题目内容
RS-232是DTE和DCE之间的接口标准，当作为DCE的MODEM和作为DTE的计算机相连时，按此标准需要连接的最少线数是 (37) 。MODEM收到呼叫信号后向计算机发送的信号是 (38) 。当数据发送完毕，计算机向MODEM发送的信号是清除 (39) ，MODEM随后向计算机发送清除 (40) 信号作为应答。当使用RS-232通信时，通常需要使用速率匹配功能，完成该功能的协议是 (41) 。
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W: Hello, Freddy.M: Hello, Mary. How nice to see you again. How’s everything goingW: Fine. Busy these daysM: Yeah. With lots of things to do. Would you like to join me for a drinkW: Ok, thanks.M: Any news recentlyW: Oh, well, I read in the local paper the other day that the government is planning to build an airport here, you know thatM: I’m afraid not.W: Well, my real objection to this idea of a new airport is, is the whole thing is so wasteful, I mean, we know we are currently in fuel crises, we know that we’ve got to conserve oil and fuel and all the rest of it, and get here the government seems quite deliberately to be encouraging people to,... to travel, to use and these jets use a hack of a lot of oil, I mean it takes a ton of oil, a ton of petrol before one of these big jets even takes off.M: Mm...W: It seems so completely short-sighted to me. Quite apart from all the ways to the land and so on, I can’t see, I can’t see the rationale behind really wanting an, an airport at all.M: Well, surely you must have to admit that the existing airport nearby are becoming swarmed. I mean, why should people, uhh...W: Well, they aren’t being swarmed.M: ... be treated like cattle when there is a chance of... a new airport here.W: But, but really people shouldn’t be traveling as much, that’s, that’s why most of the journeys, I mean, they swamped because there is far too much unnecessary tourism and soon. It isn’t necessary for people to travel so far or even so often.M: Well, you take the climate here in this country. Now just before Christmas, there was this dreadful cold spell, and there was a tremendous increase in number of people who wanted to leave and spent Christmas and the New Year in a reasonable climate of sun and, and a certain mild climate. And, and in summer, the same situation occurs. It is unbearably hot here and people want to go somewhere cool.W: Well, yes, I can sympathize with that. But it is still not really necessary to do, well, as it is necessary to, to conserve fuel and it is necessary to, well, not to Waste land. I mean land for a new airport could be used for far more important things which would benefit the people here far more. I mean it could be used for farming for instance.M: True.W: It could also he used for housing, or it could be used for parks, you know. People then could come and enjoy themselves without having to travel far.M: But, airports do bring some local advantages. They bring roads, there is obviously extra employment, for instance, new hotels, shops, restaurants will have to be built. This means more jobs for the locals and it is good for local economy.W: But you ask the people, you ask those who are now living near the airports, for instance, whether, whether they reckon that airports bring them advantages, although, all the airports bring in are noise and vast motorways, and the whole area is, is desolated, isn’t itM: But the airport infra,..., infrastructure relies on housing and other facilities for the great number of people who would be employed in the airport, the pilots even, the stewardess, they have to live somewhere near the airport, rightW: Yeah, but it’s, it’s just so, so damaging to the whole area. I think, airports, from my point of view, the whole concept is outdated, really. Umm, with modern technology, we can make a lot of travel unnecessary, really. For example, it won’t be necessary for businessmen to fly out to a foreign county to talk to somebody. They can just leave it to the telephone in the office, press the button, and say to the person they want to do business with. You see, business deals can be made without having to travel back and forth, rightM: Yes, you are right. But for a lot of people, personal contact is important. And this means travel, and means quick travel, air travel. And we just need a new airport. Mary doesn’t seem to favour the idea of a new airport because（）。
设信道带宽为3000Hz，根据Nyquist定理，理想信道的波特率为 (3) 波特，若采用QPSK调制，其数据速率应为 (4) ，如果该信道信噪比为30dB，则该信道的带宽约为 (5) 。设信道误码率为10-5，帧长为10K比特，差错为单个错，则帧出错的概率为 (6) 。若整个报文被分成5个帧，帧出错的概率为0.1，纠错重发以报文为单位，则整个报文的平均发送次数约为 (7) 次。
物理层的电气特性有多种标准，其中CCITT的V.11建议中规定 (43) 。常用的EIA。RS-232接口的电气特性与CCITT的 (44) 兼容，在这种电路中，数据“1”的接口电平规定为 (45) 。在RS-232定义的接口线中， (46) 表示计算机请求发送数据， (47) 表示计算机准备接收数据。
RS-232是DTE和DCE之间的接口标准，当作为DCE的MODEM和作为DTE的计算机相连时，按此标准需要连接的最少线数是 (37) 。MODEM收到呼叫信号后向计算机发送的信号是 (38) 。当数据发送完毕，计算机向MODEM发送的信号是清除 (39) ，MODEM随后向计算机发送清除 (40) 信号作为应答。当使用RS-232通信时，通常需要使用速率匹配功能，完成该功能的协议是 (41) 。
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