互聯(lián)網(wǎng)絡(luò)聯(lián)網(wǎng)的基本原理-3-廣域網(wǎng)(zslcn周生烈編譯摘注評(píng))(編輯修改稿)
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【文章內(nèi)容簡(jiǎn)介】 mers39。 sites by which packets of data are delivered from one to the other through the network. The section of the carrier39。s network that is shared is often referred to as a cloud. 分組包交換是一種WAN技術(shù),在那里用戶共享公共的承運(yùn)商資源。由于這能使承運(yùn)商更有效地使用其基礎(chǔ)結(jié)構(gòu),用戶所付出的代價(jià)會(huì)優(yōu)于點(diǎn)對(duì)點(diǎn)線路。在分組交換建立中,網(wǎng)絡(luò)已經(jīng)連入承運(yùn)商網(wǎng)絡(luò),并且許多用戶共享了承運(yùn)商的網(wǎng)絡(luò)。承運(yùn)商于是可以在用戶所在地之間創(chuàng)建虛電路,從而分組包數(shù)據(jù)就可以通過(guò)網(wǎng)絡(luò)從一處被交付到到另一處。承運(yùn)商的被共享的那部分網(wǎng)絡(luò)通常被稱(chēng)之為一片云。Some examples of packetswitching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and . Figure: Packet Switching Transfers Packets Across a Carrier Network shows an example packetswitched circuit. 分組交換網(wǎng)絡(luò)的一些例子包括ATM(異步傳輸模式),FR(幀中繼),SMDS(交換式多兆位數(shù)據(jù)服務(wù)),。 Figure: Packet Switching Transfers Packets Across a Carrier 分組交換傳輸數(shù)據(jù)包通過(guò)承運(yùn)商網(wǎng)絡(luò)5. WAN Virtual Circuits WAN虛電路 The virtual connections between customer sites are often referred to as a virtual circuit. 用戶所在地之間的虛連接通常稱(chēng)之為一個(gè)虛電路。A virtual circuit is a logical circuit created within a shared network between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). 一個(gè)虛電路是一個(gè)在共享的網(wǎng)絡(luò)內(nèi)兩個(gè)網(wǎng)絡(luò)設(shè)備之間產(chǎn)生的邏輯電路。有兩類(lèi)虛電路：SVC(交換式虛電路)和PVC(永久性虛電路)。 SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is plete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.SVC是在需要時(shí)動(dòng)態(tài)建立的,而在傳送完成后被終止。在一個(gè)SVC上承載的通信由三個(gè)階段組成：電路建立,數(shù)據(jù)傳輸,和電路終止。電路建立階段涉及在源設(shè)備和目的設(shè)備之間創(chuàng)建虛電路。數(shù)據(jù)傳輸涉及在虛電路上的兩個(gè)設(shè)備間傳送數(shù)據(jù),而電路終止階段則涉及在源和目的設(shè)備間關(guān)閉虛電路。 SVCs用于設(shè)備間偶有數(shù)據(jù)傳輸?shù)那闆r。PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.PVC是一種永久建立的電路,它只有一種模式：數(shù)據(jù)傳輸模式,而且保持設(shè)備間進(jìn)行傳輸?shù)那闆r不變。PVCs減少了SVCs的使用帶寬,卻由于PVCs保持不變的可用性而增加了用戶的代價(jià)。PVCs通常是由服務(wù)提供商根據(jù)用戶要求服務(wù)的訂單配置的。6. WAN Dialup Services WAN撥號(hào)服務(wù)Dialup services offer costeffective methods for connectivity across WANs. Two popular dialup implementations are dialondemand routing (DDR) and dial backup.
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