ATM cell structure and principle
ITU's definition of ATM (Asynchronous Transfer Mode) is: ATM is an asynchronous transfer mode. Asynchronous refers to the nature of ATM statistical multiplexing. Transfer mode refers to the multiplexing, switching, and transmission technologies used in the network, that is, the transfer method used to transfer information from one place to another. In this transfer mode, information is organized into cells (CELL), and individual cells from a user's information do not need to appear periodically. Therefore, ATM is a technology for statistical multiplexing, exchange and transmission in units of cells in the network.
ATM is a new type of packet technology. The cell is actually a fixed-length packet. The total length of the cell is 53 bytes, of which 5 bytes are the header and 48 bytes are the information segment, or payload. . The header contains control information such as the logical address and priority level of the cell. The information section loads information from different users and different services. The information of any business is cut and encapsulated into uniform format cells.
ATM adopts asynchronous time division multiplexing method to gather cells from different information sources together and queue them in the buffer. The cells in the queue are output to the transmission line one by one according to the priority level of the arrival, forming end-to-end cells flow. Cells with the same sign do not correspond to a fixed time slot on the transmission line, nor do they appear periodically. Asynchronous time division multiplexing gives ATM great flexibility, and any business occupies resources according to the actual amount of information, so that network resources can be used to the maximum. In addition, no matter how different the nature of the business source (such as rate, burst size, quality and real-time requirements), the network will be processed in the same mode, truly complete business synthesis. In order to improve processing speed, ensure quality, and reduce delay and cell loss rate, ATM works in a connection-oriented manner. When the communication starts, a virtual circuit is first established, and the virtual circuit logo is written into the letterhead. The network sends the cell to the destination according to the virtual circuit logo. The virtual circuit can be removed and released. All that is done on the nodes of the ATM network is the exchange of virtual circuits. In order to simplify network control, ATM hands over error control and flow control to the terminal, without the need for link-by-segment link error control and flow control. Therefore, ATM combines the advantages of circuit switching and packet switching, that is, ATM takes into account the advantages of statistical multiplexing of packet switching, flexibility and efficiency, and small transmission delay of circuit switching, and good real-time performance. It can carry multiple information media in a single main body network, carry multiple communication services, and can guarantee QoS. ATM switching is divided into VP switching and VC switching. VP exchange refers to only changing the value of VPI during the exchange process, transparently transmitting the value of VCI, while VPI and VCI both change during the VC exchange process.
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