Abstract:
An optical switch fabric with an input stage, an output stage, and a center stage coupled in a cascaded manner. The center stage includes (1) a minimum number of center switches greater than one that cause the optical switch fabric to be strict-sense nonblocking and (2) at least one additional center switch to provide redundancy for the optical switch fabric. A module is described that includes optical input and output switches coupled to an optical center stage of an optical switch fabric. The module includes an interior cavity that contains free space beams from both the optical input switch and the optical output switch. A method is described for reconfiguring a redundant optical switch into a multilevel optical switch without interrupting operation of the signals carried by the optical switch by adding additional switch components and reconfiguring the fiber interconnection between switch elements.

Description:
The present application is a divisional of U.S. patent application Ser. No. 09/876,715, filed Jun. 6, 2001 now U.S. Pat. No. 6,882,766. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of optical switches for fiberoptic networks. More particularly, the present invention relates to multistage optical switch architectures with input/output switch modules and redundant switches, and to methods for upgrading switch fabrics. 
   BACKGROUND 
   The use of fiberoptic networks is increasing due to the high bandwidth provided by such networks for transporting data, voice, and video traffic. Large switches would help to accommodate the switching needs of many of the larger fiberoptic networks, especially the high-capacity fiber backbones. 
   One disadvantage of certain prior art optical switches is that although optical signals can propagate almost losslessly while confined in optical fiber, the size of certain prior art optical switches is typically limited by diffraction of optical beams as they propagate through free space inside the switches. Moreover, large optical switching devices can be difficult to construct given the large number of optical cables and beams and complex associated electrical connection issues. In short, large optical switches can be costly and unwieldy. 
   Various types of non-optical electrical switch fabrics have been used in the prior art for telephony and network applications. One of the simplest structures has been the crossbar switch. One problem with the crossbar switch is the quadratic growth of crosspoints as the switch gets larger, which can result in far more cross-points than necessary to create all possible permutation connections. For a permutation switch, connections between input and output ports are point to point—neither one-to-many nor many-to-one connections are permitted. 
   To avoid the problem of excess crosspoints found in a single large switch, techniques have been developed for cascading small electrical switches into a multistage switch fabric in order to make large electrical permutation switches. 
   Permutation switches can be classified in terms of their blocking characteristics. On a switch, requests for connection establishment and termination can occur at random points in time. A permutation switch is rearrangeable or rearrangeably nonblocking if there exists a set of paths through the switch fabric that realizes each of any possible connection states. The rearrangeable aspect means that it may be necessary to rearrange currently active connections to support a request for a new connection between a pair of idle input and output ports. Problems with rearrangeable nonblocking switches include the fact that the required device settings to route connections through the switch are not determined easily and that connections in progress may have to be interrupted momentarily while rerouting takes place to handle the new connections. 
   Wide-sense nonblocking networks or switches are those that can realize any connection pattern without rearranging active connections provided that the correct rule is used for routing each new connection through the switch fabric. 
   Strict-sense nonblocking networks or switches require no rearrangement of active connections and no complex routing algorithms. New connection requests are allowed to use any free path in the switch. Strict-sense nonblocking switching fabrics (also referred to as strictly nonblocking switches) typically require more hardware than wide-sense nonblocking and rearrangable switching fabrics, but avoid connection disruption and provide simplicity of routing. 
   One type of cascaded permutation switch topology is a Clos switch fabric, also referred to as a Clos network, a Clos switch matrix, or a Clos switch. Various Clos switch configurations can constructed. For example, some Clos switch fabrics can be strict-sense nonblocking, other Clos switch fabrics can be wide-sense nonblocking, and others can be blocking. The blocking configurations are less useful, given that some combinations of input and output connections cannot be made. 
     FIG. 1  shows a three-stage Clos switch fabric that is strict-sense nonblocking, meaning that any input can be routed to any output at any time. The Clos switch fabric of  FIG. 1  has N inputs, N outputs, K input stage switches, 2p−1 center stage switches, and K output stage switches. Each input stage switch has p inputs and 2p−1 outputs. Each center stage switch has K inputs and K outputs. Each output stage switch has 2p−1 inputs and p outputs. 
   One disadvantage of the strict-sense nonblocking Clos switch fabric of  FIG. 1  is the lack of redundancy in switch connections. Redundancy is a desirable characteristic in a switch fabric because redundancy helps to permit rerouting in the event of a failure, the use of extra paths for test purposes during switch operation, and switch reconfiguration during switch operation. 
   SUMMARY OF THE INVENTION 
   An optical switch fabric is described that has an input stage, an output stage, and a center stage coupled in a cascaded manner. The center stage includes (1) a minimum number of center switches greater than one that cause the optical switch fabric to be strict-sense nonblocking and (2) at least one additional center switch to provide redundancy for the optical switch fabric. 
   A module is described that includes an optical input switch of an input stage of an optical switch fabric, an optical output switch of an output stage of the optical switch fabric, and an interior cavity. The input and output stages are coupled to a center stage of the optical switch fabric. The interior cavity contains free space beams from both the optical input switch and the optical output switch. 
