Asynchronous Transfer Mode (ATM) is a network protocol designed to facilitate the simultaneous handling of various types of traffic streams (voice, data, video) at very high speeds over the same physical connection. By always using 53-byte cells, ATM simplifies the design of hardware which can quickly determine the destination address of each cell. This allows simple switching of network traffic at much higher speeds than are easily accomplished using protocols with variable sizes of transfer units, such as Frame Relay and TCP/IP.
Although it was designed to operate without the requirement of any other networking protocol, other protocols are frequently segmented and encapsulated across multiple, smaller ATM cells; in effect making ATM a transport mechanism for pre-existing technologies such as Frame Relay and the TCP/IP family of protocols.
ATM relies on the concepts of Virtual Paths and Virtual Circuits. A Virtual Path, represented by a specific Virtual Path Identifier (VPI) establishes a route between two devices in a network. Each VPI may contain multiple Virtual Circuits, each represented by a Virtual Circuit Identifier (VCI).
VPIs and VCIs are local to the router, which means that only the two devices connected by the VCI or VPI need know the details of the connection. In a typical ATM network, user data might traverse multiple connections, using many different VPI and VCI connections. Each end device, just as each device in the network, needs to know only the VCI and VPI information for the path to the next device.
This chapter discusses the following ATM-specific interface properties that you can configure.