><p>　　(1) Ethernet</p><p>　　Ethernet is a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment

14、 Corporation (DEC), Intel Corporation and Xerox. The access mechanism used in an Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In CSMA/CD, before a station transmits data, it must c

15、heck the medium where any other station is currently using the medium. If no other station is transmitting, the station can send its data. If two or more statio</p><p>　　(2) Token Ring</p><p>　　

16、Token ring, a LAN standard originally developed by IBM, uses a logical ring topology. The access method used by CSMA/CD may result in collisions. Therefore, stations may attempt to send data many times before a transmiss

17、ion captures a perfect link. This redundancy can create delays of indeterminable length if traffic is heavy. There is no way to predict either the occurrence of collisions or the delays produced by multiple stations atte

18、mpting to capture the link at the same time. Token ring reso</p><p>　　(3) Fiber Distributed Data Interface (FDDI)</p><p>　　FDDI is a LAN protocol standardised by ANSI and ITU-T. It supports data

19、 rates of 100 Mbps and provides a high-speed alternative to Ethernet and token ring. When FDDI was designed, the data rate of 100 Mbps required fibre-optic cable. The access method in FDDI is also called token passing. I

20、n a token ring network, a station can send only one frame each time it captures the token. In FDDI, the token passing mechanism is slightly different in that access is limited by time. Each station keeps a tim</p>

21、<p>　　1.1.2 Wide Area Networks (WANs)</p><p>　　A WAN provides long-distance transmission of data, voice, image and video informationover large geographical areas that may comprise a country, a continen

22、t or even the world.In contrast to LANs (which depend on their own hardware for transmission), WANs can utilise public, leased or private communication devices, usually in combination.</p><p><b>　　(1)

23、PPP</b></p><p>　　The Point-to-Point Protocol (PPP) is designed to handle the transfer of data using either asynchronous modem links or high-speed synchronous leased lines.</p><p>　　The PPP

24、 frame uses the following format:</p><p>　　①Flag field: Each frame starts with a one-byte flag whose value is 7E(0111 1110). The flag is used for synchronisation at the bit level between the sender and recei

25、ver.</p><p>　?、贏ddress field: This field has the value of FF(1111 1111).</p><p>　　③Control field: This field has the value of 03(0000 0011).</p><p>　　④Protocol field: This is a two-

26、byte field whose value is 0021(0000 0000 0010 0001) for TCP/IP.</p><p>　?、?Data field: The data field ranges up to 1500 bytes.</p><p>　　⑥CRC: This is a two-byte cyclic redundancy check. Cyclic r

27、edundancy check (CRC) is </p><p>　　implemented in the physical layer for use in the data link layer. A sequence of redundant bits (CRC) is appended to the end of a data unit so that the resulting data unit b

28、ecomes exactly divisible by a predetermined binary number. At its destination, the incoming data unit is divided by the same number. If there is no remainder, the data unit is accepted. If a remainder exists, the data un

29、it has been damaged in transit and therefore must be rejected.</p><p><b>　　(2) X.25</b></p><p>　　X.25 is widely used, as the packet switching protocol provided for use in a WAN. It w

30、as developed by the ITU-T in 1976. X.25 is an interface between data terminal equipment and data circuit terminating equipment for terminal operations at the packet mode on a public data network. X.25 defines how a pack

31、et mode terminal can be connected to a packet network for the exchange of data. It describes the procedures necessary for establishing connection, data exchange, acknowledgement, flow control and</p><p>　　(3

32、)Asynchronous Transfer Mode (ATM)</p><p>　　ATM is a revolutionary idea for restructuring the infrastructure of data communication. It is designed to support the transmission of data, voice and video through

33、a high data-rate transmission medium such as fibre-optic cable. ATM is a protocol for transferring cells. A cell is a small data unit of 53 bytes long, made of a 5-byte header and a 48-byte payload.</p><p>　

34、　The header contains a virtual path identifier (VPI) and a virtual channel identifier (VCI). These two identifiers are used to route the cell through the network to the final destination. An ATM network is a connection-o

