In this article

As identified in our previous installment of next generation optical networking, Flex Spectrum: What Is It?, no real conversation around Flex Spectrum is complete until we talk about colorless, directionless and contentionless (CDC) ROADMs. 

Since the industry adoption of higher bandwidth wavelengths such as 400G, 600G and 800G, a solution such as Flex Spectrum has been necessary to allow for these greater than traditional 50Ghz and 100Ghz channel spacing wavelengths to pass through the system.

Flex Spectrum addresses the channel spacing required to allow higher bandwidth wavelengths into ROADM systems. However, it does not resolve the vast limitations of traditional standard grid ROADM systems.

Limitations of fixed grid ROADM systems

So, what does CDC-ROADM bring to the table as compared to traditional ROADM systems? In order to appreciate what CDC-ROADM has to offer, we need to understand what comprises traditional fixed grid ROADM systems limitations.

To start, fixed grid ROADMs are confined to having fixed wavelengths assigned to specific ports of WDM filters.  As displayed in Figure 1 below, we can see a traditional 40 channel DWDM filter. As we can see, the first port will only allow for that specific wavelength to be inserted into it (1530.33 in this case). If a different wavelength is input into this port the system will not be able to discern the incoming light, in which case it will not function properly.

40 channel filter
Figure 1: 40 channel filter

Moreover, these fixed wavelength ports are then assigned to a specific degree in which that wavelength is required to go out. Therefore, a wavelength must go into a pre-defined WDM filter port which it is required to exit out of the node on a specific degree. Please refer to Figure 2 below showing the node level breakdown of a traditional fixed grid ROADM site. As we can see a wavelength coming into the A filter must exit out the A degree. The same goes for the B and C degrees.

Fixed grid ROADM
Figure 2: Fixed grid ROADM

If an operator incurs a fiber break or equipment failure on the A degree, a technician must be deployed to then physically move the A filter wavelength to another degree in order to restore services. Additionally, the system must then be re-provisioned to allow this new wavelength to utilize another degree of the node. This same process would need to be completed on the terminating node as well.

Fixed grid A-degree failure
Figure 3: Fixed grid A-degree failure

Moving along, let's keep using our Figure 2 example. Retaining this same fiber break scenario, let's consider the actual wavelength coming into the A filter, which is blue. In order to swing the wavelength over to the B degree we would need to be sure that this same wavelength isn't currently being used on that degree. In our example, we are already using the blue wavelength on our B degree, so in order for our technician to be able to restore services we would be required to manually change that wavelength to another in order for the service to pass.  

In certain situations, tunable optics would give us the ability to retune that frequency port to another wavelength, and we would re-provision as we did in the first example. Nonetheless, manual intervention to this extent can add additional downtime and incur excessive operational expense.

Fixed grid A-degree failure with wavelength blocking
Figure 4: Fixed grid A-degree failure with wavelength blocking

As we can see fixed grid ROADM solutions lack the flexibility, re-programmability and automation needed in the optical transport systems of today.

Benefits of CDC-ROADM

To begin speaking towards the benefits of CDC-ROADM we can start by addressing some of the shortcomings of fixed grid ROADM solutions. First, a fixed grid solution requires the wavelength to go into a specific port of a WDM filter as referenced in Figure 1. CDC-ROADMS are colorless. This means that any filter port is capable of taking in any wavelength, as seen below. 

CDC-ROADM filter
Figure 5: CDC-ROADM filter

Continuing on, we previously discussed how our fixed grid ROADM system required a technician to move the wavelength from the A filter over to the B filter when a fiber cut was incurred. CDC-ROADMs are directionless, meaning the system is able to reroute the wavelength to other degrees in the event of a fiber or hardware failure. This is possible through manual intervention through the controller software, or it can happen automatically. That choice is up to the operator.

Additionally, we spoke about the inability of a fixed grid system to not allow for the same wavelength to be dropped onto the same path. This inability is referred to as blocking. Colorless alone won't resolve this issue of wavelength contention thus, CDC-ROADMs are contentionless. This provides a dedicated internal structure to the system which avoids such blocking mechanisms.

Figure 6: CDC-ROADM

Furthermore, there are additional benefits and features that a CDC-ROADM solution offers above and beyond addressing the shortcomings of a legacy fixed grid ROADM system. First off, a CDC-ROADM has the ability to restore wavelength level services on a failure automatically. This means that if a wavelength service between two locations fails due to a fiber cut or hardware failure, a CDC-ROADM can restore those wavelengths to an alternate path if it is available. See the video below for a brief example of how this automated process is possible.

It is important to understand that CDC-ROADMs offer many upgrades as compared to traditional fixed grid solutions, however some operators may decide that a fixed grid solution still provides some benefit to their system. It is entirely possible to run a fixed grid solution within the same ROADM environment as a CDC-ROADM. In this situation the operator has the ability to provision services as either fixed grid wavelengths or leverage their CDC portion of the spectrum to take advantage of the colorless, directionless and contentionless capabilities.


Traditional fixed grid solutions suffer several shortcomings. Those essentially break down to a lack of colorless, directionless and contentionless capabilities. Moreover, a CDC-ROADM can automatically restore failed services along another path from source to destination.

Ultimately, the ideal next generation optical transport solution lies within leveraging Flex Spectrum technology in tandem with CDC-ROADM capabilities. With this type of solution, an operator has the ease in knowing their transport system will support higher bandwidth applications of today and far into the future. Additionally, the operator empowers the network to restore services automatically in the event of a failure with the colorless, directionless and contentionless capabilities.

Questions or comments? Sound off in the comment section below or reach out directly to speak with our team of experts.