Data Communication Computer Network 简明教程
Multiplexing in Computer Network
多路复用是一种技术，通过该技术可以同时通过共享链路处理不同的模拟和数字传输流。多路复用将大容量介质分成低容量逻辑介质，然后由不同的流共享。
Multiplexing is a technique by which different analog and digital streams of transmission can be simultaneously processed over a shared link. Multiplexing divides the high capacity medium into low capacity logical medium which is then shared by different streams.
可以通过无线（射频）、使用物理介质（电缆）和光（光纤）进行通信。所有介质都具有多路复用功能。
Communication is possible over the air (radio frequency), using a physical media (cable), and light (optical fiber). All mediums are capable of multiplexing.
当多个发送方尝试通过单一介质发送时，一种称为多路复用器的设备会划分物理信道，并为每个信道分配一个。在通信的另一端，一个解复用器从单个介质接收数据，识别它们，并发送给不同的接收器。
When multiple senders try to send over a single medium, a device called Multiplexer divides the physical channel and allocates one to each. On the other end of communication, a De-multiplexer receives data from a single medium, identifies each, and sends to different receivers.
Frequency Division Multiplexing
当载波为频率时，使用 FDM。FDM 是一种模拟技术。FDM 将频谱或载波带宽划分为逻辑信道，并为每个信道分配一个用户。每个用户都可以独立使用信道频率，并对其具有独占访问权。所有信道都被划分，从而彼此不重叠。信道由保护带分开。保护带是任何信道都不使用的频率。
When the carrier is frequency, FDM is used. FDM is an analog technology. FDM divides the spectrum or carrier bandwidth in logical channels and allocates one user to each channel. Each user can use the channel frequency independently and has exclusive access of it. All channels are divided in such a way that they do not overlap with each other. Channels are separated by guard bands. Guard band is a frequency which is not used by either channel.
Time Division Multiplexing
TDM 主要应用于数字信号，但也可以应用于模拟信号。在 TDM 中，共享信道通过时隙分配给其用户。每个用户只能在提供的时隙内传输数据。数字信号被划分为帧，相当于时隙，即可以在给定的时隙内传输的最佳大小的帧。
TDM is applied primarily on digital signals but can be applied on analog signals as well. In TDM the shared channel is divided among its user by means of time slot. Each user can transmit data within the provided time slot only. Digital signals are divided in frames, equivalent to time slot i.e. frame of an optimal size which can be transmitted in given time slot.
TDM 以同步模式工作。两端（即多路复用器和解复用器）都及时同步，并且都同时切换到下一个信道。
TDM works in synchronized mode. Both ends, i.e. Multiplexer and De-multiplexer are timely synchronized and both switch to next channel simultaneously.
当信道 A 在一端传输其帧时，解复用器在另一端为信道 A 提供介质。一旦信道 A 的时隙到期，这端就会切换到信道 B。在另一端，解复用器以同步方式工作，并为信道 B 提供介质。来自不同信道的信号交错地传输路径。
When channel A transmits its frame at one end,the De-multiplexer provides media to channel A on the other end.As soon as the channel A’s time slot expires, this side switches to channel B. On the other end, the De-multiplexer works in a synchronized manner and provides media to channel B. Signals from different channels travel the path in interleaved manner.
Wavelength Division Multiplexing
光有不同的波长（颜色）。在光纤模式中，通过使用不同的波长将多个光载波信号多路复用到光纤中。这是一种模拟多路复用技术，在概念上与 FDM 相同，但使用光作为信号。
Light has different wavelength (colors). In fiber optic mode, multiple optical carrier signals are multiplexed into an optical fiber by using different wavelengths. This is an analog multiplexing technique and is done conceptually in the same manner as FDM but uses light as signals.
此外，可以在每个波长上整合时分多路复用，以容纳更多的数据信号。
Further, on each wavelength time division multiplexing can be incorporated to accommodate more data signals.
Code Division Multiplexing
可以通过使用码分多路复用通过单一频率传输多个数据信号。FDM 将频率分成较小的信道，但 CDM 允许其用户全带宽，并且使用唯一码始终传输信号。CDM 使用正交码来扩展信号。
Multiple data signals can be transmitted over a single frequency by using Code Division Multiplexing. FDM divides the frequency in smaller channels but CDM allows its users to full bandwidth and transmit signals all the time using a unique code. CDM uses orthogonal codes to spread signals.
每个站点都会分配一个称为码片的唯一码。信号在整个带宽内使用这些码片独立传输。接收器预先知道它必须接收的码片码信号。
Each station is assigned with a unique code, called chip. Signals travel with these codes independently, inside the whole bandwidth.The receiver knows in advance the chip code signal it has to receive.