35、riented cell switching network. This means that the unit of data is not a packet as in a packet switching network, or a frame as in a frame relay, but a cell. However, ATM, like X.25 and frame relay, is a connection-orie

36、nted network, which means that before two systems can communi</p><p>　　1.2 Connecting Devices</p><p>　　Connecting devices are used to connect the segments of a network together or to connect net

37、works to create an internetwork. These devices are classified into five categories: switches, repeaters, bridges, routers and gateways. Each of these devices except the first one (switches) interacts with protocols at di

38、fferent layers of the OSI model. Repeaters forward all electrical signals and are active only at the physical layer. Bridges store and forward complete packets and affect the flow control o</p><p>　　Figure 1

39、-1</p><p>　　1.2.1 Switches</p><p>　　A switched network consists of a series of interlinked switches. Switches are hardware software devices capable of creating temporary connections between two

40、or moredevices to the switch but not to each other. Switching mechanisms are generally classifiedinto three methods: circuit switching, packet switching and message switching.</p><p>　　(1)Circuit switching c

41、reates a direct physical connection between two devices such as telephones or computers. Once a connection is made between two systems, circuit switching creates a dedicated path between two end users. The end users can

42、use the path for as long as they want.</p><p>　　(2) Packet switching is one way to provide a reasonable solution for data transmission. In a packet-switched network, data are transmitted in discrete units of

43、 variable-length blocks called packets. Each packet contains not only data, but also a header with control information. The packets are sent over the network node to node. At each node, the packet is stored briefly befor

44、e being routed according to the information in its header. In the datagram approach to packet switching, each packet is t</p><p>　　1.2.2 Bridges</p><p>　　Bridges operate in both the physical and

45、 the data link layers of the OSI model. A single bridge connects different types of networks together and promotes interconnectivity between networks. Bridges divide a large network into smaller segments. Unlike repeater

46、s, bridges contain logic that allows them to keep separate the traffic for each segment. Bridges are smart enough to relay a frame towards the intended recipient so that traffic can be filtered. In fact, this filtering o

47、peration makes bridg</p><p>　　1.2.3 Routers</p><p>　　Routers operate in the physical, data link and network layers of the OSI model. The Internet is a combination of networks connected by router

48、s. When a datagram goes from a source to a destination, it will probably pass through many routers until it reaches the router attached to the destination network. Routers determine the path a packet should take. Routers

49、 relay packets among multiple interconnected networks. In particular, an IP router forwards IP datagrams among the networks to which it con</p><p>　　(1)The routing module receives an IP packet from the proce

50、ssing module. If the packet is to be forwarded, it should be passed to the routing module. It finds the IP address of the next station along with the interface number from which the packet should be sent. It then sends t

51、he packet with information to the fragmentation module. The fragmentation module consults the MTU table to find the maximum transfer unit</p><p>　　(MTU) for the specific interface number.</p><p>

52、;　　(2)The routing table is used by the routing module to determine the next-hop address of the packet. Every router keeps a routing table that has one entry for each destination network. The entry consists of the destina

53、tion network IP address, the shortest distance to reach the destination in hop count, and the next router (next hop) to which the packet should be delivered to reach its final destination. The hop count is the number of

54、networks a packet enters to reach its final destination. A rout</p><p>　　particular router is equal to the sum of the metrics of networks that comprise the</p><p>　　route. A router chooses the r

55、oute with the shortest (smallest value) metric. The metric</p><p>　　assigned to each network depends on the type of protocol. The Routing Information</p><p>　　Protocol (RIP) treats each network

56、as one hop count. So if a packet passes through 10</p><p>　　networks to reach the destination, the total cost is 10 hop counts. The Open Shortest</p><p>　　Path First protocol (OSPF) allows the a

57、dministrator to assign a cost for passing through a network based on the type of service required. A route through a network can have different metrics (costs). OSPF allows each router to have several routing tables base

