Research Issues in Next-Generation Optical Access Networks: Lessons from the SUCCESS Initiative

Optical access has long been considered a final solution to the problem of upgrading the current bottlenecked access to the one capable of delivering future broadband integrated services due to the huge capacity of an optical fiber. Because optical fibers have been used in backbone networks, Wide Area Networks (WANs), and Metropolitan Area Networks (MANs), and are also being introduced in Local Area Networks (LANs) with the advent of optical Ethernet, the implementation of the optical access would be the last step to future all-opticalnetwork revolution [1]. Traditional point-to-point optical network architectures are expensive for access configurations: Besides fiber deployment costs, they need maintenance for the outside plant active systems. These systems consist of many Electrical-to-Optical (E/O) and Optical-to-Electrical (O/E) components prone to failure, which prevent their large-scale deployment. To address these issues and expedite the introduction of Fiber To The Home/Curb/Node (FTTX), Time Division Multiplexing Passive Optical Networks (TDM-PONs) have been developed. These TDM-PONs include Ethernet PON (EPON), Broadband PON (BPON) and Gigabit PON (GPON), which are based on similar passive tree topologies but have different data transport technologies. They are internationally standardized by the IEEE and ITU, products following the standards have been developed, and are currently being deployed in the field by network service providers in several places around the world, mostly in Japan, Korea, and the United States. In a few years, once TDM-PONs are deployed, upgrading these optical access networks will be a challenge when user demand outgrows the existing access network capacities. TDMPONs have only one wavelength for downstream data and one for upstream data to share among all users, thus limiting the average bandwidth per user to a few tens of Mbit/s [2]. The very high available bandwidth of fiber is thus mostly wasted. Wavelength Division Multiplexing (WDM) technology has been considered an ideal solution to extend the capacity of TDM-PONs without drastically changing the fiber infrastructure. Many architectures incorporating WDM into access have been proposed by both academia and industry (e.g., [3] and [4]). However, integrating WDM and TDM to ensure the flexibility of the optical access networks still requires further investigation, which is a major subject of this paper. In this paper we discuss major research issues that we will face in developing and deploying next-generation optical access networks, especially based on the lessons we learned from Stanford University aCCESS (SUCCESS) initiative [5], [6] which encompasses multiple projects in optical access networks. The outline of the paper is as follows. Section II describes a new architectural trend in optical networks ranging from WANs to MANs to access together with the issues these new optical network architectures try to address. Section III reviews two projects under the SUCCESS initiative for upgrading TDM-PONs: SUCCESS-HPON and SUCCESS-DWA. Section IV discusses major research issues in next-generation optical access networks. Section V summarizes our discussions in this paper.