Fu-Tai An

SUCCESS: a next-generation hybrid WDM/TDM optical access network architecture

In this paper, the authors propose a next-generation hybrid WDM/TDM optical access network architecture called Stanford University aCCESS or SUCCESS. This architecture provides practical migration steps from current-generation time-division multiplexing (TDM)-passive optical network (PONs) to future WDM optical access networks. The architecture is backward compatible for users on existing TDM-PONs, while simultaneously capable of providing upgraded high-bandwidth services to new users on DWDM-PONs through advanced WDM techniques. The SUCCESS architecture is based on a collector ring and several distribution stars connecting the CO and the users. A semipassive configuration of the Remote Nodes (RNs) enables protection and restoration, making the network resilient to power failures. A novel design of the OLT and DWDM-PON ONUs minimizes the system cost considerably: 1) tunable lasers and receivers at the OLT are shared by all ONUs on the network to reduce the transceiver count and 2) the fast tunable lasers not only generate downstream data traffic but also provide DWDM-PON ONUs with optical CW bursts for their upstream data transmission. Results from an experimental system testbed support the feasibility of the proposed SUCCESS architecture. Also, simulation results of the first SUCCESS DWDM-PON MAC protocol verify that it can efficiently provide bidirectional transmission between the OLT and ONUs over multiple wavelengths with a small number of tunable transmitters and receivers.

SUCCESS-HPON: A next-generation optical access architecture for smooth migration from TDM-PON to WDM-PON

Optical access networks are considered to be a definite solution to the problem of upgrading current congested access networks to ones capable of delivering future broadband integrated services. However, the high deployment and maintenance cost of traditional point-to-point architectures is a major economic barrier. Current TDM-PON architectures are economically feasible, but bandwidth-limited. In this article we first discuss the possible role of WDM in access networks and investigate the associated issues. We then present the Stanford University Access Hybrid WDM/TDM Passive Optical Network (SUCCESS-HPON), a next-generation hybrid WDM/TDM optical access architecture that focuses on providing a smooth migration path from current TDM-PONs to future WDM-PONs. The first testbed for this architecture is described, along with the experimental results obtained, including feasibility of bidirectional transmission on the same wavelength on the same fiber for access networks and ONU modulation of upstream data on continuous waves provided by the OLT, eliminating the need for tunable components at the ONUs. The development of a second testbed and the issues it will address, including the implementability of the SUCCESS-HPON MAC protocol and scheduling algorithms, are also described.

Next Generation Optical Access Networks

The last mile continues to be a major bottleneck in the Internet. Its low bandwidth and flexibility prevents the deployment of new services and the development of new applications. In this paper we present a summary of current efforts in access networks research, focusing in particular on fiber optic solutions. We present the Stanford University aCCESS (SUCCESS) initiative within the Photonics & Networking Research Laboratory (PNRL). As part of this initiative, two novel network architectures have been developed, SUCCESS-HPON and SUCCESS-DWA, which propose a smooth migration path from current TDM-PONs to future higher bandwidth, cost-efficient, scalable WDM-PONs. In addition, we present SUCCESS-LCO, a spectral-shaping line coding technique that enables a cost-effective shorter-term capacity upgrade of existing TDM-PONs. We discuss as well what we believe are the main open research areas in optical access networks.

Experimental demonstration of an access point for HORNET-A packet-over-WDM multiple-access MAN

Hybrid opto-electronic ring network (HORNET) is a novel packet-over-WDM multiple-access network designed by the Stanford Optical Communications Research Laboratory (OCRL) to provide efficient bandwidth sharing among a large number of access points (APs) in a metropolitan area. The HORNET network eliminates the cost and complexity of SONET equipment by transmitting IP/ATM packets directly over the wavelength division multiplexing (WDM) layer. To improve performance above that of a conventional ring network, HORNET employs a multiple-access architecture using fast tunable transmitters and a novel carrier-sense multiple-access with collision avoidance (CSMA/CA) media access control (MAC) protocol. The OCRL has constructed a testbed to demonstrate the ability of a HORNET AP to transmit packets using a fast-tunable transmitter and a novel MAC protocol and to asynchronously receive packets in the packet-over-WDM architecture. The experimental results confirm that HORNET successfully achieves the following functions: 1) fast (low overhead) wavelength tuning using a fast-tunable transmitter; 2) collision-free packet transmission over a multiple-access network via the CSMA/CA MAC protocol; and 3) fast clock and data recovery using the embedded clock tone (ECT) technique.