58、d on the required type of service. The Border Gateway Protocol (BGP) defines the metric should specify the optimum path for the packet. The table can be either static or dynamic. A static table is one that is not changed

59、 frequently, but a dy</p><p>　　(3)A metric is a cost assigned for passing through a network. The total metric of a particular router is equal to the sum of the metrics of networks that comprise the</p>

60、<p>　　route. A router chooses the route with the shortest (smallest value) metric. The metric</p><p>　　assigned to each network depends on the type of protocol. The Routing Information</p><

61、p>　　Protocol (RIP) treats each network as one hop count. So if a packet passes through 10</p><p>　　networks to reach the destination, the total cost is 10 hop counts. The Open Shortest</p><p>

62、;　　Path First protocol (OSPF) allows the administrator to assign a cost for passing through a network based on the type of service required. A route through a network can have different metrics (costs). OSPF allows each

63、router to have several routing tables based on the required type of service. The Border Gateway Protocol (BGP) defines the metric totally differently. The policy criterion in BGP is set by the administrator. The policy A

64、defines the paths that should be chosen.</p><p>　　附錄B 外文翻譯-譯文部分</p><p><b>　　互聯(lián)網(wǎng)絡(luò)分層模型</b></p><p>　　今天，互聯(lián)網(wǎng)是一個(gè)廣泛的信息基礎(chǔ)設(shè)施，而它本質(zhì)上是一個(gè)不安全的發(fā)送郵件的通道。當(dāng)一個(gè)消息（或分組）從一個(gè)網(wǎng)站發(fā)送到另一個(gè)網(wǎng)站時(shí)，在消息中包含的

65、數(shù)據(jù)是通過一個(gè)路由的數(shù)目到達(dá)目的地之前的中間點(diǎn)?；ヂ?lián)網(wǎng)被設(shè)計(jì)為容納異構(gòu)平臺(tái)，使人們使用不同電腦進(jìn)行溝通的操作系統(tǒng)?；ヂ?lián)網(wǎng)的歷史是復(fù)雜的，涉及許多方面 ——技術(shù)，組織和社區(qū)。</p><p>　　早期ARPANET的研究人員完成了最初的packetswitching技術(shù)。在70年代末，隨著互聯(lián)網(wǎng)的增長，后來在研究界的關(guān)注規(guī)模也增大了，于此同時(shí)協(xié)調(diào)機(jī)制也需要增加。美國國防高級研究項(xiàng)目局（DARPA），成立了一個(gè)國際合

66、作委員會(huì)（工商銀行），一些歐洲國家以配合衛(wèi)星研究為中心的數(shù)據(jù)包的活動(dòng)，或者說因特網(wǎng)配置控制委員會(huì)（局）協(xié)助DARPA的管理互聯(lián)網(wǎng)活動(dòng)。1983年，DARPA認(rèn)識(shí)到，互聯(lián)網(wǎng)的持續(xù)增長導(dǎo)致了社會(huì)所要求的協(xié)調(diào)機(jī)制因此而轉(zhuǎn)型。IAB的振興互聯(lián)網(wǎng)工程任務(wù)組（IETF）為一對IAB的成員。到1985年有了巨大的增長，在實(shí)際工程方面，互聯(lián)網(wǎng)的這種增長引起了IETF的工作小組形式。 DARPA已不再是主要參與互聯(lián)網(wǎng)的資金。從那時(shí)起，互聯(lián)網(wǎng)一直是降低DA

67、RPA的活動(dòng)。 IAB公司認(rèn)識(shí)到IETF的重要性日益增加，并改組為主要標(biāo)準(zhǔn)的Internet工程指導(dǎo)小組（IESG）審查機(jī)構(gòu)。</p><p>　　20世紀(jì)80年代初以來，互聯(lián)網(wǎng)已經(jīng)超越了其主要研究的項(xiàng)目，包括廣泛的用戶群，以此來增加商業(yè)活動(dòng)。這一增長在商業(yè)界帶來了越來越多的關(guān)注。增加重視并取得進(jìn)展，最終在1991年形成了互聯(lián)網(wǎng)協(xié)會(huì)。1992年，互聯(lián)網(wǎng)活動(dòng)委員會(huì)進(jìn)行了重組和改名,互聯(lián)網(wǎng)架構(gòu)委員會(huì)（IAB）在互聯(lián)網(wǎng)

