OTN-Transport or OTN-Cross connect, What to choose in modern times?

 

Fortune favors the brave

------- Old proverb

1       Introduction:

In the optical world there has been a lot of dilemma these days about the validity of the ODU-XC or the OTN cross connect. Considering the rise of the packet traffic and the steady decline of the TDM traffic the purpose of a comprehensive cross connect device that is aggregating both TDM traffic and the packet traffic in a particular single line is slowly getting past its expiration date. Many geographies have actually moved on from the concept of OTN-XC while in some parts of the world OTN-XC is still the easiest and simplest way to groom traffic. This especially is the case when we are talking of a multi direction meshy site where the optical traffic needs to be routed in multiple directions.

1.1    Background of the OTN – XC:

In the early 2010s where in most of the areas of the world there was an explosion in the traffic requirement due to rise of data consumption there needed to be a technology that can deal with a combination of IP and traditional TDM at the same time. It was difficult and almost commercially inviable to have parallel transmission network for TDM and IP at the same time. Two parallel networks meant

1.     Higher CAPEX

2.     More maintenance.

3.     More usage of fiber cores or Channels.

4.     More manpower.

 

Arresting all these issues there was a standard quite similar to the levels of SDH multiplexing but with higher line rates. This technology was termed as OTN, Optical Transport Network. OTN has a similar hierarchal structure like that of the SDH but then there is no use of pointers over here rather it uses overheads. The understanding and the basic operations of these devices were more SDH like and thus were very appealing to the people who are especially in the transmission section of the network and are used to working with legacy, SDH like networks.

 

The OTN device has one more very unique characteristic and that is, it can aggregate several signals like IP, TDM, FC, HDI, SDI etc. and can line out this from a higher bandwidth line. This line can also be a coherent DWDM compliant interface that can easily be put as an optical channel in a WSS or in a MUX-DEMUX system. Thus, with the availability of one box the problems of multiple signal rates from different sources can be managed and multiple devices are not required for different technology. With the use of the OTN technology the transmission capabilities of a network are made more homogeneous and accommodative towards many technologies. Whatever the client rate maybe, it can be put in a OTN device and these can be aggregated and segregated at the same time.

 

There is also this good ability to add and drop multiple clients of different characteristics and switch directions of these clients by the introduction of OTN cross connects or OTN switches.

1.1.1   Structure of OTN:

The OTN, just like SDH, is very structured and this can be demonstrated in the figure below

Structure of the OTN-Frame as it evolves

 The building block and the progression from the client signal to the final Optical transport unit is somewhat similar to what is done in SDH. The difference over here is the pointers which is not existent in the OTN framework. However, if we inspect in details then the flexibility of OTN is to a great degree than that of SDH with respect to carrying of traffic. This is because of the flexibility in the number of interfaces that can be carried out with respect to client is much better than SDH.

1.1.2   The OTN Cross connect Device:

The OTN cross connect device or in short OTN-XC is a device that has the capability to have several clients converted to the OTN frame and then switch them to multiple directions. Just like the SDH cross connect this also depends on several unit of cross connections which are in the form of ODU (Optical Data Unit). This is why it is often referred to as ODU cross connect as well. In SDH the cross connects used to happen in the level of VC-12/VC-3/VC-4 and in the OTN cross connect they happen in the level of ODU0, ODU1, ODU2, ODU4 etc.

 

The ODU-XC consists of line cards that are usually coherent modules of 100G/200G/400G. These can be in the form of cards or pluggable CFP2/QSFPDD on the line cards. In addition it contains the client modules that are grey interfaces of STM1/4/16/64 1G/10G/100G FC etc. There is a back-plane connectivity and a ODU-matrix. This matrix is usually ODU-k (where k can be 0,1,2,3 and 4) or this can also be a ODU-Flex matrix where we can have several combinations of ODU in the system.

Structure of the ODU-XC device

The figure shows the flexibility of the OTN cross connect device in a similar fashion as it used to be for the SDH devices. There is a concept to cross connects that can be translated into trails in the level of topology and network. There can be a protected segment just like the SNCP-I that was there in SDH. Thus in many ways the OTN-XC resembles, operationally, the workings of the SDH cross connect albeit for higher line rates and for multiple client rates.

1.1.3   How the OTN-XC is operationally easy to use

 For a person who is coming from the traditional SDH/TDM background the OTN XC is definitely a  gradual and a comfortable method of evolution. The OTN-XC does not bring in drastic complications for the person who is into planning, provisioning and maintenance of the transmission network, as it is a gradual upgrade of the previous SDH network that was prevalent. Essentially the complications of several protocols and different service types are avoided in the transmission layer keeping it as simple as it can be.

