Abstract:
A system and method for dynamically adding/dropping wavelengths in a reconfigurable optical add-drop multiplexer (ROADM) transport network is disclosed. The system includes at least one optical transponder, a plurality of optical fan-out devices, each arranged to receive an input signal from a network degree and coupled to at least one of a plurality of optical fan-in devices, each optical fan-in device arranged to output a signal to a network degree, the optical fan-out devices comprising at least one wavelength selective switch and the optical fan-in devices comprising at least one wavelength selective switch, the optical fan-out devices and optical fan-in devices being connected so as to enable signals input from each of the plurality of network degrees to be switched to another network degree of the plurality of network degrees; a plurality of demultiplexers for locally dropping selected wavelengths; a plurality of multiplexers for locally adding selected wavelengths; and at least one fiber switch interposed between the at least one optical transponder and the plurality of demultiplexers and multiplexers. The fiber switch is coupled to wavelengths and degrees that are allocated for a bandwidth-on-demand application. Other configurations include additional fan-in and fan-out devices interposed between a mux/demux assembly and the optical transponders to support wavelength redistribution applications.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates generally to optical networks, and more particularly, to a methodology and system for enabling dynamic wavelength add/drop in a reconfigurable add-drop multiplexer (ROADM) optical transport network to support bandwidth-on-demand, mesh restoration and wavelength redistribution applications. 
       BACKGROUND OF THE INVENTION  
       [0002]    In less than a decade, the state of the art in fiber-optic transport systems has progressed from simple point-to-point chains of optically amplified fiber spans to massive networks with hundreds of optically amplified spans connecting transparent add-drop nodes spread over transcontinental distances. Cost reduction has been the primary driver for this transformation, and the primary enabler has been the emergence of the reconfigurable optical add/drop multiplexer (ROADM) as a network element (NE). 
         [0003]    Exploiting the inherent wavelength granularity of wavelength-division multiplexing (WDM), an optical add/drop multiplexer (OADM) allows some WDM channels (also referred to as wavelengths) to be dropped at a node, while the others traverse the same node without electronic regeneration. Previously, it was necessary to terminate line systems at each node served, and then regenerate the wavelength signals destined for other nodes. The ability to optically add/drop a fraction of a system&#39;s wavelengths at a node was first achieved using fixed OADMs. These were constructed from optical filters, and by enabling wavelengths to optically bypass nodes and eliminate unnecessary regeneration, they provided significant cost savings. However, because traffic growth is inherently unpredictable, it is advantageous for the add-drop capability to be reconfigurable. 
         [0004]    ROADMs provide many advantages beyond the savings achieved by optically bypassing nodes. In the future, multi-degree ROADMs with adequate reconfiguration speeds may enable shared-mesh restoration at the optical layer. Shared mesh restoration significantly reduces the number of wavelength channels that must be installed as redundant protection circuits. ROADMs also provide operational advantages. Because ROADMs can be reconfigured remotely, they enable new wavelength channels to be installed by simply placing transponders at the end points, without needing to visit multiple intermediate sites. In addition to these cost-saving benefits, ROADMs will enable new services. For example, if transponders are preinstalled, then new circuits can be provided on-demand. The rapid network reconfiguration provided by ROADMs could also become an enabler of dynamic network services, such as switched video for IPTV. For all of these reasons, ROADMs will continue to have a significant effect on the design of optical networks. 
         [0005]    Generally, a ROADM is defined as a NE that permits the active selection of add and drop wavelengths within a WDM signal, while allowing the remaining wavelengths to be passed through transparently to other network nodes. Thus, the simplest ROADM will have two line ports (East and West) that connect to other nodes, and one local port (add/drop) that connects to local transceivers. In today&#39;s networks, optical links are typically bidirectional, so each line port represents a pair of fibers. When using conventional local transceivers that can process only a single wavelength at a time, the number of fibers in the add/drop port sets the maximum number of wavelengths that can be added or dropped at a given node. 
         [0006]    A ROADM with only two line ports (East and West) is referred to as a two-degree ROADM. Practical networks also have a need for multi-degree ROADMs that can serve more than two line ports. In addition to providing local add/drop of from each of its line ports, the multi-degree ROADM must be able to interconnect any individual wavelength from one line port to another, in a reconfigurable way. The degree of a multi-degree ROADM is equal to the number of line-side fiber pairs that it supports (it does not include the number of fiber pairs used in the add/drop portion of the ROADM). 
