ATM Standards. Protocol Reference Model

The telecommunication standardization sector of the ITU, the international standards agency commissioned by the United Nations for the global standardization of telecommunications, has developed a number of standards for ATM networks.

Other standards bodies and consortia (e.g., the ATM Forum, ANSI) have also contributed to the development of ATM standards. This section presents an overview of the standards, with particular emphasis on the protocol reference model used by ATM (2).

Protocol Reference Model. The B-ISDN protocol reference model, defined in ITU-T recommendation I.321, is shown in Fig. 1 (1). The purpose of the protocol reference model is to clarify the functions that ATM networks perform by grouping them into a set of interrelated, function-specific layers and planes. The reference model consists of a user plane, a control plane, and a management plane. Within the user and control planes is a hierarchical set of layers.

Figure 1. Protocol reference model for ATM

The user plane defines a set of functions for the transfer of user information between communication endpoints; the control plane defines control functions such as call establishment, call maintenance, and call release; and the management plane defines the operations necessary to control information flow between planes and layers and to maintain accurate and fault-tolerant network operation.

Within the user and control planes, there are three layers: the physical layer, the ATM layer, and the ATM adaptation layer (AAL). Figure 2 summarizes the functions of each layer (1). The physical layer performs primarily bit- level functions, the ATM layer is primarily responsible for the switching of ATM cells, and the ATM adaptation layer is responsible for the conversion of higher-layer protocol frames into ATM cells. The functions that the physical, ATM, and adaptation layers perform are described in more detail next.

Figure 2. Functions of each layer in the protocol reference model

Physical Layer. The physical layer is divided into two sublayers: the physical medium sublayer and the transmission convergence sublayer (1).

Physical Medium Sublayer. The physical medium (PM) sublayer performs medium-dependent functions. For example, it provides bit transmission capabilities including bit alignment, line coding and electrical/optical conversion. The PM sublayer is also responsible for bit timing (i.e., the insertion and extraction of bit timing information). The PM sublayer currently supports two types of interface: optical and electrical.

Transmission Convergence Sublayer. Above the physical medium sublayer is the transmission convergence (TC) sublayer, which is primarily responsible for the framing of data transported over the physical medium. The ITU-T recommendation specifies two options for TC sublayer transmission frame structure: cell-based and synchronous digital hierarchy (SDH). In the cell-based case, cells are transported continuously without any regular frame structure. Under SDH, cells are carried in a special frame structure based on the North American SONET (synchronous optical network) protocol (3).

Regardless of which transmission frame structure is used, the TC sublayer is responsible for the following four functions: cell rate decoupling, header error control, cell delineation, and transmission frame adaptation. Cell rate decoupling is the insertion of idle cells at the sending side to adapt the ATM cell stream’s rate to the rate of the transmission path. Header error control is the insertion of an 8-bit CRC in the ATM cell header to protect the contents of the ATM cell header. Cell delineation is the detection of cell boundaries. Transmission frame adaptation is the encapsulation of departing cells into an appropriate framing structure (either cell- based or SDH-based).