Abstract:
An optical switch and switching system includes a large scale switching device with a first input, a second input, a first output, and a second output. A small scale switching device includes a third input, a fourth input, and a third output, wherein the third input of the small scale switching device is coupled to the first output of the large scale switching device and the fourth input of the small scale switching device is coupled to the second output of the large scale switching device. A controller establishes a cross connect in the large scale switching device between the second input and the second output. The small scale switching device switches from the third input to the forth input upon establishment of the cross connect in the large scale switching device,

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 11/610,954, filed Dec. 14, 2006, the entirety of which is incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    Optical networks include various optical switches or nodes coupled through a network of optical fiber links. Optical network failures or faults may be caused by any number of events or reasons, including damaged or severed optical fibers, or equipment failure. Because optical fibers may be installed virtually anywhere, including underground, above ground or underwater, they are subject to damage through a variety of ways and phenomena. Optical fibers and optical equipment may be damaged or severed, for example, by lightning, fire, natural disasters, traffic accidents, digging, construction accidents, and the like. 
         [0003]    Because optical fibers carry far greater amounts of information than copper wires used to transmit electrical telecommunications signals, the loss of an optical fiber can cause far more user disruptions when compared with the loss of a copper wire. For example, the loss of a single optical link, such as an optical link carrying a Wavelength Division Multiplexed (“WDM”) signal, may result in the loss of hundreds of thousands of phone calls and computer data transmissions. Additionally, dozens of fibers may be routed within a single cable or conduit, substantially increasing the risk of loss associated with a damaged cable or conduit. 
         [0004]    To reduce the negative effects of optical network failures, optical network topologies are provided in arrangements and configurations, such as mesh or ring topologies, so that telecommunications traffic may traverse the optical network using multiple optical links. This allows such optical networks to be reconfigured to route around network failure point. An optical network may include both working links or paths and spare links or paths that may be used to assist with optical network restoration. Due to the large amount of data or bandwidth an optical network carries, the amount of time it takes to identify the location of an optical network failure, and the time it takes then to reconfigure the optical network, may result in significant amounts of telecommunications traffic being lost. In particular, the reconfiguration of an optical network may result in the loss of other telecommunications traffic if not done efficiently or optimally. 
         [0005]    Known restoration techniques and methodologies are generally designed to restore telecommunications networks operating in the electrical domain as opposed to the optical domain, which presents additional challenges. Unfortunately, switching and restoration in the electrical domain, while fast, results in a significant waste of fiber resources, in that entire optical links are removed from service, even when only a small portion of the fiber is in need of maintenance. 
         [0006]    Another technique involves the use of a central control and database to model the network, monitor network operations, and communicate instructions to each node or optical switch in the network in response to a failure. Unfortunately, as fiber counts and network bandwidth requirements increase, the ability to efficiently switch signals in the optical domain is significantly reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more exemplary embodiments of the invention and, together with the description, explain the invention. In the drawings, 
           [0008]      FIG. 1  is a block diagram illustrating an exemplary communications system  100  in which systems and methods consistent with the exemplary embodiments described herein may be implemented; 
           [0009]      FIG. 2  is a block diagram conceptually illustrating a high data rate optical span for a fiber optic cable including a number of working fibers; 
           [0010]      FIG. 3  illustrates one exemplary implementation of a high data rate optical span as depicted in  FIG. 2 ; 
           [0011]      FIG. 4   a - 4   c  are exemplary block diagrams illustrating a fiber node configured to include hybrid switching components; and 
           [0012]      FIGS. 5   a  and  5   b  are block diagrams illustrating exemplary embodiments of traveling wave triggers for triggering switchover of the hybrid switch of  FIGS. 3 and 4 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0013]    The following detailed description of implementations consistent with the present invention refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents. 
         [0014]    Systems and methods consistent with aspects described herein provide a hybrid optical switch for enabling rapid switchover from a working fiber to a backup fiber in the event of required downtime or other maintenance. In one implementation, for example, a combination of a slower, large scale switch and a faster, small scale switch may be used to affect the switchover. 
