Control. This plane is responsible for generating and managing signalling requests, i.e. call controls and connection control functions. User. This plane is responsible for managing the transfer of data along with error and flow control. Management. This plane contains two components: (a) Layer management manages layer-specific functions, such as the detection of failures and protocol problems. (b) Plane management manages and coordinates functions related to the complete system and also provides coordination between all the planes. The ATM reference model is composed of the following ATM layers: Physical layer. Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the medium dependent transmission. This layer consists of two sublayers: the physical medium sublayer and the transmission convergence sublayer. Transmission Network Planning and Optimisation Management Plane OSI Reference Model Control Plane User Plane Application Higher Layer Presentation Session ATM Adaption Layer Transport Network ATM Layer Physical Layer Physical Figure 3.30 B-ISDN ATM protocol reference model ATM layer. Combined with the ATM adaptation layer, the ATM layer is roughly analogous to the data link layer of the OSI reference model. The ATM layer is responsible for establishing connections and passing cells through the ATM network. To do this, it uses information in the header of each ATM cell. ATM adaptation layer (AAL). Combined with the ATM layer, the AAL is roughly analogous to the data link layer of the OSI model. The AAL is responsible for isolating higher layer protocols from the details of the ATM processes. Finally, the higher layers residing above the AAL accept user data, arrange it into packets and hand it to the AAL. Figure 3.30 illustrates the ATM protocol reference model. The Physical Layer The physical layer of any architecture model takes care of the transfer of information from the higher layers above it to the physical medium the system uses. The ATM physical layer has four basic 