   A method is described for reconfiguring an optical switch without interrupting working optical signals. 
   Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
       FIG. 1  shows a prior art multistage switching fabric with a Clos architecture. 
       FIG. 2  shows a multistage Clos switching fabric with external 1×2 switches and 2×1 switches for redundancy. 
       FIG. 3  shows a Clos switch fabric with redundancy input and output stage switches. 
       FIG. 4  illustrates a redundant Clos switch fabric that includes an additional center stage switch. 
       FIG. 5  shows an 8,000-port optical redundant Clos switch fabric that has 8-port input stage switches and 8-port output stage switches. 
       FIG. 6  illustrates the architecture of an optical Clos input/output module for a redundant Clos optical switch fabric. 
       FIG. 7  shows a redundant Clos switch fabric with relatively small center stage switches. 
       FIG. 8  shows a redundant Clos switch fabric with partially populated center switches. 
       FIG. 9  illustrates a switch fabric that uses 125 Clos input/output modules and two 1,000-port switches. 
       FIG. 10  is a simplified schematic of the switch fabric of  FIG. 9  with the 8×16 input stage switches shown as a single box and the 16×8 output stage switches shown as a single box. 
       FIG. 11  shows a partial upgrade from a switch fabric that uses Clos input/output modules and two 1,000-port center switches to a redundant Clos switch fabric. 
       FIG. 12  shows a completed upgrade to a redundant Clos 1,000 port switch fabric. 
       FIG. 13  shows a redundant Clos 2,000-port switch fabric configured by the addition of fiber interconnections. 
       FIG. 14  shows a redundant Clos 4,000-port switch fabric configured by repositioning fiber cables. 
       FIG. 15  shows a redundant Clos 8,000-port switch fabric. 
       FIG. 16  shows a redundant 16,000 port Clos switch fabric configured by upgrading center stage switches from 1,000-ports to 2,000-ports. 
   

   DETAILED DESCRIPTION 
   A high availability optical switch fabric or matrix is described that uses a Clos multistage architecture. 
   As will be described in more detail below, for one embodiment an additional center stage switch is added to a strict-sense nonblocking optical Clos switch fabric to provide redundancy. An intended advantage of the embodiment is to provide a large capacity optical switch that is easier to construct given that is comprised of a number of smaller optical switches. Another intended advantage is the capability of providing both working and protection (i.e., test) connections and yet have the working connections be strict-sense nonblocking. A further intended advantage includes an enhanced ability to reroute connections during failure. Another intended advantage of the redundant Clos switch fabric is that it is amendable to switch fabric reconfiguration and upgrades while live traffic is being carried, thereby helping to minimize service disruptions. 
   An embodiment is described wherein input stage switches and output stage switches are combined to form Clos input/output modules (“CIO modules”). An intended advantage of this embodiment is to minimize switch granularity as compared with separate input stage switch modules and output stage switch modules. Other intended advantages include cost minimization and modularity to help to facilitate switch fabric upgrades. 
   Methods are also described for upgrading switch fabrics into large redundant Clos switch fabrics while those switch fabrics carry live traffic. Intended advantages of the methods include minimizing service disruption, providing various flexible upgrade paths, and providing the ability to reuse at least some existing equipment, thereby helping to minimize costs. 
   One way to add redundancy to a strict-sense nonblocking Clos switch fabric is shown in  FIG. 2 . In  FIG. 2 , optical protection switches (1×2 and 2×1) are placed at the respective input  2  and output  3  optical switch stages and an identical redundant three-stage optical Clos switch  12  is added to the original three-stage optical Clos switch  8 . 
     FIG. 3  shows a simplified version of the redundant Clos network of  FIG. 2 . In the redundant Clos optical switch fabric  10  of  FIG. 3 , each 1×2 protection switch and each of two corresponding p×(2p−1) switches of the switch fabric of  FIG. 2  are combined into one p×(4p−2) switch. The output stage (2p−1)×p switches and the 2×1 protection switches are similarly combined into (4p−2)×p switches. This allows the optical switch fabric  10  of  FIG. 3  to eliminate the protection switches (1×2 and 2×1) of  FIG. 2 , even though the switch fabric of  FIG. 3  maintains a redundancy similar to that of the switch fabric of  FIG. 2 . The switch configurations of  FIGS. 2 and 3  achieve redundancy through the use of a large number of components and, accordingly, there is an increase in complexity. 
     FIG. 4  illustrates an optical Clos switch fabric  20  that is strict sense nonblocking and is fully redundant. Redundancy is achieved in the switch fabric  20  by the inclusion of an additional center stage switch  46 . Switch fabric  20  is also referred to as switch matrix  20 , network  20 , multilevel switch  20 , multistage switch  20 , or simply switch  20 . 
   The Clos switch fabric  20  has an input stage  30 , a center stage  40 , and an output stage  50 . The input stage  30  is coupled to the center stage  40  via fiber optic interconnect lines  38 . The center stage  40  is coupled to the output stage  50  via fiber optic interconnect lines  48 . 
