Performance assessment between tree- and star-ring-based EPONs

Ethernet Passive Optical Network (EPON) was proposed to overcome the bandwidth bottleneck at the Access Networks due to its simplicity, cost effectiveness and wide spread deployment. The architecture in access networks can affect the system performance, because access networks transmit aggregated high-speed traffic from hundreds of subscribers. Dynamic bandwidth allocation (DBA) mechanisms have been applied in backbone networks, but are not considered significantly in the scope of star-ring architecture. In this paper, we propose a distributed QoS-Based Dynamic Bandwidth Allocation (DQ-DBA) scheduling for a modified star-ring-based EPON architecture. The modified star-ring architecture includes OLT, Sub-OLT (an intermediary between OLT and ONUs) and ONUs. The DQ-DBA is making the highest priority traffic (EF) of each ONU is sent to OLT by tree structure, and the minor priority traffic (AF and BE) of each ONU are transmitted to Sub-OLT by ring architecture. Furthermore, this approach can effectively resolve the idle period problem and reduce overload of OLT in traditional DBA mechanism. We perform exhaustive simulation experiments to study the system performance between tree architecture and star-ring architecture to validate the usefulness of the proposed mechanism. The simulation results show that the proposed DQ-DBA mechanism in modified star-ring architecture can reduce the packet delay and packet delay variation for high priority ONUs to ensure Quality of Service (QoS).