Fair Queuing with Service Envelopes (FQSE): a cousin-fair hierarchical scheduler for Ethernet PONs

Abstract: Non-hierarchicalalgorithms are not scalable for scheduling in EPON, while hierarchical algorithms do not provide fairness among,the end consumers. This study presents a novel hierarchical EPON scheduler, which guarantees fairness among all subscribers. ©2003 Optical Society of America OCIS codes: (060.4250) Networks 1.,Introduction Ethernet Passive Optical Networks (EPON) are point-to-multipoint optical access networks. An Optical Line Terminal (OLT) at the Central Office (CO) is connected to many Optical Network Units (ONUs) at remote ends using optical fiber and passive splitter[1]. Each ONU acts as an access point for multiple subscribers, each of whom has one or more queues. Since the ONUs share the same optical channel for upstream transmission, the OLT must schedule timeslots for the ONUs to transmit data. The timeslot assignment is facilitated by using GATE and REPORT control messages to grant and request bandwidth respectively. A fundamental requirement for an access network is to beable to guarantee minimum service (bandwidth) to each subscriber and ensure fairness in distributing excess bandwidth, if any is available. One way to achieve this is to assign timeslot to each subscriber independently. However, such method is not scalable with the number of subscribers as it requires a separate GATE and REPORT message to each subscriber, and given a large number of subscribers, may consume a significant portion of EPON bandwidth. Alternatively, EPON may employ a hierarchical scheduling scheme, where central scheduler in the OLT assigns timeslots to individual ONUs. In turn, each ONU subdivides its assigned timeslot among the subscribers connected to it. Such hierarchical schemes are free from the scalability issues mentioned above, however, they cannot guarantee fairness among the subscribers. Most hierarchical scheduling algorithms that we surveyed in literature, allow fairness only between siblings (i.e., nodes having the same parent). We call these schemes sibling-fair.A sibling-fair algorithm may discriminate between users connected to different ONUs. Hence, there is a need for a cousin-fair scheduler, which schedules fairly to leaf nodes(subscribers) in the hierarchy. Figure 1 illustrates the