计算机专业英语教程(第4版)译文6.2

《计算机专业英语教程》（第4版）6.2 Carrier Frequencies and Multiplexing载波频率和多路复用@ Computer networks that use a modulated carrier wave to transmit data are similar to television stations that use a modulated carrier wave to broadcast video.modulated 已调制的使用调制载波发送数据的计算机网络和利用调制载波广播视频信息的电视台相类似。

The similarities provide the intuition needed to understand a fundamental principle:similarity 相似性intuition 直观这一相似性给理解下述基本原理提供了启示：Two or more signals that use different carrier frequencies can be transmitted over a single medium simultaneously without interference.signal 信号simultaneously 同时地interference 干扰两个或多个使用不同载波频率的信号可以在单一介质上同时传输而互不干扰。

@ To understand the principle, consider how television transmission works.为理解这一原理，考虑（有线）电视传输是如何工作的。

Each television station is assigned a channel number on which it broadcasts a signal.channel 频道每个电视台都分配一个频道号，它在该频道上广播信号。

In fact, a channel number is merely shorthand for the frequency at which the station's carrier oscillates.shorthand 速记oscillate 振荡事实上,频道号就是电视台载波用的振荡频率的速记。

To receive a transmission, a television receiver must be tuned to the same frequency as the transmitter.tune to 调谐到为接收一个频道传输，电视接收器必须调谐到与发送器相同的频率。

More important, a given city can contain many television stations that all broadcast on separate frequencies simultaneously.更重要的是，一个城市可以有许多电视台，它们彼此在不同的频率上同时广播。

A receiver selects one to receive at any time.一个接收器在任一时间选择接收其中一个。

@ Cable television illustrates that the principle applies to many signals traveling across a wire.有线电视这一例子说明了以上原理应用于许多信号在一根导线上同时传输的情况。

Although a cable subscriber may have only one physical wire that connects to the cable company, the subscriber receives many channels of information simultaneously.虽然一个有线电视用户可能只有一根线连向有线电视公司，但用户可同时收到许多频道的信息。

The signal for one channel does not interfere with the signal for another. 一个频道中的信号不会与另一频道中的信号相互干扰。

@ Computer networks use the principle of separate channels to permit multiple communications to share a single, physical connection.permit 允许计算机网络使用分离频道的原理使多个通信共享单根物理连线。

Each sender transmits a signal using a particular carrier frequency.每个发送器使用一个特定载波频率传输信号。

A receiver configured to accept a carrier at a given frequency will not be affected by signals sent at other frequencies.一个接收器被设置成接收给定频率的载波，它将不会受其它频率信号的影响（或干扰）。

All carriers can pass over the same wire at the same time without interference.所有载波可在同一时间通过同一导线而互不干扰。

Frequency Division Multiplexing 频分多路复用@ Frequency division multiplexing(FDM) is the technical term applied to a network system that uses multiple carrier frequencies to allow independent signals to travel through a medium.频分多路复用（FDM）是使用多个载波频率在一个介质中同时传输多个独立信号的计算机网络系统术语。

FDM technology can be used when sending signals over wire, RF, or optical fiber.当通过导线、RF或光纤发送信号时便可使用FDM技术。

Figure 6-1 illustrates the concept, and shows the hardware components needed for FDM.图6-1说明了这一概念并显示了FDM所需的硬件部件。

@ In theory, as long as each carrier operates at a different frequency than the others, it remains independent.理论上，工作在不同频率上的载波将保持相互独立。

In practice, however, two carriers operating at almost the same frequency or at exact multiples of a frequency can interfere with one another.但实际上，两个频率相近或频率成整倍的载波会彼此干扰。

To avoid problems, engineers who design FDM network systems choose a minimum separation between the carriers.为了避免这些问题，设计FDM网络系统的工程师们在各载波之间选择一个最起码的频率分隔。

The mandate for large gaps between the frequencies assigned to carriers means that underlying hardware used with FDM can tolerate a wide range of frequencies.mandate 授权underlying 相关的tolerate 容纳在各载波频率之间要求有较大的间隔意味着FDM所用的相关硬件能容纳很宽的频率范围。

Consequently, FDM is only used on high-bandwidth transmission channels.因此，FDM仅用于高带宽传输通道中。

In summary, 概括地说，Frequency division multiplexing(FDM) allows multiple pairs of senders and receivers to communicate over a shared medium simultaneously.频分多路复用允许多对发送和接收装置在一个共享介质上同时通信。

The carrier used by each pair operates at a unique frequency that does not interfere with the others.每对装置使用的载波频率唯一的，并彼此互不干扰的频率运行。

Time Division Multiplexing 时分多路复用@ The general alternative to FDM is time division multiplexing (TDM), in which sources sharing a medium take turn.与FDM不同的另一种复用形式是时分多路复用(TDM)，按这种方式，各发送源轮流使用共享的通信介质。

For example, some time-division multiplexing hardware use a round-robin scheme in which the multiplesor sends a packet from sourcel, then sends a packet from souces2, and so on.round-robin 循环例如，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.arbitraryprompt 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”。