         [0007]    A full ROADM provides add/drop (de)multiplexing of any arbitrary combination of wavelengths supported by the system with no maximum, minimum, or grouping constraints. A partial ROADM only has access to a subset of the wavelengths, or the choice of the first wavelength introduces constraints on other wavelengths to be dropped. The drop fraction of a ROADM is the maximum number of wavelengths that can be simultaneously dropped, divided by the total number of wavelengths in the WDM signal. If a given add or drop fiber is capable of handling any wavelength, it is said to be colorless. If a given add or drop fiber can be set to address any of the line ports (e.g., east or west for a 2-degree ROADM), it is said to be “steerable.” A NE can be configured such that no single failure that will cause a loss of add/drop service to any two of its line ports. 
         [0008]    Carriers wish to deploy systems in the most cost-effective manner possible. Today, it is far more cost-effective to initially deploy the minimal amount of equipment that can smoothly evolve to meet future needs, rather than to deploy a fully loaded system configuration from the very beginning. Currently and for the foreseeable future, transponders make up the dominant cost of a fully loaded optical communication system. If a full set of transponders were included in the initial deployment, then a substantial cost would be incurred before the network had sufficient traffic to support the expense. Therefore, systems are routinely designed to permit incremental deployment of transponders on an as-needed basis. Similar considerations also apply to multiplexers, although the economic drivers are not as strong. In general, modular growth will be supported whenever the additional cost and complication of upgrading to higher capacity in the future is small compared to the financial impact of a full equipment deployment at startup. By designing this pay-as-you-grow approach into ROADMs, the network itself can grow in a cost-effective manner. Traditional networks grow by adding and interconnecting stand-alone line systems, incurring substantial cost and complexity. By using ROADMs that allow for modular deployment of additional ports, network growth can benefit from both the equipment and operational efficiencies of integrating line systems as they are needed into a seamless network. Because networks are deployed over the course of years, carriers prefer to be able to grow the nodes of the network from terminals or amplifiers into multi-degree ROADMs. This not only allows the expense to be spread out over years, it also enables the network designers to respond to unforeseen traffic growth patterns. 
         [0009]      FIG. 1  is a schematic of a prior art multi-degree ROADM system  100  (four network degrees are shown). Each network degree is coupled to a pair of optical amplifiers  102 , with an input connected to a 1×N optical fan-in device, i.e., a power splitter (PS) or wavelength selective switch (WSS)  104 ), and an output connected to a N×1 optical fan-out device, i.e., WSS  106 . Multiplexed optical signals on input  108   1  from network degree  1  are selectively directed via PS/WSS  104  to WSSs  106  and associated outputs  110   2 ,  110   3  and/or  110   4  for network degrees  2 ,  3  and/or  4 , respectively. In the same manner, multiplexed optical signals on inputs  108   2 ,  108   3  and  108   4  (network degrees  2 ,  3  and  4 ) may be similarly routed to the other network degrees of the system. A plurality of multiplexer/demultiplexer assemblies  112   1 ,  112   2 ,  112   3 , and  112   4  are connected to the WSSs  106  and PS/WSSs  104  for locally adding/dropping wavelengths to/from each network degree  1 ,  2 ,  3  and  4  by WSSs  106  and PC/WSSs  104 . In this implementation, the add/drop wavelengths cannot be redirected between the network degrees to support dynamic wavelength applications such as bandwidth-on-demand, mesh restoration and wavelength redistribution. 
         [0010]    An existing ROADM system for providing dynamic add/drop wavelengths uses a degree for the add/drop wavelengths such that the mux/demux is shared among all the other degrees on the node. Another known approach employs a fiber switch that is disposed between the transponders and the mux/demux to provide a centralized transponder manager such that any transponder can be switched to any add/drop port on any degree. However, none of the previous solutions have proven to be economically practical, and they all suffer from limited scalability. 
         [0011]    In view of the above, there exists a need for a new type of multi-degree ROADM system that is specifically adapted for bandwidth-on-demand, mesh restoration or wavelength redistribution applications. 
