# Optimization of Optical Networks Based on CDC-ROADM Technology
## 1 Introduction  
Colorless, Directionless and Contentionless Reconfigurable Optical Add Drop Multiplexers (CDC-ROADMs) have become an important element of optical transmission systems deployed by major network operators across the world. This is because Conventional Reconfigurable Optical Add Drop Multiplexers (C-ROADMs) fail to provide wavelength routing flexibility sufficient to meet system requirements resulting from rapid data traffic growth in optical networks [1] (CDC) [2; 3; 4]. Figure 1 shows a schematic diagram of C-ROADM. C-ROADMs are limited by fixed wavelength assignment to specific ports and fixed direction assignment for multiplexers and thus can add/drop a wavelength only to a fixed outgoing direction. For instance, in Figure 1 a transponder connected to the Mux West can only add/drop a wavelength to the West direction and not to any other (i.e., North or East). Thus rerouting the wavelength emitted by a transponder connected to Mux West to a different direction requires a visit of an engineer on site in order to manually connect the transponder to the correct mux/demux port. Hence, C-ROADM technology is also incompatible with the concept of the software defined network (SDN). On the positive side C-ROADM architectures, based on Wavelength Selective Switch (WSS), allow for simple adding, dropping and express routing traffic through networknodes and hence offer benefits such as simple planning, simple and robust bandwidth use and low cost network maintenance. Nonetheless, the growing traffic in optical networks renders C-ROADM technology more and more obsolete due to their low routing flexibility and lack of ability to conform with SDN paradigm. CDC-ROADMs are of great interest to next generation optical telecom networks since they offer much greater wavelength routing flexibility than C-ROADMs and also because they conform with the SDN paradigm. Figure 2 shows a schematic diagram of a CDC-ROADM. Using CDC-ROADMs optical network operators can remotely connect any wavelength from any transponder to any direction (Figure 2). Thus regardless of specific architecture details, the final result is that any wavelength (color) can be assigned to any port at the add/drop site, using remote software control without any technician intervention on site [5; 6]. This combined with now widely deployed remotely tunable coherent transponders gives a network operator unprecedented capability to manage the optical network within the SDN paradigm thus enhancing network flexibility and survivability while reducing capital and operational expenses. This is because with tunable transponders network operators do not need to pay high capital expense by installing transponders for all wavelengths, but can implement an investment strategy of \"pay as you grow\". It is noted that there are also intermediate optical node architectures between C-ROADM and CDC-ROADM. Colorless ROADM node architectures can remotely assign any wavelength to a particular Mux port and thus provide the means for building ROADMs that automate the assignment of add/drop wavelengths. Additionally, having colorless functionality, one can use different wavelengths for different sections in the optical path to avoid congestion in the network. Directionless ROADMs, by contrast, allow any wavelength to be routed to any direction served by a node using software control without any physical rewiring. Colorless and directionless ROADMs combine the advantages of both technologies [6; 7; 8]. However, even with colorless and directionless functionality, an optical network still has limitations that could require manual intervention on site in some cases. In other words,  
Figure 1: Schematic diagram of conventional C-ROADM. the colorless, directionless network is still not completely flexible and compatible with the SDN concept. Only a contentionless architecture, allows multiple copies of the same wavelength on a single add/drop Mux.