Figure is a reference model that illustrates the organization of ATM functionality and the interrelationships between the layers of functionality.
Figure : ATM Reference Model
In the ATM reference model, the ATM layer and the ATM adaptation layers are roughly analogous parts of the data link layer of the Open System Interconnection (OSI) reference model, and the ATM physical layer is analogous to the physical layer of the OSI reference model. The control plane is responsible for generating and managing signaling requests. The user plane is responsible for managing the transfer of data. Above the ATM adaptation layer are higher-layer protocols representing traditional transports and applications.
The ATM physical layer controls transmission and receipt of bits on the physical medium. It also keeps track of ATM cell boundaries and packages cells into the appropriate type of frame for the physical medium being used.
The ATM physical layer is divided into two parts: the physical medium sublayer and the transmission convergence sublayer. The physical medium sublayer is responsible for sending and receiving a continuous flow of bits with associated timing information to synchronize transmission and reception. Because it includes only physical-medium-dependent functions, its specification depends on the physical medium used.
ATM can use any physical medium capable of carrying ATM cells. Some existing standards that can carry ATM cells are SONET (Synchronous Optical Network)/SDH, DS-3/E3, 100-Mbps local fiber (Fiber Distributed Data Interface [FDDI] physical layer), and 155-Mbps local fiber (Fiber Channel physical layer). Various proposals for use over twisted-pair wire are also under consideration.
The transmission convergence sublayer is responsible for the following:
- Cell delineation—Maintains ATM cell boundaries.
- Header error control sequence generation and verification—Generates and checks the header error control code to ensure valid data.
- Cell rate decoupling—Inserts or suppresses idle (unassigned) ATM cells to adapt the rate of valid ATM cells to the payload capacity of the transmission system.
- Transmission frame adaptation—Packages ATM cells into frames acceptable to the particular physical-layer implementation.
- Transmission frame generation and recovery—Generates and maintains the appropriate physical-layer frame structure.
The ATM layer is responsible for establishing connections and passing cells through the ATM network. To do this, it uses the information contained in the header of each ATM cell. For more information about how information in the header is used to pass cells through an ATM network, see the section “ATM Switching,” later in this chapter.
ATM Adaptation Layer
The ATM adaptation layer (AAL) translates between the larger service data units (SDUs) (for example, video streams, and data packets) of upper-layer processes and ATM cells. Specifically, the ATM adaptation layer (AAL) receives packets from upper-level protocols (such as AppleTalk, Internet Protocols [IP], and NetWare) and breaks them into the 48-byte segments that form the payload field of an ATM cell. Several ATM adaptation layers are currently specified. Table 1 summarizes the characteristics of each AAL.
Table 1 : ATM Adaptation Layers
|Requires timing between source and destination||Yes||No||No||No|
|Traffic types||Voice and circuit emulation||Data||Data||Data|