68、的贊助下運(yùn)作。IAB中，IESG和IETF相互支持的關(guān)系導(dǎo)致他們采取更多的措施，審批標(biāo)準(zhǔn)，隨著規(guī)定服務(wù)和其他措施,將促進(jìn)IETF的工作。</p><p><b>　　1.1網(wǎng)絡(luò)技術(shù)</b></p><p>　　數(shù)據(jù)信號(hào)從一個(gè)設(shè)備傳送到另一個(gè)設(shè)備時(shí)需使用一個(gè)或多個(gè)類型的傳輸介質(zhì)，包括雙絞線電纜，同軸電纜和光纖電纜。一個(gè)信息傳送是網(wǎng)絡(luò)通信的基本單位。一個(gè)消息可能是一個(gè)或多

69、個(gè)單元格，幀或數(shù)據(jù)包為單位的網(wǎng)絡(luò)元素通訊。網(wǎng)絡(luò)技術(shù)包括一切從局域網(wǎng)在有限的地理區(qū)域，如單個(gè)建設(shè)，部門或校園廣域網(wǎng)（WAN）在較大的地理區(qū)域，可能包括一個(gè)國家，非洲大陸乃至整個(gè)世界。</p><p>　　1.1.1局部區(qū)域網(wǎng)絡(luò)（局域網(wǎng)）</p><p>　　局域網(wǎng)（LAN）對本地區(qū)域網(wǎng)絡(luò)（LAN）是一種通信系統(tǒng)，允許獨(dú)立的數(shù)目設(shè)備直接與每一個(gè)有限的地理區(qū)域等作為一個(gè)單一的辦公樓，倉庫或校園。

70、三是規(guī)范局域網(wǎng)建筑結(jié)構(gòu)：以太網(wǎng)，令牌環(huán)網(wǎng)和光纖分布式數(shù)據(jù)接口（FDDI）。</p><p><b>　　(1)以太網(wǎng)</b></p><p>　　以太網(wǎng)是局域網(wǎng)標(biāo)準(zhǔn),最初是由施樂公司開發(fā)的，后來延長了聯(lián)合企業(yè)之間的數(shù)字設(shè)備公司（DEC），英特爾公司和施樂公司。訪問機(jī)制用在以太網(wǎng)稱為載波偵聽多路訪問碰撞檢測。在CSMA / CD中，前一站傳送的數(shù)據(jù)，它必須檢查目前使用的

71、媒介。如果沒有其他發(fā)射站，該站可發(fā)送其數(shù)據(jù)。如果在兩個(gè)或更多的站發(fā)送數(shù)據(jù)的同時(shí)，那它可能會(huì)導(dǎo)致沖突。因此，要不斷檢查各站的任何碰撞。如果碰撞發(fā)生時(shí)，所有站的數(shù)據(jù)可忽略收到。發(fā)送站等待一段時(shí)間才能重新發(fā)送數(shù)據(jù)。以至減少第二碰撞的可能性，發(fā)送站隨機(jī)生成的決定因素是應(yīng)該等待多久才能重新發(fā)送數(shù)據(jù)。</p><p><b>　　(2)令牌環(huán)</b></p><p>　　令牌環(huán)網(wǎng)

