计算机专业英语教程.pptVIP

Chapter 4 Network Communication All carriers can pass over the same wire at the same time without interference. 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. FDM technology can be used when sending signals over wire, RF, or optical fiber. Figure 6-1 illustrates the concept, and shows the hardware components needed for FDM. 所有载波可在同一时间通过同一导线而互不干扰。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。当通过导线、RF或光纤发送信号时便可使用FDM技术。 图6-1说明了这一概念并显示了FDM所需的硬件部件。 Chapter 4 Network Communication In theory, as long as each carrier operates at a different frequency than the others, it remains independent. In practice, however, two carriers operating at almost the same frequency or at exact multiples of a frequency can interfere with one another. To avoid problems, engineers who design FDM network systems choose a minimum separation between the carriers. 理论上，工作在不同频率上的载波将保持相互独立。但实际上，两个频率相近或频率成整倍的载波会彼此干扰。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。 Chapter 4 Network Communication 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. Consequently, FDM is only used on high-bandwidth transmission channels. Frequency division multiplexing(FDM) allows multiple pairs of senders and receivers to communicate over a shared medium simultaneously. The carrier used by each pair operates at a unique frequency that does not interfere with the others. 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。 因此，FDM仅用于高带宽传输通道中。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。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 Chapter 4 Network Communication The general alternative to FDM is time division multiplexing (TDM), in which sources sharing a medium take turn. For example, some time-division multiplexing hardware use a round-robin scheme in which the multiplesor sends