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Medium access control (MAC) is one of the most crucial issues in Ethernet passive optical networks (EPONs). To prevent data of different optical network units (ONUs) from collision in the upstream direction, an EPON system must employ a MAC mechanism to arbitrate access to the shared upstream channel and at the same time efficiently share the bandwidth of the upstream channel among all ONUs. In this paper, we present an adaptive MAC polling protocol for an EPON system. This polling protocol uses an adaptive scheduling algorithm called the earliest-packet-first (EPF) algorithm that schedules the transmission order of different ONUs based on the arrival time of the first packet waiting in the queue of each ONU and always schedules the ONU with the earliest packet to transmit first in each polling. The purpose is to reduce the packet delay in the system and thus provide better quality of service for end users.