ATM is a packet switching protocol that encodes data into small fixed-sized cells and provides data link layer services that run over OSI Layer 1 physical links. ATM cells is 53 bytes in length, which include a 5-byte header and 48-byte payload. Their small and fixed size makes them perfect for the transmission of time-sensitive traffic like voice and video. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used.
ATM uses a connection-oriented model and establishes a virtual circuit between two endpoints before the actual data exchange begins. Virtue circuits provide a bidirectional communications path from one ATM endpoint to another. Every ATM cell has an 8- or 12-bit Virtual Path Identifier (VPI) and 16-bit Virtual Channel Identifier (VCI) pair defined in its header. Together, these identify the virtual circuit used by the connection. As these cells traverse an ATM network, switching takes place by changing the VPI/VCI values.
ATM can build virtual circuits either statically or dynamically.
- switched virtual circuits (SVCs) -- ATM networks build and tear down switched virtual circuits (SVCs) on demand when requested by an end equipment. Through the use of switched virtual circuits, an ATM network can behave almost like a typical circuit-switched network.
- permanent virtual circuits (PVCs) -- When PVCs are used, a path must be defined between endpoints across an ATM internetwork, including on intermediary switches. While a PVC provides the benefit of reducing the overhead associated with call setup and teardown, it also limits data to a single path across the network, eliminating redundant paths.
Two main types of equipment exist on ATM networks:
- ATM switches -- The ATM switch handles transmission of cells through the ATM network. Its functions are: accepting the incoming cell from an ATM end station or another ATM switch; reading and updating the cell-header information and switching the cell toward its destination.
- ATM end points-- An ATM end point is a network device equipped with an ATM network interface card, such as a router, computer, LAN switch, and so forth.
An ATM network is made of ATM switches connected by ATM interfaces.
There are two main types of interfaces:
- The UNI connects ATM end systems to an ATM switch.
- The NNI connects two ATM switches.