The ATM Protocol Reference Model

The ATM protocol reference model is consistent with the telecommunications protocol approach. Basically it consists of three planes: The user plane, control plane and the management plane. The user and control planes include a physical layer, the ATM layer and the AAL.

The ATM layer provides cell-transfer capabilities in a per-virtual-circuit fashion by multiplexing/demultiplexing different virtual connections in/from a single flow of ATM cells. It is responsible, at the transmitting side, for assigning the correct VPI/VCI values to the cells belonging to a virtual connection and for delivering incoming cells to the related logical connection.

The AAL performs service-dependent functions: four different AAL types have been defined in order to adapt the application requirements to the ATM cell transport. The control plane has its own AAL, conventionally called (signalling) SAAL. The AAL types take into account the application needs in terms of timing relation between source and destination, traffic emission profile (CBR or VBR) and connection mode (connection oriented or connectionless). Each AAL type is subdivided into two layers: SAR (segmentation and reassembly) sublayer performs the higher layer information framing/ reconstruction in/from a sequence of small units fitting into the ATM cell payload. The CS (convergence sublayer) provides service specific functions (timing, frame error detection, and frame sequence checking and recovery).

Out of the four AAL types, AAL5 is favored for connection-oriented variable-rate data (VBR) services. AAL5 offers to the majority of data applications the best tradeoff between the reliability of data delivery, supported by error-detection mechanisms, and the associated overhead. (ATM and AAL protocols’ overhead is high: it spans up to 17%, that is, up to 9 bytes over the 53-byte-long cell). AAL1 supports connection-oriented services that require CBR’s and have specific timing and delay requirements, while AAL2 supports connection-oriented services that do not require constant bit rates. AAL3/4 is intended for both connectionless and connection-oriented VBR services.