Optimized colorless, directionless, and contentionless ROADM in a module

A Reconfigurable Optical Add/Drop Multiplexer (ROADM) node with a Colorless, Directionless, and Contentionless (CDC) architecture, targeting smaller degree nodes, includes an integrated ROADM degree and add/drop module having M common input and output ports and N add/drop input and output ports, wherein the integrated ROADM degree and add/drop module is formed by an M×N demultiplexer Contentionless Wavelength Selective Switch (CWSS) and an M×N multiplexer CWSS; and X degree modules, each having an input and output port connected to common ports of the integrated ROADM degree and add/drop module, a first set of ports of the N add/drop input and output ports are connected for degree-to-degree connectivity and a second set of ports of the N add/drop input and output ports are utilized for local add/drop, such that the integrated module provides both the degree-to-degree connectivity and the local add/drop.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to optical networking. More particularly, the present disclosure relates to systems and methods for an optimized Colorless, Directionless, and Contentionless (CDC) Reconfigurable Optical Add/Drop Multiplexer (ROADM) in an integrated module.

BACKGROUND OF THE DISCLOSURE

Optical networks utilize Reconfigurable Optical Add-Drop Multiplexers (ROADMs) to realize selective and reconfigurable add/drop of wavelengths or spectrum locally and between various degrees. ROADMs generally utilize Wavelength Selective Switches (WSSs) in different configurations. Flexibility in add/drop requirements has led to so-called colorless, directionless, and optionally contentionless add/drop multiplexer structures, such as in ROADM devices, nodes, architectures, and structures. A colorless add/drop device supports any wavelength or spectral occupancy/band being added to any port of an add/drop device, i.e., ports are not wavelength specific. A directionless add/drop device supports any port being directed to any degree. Finally, a contentionless add/drop device supports multiple instances of the same channel (wavelength) in the same device (albeit to different degrees). A colorless, directionless add/drop device can be referred to as a CD device, and a colorless, directionless, and contentionless add/drop device can be referred to as a CDC device.

CDC ROADM deployments are common and offer the most flexibility, albeit at higher costs and equipment requirements. Of note, conventional CDC configurations are less cost effective for smaller degree nodes. For this reason, network operators typically opt for CD or Colorless Direct Attach (CDA) configurations for smaller degree nodes (e.g., four or fewer degrees). It would be advantageous to provide a configuration which supports CDC in smaller degree and add/drop nodes with lower costs and equipment requirements.

BRIEF SUMMARY OF THE DISCLOSURE

In an embodiment, a Reconfigurable Optical Add/Drop Multiplexer (ROADM) node with a Colorless, Directionless, and Contentionless (CDC) architecture includes an integrated ROADM degree and add/drop module having M common input and output ports and N add/drop input and output ports, wherein the integrated ROADM degree and add/drop module is formed by an M×N demultiplexer Contentionless Wavelength Selective Switch (CWSS) and an M×N multiplexer CWSS, M and N are integers; and X degree modules, X is an integer and represents a number of degrees of the ROADM node, each having an input and output port connected to associated common ports of the integrated ROADM degree and add/drop module, wherein a first set of ports of the N add/drop input and output ports are connected between the demultiplexer CWSS and the multiplexer CWSS for degree-to-degree connectivity and a second set of ports of the N add/drop input and output ports are utilized for local add/drop of channels, such that the integrated ROADM degree and add/drop module provides both the degree-to-degree connectivity and the local add/drop of channels utilizing the demultiplexer CWSS and the multiplexer CWSS. X can be ≤4.

The first set of ports can be X*(X−1) input and output ports and the second set of ports can be N−X*(X−1) input and output ports. M−X input and output ports of the M common input and output ports can be unequipped. The first set of ports can include input and output ports for each degree to connect to every other degree. The demultiplexer CWSS and the multiplexer CWSS each can include M 1×N Wavelength Selective Switches (WSSs) each connected to one of M common ports; and N M×1 selector switches each connected to each of the M 1×N WSSs and connected to N add/drop ports. The M 1×N WSSs can be each formed using Liquid Crystal On Silicon (LCOS) and the N M×1 selector switches can be formed using Microelectromechanical systems (MEMS) mirrors or a Planar Lightwave Circuit (PLC). The X degree modules each can include a pre-amplifier, a post-amplifier, and an Optical Service Channel (OSC) module.

