Networking

Build Your Skills: ATM's fixed-length cells speed data transit

Take a look back at the Asynchronous Transfer Mode switching model to see how it enhances network speed.


You’ve probably experienced it in your enterprise. The number of enterprise networks that must support data, voice, and video is growing rapidly.

Supporting such networks requires enormous amounts of bandwidth and throughput. What’s the solution?

For many companies, ATM switching is leading the way.

What is ATM?
Asynchronous Transfer Mode (ATM) is an ITU standard for cell relay that supports voice, video, and data transport using 53 byte (48 bytes of data and 5 bytes of addressing), fixed-length cells.

Because ATM uses fixed-length cells, switching occurs at the hardware level. As a result, network latency is greatly reduced.

Additionally, ATM provides asynchronous communications and is more efficient than synchronous technologies, such as time division multiplexing (TDM). ATM is a flexible protocol designed to support high bandwidth transport ranging from a few megabits per second up to many gigabits per second.

The ATM model
ATM is defined at the physical layer (Layer 1) and data link layer (Layer 2) of the OSI reference model. The ATM protocol consists of three layers: the ATM physical layer, the ATM layer, and the ATM adaptation layer (AAL).

The ATM physical layer operates at the physical layer of the OSI reference model and is responsible for sending and receiving bits across the physically attached media. ATM is physical medium dependent and will run only on media specifically designed for ATM, such as Fiber Distributed Data Interface (FDDI), SONET, and 155 Mbps local fiber.

The ATM layer spans both the physical and data link layers of the OSI reference model and is responsible for multiplexing and demultiplexing ATM cells from different virtual connections. The ATM layer also manages flow control and delivers cells to and from the AAL.

The AAL functions at the data link layer of the OSI reference model and is responsible for providing communication with upper-layer protocols. Because ATM uses 53 byte segments, the AAL must break up packets received from upper-layer protocols and reassemble packets sent to the upper-layer protocols.

It is at the AAL level that integration of ATM into traditional Ethernet and Token Ring networks occurs. The service that supports the integration of ATM into a LAN environment is called LAN emulation or LANE.

For more information on introducing ATM into you network, check out: "Use LAN emulation to integrate ATM with Ethernet, Token Ring networks."

Warren Heaton CCDA, CCNA, MCSE+I, is the Cisco program manager for A Technological Advantage in Louisville, KY.

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