For flexible and agile service adaptation at a low cost in the next-generation optical access system, a new system architecture, which is based on SDN/NFV technologies, should be required. The most promising way to pursue the new architecture is disaggregating PON functions to functional modules with open interfaces. The project propose a new work item to progress the architecture further by specifying a PON abstraction interface for time-critical processing functions, e.g. Dynamic Bandwidth Assignment (DBA), which is not covered existing BBF documents and on-going projects.