4. Protocol model, functional architecture, network and interfaces

4.1 Protocol reference model

As is customary in the world of communications protocols, ATM has been endowed with a protocol reference model that organizes the various functions into layers and planes that are in principle homogeneous and independent of each other (figure 22 ). In the user plane, which describes the actual transfer of information, the protocol stack emphasizes the common ATM layer, which embodies the multiservice nature of the technology. This layer is common in two respects: different AAL adaptation layers enable different applications to be carried, while different physical layers (PHY) reflect ATM's compatibility with different underlying transmission systems.