题目内容
若卫星信道的数据传输率为1Mb/s，帧长为1000bit，利用卫星信道的两个站点从一方到另一方的传播时延为250ms。忽略确认帧长和处理时间，则：若帧的出错概率为0.1，而假设应答帧不出现错误，当采用停等协议时，其协议效率是 (1) 。若采用连续 ARQ协议，发送窗口Wt＝7，接收窗口Wr＝1，在不出错的情况下，信道的利用率为 (2) 。若帧传输过程中出错是突发式的，突发位数为100bit，信道的误码率为10-3，则帧的出错概率变为 (3) 。在连续ARQ协议中，若发送窗口大于2K(K为编号位数)，则会 (4) ，停等协议可以看成是连续ARQ协议的特例，即 (5) 。 5（）
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根据协议规定，以太网网段的最大连接距离是（）m。
在时间和数值上都是连续的信号的通信称为 (1) 。对模拟信号进行一次测量称为 (2) 。 1（）
Linux是目前较为流行的网络操作系统，如同Unix操作系统一样，它也可以通过手工编辑配置文件达到对系统进行配置的目的。在Linux网络配置文件中的几个较为重要的配置文件如下：(1) 用于存放本机主机名以及经常访问IP地址的主机名，在对IP进行域名解析时，可以设定为先访问该文件，再访问DNS，最后访问NIS。Linux下存在两个网络服务守候进程的配置文件。通过修改 (2) ，可以达到关闭或开放某种对应服务的目的。以FTP服务为例，将文件中“#FTP Stream TCP nowait root/usr/sbin/tcpd in.ftpd-1-a”前的“#”去掉就可以开启FTP服务。修改 (3) ，可对允许访问本机提供服务的主机范围进行限制。例如在文件中加入“in.ftpd:202.112.207.0/255.255.255.0”就可以允许202.112.207.0子网的用户主机访问本机提供的FTP服务。在Linux下进行DNS服务时， (4) 是控制解析器配置的最主要文件，规定了解析使用的服务，以及服务的顺序，比如可以通过在文件中加入“Order hosts，DNS，NIS”设定前面提到的域名解析顺序， (5) 是配置DNS服务器的主要文件，指定了本机使用的域名服务器以及多个服务器之间查找的顺序等。 1（）
A new study claiming to document a connection between violence on television and violence in real life is already coming under attack from academics. They say that the author is demanding action on his report before producing detailed findings to substantiate it.(47) Dr. William Belson told the British Association for the Advancement of Science in Birmingham last week that his research suggested that boys exposed to high levels of television violence were 50 percent more likely to commit acts of violence than boys who had not been exposed.His 110,000 pounds survey, paid for by CBS, the American television company, studied more than 15,000 London boys aged between 12 and 16. He closed his paper with a call for immediate action on his recommendations to reduce levels of TV violence and specific kinds of violence which he claimed were more damaging than others. His recommendations were enthusiastically endorsed by Mrs. Mary Whitehouse.Social scientists familiar with the field have a number of specific queries about Belson’s work. (48) They pointed out that a statistical technique invented by him and central to his research has been criticized by some academics in the past. Robin MrCorn, of the Mass Communication Research Centre at Leicester University, says," Self reporting—asking the subject to give his own account of the evidence is notoriously unreliable. Studies have put the possible error as high as 20 percent, and we don’t know what checks there were in this work. The fact that Belson paid the boys may’ have had an influence. Without the full data, it can’t be checked."McCorn adds:" His questions on the programmes go back 12 years. If the boys were aged between 13 and 16 it means the oldest was only four years old when the first programmes were broadcast. How reliable is the memory of a child that young likely to be on the programmes he watched Dr. Belson may have answers, but we just don’t know."(49) The Nelson affair highlights the difficulties faced by researchers into television violence-problems so severe that at least one British group has withdrawn from the field completely. "It’s impossible to do serious scientific work in his area now," says Robin McCron. "It has moved out of the academic world and it has been taken over by pressure groups and politics."Indeed, experience in television research in America reveals how treacherous this field has become. (50) Results of nervous projects there have been found, at worst, contradictory, at best, inconclusive. Dr. William Belson told the British Association for the Advancement of Science in Birmingham last week that his research suggested that boys exposed to high levels of television violence were 50 percent more likely to commit acts of violence than boys who had not been exposed.
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