 

On top of that the OTN-XC can do the same kind of provisioning which is closely aligned with the processes of the erstwhile TDM architecture. This is extremely important when a planning team and an operational team in the system is generally evolving from the SDH architecture. 

2       Challenges with the OTN – XC

It may be very easy operationally to scale from the SDH network to the OTN-XC network as a natural choice to increase the bandwidth and accommodate changes but OTN-XC comes with its set of challenges and this need to be really considered when considering OTN-XC for a long term solution for the network.

2.1    Bulky:

The OTN-XC can be extremely bulky resulting to heavy footprints in the network. The general consensus over here is that with the increase in the bandwidth in the access there will be requirement to increase of the OTN-XC capacity. From 2T, to 4T to 16T to 32 T and so on. As the access bandwidth increases in meshy sites and the line bandwidth increases it results to bulky OTN-XCs and thus make the solution not really OPEX efficient.

2.2    Heavy consumption of Power:

With the increase in the size of the OTN-XC there is also a demand of increase in the power consumption. The OTN – XC has the cross connect component and the combination of line cards and client cards that make the solution extremely power inefficient. Networks where there are requirements of power efficiency due to frequent outages in the system and heavy battery backup, the OTN-XC solution can become extremely Opex hungry. In the later sections we will come across the understanding of the comparison of power consumption against alternative solutions like OTN transport.

2.3    Not efficient for point-to-point traffic

Generally, when we talk about wholesale bandwidth providers in the industry most of the nature of their traffic is point to point. The traffic is either between two data center points or specific leas of bandwidth, protected or unprotected across two different endpoints. Realizing an OTN-XC under such a condition is extremely expensive option.

Let us assume that there is a requirement of 4x100G across two points that are 500kms apart. The most efficient way would be to have a Muxponder with 4x100G groomed to a 400G QPSK line.

OTN on Card for Point to Point traffic

 

As we can see in the figure the Muxponder card provides a localized OTN – XC on a  card function and the traffic can be groomed in one unit and provide the aggregate traffic to the two different points.

The same system can also be used for 800G wavelengths and for 1.2Tb/s wavelengths as well. Here the main thing is that we are saving a lot of Opex when it comes to service delivery and cost by introducing the concept of Muxponder.

2.4    Not suitable for a full packet environment

As we have seen from 2017 there has been a consistent decline in the traditional TDM traffic that used to ride on SDH. Even international connectivity is now no more on SDH but on pure ethernet or IP peering. If we analyze the data from 2017 to 2023 then we will see that today almost 95% of the traffic that is there in the network is IP or Ethernet.

The utility of the OTN-XC was when we had a mixture of clients that were from the ethernet zone and the traditional SDH zone with some elements of fiber channel as well. However, with most of the traffic migrating to IP/Ethernet this mixture is becoming far from homogenous. The ethernet share of the traffic is increasing to a great extent.

This has further led to a thought process which mandates the coherent interfaces of 100G/200G/400G and even 800G to be used as a pluggable in the router itself which totally eliminates the need for the OTN-XC and to some extent even transponders and muxponders. The coherent interfaces can directly interact with the WSS or the Optical line system.

In the later section as we study the IPoDWDM system this is explained better.

3       OTN – Transport system

A possible alternative that is low cost on the CAPEX and OPEX is the OTN -Transport system. Basically this consists of grooming of bandwidth by means of transponder and Muxponder. The OTN-TX as it is called with abbreviation can eliminate the need of a back-plane based OTN-XC and can be accommodated in the optical photonic user shelf itself that is used for the optical line system.

3.1    Understanding the OTN-TX components:

In order to delve more into the OTN – TX we need to first understand the components of the OTN-Transport. There are many elementary components that comprise of the OTN-Transport and this is what we are exploring in the sections below. 

3.1.1   Transponder:

A transponder is the simplest form of a service card that can be realized in a OTN – Transport scenario. The basic work of a transponder is to take in a signal of a particular rate (Eth/OTU etc.) and then provide the OTN wrapping in addition to tuning this signal to a particular wavelength and frequency of a particular width.  Transponders are used generally in network when we have a situation where the line and the client need to be of the same rate.

The structure of a Transponder

As we can see in the picture this is a module where the input is essentially a signal of 100G / 400G and the output is a colored signal of OTU4/OTUC4.

