Patent Publication Number: US-2005129403-A1

Title: Method and system for communicating optical traffic at a node

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention relates generally to optical communication systems and, more particularly, to a method and system for communicating optical traffic at a node.  
     BACKGROUND OF THE INVENTION  
      Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of transmitting the signals over long distances with very low loss.  
      Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at different wavelengths or frequencies. Network capacity can be defined based on the number of wavelengths, or channels, in each fiber, and the bandwidth, or size, of the channels.  
      Optical networks add and drop optical traffic at nodes on the network. Traffic may be added to the optical network at add switches or ports which may selectively pass through to other nodes optical traffic currently being communicated on the network or optical traffic desired to be added to the network from a local interface or client signal.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method and system for communicating optical traffic at a node that substantially eliminates or reduces at least some of the disadvantages and problems associated with previous methods and systems.  
      In accordance with a particular embodiment of the present invention, a method for communicating optical traffic at a node includes receiving optical traffic on a network and demultiplexing the optical traffic into component signals of the optical traffic. The method includes splitting at least one of the component signals into a drop signal and a continue signal and receiving and recovering the drop signal. The method also includes selecting between an add signal and the continue signal for communication on the network and multiplexing the selected signal with other signals for communication on the network.  
      Demultiplexing the optical traffic into component signals may comprise demultiplexing the optical traffic into component wavelengths, such as approximately forty component wavelengths. Demultiplexing the optical traffic may comprise demultiplexing the optical traffic at a demultiplexer card, and splitting the at least one of the component signals may comprise splitting the at least one of the component signals at the demultiplexer card. The method may also include splitting the drop signal into a first drop signal and a second drop signal, receiving the first drop signal at a work receiver and receiving the second drop signal at a protect receiver. Selecting between an add signal and the continue signal may comprise selecting between an add signal and the continue signal at a 2×1 switch.  
      In accordance with another embodiment, a system for communicating optical traffic at a node includes a node operable to receive optical traffic on a network and a demultiplexer operable to demultiplex the optical traffic received at the node into component signals of the optical traffic. The system includes a splitter coupled to the demultiplexer. The splitter is operable to split at least one of the component signals into a drop signal and a continue signal. The system includes a receiver coupled to the splitter. The receiver is operable to receive and recover the drop signal. The system also includes a switch coupled to the splitter. The switch is operable to select between an add signal and the continue signal for communication on the network. The system includes a multiplexer coupled to the switch. The multiplexer is operable to multiplex the selected signal with other signals for communication on the network.  
      The demultiplexer and the splitter may be positioned upon a demultiplexer card. The splitter may be operable to split at least one of the component signals into a drop signal and a continue signal on the demultiplexer card using array waveguide technology or using thin film filters. The system may include a second splitter coupled to the splitter. The second splitter may be operable to split the drop signal into a first drop signal and a second drop signal. The system may also include a work receiver coupled to the second splitter. The work receiver may be operable to receive the first drop signal. The system may also include a protect receiver coupled to the second splitter. The protect receiver may be operable to receive the second drop signal.  
      Technical advantages of particular embodiments of the present invention include a method for communicating optical traffic at a node that utilizes a splitter to split demultiplexed optical traffic prior to the switch where traffic is added back to the network. This reduces the need for more complex switches at the same location where traffic is added back to the network and reduces interference caused by the use of such switches. Dropping the traffic at a separate location from the add switch provides unique physical locations to drop and add the optical signal and further reduces the complexity of the switches utilized. Moreover, having a separate splitter from the switch facilitates dealing with failure of either of the two components. Accordingly, labor and expense incurred in manufacturing the node and its components are reduced.  
      Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some or none of the enumerated advantages.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of particular embodiments of the invention and their advantages, reference is now made to the following descriptions, taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  illustrates an optical network, in accordance with one embodiment;  
       FIG. 2  illustrates a node with a splitter for dropping optical traffic, in accordance with a particular embodiment;  
       FIG. 3  illustrates a node with a splitter for dropping optical traffic at a demultiplexer element, in accordance with another embodiment; and  
       FIG. 4  illustrates a method for communicating optical traffic at a node, in accordance with a particular embodiment.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates an optical network  100  that communicates information between network nodes  200  using optical links  102 , in accordance with a particular embodiment. Optical network  100  generally represents any collection of hardware and/or software that communicates information between network nodes  200  in the form of optical signals. In a particular embodiment, optical network  100  uses wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to communicate information on multiple channels, each channel using a different wavelength. Network nodes  200 , referring generally to nodes  200   a ,  200   b ,  200   c  and  200   d , represent any hardware and/or software that receives information carried in optical network  100  in the form of optical signals, processes that information in any suitable fashion, and/or communicates information to optical network  100 .  