72、，局域網(wǎng)標(biāo)準(zhǔn)最初是由IBM開發(fā)，采用了邏輯環(huán)的拓?fù)?。訪問方法使用CSMA / CD的可能導(dǎo)致沖突。因此，車站的互聯(lián)網(wǎng)絡(luò)和三分層模型前一個(gè)傳輸，嘗試將數(shù)據(jù)發(fā)送多次捕捉一個(gè)完美的鏈接。這種冗余可以創(chuàng)建不可估計(jì)長度的延遲，如果在交通繁忙的時(shí)候，有沒有辦法試圖預(yù)測碰撞和多個(gè)電視臺(tái)制作延誤在同一時(shí)間的聯(lián)系。令牌環(huán)網(wǎng)解決這種不確定性使站輪流發(fā)送數(shù)據(jù)。作為訪問方法，令牌傳遞順序從車站到車站，直到遇到一個(gè)站發(fā)送數(shù)據(jù)。該站為發(fā)送的數(shù)據(jù)等待令牌。該車站然后

73、捕獲令牌和發(fā)送的數(shù)據(jù)幀。此數(shù)據(jù)幀左右的收益環(huán)，每個(gè)站再生框架。每個(gè)中間站檢查目的地址，如果發(fā)現(xiàn)該幀是給另一個(gè)車站，它繼電器鄰近車站。預(yù)期的收件人確認(rèn)自己的地址，復(fù)制信息，進(jìn)行錯(cuò)誤檢查和變化中的四個(gè)幀的最后一個(gè)字節(jié)表示位該地址已被確認(rèn)和幀復(fù)制。完整的數(shù)據(jù)包，然后周圍環(huán)，直到它返回到車站。</p><p>　　(3)光纖分布式數(shù)據(jù)接口（FDDI）</p><p>　　FDDI的網(wǎng)絡(luò)協(xié)議是一個(gè)由

74、ANSI和ITU - T的標(biāo)準(zhǔn)化。它支持的數(shù)據(jù)傳輸速率100 Mbps和提供高速替代以太網(wǎng)和令牌環(huán)。當(dāng)光纖分布式數(shù)據(jù)介面是設(shè)計(jì)，100 Mbps的數(shù)據(jù)速率所需的光纖電纜。在FDDI的訪問方法又稱為令牌傳遞。在令牌環(huán)網(wǎng)，一站可以只發(fā)送一幀每次捕獲令牌。在光纖分布式數(shù)據(jù)介面，令牌傳遞機(jī)制略有不同的是訪問時(shí)間的限制。每個(gè)站保持一個(gè)計(jì)時(shí)器，顯示該令牌時(shí)，應(yīng)離開車站。如果一個(gè)站接收指定的令牌時(shí)間早，它可以保持令牌和發(fā)送數(shù)據(jù)，直到離開預(yù)定的時(shí)間。另

75、一方面，如果一個(gè)站接收到令牌在指定時(shí)間或晚于這個(gè)時(shí)候，應(yīng)該讓令牌傳遞給下一個(gè)車站，等待它的下一個(gè)轉(zhuǎn)折。FDDI的實(shí)現(xiàn)為雙環(huán)。在大多數(shù)情況下，數(shù)據(jù)傳輸，僅局限于主環(huán)。二次環(huán)是提供了與主環(huán)的故障情況。一個(gè)問題在主環(huán)發(fā)生時(shí)，第二圈可激活完成數(shù)據(jù)電路和維護(hù)服務(wù)。</p><p>　　1.1.2廣域網(wǎng)（WAN） </p><p>　　一個(gè)廣域網(wǎng)提供長途數(shù)據(jù)，語音，圖像和視頻傳輸?shù)男畔?在廣大的地理區(qū)

76、域，可能包括一個(gè)國家，一個(gè)大陸乃至世界。 相對于局域網(wǎng)，廣域網(wǎng)可以利用公共出租或私人通訊設(shè)備，通常將他們結(jié)合使用。 </p><p><b>　　(1)購買力平價(jià) </b></p><p>　　點(diǎn)對點(diǎn)協(xié)議（PPP）是用于設(shè)計(jì)處理數(shù)據(jù)傳輸或使用異步調(diào)制解調(diào)器鏈接或高速同步租用線路。PPP幀使用格式如下：</p><p>　?、贅?biāo)志字段：每個(gè)幀開頭

