Revolutionizing optical networks with key new technologies
Modern colorless, directionless, contentless, and flexible grid (CDC-F) ROADM networks have gained wide industry acceptance due to their flexibility, simplified operations, and improved efficiency. These new ROADM architectures automatically move, steer, and re-route wavelengths without the need for costly, time-consuming, manual changes required on previous generations of mux/demux-based ROADMs. Since their introduction in 2016, the industry has a seen a dramatic shift to these newer, more modern, more flexible CDC-F ROADM architectures.
ROADM Add/Drop – Technology Engine
Modern CDC-F ROADM nodes are composed of ROADM line units, CDC-F add/drop blocks, transponders, and a fiber patch panel that provides interconnection between the functional blocks (Figure 1). The add/drop block enables any transponder to be connected to any incoming port (colorless), any port to be connected to any ROADM line degree (directionless), while avoiding wavelength interference (contentionless), all without requiring manual fiber changes or manual intervention. The add/drop block is the engine of a CDC-F ROADM node.
Figure 1) CDC-F ROADM Node Architecture (MCS)
When first introduced, CDC-F ROADMs utilized multi-cast switch (MCS) modules to implement the add/drop architecture. As shown in Figure 2, an MCS module consists of an array of splitters, couplers, and optical switches. In the drop direction, wavelengths are split to each client-side port, where optical switches select the provisioned degree (i.e., direction). In the add direction, optical couplers simply combine all the wavelengths provisioned to the same WDM direction.
Figure 2) Multi-Cast Switch (MCS)
Recently, contentionless MxN wavelength-selective switch (WSS) technology was introduced as an alternative option to support CDC-F add/drop architectures. Within an MxN WSS unit, the optical splitters and couplers used in an MCS architecture are replaced by dual 8x24 or dual 16x24 WSS modules.
Replacing the optical splitters and couplers used in an MCS architecture with MxN WSS modules reduces the insertion loss. However, the larger, more complex MxN WSS components result in physically larger cards, along with slightly higher ROADM prices.
MCS vs MxN WSS Comparison
The optical networking industry supports both MCS- and MxN WSS-based architectures, with both technologies providing the same colorless, directionless, contentionless, flexible-grid interconnections between transponders and WDM line degrees. While MCS- and MxN WSS-based architectures offer similar operation and performance, there are important differences with each technology.
MCS-based ROADM architectures can be configured with 8-degree (8D) or 16-degree (16D) nodes and support node capacities with up to approximately 500 transponder add/drop ports. Due to their simpler internal technology, MCS modules tend to be smaller in size, requiring fewer ROADM shelf slots, than MxN WSS alternatives. As a mature technology, MCS components are widely available from multiple component suppliers, ensuring a diverse supply chain with independent production capacity.
MxN WSS-based architectures are ideal for nodes with very large add/drop port requirements. When combined with amplified splitter/coupler cards, MxN WSS architectures can support up to 1000 transponder ports. As with MCS-based systems, MxN WSS modules are also available in both 8D and 16D configurations.
As a result of their more complex underlying technology, MxN WSS cards tend to be higher priced and physically larger than MCS options. In addition, the MxN WSS components are currently single-sourced in the industry.
Figure 3) Comparison Summary
MCS units have moderately high insertion losses, requiring amplifier arrays in MCS add/drop blocks to boost the optical power levels, as shown in Figure 1. One of the rationales for MxN WSS-based architectures was the hope that the lower MxN WSS insertion loss might preclude the need for amplification in the MxN WSS add/drop block. Unfortunately, most MxN WSS-based applications will require similar add/drop amplification to ensure proper internal power levels and achieve comparable OSNR performance.
Future-Proofing Optical Networks
The additional optical layer flexibility provided by CDC-F ROADMs enables powerful new optical layer protection and restoration techniques, improves network capacity and utilization, and reduces overall networking costs. Network operations that once required costly, time-consuming, manual changes can now be fully automated.
Both MCS and MxN WSS technologies functionally perform the same CDC-F function for ROADM add/drop architectures and both can be configured to provide similar network performance. MCS- based architectures are offer slightly lower ROADM node costs than MxN WSS versions and result in slightly smaller physical node sizes. MxN WSS-based systems can support very large add/drop port counts (> 600 ports) and are better suited for use with new, low Tx power coherent pluggables, when coupled with amplified splitter/coupler cards. Fortunately, Nokia 1830 products support a wide array of both MCS and MxN WSS solutions, so carriers can choose the best fit for their individual network applications.