Abstract:
Optical networks occasionally experience a fault along a communications path. Service providers prefer to have an alternative communications path available to enable users to still communicate in a seamless manner. Accordingly, a method and corresponding apparatus for providing path protection for dedicated paths in an optical network is provided.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/147,981, filed on Jan. 28, 2009. 
         [0002]    The entire teachings of the above application are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Electrical and optical networks occasionally experience a fault along a communications path, such as at nodes along a path, due to power failure, for example, or on physical transmission lines or links, due to backhoe cuts, for example. Whatever the cause of the fault, service providers generally prefer to have an alternative communications path available to enable users to continue communications in a seamless manner. 
       SUMMARY OF THE INVENTION 
       [0004]    Example embodiments of the present invention may be implemented in the form of a method or corresponding apparatus that provides path protection for dedicated paths in an optical network. A method and corresponding apparatus according to one embodiment of the present invention include accessing optical link information representing optical links forming optical paths from a source optical switch to a destination optical switch in an optical network. The optical paths include dedicated optical paths. In an event of a fault in a dedicated optical path, based on the optical link information accessed, the method determines, from among the optical paths, a protection optical path from the source optical switch to the destination optical switch, where the optical link information of the protection optical path meets an optical criterion. After determining a protection optical path, the method causes the destination optical switch and intermediary optical switches to instantiate links forming the protection optical path from the source optical switch to the destination optical switch to provide path protection for the dedicated path without the fault in the optical network. 
         [0005]    As used herein, to instantiate a link means to active a concrete representation of the link to enable communications, such as optical signals, to be physically transmitted over the link. The representation includes hardware, software or a combination of hardware and software network elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0007]      FIGS. 1A-C  are network diagrams of an example optical network where example embodiments may be employed; 
           [0008]      FIG. 2  is a network diagram of a management network; 
           [0009]      FIG. 3  is a message diagram of signaling messages, in accordance with an example embodiment; 
           [0010]      FIGS. 4A-C  are flowcharts of example processes for providing path protection for dedicated paths in an optical network, in accordance with example embodiments; and 
           [0011]      FIGS. 5A-B  are block diagrams of example apparatuses to provide path protection for dedicated paths in an optical network, in accordance with example embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    A description of example embodiments of the invention follows. 
         [0013]      FIGS. 1A-C  illustrate an optical network  100  with optical paths from a source optical switch  105  to a destination optical switch  110 . The optical paths include intermediary nodes A-G,  115   a - g , and optical links  120  and  125  therein between. The optical links  120  and  125  forming the optical paths are represented by optical link information. 
         [0014]    The optical link information includes information about the links  120  and  125 . The optical link information may include, for example, optical signal-to-noise ratio, chromatic dispersion, polarization mode dispersion, and other optical characteristics of the optical links  120  and  125 . 
         [0015]    The optical link information maybe maintained by individual optical switches and then communicated or otherwise shared with other optical switches using a link-state routing protocol such as Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), Private Network-to-Network Interface (PNNI), and Nimrod Routing Architecture. 
         [0016]    Alternatively, the optical link information may be maintained by a central entity, such as an element management system (EMS) or Network Management System (NMS), and then communicated to individual optical switches. 
         [0017]    Independent of a specific mechanism for maintaining and communicating optical link information, an example embodiment accesses the optical link information. The optical link information represents optical links that form optical paths, including dedicated optical paths, from a source optical switch to a destination optical switch in an optical network. In one embodiment, an optical control plane in a configuration entity accesses the optical link information. In another embodiment, an optical control plane in a source optical switch accesses the optical link information. 
         [0018]    In  FIG. 1A , optical paths from the source optical switch  105  to the destination optical switch  110 , through intermediary optical switch A  115   a , and from the source optical switch  105  to the destination optical switch  110 , through intermediary optical switches B  115   b  and C  115   c , are dedicated optical paths  135  and  140 , respectively. Additionally, the dedicated optical paths  135  and  140  provide fault tolerance, as described immediately below in reference to  FIG. 1B . 
