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Dynamic bandwidth allocation (DBA) is an open and hot topic in the Ethernet passive optical network (EPON). In this paper, we propose an efficient DBA scheme for EPONs. Unlike traditional DBA schemes, the proposed DBA scheme adapts the polling cycle time not only to the uplink traffic load but also to the downlink load. Thus it can adapt polling duration to the downstream load, and keep the high throughput of downlink by reducing the protocol overhead (GATE messages) related bandwidth loss. In the uplink, we divide the polling cycle into two subcycles-BG-subcycle and EBR-subcycle, and grant a window size not exceeding the minimum guaranteed bandwidth for each ONU in GB-subcycle, then reallocate the excessive bandwidth to those overloaded ONUs in EBR-subcycle. This scheme can avoid the walk-time bandwidth waste and reduce the packets delay of overloaded ONUs.