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The ATM reference model contains the adaptataion layer (AAL), which provides network models (connection-oriented, connectionless, time priorities or not); the ATM layer, which mostly does routing and control signalling; and the physical layer, which defines signal levels on various transmission media, etc. The two most widely accepted physical standards are currently SONET and HDS.
Remaining challenges for ATM include good emulation of existing LAN standards (e.g. IEEE 802.x), hardware incompatibilities among vendors, QOS scheduling in software (ATM provides the mechanism for doing this but no policy), creating fast switches, and implementing successful flow and congestion control.