Abstract
Time Division Multiplexing Passive Optical Networks (TDM-PONs) has been a target for energy saving activities in the past years. Under energy efficient TDM-PON, there can be three types of a grant in a TDM-PON (i.e. upstream grants, listening grants, and gratuitous grants). These grants are assigned by the Optical Line Terminal (OLT) to its connected Optical Network Units (ONUs). The upstream grants are assigned to active ONUs, whereas; the listening (scheduled at the end of the sleep period) and gratuitous (periodically scheduled during the sleep period) grants are assigned to the ONUs which invoked to enter into the energy efficient mode. In this paper, we investigate the influences of the gratuitous grant on the energy efficient TDM-PON. We reveal three factors that can affect the total upstream grant time during a Dynamic Bandwidth Allocation (DBA) cycle. Those factors are: (1) the number of ONUs of a TDM-PON, (2) gratuitous grant interval and (3) gratuitous grant size. Results indicate that the larger number of ONUs and the larger amount of the gratuitous grant size can decrease the total upstream grant time remarkably. On the other hand, results indicate that increasing the gratuitous grant interval can increase the total upstream grant time. And therefore, in this paper, we point out that it is significant to develop a method to decide the gratuitous grant interval and size to improve the performance of an energy efficient TDM-PONs.