Multiplexing

本文讨论"Multiplexing"的含义，它是由: "multi+plexing" 组成

wikipedia Multiplexing

NOTE:

1、"Multiplexing"的意思是"多路复用"

2、在现实application中，都需要有reverse operation: demultiplexing，**demultiplexing**其实非常类似于dispatch，关于此的文章有:

a、zhihu 如何深刻理解reactor和proactor？ # A

In telecommunications and computer networks, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium（将多个模拟信号或电子信号合并为一个，通过一个共享的媒介进行发送）. The aim is to share a scarce resource（稀缺资源）. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the 1870s, and is now widely applied in communications. In telephony, George Owen Squier is credited with the development of telephone carrier multiplexing（电话运营商多路复用） in 1910.

Multiplexing and demultiplexing

The multiplexed signal is transmitted over a communication channel such as a cable. The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, extracts the original channels on the receiver end.

NOTE:

1、上面这段话非常重要，它对于理解Multiplexing 的本质有着帮助

2、physical and logical :

physical: communication channel

logical: logical channels

A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX or DMX).

Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream.

I/O multiplexing

In computing, I/O multiplexing can also be used to refer to the concept of processing multiple input/output events from a single event loop, with system calls like poll[1] and select (Unix).[2]

NOTE: 如何来理解multiplexing？multiplexing的关键在于many和one，下面是一些例子:

1、在telecommunications中multiple是multiple analog or digital signals ，one是 combined into one signal over a shared medium 。

many logical and one physical ；

2、在 computing的 **I/O multiplexing**中multiple是multiple input/output events 而one则是a single event loop object；

many IO event and one event loop object；

显然multiplexing的目标在于实现性能的提升，资源的节省；

Types

Space-division multiplexing

Frequency-division multiplexing

Time-division multiplexing

NOTE:

1、下面的OS timesharing就属于这种范畴

Polarization-division multiplexing

Orbital angular momentum multiplexing

Code-division multiplexing

Examples

rabbitmq AMQP 0-9-1 Model Explained

其中的channel和connection就是multiplexing；

OS time sharing

在《Understanding.The.Linux.kernel.3rd.Edition》中有这样的话：

Linux scheduling is based on the time sharing technique: several processes run in "time multiplexing" because the CPU time is divided into slices, one for each runnable process. [*] Of course, a single processor can run only one process at any given instant. If a currently running process is not terminated when its time slice or quantum expires, a process switch may take place. Time sharing relies on timer interrupts and is thus transparent to processes. No additional code needs to be inserted in the programs to ensure CPU time sharing.

可以看到，time sharing也是一种multiplex技术，它共享的是CPU。