In this paper, we present a detailed and marketing - free study of all components of the upstream and downstream channel transmission overhead for Ethernet Passive Optical Networks (EPONs), excluding optional Forward Error Correction (FEC). First, the general Ethernet framing overhead is evaluated, followed by in depth analysis of each overhead component related to operation of the EPON MPCP protocol. Their overall impact on the performance of the system (especially in the upstream channel) is quantified for both best and worst case scenarios using typical system parameters, characteristic of standard EPON deployments. Based on a set of standard-based and typical EPON parameters, an estimated throughput of approximately 820 to 900 Mbit/s is available in the upstream direction, while in the downstream the effective throughput ranges from around 915 to 935 Mbit/s, producing the channel efficiency figures of 82 to 90% and 91.5 to 93.5% , respectively, relative to the effective data rate of 1 Gbit/s.