In another embodiment, an integrated Reconfigurable Optical Add/Drop Multiplexer (ROADM) degree and add/drop module with a Colorless, Directionless, and Contentionless (CDC) architecture includes M common input and output ports; and N add/drop input and output ports, an M×N demultiplexer Contentionless Wavelength Selective Switch (CWSS) and an M×N multiplexer CWSS, M and N are integers, configured to optically connect the M common input and output ports and the N add/drop input and output ports, wherein the integrated ROADM degree and add/drop module is utilized in an X degree ROADM node, X is an integer, and wherein a first set of ports of the N add/drop input and output ports are connected between the demultiplexer CWSS and the multiplexer CWSS for degree-to-degree connectivity and a second set of ports of the N add/drop input and output ports are utilized for local add/drop of channels, such that the integrated ROADM degree and add/drop module provides both the degree-to-degree connectivity and the local add/drop of channels utilizing the demultiplexer CWSS and the multiplexer CWSS. X can be ≤4.

The first set of ports can be X*(X−1) input and output ports and the second set of ports can be N−X*(X−1) input and output ports. M−X input and output ports of the M common input and output ports can be unequipped. The first set of ports can include input and output ports for each degree to connect to every other degree. The demultiplexer CWSS and the multiplexer CWSS each can include M 1×N Wavelength Selective Switches (WSSs) each connected to one of M common ports; and N M×1 selector switches each connected to each of the M 1×N WSSs and connected to N add/drop ports. The M 1×N WSSs can be each formed using Liquid Crystal On Silicon (LCOS) and the N M×1 selector switches can be formed using Microelectromechanical systems (MEMS) mirrors or a Planar Lightwave Circuit (PLC). Each of X of the M common input and output ports can be each connected to an associated degree module each including a pre-amplifier, a post-amplifier, and an Optical Service Channel (OSC) module.

In a further embodiment, a method includes providing an integrated Reconfigurable Optical Add/Drop Multiplexer (ROADM) degree and add/drop module with a Colorless, Directionless, and Contentionless (CDC) architecture, including M common input and output ports; and N add/drop input and output ports, an M×N demultiplexer Contentionless Wavelength Selective Switch (CWSS) and an M×N multiplexer CWSS, M and N are integers, configured to optically connect the M common input and output ports and the N add/drop input and output ports, wherein the integrated ROADM degree and add/drop module is utilized in an X degree ROADM node, X is an integer, and wherein a first set of ports of the N add/drop input and output ports are connected between the demultiplexer CWSS and the multiplexer CWSS for degree-to-degree connectivity and a second set of ports of the N add/drop input and output ports are utilized for local add/drop of channels, such that the integrated ROADM degree and add/drop module provides both the degree-to-degree connectivity and the local add/drop of channels utilizing the demultiplexer CWSS and the multiplexer CWSS. The method can further include providing X degree modules each having an input and output port connected to associated common ports of the integrated ROADM degree and add/drop module. X can be ≤4. The first set of ports can be X*(X−1) input and output ports and the second set of ports can be N−X*(X−1) input and output ports.

DETAILED DESCRIPTION OF THE DISCLOSURE

In various embodiments, the present disclosure relates to an optimized Colorless, Directionless, and Contentionless (CDC) Reconfigurable Optical Add/Drop Multiplexer (ROADM) in an integrated module. Specifically, the proposed CDC ROADM described herein provides a small CDC architecture within a single module, e.g., supporting a four degree or less ROADM node. Variously, the CDC architecture proposed herein utilizes a same Wavelength Selective Switch (WSS) module for both degree connectivity and for local add/drop, enabling a single module to support a cost-reduced CDC ROADM. Thus, the switching elements of a CDC architecture is self-contained in the single module, providing cost reduction, less equipment, reduced power consumption, etc. versus a conventional, multi-module CDC architecture. The CDC architecture proposed herein is ideal for smaller degree nodes.