      Nodes  200  are each operable to passively add and drop traffic to and from links  102 . In particular, each node  200  receives traffic from local clients and adds that traffic to links  102 . At the same time, each node  200  receives traffic from links  102  and drops traffic destined for the local clients. As used throughout this description and the following claims, the term “each” means every one of at least a subset of the identified items. In adding and dropping traffic, nodes  200  may combine data from clients for transmittal in links  102  and may drop channels of data from links  102  for clients. Traffic may be dropped by making the traffic available for transmission to the local clients. Thus, traffic may be dropped and yet continue to circulate on a link. Nodes  200  communicate the traffic on links  102  regardless of the channel spacing of the traffic—thus providing “flexible” channel spacing in nodes  200 . Nodes  200  may include multiplexers, demultiplexers, optical switches, amplifiers such as erbium doped fiber amplifiers (EDFAs), optical-electronic converters or any other suitable hardware and/or software for processing optical signals.  
      Links  102  represent any suitable links for communicating optical signals  104  between network nodes  200 . As such, links  102  may include any manner of optical communication medium, including optical fibers such as single-mode fiber, dispersion compensation fiber, dispersion-shifted fiber, non-zero dispersion shifted fiber. Links  102  may also include any other suitable optical components, such as EDFAs, repeaters, or optical-electronic-optical (OEO) converters. Links  102  may carry information using any suitable format or protocol, including frame relay, asynchronous transfer mode (ATM), synchronous optical network (SONET), or any other suitable method of communication. Links  102  may also perform any necessary signal and/or protocol conversion necessary to communicate information between nodes  200 . Links  102  may be unidirectional or bidirectional. In many networks, there is an “eastbound” path traveling clockwise around optical network  100 , and a “westbound” path, which communicates information counterclockwise around optical network  100 . Each link  102  may include one or multiple optical fibers or other media for communicating optical signals  104 , and nodes  200  of optical network  100  may be arranged in any suitable configuration, including ring, star, linear or other suitable network configuration. In a particular embodiment, network  100  may be an Optical Unidirectional Path-Switched Ring (OUPSR) network in which traffic sent from a first node  200  to a second node  200  is communicated over both directions of links  102 . The use of such dual communication allows traffic to get from one node  200  to another over at least one link  102  in the event of a line break or other damage to the other of the links  102 .  
      In a particular embodiment, links  102  carry optical signals  104  that have a wavelength spectrum of the form shown in  FIG. 1 . Digital as well as analog signals may be communicated on optical links  102 . In signal  104 , the optical information is apportioned in several different wavelengths  108 . Each wavelength  108  represents a particular channel. Information carried on links  102  may be assigned to any particular wavelength  108  and optical signal  104 . Using appropriate equipment, wavelengths  108  may be added, dropped, switched, or otherwise processed separately. Signal  104  may also include an optical supervisory channel (OSC) that represents one or more wavelengths assigned to carry information used for management of network  100 . For example, the OSC may communicate status information for the channels  108  indicating whether each channel  108  is provisioned and whether there has been an error detected in communication of channel  108 . Any number of wavelengths may be assigned to the OSC for carrying network management information.  
       FIG. 2  is a block diagram illustrating details of a node  200 , in accordance with one embodiment. Node  200  includes demultiplexer elements  202 , multiplexer elements  204 , node elements  206  and  208  and transmitter/receiver elements  210 . In the illustrated embodiment, node elements  206  and  208  include identical or similar components and provide similar functionality for optical traffic communicated in respective directions. In one embodiment, elements  202 ,  204 ,  206 ,  208  and  210 , as well as components within the elements, may be interconnected with optical fiber links. Interconnection may occur at an optical backplace in particular embodiments. Any other suitable connections may alternatively be used. In addition, such elements may each be implemented as one or more discrete cards or plug in units within a card shelf of node  200 . Connectors may be used in a card shelf embodiment to allow efficient and cost effective replacement of failed components.  
