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Ethernet Passive Optical Network (EPON) has been considered in the past few years as the best candidate for offering cost-effective and efficient broadband access. To save energy in EPON, it has been suggested to allow the optical network unit (ONU) to go into sleep mode. This nonetheless still wastes energy due to the overhead time caused by switching from sleep to active. To address this problem, the paper investigates the scenario where burst-mode transmission is enabled in downstream and upstream channels. The ONU can go into sleep mode for a certain amount of time before waking up to send/receive a batch of buffered traffic. To determine the maximum ONU sleep time without impairing the required quality-of-service (QoS) for all types of traffic, we develop an analytical model whereby each ONU is modeled as an M/G/1 system with vacations. Our model is able to extract an expression for the …