Synchronous and asynchronous
In order to better understand the ATM, it is necessary to make a simple review to the time division multiplexing (TDM) and synchronous transmission (STM). TDM is a time block divided into a frame in a communication line (e.g., 125 ns), and in each frame is divided into several time slots, each time slot can carry the corresponding user information. When a user is established by a call, during which the signal will fix a certain sport in each frame until the communication ends, as shown in FIG.
For synchronous transmission, the exchange is done between fixed time gaps. For example, in FIG. 1, the input frame occupies a signal of the second time slot. If the fourth time slot is occupied in the output frame, this correspondence is fixed until the corresponding communication process ends. as shown in picture 2.
In this transmission and switch mode of this fixed time slot, if a certain time in the communication process, the user has no data transfer, but its fixed-occupied time slot is all all of them, although at this moment, it is in an idle state; There is a large amount of burst data to be transmitted, although this may cause the signal to be delayed or even the loss of the cell, it is still only transmitted and exchanged by a fixed time slot. In contrast, in the asynchronous transmission mode (ATM), the time slot occupied by the cell transmission is not fixed, which is also a so-called statistical time division multiplexed. Further, the number of time slots occupied in one frame is not fixed, and there may be 1 to more time slots, which is completely based on the case where the user communicates at the time. And there is no need to continuous between the time slots, purely "see the needle". It is also similar to when exchange. This process is shown in Figure 3.
