Bandwidth Drivers and Emerging Networking Requirements
Wavelength networking is no longer the preserve of transporting highly aggregated traffic between a few major nodes in a network.

The explosion of video services traffic across business and residential users is driving new architectures where traditional metro and regional network boundaries are blurring. According to a recent Nortel study, video on demand (VoD) services traffic will start dominating networks even at modest penetration levels. Add to this the growth in private networking over dark fiber across a range of enterprise and government verticals, which can be better supported via managed wavelengths over shared infrastructure. Together these considerations are driving a change in network architectures where wavelength-level networking is a major force du jour. A flexible any-to-any reconfigurable connectivity at the wavelength layer delivers the true capability required to serve the different range of bandwidth demands, particularly those driven by emerging content provider architectures. The paper by Verizon Labs referenced in [1] provides more detailed insight.

Satisfying a cast of many
How can suppliers mold an optical network solution suited both for the most ambitious operators and also address the requirements of those that are steadfastly improving their existing network and operations but yet need future-proofing to get a good return on their incremental capital expenses (CAPEX)?

The requirements in Figure 1, from a recent survey of "service provider" from Heavy Reading [2], highlight the common need of all operators for significant improvements in the speed and efficiency of provisioning wavelengths. Note additionally there is also an imperative to remove non-essential electrical regeneration points to reduce the CAPEX of regenerating the many wavelengths these systems support. Add to this the new architectural requirement for wavelength layer routing and we have a number of interrelated solution requirements to consider.

Figure 1: Long-Haul DWDM: Market and Technology Outlook (Source: Heavy Reading, February 2007)

These combined requirements can be addressed by leveraging new optical branching technologies and developments in transponder technologies, but to truly exploit all the values and make the transition future-proof, the challenge is only properly addressed under a structured framework that includes future requirements. Let us define this framework as the "adaptive intelligent all-optical network."

We first define the goals of an adaptive intelligent all-optical network and discuss the key building blocks that are required to deliver these as follows:

• Adaptive — To cost-effectively operate a network where the any-to-any wavelength paths are readily configured and reconfigured at 10 Gbps, 40 Gbps and ultimately at 100 Gbps with minimum manual intervention at intermediate sites
• All Optical — To innovative transponder designs with a system reach that avoids the OEO regeneration, wavelength signal termination for branching, and associated costs, as well as remove fixed dispersion management schemes to leverage an optical branch's wavelength routing capability while also simplifying operations
• Intelligent — To continuously optimize wavelength paths thus speeding up provisioning and leveraging the above attributes to create a dynamic, adaptive, and reconfigurable network.

  Figure 2 shows a generic network scenario with the key building blocks for adaptive intelligent all-optical networks. Note the key requirements are to efficiently route wavelengths across any path on any fiber type across the longest distance achievable without regeneration so that only two endpoint optical transponders are ever required.

  
  Figure 2: Key Building Blocks

  Key Building Blocks for All-Optical Networking

  Next-Generation DWDM Transponders

  Tunable transponders: a main driver for tunable transponders to date has been to reduce spare inventory. Full C-band tunable sources are available in the market. As far as all optical networking is concerned, the flexibility to choose any wavelength for any path is extremely powerful, so tunable transponders are a key requirement.

  
  Figure 3:

  Dynamic electronic dispersion equalization [3]: A large proportion of installed fiber suffers from chromatic dispersion, so compensators that reverse this impairment are used at many sections. Bulk chromatic dispersion compensation using traditional dispersion compensation fiber (DCF) cannot support the values of dynamically routing any wavelength over any path/fiber in a network that supports optical branching.

  An electronic compensating scheme is available, which can dynamically pre-compensate for dispersion management on a per-wavelength and -route basis (over distances more than 2,000 km), making rapid any-to-any path wavelength routing realizable. Figure 3 summarizes, in outline, such a scheme. Together with extended optical reach of transponders described below, these are key enablers to efficient all-optical networking.

  Extended all optical reach: powerful forward error correction (FEC) coded into transmitted information provide greater system budget and reach. Coupled with improvements in system loss (no DCMs) and per-wavelength dispersion management, state-of-the-art transponders can provide 10 Gbps network reach exceeding 2,000 km with little or no price premium compared to older transponder technologies. The key to this is use of advanced silicon in the transponders.

  Continuous system balancing, power equalization, and managed routing: To provision wavelengths quickly and efficiently over a multi-section network with distributed resource information, a domain level controller is required. This software enables in-service changes, e.g., addition/deletion of wavelengths/wavelength groups. The system automatically re-optimizes for short- and long-term power changes. The result is provisioning time reduced to a few tens of minutes, as opposed to hours or even days where intermediate regeneration sites were required.

  Colorless Optical Branching
  A remote optical add-drop multiplexer (ROADM) based on wavelength selective switch (WSS) technology is the key building block for all-optical networks. WSS ROADM come in several flavors, but for ultimate flexibility to route any wavelength to any port, the use of tunable filter technology incorporated into the solution is essential. This is referred to as colorless optical branching.

  Control Planes
  Next-generation core data networks have been converging to IP MPLS. The optical network design should be an integral part of the overall data network architecture because when this is done, the sum of the parts results in a more efficient, resilient, and agile solution capable of auto restoration and reconfiguration. Generalized MPLS (GMPLS) is the International Telecommunication Union (ITU)-defined control plane standard that interworks with the MPLS control plane. Knowledge of the physical layer is made available through distributed intelligence that resides in the active optical network elements. Key building blocks are the ability to discover topology and incorporate a wavelength path computation engine and a signaling protocol to reserve resources. The benefits are the ability to restore networks even in complex failure scenarios with bandwidth efficiency and, ultimately, client plane-signaled auto-provisioning for applications such as time-of-day services that also lead to improved network utilization.

  The Final Frontier As demands on networks evolve, the values of adaptive intelligent all-optical networks need to be preserved, so the "optical reach" and any-to-any path flexibility also has to apply at 40 Gbps rates. 40 Gbps deployment will accelerate as it provides efficiency gains for router port utilization. Unprecedented triple-play and multimedia IP service traffic is driving high-capacity router deployments, so the same overall requirements for all optical networking will apply for 40 Gbps wavelengths to minimize stranded network investments.

  Conclusion
  Creating the right foundation for adaptive intelligent all-optical networking is viable with mature technologies available today when approached in a structured way. The outcome addresses all of the requirements such as CAPEX and OPEX reduction while providing operational agility, flexibility, and simplicity required by service providers to develop network architectures and new processes to support higher volumes and new high-bandwidth services and related business models.

  References
  [1] "Architectural Trade offs of Reconfigurable DWDM Systems," E. Bert Basch et al (Verizon Laboratories MA), IEEE Journal of Selected Topics in Quantum Electronics, Vol. 14 No. 4 July/August 2006. [2] "Long-Haul Market and Technology Outlook," Heavy Reading, Vol. 5, No. 2 (February 2007). [3] Network cost impact of solutions for mitigating optical impairments, M. Belanger et al, IEEE Journal of Lightwave Technology, Vol. 24 No. 1 January 2006. [4] "Telco Video: A Sporting Chance," www.lightreading.com/document.asp?doc_id=116278&WT.svl=deptop_1