   The input stage  30  comprises K optical input switches  36 . K is an integer. Each of the input switches  36  has P inputs and 2p outputs. P is an integer. There are N inputs  34  to the input stage  30 . N is an integer. The inputs  34  are divided evenly between the switches  36  so each switch has N divided by K number of inputs. Therefore, P equals N divided by K. 
   The 2P outputs of each input stage switch  36  are coupled to the center stage optical switches  46 . Each output of each of the input stage switches  36  is coupled to one of the center stage switches  46  such that each switch  36  is coupled to each of the center stage switches  46 . 
   For one embodiment, the number of center stage switches  46  is 2P, which equals the number of outputs of each input stage switch  36 . Each of the center stage switches  46  has K inputs and K outputs. K equals the number of input stage switches  36 . 
   The K outputs of center switches  46  are coupled to output optical switches  56  of output stage  50 . Each of the output switches  56  has 2P inputs and P outputs. There are K switches  56  in output stage  50 . There are N outputs  54  of output stage  50 . Each of the outputs K of each center stage switch  46  is coupled to one of the switches  56  of output stage  50 . 
   Redundancy is obtained in switch fabric  20  by adding an additional center stage switch  46  beyond the number of center stages switches required for the switch fabric to be strict-sense nonblocking. Thus, switch fabric  20  has 2P center stage switches  46 . This differs from the prior art switch fabric shown in  FIG. 1 , which only has 2P−1 center stage switches. For the switch fabric  20  shown in  FIG. 4 , each of the input stage switches  36  has an extra output as compared to the input stage switches of the prior art switch fabric shown in  FIG. 1 . In addition, each of the output stage switches  56  shown in  FIG. 4  has an additional input as compared to the output stage switches of the prior art switch fabric shown in  FIG. 1 . 
   The switch fabric  20  shown in  FIG. 4  is more reliable than the prior art switch shown in  FIG. 1  because the switch fabric  20  is fully redundant. Moreover, switch fabric  20  has fewer center stage switches and fewer fiber interconnects between the center stage switches and the input and output switches than the switches shown in  FIGS. 2 and 3  and thus is more efficient. In particular, switch fabric  20  has 2P center stage switches, which contrasts with the 4P−2 center stage switches of the switch fabrics of  FIGS. 2 and 3 . In addition, each input stage switch of the switch fabric shown in  FIG. 3  has 4P−2 outputs and each output stage switch of that prior art switch fabric has 4P−2 inputs. In contrast, each of the input stage switches  36  of  FIG. 4  has 2P outputs and each of the output stage switches  56  has 2P inputs. 
   The switch fabric  20  of  FIG. 4  has one additional center stage switch  46  added for redundancy. For alternative embodiments, however, additional center stage switches  46  could be added for more redundancy. There need only be P center switches  46  to make switch fabric  20  rearrangeably nonblocking. 
   The center stage  40  of Clos switch fabric  20  of  FIG. 4  has the capability of establishing two times the number of total possible connections in order for the switch fabric  20  to still be strict-sense nonblocking. Nevertheless, if switch fabric  20  is configured to be only rearrangeably nonblocking, then the center stage  40  need only have capacity for the total possible connections, rather than two times the number of total possible connections. 
   A protection path can be set up for every working path. If the working connections through the Clos switch fabric  20  must be strict-sense nonblocking, but the protection connections are allowed to be rearrangeably nonblocking, then it is possible to only require two times the number of total possible working connections (strictly K equals 2 times N) and yet have the capability of providing both working and protection connections. The establishment of any working connection must be able to preempt any protection connection. The establishment of any protection connection may require rearrangement of all of the other protection connections. 
     FIG. 5  shows an optical redundant Clos switch fabric  60  that has an input stage  70 , a center stage  80 , and an output stage  90 . The switch fabric  60  has 8,000 input ports  74  and 8,000 output ports  98 . The input stage  70  includes 1,000 optical input stage switches  76 . The center stage  80  includes 16 optical center stage switches  86 . The output stage  90  includes 1,000 optical output stage switches  96 . 
   Each input stage switch  76  has eight inputs for working signals. In addition, each input stage switch  76  has two inputs  71  for the outputs of test source  73 . Thus, each of the input stage switches  76  has ten inputs—i.e., eight working inputs IN  1 - 1  through IN  1 - 8 , for example, and 2 test inputs  71 . Lasers  73  provide the test light for the test signals. The test signals can be used for setting up protection paths. 
   The outputs of the test sources  73  can be routed to unused paths of the Clos switch fabric  60  to verify operation of all of the optical paths and to preconfigure the redundant center switch of center stage  80  with all the settings needed to replace any of the other center switches  86  should one of them fail. The test sources  73  may also be used to set up protection paths through unused ports in working switches. 
   Detectors  95 , each of which contains two detectors, are coupled to each of the output switches  96  in order to allow monitoring of the test signals through the unused Clos switch paths. 