77、有一個(gè)字節(jié)標(biāo)志，它的值是7E（0111 1110）。該標(biāo)志用于同步在發(fā)送者和接收者之間的水平位。 </p><p>　?、诘刂窓冢捍藱谟蟹ɡ桑?111 1111）值。 </p><p>　?、劭刂谱侄危涸撟侄蔚闹?3（0000 0011）。 </p><p>　?、軈f(xié)議字段：這是一個(gè)兩個(gè)字節(jié)的字段的值是0021（0000 0000 0010 0001） 為TCP /

78、 IP協(xié)議。 </p><p>　?、輸?shù)據(jù)域：數(shù)據(jù)域范圍達(dá)1500個(gè)字節(jié)。 </p><p>　?、奕A潤：這是一個(gè)兩個(gè)字節(jié)的循環(huán)冗余校驗(yàn)。循環(huán)冗余校驗(yàn)（CRC） 實(shí)現(xiàn)在物理層的數(shù)據(jù)鏈路層。一個(gè)序列 （CRC）的冗余位是追加到一個(gè)數(shù)據(jù)單元結(jié)束時(shí)，使所產(chǎn)生的數(shù)據(jù) 單位變成了一個(gè)預(yù)定的整除的二進(jìn)制數(shù)。在它的目的地，傳入的數(shù)據(jù)單元除以相同的號(hào)碼。如果沒有余下的時(shí)間， 數(shù)據(jù)單元被接受。如果其余的存在

79、，數(shù)據(jù)單位已在運(yùn)輸途中受損 因此必須予以拒絕。</p><p><b>　　(2) X.25 </b></p><p>　　X.25作為交換成廣域網(wǎng)的使用提供了一個(gè)協(xié)議數(shù)據(jù)包。這是由ITU - T制定于1976年。X.25是一個(gè)終端設(shè)備之間的數(shù)據(jù)接口和數(shù)據(jù)電路終端行動(dòng)終止在設(shè)備上的數(shù)據(jù)包模式公共數(shù)據(jù)網(wǎng)絡(luò)。X.25定義了如何將一個(gè)終端連接到一個(gè)數(shù)據(jù)包網(wǎng)絡(luò)的數(shù)據(jù)交換。它描

80、述了必要的程序，建立了連接和數(shù)據(jù)的交換，然后確認(rèn)流量控制和數(shù)據(jù)控制。 </p><p>　　(3)異步傳輸模式（ATM） </p><p>　　ATM是一種重組的數(shù)據(jù)通信基礎(chǔ)設(shè)施的革命思想。它旨在通過支持高數(shù)據(jù)率的數(shù)據(jù)傳輸，語音和視頻傳輸介質(zhì)，如光纖電纜。ATM是一個(gè)細(xì)胞轉(zhuǎn)移協(xié)議。一 個(gè)細(xì)胞是一個(gè)53字節(jié)的小數(shù)據(jù)單元長，一個(gè)5字節(jié)的報(bào)頭，并建立一個(gè)48字節(jié)的有效載荷。 互聯(lián)網(wǎng)絡(luò)和5分層模型

81、標(biāo)頭包含一個(gè)虛擬路徑標(biāo)識(shí)符（VPI）和一個(gè)虛擬通道標(biāo)識(shí)符（VCI的）。 這兩個(gè)標(biāo)識(shí)符用于路由通過網(wǎng)絡(luò)向最終目的地的細(xì)胞。 ATM網(wǎng)絡(luò)是一個(gè)面向連接的細(xì)胞交換網(wǎng)。然而，自動(dòng)柜員機(jī)，如X.25和幀中繼是面向連接的網(wǎng)絡(luò)， 這意味著前兩個(gè)系統(tǒng)可以溝通，他們必須進(jìn)行連接。至 啟動(dòng)一個(gè)連接，系統(tǒng)使用一個(gè)20字節(jié)的地址。</p><p><b>　　1.2連接器件</b></p><p

