21.高速率DWDM系统和相干检测系统中必须采用光的(      )

A.内腔调制                                                      B.幅度调制

C.频率调制                                                      D.外腔调制

22.在单模光纤中，只传输m值(      )

A.大于零的单模                                               B.小于零的单模

C.等于零的单模                                               D.必须大于1的单模

23.1.55μm光纤的损耗通常只有(      )

A.0.4dB                                                           B.0.2dB

C.0.1dB                                                           D.0.05dB

24.VSAT入向信道上采用的随机分配方式主要是(      )

A.帧ALOHA                                                   B.同步ALOHA

C.时隙ALOHA                                                D.时隙ALAHO

25.我国的支线或专用网微波通信常用(      )

A.3、4、5GHz频段                                         B.1、5、6、7GHz频段

C.2、7、8GHz频段                                         D.2、7、8、11GHz频段