The concept of transponder is slowly getting faded away with the arrival of IPoDWDM devices, which we will discuss later. However, transponder have a very good case where we need to connect third party grey interfaces to an optical line system of the same line rate. Eg. Say all the routers in the data center have 400G line out and this needs to be put on different wavelengths of 400G in the optical line system. Here the routers do not have colored interfaces and we need a device to convert the 400G to a proper coherent OTUC4 that can ride on the optical line system. This is where we take the help of a transponder.

 

3.1.2   Muxponder:

As the name suggests a Muxponder is a device that does multiplexing of signals of lower rates to a line of higher rate. In a very layman language we can imagine this as an OTN-XC on a card where the card has 1 line output of a particular line rate and several clients of lower client rates. The Muxponder enables to create ODU-XC trails across several clients and lines in the same subsystem of the card.

Structure of a Muxponder

As we can see in the figure this is a simple layout of a Muxponder that can be inserted on one of the slots of the Optical photonic user shelf. Here we can see an example of a Muxponder that has a 400G/OTUC4 line out and 2x100G and 20x10G interfaces thus giving the flexibility to the user of having several clients of several rates groomed to a particular Muxponder. The Muxponder makes traffic grooming, if it is pre-planned and predetermined, extremely efficient and provides the right mix of low order and high order traffic especially for the whole-sale bandwidth environment.

3.1.3   ADM on Card (AoC)

As we see the structure of the Muxponder a question arises. “How can we use the muxponder in sites that are add drop in nature?” The Muxponder is having only one-line out port so how can we have a east west kind of a scenario where we can add and drop traffic and do a bit of 1+1 electrical protection as well. Here comes the concept of ADM on a card. The ADM on a card stands for Add drop multiplexer on a card, in short in can be called as AoC as well.

The concept remains same as the Muxponder, however, there is also an ability to do east west routing of traffic using protected cross connect. There can be Y type of protected cross connect form using the ADM on card. The ADM on card can actually solve the problem of several add and drop over a particular channel and can also provide ring protection with east-west kind of cross connection.

Structure of the ADM on a Card (AoC)

The picture shows an extension of the concept of Muxponder to a system that can be providing an east-west kind of a topology. The AoC concept is extremely beneficial to use if we know the proper drop points and plan the traffic in a way that it will properly adhere to the traffic matrix with a growth margin. The AoC in a way simulates the multi directional attribute of the OTN-XC in a very compact manner and gets accommodated in one of the slots of the photonic shelf. The AoC can seamlessly interoperate with a OTN-XC on a mesh side and a Muxponder on a terminal site and this can provide a comprehensive way of dealing with networks that have multiple layers. In the later sections it will be more apparent to understand how the traffic can be groomed across different kinds of the networks. 

3.1.4   Transponder/Muxponder:

In some scenarios a particular card can perform the dual role of a Transponder and a Muxponder. This can be considered to have a sort of dual function in multiple directions. A reason for this is also to save the amount of slot space for grooming more traffic. Taking one such example is a card/module that has two-line interfaces of OTUC4 however on the client side it can be 1 client interface of 400GE or 4 client interfaces of 100GE. When the mapping is from 400GE to the OTUC4 it is working as a transponder. When the mapping is from 4x100GE to a OTUC4 it is working as a Muxponder.

Layout of a Transponder/Muxponder 

As we can see in the picture here, we have an example of a transponder/Muxponder. Another thing to note down is the 4x100GE realization on a single port. Here we use a concept that is called the fan-out. The fanout takes the advantage of grooming lanes of 100G on a 400G DR4/LR4 mapping. Generally, the QSFP slot of a 400GE takes the LR4/DR4 of 400G. This essentially means 4 lanes of 100G combining to make the 400Ge interface. Now this can be further distributed using a MPO cable and a passive fanout device to 4x100GE separate inputs. However, here the thing needs to be noted that the router should have the 100GE LR1 instead of the conventional 100GE LR4.

3.2    How the protection works?

It may be a common argument from the operator side that the realization of a simple level protection is difficult in the OTN-TX scenario as compared to a full-Fledged OTN cross connect device. There may be arguments that the OTN-XC has redundant mechanisms like the ASON in the electrical level and the SNCP when it comes to 1+1.  As these arguments are true, a point needs to be considered that with the inclusion of more an more electrical protection layers the cost increases drastically. This is because the most expensive component in a optical scenario is the Optical line transceiver. While providing multiple transceiver for the line is a solution to enhance protection but then it increases the cost to exorbitant levels. This is why alternative protection schemes should be envisaged looking at the MTBF of the line transceivers.