82、>　　連接設(shè)備是用于將網(wǎng)絡(luò)的一個(gè)部分連接在一起或互聯(lián)的網(wǎng)絡(luò)。這些設(shè)備可分為五類：交換機(jī)，中繼器，網(wǎng)橋，路由器和網(wǎng)關(guān)。這些設(shè)備的每一個(gè)除第一（交換機(jī)）與交互在OSI模型的不同層協(xié)議.中繼器積極的轉(zhuǎn)發(fā)所有的電子信號(hào)，只在物理層。網(wǎng)橋完整的存儲(chǔ)和轉(zhuǎn)發(fā)數(shù)據(jù)包并影響一個(gè)單一的局域網(wǎng)流量控制。網(wǎng)橋活躍在物理和數(shù)據(jù)鏈路層。路由器提供兩個(gè)單獨(dú)的鏈接局域網(wǎng)和活躍于物理層，數(shù)據(jù)鏈路層和網(wǎng)絡(luò)層。最后，網(wǎng)關(guān)不兼容的局域網(wǎng)之間提供翻譯服務(wù)或應(yīng)用程序，并積極

83、在所有層次。連接設(shè)備，在OSI模型的不同層協(xié)議見圖1-1:</p><p><b>　　圖 1-1</b></p><p><b>　　1.2.1開關(guān)</b></p><p>　　交換式網(wǎng)絡(luò)由一組相互關(guān)聯(lián)的開關(guān)系列組成。交換機(jī)是在硬件/軟件設(shè)備之間建立的，它是連接兩個(gè)或多個(gè)臨時(shí)網(wǎng)絡(luò)到交換機(jī)的設(shè)備。開關(guān)機(jī)制一般分為三種方法：

84、電路開關(guān)，分組交換和信息交換。</p><p>　　(1)電路交換兩個(gè)設(shè)備之間建立了直接的物理連接，如電話或電腦。一旦建立了連接了兩個(gè)系統(tǒng)，那么電路開關(guān)兩端就會(huì)建立一個(gè)用戶專用的道路。</p><p>　　(2)分組交換是一種為數(shù)據(jù)傳輸提供合理的解決方案的設(shè)備。每個(gè)數(shù)據(jù)包不僅包含數(shù)據(jù)，而且還可控制信息。數(shù)據(jù)包通過網(wǎng)絡(luò)發(fā)送節(jié)點(diǎn)到節(jié)點(diǎn)信息。在分組交換數(shù)據(jù)包方式，每個(gè)數(shù)據(jù)包被視為是獨(dú)立的，它是單

85、獨(dú)存在的，在虛電路方式分組交換中，如果是是發(fā)送者和接收者之間選擇在開始會(huì)議期間的，所有的數(shù)據(jù)包會(huì)沿著這條路線一個(gè)接一個(gè)。盡管這兩種方法似乎是相同的，但是它們之間存在著根本區(qū)別。在電路交換中，兩者之間的最終用戶的路徑只包含一個(gè)通道。在這種方法中，計(jì)算機(jī)（或節(jié)點(diǎn)）接收到相應(yīng)的路由信息,那么它是免費(fèi)的，然后發(fā)送它,該方法已被淘汰。</p><p><b>　　1.2.2 網(wǎng)橋</b></p&

86、gt;<p>　　網(wǎng)橋橫跨在兩個(gè)物理鏈路層和數(shù)據(jù)鏈路層的OSI模型間，將不同類型的網(wǎng)絡(luò)互聯(lián)起來，并促進(jìn)網(wǎng)絡(luò)之間的聯(lián)系。網(wǎng)橋部分劃分成較小的一個(gè)大型網(wǎng)絡(luò)。網(wǎng)橋有足夠的能力朝著預(yù)期的收件人，使之能夠被過濾。其實(shí)，這使得網(wǎng)橋的過濾操作控制功能有用，并使得能夠安全通過這種交通分區(qū)。網(wǎng)橋可以訪問連接到它的所有車站的物理地址。當(dāng)幀進(jìn)入網(wǎng)橋，網(wǎng)橋不僅重新生成了信號(hào)，而且還檢查地址，目的地和新的副本轉(zhuǎn)發(fā)給該段地址所屬。當(dāng)遇到一包的網(wǎng)橋，它

