A multidimensional model, with three planes and four layers, as illustrated in Figure A-9.The lower two layers of this reference model loosely compare to the Physical Layer of the OSI Reference Model.As in the OSI model, each layer of the ATM model functions independently, yet all layers are tightly linked and the functions are highly coordinated.The layers of the ATM reference model are the Physical Layer, the ATM Layer, ATM Adaptation Layer, and higher layers and functions.