       Exemplary Architecture 
       [0015]      FIG. 1  is a block diagram illustrating an exemplary communications system  100  in which systems and methods consistent with the exemplary embodiments described herein may be implemented. Communications system  100  may include multiple sites  102   a,    102   b,    102   c,    102   d,  and  102   e  connected together by links  104  and  106 . Links  104  and  106  may be implemented using electrical cables, satellites, radio or microwave signals, or optical connections and can stretch for tens or hundreds of miles between sites. Through these links, the communications system  100  carries data signals among the sites  102   a - 102   e  to effectively interconnect data equipment  108 ,  110 ,  112 ,  114 , and  116  (e.g., computers, remote terminals, servers, etc.) In the case of fiber optic links, each link may be configured to provide a number of high speed (e.g., 10 Gbps) connections using known WDM techniques. One or more links  104  and  106  that connect two sites are collectively referred to as a span  118 . 
         [0016]    A span  118  often includes multiple parallel links to increase working and spare capacity. As discussed above, to protect against data loss from fiber link or other equipment failures, redundant spare links may be commonly added between sites with the intent that they usually carry no data traffic but are available as alternate routes in the event of partial network failure affecting working links. If the network detects a link failure such as a fiber failure, cable cut, or transmitter/receiver nodal failure, traffic may be automatically switched from the failed link to an available spare link. 
         [0017]      FIG. 2  is a block diagram conceptually illustrating a high data rate optical span  200  for a fiber optic cable including four working fibers  202   a - 202   d.  Span  200  may include a number of transmitting line terminal ends (LTEs)  204   a - 204   d,  a number of optical amplifiers  206   a - 206   h,  an upstream switch, a downstream switch  210 , a number of receiving LTEs  212   a - 212   d,  and a backup fiber  214 . In accordance with embodiments described below, fiber lengths between optical amplifiers  206  may be referred to as nodes. By incorporating upstream and downstream switches  208  and  210  for each node, traffic may be switched in the optical domain from a working fiber requiring maintenance (e.g., one of fibers  202   a - 202   d ) to backup fiber  214 . Although only three working fibers  2021 - 202   c  and one backup fiber  202   d  are illustrated in  FIG. 2 , it should be understand that any suitable number of fibers may be switched in accordance with embodiments described herein. 
         [0018]    One exemplary implementation of a high data rate optical span is depicted in  FIG. 3 . In  FIG. 3 , a given site A is connected to another site B by a span (Span A-B) consisting of two optical fibers  302  and  304 . Additionally, each span may include upstream and downstream digital cross connected switches (DCSs)  312  and  313 , transmitting LTEs  314  and  315 , optical amplifiers  316 ,  317 ,  318 , and  319 , an upstream hybrid switch  320 , a backup fiber  322 , a downstream hybrid switch  324 , receiving LTEs  326  and  327 , and controller  350 . 
         [0019]    Two electrical data signals are presented at site A via inputs  328  and  330 . In normal operation, these signals may be carried through the network span and recovered at Site B as electrical signal outputs  332  and  334  respectively. For example, these data signals may be STM-64 synchronous data signals each bearing digital data at about 10 Gbps or the equivalent of 128,000 telephone-quality voice channels. As mentioned briefly above, each fiber  302  and  304  may be configured to carry multiple signals using different wavelengths. 
         [0020]    At site A, the signal presented at input  328  enters DCS  312 , and under normal conditions appears as an electrical signal along connection  329 . The signal at connection  329  enters LTE  314 , shown to include an optical transmitter  340 , such as a semiconductor laser. Light emitted by transmitter  340  may be wavelength division multiplexed by the electrical data signal that enters along connection  329  to deliver a modulated optical output signal over optical fiber  302 . 