Design and performance analysis of scheduling algorithms for WDM-PON under SUCCESS-HPON architecture

Results of the design and performance analysis of two new algorithms for efficient and fair scheduling of variable-length frames in a wavelength division multiplexing (WDM)-passive optical network (PON) under the Stanford University aCCESS-Hybrid PON (SUCCESS-HPON) architecture are reported. The WDM-PON under the SUCCESS-HPON architecture has unique features that have direct impacts on the design of scheduling algorithms. First, an optical line terminal (OLT) uses tunable transmitters and receivers that are shared by all the optical network units (ONUs) served by the OLT to reduce the number of expensive dense WDM (DWDM) transceivers. Second, also for cost reduction, ONUs have no local DWDM light sources but use optical modulators to modulate optical continuous wave (CW) bursts provided by the OLT for upstream transmissions. Therefore, the tunable transmitters at the OLT are used for both upstream and downstream transmissions. To provide efficient bidirectional communications between the OLT and the ONUs and guarantee fairness between upstream and downstream traffic, two scheduling algorithms have been designed: 1) batching earliest departure first (BEDF); and 2) sequential scheduling with schedule-time framing (S/sup 3/F). The BEDF is based on the batch scheduling mode where frames arriving at the OLT during a batch period are stored in virtual output queues (VOQs) and scheduled at the end of the batch period. It improves transmission efficiency by selecting the frame with the earliest departure time from a batch of multiple frames, which optimizes the usage of tunable transmitters in scheduling. Considering the high complexity of the optimization process in BEDF, the S/sup 3/F based on the sequential scheduling mode has also been designed as in the original sequential scheduling algorithm proposed earlier. In S/sup 3/F, the authors use VOQs to provide memory space protection among traffic flows and a granting scheme together with schedule-time framing for both upstream and downstream traffic to reduce framing and guard band overhead. Through extensive simulations under various configurations of the tunable transmitters and receivers, it has been demonstrated that both the BEDF and S/sup 3/F substantially improve the throughput and delay performances over the original sequential scheduling algorithm, while guaranteeing better fairness between upstream and downstream traffic.

Experimental demonstration of a novel media access protocol for HORNET: a packet-over-WDM multiple-access MAN ring

As packet-based traffic in the metropolitan area network continues to exponentially increase, and as content and applications become more distributed, conventional metropolitan area networks fail to function efficiently. Sprint Advanced Technology Laboratories and Stanford Universitys Optical Communications Research Laboratory are developing a new network called HORNET (hybrid opto-electronic ring network), which is optimized for bursty, unpredictable, packet-based traffic patterns with distributed sources and destinations. HORNET uses fast tunable transmitters and a novel media access control scheme to realize a packet-over-WDM multiple-access ring architecture. The experimental demonstration of the novel media access control scheme is presented in this work.

A new media access control protocol guaranteeing fairness among users in Ethernet-based passive optical networks

We propose a new EPON MAC protocol guaranteeing fairness among users by allocating excess bandwidth proportional to their subscription rates. The novelty of the protocol is in the use of scalable per-subscription-rate-queuing with round-robin scheduling and packet reclassification at ONU.

Batch scheduling algorithm for SUCCESS WDM-PON

In this paper we study the problem of scheduling variable-length frames in WDM-PON under Stanford University access (SUCCESS), a next-generation hybrid WDM/TDM optical access network architecture. The SUCCESS WDM-PON architecture has unique features that have direct impact on the design of scheduling algorithms: first, tunable transmitters and receivers at OLT are shared by ONU to reduce transceiver counts; second, the tunable transmitters not only generate downstream data traffic but also provide ONU with optical continuous wave (CW) bursts for upstream transmissions. To provide efficient bidirectional transmissions between OLT and ONU, we propose a batch scheduling algorithm based on the sequential scheduling algorithm previously studied. The key idea is to provide room for optimization and priority queueing by scheduling over more than one frame. In the batch scheduling, frames arriving at OLT during a batch period are stored in virtual output queues (VOQ) and scheduled at the end of the batch period. Through simulation with various configurations, we demonstrate that the proposed batch scheduling algorithm, compared to the original sequential scheduling algorithm, provides higher throughput, especially when the system load is high, and better fairness between upstream and downstream transmissions.

Gain-clamped S-band discrete Raman amplifier

In summary, we have demonstrated, for the first time to our knowledge, a gain-clamped discrete Raman amplifier in the S-band. The device achieves peak net gain over 22 dB, and a gain variation of only 0.3 dB for signal input power ranging from -20 dBm to 2.7 dBm. Unlike conventional gain-clamping designs, an extinction ratio between signal and lasing wavelength of over 30 dB is achieved. This new technique suppresses instability due to randomly-polarized ASE and double Rayleigh scattering (DRS) (hence noise figure) for small signal input power under high pump power, as well as power surges due to transient effects in Raman amplifiers. This promises a viable technology to provide gain bands not currently available from traditional doped fiber amplifiers.

Fast transient response of L-band tellurite-based EDFAs and their optically gain-clamped behavior

We reported on the fast transient response of EDTFAs. The transient time of EDTFAs can be one-ninth as short as that of the EDSFAs with similar gain shape in L-band. We also demonstrated optically gain-clamped EDTFAs and showed that the power excursion due to SHB is comparable to or smaller than that in EDSFAs. The relaxation oscillation frequencies also give us information about the fast dynamics within EDTFAs. In summary, EDTFAs show superior controllability over EDSFAs for automatic gain control via either electrical or optical schemes. Further research will include modeling of the dynamic behavior of EDTFAs.