       SUMMARY OF THE INVENTION  
       [0012]    In accordance with an aspect of the invention, there is provided a system for dynamically adding/dropping wavelengths in a reconfigurable optical add-drop multiplexer (ROADM) transport network. The system includes a plurality of optical fan-out devices, each arranged to receive an input signal from a network degree and coupled to at least one of a plurality of optical fan-in devices, each optical fan-in device arranged to output a signal to a network degree, the optical fan-out devices comprising at least one wavelength selective switch and the optical fan-in devices comprising at least one wavelength selective switch, the optical fan-out devices and optical fan-in devices being connected so as to enable signals input from each of the plurality of network degrees to be switched to another network degree of the plurality of network degrees; a plurality of demultiplexers for locally dropping selected wavelengths; a plurality of multiplexers for locally adding selected wavelengths; and at least one fiber switch interposed between at least one optical transponder and the plurality of demultiplexers and multiplexers, the fiber switch being coupled to wavelengths and degrees that are allocated for a bandwidth-on-demand application. 
         [0013]    The multiplexers and demultiplexers have fixed-wavelength ports, and the at least one transponder is tunable to any wavelength supported by the ROADM. 
         [0014]    In one embodiment, the fiber switch is an M×N fiber switch adapted for coupling with M transponders and N wavelengths or degrees through the multiplexers and demultiplexers. 
         [0015]    In another embodiment, first and second fiber switches are interposed between a plurality of optical transponders and the multiplexers and demultiplexers to provide at least one redundant path through the ROADM. In this expedient, at least one optical transponder includes a protection port, and the transponder is coupled to the first fiber switch and the second fiber switch. 
         [0016]    In yet another embodiment, a pair of optical transponders is respectively connected to the first fiber switch and the second fiber switch, and a Y-splitter couples the pair of optical transponders. 
         [0017]    In still another embodiment, first and second 1×N fiber switches are employed, with the first fiber switch coupled to the plurality of multiplexers and the second fiber switch coupled to the plurality of demultiplexers. Each multiplexer and demultiplexer comprises a wavelength selective switch, and the first and second fiber switches are further coupled to at least one optical transponder. 
         [0018]    In accordance with another aspect of the invention, there is provided a system for dynamically adding/dropping wavelengths in a reconfigurable optical add-drop multiplexer (ROADM) transport network. The system comprises a plurality of optical fan-out devices, each arranged to receive an input signal from a network degree and coupled to at least one of a plurality of optical fan-in devices, each optical fan-in device arranged to output a signal to a network degree, the optical fan-out devices comprising at least one wavelength selective switch and the optical fan-in devices comprising at least one wavelength selective switch, the optical fan-out devices and optical fan-in devices being connected so as to enable signals input from each of the plurality of network degrees to be switched to another network degree of the plurality of network degrees; a plurality of demultiplexers for locally dropping selected wavelengths; a plurality of multiplexers for locally adding selected wavelengths; an optical fan-in device coupling each demultiplexer to an optical fan-out device for dropping a wavelength from a network degree; and an optical fan-out device coupling each multiplexer to an optical fan-in device for adding a wavelength to a network degree. 
         [0019]    In accordance with yet another aspect of the invention, there is provided a method for dynamically adding/dropping wavelengths in a reconfigurable optical add-drop multiplexer (ROADM) transport network, comprising: a plurality of optical fan-out devices, each arranged to receive an input signal from a network degree and coupled to at least one of a plurality of optical fan-in devices, each optical fan-in device arranged to output a signal to a network degree, the optical fan-out devices comprising at least one wavelength selective switch and the optical fan-in devices comprising at least one wavelength selective switch, the optical fan-out devices and optical fan-in devices being connected so as to enable signals input from each of the plurality of network degrees to be switched to another network degree of the plurality of network degrees; a plurality of demultiplexers for locally dropping selected wavelengths; a plurality of multiplexers for locally adding selected wavelengths. The method comprises the steps of: adding an optical fan-in device and a demultiplexer and coupling the demultiplexer to an optical fan-out device for receiving a dropped wavelength from a network degree for a bandwidth-on-demand application; and adding an optical fan-out device and a multiplexer and coupling the multiplexer to an optical fan-in device for adding a wavelength to a network degree for a bandwidth-on-demand application. 