         [0021]    As shown, fiber length  302  may include several optical amplifiers  316  and  318  for facilitating long haul lengths of optical fibers. Prior to the introduction of WDM, prior optical networks required a separate electrical regenerator every 60 to 100 kilometers. Traffic on each fiber would be converted from the optical to electrical domain and then regenerated for the next span. Conversely, optical amplifiers  316  and  318  may reamplify all of the channels on a WDM fiber in the optical domain without the need to de-multiplex, convert to the electrical domain, and individually process the included signals. In one implementation, optical amplifiers  316  and  318  may be spaced approximately every 1000 kilometers or so. Although only a pair of optical amplifiers is disclosed for each fiber  302  and  304  (i.e., optical amplifiers  316 ,  317 ,  318 , and  319 ), it should be noted that any number of optical amplifiers may be supported to facilitate transmission of optical signals from site A to site B. In accordance with exemplary embodiments described herein, each optical amplifier may be connected to upstream and downstream hybrid switches to facilitate switchover from a working fiber to a backup fiber in the event of outage or maintenance needs. 
         [0022]    In normal operation, each optical signal would proceed through upstream hybrid switch  320 , along fiber  302 , through downstream hybrid switch  324  to receiving LTE  326  located at site B. Receiving LTE  326  may include receiver  342  for receiving the optical signals from fiber  302  and outputting electrical versions thereof In one implementation, receiver  342  may include a photodiode. In the illustrated implementation, receiver  342  is shown to be a part of receiving LTE  326  that amplifies and conditions the signal to render a faithful electrical reproduction at output  344 . 
         [0023]    Although only described above with respect to fiber  302 , traffic along fiber  304  may be similarly processed, with an electrical data signal presented at input  330  being transported from DCS  312  through connection  331 , through transmitting LTE  315  via transmitter  341 , through optical amplifier  317 , upstream hybrid switch  320 , fiber  304 , downstream hybrid switch  324 , optical amplifier  319 , receiving LTE  327  via receiver  343  to output port  345 . 
         [0024]    Under normal operation, upstream and downstream hybrid switches  320  and  324  simply connect upstream sections of fibers  302  and  304  to downstream sections of fibers  302  and  304  to complete the end-to-end connection of inputs  328  and  330  to outputs  332  and  334 , respectively. 
         [0025]    In  FIG. 3 , fibers  302  and  304  are referred to as working fibers because they both carry data traffic when all network elements are functioning properly. In contrast, fiber  322  may be referred to as a spare or backup fiber because it carries data traffic only in the event of failure of one of the working fibers  302  and  304  or of the associated LTEs  314 ,  315 ,  326 , and  327 . Under normal circumstances, backup fiber  322  does not carry an optical data signal. It should be understood that while only two working fibers  302  and  304  are disclosed in  FIG. 3 , this number is merely exemplary and provided for the purposes of brevity and simplicity only. Real world fiber networks may incorporate a significantly higher number of working fibers for each backup fiber, depending on network demands. Additionally, real world fiber networks may incorporate more than one backup fiber. 
         [0026]    When maintenance is required for one of working fibers  302  and  304 , hybrid switches  320  and  324  switch data traffic onto backup fiber  322 . For example, if fiber  302  requires maintenance, upstream hybrid switch  320  may connect the output from optical amplifier  316  to backup fiber  322 . At the same time, downstream hybrid switch  324  may connect backup fiber  322  to the input of optical amplifier  318 . This switching action restores end-to-end connectivity between input  328  and output  332  during a maintenance period for working fiber  302 . Details regarding the configuration of upstream and downstream hybrid switches  320  and  324  are set forth in additional detail below. 
         [0027]    By providing high speed switching between optical amplifiers in the optical domain, rapid switchover to backup fiber  322  may be realized in the event that a working fiber  302  or  304  needs to be taken out of service. 
         [0028]    To successfully perform backup fiber switching, controller  350  may be configured to direct upstream hybrid switch  320  and downstream hybrid switch  324  to switch an output of optical amplifier  316  or  317  to backup fiber  322 . As will be described in additional detail below, a final switchover of downstream hybrid switch  324  may be accomplished without controller direction, such that losses resulting from data “trapped” in fiber  302  or  304  at the time of switchover may be minimized. More specifically, the final switchover of downstream hybrid switch  324  may be performed in direct response to the switchover of upstream hybrid switch  320  by using light detection techniques as described below. Controller  350  may include an imbedded microprocessor, computer, workstation, or other type of processor for controlling the switching of upstream hybrid switch  320 . 
