SUCCESS-HPON: Migrating from TDM-PON to WDM-PON David Gutierrez (1), Kyeong Soo Kim (2), Fu-Tai An (1) and Leonid G. Kazovsky (1) 1 : Photonics & Networking Research Laboratory, Stanford University, Stanford CA 94304, {degm,kazovsky}@stanford.edu, [email protected] 2 : Advanced System Technology, STMicroelectronics, San Jose CA 95131, [email protected] Abstract We present a summary of SUCCESS-HPON, a hybrid WDM/TDM PON architecture that enables the smooth and cost-effective migration from TDM-PONs to WDM-PONs using centralized light sources and novel scheduling algorithms to share tunable components. Introduction The Stanford University aCCESS (SUCCESS) initiative within the Photonics & Networking Research Laboratory encompasses multiple projects in access networks. One of these projects is the Hybrid TDM/WDM PON or SUCCESS-HPON [1]. The two main motivations behind this network architecture are: (1) Provide a smooth migration from current TDM based PONs (i.e., EPON, BPON and GPON) to higher bandwidth WDM-PONs and (2) Make WDMPON cost-efficient for access. The SUCCESS-HPON architecture proposes a migration path from TDM to WDM-PON and uses a centralized light sources (CLS) approach and tunable WDM component sharing for cost-efficiency. CO

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the functionality of the optical access network is exactly the same. Therefore, existing TDM-PON ONUs can continue their normal operation as before. As more users demand higher bandwidths, RNs that contain an AWG as a DWDM mux / demux are inserted in the ring, as shown in Fig. 1(c). The WDM ONUs that are connected to these RNs have a dedicated DWDM channel between them and the OLT at the CO. In order to avoid the need for tunable sources at the ONU, we use CLS with RSOAs as modulators [2], using intensity modulation in both directions. In this manner, we can provide up to 1.25 Gbps half-duplex downstream (DS) and upstream (US) transmission with current technologies.

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Figure 1. From TDM-PON to WDM-PON TDM to WDM-PON Migration Fig. 1 illustrates the transition path from TDM-PON to WDM-PON. In Fig. 1(a), a Central Office (CO) provides service for three existing tree TDM-PONs. Each of these requires exclusive cabling and OLT inside the CO. As a first step, in Fig 1(b), the passive splitters of the TDM-PONs are replaced by Remote Nodes (RNs) consisting of passive couplers and thin film add/drop filters to introduce CWDM. The feeder fiber of each TDM-PON is replaced with a single fiber ring that strings all the RNs served by this CO. This is the most dramatic step during the migration process, but its benefits in terms of resource sharing and protection are considerable as we will describe. Note that the distribution fibers are not changed in this migration. From the TDM ONUs’ point of view,

(c) Fig. 2. Experimental results: (a) DS data and CW on λ1, (b) DS data and CW on λ2, (c) US data from on λ1 and λ2 on modulated CW, (d) DS eye diagram and (e) US eye diagram at 1.25 Gbps. The RNs may implement basic protection and restoration functionality using semi-passive switches. Fig. 1(d) shows even further flexibility: since there is a dedicated wavelength at the output of the AWG, it is possible to use the collector ring as a backhaul for PONs with tree topology. The two feeder fibers of the PON can connect to different RNs to form a protection path. To upgrade the capacity even further, the RSOA modulator can be replaced by a stabilized laser source ONU for full-duplex communication. Experimental results shown in Figure 2 illustrate how bidirectional transmission on the same wavelength at

1.25 Gbps is possible. In this experiment, we did not yet use the RSOAs, but a combination of circulator + MZ modulator. For further details, please refer to [3]. The OLT, RNs and WDM ONUs are shown in Fig. 3. Given that all the ONUs in the network share the same ring and that a CLS approach is being used, just a few expensive tunable DWDM components can be shared at the OLT. As the number of users and/or traffic in the network increases, more tunable transmitters or receivers are easily added at the OLT. The main benefits of this network architecture are that (1) it allows for TDM and WDM-PONs to coexist, making a smooth transition possible, (2) it is costefficient by sharing the expensive WDM components and using a CLS approach and (3) it scales easily by simply adding more tunable components at the OLT.

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transmissions according to traffic demands. Several algorithms have been developed as part of this project. These algorithms including sequential mode, in which the scheduling is done as packets are received or transmitted, and batch mode (Bath Earliest Departure First, BEDF), in which the scheduling is performed every certain period of time, allowing for some throughput optimization at the expense of some delay. The Sequential Scheduling 3 with Schedule-Time Framing (S F) algorithm has relatively low computational complexity, high throughput and low delay. A comparison of the throughput of these three algorithms in a 16 ONU PON using 10 Gbps rates, with 8 transmitters and varying number of receivers (N) is shown in Figure 4. For a detailed description and analysis of these algorithms, please see [1,4,5].

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(d) Figure 3. SUCCESS-HPON components: (a) OLT, (b) CWDM RN, (c) DWDM RN, (d) WDM-PON ONU. Resource Sharing A single OLT equipped with tunable lasers and receivers can serve multiple TDM and WDM ONUs. In order to do this, particular MAC protocols and scheduling algorithms need to be implemented to enable the efficient sharing of these expensive tunable WDM resources among many users for both DS and US transmission. The algorithm has to keep track of the status of the tunable transmitters, tunable receivers, ONU wavelength assignments and round trip times and arrange them properly in both the time and wavelength domains for both DS and US data

Future work Future work in SUCCESS-HPON includes achieving higher transmission rates with the RSOAs, implementing simplified versions of the scheduling algorithms on an FPGA testbed and providing QoS guarantees over this architecture. Conclusions The SUCCESS-HPON architecture provides a smooth migration path from current TDM-PON to future WDM-PON. We present the architecture and design for the OLT, RNs and ONUs. Physical layer transmission experiments show the CLS approach feasibility. Scheduling algorithms have been developed to share expensive WDM resources and the throughput simulation results presented here. Future work in this project is briefly mentioned. References [1] F.-T. An et al, IEEE JLT, Vol. 22, No. 11, p. 2557. [2] C. Arellano et al, OFC 2006, OTuC1. [3] F.-T. An et al, IEEE Comm Mag, Vol. 43, No. 11, p. S40. [4] K.S. Kim et al, IEEE JLT, Vol. 23 No. 11, p. 3716. [5] J.W. Lee et al, submitted to Globecom 2006.