1 引言开关在通讯中非常重要，但人们可能会搞混它，回为它有两种截然不同的理解方式。电话网络包括许多又大又贵的称作交换器的箱子，里边有专用的、计算机控制的、一些称作开关的小元件。对于网络工程师来说，大箱子就是开关；但对于光学工程师而言，开关是大箱子里的一个小 1 Introduction Switches are very important in communication, but people may confuse it with two completely different ways of understanding it. The telephone network consists of many large and expensive boxes called switches with special, computer-controlled, small components called switches. For network engineers, the big box is the switch; but for the optical engineer, the switch is a small box