3.2.1   OLP Level Protection:

One of the basic levels of protection that can be achieved while keeping the service intact is the OLP level protection. Here we are looking at the protection only at the fiber level to provide a 1+1 protection at sub 50ms. OLP level protection is very easy to configure as the protection works at Layer – 0. This saves the idea of keeping redundant line interfaces that is extremely costly.  A general OLP level protection can work perfectly well if we are considering a point-to-point bandwidth between two different locations.

Below is an example of an OLP based configuration on one side of the network. The similar configuration is replicated on the other side of the network also. Please note that since this is using the collector and the ROADM network the OLP mechanism can also be implemented in a network that consists of a WSS Mesh and is not limited only to a network that is traditionally point to point.

 

With the introduction of OLP level protection we can provide

1.     1+1 protection for multiple number of client services with less cost.

2.     Optimize the number of line coherent interfaces.

3.     Reduce a lot of cost on the footprint by providing the same amount of redundancy on the line.

4.     Provide business services in a converged meshy network with desired amount of protection.

 

OLP level protection setup with dual collector

 

3.2.2  WSON (Wavelength Switched Optical Networks)

There would be a lot of demand that is based on multiple restoration if more paths are available in the network. A network that is complex and meshy demands multiple switching paths of the traffic with respect to the availability of paths after a failure event occurs on multiple sections. This also takes into account the availability of resources along the path. A classical way to achieving this in the OTN based network is ASON (Automatic switched optical network) that is based on the GMPLS protocol. A similar extension of this in the layer-0 is WSON. In the case of ASON it is the ODU that is the main unit of switching and there needs to be several electrical options and line interface availability for the switching of the network. This, however, is eliminated in the WSON network.

 

The WSON enabled network can do multiple level restoration and switching at layer-0 but switching wavelengths. This however needs at least a colorless-directionless configuration in the nodes or best a CDC configuration that makes the switching of wavelength easy for the traffic.

 

For networks that are metro level and connecting to several of data centers and traffic aggregation points over a region say spanning around 400-500 kms, WSON is a very good solution to provide cost effective services with multiple restoration. Here the requirement of line interfaces are less as there is no O-E-O switching and the cost of the infrastructure is one time.

Example of a WSON Network

 In the picture above we see an example of a WSON network that is built out of the CDC ROADMs. There are multiple paths for the restoration of the main path and here three options of the alternate paths are shown. A Thing to note over here is that the WSON restoration paths do not need any additional line interface hardware to be constructed or generated. On the basis of availability, the system will create optical cross connects at the OCh level in the system. These OCH level cross connects will lead to the creation of new paths.

So to summarize when we do the network using OTN-Transport and WSON the following are the advantages of WSON

1.   

1.     1. Economical to implement in the network.

2.    2.  Does the restoration at the OCH level so there is no need for additional hardware.

3.    3. In the event more services are added extra line cards need not be added in the transit as the network is self-healing in the optical mode.

4.     4. GMPLS infrastructure has to be developed only once and not again and again. 

3.2.3   OLP + WSON

As we know that GMPLS restoration has its challenges or restoration times that can exceed the limit of 50ms. This is the reason why in ASON also we combine it with the protection of SNCP where there is a pre-provisioned protection path so that we have 50ms switching always.  In the case of WSON also we can combine this with OLP protection, and this can provide the 50ms switching always.

In the ROADM-CDC sites we need to ensure dual collector and ensure that the OLP connectivity is done properly along the lines. In case we use the CDC-collector, this will be better as we will have more flexibility in the wavelength re-usage in the whole network.

 

In the OLP+WSON mechanism we have complete end to end protection mechanism that is carrier grade in nature.

4       The trend to choose OTN- transport over OTN-XC

As we see the operator network from end to end today there is more trend to choose the OTN-Transport solution over a bulky OTN-XC solution. The question is why? There needs to be a close examination on this aspect that leads to this choice for the operator network and the below sections reveal the rationale behind this choice that the operator networks are making.

4.1    Comparison over cost and power

When a network is constructed plainly to provide bandwidth as a service then the primary question that comes into the mind of an operator is how much would be the CAPEX and OPEX load on the system? This figure is a major factor that ultimately provides the figures for pricing of the bandwidth and determines the profitability of the service over the years. If we compare the OTN-XC cost with the cost involved in providing a service of the same bandwidth with OTN-Transport then the OTN-Transport wins hands down. Same happens when the comparison is made on the basis of power consumption.

Statistically speaking the following inferences can be drawn

1.     The OTN-XC for a 400G lambda is 3 times as costly than the OTN – Transport.
2.     In case we are introducing ASON in the OTN-XC then the cost jumps 6 times.
3.     Power consumption is 4 times in the OTN-XC than the OTN-Transport.
4.     With ASON the power consumption is around 10 times more for the OTN-XC.

