计算机专业英语教程第5版翻译(21)

计算机专业英语教程第5版翻译 绝对准确

例如，某些TDM硬件使用循环法方案共享介质，用这种方法，多路复用器先发送源1的一个包，然后发送源2的一个包，如此循环。

Figure 6-2 illustrates the idea.

图6-2说明了这种思想。

@ Dividing data into small packets ensures that all sources receive prompt service because it prohibits one source from gaining exclusive access for an arbitrarily long time.arbitrary

prompt 立刻地 prohibit禁止 arbitrarily 任意地 exclusive 独占

把数据分成许多小包保证了所有源都接受到即时服务，因为它禁止一个源任意长时间独占介质”。

In particular, if one source has a few packets to send and another has many, allowing both sources to take turns sending packets guarantees that the source with a small amount of data will finish promptly. 特别是，如果一个源只有少量的包发送，而另一个源有许多包发送，于是允许两个源轮流发送包就保证只有少量数据的源会很快结束。

@ In fact, most computer networks use some form of time division multiplexing. demultiplexor 实际上，大多数计算机网络使用某种形式的TDM。

《计算机专业英语教程》（第4版）

7.1 An Overview of a Database System

数据库系统概述

@ Let us consider an enterprise, such as an airline, that has a large amount of data kept for long periods of time in a computer.

让我们考虑航空公司这样的企业，它有大量的数据长时间保存在计算机中。

This data might include information about passengers, flights, aircraft, and personnel, for example. 这些数据，例如，可能包括关于乘客、航班、飞机与乘务组人员的信息。

Typical relationahips that might be represented include bookings (which passengers have seats on which flights? ) flight crews (who is to be the pilot, copilot, etc., on which flights?), and service records (when and by whom was each aircraft last serviced? ).

booking 预订 flight crew 机组人员 pilot 驾驶员 copilot 副驾驶员

可能表示的典型关系包括订票（哪些乘客在那些航班上订有座位？）、航班乘务组（在哪些航班上谁是驾驶员，谁是副驾驶员，等等），以及服务记录（每架飞机最后是何时由谁服务？）。

@ Data, such as the above, that is stored more-or-less permanently in a computer we term a database. term 把...叫做

我们把上述那样的不同程度地长久存储在计算机中的数据称为数据库。

@ The software that allows one or many persons to use and/or modify this data is a database management system(DBMS).

使得一个人或许多人能使用和/或修改这些数据的软件是数据库管理系统（DBMS）。

The primary goal of a DBMS is to provide an environment that is both convenient and efficient to use in retrieving information from and storing information into the database.

retrieving 检索

DBMS的主要目标是提供这样的环境，在从数据库中检索信息和把信息存储在数据库中时使用起来既方便，功效又高。

Data Abstraction 数据抽象

@ It should be obvious that between the computer, dealing with bits, and the ultimate user dealing with abstracions such as flights or assignment of personnel to aricraft, where will be many levels of abstraction. 显然，在处理二进位的计算机和处理像航班或把乘务组人员分派到飞机这样一些抽象的最终用户之间将存在很多层抽象。

A fairly standard viewpoint regarding levels of abstraction is shown in Figure 7-1.

关于抽象层的相当标准的观点如图7-1所示。

There we see a single database, which may be one of many databases using the smae DBMS software, at three