         [0029]      FIGS. 4   a - 4   c  are a block diagram illustrating a node of fibers  302  and  304  positioned between optical amplifiers  316  and  318  and  317  and  319 , respectively, in various stages of switchover. As illustrated in  FIG. 4   a , upstream hybrid switch  320  may include two small scale switching components  405   a  and  405   b  associated with working fibers  302  and  304 , respectively. Upstream hybrid switch  320  may also include a large scale switching component  410  positioned downstream of small scale switching components  405   a  and  405   b.  Similarly, downstream hybrid switch  324  may include a large scale switching component  415  and small scale switching components  420   a  and  420   b.    
         [0030]    In one implementation, small scale switching components  405   a  may include a fast switch configured to switch an output from optical amplifier  316  between one of two inputs to large scale switching component  410 . In one exemplary implementation, small scale switching components  405   a - 405   b  and  420   a - 420   b  may include a 2×2 optical switch capable of switching outputs in approximately 20 microseconds. 
         [0031]    Large scale switching component  410  may include an N×N switch capable of switching a large number of input signals between a large number of output ports, where N may be a number between 10 and 1000. In one exemplary embodiment, large scale switching components  410  and  415  may include 320×320 optical switches capable of switching inputs in approximately 20 milliseconds. As shown, large scale switching component  410  may output to working fibers  302  and  304  or backup fiber  322 . 
         [0032]    By nature, large scale switching component  410  accomplishes its switching functions significantly more slowly than small scale switching components  405   a  and  405   b.  Because of this factor, using a large scale switching component alone to effect a switchover between one of working fibers  302  and  304  and backup fiber  322  would result in unacceptable loss of data. At the same time, performing backup using only small scale switches would result in the deployment and construction of hundreds or thousands of backup fibers in the optical network, since each working fiber would need to be associated with a backup fiber on a 1 to 1 or perhaps 2 to 1 basis. Neither of these alternatives is tenable to a successful optical network configuration. 
         [0033]    Unlike either of these approaches, the described hybrid switching pair  320  and  324  enable fast switchover to backup fibers without requiring deployment of large number of backup fibers. As shown in  FIG. 4   b , upon receipt of a command from controller  350  (not shown) to switch over working fiber  302  to backup fiber  322 , large scale switching component  410  may set up an optical cross connect  425  between a second output from small scale switching component  405   a  associated with fiber  302 . 
         [0034]    Following configuration of large scale switching component  410 , backup fiber  322  may be configured to receive an output from small scale switching device  405   a . Similarly, downstream large scale switching component  415  may be configured to set up an optical cross connect  430  between backup fiber  322  and a second output associated with small scale switching component  420   a.  In one exemplary implementation, successful configuration of cross connects  425  and  430  may be monitored by testing devices  435  to ensure that signals will be properly routed following switchover. 
         [0035]    During the configuration of cross connects  425  and  430  within upstream and downstream large scale switching components  410  and  415 , respectively, working fiber  302  may continue to transmit data via previously established cross connect  440  in small scale switching component  405   a  and cross connect  445  in small scale switching component  420   a  as well as cross connect  450  in large scale switching component  410  and cross connect  455  in large scale switching component  415 . 
         [0036]    Once configuration of cross connects  425  and  430  have been established, switchover from working fiber  302  to backup fiber  322  may be performed. As shown in  FIG. 4   c , switchover between fiber  302  and backup fiber  322  may be effected by switching small scale switching component  405   a  between a first output associated with working fiber  302  and a second output associated with cross connect  425 . Because cross connect  425  has been previously established, switching of small scale switching component  405  enables the output from optical amplifier  316  to reach backup fiber  322 . 
         [0037]    On the downstream side, small scale switching component  420   b  may be triggered in a variety of manners. In one implementation, controller  350  (not shown) may trigger the switching of small scale switching component  420   b.  However, because light “data” present in fiber  302  at the time small scale switching component  405   a  is triggered should be received by optical amplifier  318  prior to switchover of small scale switching component  420   b,  controller  350  may need to account for a transmission delay related to the length of fiber node between optical amplifiers  316  and  318  as well as other factors. 
