Owing to environmental concerns, reducing energy consumption of optical access networks has become an important problem for network designers. This paper focuses on reducing energy consumption of optical network units (ONUs) in an Ethernet passive optical network (EPON). In an EPON, the optical line terminal (OLT) located at the central office broadcasts the downstream traffic to all ONUs, each of which checks all arriving downstream packets so as to obtain the downstream packets destined to itself. Thus, receivers at ONUs always have to stay in the awake status and consume a large amount of energy. To address the downstream challenge, we propose a novel sleep control scheme that can efficiently put ONU receivers into sleep without modifying the standardized EPON media access control protocol. The proposed scheme contains two main parts: downstream traffic scheduling rules at the OLT and sleep control schemes at ONUs. By letting ONUs be aware of the downstream traffic scheduling rules, ONUs can infer their own downstream queue status and switch into the sleep status properly; by letting the OLT know the sleep control scheme implemented at ONUs, the OLT can accurately infer the sleep status of ONUs and buffer traffic of asleep ONUs accordingly. We also theoretically analyze the impacts of different parameters in the sleep control scheme on the delay and energy-saving performances by using semi-Markov chains. It is shown that, with proper settings of sleep control parameters, the proposed scheme can save as much as 50% of the ONU receiver energy.
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