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ATM switch


ATM switch technology is mainly used in the backbone network segment of telecommunications and postal networks, so its switch products are rarely seen in the market. For example, if the PPPoA protocol is used in the ADSL broadband access method we will talk about below, an ATM switch needs to be configured at the central office (NSP side), and the Cable Modem Internet access method of cable TV also uses an ATM switch at the central office. Its transmission medium generally uses optical fiber, and there are generally two types of interfaces: Ethernet RJ-45 interface and optical fiber interface. These two interfaces are suitable for interconnecting with different types of networks.

Queuing principle

ATM switching has two fundamental points: cell switching and statistical multiplexing between virtual connections. Cell exchange means that ATM cells are switched from one VP/VC to another VP/VC through various forms of exchange media. Statistical multiplexing is manifested in that the cells of each virtual connection compete for switching resources such as the exchange medium for transmitting the cells. In order to solve the competition of the cells for these resources, the cells must be queued, each cell is separated in time, and the circuit is borrowed. The idea of ​​exchange can be thought of as statistical multiplexing embodied in the exchange as time-division exchange and realized through the queuing mechanism.

The queuing mechanism is an extremely important content in ATM switching. Overflow of the queue will cause cell loss. Cell queuing is the main reason for the switching delay and delay jitter. Therefore, the queuing mechanism affects the performance of ATM switches. Has a decisive influence. There are three basic queuing mechanisms: input queuing, output queuing and central queuing. These three methods have their own shortcomings. For example, the input queuing has letterhead blocking, and the load of the switch is less than 60%; the output queuing memory is low in utilization, the average queue length is long, and the central queuing memory requires high speed and complex memory management. At the same time, the three methods have their own advantages. The input queue requires low memory speed, the central queuing efficiency is high, and the output queue is somewhere in between. Therefore, these three methods are not directly used in actual applications, but are integrated. , Has taken some improvement measures. The improved methods mainly include:

Reduce the head-of-line blocking of input queuing;

Use input and output queuing mode with back pressure control;

With loopback mechanism Queuing mode;

Shared output queuing mode;

Set multiple output sub-queues on one output line, and these output sub-queues logically operate as a single output queue .

Switching mechanism

In order to achieve large-capacity switching and to increase the scalability of ATM switches, small-capacity basic switching units are often constructed, and these switching units are The structure is structured as an ATM switching mechanism (Fabric). For the ATM switching mechanism, the main research issues are the transmission medium structure and routing method between each switching unit, and how to reduce competition and reduce congestion.

There are different classification methods for ATM switching institutions. There is a classification method: time division switching and space division switching. Time division switching includes shared bus, shared ring, and shared memory structure. Space division switching includes full Internet and multiple Level Internet.


The general model of ATM cell switch has some input lines and some output lines, which are usually equal in number (because the lines are bidirectional). Get one cell (if any) from each input line in each cycle. Through the internal switching fabric (switching fabric), and gradually transmit on the appropriate output line. From this point of view, ATM switches are synchronous.

The switch can be pipelined, that is, the incoming cell may only appear on the output line after several cycles. Cells actually arrive at the input line asynchronously, so there is a master clock that indicates the beginning of the cycle. Any cell that arrives completely when the clock is ticking can be exchanged in that cycle. Cells that have not fully arrived must wait until the next cycle.

Cells usually arrive at the ATM rate, generally around 150Mb/s, which means more than 360,000 cells/s, which means that the cycle of the switch is about 2.7um. A commercial switch may have 16 to 1,024 input lines, that is, it must be able to receive and exchange 16 to 1,024 cells per 2.7um. At a rate of 622Mb/s, a batch of cells enter the switching structure every 700ns. Since the cell is of fixed length and small (53 bytes), it is possible to make such a switch. If you use longer variable-length packets, high-speed switching will be more complicated, which is why ATM uses short, fixed-length cells.


Due to different applications, various ATM switching equipment has slightly different functions. The main differences are interface type, switching capacity, and signaling processing. .

In the public network, there are access switches, node switches and cross-connect equipment. The position of the access switch in the network is equivalent to the subscriber switch in the telephone network. It is located at the edge of the ATM network and connects various service terminals into the ATM network. The position of the node switch is similar to that of the office switch in the existing telephone network. It completes VP/VC switching and requires a larger switching capacity, but the interface type is simpler than that of the access switch. There are only standard ATM interfaces, mainly NNI interfaces. There is a UNI interface or a B-ICI interface. In terms of signaling, only ATM signaling is required. The cross-connect equipment is similar to the cross-connect equipment in the existing telephone network. It completes the VP exchange in the backbone network without signaling processing, thereby realizing extremely high-speed exchange.

In the ATM private network, there are private network switches and ATM LAN switches. The function of the private network switch is equivalent to the node switch in the public network. It has the UNI and NNI interfaces of the private network, completes the signaling processing of P-UNI and P-NNI, and has strong management and maintenance functions. The ATM LAN switch completes the access of the LAN service. The ATM LAN switch should have a LAN interface and an ATMP-UNI interface to handle the various layer protocols of the LAN and ATM signaling.

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