         [0019]    In  FIG. 1B , despite the dedicated optical path  140  experiencing a fault  145  (e.g., a backhoe cutting a link  120 ), the dedicated optical path  135  still carries traffic  130  and customer services are uninterrupted. As such, the dedicated optical paths  135  and  140  may be said to tolerate the fault  145 . Further, it may be convenient to label one of the dedicated optical paths as a “working path,” and other dedicated optical paths as “protection paths.” 
         [0020]    In an event of the fault  145 , based on the optical link information accessed, the embodiment determines, from among the optical paths, a protection path  150  from the source optical switch  105  to the destination optical switch  110 . In the example illustrated in  FIG. 1B , the protection optical path  150  is from the source optical switch  105  to the destination optical switch  110 , through intermediate optical switches D-G,  115   d - g.    
         [0021]    The protection optical path  150  is selected because its corresponding optical link information is determined to meet an optical criterion. For example, the optical link information can include several parameters or metrics, such as optical signal-to-noise ratio, chromatic dispersion, polarity mode dispersion, or other optical characteristics of the links  125  forming the protection optical path  150  and the protection optical path  150  may be selected because the parameters or metrics meet or otherwise satisfy respective thresholds. Alternatively, a combination of parameters or metrics may meet or otherwise satisfy respective combined thresholds. One of ordinary skill in the art will readily recognize that other optical criteria or comparisons are contemplated by embodiments of the present invention. 
         [0022]    Continuing with the current embodiment, the embodiment causes the destination optical switch  110  and the intermediate optical switches D-G,  115   d - g  to instantiate the links  125  forming the protection optical path  150 . 
         [0023]    As used herein, to instantiate a link means to active a concrete representation of the link to enable communications, such as optical signals, to be physically transmitted over the link. The representation includes hardware, software or a combination of hardware and software network elements. 
         [0024]    The embodiment, with the protection optical path  150  from the source optical switch  105  to the destination optical switch  110 , provides path protection for the still active dedicated optical paths  135  and  140  in the optical network  100 . 
         [0025]    In  FIG. 1C , given the dedicated optical path  140  experiencing the fault  145 , in an event of a second fault in the remaining dedicated optical paths, another embodiment uses the instantiated links  125  that form the protection optical path  150 . In the example illustrated in  FIG. 1C , the dedicated optical path  135  (i.e., the remaining dedicated optical path) is experiencing a fault  155  (i.e., the second fault). In this embodiment, despite the dedicated optical paths  135  and  140  experiencing faults  145  and  155 , respectively, the protection optical path  150  carries traffic  130 , so customer service is uninterrupted. In an event of a fault in a dedicated optical path is corrected (e.g., the fault  155  is corrected), an embodiment of the invention reverts use of the links  125  forming the protection optical path  150  to use of the links  120  forming the corrected dedicated optical path. 
         [0026]      FIG. 2  is a network diagram of a management network  200  that networks a source optical switch  205 , destination optical switch  210 , and intermediary optical switches A-G,  215   a - g . The management network  200  differs from an optical network (e.g., the optical network  100  of  FIGS. 1A-C ). The optical network networks optical switches. Links between the networked optical switches carry application-layer data, such as Web traffic, Voice over Internet Protocol (VoIP) traffic, and e-mail traffic. 
         [0027]    In contrast, the management network  200  carries communication  255  supporting or otherwise enabling an embodiment to access optical link information representing optical links forming optical paths between the source optical switch  205  and the destination optical switch  210 , and to cause the destination optical switch  210  and the intermediary optical switches A-G,  215   a - g  to instantiate links forming optical paths between the source optical switch  205  and the destination optical switch  210 , as discussed previously in reference to  FIGS. 1A-B . 
         [0028]    The communications  255  may be unidirectional (e.g., request message) or bidirectional (e.g., request and response messages). The communications  255  may be packets, such as Internet Protocol (IP) packets. Accordingly, the management network  200  may be a packet-based or packet switched network, such as an IP network. The communications  255  may be one-to-one (e.g., IP unicast), one-to-many (e.g., IP multicast), or one-to-all (e.g., IP broadcast). 
