题目内容
对照ISO/OSI参考模型各个层中的网络安全服务，在物理层可以采用 (1) 加强通信线路的安全；在数据链路层，可以采用 (2) 进行链路加密；在网络层可以采用 (3) 来处理信息内外网络边界流动和建立透明的安全加密信道；在传输层主要解决进程到进程间的加密，最常见的传输层安全技术有 (4) ；为了将低层安全服务进行抽象和屏蔽，最有效的一类做法是可以在传输层和应用层之间建立中间件层次实现通用的安全服务功能，通过定义统一的安全服务接口向应用层提供 (5) 安全服务。 3（）
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帧中继网的虚电路建立在 (1) ，在用户层面采用的协议是 (2) 。这种网络没有流量控制功能，但增加了拥塞控制功能。如果沿着帧传送方向出现了拥塞，则把帧地址字段中的 (3) 位设置为1，这样接收方就可通过 (4) 协议要求发送方降低数据速率。最适合提供帧中继业务的网络是 (5) 。 4（）
采用可变长子网掩码技术可以把大的网络分成小的子网，例如把子网掩码为 255.255.0.0的网络40.15.0.0分为两个子网，假设第一个子网为40.15.0.0/17,则第二个子网为 (1) 。假设用户X1有2000台主机，则至少应给他分配 (2) 个C类网络，如果分配给用户X1的网络号为196.25.64.0，则指定给X1的子网掩码为 (3) ；假设给用户X2分配的C类网络号为196.25.16.0～196.25.31.0，则X2的子网掩码应为 (4) ；如果路由器收到一个目标地址为11000100.00011001.01000011.00100001的数据报，则该数据报应送给 (5) 用户。 1（）
CSMA(载波监听多路访问)控制策略中有3种坚持退避算法，其中一种是：“一旦介质空闲就发送数据，假如介质是忙的，继续监听，直到介质空闲后立即发送数据；如果有冲突就退避，然后再监听”这种退避算法称为 (1) 算法。这种算法的主要特点是 (2) 。CSMA/CD在CSMA的基础上增加了冲突检测功能。网络中的某个发送站点一旦检测到冲突，它就立即停止发送，并发冲突码，其他站点都会 (3) 。如果站点发送时间为1，任意两个站之间的传播延迟为t，若能正常检测到冲突，对于基带总线网络，t的值应为 (4) ；对于宽带总线网络，t的值应为 (5) 。 3（）
[A] Why is foreign text "rendered meaningless" when passed through an on-line translation tool According to Sabine Reul, who runs a Frankfurt-based translation company, translation tools have limited uses, and problems arise when web users expect too much from them. "A translation tool works for some things," says Reul, "Say a British company wants to order a box of screws from a German supplier. A sentence like ’We need one box of a certain type of screw’ is something that a machine could translate reasonably accurately--though primitively."[B] Yet when it comes to translating blocks of text—words and sentences that convey thoughts and sentiments, on-line tools are bound to fail, she adds. "Beyond simple sentences, the on-line process simply doesn’t work because machines don’t understand grammar and semantics, never mind idiom and style." "Language is not a system of signs in the mechanical sense of the word", says Reul, "It is a living medium that is used to convey thought. And that is where machines fail. Human input is indispensable as long as computers cannot think." Reul and other translators look forward to the day when clever computers might help to ease their workload--but that time has not arrived yet.[C] Earlier this month the small German town of Homberg-an-der-Efze, north of Frankfurt, had to pulp an entire print run of its English language tourism brochure after officials used an Internet translating tool to translate the German text. According to one report, the brochure was "rendered meaningless" by the on line tool. Martin Wagner, mayor of Homherg-an-der-Efze, admits that the town made a "blunder". As a result of officials trying to save money by getting the Internet to do a translator’s job, a total of 7,500 brochures had to be binned.[D] "It would be nice if computers could do the job. And certainly the quest for machine translation has prompted a lot of linguistic research that may prove valuable in unforeseen ways. But experience to date confirms that even the most subtle computer program doesn’t think and you need to be able to think in order to translate."[E] This story highlights some of the pitfalls of translating on line. There are many instant translation tools on the web, but they are best used for individual words and short phrases, rather than for brochures, books or anything complex. For example, one of the joys of the web is that it grants you access to an array of foreign news sources. Yet if you were to use a translation tool to try to make sense of such reports, you could end up with a rather skewed and surreal view of the world.[F] Until the dawn of thinking computers, on-line translation tools are best reserved for words, basic sentences and useful holiday phrase. For tourism brochures, newspaper reports and the rest, you will have to rely on some old fashioned "human input".[G] Relying on on-line translation tools can be a risky business, especially if you expect too much of it. For the time being, might translation be something best left to the humansOrder: 43
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