  Environmental and economic concerns promote research on designing
energy-efficient Time and Wavelength Division Multiplexed Ethernet Passive
Optical Network (TWDM-EPON), which is the future extension to TDM-EPON. In
TDM-EPON, a plethora of research is already present to achieve energy savings
at Optical Network Units (ONUs) which can easily be applied for TWDM-EPON ONUs.
However, TWDM-EPON provides an additional opportunity for saving energy at the
Optical Line Terminal (OLT). All existing protocols have primarily been
designed for saving energy at the OLT receivers. The protocols to save energy
at the OLT receives depends only on the Up-Stream(US) traffic scheduling while
its transmitter counterpart depends on both US and Down-Stream (DS) scheduling
since the OLT transmits GATE message along with DS traffic. The US and DS
scheduling have a basic difference. The MAC protocol doesn't allow scheduling
of US traffic of an ONU after its REPORT arrival at multiple disjoint time
slots. However, this restriction is absent for DS traffic and hence, the
grant-size of an ONU can be partitioned and every part can be scheduled at
different times. In this paper, we propose a method for saving energy at the
OLT transmitters in TWDM-EPON while satisfying the SLAs. This includes a
heuristic algorithm to partition the DS grant and schedule them. Through
extensive simulations, we demonstrate that the proposed method provides a
significant improvement in energy efficiency as compared to existing protocols
(up to 45%).
