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Asynchronous Transfer Mode (ATM) is a Layer 2 WAN technology that operates using the concept of PVCs and SVCs. However, ATM uses fixed-length cells as its protocol data unit (PDU), as opposed to the variable frames used by Frame Relay. An ATM cell contains a 48-byte payload and a 5-byte header.

An ATM cell’s 48-byte payload size resulted from a compromise between the different countries as an international standard for ATM was being developed. Some countries, such as France and Japan, wanted a 32-byte payload size because smaller payload sizes worked well for voice transmission. However, other countries, including the United States, requested a 64-byte payload size because they felt such a size would better support the transmission of both voice and data. In the end, the compromise was to use the average of 32 bytes and 64 bytes (that is, 48 bytes).

Although ATM uses VCs to send voice, data, and video, those VCs are not identified with DLCIs. Instead, ATM uses a pair of numbers to identify a VC. One of the numbers represents the identifier of an ATM virtual path. A single virtual path can contain multiple virtual circuits.

A virtual path is labeled with a virtual path identifier (VPI), and a virtual circuit is labeled with a virtual circuit identifier (VCI). Therefore, an ATM VC can be identified with a VPI/VCI pair of numbers. For example, 100/110 can be used to represent a VC with a VPI of 100 and a VCI of 110.

Interconnections between ATM switches and ATM endpoints are called user-network interfaces (UNI), and interconnections between ATM switches are called network-node interfaces (NNI).

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