      Demultiplexer elements  202  each include an optional amplifier  212  and a demultiplexer  214 . Demultiplexer elements  202  each receive optical traffic from an optical link  102  coupled to node  200 . In the illustrated embodiment, demultiplexer element  202   a  receives optical traffic traveling along optical link  102   a  in one direction, and demultiplexer element  202   b  receives optical traffic traveling along optical link  102   b  in the other direction. Demultiplexers  214  demultiplex the optical traffic received into its constituent wavelengths or channels. In particular embodiments, demultiplexers  214  may demultiplex optical traffic into forty separate channels. It should be understood that while the illustrated embodiment specifically shows what happens to optical traffic of one channel demultiplexed onto optical links  215   a  and  215   b  at demultiplexers  214   a  and  214   b , respectively, optical traffic in other channels demultiplexed onto optical links may encounter similar components and undergo similar action. As illustrated, demultiplexer elements  202  each also include OSC taps  219  to split the OSC signal before encountering demultiplexers  214 .  
      Traffic traveling along optical link  215   a  encounters node element  206  after demultiplexer element  202   a , and traffic traveling along optical link  215   b  encounters node element  208  after demultiplexer element  202   b . At node element  206 , traffic traveling along optical link  215   a  encounters a splitter  216  which allows for traffic along optical link  215   a  to be dropped to another splitter  218  and also continued to node element  208 . As used herein, the term “splitter” may comprise any suitable coupler, splitter, tap, combiner or other element able to receive one or more input optical signals and either split or combine the input optical signal(s) into one or more output optical signals. In the illustrated embodiment, splitter  216  comprises a drop coupler that passively splits the signal from the demultiplexer element into two generally identical signals: a through signal that is forwarded to another node element and a drop signal that is forwarded to the associated transmitter/receiver element. The split signals are copies in that they are identical or substantially identical in content, although power and/or energy levels may differ. In particular embodiments the drop signal may comprise approximately ten percent of the power of the original signal entering the splitter; however, in other embodiments the drop signal may comprise another percentage of the power of the original signal.  
      Traffic from splitter  218  travels to transmitter/receiver elements  210   a  and  210   b . It should be understood that other embodiments may utilize other components, such as a switch, in place of splitter  218 . Particular embodiments may not include multiple transmitter/receiver elements  210 , in which case splitter  218  may not be implemented or used.  
      Traffic continuing along optical link  215   a  from splitter  216  continues to node element  208  where it encounters a 2×1 optical switch  224 . Optical switch  224  is operable to select traffic from either optical link  215   a  or optical link  217   a  to continue to multiplexer element  204   a . Traffic from optical link  217   a  may comprise add signals that a user desires to add to the network from transmitter/receiver elements  210 . Traffic on optical link  217   a  comes from a 2×1 switch  226  coupled to both transmitter/receiver elements  210 . Switch  226  selects traffic transmitted from either transmitter/receiver element  210   a  or  210   b  to continue along optical link  217   a . As indicated above, particular embodiments may not include multiple transmitter/receiver elements  210 , in which case 2×1 switch  226  may not be implemented or used. It should be understood that switches of particular embodiments, including switch  224  and switch  234  discussed below, are reconfigurable in that their states may be changed to pass through an add signal or the continued signal according to particular needs. For example, if a user desires to add to the network a signal in the channel demultiplexed onto optical link  215   a , then the user may change the operation of switch  224  to pass through the add signal on optical link  217   a  to multiplexer  228   a.    
      Traffic selected to pass through switch  224  continues to multiplexer  228   a  of multiplexer element  204   a  where the traffic is multiplexed with other optical traffic in other channels or wavelengths into one traffic stream. Multiplexer element  204   a  also includes an optional amplifier  230  and an OSC tap  232  where the OSC signal is added back to optical link  102   a.    
      In a similar manner but opposite direction to traffic along optical link  215   a  from demultiplexer  214   a , demultiplexed traffic in one channel traveling along optical link  215   b  encounters splitter  220  which drops a copy of such traffic to another splitter  222 . It should be understood that other embodiments may utilize other components, such as a switch, in place of splitter  222 . Traffic from splitter  222  travels to transmitter/receiver elements  210   a  and  210   b . As indicated above, particular embodiments may not include multiple transmitter/receiver elements  210 , in which case splitter  222  may not be implemented or used.  
      Traffic continuing along optical link  215   b  from splitter  220  continues to node element  206  where it encounters a 2×1 optical switch  234 . Optical switch  234  is operable to select traffic from either optical link  215   b  or optical link  217   b  to continue to multiplexer element  204   b . Traffic on optical link  217   b  comes from a 2×1 switch  236  coupled to both transmitter/receiver elements  210 . Switch  236  selects traffic transmitted from either transmitter/receiver element  210   a  or  210   b  to continue along optical link  217   b . As indicated above, particular embodiments may not include multiple transmitter/receiver elements  210 , in which case 2×1 switch  236  may not be implemented or used.  
