ATM architecture
ATM has a three-dimensional architecture. It consists of three planes: user plane, control plane, and management plane. Both the user plane and the control plane are divided into four layers: physical layer, ATM layer, ATM Adaptation Layer (AAL), and upper layer, each of which are further divided into the following sub-layers:
The control plane establishes and tears down connections with signaling protocols.
The management plane consists of layer management and plane management. The former manages the layers in each plane and has a layered structure corresponding to other planes. The latter manages the system and the communications between different planes.
Figure 65 illustrates the relationships between layers and planes in ATM.
Figure 65: ATM architecture
The functions of the four ATM layers are as follows:
Physical layer—Provides transmission channels for ATM cells. At this layer, cells received from the ATM layer are transferred into a continuous bit stream after transmission overheads are added to them. Meanwhile, continuous bit streams received from physical media are restored to cells, which are then passed to the ATM layer.
ATM layer—Resides over the physical layer, and implements cell-based communication with its peer layer by invoking the services provided by the physical layer. It is independent of physical media, implementation of the physical layer, and types of services being carried. Data passed to this layer takes the form of 48-byte payloads, which are segmentation and reassembly protocol data units (SAR-PDUs); and data passed from this layer to the physical layer is 53-byte cells, with the 48-byte payload being encapsulated in a five-byte header. Other functions of the ATM layer include VPI/VCI transmission, cell multiplexing/demultiplexing, and generic flow control.
ATM Adaptation Layer (AAL)—Provides interfaces between high-level protocols and the ATM Layer. It forwards information between the ATM layer and upper layer protocols. Four types of AAL are available: AAL1, AAL2, AAL3/4, and AAL5, each of which supports specific services provided in an ATM network. Most ATM equipment vendors adopt AAL5 for data communication services.
ATM upper layer protocols—Responsible for WAN interconnection, voice interconnection, Layer 3 interconnection, encapsulation, LAN emulation, multiprotocol over ATM, and traditional IP.