Conventional CDC ROADM Architecture

FIG. 1is a block diagram of an example four degree ROADM node100utilizing multiple modules102,104,106to form the degrees and the local add/drop. Specifically, the ROADM node100includes four-degree modules102A,102B,102C,102D, a Fiber Interface Module (FIM)104for managing fiber connectivity between the modules102,106, and a local add/drop module106. The degree modules102A,102B,102C,102D each include a Wavelength Selective Switch (WSS) demultiplexer110, a WSS multiplexer112, pre-amplifier116, a post-amplifier118, and an Optical Channel Monitor (OCM)114. The degree modules102A,102B,102C,102D can further include an Optical Service Channel (OSC), and other components. The FIM module104can be a passive device which provides optical fiber connectivity between the degree modules102A,102B,102C,102D, between the degree modules102A,102B,102C,102D and the local add/drop module106. The local add/drop module106provides connectivity to local optical transceivers, modems, etc. to the degrees via the degree modules102A,102B,102C,102D. The local add/drop module106includes a WSS120for channel adds, a WSS122for channel drops, and amplifiers124(which can be optional).

The four-degree ROADM node100includes a CDC architecture which is flexible, operationally simple, and future-proof. Any wavelength can be added/dropped or expressed through any degree, through software configuration. However, the CDC architecture illustrated in the four degree ROADM node100has significant cost, required equipment, and power consumption. Specifically, the four-degree ROADM node100has eight WSS modules110,112and two WSS modules120,122for a total of 10 WSS modules.

Optimized CDC ROADM Architecture

Accordingly, embodiments are presented directed to an optimized CDC ROADM architecture which reduces the equipment, footprint, and cost/power associated with the four degree ROADM node100.FIG. 2is a block diagram of an optimized ROADM node200utilizing a single module202to form the degrees and the local add/drop degrees and local add/drop switching. Specifically, the optimized ROADM node200provides the same CDC architecture as the four-degree ROADM node100albeit with reduced equipment. The optimized ROADM node200includes a ROADM degree and add/drop module202which is a single module providing WSS functionality for both degree-to-degree connectivity and for local add/drop. The ROADM degree and add/drop module202provides the degree functionality of the degree modules102A,102B,102C,102D. Instead, the optimized ROADM node200includes amplifier modules204A,204B,204C,204D instead of the degree modules102A,102B,102C,102D. The amplifier module204includes a pre-amplifier206, a post-amplifier208, and an OSC210. Note, the amplifier modules204do not require WSS components as the degree modules102include. Accordingly, the amplifier modules204have reduced cost, space, and power relative to the degree modules102.

Note, the four-degree ROADM node100and the optimized ROADM node200are both shown with four degrees for illustration purposes. As described herein, the four-degree ROADM node100has a total of 10 WSS modules whereas the optimized ROADM node200requires only 2 WSS modules, namely a contentionless WSS250(one for the multiplexer and one for the demultiplexer). Those of ordinary skill in the art will recognize the single module202can be used to form other nodal architectures, i.e., one, two, three-degrees. Also, the single module202can be used to form larger degrees, i.e., five or more, at the expense of a reduction in local add/drop. The proposed solution advantageously enables implementation of smaller degree nodes with a CDC approach.

The four-degree ROADM node100can also include the OCM114to provide monitoring functionality. In an embodiment, the OCM114can be integrated in the single module202. In another embodiment, the OCM114can be in each of the amplifier modules204A,204B,204C,204D.

FIG. 3is a block diagram of a contentionless WSS (CWSS)250utilized in the optimized ROADM node200. The CWSS250is utilized to realize the CDC architecture. Previously, the CDC architecture was formed through Multicast Switches (MCS). Advantageously, the CWSS250has, relative to the MCS implementation of a CDC architecture, a significantly lower loss, the potential to scale to higher port counts, and channel filtering is built-in in the multiplexing direction to reduce noise funneling. The systems and methods herein utilize the CWSS250to realize the CDC architecture in the optimized ROADM node200. An example of the CWSS250is described in Colbourne, P. D., McLaughlin, S., Murley, C., Gaudet, S., & Burke, D. (2018, March), “Contentionless Twin 8×24 WSS with Low Insertion Loss,” inOptical Fiber Communication Conference(pp. Th4A-1), Optical Society of America, the contents of which are incorporated by reference herein.

The CWSS250includes an M-array of 1×N WSSs252and an N-array of M×1 selector switches254. The CWSS250requires two switching elements, namely the M-array of 1×N WSS252and the N-array of M×1 selector switches254(whereas the MCS has a single switching element with combiners/splitters). Thus, the CWSS250can be referred to as an M×N device (M, N are integers, such as 8×24, etc.).

