A wireless data network architecture supports both centralized mode
operation and distributed mode operation. In the centralized mode, all of
the Open Systems Interconnection (OSI) reference model data link layer,
also known as layer 2, protocol functions are implemented within the
radio access network control entity, such as at a base station controller
(BSC) or similar network controller, before the data packets are
delivered to one or more base station transceiver subsystems (BTSs). In
the distributed mode, some of the layer 2 protocol functions are
implemented at the BSC or similar network controller, and some of the
layer 2 protocol functions are implemented at the BTSs. The data
transmission is dynamically switched from one of the modes to the other
as a function of various implementation-specific triggers.