什么是比特率?什么是波特率? 比特率是一种数字信号的传输速率,它表示单位时间内所传送的二进制代码的有效位数,单位用比特每秒或千比特每秒表示。波特率是一种调制速率,也称波形速率。它是针对在模拟信道上进行数字传输时,从调制解调器输出的调制信号,每秒钟载波调制状态改变的次数。或者说,在数据传输过程中,线路上每秒钟传送的波形个数就是波特率,其单位为波特。

题目标签：传输速率调制解调器二进制代码

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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.

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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.