      Traffic selected to pass through switch  234  continues to multiplexer  228   b  of multiplexer element  204   b  where the traffic is multiplexed with other optical traffic in other channels or wavelengths into one traffic stream. Multiplexer element  204   b  also includes an optional amplifier  230  and an OSC tap  232  where the OSC signal is added back to optical link  102   b.    
      Transmitter/receiver elements  210  each include a switch  240 , a distributing splitter  242 , a transmitter  244 , a filter  246  and a receiver  248 . Transmitter/receiver elements  210  each transmit locally-derived add traffic from local clients, subscribers, another network, or any other appropriate source to optical links  102  and each receive from optical links  102  locally-destined drop traffic for local clients, subscribers, another network or any other appropriate destination. Two transmitter/receiver elements  210  are illustrated to add work and protect functionality to node  200  (one may act as a work transmitter/receiver while the other acts as a protect transmitter/receiver). Transmitter/receiver elements  210  handle traffic for optical links  215   a  and  215   b . Other transmitter/receiver elements may be utilized for traffic in other channels carried on other optical links between demultiplexer elements  202  and multiplexer elements  204 . It should be understood that transmitter/receiver elements described herein with respect to various embodiments may be located on other equipment and at other locations from other components described herein.  
      Switches  240  each receive optical traffic from optical links  215   a  and  215   b  and selectively pass one received traffic stream for receipt at filters  246 . Filters  246  may be implemented such that each filter allows a different channel to be forwarded to its associated receiver  248 . Distributing splitters  242  distribute copies of optical traffic transmitted from transmitters  244  to optical links  215   a  and  215   b  for communication on optical links  102   a  and  102   b . It should be understood that transmitter/receiver elements in other embodiments may include other types of optical components for transmitting and receiving optical traffic to and from an optical network.  
      As illustrated and discussed above, the demultiplexed traffic traveling along optical link  215   a  is dropped at splitter  216  of node element  206 . The splitting of such traffic prior to switch  224  where traffic is added back to the network via transmitter/receiver elements  210  reduces the need for more complex switches at the same location where traffic is added back to the network and reduces interference caused by the use of such switches. For example, using a 2×2 optical switch in place of 2×1 switch  224  to provide both add and drop functionality in a single switch can add interference and other problems within the network. However, dropping the traffic at a separate location from the add switch provides unique physical locations to drop and add the optical signal and further reduces the complexity of the switches utilized. Moreover, as discussed above a passive drop coupler may be used to drop the signal allowing the remaining signal to be used for pass through applications.  
      As discussed above, switch  224  is operable to select between traffic added from one of transmitter/receiver elements  210  or demultiplexed traffic along optical link  215   a  for continuation to multiplexer  228   a . If a client desires to add and drop traffic at node  200  (via transmitter/receiver elements  210 ) in the same channel as the demultiplexed traffic traveling along optical link  215   a , then switch  224  may be set to allow traffic from optical link  217   a  to pass through to multiplexer  228   a  for communication along the network. However, if no client traffic is being added in the same channel as the demultiplexed traffic traveling along optical link  215   a , then switch  224  may be set to allow the traffic traveling along optical link  215   a  to continue to multiplexer  228   a  for communication along the optical network.  
       FIG. 3  is a block diagram illustrating details of a node  300 , in accordance with one embodiment. Node  300  includes demultiplexer element  302 , multiplexer element  304 , node elements  306  and  308  and transmitter/receiver elements  310 . For ease of illustration, node  300  is illustrated as coupled to optical link  301  carrying optical traffic in one direction; however node  300  may also be coupled to an optical link carrying optical traffic in the opposite direction. Node  300  may include other components for adding and dropping optical traffic to and from the optical link carrying traffic in the opposite direction as the traffic carried by optical link  301 .  
      Demultiplexer element  302  includes an optional amplifier  312 , a demultiplexer  314  and an OSC tap  319  to split the OSC signal before encountering demultiplexer  314 . As is the case with node  200  of  FIG. 2 , node  300  of  FIG. 3  illustrates what happens to optical traffic of one channel demultiplexed onto optical link  315 . Optical traffic in other channels demultiplexed onto optical links may encounter similar components and undergo similar action.  
      Optical traffic demultiplexed onto optical link  315  encounters a splitter  316  which in the illustrated embodiment passively drops and continues the optical traffic. Splitter  316  is similar in functionality to splitter  216  of node  200  of  FIG. 2 ; however splitter  316  is located on demultiplexer element  302  of node  300  instead of on a separate node element or switch card. In particular embodiments, splitter  316  may be implemented on demultiplexer element  302  using array waveguide technology, using thin film filters or using other technology or components.  