         [0038]    In another implementation, traveling wave synchronization may be used to trigger small scale switching component  420   a.    FIG. 5   a  is a block diagram illustrating one exemplary embodiment of a traveling wave trigger  500  for triggering switchover of small scale switching component  420   a  from a first input to a second input. As shown, trigger  500  may include a switch control  505 , an optical tap  510  and a photo detector  515 . 
         [0039]    In operation, optical tap  510  may be configured to connect the output of small scale switching component  420   a  to photo detector  515  via optical tap  510 . Once small scale switching component  405   a  has affected a switchover at the upstream end of fiber  302 , light transmitted in fiber  302  will cease. Photo detector  515  may monitor the output of small scale switching component  420   a  to determine when this has occurred. In one implementation, photo detector  515  may include a photodiode. Once photo detector  515  observes a loss of light, a signal indicative thereof may be sent to switch control  505 . Switch control  505  may then, in turn, effect the switchover of small scale switching component  420   a  from the first input associated with working fiber  302  to a second input associated with backup fiber  322 . 
         [0040]    In yet another implementation, a reverse principle of traveling wave synchronization may be used to trigger small scale switching component  420   a.    FIG. 5   b  is a block diagram illustrating another exemplary embodiment of a traveling wave trigger  520  for triggering switchover of small scale switching component  420   a  from a first input to a second input. As shown, trigger  520  may include a switch control  525 , an optical tap  530  and a photo detector  535 . However, instead of positioning tap downstream of small stream switching component  420   a,  tap  530  may be positioned downstream of large scale switching component  415  and upstream of small scale switching component  420   a.    
         [0041]    Once small scale switching component  405   a  has affected a switchover at the upstream end of fiber  302  onto backup fiber  322 , light transmitted in fiber  322  will begin. Photo detector  535  may monitor the output of large scale switching component  415  associated with backup fiber  322  via tap  530  to determine when light is present on backup fiber  322 . Once photo detector  535  observes the presence of light at this output, a signal indicative thereof may be sent to switch control  525 . Switch control  525  may then, in turn, effect the switchover of small scale switching component  420   a  from the first input associated with working fiber  302  to a second input associated with backup fiber  322 . 
         [0042]    By using the absence of light in working fiber  302  or the presence of light in backup fiber  322 , triggers  505  and  520  may perform switching of small scale switching component  420   a  with minimal delay and zero loss in data previously transmitted on working fiber  302 . More specifically, along with the eliminated loss of previously transmitted data, switchover from working fiber  302  to backup fiber  322  may be accomplished in an amount of time consistent with the switching times of small scale switching components  405   a  (T1)+small scale switching component  420   a  (T2)+processing of switch control  505  or  525  (Tp). 
       CONCLUSION 
       [0043]    Implementations consistent with aspects described herein enable rapid and efficient switchover from a working fiber to a backup fiber. More particularly, in one implementation, a combination of slower, large scale optical switches, and faster, small scale optical switches may be used to implement the switchover. Moreover, traveling wave synchronization techniques may be used to reduce switch delay and data loss. 
         [0044]    The foregoing description of exemplary embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. 
         [0045]    It will be apparent to one of ordinary skill in the art that features of the invention, as described above, may be implemented in many different forms of hardware, software, or firmware in the implementations illustrated in the figures. The actual hardware or control software used to implement the described features is not limiting of the invention. Thus, the operation and behavior of these features were described without reference to specific hardware or control software—it being understood that one of ordinary skill in the art would be able to design hardware and software to implement the aspects based on the description herein. 
         [0046]    Further, certain portions of the invention may be implemented as “logic” that performs one or more functions. This logic may include hardware, such as an application specific integrated circuit or field programmable gate array, software, or a combination of hardware and software. 
         [0047]    No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise. The scope of the invention is defined by the claims and their equivalents.