         [0020]    These aspects of the invention and further advantages thereof will become apparent to those skilled in the art as the present invention is described with particular reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]      FIG. 1  is a schematic of a prior art system for adding and dropping wavelengths in a multi-degree ROADM transport network; 
           [0022]      FIG. 2  is a schematic of an exemplary system for dynamically adding/dropping wavelengths in a ROADM transport network utilizing a M×N fiber switch in accordance with an aspect of the invention; 
           [0023]      FIG. 3  is a schematic of an exemplary system for dynamically adding/dropping wavelengths in a ROADM transport network utilizing a pair of M×N fiber switches in accordance with another aspect of the invention; 
           [0024]      FIG. 4  is a schematic of an exemplary system for dynamically adding/dropping wavelengths in a ROADM transport network employing a pair of 1×N fiber switches for each transponder, and multiplexers/demultiplexers comprising wavelength selective switches in accordance with yet another aspect of the invention; 
           [0025]      FIG. 5  is a schematic of an exemplary system for supporting a wavelength redistribution application in accordance with an aspect of the invention; and 
           [0026]      FIG. 6  is a schematic of an illustrative deployment of a plurality of wavelength add/drop applications in an ROADM optical transport network in accordance with aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout to the extent possible. Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the examples set forth in the following description or illustrated in the figures. The invention is capable of other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0028]      FIG. 2  is a schematic of an exemplary multi-degree ROADM system  200  in accordance with an aspect of the present invention. Four network degrees are depicted in the figure, with each degree having an input  208   1 ,  208   2 ,  208   3 , and  208   4 , respectively coupled to an amplifier  202  and a 1×N PS or WSS  204 . Similarly, each degree has an output  210   1 ,  210   2 ,  210   3  and  210   4 , respectively, coupled to an amplifier  202  and an N×1 WSS  206 . As described above, multiplexed optical signals on any of the inputs  208   1-4  may be switched to any of the outputs  210   1-4 by the PSs/WSSs  204  and WSSs  206  serving each network degree. A multiplexer (mux)/demultiplexer (demux) assembly  212   1 ,  212   2 ,  212   3  and  212   4  is respectively connected to each network degree  1 - 4  to facilitate local add/drop of wavelengths. Each mux/demux  212   1-4  includes a mux  214  and demux  216 . The mux  214  comprises a plurality of input ports  218   1 ,  218   2 , . . .  218   N , and an output port  220 . The demux  216  comprises an input port  222  and a plurality of output ports  224   1 ,  224   2 , . . .  224   N . For each network degree, the output port  220  of each mux  214  is connected to one of the N input ports of a respective WSS  206 . Similarly, the input port  222  of each demux  216  is connected to one of the N output ports of a respective PS/WSS  204 . An M×N fiber switch  225  is disposed between a plurality of optical transponders  226  and the mux/demux assemblies  212   1-4 . In accordance with the invention, the fiber switch  225  is constructed and arranged with connections to those wavelengths and degrees that are predetermined to be used for a bandwidth-on-demand (BWOD) application. In the example shown in  FIG. 2 , wavelengths  1  and  2  are added/dropped to/from network degree  1 , wavelength  2  added/dropped to/from network degree  2 , and wavelength  1  added/dropped to/from network degree  4 . In this regard, each mux/demux  212   1-4  are arrayed waveguide gratings or the like provided with fixed-wavelength ports. The M×N fiber switch  225  can serve M transponders and access/switch up to N wavelengths or degrees between the M transponders and the mux/demux assemblies  212   1-4 . As will be appreciated by those skilled in the art, each transponder  226  may be tuned to transmit and receive any wavelength supported by the ROADM system. Wavelengths added at the ROADM are transmitted from each transponder  226   1-M  to one of ports  228   1-M  of fiber switch  225 . Similarly, wavelengths dropped at the ROADM are communicated from ports  230   1-M  of fiber switch  225  to the transponders tuned to receive those wavelengths. On the mux/demux side, the added wavelengths are communicated from ports  232   1-N  of fiber switch  225  to the mux/demux, and dropped wavelengths from the selected network degrees are input to the fiber switch  225  at ports  234   1-N . 
