Scheduling and wavelength assignment issues in metro-scale hybrid WDM/TDMA EPONs

Online upstream scheduling and wavelength assignment (USWA) for hybrid WDM/TDMA EPON networks has been shown to perform much better in terms of frame delay compared to offline USWA. However, when simplistic online USWA schemes are applied, especially in metro-scale EPONs, wavelength channels tend to be underutilized due to the formation of scheduling voids. In this work we propose schemes to overcome the aforementioned inefficiency following two different directions. The first one, represented by the Earliest Finish Time with Void Filling (EFT-VF) algorithm, takes into account voids when scheduling upstream transmissions and fills them when possible. A different approach, followed by the Distance Based Grouping (DBG) concept, is based on the observation that voids emerge as a result of significant differential OLT-ONU distances among ONUs. Therefore, when similarly distanced ONUs are scheduled on the same wavelengths the void formation effect is reduced. All proposed schemes are presented in detail and evaluated using simulations.