ATM architecture
ATM has a three-dimensional architecture. It contains the user plane, control plane, and management plane.
Both the user plane and the control plane are divided into the following layers: physical layer, ATM layer, ATM Adaptation Layer (AAL), and upper layer. Each layer is further divided into sublayers.
The control plane establishes and tears down connections with signaling protocols. The management plane contains layer management and plane management. Layer management manages the layers in each plane and has a layered structure corresponding to other planes. Plane management manages the system and the communications between different planes.
Figure 42 shows the relationships between layers and planes in ATM.
Figure 42: ATM architecture
ATM layers have the following functions:
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. AAL passes 48-byte payloads, which are called segmentation and reassembly protocol data units (SAR-PDUs) to the ATM layer. The ATM layer encapsulates the 48-byte payloads in 5-byte headers, and passes 53-byte cells to the physical layer. Other functions of the ATM layer include VPI/VCI transmission, cell multiplexing/demultiplexing, and generic flow control.
ATM Adaptation Layer—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. Hewlett Packard Enterprise uses AAL5 for data communication services.
ATM upper-layer protocols—Responsible for WAN interconnection, Layer 3 interconnection, and multiprotocol over ATM (such as IP, IPoE, PPP, and PPPoE).