   The optical Clos switch fabric  60  of  FIG. 5  differs from the prior art. One prior art Clos electrical switch fabric would have input switches with 15 outputs, 15 center switches with 1,000 inputs and 1,000 outputs, and output stages with 15 inputs. In contrast, Clos optical switch fabric  60  of  FIG. 5  has redundancy by the addition of another switch output to each input stage switch  76 , by the addition of another center stage switch  86 , and by the addition of another input to each output stage switch  96 . 
   Each of the center stage switches  86  of switch fabric  60  includes an internal optical tap  87  that allows substantially noninvasive real-time monitoring of any of the optical signals. The internal optical taps  87  in center stage switches  86  have the ability to provide high speed samples of the optical signals passing through switches  86 . 
   The input stage  70  of input switch fabric  60  includes detectors  72  for monitoring optical signals provided as inputs to switch fabric  60 . The input stage  70  also includes detectors  74  for monitoring optical signals from the outputs of input stage switches  76 . Output stage  90  of switch fabric  60  includes detectors  92  for monitoring optical signals that are sent as inputs to output stage switches  96 . Output stage  90  also includes detectors  94  for monitoring the output optical signals from output stage switches  96 . 
   For alternative embodiments of the invention, switch fabric  60  of  FIG. 5  could be larger or smaller, but still meet the relationships among K, P, and N of Clos switch fabric  20  of  FIG. 4 . For example, binary sequences can be used, such as K=1,024, P=8, and N=8,192. Alternatively, larger and smaller numbers can be used for K, P, and N. 
   For switch fabric  60  shown in  FIG. 5 , the input stage switches  76  and the output stage switches  96  are grouped together to form Clos input/output (“CIO”) modules, which minimizes the switch granularity compared to having separate modules for input stage switches and separate modules for output stage switches. The functionality of the 10×16 optical input switches and the 16×10 optical output switches can be combined into one 26×26 optical switch. 
     FIG. 6  illustrates Clos input/output module  110 , which is a 26×26 optical port switch that combines the functionality one of the 10×16 optical input stage switches  76  and one of the 16×10 optical output stage switches  96  of the switch fabric  60  of  FIG. 5 . 
   Clos I/O module  110  includes a housing  119  that encloses an array  139  of microelectromechanical (“MEMs”) mirrors that includes an input beam mirror array  145  and an output beam mirror array  147 . For one embodiment, the MEMs array  139  includes 52 working mirrors for the 26 input optical beams and the 26 output optical beams. For an alternative embodiment, the MEMS array  139  includes 64 mirrors, which includes 12 mirrors for redundancy. Also enclosed within housing  119  is a fixed mirror  137 . Light beams  141  within housing  119  are reflected by input mirror array  145 , then reflected by fixed mirror  137  then reflected by output mirror array  147  to form output optical beams  143 . 
   Collimator array  131  holds the 26 input fiber lines and the 26 output fiber lines within housing  119 . For an alternative embodiment, collimator array  131  also holds 12 fiber lines for redundancy, for a total of 64 fiber lines. Monolithic lens array  132  focuses or collimates the optical outputs of the fiber lines of collimator array  131  into beams  141 . Lens array  132  also receives beams  143  and focuses them into the fiber lines that are carrying optical signals out of Clos I/O module  110 . 
   Tap  135  is also enclosed within housing  119 . Tap  135  permits substantially noninvasive optical power monitoring of optical beams  141 . Tap  135  sends optical signals to 26 photodetectors  133  within housing  119  for optical input signal detection and monitoring. 
   Clos input/output module  110  is a 26×26 port optical switch, so there are twenty-six inputs and twenty-six outputs. The eight Clos module inputs  114  are the same as the inputs to an individual input stage switch  76  shown in  FIG. 5 , and thus are part of the inputs  74  to switch matrix  60 . 
   Inputs  128  to Clos input/output module  110  are 16 outputs from one of the 1,000×1,000 port center stage switches  86  shown in  FIG. 5 . Thus, the optical signals on lines  128  are part of the signals carried by fiber lines  88  shown in  FIG. 5 . 
   Inputs  111  to the Clos I/O module  110  of  FIG. 6  are two optical fibers for two test lasers  113 . Test lasers  113  send optical test signals through inactive optical channels within Clos input/output module  110 . 
   Two corresponding optical output detectors  125  are provided on the output side of Clos input/output module  110 . Switch outputs not used for working signals send optical signals to the detectors  125  over two fiber lines  121  to allow detectors  125  to monitor inactive optical channels of Clos input/output module  110 . These detectors  125  correspond to detectors  95  of  FIG. 5 . 
   Outputs  134  comprise eight optical Clos outputs from Clos input/output module  110 . Outputs  134  correspond to the outputs of one of the output stage switches  96  shown in  FIG. 5  and are part of the outputs  98  from switch fabric  60 . 