The M ports connected to the 1×N WSSs252can be referred to as common ports256of the CWSS250and each is connected to a fully independent 1×N WSS252, enabling individual wavelengths to be routed independently to any of N Add/Drop ports258connected to the M×1 selector switches254. Each of the N Add/Drop ports258can be coupled to any common port256of the CWSS250via the bank of M×1 selector switches254. Note that each add/drop port258can be connected to only one common port256at one time (there is no wavelength selectivity in the M×1 selector switches254). The function is similar to a multicast switch, but with 1×N splitters replaced by 1×N WSS's. Up to M instances of a given wavelength can be routed independently through the M×N CWSS250without contention.

In an embodiment, the CWSS250can be 8×24 (M=8, N=24) and the 1×24 WSS's252can be implemented using Liquid Crystal On Silicon (LCOS) phase modulator beam steering. One LCOS panel can be sub-divided into several independent sections, to control multiple independent WSS's within the same device, plus the LCOS steering engine enables flexible spectrum operation with variable channel widths. To minimize insertion loss, the 8×1 selector switches254can be implemented using an array of Microelectromechanical systems (MEMS) mirrors (a Planar Lightwave Circuit (PLC) design also possible). An advantage of MEMS mirrors as the switch elements is high isolation, thus preventing same-wavelength signals from different common ports256from causing interference.

The foregoing description utilizes the CWSS250as an 8×26 device (M=8, N=26) for describing the implementation of the ROADM degree and add/drop module202. Those of ordinary skill in the art will recognize that different values of M and N are contemplated.

Single ROADM Degree and Add/Drop Module

FIG. 4is a block diagram of nodal connectivity associated with the ROADM degree and add/drop module202and two CWSSs250in the optimized ROADM node200.FIG. 5is a block diagram of module connectivity between the ROADM degree and add/drop module202and the amplifier modules204A,204B,204C,204D in the optimized ROADM node200. Again, for illustration purposes,FIGS. 4 and 5illustrate a four-degree configuration and other degree configurations are also contemplated.

The present disclosure contemplates a single ROADM degree and add/drop module202which performs the degree connectivity and the local add/drop connectivity in a single, integrated module. The ROADM degree and add/drop module202provides the functionality of the local add/drop module106and the WSS demultiplexer110and the WSS multiplexer112in the degree modules102.

The single ROADM degree and add/drop module202includes two CWSS250modules which are denoted as CWSS250D for a demultiplexer WSS and CWSS250M for a multiplexer WSS, i.e., the single ROADM degree and add/drop module202contains both the multiplexer and demultiplexer WSS functions. In an embodiment, the CWSS250D,250M can be a twin contentionless 8×26 WSS module. The CWSS250D,250M is generally designed to act as CDC multiplexer/demultiplexer when used in combination with a high port count twin WSS on the line side (such as the WSSs120,122inFIG. 1). However, through remapped internal connectivity, the CWSSs250D,250M are also repurposed to provide a multi-degree CDC ROADM along with the add/drop functionality.

FIG. 4illustrates logical connectivity using the single ROADM degree and add/drop module202to provide a four degree CDC architecture and to locally add/drop14channels. The optimized ROADM node200includes four degrees, labeled D1, D2, D3, D4.FIG. 4is illustrated logically from right to left with the right side showing node ingress via four pre-amplifiers206, one for each degree D1, D2, D3, D4, and each input into an associated common port256of the CWSS250D.

The CWSS250D has ports258, which are denoted as add/drop ports258A and express ports258B. The add/drop ports258A are used for local add/drop260and the express ports258B are used for degree-to-degree connectivity262. On the CWSS250D, the add/drop ports258A are used for dropping channels from the degrees D1, D2, D3, D4. The express ports258B on the CWSS250D connect to respective express ports258B on the CWSS250M. For example, a degree D1-D2express port258B on the CWSS250D connects to a corresponding degree D1-D2express port258B on the CWSS250M, and the like. The CWSS250M also has add/drop ports258A used for local add/drop260. On the CWSS250M, the add/drop ports258A are used for adding channels locally to the degrees D1, D2, D3, D4.