      The traffic dropped at splitter  316  is split at splitter  318  for communication to both transmitter/receiver elements  310 . Particular embodiments may not include multiple transmitter/receiver elements in which case splitter  318  may not be implemented or utilized.  
      The continued traffic from splitter  316  passes through to 2×1 switch  324  at node element  308 . Optical switch  324  is operable to select traffic from either optical link  315  or optical link  317  to continue to multiplexer element  304 . Traffic on optical link  317  comes from a 2×1 switch  326  coupled to both transmitter/receiver elements  310 . Switch  326  selects traffic transmitted from either transmitter/receiver element  310   a  or  310   b  to continue along optical link  317 . As indicated above, particular embodiments may not include multiple transmitter/receiver elements  310 , in which case 2×1 switch  326  may not be implemented or utilized.  
      Traffic selected to pass through switch  324  continues to multiplexer  328  of multiplexer element  304  where the traffic is multiplexed with other optical traffic in other channels or wavelengths into one traffic stream. Multiplexer element  304  also includes an optional amplifier  330  and an OSC tap  332  where the OSC signal is added back to optical link  301 .  
      Node  300  may also include additional similar components for adding and dropping traffic communicated in a direction opposite from the traffic communicated on optical link  301 . Moreover, node  300  may include additional components for handling traffic in demultiplexed channels other than the channel or wavelength communicated on optical link  315  from demultiplexer  314 . Transmitter/receiver elements  310  may be similar in functionality to, and include similar components as, transmitter/receiver elements  210  of  FIG. 2 .  
      As discussed above, node  300  is similar to node  200  of  FIG. 2  except that demultiplexed traffic is dropped at the demultiplexer element (using switch  316 ) instead of at a subsequent node element or card. However, node  300  provides similar advantages to node  200  of  FIG. 2  in that the dropping of optical traffic prior to switch  324  where traffic is added back to the network reduces the need for more complex switches at the same location where traffic is added back to the network and reduces interference caused by the use of such switches. Dropping the traffic at a separate location from the add switch also provides unique physical locations to drop and add the optical signal.  
       FIG. 4  is a flowchart illustrating a method for communicating optical traffic at a node, in accordance with a particular embodiment. The method begins at step  400  where optical traffic is received at a node on a network. At step  402 , the optical traffic is demultiplexed into component signals, for example component wavelengths or channels. In particular embodiments, the optical traffic may be demultiplexed into approximately forty component wavelengths with approximately 100 GHz spacing.  
      At step  404 , at least one of the component signals is split into a drop signal and a continue signal at a splitter. The drop signal may comprise approximately ten percent of the power of the continue signal. At step  406 , the drop signal is received and recovered for use at a client. In particular embodiments, the drop signal may be further split and communicated to both a work receiver and a protect receiver, wherein the protect receiver is used in the event of failure of the work receiver. In some embodiments, the protect receiver may be used as a secondary receiver in parallel with the work receiver instead of merely in the event of failure of the work receiver. At step  408 , a switch selects between an add signal and the continue signal for communication on the network. It should be understood that the splitting of at least one of the component signals into a drop signal and a continue signal of step  404  may take place on the same card or plug in unit as the demultiplexing of step  402 . Such splitting of step  404  may also take place on a card or plug in unit located between a card or plug in unit where the demultiplexing of step  402  takes place and the card or plug in unit where the switch selection of step  408  takes place. At step  410 , the selected signal is multiplexed with other signals for communication on the network.  
      Some of the steps illustrated in  FIG. 4  may be combined, modified or deleted where appropriate, and additional steps may also be added to the flowchart. Additionally, steps may be performed in any suitable order without departing from the scope of the invention.  
      Although the present invention has been described in detail with reference to particular embodiments, it should be understood that various other changes, substitutions, and alterations may be made hereto without departing from the spirit and scope of the present invention. For example, although the present invention has been described with reference to a number of components included within switch cards and other node elements, other and different components may be utilized to accommodate particular needs. The present invention contemplates great flexibility in the arrangement of these elements as well as their internal components.  
      Numerous other changes, substitutions, variations, alterations and modifications may be ascertained by those skilled in the art and it is intended that the present invention encompass all such changes, substitutions, variations, alterations and modifications as falling within the spirit and scope of the appended claims. Moreover, the, present invention is not intended to be limited in any way by any statement in the specification that is not otherwise reflected in the claims.