广域网技术介绍.ppt

1、,CCNA 4 Chapter 1,WAN technologies (广域网技术 ),1.1 WAN Overview (广域网概述 ),A WAN is a data communications network that operates beyond the geographic scope of a LAN. 广域网是一种运行的地域超过局域网的数据通信网络。,各种速率的广域网链路,1.2 WAN devices and module（广域网设备与组件）,Devices on the subscriber premises are called customer premises eq

2、uipment (CPE, 客户端设备) The subscriber owns the CPE or leases the CPE from the service provider. A copper or fiber cable connects the CPE to the service providers nearest exchange or central office (CO, 中心局 ). This cabling is often called the local loop(本地环路 ), or last-mile“(最后一英里) .,example,In order f

3、or the local loop to carry data, a device such as a modem is needed to prepare the data for transmission. Devices that put data on the local loop are called data circuit-terminating equipment, or data communications equipment (DCE, 数据电路终端设备) .The customer devices that pass the data to the DCE are ca

4、lled data terminal equipment (DTE, 数据终端设备).,CSU/ DSU,The communications link needs signals in an appropriate format. For digital lines, a channel service unit (CSU,信道服务单元 ) and a data service unit (DSU,数据服务单元 ) are required. The two are often combined into a single piece of equipment, called the CSU

5、/DSU. The CSU/DSU may also be built into the interface card in the router.,MODEM(调制解调器 ),A modem is needed if the local loop(本地环路) is analog(模拟) rather than digital(数字).,Demarcation point (分界点 ),The demarcation point, or demarc as it is commonly known, is the point in the network where the responsib

6、ility of the service provider or telco ends. United States International,1.3 WAN Standards（广域网标准）,1.3.1 Physical Layer Standards （物理层标准）,The physical layer protocols describe how to provide electrical(电气), mechanical(机械), operational(操作), and functional(功能) connections to the services provided by a

7、communications service provider.,WAN connectors,1.3.2 Data Link layer Standards（数据链路层标准）,The data link layer protocols define how data is encapsulated for transmission to remote sites, and the mechanisms for transferring the resulting frames. These protocols use the same basic framing mechanism, hig

8、h-level data link control (HDLC,高级数据链路控制), an ISO standard, or one of its sub-sets(子集) or variants.,1.4 WAN Encapsulation（广域网封装）,1.4.1 Common Encapsulation Methods（常用封装方法）,Most framing is based on the HDLC standard.,1.4.2 HDLC,HDLC framing gives reliable delivery of data over unreliable lines and in

9、cludes signal mechanisms for flow and error control. The frame always starts and ends with an 8-bit flag field, the bit pattern is 01111110.,The control field indicates the frame type, which may be information, supervisory, or unnumbered: Unnumbered frames (无编号帧) carry line setup messages. Informati

10、on frames (信息帧) carry network layer data. Supervisory frames (监控帧) control the flow of information frames and request data retransmission in the event of an error.,1.4.3 Configuration HDLC（配置HDLC ）,router(config) # interface serial slot_# / port_# router(config-i f) # encapsulation hdlc,1.5 WAN conn

11、ection（广域网连接）,1.5.1 Circuit Switching (电路交换),When a subscriber makes a telephone call, the dialed number is used to set switches in the exchanges along the route of the call so that there is a continuous circuit from the originating caller to that of the called party. Because of the switching operat

12、ion used to establish the circuit, the telephone system is called a circuit-switched network. If the telephones are replaced with modems, then the switched circuit is able to carry computer data.,1.5.2 Packet Switching(分组交换),Packet-switched networks were developed to overcome the expense of public c

13、ircuit-switched networks and to provide a more cost-effective WAN technology. If the circuit is to be shared, there must be some mechanism to label the bits so that the system knows where to deliver them. It is difficult to label individual bits, therefore they are gathered into groups called cells(

14、信元), frames, or packets.,The packet passes from exchange to exchange for delivery through the provider network. Networks that implement this system are called packet-switched networks（分组交换网络）. The set of entries in the tables identifies a particular route or circuit through the system. If this circu

15、it is only physically in existence while a packet is traveling through it, it is called a Virtual Circuit (VC,虚电路).,The table entries that constitute（组成） a VC can be established by sending a connection request through the network. In this case the resulting circuit is called a Switched Virtual Circu

16、it (SVC,交换虚电路). Where a circuit is required to be always available, a Permanent Virtual Circuit (PVC,永久虚电路) will be established.,电路交换和分组交换,1.5.3 Circuit and Packet switching (电路和分组交换),1.6 WAN Technologies（广域网技术）,1.6.1 Analog Dial-up （模拟拨号）,public switched telephone network (PSTN,公共交换电话网络). 33 kbps 5

17、6 kbps The advantages of modem and analog lines(模拟线路) are simplicity(简单), availability, and low implementation cost. The disadvantages are the low data rates and a relatively long connection time.,1.6.2 ISDN,Integrated Services Digital Network (ISDN,综合业务数字网络) turns the local loop into a TDM(时分多路) di

18、gital connection. 64 kbps bearer channels (B) 16 kbps delta channel (D),Basic Rate Interface (BRI) ISDN Primary Rate Interface (PRI) ISDN,1.6.3 Leased Lines(租用线),Point-to-point（点到点） lines are usually leased（租借） from a carrier and are called leased lines. Leased lines are used extensively for buildin

19、g WANs and give permanent（永久） dedicated（专用） capacity（容量）.,1.6.4 X.25,X.25 is a network-layer protocol and subscribers（用户） are provided with a network address. Virtual circuits（虚电路） can be established through the network with call request packets to the target address. maximum of 48 kbps Frame Relay

20、has replaced X.25,1.6.5 Frame Relay （帧中继）,Most importantly, it is a much simpler protocol that works at the data link layer rather than the network layer. Frame Relay implements no error or flow control（无差错控制或流量控制）. Most Frame Relay connections are PVCs rather than SVCs.,1.6.6 ATM,Asynchronous Trans

21、fer Mode (异步传输模式) It is built on a cell-based(基于信元) architecture rather than on a frame-based（基于帧） architecture. 53 byte ATM cell (5 byte ATM header + 48 bytes of ATM payload ) ATM offers both PVCs and SVCs,1.6.7 Digital Subscriber Line （数字用户线）,Digital Subscriber Line (DSL,数字用户线 ) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers.,The term xDSL covers a number of similar yet competing forms of DSL technologies,1.6.8 Cable Modem