   Further outputs from Clos input/output module  110  of  FIG. 6  are the optical signals of fiber lines  118 , which are 16 signals to be sent to the center stage switches  86  shown in  FIG. 5 . The sixteen signals on fiber lines  118  are part of the signals carried on connections  78  shown on  FIG. 5 . 
   Clos input/output module  110  also includes twenty-four optical detectors  124  for monitoring the output optical signals from Clos input/output module  110 . Optical fiber power splitters  146  send optical signals to detectors  124 . Detectors  124  comprise a combination of a detector  74  and a detector  94  of  FIG. 5 . 
   Thus, eight optical inputs  114  are switched among sixteen optical outputs  118 , which in turn go to the center switches  86 . Sixteen optical inputs  128  to Clos I/O module  110  coming from center switches  86  are switched among optical outputs  134 . 
   For alternative embodiments, more than twenty-six input and output ports would be fabricated for the Clos input/output optics module  110  to allow for production yield. For the alternative embodiments, other parts of the module  110  would be larger, such as the mirror array. For other alternative embodiments, the Clos I/O module  110  could be smaller, with fewer than 26 respective input and output ports. 
   For an alternative embodiment of the invention, the twenty-four photo detectors  124  sampling the output signals and the two photo detectors  125  used to monitor inactive channels may be placed inside the housing  119  of the optics module by routing the coupled output fibers  144  back into the collimator array  131 . 
   The cost of a large capacity optical switch with a Clos architecture like the switch fabric  60  of  FIG. 5  can be dominated by a large number of small switches, which in this case are the 1,000 Clos input/output modules needed for an 8,000 port Clos switch, assuming the input and output stage switches  76  and  96  are combined into Clos input/output modules  110 . The total cost of the sixteen center stage switches  86  is lower than the total cost of the 1,000 Clos input/output modules  110  formed by switches  76  and  96 , even though the cost per switch is higher for the center stage switches  86 . The reduced cost is due to the fact that there are only sixteen center stage switches  86  versus 1,000 Clos I/O modules  110 . 
   For one embodiment of the invention, customers would be able to purchase expandable switch fabrics, where initially a small number of ports are purchased, but additional ports could be added as required. 
   One approach to an upgrade path for a switch fabric is to initially install the full set of center stage switches and install the Clos input/output modules (that form the input and output switch stages) as needed. For example, the 8,000 port switch fabric  60  shown in  FIG. 5  initially could be used as a 2,000 port switch fabric by installing one fourth of the Clos input/output modules  110  that combine input switches  76  and output switches  96 . Such a 2,000 port switch would retain all sixteen center stage switches  86 . For this approach, however, the cost of the center stage switches  86  becomes a larger fraction of the initial cost of the switch fabric  60  because there are fewer Clos input/output modules  110  than with having 1,000 Clos input/output modules  110 . For alternative embodiments, this approach could be used for smaller or larger switch fabrics. 
   For yet other alternative embodiments, a reverse approach can be used to downgrade capacity of a switch fabric. Clos input/output modules could be removed to lower switch capacity in a manner opposite to the upgrade approach. 
   Another way to reduce the initial cost of installing a portion of a large capacity optical switch is to initially install smaller center stage switches, which is shown by the switch fabric  200  in  FIG. 7 . Switch fabric  200  is a Clos multistage switch fabric containing input stage  210 , center stage  220 , and output stage  230 . Instead of being configured as an 8,000 port switch fabric such as switch fabric  60  shown in  FIG. 5 , the switch fabric  200  is instead initially configured as a 2,000 port Clos switch fabric with one fourth of the Clos input/output modules as switch fabric  60 . For switch fabric  200 , the total number of inputs  214  is 2,000 and the total number of outputs  234  is 2,000. The switch fabric  200  of  FIG. 7  has 16 center stage switches  26  that need only 250 input ports and 250 output ports. Therefore, each center stage switch  226  is a 250 by 250 port switch. Input stage  210  has 250 input stage switches  216  and output stage  230  includes 250 output stage switches  236 . 
   For the sake of simplicity, the test and monitoring functions are not shown in the switch fabric  200  of  FIG. 7 . Each of the input stage switches  216  has eight inputs and sixteen outputs. Each of the output stage switches  236  has sixteen inputs and eight outputs. If the testing and monitoring functions were shown, then each of the input stage switches  216  would have ten inputs and each of the output stage switches  236  would have ten outputs. 
   For switch fabric  200 , the input stage switches  216  and the output stage switches  236  are combined as Clos input/output modules. 
   The fiber routing between modules is unchanged when the switch fabric  200  is upgraded by upgrading the center switches  226  to 1,000 port by 1,000 port switches and by adding more Clos input/output modules. In other words, the center switches  226  are replaced by sixteen 1,000 port by 1,000 port center stage switches. Additional Clos I/O modules are added so that there are 1,000 input switches  216  and 1,000 output switches  236 . 
   This upgrade approach has the disadvantage that the 250 port center stage switches  226  need to be replaced. Nevertheless, the center stage switches  226  may be reused in some other smaller capacity optical switch somewhere in the optical network. 
