本文内容为CCNA题库解析：CCNA640-802。
CCNA题库解析一：
Refer to the exhibit. Thenetworkadministrator must complete the connection between the RTA of the XYZ Company and the service provider. To accomplish this task, which two devices could be installed at the customer site to provide a connection through the local loop to the central office of the provider? (Choose two.)
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A:WANswitch
B:PVC
C:ATM switch
D:multiplexer
E:CSU/DSU
F:modem
Correct Answers:E, F
通道服务单元/数据服务单元(CSU/DSU)，两个单元通常结合为一个单元，是将终端用户设备连接到当地数字电话环路的数字界面设备。通道服务单元(CSU)是将终端连接到数字线路的设备，而数据服务单元则被使用在将数据终端设备(DTE)的物理界面转换为传输设备(如，T1或E1)的设置。
CCNA题库解析二：
Refer to the exhibit. The network administrator is in a campus building distant from Building B. WANRouter is hosting a newly installedWANlink on interface S0/0. The new link is not functioning and the administrator needs to determine if the correct cable has been attached to the S0/0 interface. How can the administrator accurately verify the correct cable type on S0/0 in the most efficient manner?
A: Telnet to WANRouter and execute the command show interfaces S0/0
B: Telnet to WANRouter and execute the command show processes S0/0
C: Telnet to WANRouter and execute the command show running-configuration
D: Telnet to WANRouter and execute the command show controller S0/0
E: Physically examine the cable between WANRouter S0/0 and theDCE.
F: Establish a console session on WANRouter and execute the command show interfaces S0/0
Correct Answers:D
要检验端口s0/0的线缆类型，首先要登陆到路由器上去，我们可以通过telnet的技术登上去，查看接口所接的线缆的类型使用的命令为show controller s0/0,可以看到接口的状态为DTE，或是DCE。
CCNA题库解析三：
Which three Layer 2 encapsulation types would be used on a WAN rather than a LAN? (Choose three.)
A:HDLC
B:Ethernet
C:Token Ring
D:PPP
E:FDDI
F:Frame. Relay
Correct Answers:A, D, F
连接到WAN，有三种连接方式：专线连接，电路交换和包交换。专线使用同步串行线，他的二层的封装协议常用的有：HDLC，PPP;电路交换使用同步串行线，他使用的二层的封装协议有：HDLC，PPP包交换使用的是虚电路(VC)，而VC又分为PVC(***虚电路)和SVC(交换虚电路)，他使用的二层的封装协议有：X.25，Frame-relay，ATM。
CCNA题库解析四：
Which of the following describes the roles of devices in a WAN? (Choose three.)
A:A CSU/DSU terminates a digital local loop.
B:A modem terminates a digital local loop.
C:A CSU/DSU terminates an analog local loop.
D:A modem terminates an analog local loop.
E:A router is commonly considered a DTE device.
F:A router is commonly considered a DCE device.
Correct Answers:A, D, E
At which layers of the OSI model do WANs operate? (Choose two.)
A. application layerB. session layerC. transport layerD. network layer
E. datalink layerF. physical layer
Correct Answers: E，F
WAN是工作在一层和二层的，比如现在常用的Frame-relay,ATM等这些都是工作在OSI的二层的。
这就是CCNA题库解析：CCNA640-802的相关内容。
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