第52題： IEEE802.11采用了類似于802.3CSMA/CD協(xié)議，之所以不采用CSMA/CD協(xié)議的原因是（ ）。
A.CSMA/CD協(xié)議的效率更高
B.為了解決隱蔽終端問題
C.CSMA/CD協(xié)議的開銷更大
D.為了引進其他業(yè)務(wù)
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第53題： 無線局域網(wǎng)（WLAN）標準IEEE802.llg規(guī)定的最大數(shù)據(jù)速率是（ ）。
A.1Mb/s
B.11 Mb/s
C.5 Mb/s
D. 54Mb/s
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第54題： 大型局域網(wǎng)通常組織成分層結(jié)構(gòu)（核心層、匯聚層和接入層），以下關(guān)于網(wǎng)絡(luò)核心層的敘述中，正確的是（ ）。
A.為了保障安全性，應(yīng)該以分組進行盡可能多的處理
B.將數(shù)據(jù)分組從一個區(qū)域高速地轉(zhuǎn)發(fā)到另一個區(qū)域
C.由多臺二、三層交換機組成
D.提供用戶的訪問控制
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第56題： A transport layer protocol usually has several responsibilities .One is to create a process-to-process communication;UDP uses (1)numbers to accomplish this.Another Responsibility is to provide control mechanisms at the  transpt level.UDP does this task at a very minimal level.There is no flow control mechanism and there Is no(2) for received packet. UDP,however ,does provide error control to some extent.If UDP detects an error in the received packet,it will silently drop it.The transport layer also provides a connection mechanism for the processes.The(3)must be able to send streams of data to the transport layer.It is the responsibility of the transport layer at (4)station to make the connection with the receiver.chop the stream into transportable units,number them,and send them one by one.It is the responsibility of th transorort layer at the receiving end to wait until all the different units  belong to th same process have arrived,check and pass those that are(5) free ,and deliver them to the receiving process as a stream.

(1)A. hop   
B. port    
C. route  
D.packet
(2)A. connection   
B. window   
C. acknowledgement  
D.destination
(3)A. jobs   
B. processes   
C. proarams   
D.users
(4)A. sending   
B. routing    
C. switching  
D.receiving
(5)A. call   
B. state    
C. cost  
D.error

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