ATM protocol reference model and functions of each layer
The B-ISDN protocol reference model is defined in ITU-T Recommendation I.321. This model is a three-dimensional model, including three planes: user plane U, control plane C, and management plane M, and in each plane It is layered and divided into physical layer, ATM layer, AAL layer and high layer.
The three faces in the protocol reference model perform different functions:
1. User plane U: provides the function of user information flow transmission, and also has certain control functions, such as flow control and error control;
2. Control plane C: provides call control and connection control functions, and uses signaling to establish, monitor, and release calls and connections;
3. Management plane M: provides two management functions: including layer management and plane management. Layer management (layered), complete management functions related to the resources and parameters of each protocol layer entity, such as meta-signaling, and also handle OAM information flow related to each layer; plane management (non-layered), it completes with Related management functions of the entire system, and play a coordinating role on all planes.
The functions of each layer in the protocol reference model of ATM:
1. The physical layer is divided into two sublayers: PM (physical media sublayer) and TC (transmission convergence sublayer). PM (Physical Media Sublayer) is responsible for line coding photoelectric conversion and bit timing to ensure the correct transmission of the data bit stream; the function of the transmission convergence sublayer is cell rate decoupling; HEC generation / checking; cell delimitation; transmission Frame adaptation; transmission frame generation / recovery.
2. The ATM layer mainly completes four functions: general flow control; letterhead generation and extraction; cell VPI / VCI translation; cell multiplexing and branching.
3. AAL (ATM Adaptive Layer) whose function is to adapt high-level functions into ATM cells. The purpose of the AAL layer is to enable different types of services, including management plane and control plane information, to be transmitted in a unified ATM cell form after adaptation. The AAL layer is directly related to the business. The AAL layer performs different adaptations to different types of services. For ATM users, AAL is implemented in user terminal equipment; for non-ATM users, AAL is implemented in the network side equipment of UNI. The AAL layer is further divided into two sublayers: disassembly and assembly sublayer SAR and convergence sublayer CS. At the sending end, the service flow needs to be adapted to the ATM layer, and SAR segments the high-level information into ATM cells of fixed length and standard format; at the receiving end, when transferring ATM layer information to the upper layer, SAR receives ATM cells , Reassemble it into a high-level protocol information format. The CS performs functions such as delivery of timing information, error detection and processing, processing of cell transmission delays, and identification and processing of user data units.
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