Colorless Directionless Contentionless ROADM Networks

Introduction:

Often when we are looking at the DWDM networking we come across the terminologies of CDC. The full form of CDC is Colorless-Directionless-Contentionless. However, transmission engineers who are relatively new to the industry and telecom engineers in general who are exploring the world of the DWDM transmission do not really comprehend the CDC in the essence that it is.  

Few doubts that come in mind are.

DWDM is always colored, so what is colorless?

What is directionless, when we know that traffic is essentially directional?

Last but not the least, what is this term contentionless?

When I was a rookie in this field I also had the same questions and I would like to properly address that for people who are still trying to explore this idea of CDC. 

So let us do step by step dissection of this mystical world of ROADM networks.

ROADM = Reconfigurable Add Drop Multiplexer

 Exploring the ROADM

Before we dive deep in to the CDC level we need to understand what is ROADM. ROADM stands for reconfigurable add drop multiplexer. Essentially in a DWDM network this is an advance version of OADM where you can actually program the channels and frequencies that you want to add and drop in a particular location.

In the figure we are seeing a 3degree ROADM configuration in its most simple arrangement. There are three ROADMs in an optical junction points and these are connected to each other by means of express ports.  The configuration is made such that Channel 21 comes from degree 1 and is dropped over there and the same channel 21 is rerouted as an add towards degree 3.

Similarly from Degree 2 there is a channel 22 coming and dropping and this is added and rerouted to Degree-1. 

General 3 Degree ROADM Site Configuration

This is a very simple arrangement of the ROADM and the optical cross connects are created in the ROADM in the similar fashion. 

While everything seems very simple there are certain operational challenges to this configuration that needs to be addressed. 

Suppose the channel 21 now needs to be nor rerouted through degree 3 but through degree 2 then there needs to be physical presence of an engineer on the site to change the ports. 

As all the channels drop through a Mux-Demux port there will be need to physically be present on the site and change the port allocation. 

Similarly if the same Channel 21 needs to be rerouted to say Deg-3 and Deg2 this will not be possible because the port on Deg-1 for channel 21 has already been reused. Thus this will prevent any sort of channel reusability under such optical junction points. 

What are the things that we are missing out over here?

1. We are always mandated to have a physical presence of an engineer on site in order to do manipulation for port add and drops. 

2. It is not possible to reuse the channels in case we want to do that for efficiency. 

3. The flexibility is less in such kind of configurations. 

Directionless Configuration

In order to mitigate the some of the drawbacks of the simple ROADM configuration we have something called as the directionless configuration. The directionless configuration allows the a particular channel to be rerouted or reprogrammed to any direction across the degrees without the presence of an engineer on site. Let us explore how the directionless configuration actually looks like. 

Directionless Configuration

The figure here shows the directionless configuration. Here we are solving one problem and that is not to invest any physical resources to route the channel from one degree to another. As we can see there is a ROADM termed as ROADM-D. This is connected to a Mux/Demux that is having one input of Channel 21. Because we also have the ROADM-D we are able to re-route the channel 21 without any presence of a person on site to change the ports. Hence the channel routing is now a bit more centralized. 

As you can see over here that in order to manipulate the direction for Channel 21 add and drop there only needs to be manipulation in the optical cross connects in the ROADM. This can be done centrally. 

The directionless configuration works well but then it has its own drawbacks for which it is now become more of an obsolete configuration. 

Which problems are not solved by the Directionless Configuration? 

As I mentioned above that there are some problems that are not solved by the directionless configuration eventhough the manipulation of directions can be done by centralized routing. Let us explore these problems over here. 

1. While it is clear that channel 21 is sorted, if the requirement comes to change the channel of the path say the same transceiver now needs to run on channel 22 then in order to make the changes there needs to be a physical shift on the mux demux port on site from channel 21 to 22. 

2. We are not seeing the benefit of a frequency reusage or a channel reusage over here. In the junction point if I have to send Channel 21 in all the directions simultaneously then I will not be able to do so as my Mux-Demux will have only one port of Channel 21 and not multiple ports of the same channel. 

Colorless-Directionless (CD) configuration:

We saw how partially we could mitigate the physical presence of engineers on site with the directionless approach. However, this was only solving the problem to a small extent. Networks have much bigger problems and there will be more number of junction sites in the network that will need manipulations of channels more often than not. In order to mitigate some of these aspects we have the Colorless and Directionless approach that is taken. 

In the CD configuration we use a ROADM on the site that is apart from the degree and this is called as the collector ROADM.  The collector ROADM has a combination of Degree ports that connect to the degree ROADMs and client ports that can directly connect to a tunable client in this case.

Colorless-Directionless Configuration

The best part of the client port is the fact that they can be tuned centrally to any channels/wavelengths that we want. So if a particular tunable interface decides to change the wavelength of transmission from say channel 21 to channel 22 all we need to do is to manipulate the client port wavelength configuration centrally and the optical cross connect centrally.  

A point to be noted over here is that two of the client  port of the collector ROADM cannot be of the same frequency of wavelength. 

As we can see in the figure that now since the client port is tunable we can route any color to any direction without having any physical presence on the site. 

