High Throughput

High throughput has always been one of the most compelling benefits of ATM. At the time ATM was conceived, routers were slow devices that required software-based processing to handle the complex variable-length and variable-format multiprotocol (IP, IPX, and so forth) traffic. The variable data lengths resulted in inefficient processing and many complex buffering schemes (to illustrate this point, just issue a show buffers command on a Cisco router). The variable data formats required every Layer 3 protocol to utilize a different set of logic and routing procedures. Run this on a general-purpose CPU and the result is a low-throughput device.

The ATM cell was designed to address both of these issues. Because cells are fixed in length, buffering becomes a trivial exercise of simply carving up buffer memory into fixed-length cubbyholes. Because cells have a fixed-format, 5-byte header, switch processing is drastically simplified. The result: it becomes much easier to build very high-speed, hardware-based switching mechanisms.

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