- Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and adopted by the ITU-T.
- Cell Networks: ATM is a cell network. A cell is a small data unit of ﬁxed size that is the basic unit of data exchange in a cell network.
- ATM is a switched network. The user access devices, called the end points, are connected to the switches inside the network. The switches are connected to each other using high-speed communication channels. Figure1 shows an example of an ATM network.
Virtual Connection: Connection between two end points is accomplished through transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs).
- A transmission path (TP) is the physical connection (wire, cable, satellite, and so on) between an end point and a switch or between two switches.
- A transmission path is divided into several virtual paths. A virtual path (VP) provides a connection or a set of connections between two switches.
- Cell networks are based on virtual circuits (VCs). All cells belonging to a single message follow the same virtual circuit and remain in their original order until they reach their destination.
ATM Layers: The ATM standard defines three layers. They are, from top to bottom, the application adaptation layer, the ATM layer, and the physical layer as shown in Figure2.
AAL Layer: The application adaptation layer (AAL) allows existing networks (such as packet networks) to connect to ATM facilities.
AAL5: AAL5, which is sometimes called the simple and efficient adaptation layer (SEAL), assumes that all cells belonging to a single message travel sequentially and that control functions are included in the upper layers of the sending application.