构成SDH/SONET传送网的主要网元设备有哪些,它们在网络中的作用是什么?
在SDH中,虚容器的含义是什么,它在SDH中起什么作用?
Match the words or phrases with their explanations. All of them are from the text. (Pay special attention to the words and phrases on the side of each page.)____: following behind 跟随
____: one feels a lot of sympathy towards sb. 对某人表示同情或慰问