第5套
Data communications equipment(DEC) (1) the serial data stream from  the DTE and converts it to some form of  (2) suitable of  transmission on voice grade lines. At the (3),the DCE performs the  reverse function of convering the received analog signal to serial  digital data stream .The simplest form of DCE is a (4) or data set . At the transmit end, the modem can be considered a form of (5),while  at the receive end, it can be considered a form of  analog to  digital converter. The most common forms of modulation by modems are frequency shift keying(FSK),phase shift keying(PSK) ,and quadrature  amplitude modulation(QAM). If the cables and signal levels used to  interconnect the DTE and DCE were left unregulated,the variations  generated would probably be proporation to the number of  manufactures. Electronics Industries Association(EIA) ,an  organization of manufactuuers concerned with establishing industry  standards,have agreed on the RS-232C as the standard interface  between the DTE and the modem .This is a 25 pin cable pins have  designated funtons and specified signal levels. 
(1) A. accepts    B.sends        C.delivers      D.receives 
(2) A. digital signal  B.analog signal C.light signal  D.sound signal 
(3) A. back end  B.front end    C.sending end  D.receive end 
(4) A. modem      B.cpu          C.demodulator  D.modulator. 
(5) A. host computer              B.digital to analog converter 
C. analog to digital converter D.secondaries 
答案: 
1:A,2:B,3:D,4:A,5:B