         [0029]      FIG. 3  is a schematic of an exemplary ROADM system  300  in accordance with another aspect of the invention. Four network degrees are depicted in the figure, with each degree having an input  308   1 ,  308   2 ,  308   3 , and  308   4 , respectively coupled to an amplifier  302  and a 1×N PS or WSS  304 . Similarly, each degree has an output  310   1 ,  310   2 ,  310   3  and  310   4 , respectively, coupled to an amplifier  302  and an N×1 WSS  306 . A mux/demux assembly  312   1 ,  312   2 ,  312   3  and  312   4  is respectively connected to each network degree  1 - 4  to facilitate local add/drop of wavelengths. Each mux/demux  312   1-4  includes a mux  314  and demux  316 . The mux  314  comprises a plurality of input ports  318   1 ,  318   2 , . . .  318   N , and an output port  320 . The demux  316  comprises an input port  322  and a plurality of output ports  324   1 ,  324   2 , . . .  324   N . For each network degree, the output port  320  of each mux  314  is connected to one of the N input ports of a respective WSS  306 . Similarly, the input port  322  of each demux  316  is connected to one of the N output ports of a respective PS/WSS  304 . In this embodiment, a pair of M×N fiber switches  325   a,    325   b,  is employed in lieu of the single fiber switch  225  in the embodiment of  FIG. 2 . This arrangement eliminates the potential for a single point of failure in the ROADM  300 . Each fiber switch  325   a,    325   b  has a plurality of M output ports  328  and input ports  330  on the transponder side, and a plurality of N input ports  334  and output ports  332  on the mux/demux side. The exemplary system of  FIG. 3  includes an unprotected transponder  326   1 , and three protected transponders  326   2 ,  326   3  and  326   4 . Transponders  326   2 ,  326   3  are coupled to a Y-splitter  340 . Transponder  326   4  includes conventional and protection ports for servicing the same wavelengths through both fiber switches  325   a  and  325   b.  For unprotected wavelengths, either switch  325   a,    325   b  may be employed to access any wavelength/degree in the ROADM system  300 . As shown, network degree  2  has an add/drop path through both switches  325   a,    325   b  for wavelengths  3  and  1  respectively. 
         [0030]      FIG. 4  is schematic of an exemplary ROADM  400  in accordance with another aspect of the invention. Four network degrees are depicted in the figure, with each degree having an input  408   1 ,  408   2 ,  408   3 , and  408   4 , respectively coupled to an amplifier  402  and a 1×N PS or WSS  404 . Similarly, each degree has an output  410   1 ,  410   2 ,  410   3  and  410   4 , respectively, coupled to an amplifier  402  and an N×1 WSS  406 . A mux/demux assembly  412   1 ,  412   2 ,  412   3  and  412   4  is respectively connected to each network degree  1 - 4  to facilitate local add/drop of wavelengths. Each mux/demux  412   1-4  includes a mux  414  and demux  416 . The mux  414  comprises a plurality of input ports  418   1 ,  418   2 , . . .  418   N , and an output port  420 . The demux  416  comprises an input port  422  and a plurality of output ports  424   1 ,  424   2 , . . .  424   N . For each network degree, the output port  420  of each mux  414  is connected to one of the N input ports of a respective WSS  406 . Similarly, the input port  422  of each demux  416  is connected to one of the N output ports of a respective PS/WSS  404 . In this expedient, a first 1×N fiber switch  425   a  has a single input port  430  connected to the transmit port of a transponder  426 , and a plurality of N output ports  432  that may be coupled to the mux/demux assembly. Similarly, a second 1×N fiber switch  425   b  has a plurality of input ports  434  that may be coupled to the mux/demux assembly and a single output port  428  that connects to the receive port of transponder  428 . As shown, fiber switch  425   a  is connected to mux  414  for adding wavelength  1  to network degrees  1 - 4 , and fiber switch  425   b  is connected to demux  416  for dropping wavelength  1  from network degrees  1 - 4 . In this arrangement, the ports can accept any of the wavelengths supported by the ROADM system, and are thus referred to as “colorless.” A separate 1×N switch is utilized for the transmit direction (add) and the receive direction (drop) for each transponder  426  that requires dynamic add/drop wavelength capability. Since the add/drop ports are colorless, each transponder can access any wavelength up to N degrees. This configuration may be used either for applications with predetermined wavelengths and routes, or for applications with real-time selection of any wavelength and route. 