87、讀取包含在框架中的地址并比較所有環(huán)節(jié)上站表的地址。當(dāng)它找到互聯(lián)網(wǎng)絡(luò)和7種分層模型時(shí)，就會(huì)發(fā)現(xiàn)有部分車站所屬的中繼到該數(shù)據(jù)包片段。</p><p><b>　　1.2.3路由器 </b></p><p>　　路由器工作在物理層，數(shù)據(jù)鏈路和網(wǎng)絡(luò)層的OSI模型。該互聯(lián)網(wǎng)是由路由器連接的網(wǎng)絡(luò)的結(jié)合。路由器使用 在數(shù)據(jù)報(bào)目的地的地址選擇下跳到轉(zhuǎn)發(fā)數(shù)據(jù)報(bào)。 路由器 提供連接到許多

88、不同的物理網(wǎng)絡(luò)類型：以太網(wǎng),令牌環(huán)，點(diǎn)至點(diǎn)鏈接，F(xiàn)DDI等。 </p><p>　　(1)模塊的路由處理模塊接收來自一個(gè)IP數(shù)據(jù)包。如果數(shù)據(jù)包要轉(zhuǎn)發(fā)，那它應(yīng)該傳遞給路由模塊。它發(fā)現(xiàn)IP地址應(yīng)該從該數(shù)據(jù)包接口數(shù)目的下一站被發(fā)送。然后處理被它發(fā)送的信息包的碎片模塊。該協(xié)商的成體系模塊表的MTU最大傳輸單元（MTU）為特定的接口。 </p><p>　　(2)路由器是由路由模塊用來確定下一跳地址

89、的數(shù)據(jù)包。每個(gè)路由器的路由表中保存了每一個(gè)目的地入境的網(wǎng)絡(luò)。該項(xiàng)目是目標(biāo)網(wǎng)絡(luò)的IP地址，最短距離 到達(dá)目的地的跳數(shù)，下一個(gè)路由器應(yīng)該以該數(shù)據(jù)包發(fā)送到最終目的地。路由器應(yīng)該是當(dāng)有一個(gè)路由表咨詢時(shí)，隨時(shí)可以轉(zhuǎn)發(fā)。路由表應(yīng)指定為數(shù)據(jù)包的最佳路徑。該表可以是靜態(tài)或動(dòng)態(tài)的。靜態(tài)表不是經(jīng)常更換的，而動(dòng)態(tài)表在自動(dòng)更新時(shí)，某處可能有網(wǎng)絡(luò)在變化。今天，互聯(lián)網(wǎng)需要?jiǎng)討B(tài)路由表來實(shí)現(xiàn)。 </p><p>　　(3)度量是通過網(wǎng)絡(luò)傳遞的分

90、配成本。一個(gè)總指標(biāo)特別是路由器等于組成該網(wǎng)絡(luò)的數(shù)據(jù)總和路線。路由器選擇具有最短（最小價(jià)值線）的指標(biāo)。度量分配給每個(gè)網(wǎng)絡(luò)取決于協(xié)議類型。路由信息協(xié)議（RIP）對待每一個(gè)網(wǎng)絡(luò)跳數(shù)為1。因此，如果一個(gè)數(shù)據(jù)包通過10次網(wǎng)絡(luò)到達(dá)目的地，總成本為10跳計(jì)數(shù)。開放式最短路徑優(yōu)先協(xié)議（OSPF）允許管理員指定途經(jīng)成本對服務(wù)所需的類型為基礎(chǔ)的網(wǎng)絡(luò)。一個(gè)通過網(wǎng)絡(luò)路由可以有不同的指標(biāo)。OSPF協(xié)議允許每個(gè)路由器有多個(gè)路由表構(gòu)成的所需服務(wù)類型，是邊界網(wǎng)關(guān)協(xié)議（