         [0029]    A convenient embodiment causes links forming a protection optical path (e.g., the protection optical path  150  of  FIG. 1B ) to be instantiated by signaling (e.g., with the communications  255 ) a respective optical control plane on the destination optical switch  210  and the intermediary optical switches of the protection optical path. 
         [0030]    The embodiment, based on the signaling, causes the destination optical switch  210  and the intermediary optical switches of the protection optical path to be configured to make available use of the links forming the protection optical path. 
         [0031]    In another embodiment, signaling includes reserving optical layer resources on the destination optical switch  210  and intermediary optical switches of the protection optical path with a network-layer reservation protocol, such as Resource ReSerVation Protocol (RSVP), Next Step In Signaling (NSIS), Network Layer Signaling (NLS), Private Network-to-Network Interface (PNNI), Constraint-based Label Distribution Protocol (CR-LDP), ITU-T Recommendation Q.931, and extensions thereto. 
         [0032]    It should be noted that illustrated embodiments described herein are described as using RSVP to reserve optical layer resources in an optical network (e.g., the optical network  100  of  FIGS. 1A-C ). It should be noted that other protocols that reserve resources, such as optical layer resources, in a communications network, such as an optical network, may take advantage of the present invention. RSVP is described by R. Braden et al., “Resource ReSerVation Protocol (RSVP),” Request For Comments (RFC) 2205, September 1997, available from the Internet Engineering Task Force (IETF). 
         [0033]      FIG. 3  illustrates a protection optical path  300  from a source optical switch  305  to a destination optical switch  310 , through intermediary optical switch D-G,  315   d - g . An embodiment reserves optical layer resources on the destination optical switch  310  and the intermediary optical switch D-G,  315   d - g  using RSVP. 
         [0034]    In accordance with RSVP described in RFC 2205, RSVP Path messages are used by a sender to indicate its presence as well as specify resources needed to accommodate a data flow between the sender and a receiver. The receiver establishes a new reservation for resources for a data flow between a sender and a receiver by responding to a sender&#39;s Path message with a reservation request (Resv) message. The Resv message travels upstream hop-by-hop along the path used by the Path message from the receiver to the sender. The Resv message contains information that is used by intermediate nodes along the path to reserve resources for the data flow. 
         [0035]    Typically, RSVP (also NSIS and NLS) is used to reserve network-layer resources for data flows (corresponding to layer-3 (L3) of the Open Systems Interconnection Reference Model (OSI-RM). In contrast, in the context of the illustrated embodiments, RSVP is used atypically to reserve optical-layer resources (corresponding to layer-1 (L1) of the OSI-RM). Further, RSVP (also NSIS and NLS) is an “on-path” resource reservation protocol in which protocol messaging or signaling are communicated or otherwise exchanged over the datapath of the dataflow between the sender and receiver. In contrast, in the context of the illustrated embodiments, RSVP signaling is not communicated over a datapath of a dataflow between a sender and receiver (e.g., the links  120  and  125  of  FIG. 1A-C ), but over a different datapath (e.g., the management network  200  of  FIG. 2 ). A convenient embodiment extends the RSVP Path and RSVP Resv message described in RFC 2205 as extended Path message  355  and extended Resv message  360  to accommodate the optical network environment. 
         [0036]      FIG. 4A  illustrates an example process  400  for providing path protection for dedicated paths in an optical network. The process  400  starts ( 401 ). The process  400  accesses ( 405 ) optical link information representing optical links forming optical paths from a source optical switch to a destination optical switch in the optical network. The optical link information includes information about links forming the optical paths, which include dedicated optical paths. 
         [0037]    The process  400  determines ( 410 ) whether a fault in a dedicated optical path has occurred, for example, by receiving a fault indication or signal from another process or by monitoring dedicated optical paths. If the process  400  determines ( 410 ) that a fault in a dedicated optical path has occurred, based on the optical link information accessed ( 405 ), the process  400  determines ( 415 ) from among the optical paths, a protection optical path from the source optical switch to the destination optical switch; else the process  400  starts ( 401 ) again. 
