Report duration computation schemes in reduced-buffer optical network units for passive optical networks

Reduction of packet buffers in an optical network unit (ONU) has been proposed previously as a power savings mechanism. To support smaller buffer sizes at an ONU, the polling scheme used must be extended to two levels. Similar to an ONU reporting its queue length to the optical line terminal (OLT), with two-level polling the end nodes (ENs) will report their queue lengths to the connected ONU. We present ONU report computation schemes that take reported EN queue lengths as the input and compute the report duration to be sought from the OLT. This problem can be mapped onto the minimum makespan scheduling (MMS) problem that is NP-complete. Three different heuristics were studied: longest duration first (LD), wrapped round robin (WR), and earliest free first (EF). To incorporate the small-buffer case, three sequencing strategies were also proposed. These strategies are executed before (PRE), after (PST), or during (INL) the execution of a given MMS heuristic. The performances of nine combinations (3 heuristics × 3 sequencing strategies) were evaluated by using a simulation-based model. Based on simulations, the EF-PST combination emerged as the best possible solution for Poisson, video, and WAN traffic types.