TK的题目，请高手指点
<P>The Testking Corporation consists of the head office in New York with its regional<BR>offices in: Chicago, Detroit, Philadelphia, Toronto, and Atlanta. These offices need<BR>to be connected in a WAN, and Testking wishes to do this via a hub and spoke<BR>arrangement that will utilize packet-switched technology.<BR>Which one of the WAN technologies below would be the best choice for Testking?<BR>A. ISDN<BR>B. Wireless<BR>C. Frame Relay<BR>D. T1 leased line<BR>E. ATM<BR>F. VPN</P><P> </P><P> </P><P>这题是什么意思啊，有高手解释一下吗，为什么是C啊，A为什么不行</P> <P>These offices need<BR>to be connected in a WAN,</P><P>ISDN只是拨号技术,不能把不同地方的OFFICE通过WAN连接到一起</P> <P>via a hub and spoke<BR>arrangement that will utilize packet-switched technology</P><P>星型＋包交换技术，想想看，应该是什么？</P><P> </P> <P>哦，上题明白了，Frame relay是包交换技术，</P><P> 那这题的关键在哪呢</P><P>You are a systems administrator of an HR company in Dallas. You want to connect<BR>your company’s head office with a branch office in Detroit. To do this, you want to<BR>use two data link layer encapsulations: one exclusively for data and the other<BR>exclusively for signaling. Which one of the following WAN services would best suit<BR>your needs?<BR>A. ISDN<BR>B. ATM<BR>C. FDDI<BR>D. ATX<BR>E. Frame Relay</P><P>选A，为什么</P> <P>NA中主要要求掌握FR和ISDN这两种WAN技术。</P><P><FONT size=3>I S D N是基于电路交换的数字化数据服务。I S D N本来是用来提供增强的语音和数据服务的。它有BRI和PRI两种，我们生活中主要用到BRI，spoto就拥有两条ISDNbri线路。<FONT color=#e61a42>这种bri有两个B信道，当电话信号用来拨打时只占用一个B信道，另外一个B信道可以用来传输数据，就像当年的猫一样可以拨号上网。还可以将两个B信道都用来传数据那样就有64K*2的速度。</FONT>ISDN对企业来说解决了两个问题：1.网速的提升 2.打电话发传真和上网同时进行。但现在ISDN技术已经逐渐不敌了！上网被ADSL取代，打电话发传真用普通电话线就够了！所以ISDN现在仅在作线路备份方面有一席之地。</FONT></P><P><FONT size=3>题中谈到的就是用ISDN才能做到two data link layer encapsulations: one exclusively for data and the other<BR>exclusively for signaling。</FONT></P>
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