  Economical and environmental concerns necessitate research on designing
energy-efficient optical access network especially Ethernet Passive Optical
Network (EPON) which is one of the most widely accepted and deployed last-mile
access network. In this paper, our primary focus is on designing a protocol for
saving energy at Optical Network Units (ONUs) while satisfying the Service
Label Agreement (SLA). The SLA of both Up-Stream (US) and Down-Stream (DS)
traffic can be satisfied only if the EPON network can react to their
instantaneous load change during sleep periods of ONUs and to the best of our
knowledge, there doesn't exist any such proposal. Towards this target, we
propose a mechanism that allows the Optical Line Terminal (OLT) to force ONUs
to wake-up from sleep mode. Here, we demonstrate that if the OLT can distribute
the active ONUs (transceivers are active) fairly among cycles then it provides
a significant improvement in energy-efficiency. To achieve this, we formulate
an ILP for fairly distributing active ONUs among cycles while satisfying the
SLA of both US and DS traffic at the same time. A polynomial time
$1$-approximation algorithm is proposed for solving this ILP. The convergence
and the complexity analysis of the algorithm are also performed. Extensive
simulations depict that fair distribution of ONUs reduces the power consumption
and average delay figure at the same time and the reduction increases with an
increment of the number of ONUs and round-trip time.
