Applications for two-degree and multi-degree reconfigurable add/drop multiplexers
Today's network operators are finding distinct applications for two-degree and multi-degree ROADMs. In many cases, the two ROADM types may coexist and complement each other to redefine the DWDM layer in new optical networks.
By Sorin Tibuleac, Movaz Networks
The first generation of commercially available reconfigurable optical add/drop multiplexers (ROADMs) is being deployed with increasing volume in metro and long-haul DWDM networks. ROADMs provide several benefits over fixed optical add/drop multiplexers (OADMs), including:
• Simplified network engineering and operation;
• Accelerated turn-up of new services;
• Improved performance of DWDM system provided by power equalization; and
• Increased network availability.
These features ultimately translate into lower operating costs for DWDM networks, which is a top priority among service providers in the new telecommunications market environment.
ROADMs with 1x1 wavelength-selective switches
ROADM architecture is based either on 1x1 wavelength-selective switches (WSS), also known as wavelength blockers, or 1x2 switch and variable optical attenuator (VOA) arrays. The blocker alternative uses a broadcast-and-select approach, with a passive splitter and combiner to drop and add all channels and a 1x1 WSS with per-wavelength optical attenuation capabilities to block the wavelengths terminated at the ROADM node. These devices typically were built with free-space optical diffraction gratings for wavelength multiplexing and demultiplexing with micro-electromechanical systems (MEMS) or liquid crystal arrays for wavelength switching. The wide, flat passbands of these 1x1 WSS ROADMs--with high adjacent-channel isolation, low polarization-dependent loss (PDL), and low chromatic dispersion (CD)--yield excellent optical performance in any DWDM network, including ultra long haul and 40-Gbit/sec systems.
ROADMs with 1x2 switch array
The 1x2 switch array ROADM, based on arrayed-waveguide gratings (AWG), has benefited from recent advances in planar waveguide technology. Reduction in insertion loss and improvements in passband shape have generated a practical alternative to the 1x1 WSSs based on free-space optics. A ROADM 'add' module can be built by inserting arrays of 1x2 switches and VOAs between a pair of AWG multiplexers/demultiplexers. A set of photodiodes performs wavelength monitoring and power equalization for add and pass-through channels. Similar to the 1x1 WSS-based ROADM, the 'drop' module uses a passive splitter and an AWG demultiplexer.
The optical performance monitoring and multiplexer/demultiplexer units required for add/drop in a single shelf provide a compact and economical solution for full-capacity wavelength reconfigurability (See Figure 1). Furthermore, if passive AWG multiplexers and demultiplexers are used in the ROADM, a reduction in power consumption and a simplification of ROADM management is achieved.It is likely that future development of these devices will lead to integration of some or all key ROADM functions, including multiplexing/demultiplexing, attenuation, switching, and power monitoring, on the same opto-electronic chip assembly. This could yield an improvement in performance, an increase in reliability, and a significant reduction in cost.
ROADMs with 1xN wavelength-selective switches
While employing different underlying technologies for wavelength blocking or switching, these first-generation ROADMs share a common architecture. They are network elements with two-degree connectivity deployed in a ring configuration with fixed wavelengths for each add/drop port. As these two-degree ROADMs (2D-ROADMs) continue to improve, a new generation of ROADMs is emerging based on WSS devices with one input (or output) port and N output (or input) ports. Such 1xN WSS units can be configured in a network as "colorless" 2D-ROADMs, or they can form the building blocks for a multi-degree ROADM (MD-ROADM).
The core of a colorless ROADM typically consists of a 1xN WSS used as a drop module (one input port and N output ports), where one output port is used for pass-through traffic, and N output ports are used for drop traffic. This drop unit can be paired with a passive N:1 coupler to add wavelengths from tunable laser transponders. Unlike the 2D-ROADMs discussed previously, the resulting 2D-ROADM offers the flexibility of adding and dropping any wavelength on any N port. This flexibility enables the network operator to offer new services based on dynamic provisioning of wavelengths on a DWDM ring.
However, this add/drop flexibility may limit the add/drop capacity at a network node if the number of available ports on a 1xN WSS is lower than the number of channels that need to be terminated. Commercially available 1xN WSS units typically have up to nine output ports, while the number of channels per fiber usually is 40 for metro/regional networks and more for long-haul networks. While an increased number of ports can be expected with improvements in technology, a colorless ROADM with full add/drop capability will require passive couplers with an equivalent increase in port count. The higher split ratio of the passive coupler with an increased number of ports adds significant loss to the 'add' and 'pass-through' paths.
Multi-degree ROADMs
Nx1 WSSs also are used in a multi-degree ROADM (MD-ROADM) to switch individual wavelengths between different DWDM paths as required for inter-ring connectivity and mesh networks. Typically, inter-ring connectivity is performed in the electrical domain. However, with increased traffic and higher modulation rates, optical-electrical-optical (OEO) conversion and electronic switching between interconnecting rings carries a prohibitively high cost.
A wavelength-selective optical cross-connect can be built using Nx1 WSSs as building blocks (See Figure 2). This function can be performed by a set of N passive splitters with N:1 split ratio and Nx1 WSSs. One port is used for local add/drop while the others are dedicated to inter-node connectivity. As an example, a set of five 5x1 WSSs with corresponding passive splitters can be used for add/drop and for interconnecting two rings, enabling traffic to be redirected from any ingress network fiber to any egress network fiber, including optical loop-back for testing and fault-isolation. Using Nx1 WSSs and passive splitters rather than a single NxN WSS results in a mode modular solution, thus reducing the cost of initial deployments and allowing in-service upgrades as new routes and external equipment are connected through the wavelength-switching node.
ROADM and fixed OADM
While ROADMs are experiencing an impressive growth in the market, service providers cannot neglect their large embedded DWDM infrastructure with fixed OADMs.
Upgrading such legacy networks from OADMs to ROADMs with minimum cost requires a ROADM solution that can reuse existing equipment, including amplifiers, dispersion compensation modules (DCMs), transponders, line and tributary protection cards, and shelves with common equipment for control and management. In figures 1 and 2, the ROADM can replace a multiplexer/demultiplexer pair used for fixed OADM, while all other circuit packs are reused in the ROADM node.
Upgrade paths that need to be considered when planning and designing a network include:
• Fixed OADM to 2D-ROADM;
• Fixed OADM to MD-ROADM; and
• 2D-ROADM to MD-ROADM.
In addition, the cost difference between fixed and reconfigurable OADMs maintains a place for fixed OADMs either in hybrid networks or in rings subtended from core rings with ROADMs (See Figure 3).
Network operators are finding distinct applications for both 2D-ROADMs and MD-ROADMs, and in many cases, the two ROADM architectures may coexist and complement each other to redefine the DWDM layer in new optical networks. A mixture of 2D-ROADMs and MD-ROADMs is likely to dominate the core of next-generation DWDM networks. MD-ROADMs will be used at hub sites, while 2D-ROADMs with fixed wavelengths or colorless add/drop ports will be used at other locations as needed. A fixed OADM solution may be adequate for subtending rings or linear segments. The ability of a DWDM system to support multiple types of ROADMs and fixed optical add/drop nodes on the same optical transmission path allows network operators to design an agile, yet cost-effective network.
Sorin Tibuleac is director of product management at Movaz Networks (Norcross, GA). He may be reached via the company's Web site at www.movaz.com.