Of note, an aspect of the proposed solution is the unique connectivity between the express ports258B on the CWSS250D and the express ports258B on the CWSS250M for the degree-to-degree connectivity. Because the CWSS250D,250M uses MEMS for the add/drop ports258, e.g., the M×1 selector switches254, the CWSS250D,250M can only route spectrum from/to a specific degree. As such, it is not possible to simply connect the multiplexer and demultiplexer halves of a module and route traffic arbitrarily between degrees.

For example, traffic incident on degree D1has spectrum that needs to be routed to degrees D2, D3, D4. Since the input to the CWSS250M module has a MEMs switch which selects a given degree (stripe of LCOS), it is not possible to send all the express traffic to one port. Express traffic from degree D1needs to be routable to an input dedicated to degree D2, D3, D4.FIG. 6is a block diagram of the CWSS250D illustrating degree routing. Here, degree D1is input to a 1×N WSS252A which steers light towards a given output port's MEMs switch. As such, the express ports258B require a port for each degree, namely D1-D2, D1-D3, etc.

For X degrees, X being an integer, the configuration of the CWSS250D,250M requires X*(X−1) ports to route express traffic. Thus, for four degrees, the optimized ROADM node200requires 4*(4−1)=12 port connections between the CWSS250D and the CWSS250M halves of the module.

Assume the CWSS250D,250M are M×N devices (M, N are integers, typically M<N, but not required) and there are X degrees, X is an integer (X must be less than or equal to M), the following provides the port numbers available for local add/drop.

Number of the express ports258B required for degree-to-degree connectivity262=X*(X−1).

The M common ports256on each of the CWSS250D,250M are connected to the X degrees, and if M>X, these ports are unequipped.

Number of the express ports258B for the local add/drop260=N−X*(X−1).

Assume the CWSS250D,250M are 8×26 (M=8, N=26), the following table illustrates capabilities for a different number of degrees (these numbers apply to one of the CWSS250D,250M):

As seen in Table 1, the CWSS250D,250M in the optimized ROADM node200provide reasonable add/drop counts for degrees four and lower, at the expense of unused/unequipped common ports and at a significantly lower cost, power, and footprint relative to the CDC architecture of the four-degree ROADM node100.

Of course, other values of M×N are contemplated. For example, it is expected that N will increase, e.g.,26to40, etc. This would enable more degrees, e.g., an 8×40 CWSS250D,250M would enable the 5 degrees with 20 local add/drop260. Thus, when N is larger, it may be possible to deploy the optimized ROADM node200at higher degree nodes (e.g., 5 or more). In this manner, the optimized ROADM node200may support the CDC architecture at all nodes in a network.

FIG. 5illustrates a module configuration for realizing the optimized ROADM node200using the CWSS250D,250M. Of note,FIG. 5has the same functionality asFIG. 1albeit with less equipment and the ROADM degree and add/drop module202for providing both local add/drop260and degree-to-degree connectivity262. From a hardware perspective, the ROADM degree and add/drop module202can be a rack-mountable module (e.g., 1-2 Rack Units (RU) high) or circuit pack inserted into a shelf with 2M common ports270and2N add/drop ports272. The ports270,272are optical ports configured for an optical fiber, patch cord, etc. The ROADM degree and add/drop module202includes both the CWSS250D,250M and thus has 2M common ports270and 2N add/drop ports272.

In this example, four degrees and 8×26 CWSS250D,250M, the 2M common ports270include 8 ports on the CWSS250D and 8 ports on the CWSS250M, four of which on each are unused/unequipped as described herein. The 2N add/drop ports272includes 14 local add/drop260channels on each of the CWSS250D,250M and12express connections between the express ports258B for the degree-to-degree connectivity262. In an embodiment, express connections280can be internally connected inside the ROADM degree and add/drop module202(as illustrated inFIG. 5). In another embodiment, the express ports258B can also have faceplate ports on the ROADM degree and add/drop module202and the express connections280can be formed by cabling between the faceplate ports.

Note, each of the CWSS250D,250M is M×N, so the overall ROADM degree and add/drop module202can have 2M common ports and 2N add/drop ports on the faceplate, i.e., each port can connect to one optical fiber and a channel can be an input and an output port. Additionally, the term port used herein can refer to two physical connections on the ROADM degree and add/drop module202. For example, an input and output port physically has two connections—one each for input and output. For example, the CWSS250D,250M are deployed in a so-called twin module. The express connections280can be ports of each of the twin connected to one another.