   For alternative embodiments, this upgrade approach could be used for smaller or larger switch fabrics. 
   For yet other alternatives embodiments, the upgrade approach can operate in reverse to downgrade switch fabric capacity. This downgrade path would be accomplished by replacing large center stage switches (such as 1,000 port×1,000 port center stage switches) with smaller center stage switches (such as 250 port by 250 port center stage switches). 
   Another upgrade approach is to use 1,000 port by 1,000 port center stage switches, but to install only a portion of them.  FIG. 8  illustrates switch fabric  250  that uses this upgrade approach. For switch fabric  250 , the fiber routing is unchanged as the switch fabric  250  is upgraded. Switch fabric  250  includes input stage  260 , center stage  270 , and output stage  280 . Input stage  260  has 500 input stage switches  266 . Center stage  270  has eight center stage switches  276 . Output stage  280  has 500 output stage switches  286 . For the initial configuration of switch fabric  250 , there are 2,000 inputs  264  and  2 , 000  outputs  284 . 
   The approach shown in  FIG. 8  starts with partially populated center switches  276 . For switch fabric  250 , only one half of the 1,000 port by 1,000 port center switches  276  are loaded so there are only 8 center stage switches  270  in  FIG. 8 . Therefore, center stage switches  276  are only acting functionally as 500 port by 500 port switches. The eight port by sixteen port input stage switches  266  are only operating functionally as four inputs by eight outputs switches because only half of the center stage switches  256  are loaded as compared to switch fabric  60  shown in  FIG. 5 . Likewise, the sixteen port by eight port output stage switches  286  are only operating functionally as eight inputs by four outputs stage switches. 
   For the sake of simplicity, the test and monitoring functions are not shown for switch fabric  250 . Therefore, the two additional inputs for each input stage switch  266  for test and monitoring functions are not shown. Likewise, the two additional output ports for test and monitoring for each of the output stage switches  286  are not shown. 
   The input stage switches  266  and the output stage switches  286  are combined into Clos input/output modules. One half of the Clos input/output modules need to be installed to realize a 2,000 port switch fabric, so this approach is most useful for small initial implementations where the benefit of not needing to replace the center stage switches  276  outweighs the inefficient utilization of the Clos input/output modules that comprise the input  266  and output  286  switches. The approach shown in  FIG. 8  has the advantage that the switch fabric capability is doubled when the center stage switches  276  are fully installed without installing any additional Clos input/output modules. In other words, if eight additional 1,000 port center stage switches  276  are added to switch fabric  250 , the capacity of switch fabric  250  doubles, and switch fabric  250  becomes a 4,000 port Clos switch. Once the eight additional center stage switches are added (resulting in sixteen center stage switches  276 ), then the input stage switches  266  begin to operate as eight inputs by sixteen outputs switches and the output stage switches  286  begin to operate as sixteen inputs by eight outputs switches. This results in the number of inputs  264  becoming 4,000 and the number of outputs  286  becoming 4,000. To further upgrade the switch fabric  250  to a 8,000 port Clos redundant strict sense nonblocking switch, 500 additional eight by sixteen port input stage switches  266  can be added and 500 additional sixteen by eight port output stage switches  286  can be added. The number of center stage 1,000 port by 1,000 port switches  276  remains at sixteen. Given that input stage switches  266  and output stages switches  286  are combined into Clos input/output modules, this means that only five hundred Clos input/output modules need to be added to make the switch fabric go from 2,000 ports to 8,000. 
   For alternative embodiments, this upgrade approach could be used for smaller or larger switch fabrics. 
   For yet other alternative embodiments, this upgrade approach can operate in reverse to downgrade switch fabric capacity. In other words, center stage switches and Clos input/output modules can be removed to lower capacity of the switch fabric in a manner that is the reverse of the upgrade path. 
   Another switch fabric upgrade path is shown with respect to  FIGS. 9 through 16 . This upgrade path is referred to as a fiber backplane upgrade. For the fiber backplane upgrade shown in  FIGS. 9 through 16 , the fiber routing is changed as the switch fabric is upgraded. 
   This fiber backplane upgrade starts with switch fabric  300  shown in  FIG. 9 . Switch fabric  300  is a 1,000 port switch fabric using Clos components and Clos backplane wiring. 
   Switch fabric  300  includes input stage  310 , center stage  320 , and output stage  330 . Input stage  310  includes 125 stage switches  316 . Each input stage switch  316  is an eight port by sixteen port switch. Center stage  320  is made up of two center stage switches  325  and  326 . Switches  325  and  326  are each a 1,000 port by 1,000 port switch. Output stage  330  comprises 125 output stage switches  336 . Each of the output stages switches  336  is a sixteen port by eight port switch. There are 125 output stage switches  336 . There are 1,000 optical inputs  314  applied to switch fabric  300 . There are 1,000 optical outputs  334  from switch fabric  300 . 
   For one embodiment, the input stage switches  316  and the output stage switches  336  are combined to form 125 Clos input/output modules. 
