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56Kbps MODEM
目标：56Kbps MODEM，更恰当地说叫做脉码调制(PCM)MODEM，在1996年底初次亮相，生产厂商允诺其桌面数据传送速度是28.8Kbps MODEM的两倍，而价格则将与现有33.6Kbps MODEM的价格差不多。
提交：即使是在美国现有通信线路的条件下，56Kbps MODEM的数据传送速度也根本无法达到56Kbps，与许多类似的MODEM相比，56Kbps MODEM对线路条件敏感得多，而且几乎线路上所有的不利因素，包括公共话音的增强，都会降低56Kbps MODEM的数据传送速度。一般的56Kbps MODEM的传送速度均低于45Kbps，理想条件下可达到53.5Kbps，而且只有在下载数据时才能达到较高的速率，上载时的速率会更低。
进展：1998年更好的56Kbps驱动程序面世，它能提高速率，解决联网方面的问题。另外，ISP们进一步支持56Kbps MODEM。然而令人遗憾的是，迄今为止还没有出现一个统一的标准。业界究竟是选择3Com/U.S.Roboticsx2还是Lucent/Rockwell的K56flex，尚需数月时间。不过很有可能您看到的会是这两个标准的折中方案。
评述：尽管就性能而言56Kbps MODEM的宣传存在着一定的夸大其辞，它仍有可能是PC历史上最畅销的外设之一，越来越多的用户开始使用更新型的56Kbps MODEM取代旧的MODEM。然而它缺乏统一标准的现实仍旧令人感到失望。与此同时，可以预见的是，56Kbps MODEM不久将被各种高速Internet技术如线缆调制解调器、卫星通信技术、DSL，甚至新型调制解调器技术等所取代。
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题目标签：折中方案调制解调器驱动程序

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【单选题】Online services are managed by a host system that maintains a base of information available to satellite users. Users of so-called "dumb" terminals (i. e., those without processing capability)simply a...

Online services are managed by a host system that maintains a base of information available to satellite users. Users of so-called "dumb" terminals (i. e., those without processing capability)simply access the information base via programs stored on the system. Personal computer (PC) users typically access the host through a modem (调制解调器). A PC software program serves as an interface (接口)between the server and a PC, allowing the user to operate through the online system and select different databases using a keyboard or a mouse.

B.
National and regional online systems usually have local telephone numbers that PC modems can call to access either a local information base or an indirect long-distance connection, thus reducing long-distance telephone fees. Some online systems allow users ’to copy large volumes of information onto a local memory storage device, which also reduces the time the user is connected to the online system.

Besides offering a great number of different information based, ranging from full-text journal li- braries to reports of missing children, online services allow users to, for example, reserve airline tick- ets, buy stocks, purchase goods, and communicate with other users. In exchange for the service, users usually pay a monthly membership fee. They may also pay to connect to various databases on the service or to download information.