         [0038]    A convenient embodiment uses a link-state routing protocol, such as Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), Private Network-to-Network Interface (PNNI), and Nimrod Routing Architecture, to determine from the optical path information accessed, the protection optical path from the source optical switch to the destination optical switch. 
         [0039]    Returning to  FIG. 4A , the process  400  causes ( 420 ) the destination optical switch and intermediary optical switches to instantiate links forming the protection optical path from the source optical switch to the destination optical switch. 
         [0040]    The process  400  ends ( 421 ) with path protection for the dedicated paths in the optical network provided. 
         [0041]    The foregoing example process  400  may be executed by an optical control plane in a configuration entity. Alternatively, the process  400  may be executed by an optical control plane in a source optical switch. 
         [0042]      FIG. 4B  illustrates an example process  450  for providing path protection for dedicated paths in an optical network. The process  450  starts ( 401 ). The process  450  accesses ( 455 ) optical link information representing optical links forming optical paths from a source optical switch to a destination optical switch in the optical network. The optical link information includes information about links forming the optical paths, which include dedicated optical paths, 
         [0043]    The process  450  determines ( 460 ) whether a fault in a dedicated optical path has occurred, for example, by receiving a fault indication or signal from another process or by monitoring dedicated optical paths. If the process  450  determines ( 460 ) that a fault in a dedicated optical path has occurred, based on the optical link information accessed ( 455 ), the process  450  determines ( 465 ) from among the optical paths, a protection optical path from the source optical switch to the destination optical switch; else the process  450  starts ( 451 ) again. 
         [0044]    The process  450  causes ( 470 ) the destination optical switch and intermediary optical switches to instantiate links forming the protection optical path from the source optical switch to the destination optical switch. 
         [0045]    Continuing with  FIG. 4C , the process  450  determines ( 475 ) whether a second fault in the remaining dedicated optical paths has occurred, for example, by receiving a fault indication or signal from another process or by monitoring dedicated optical paths. If the process  450  determines ( 475 ) that a second fault in the remaining dedicated optical paths has occurred, the process  450  uses ( 480 ) the instantiated links forming the protection optical path. 
         [0046]    The process  450  determines ( 485 ) whether a fault in one of the dedicated optical paths is corrected. If the process  450  determines ( 485 ) that a fault in one of the dedicated optical paths is corrected, the process  450  reverts ( 490 ) use of links forming the protection optical path to use of links forming the respective dedicated optical path. 
         [0047]    The process  450  ends ( 491 ) with path protection for the dedicated paths in the optical network provided. 
         [0048]    The foregoing example process  450  may be executed by an optical control plane in a configuration entity. Alternatively, the process  450  may be executed by an optical control plane in a source optical switch. 
         [0049]      FIG. 5A  illustrates an example apparatus  500  to provide path protection for dedicated paths in an optical network. The example apparatus  500  has an accessing  505  unit, determining unit  510 , and causing unit  515 . The accessing  505  unit being communicatively coupled to the determining unit  510 , and the determining unit  510  communicatively coupled to the causing unit  515 . 
         [0050]    The accessing unit  505  accesses optical link information  501  representing optical links forming optical paths from a source optical switch to a destination optical switch in an optical network, resulting in accessed optical link information  506 . The accessed optical link information  506  includes information about links forming the optical paths, which includes dedicated optical paths. 
         [0051]    In an event of a fault in a dedicated optical path, denoted by a fault indication  509 , the determining unit  510 , based on the accessed optical link information  506 , determines, from among the optical paths, a protection optical path from the source optical switch to the destination optical switch. A protection optical path indication  511  denotes the protection optical path determined by the determining unit  510 . Optical link information of the protection optical path is determined to meet an optical criterion. 
         [0052]    With the protection optical path indication  511 , the causing unit  515  causes the destination optical switch and the intermediary optical switches to instantiate links forming the protection optical path from the source optical switch to the destination optical switch. With the protection optical path, the apparatus  500  provides path protection for dedicated paths in the optical network. 