   For the sake of simplicity, the test and monitoring functions are not shown for switch fabric  300 . If the test and monitoring functions were shown, then input stage switches  316  would have ten inputs each instead of eight, and output stage switches  336  would have ten outputs instead of eight. 
   The switch fabric  300  of  FIG. 9  is generally not considered a Clos architecture given that the Clos input/output modules formed by input and output stage switches  316  and  336  are only functioning as respective one-by-two protection switches and two-by-one protection switches. Therefore, the second center stage switch  326  is redundant. 
   Switch fabric  300  shown in  FIG. 9  depicts a fully loaded 1,000 port center switch configuration. For an alternative embodiment, however, center stage switches  325  and  326  initially could be smaller than 1,000 ports. For that alternative embodiment, the center stage switches  325  and  326  could be upgraded as Clos input/output modules (comprising input switches  316  and output  336 ) are added to switch fabric  300 , while the same fiber backplane is maintained. 
   Switch fabric  300  has an optical backplane comprising sixteen fiber bundles or fiber cables, each with 250 fibers. There are 250 fibers in each bundle because the Clos input/output modules combine the 125 input stage switches  316  and the 125 output stage switches  336 . Accordingly, one end of each fiber cable contains one output fiber and one input fiber with respect to each Clos input/output module in the switch fabric  300 . These fibers are connected to different Clos input/output module fiber connectors. For one embodiment, there could be a single 32-fiber connector for each Clos input/output module. If test and monitoring functions are included in the switch fabric then the fiber connector for each Clos input/output module would be bigger than 32 fibers because it would include additional overhead fibers. 
   At the other end of each of the sixteen fiber cables, all 250 fibers terminate in a single fiber connector that goes to 125 input ports and 125 output ports of the corresponding center stage  320  switch—i.e., either center switch  325  or center switch  326 . For the switch fiber  300  shown in  FIG. 9 , which has 1,000 port by 1,000 port center stage switches  325  and  326 , all of the sixteen fiber cables terminate in the center stage  320 . 
     FIG. 10  illustrates switch fabric  350 , which is a simplified schematic representation of the same 1,000 port switch configuration  300  shown in  FIG. 9 . Switch fabric  350  of  FIG. 10  is also referred to as switch fabric subsystem  350 , switch subsystem  350 , or subsystem  350 . For switch fabric  350 , all of the 8 port by 16 port input stage switches  316  are shown as single switchbox  366  of input stage  360 . Likewise, all of the 16 port by 8 port output stage switches  336  of  FIG. 9  are shown in  FIG. 10  as switchbox  386  of output stage  380 . In addition, the center stage switches  325  and  326  of  FIG. 9  are combined in  FIG. 10  to form center stage switchbox  376 , which comprises 1,000 port by 1,000 port switch  375  and 1,000 port by 1,000 port center stage switch  377  of center stage  370 . 
     FIG. 10  shows eight fiber cables  368  connecting input stage switches  366  with center stage switch  375 .  FIG. 10  also shows eight fiber cables  369  connecting input stage switches  366  with center stage switch  377 . Each of the eight fiber cables  368  has 125 fibers. Likewise, each of the fiber cables  369  has 125 fibers. 
   Fiber cables  378  comprise eight fiber cables connecting center stage switch  375  with output stage switches  386 . Fiber cables  379  comprise eight fiber cables connecting center stage switch  377  with output stage switches  386 . Each fiber cable of the eight fiber cables  378  has 125 fibers. Likewise, each of the eight fiber cables  379  has 125 fibers. 
   For one embodiment, the input stage switches  366  and output stage switches  386  are combined into Clos input/output modules. The fiber cables  368 ,  369 ,  378 , and  379  are thus coupled to the Clos input/output modules as well as being coupled to the center stage switches  376 . For Clos I/O modules, the fiber cables  368  and  378  would be combined and have 250 fibers, and the fiber cables  369  and  379  would be combined and have 250 fibers. 
   The switch fabric  350  thus represents a switch subsystem with 1,000 inputs  364  and  1 , 000  outputs  384 . Fiber connectors may be moved by disconnecting a fiber connector from a 1,000 port switch subsystem such as subsystem  350  and moving the fiber connector to another 1,000 port switch subsystem. The fiber backplanes of these switch subsystems may be configured at the factory to connect to all of the Clos input/output modules. For one embodiment, only the large 250 port fiber connectors are reconfigured to change the overall switch fabric size. 
     FIG. 11  shows a upgrade path for transitioning from the 1,000 port switch fabric  350  (of  FIG. 10 ) with one-to-one protection to a 2,000 port Clos switch fabric while live traffic is being carried. 
     FIG. 11  shows switch fabric subsystems  350  and  400 . Switch fabric subsystem  350  includes an input stage  360 , a center stage  370 , and an output stage  380 . For subsystem  350 , there are 1,000 inputs  364  and 1,000 outputs  384 . Switch fabric  350  includes 125 eight port by sixteen port input stage switches  366  and  125  sixteen port by eight port output stage switches  386 . For the sake of simplicity, test and monitoring functions are not shown for switch fabrics  350  and  400 . Switch fabric  350  also includes center stage switches  376 , which comprise 1,000 port by 1,000 port switch  375  and 1,000 port by 1,000 port switch  377 . 
