ATM Protocol Architecture Mode

ATM architecture uses a logical reference model to describe its functions ATM functions correspond to physical layer and part of the Data Link layer of the OSI model On its own ATM has function at layers 1and 2 of the OSI model, but today TCP/IP is routed over ATM networks which means it can also function at layers 3 and 4 of the OSI model. Acts as an interface b/w user and network.

ATM Protocol Architecture (Reference Model)

Physical layer - responsible for transferring cells via physical medium. Physical Media Dependent Sub layer - transmission of physical signals, bit synchronization. Transmission Convergence Sub layer – cell discovering, mapping into transmission slots, scrambling/descrambling, header error control (HEC) processing. ATM Layer – Common to all services; Provides packet transfer capabilities – cell multiplexing, switching and flow control. AAL – ATM Adaptation Layer – Service Dependent; Maps higher layer info in to ATM cells and then collects info from ATM cells for delivery to higher layers.

Has 2 sublayers: Convergence Sublayer ( CS ) Determines the Class of service (CoS) for the incoming traffic. Provides a specific AAL service at an AAL network service access point (NSAP). Segmentation and Reassembly Sublayer (SAR) Segments higher – level user data into 48 – byte cells plus necessary overhead at the sending node and reassembles cells at the receiving node

The ATM reference model constructed as 3 planes which span all the layers : Control – generates and manages the signaling requests User – manages the transfer of data e.g. Error control, flow control. Management – contains 2 parts: – Layer management : manages layer specific functions – Plane management : manages and co – ordinates functions related to the whole system