         [0053]      FIG. 5B  illustrates an example optical node  550  that supports the instantiation of a third path (second protection path)  595 . According to the example embodiment, the optical node  550  includes at least two transponders  555 ,  560 . The first transponder  555  is coupled to a first reconfigurable optical add-drop multiplexer (ROADM)  565 , and the second transponder  560  is coupled to a colorless directionless switch module (CDXM)  580 . The CDXM  580  is further coupled to two more ROADMs (a second ROADM  570  and third ROADM  575 ). 
         [0054]    The first transponder  555  and first ROADM  565  are dedicated to carrying traffic along a first path  585 . The first path  585  may be, for example, a working path between the optical node  550  and another optical node (not shown) or may be a protection path assigned to support a working path. The second ROADM  570  is dedicated to carrying the traffic along a second path  590 , which may be, for example, a protection path between the optical node  550  and the other optical node (not shown). As a protection path, the second path  590  may be assigned to support a working path, such as the first path  585 . Alternatively, the second path  590  may be a working path between the optical node  550  and the other optical node (not shown) and may be supported by the first path  585 , in which case the first path  585  would be a protection path. 
         [0055]    The third ROADM  575  is used to support a third path  595  that can carry the traffic between the optical node  550  and the other optical node (not shown) in an event that both the first and second paths  585 ,  590  fail. According to the example embodiment, the third path  595  is instantiated upon a failure along either the first or second path  585 ,  590 . When instantiated, the third path  595  is assigned to protect the path (from among the first and second paths  585 ,  590 ) that did not include the failure; thus, becoming a protection path for whichever of the first and second paths  585 ,  590  is the working path after the failure. This functionality allows the example embodiment to maintain “1+1 protection.” 
         [0056]    It should be noted that the second transponder  560  is not dedicated to carrying the traffic along any particular path, but may be used to carry the traffic along either the second path  590  or the third path  595 . Configuring the CDXM  580  to be coupled to the second transponder  560 , the second ROADM  570 , and the third ROADM  575  makes this possible. This configuration also reduces the number of transponders included in the optical node  550 , thereby reducing overall equipment costs. 
         [0057]    A number of example embodiments and their features have been described. Additional embodiments and features include a method of providing highly reliable optical networking in a cost effective manner. The method combines dedicated protection resources and shared protection resources to provide fast protection switching for multiple network faults while reducing network costs. The method uses technologies, such as connectionless and directionless switching and optical layer control plane to enable the method. 
         [0058]    Some optical networks require tolerance of more than one fault due to frequent occurrences of optical fiber failures. At the same time, fast protection is still desirable to have minimal impact to services using the optical network. 
         [0059]    The example embodiments described are contrasted with other solutions that solve the foregoing problem at the electrical layer. Solving at the electrical layer results in large or lengthy restoration times and high costs. Other solutions include using true optical cross-connects, however, they typically do not have enough ports or monitoring capabilities, and thus are not cost effective. 
         [0060]    In contrast, the example embodiments described above solve the foregoing problem at the optical layer, thereby minimizing the amount of switching required compared to the electrical layer. Further, with an optical layer solution, the overall network costs are reduced. 
         [0061]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 
         [0062]    It should be understood that the block, flow, and network diagrams may include more or fewer elements, be arranged differently, or be represented differently. It should be understood that implementation may dictate the block, flow, and network diagrams and the number of block, flow, and network diagrams illustrating the execution of embodiments of the invention. 
         [0063]    It should be understood that elements of the block, flow, and network diagrams described above may be implemented in software, hardware, or firmware. In addition, the elements of the block, flow, and network diagrams described above may be combined or divided in any manner in software, hardware, or firmware. If implemented in software, the software may be written in any language that can support the embodiments disclosed herein. The software may be stored on any form of computer readable medium, such as random access memory (RAM), read only memory (ROM), compact disk read only memory (CD-ROM), and so forth. In operation, a general purpose or application specific processor loads and executes the software in a manner well understood in the art.