21.高速率DWDM系統(tǒng)和相干檢測系統(tǒng)中必須采用光的(      )

A.內(nèi)腔調(diào)制                                                      B.幅度調(diào)制

C.頻率調(diào)制                                                      D.外腔調(diào)制

22.在單模光纖中，只傳輸m值(      )

A.大于零的單模                                               B.小于零的單模

C.等于零的單模                                               D.必須大于1的單模

23.1.55μm光纖的損耗通常只有(      )

A.0.4dB                                                           B.0.2dB

C.0.1dB                                                           D.0.05dB

24.VSAT入向信道上采用的隨機分配方式主要是(      )

A.幀ALOHA                                                   B.同步ALOHA

C.時隙ALOHA                                                D.時隙ALAHO

25.我國的支線或?qū)Ｓ镁W(wǎng)微波通信常用(      )

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

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