ROADM Node with an Optimized CDC Architecture

In an embodiment, a ROADM node with an optimized CDC architecture includes an integrated ROADM degree and add/drop module202having M common input and output ports270and N add/drop input and output ports272, wherein the integrated ROADM degree and add/drop module202is formed by an M×N demultiplexer Contentionless Wavelength Selective Switch (CWSS)250D and an M×N multiplexer CWSS250M, M and N are integers; and X degree modules204, X is an integer and represents a number of degrees of the ROADM node, each having an input and output port connected to associated common ports270of the integrated ROADM degree and add/drop module202, wherein a first set of ports258B of the N add/drop input and output ports272are connected between the demultiplexer CWSS250D and the multiplexer CWSS250M for degree-to-degree connectivity262and a second set of ports258A of the N add/drop input and output ports272are utilized for local add/drop260of channels, such that the integrated ROADM degree and add/drop module202provides both the degree-to-degree connectivity262and the local add/drop260of channels utilizing the demultiplexer CWSS250D and the multiplexer CWSS250M. Optionally, the number of degrees is X≤4.

The first set of ports258B is X*(X−1) input and output ports and the second set of ports258A is N−X*(X−1) input and output ports. M−X input and output ports of the M common input and output ports270are unequipped. The first set of ports258B include input and output ports for each degree to connect to every other degree. The demultiplexer CWSS250D and the multiplexer CWSS250M each include M 1×N Wavelength Selective Switches (WSSs)252each connected to one of M common ports256; and N M×1 selector switches254each connected to each of the M 1×N WSSs252and connected to N add/drop ports258. The M 1×N WSSs252are each formed using Liquid Crystal On Silicon (LCOS) and the N M×1 selector switches254are formed using Microelectromechanical systems (MEMS) mirrors. The X degree modules204each can include a pre-amplifier206, a post-amplifier208, and an Optical Service Channel (OSC) module210.

In another embodiment, an integrated ROADM degree and add/drop module202with an optimized CDC architecture includes M common input and output ports270; and N add/drop input and output ports272, an M×N demultiplexer Contentionless Wavelength Selective Switch (CWSS)250D and an M×N multiplexer CWSS250M, M and N are integers, configured to optically connect the M common input and output ports270and the N add/drop input and output ports272, wherein the integrated ROADM degree and add/drop module202is utilized in an X degree ROADM node, X is an integer, and wherein a first set of ports258B of the N add/drop input and output ports272are connected between the demultiplexer CWSS250D and the multiplexer CWSS250M for degree-to-degree connectivity262and a second set of ports258A of the N add/drop input and output ports272are utilized for local add/drop260of channels, such that the integrated ROADM degree and add/drop module202provides both the degree-to-degree connectivity262and the local add/drop260of channels utilizing the demultiplexer CWSS250D and the multiplexer CWSS250M.

Redundant Configuration

FIG. 7is a block diagram of module connectivity in an optimized ROAM node200A which includes two ROADM degree and add/drop modules202A,202B for redundancy and/or increased port count. The optimized ROAM node200A inFIG. 7is similar to the optimized ROAM node200inFIG. 5with four amplifier modules204A,204B,204C,204D. Again, for illustration purposes,FIG. 7illustrates a four-degree configuration and other degree configurations are also contemplated. The optimized ROAM node200A includes two of the ROADM degree and add/drop modules202A,202B along with a splitter300located between the ROADM degree and add/drop modules202A,202B and the amplifier modules204A,204B,204C,204D. The splitter300is a passive splitter array in both the transmit and the receive direction.

Functionally, the optimized ROAM node200A provides redundancy, namely the ROADM degree and add/drop modules202is not a single point of failure. However, the optimized ROAM node200A can also support2X the port count due to the presence of two ROADM degree and add/drop modules202A,202B. For example, the optimized ROAM node200A can support additional local add/drop ports.

On the add/drop ports272, there can be a second transceiver for 1+1/1:1/etc. protection or an Optical Protection Switch (OPS) which only utilizes a single transceiver. With the optimized ROAM node200A for redundancy, there is protection against a complete node failure in the event a single ROADM degree and add/drop module202fails. Also, the optimized ROAM node200A can also provide express redundancy, software could detect a failed ROADM degree and add/drop module202A and route the express traffic via the second ROADM degree and add/drop module202B.