         [0031]    Another proposed application for the dynamic add/drop of optical wavelengths is the redistribution wavelengths to support a migration from a ring-based network topology to a mesh topology. In this application, when capacity is exhausted in a network consisting of interconnected rings, an express route can be added between large nodes that bypasses smaller intermediate nodes on the original ring. In order to free capacity to these smaller nodes, the express wavelengths must then be redistributed to the new route. This application requires a shared mux/demux configuration to allow the add/drop wavelengths to be moved to the new route. This application requires that the capability to share the mux/demux be reserved for use by the future overlay route(s). However, since it is necessary to insure that the redistribution of the wavelengths doesn&#39;t collapse diversely-routed wavelengths onto the same shared risk link group (SRLG), the degrees that will ultimately share the mux/demux can be limited based on the SRLG. In addition, the deployment of this shared mux/demux capability can be limited to degrees at locations with large local add/drop demand for express wavelengths. 
         [0032]      FIG. 5  is a schematic of an exemplary ROADM system  500  in accordance with another aspect of the invention for supporting a wavelength redistribution application. Four network degrees are depicted, each having a respective input  508   1 ,  508   2 ,  508   3 , and  508   4 , coupled to an amplifier  502  and a PS/WSS  504 , and a respective output  510   1 ,  510   2 ,  510   3  and  510   4 , coupled to an amplifier  502  and a WSS  506 . In this embodiment, a mux/demux  512   1  is shared between network degrees  1  and  3 , and a mux/demux  512   2  is shared between network degrees  2  and  4 . Mux/demux  512   1  comprises muxs  514   1 ,  514   2 , and demuxs  516   1 ,  516   2 , and mux/demux  512   2  comprises muxs  514   3 ,  51   4  and demuxs  516   3 ,  516   4 . Demux  516   1  is connected to network degrees  1  and  3  via N×1 WSS/PS  550   1  and demux  516   2  is connected to network degrees  1  and  3  via WSS/PS  550   2 . If a passive PS  504  is used instead of a WSS  504 , then a WSS (in lieu of a PS) must be used for  550   1 ,  550   2 . Muxs  514   1  and  514   2  are coupled to network degrees  1  and  3  via 1×N WSSs/PSs  550   3  and  550   4  that are in turn connected to WSSs  506 . Similar to mux/demux  512   1 , demuxs  516   3 ,  516   4  are adapted to drop wavelengths from network degrees  2  and  4  via N×1 WSS/PS  550   5 ,  550   6 , and muxs  514   3 ,  514   4  can add wavelengths to network degrees  2  and  4  through 1×N WSS/PS  550   7 ,  550   8 . In this manner, selected wavelengths can be added and dropped as required to create an express route between large network nodes that bypass smaller intermediate nodes on the original ring. This permits freeing capacity to the smaller nodes by redistributing the wavelengths on the new route. In the example shown in  FIG. 5 , the dashed lines depict the routing of redistributed wavelengths to and from network degrees  1 - 4  by the ROADM system  500 . 
         [0033]      FIG. 6  is a schematic depicting an exemplary cascade of the foregoing embodiments in a fiber optic network  600  comprising a first ring  602 , a second ring  604  and a third ring  606 . Ring  604  includes a ROADM  608  serving a BWoD application  610  through a fiber switch arrangement as described above. Ring  602  is coupled to ring  604  by ROADMs  612  and  614 . Ring  602  further includes ROADMs  616  and  618 . ROADM  614  also includes a fiber switch for service to BWoD application  620 , and a shared mux/demux for enabling a future express overlay  622  as described above in the embodiment of  FIG. 5 . ROADM  624  is similarly configured and enables service to BWoD application  626  through a fiber switch, and the express overlay  622 . ROADM  624  also connects ring  604  to ring  606 . A ROADM  628  on ring  606  supports BWoD service to BWoD application  630  through a fiber switch analogously to ROADM  608  on ring  604 . Ring  604  further includes ROADMs  632  and  634 , which may be similarly configured to provide additional BWoD service as required. Ring  606  also includes additional ROADMs  636  and  638  that may operate using the same principles. Cascading these solutions provides support for all of the dynamic add/drop applications while only requiring that the equipment for each application be placed when and where needed. 
         [0034]    The above-described expedients provide an economic and scalable solution for supporting dynamic add/drop applications without the need for ubiquitous deployment of dynamic add/drop equipment for all wavelengths in a WDM optical system. This methodology enables new dynamic wavelength services to be deployed and supports migration to a mesh network topology with more efficient utilization of wavelength capacity. 
         [0035]    The foregoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the description of the invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.