IEEE/OSA JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. XX, NOVEMBER 2004 1 Dynamic Bandwidth Allocation Schemes to Improve Utilization under Non-Uniform Traffic in Ethernet Passive Optical Networks Kyuho Son, Hyungkeun Ryu, Student Member, IEEE and Song Chong, Member, IEEE Abstract— Ethernet Passive Optical Networks (EPONs) are an emerging access network technology that provides a low cost solution for fiber-to-the-home (FTTH) and fiber-to-the-business (FTTB). In this paper, conventional bandwidth allocation schemes in EPON are shown to suffer from poor utilization under non- uniform traffic conditions, particularly as the number of ONUs, guard time and round-trip time increase. To resolve this problem, we propose a new scheme that allocates a timeslot intelligently by considering other ONUs’ queue occupancy, instead of strictly enforcing a maximum timeslot size. The analysis and simulation results show that the proposed scheme can provide significantly higher utilization than conventional schemes and can support max-min fairness under non-uniform traffic conditions. Index Terms— access network, Ethernet Passive Optical Net- work (EPON), bandwidth allocation scheme, non-uniform traffic, maximum utilization, max-min fairness, performance analysis. I. I NTRODUCTION Recently, the capacity of backbone networks has been increased dramatically. However, in access networks, dial-up, xDSL(x Digital Subscriber Line) and cable modem technolo- gies provide relatively small increases in data transfer. With the rapid growth of the number of Internet users and multimedia services, access networks have become bottlenecks. A passive optical network (PON), which is a point-to-multipoint optical network composed solely of passive elements, is regarded as the technology most likely to offer a solution to this problem, [1], [2] and is considered to be the best way to sup- port emerging services such as high-quality digital television broadcast services and online education multicast services. In particular, Ethernet PON (EPON) is the best candidate for a next generation optical access network because Ethernet is cheap, simple, scalable and popular, and is regarded as an economical method for deploying fiber-to-the-X (FTTX) [3]. Fig. 1 shows a typical topology of EPON. An optical line terminal (OLT), located at a local exchange, is connected to multiple optical network units (ONUs) through passive elements, splitter and coupler. In downlink transmission from OLT to ONUs (using 1550nm wavelength), a packet is broad- casted automatically to all ONUs through an optical splitter and each ONU filters the received packet according to its destination address. In uplink transmission from ONUs to OLT (using 1310nm wavelength), a packet from all ONUs can only reach the OLT, not other ONUs. As a result, ONUs Manuscript received January xx, 2004; revised November xx, 2004. This work was supported in part by University IT Research Center Project and Electronics and Telecommunications Research Institute, Korea. must share a single uplink optical fiber trunk. To prevent data from being corrupted due to multiple ONUs transmitting at the same time, a medium access control protocol is required. Therefore, a Multi-Point Control Protocol (MPCP) is being developed by the IEEE 802.3ah task force to arbitrate between transmissions by ONUs to avoid collisions [4]. The MPCP is operated principally by two control messages, GATE and REPORT. Each ONU informs the OLT of the queue occupancy by REPORT to help the OLT make a decision about bandwidth allocation. The OLT assigns the transmission timeslot by GATE. In EPON, one of the most important issues is how to share uplink resources efficiently and fairly (i.e., the bandwidth allocation problem), and much research is currently being done by experts on this topic. Kramer et al. [5] suggested a fixed service scheme that always grants the maximum timeslot size to each ONU. They determine the optimal max- imum timeslot size by using a utilization-delay optimization approach. In Interleaved Polling with Adaptive Cycle Time (IPACT) [6], they also proposed a well-designed bandwidth allocation framework that minimizes the unused timeslot by interleaving polling messages. IPACT can reduce the polling delay while maintaining high link utilization, and can be used with a variety of allocation schemes: fixed, gated, limited, constant/linear credit service, etc. Kramer et al. compared the above schemes through simulations and concluded that neither of the discussed service schemes is better than the limited service scheme. In [7], the authors introduced the concept of threshold report to achieve higher bandwidth efficiency. In addition to research on the bandwidth allocation problem, there is much research being done on differentiating quality of services (or classes of services) in EPON [8]–[11]. Previous research mostly has performed simulations under the assumption of uniform traffic conditions. In this paper, we show that not only do conventional bandwidth allocation schemes suffer from utilization degradation under non-uniform traffic conditions, but also that the degradation can be se- vere when the number of ONUs, guard time and round-trip time increase. We note that this problem can be resolved if an intelligent decision is made about bandwidth allocation without maximum timeslot restriction, which decision takes into account not just one, but all queues of ONUs. Our pro- posed scheme can provide higher utilization than conventional schemes and can support max-min fairness under non-uniform traffic conditions. The remainder of this paper is organized as follows. In