To understand this simply let us take an example.  Suppose port 1 of the client of collector ROADM-C is connected to a tunable interface of say channel 21. Now due to the fact that the collector ROADM is having properties of optical cross connects we can divert this channel to any direction or any degrees.  This is purely the directionless principle. Now say the user decides to change the wavelength of the interface from channel 21 to channel 22. In the case of Directionless we would have to move ports on the Mux-Demux and send a physical engineer, however in the case of CD we just need to change the color of the port connected to channel 22 (Assuming channel 22 is not used in any other ports of the collector). This prevents any physical presence of the person on the site. 

What are we not able to achieve from the CD Configuration?

For the medium sized meshed networks CD configuration solves most of the problems of centralized control of optical cross connects and channel re-routing. However, there is one case, which is apparent in the case of highly meshed networks that the CD configuration is not able to solve.

 

Suppose we have three interfaces on the site of add and drop and the following case applies.

Ø  Interface 1 wants to send to degree 1 on channel 21

Ø  Interface 2 wants to send to degree 2 on channel 21

Ø  Interface 3 wants to send to degree 3 on channel 21

 

Now we have a problem with the CD configuration. All these three interfaces need to be channel 21 but the collector ROADM can have only one port with channel 21. How are we going to achieve this?

Can multiple collector ROADMs handle this problem? 

A layman way of achieving this would be have multiple collectors. So let us have a look at the figure what happens when we have multiple collectors. In this we have ROADMC1 ROADMC2 and ROADMC3 as three collector ROADMs in the drop site. As impractical as it may seem to be let us have a look at this for understanding. 

Because now we have three collector RAODMs we can reuse the same channels in three collectors and connect our interfaces to different collectors and get them sorted. We will need to create separate optical cross connects over here and this will ensure that the directions are routed appropriately and the frequency reusage is done.  Here there will be central control as well and there will be reusage of frequency in these junction points. So in a way we have been able to solve the problem of the channel reusage which was not mitigated in the CD configuration by adding multiple collectors. 

Channel re-usage using multiple collectors

Although we have solved the problem the question that we need to address is much bigger over here.  And the questions are written as follows.

1.       Is this a practical solution?

2.      What happens when I need to add one more degree to the site?

3.      If I have many such junction points in my network what would be my ROADM investment? 

Well, the answers of these question are tough and definitely the solution is not scalable and commercially feasible let us see how. 

Why the multiple collectors will not work after a particular point?

The multiple collector solution is assuming 1:1 provisioning of collectors per degree ROADM. So a site with say 7 degrees will have 7 collectors in order to achieve the channel reusage function. The channel reusability will then cost the provider a huge amount of cost and heavy footprints considering the network. Also the sites will become bulky and meshy as each collector needs to be connected to all the degree ROADMs that we have. As the number of degrees increase and more number of such sites come into existence this solution becomes awefully expensive and non-viable.

Colorless - Directionless - Contentionless (CDC):

From the CD configuration we have only one drawback to mitigate and that is the channel reusage in all the directions. We tried to do the multiple collector arrangement but then the solution seemed to be non-viable with respect to scale and numbers of a network.  So what we need is a kind of a collector ROADM that is having the internal cascading of many collector ROADMs at the same time. Basically it is an internal ROADM switch that allows to have same channel configuration on different ports ad the same time.

CDC Configuration

This kind of collector ROADM is called the contentionless ROADM.  In the figure that we are seeing we have a collector ROADM which is called as ROADM-CN. This is a kind of contentionless ROADM.  Here we can see we can have many client interfaces with the same frequency aligned.

While it is easier said than done the contentionless ROADM is coming with a lot of complexities inside. Technically a Contentionless ROADM collector is a MxN ROADM where there are N number of collectors with M number of degrees. So the ROADM internally is a collection of many ROADMs and is a kind of a switch. However, the contentionless configuration actually enables the ease of operation to a great degree especially in the field of dynamically changing networks. 

Comparison of different kind of configurations

Now that we have an idea of ROADM sites, CD and CDC, let us make a small analysis in terms of a table as to what configuration is to be used where.

Aspect	Normal ROADM	CD Configuration	CDC Configuration
Cost	$	$$	$$$$
Complexity	Simple	Moderate	Complex
Flexibility	Limited	No frequency reusage	Fully Flexible
Type of sites	Can be used for 2D or in some cases even 3D sites which have less channel re-route	Recommended for 3D – 5D sites with moderate traffic and channels	Recommended for a high mesh 5D onwards with higher possibility of traffic rerouting.
Centralized management	Physical presence needed for change of channels or ports	Can be centrally configured	Can be centrally configured.
GMPLS*	Difficult to achieve	Supported	Supported

 

Summary: 

In order to understand what configuration is suitable to your network it is very essential to make a proper planning of wavelengths. Proper wavelength planning and keeping room for the future makes it very easy to ascertain what technology to go for and in which node.

Most of the time it is at the planning stage where we need to decide if we go for the CD configuration or for the CDC configuration. Depending on traffic, flexibility and multiple add-drop points we need to make this decision. 

Cheers

Kalyan