5.   As we increase the lambdas from 400G to 800G the cost even goes up.

 

CAPEX and Power consumption is extremely high as we go more towards the OTN-XC. Here this is the case because there will be expansion on the line side and the client side separately. On the other side there will be a good amount of increase on the line side when ASON is added. OTN – XC expansion will create more number of cards and line modules. In addition, it will increase the load on the matrix and the cross connect. These more modules and more number of processes increase the power consumption to a great deal.

4.2    Comparison on Space and Density:

Another very important part of the OPEX is the space and density. Most of the operators who have their networks running leased space environment. Space comes at a premium in some of the cases just like power. Space prices appreciate over the years, and it is very imminent to have efficiency on the space and increase the density of the product to have more interfaces packed in less of a footprint. In this case the OTN-Transport, especially the compact modular version comes of a greater use.

A thing to note is that more density means less space consumed and thus less OPEX. 

4.3    Services types

The services types are also a candidate for the operators to choose the OTN-Tranport over the OTN-XC. As we go to the level of multiples of 100G services it is important to note that in the case of OTN-XC the space consumption and the power consumption will be very high. In case most of the services are point to point bandwidth delivery and grooming kind of services the OTN-Tranport makes a much better solution option than the OTN-XC. There can be in the network some services that are completely based on 10G and lower rates and for them a separate channel of 400G + can be used for small OTN-XC devices giving them a sort of diversity in the network.

Blending the OTN-Transport and the OTN-XC making an efficient solution

The figure above represents a comprehensive device for the OTN-XC and the OTN-Transport blending. For the present case of services which the customers have, especially in the region of wholesale bandwidth and providing bandwidth as a service, this model can be extremely efficient based on services.

5       IPoDWDM (IP over DWDM)

The purpose of the OTN-XC as we discussed before is more when there is a mix of services that are having legacy TDM and modern IP. However, if we consider the present situation that is prevalent in the market then we will see that most of the services are ethernet/ip natured and the demands are also going to those levels.

So, there is a maximum need to have a system that has most of the clients of the IP nature. This is the router. However, today we have a router connected to an OTN-XC and then connected to an Optical line system. 

How the combination of transport and IP is in most of the cases today

The figure above was pertinent if there is a proper mix of the TDM clients and the IP clients. TDM aggregation coming from the legacy TDM mux while the IP clients coming from the IP interfaces. Both of them merging to the OTN-XC and then further the OTN-Wrapping is done to take this over the optical line system. Here we have a complete segregation of network and services. There is a layer of TDM, IP, OTN and Optical separately.

Today, however, the situation is different. The number of TDM-Mux in the network has reduced substantially and there is a good amount of increase in the IP-Traffic in the network. Most of the services are IP oriented so the relevance of the TDM device has reduced considerably. On top of that the bandwidth requirements have increased to a great extent. The need is thus of a router that can have line interfaces as coherent line.

How the IPoDWDM is setup

 

In the IPoDWDM structure we are able to groom the services of IP on a line that is coherent in nature. This line can be C-band or L-Band and thus can be integrated directly to the Optical Line system. Here, the OTN-XC and the OTN-Tranport both are eliminated from the network and the network becomes even more economical.

 

The carriage of the legacy bandwidth of TDM can be done over CES, Circuit emulated service. The CES can carry E-1, STM1/4/16 over encapsulation when needed and the IP services go as it is.

The line interfaces can be 100G/200G/400G coherent. The protection can also be based out of LDP-FRR and other IP protection mechanisms.

6      Conclusion

As we look at the various aspects of the network that the operator takes into account the following points come out as inferences.

1.     OTN-Transport solutions involving transponders, muxponders, compact-modular serve to be much economical and efficient ways of delivering bandwidth especially in a wholesale bandwidth scenario. 

2.     OTN-Transport solutions have much higher densities, especially when we go to the options of compact modular to delivery high amounts of bandwidth with better efficiency. 

3.     The protection mechanisms and the reliability of the network is hardly compromised with options like OLP and WSON also in the offering. 

4.   The deployment ease and upgrade in an OTN transport network is much higher than the OTN-XC.

5.     IPoDWDM devices can be a better option considering the futuristic growth of the network. 

And Finally:

      So friends, fairly long article and the article is put up after a lot of research and observation. Hope all of you will like it. Please comment, share and like.... 

Cheers, 

Kalyan.....

 

 

 

 

 

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