   Fiber bundle  368  is comprised of four fiber cables  391  and four fiber cables  392 . Fiber bundle  369  is comprised of four fiber cables  393  and four fiber cables  394 . Fiber bundle  378  is comprised of four fiber cables  395  and four fiber cables  396 . Fiber bundle  379  is comprised of four fiber cables  397  and four fiber cables  398 . For one embodiment, each fiber cable of fiber cables  391  through  398  is comprised of 125 optical fibers. 
   Switch fabric subsystem  400  includes input stage  410 , center stage  420 , and output stage  430 . Subsystem  400  has 1,000 optical inputs  414  and 1,000 optical outputs  434 . Input stage  410  is comprised of 125 eight port by sixteen port optical switches  416 . Output stage  430  is comprised of 125 sixteen port by eight port optical output stage switches  436 . Center stage  420  is comprised of center stage switches  426 , which comprise 1,000 port by 1,000 port center stage switch  425  and 1,000 port by 1,000 port center stage switch  427 . 
   Initially, switch fabric subsystem  350  is configured as shown in  FIG. 10 , with eight fiber bundles going to each center switch. Thus, eight fiber bundles  368  go to center stage switch  375 , eight fiber bundles  369  go to center stage switch  377 , eight fiber bundles  378  leave center switch  375 , and eight fiber bundles  379  leave center stage switch  377 . 
   In order to switch the fiber cables over to another subsystem while live traffic is being carried, all connections first are routed to one center stage switch. Therefore, for one embodiment, all connections are first routed to center stage switch  375 . Four of the cables  394  from switch fabric subsystem  350  are routed to the second switch fabric subsystem  400  that has been added, as shown in  FIG. 11 . This moving of the cables  394  to subsystem  400  is done while fiber cables  391 ,  392 ,  395 , and  396  continue to carry live traffic. 
   As shown in  FIG. 11 , fiber cables  394  are routed from input stage switches  366  of switch fabric  350  to center stage switch  425  of switch fabric  400 . Likewise, fiber cables  398  are routed from center stage switch  425  of switch fabric  400  to output stage switches  386  of switch fabric  350 . The switch fabric  350  is not fully redundant during this fiber reconfiguration given that there is not a backup to the 1,000 port by 1,000 port center stage switch  375  while the fiber cables are being rerouted. During this rerouting, all live traffic is carried through center stage switch  375 . 
   The fiber bundles  391 ,  393 ,  394 ,  395 ,  397 , and  398  can be used to form a 1,000 port Clos switch shown in  FIG. 11  that includes subsystems  350  and  400 . The input stage switches  366  and output stage switches  386  are combined to form Clos input/output modules. In addition, the input stage switches  416  and output stage switches  436  are combined into Clos input/output modules. For Clos I/O modules, each fiber cable pair  391 / 395 ,  392 / 396 ,  393 / 397 , and  394 / 398  would be combined to form a fiber cable with 250 fibers. 
   After the rerouting of fiber cables  394  and  398  to center switch  425 , the Clos input/output module switches of  FIG. 11  can be partitioned into two port by four port input stage Clos switches and four port by two port output stage Clos switches instead of one port by two port protection switches and two port by one port protection switches. Consequently, all live traffic can then be shifted off of the fiber bundles  392  and  396  shown in  FIG. 11 . Live traffic would then be carried by fiber bundles  391 ,  393 ,  394 ,  395 ,  397 , and  398 . That transition of live traffic represents a transition from an unprotected 1,000 port switch fabric to an unprotected Clos 1,000 port switch fabric made up of subsystems  350  and  400 . 
   Redundancy of the center stage switches  376  and  426  can be added by moving the extra sets of fiber bundles  392  and  396  to the 1,000 port by 1,000 port center stage switch  427  as shown in  FIG. 12 . Fiber cables  392  are coupled between input stage switches  366  and center stage switch  427 . Fiber cables  396  are coupled between center stage switch  427  and output stage switches  386 . 
   The upgrade to a redundant Clos 2,000 port switch fabric  480  by the connection of fiber cables  461 ,  462 ,  463 ,  464 ,  471 ,  472 ,  473 , and  474  is shown in  FIG. 13 . Each of the fiber cables contain 125 fibers. For Clos I/O modules, each fiber cable pair  461 / 463 ,  462 / 464 ,  471 / 473 , and  472 / 474  would be combined to form a fiber cable with 250 fibers. Switch fabric  480  has 2,000 optical inputs  444  and 2,000 optical outputs  454 . Four fiber cables  461  are coupled between input stage switches  416  and center stage switch  375 . Four fiber cables  463  are coupled between center stage switch  375  and output stage switches  436 . Four fiber cables  462  are coupled between input stage switches  416  and center stage switch  377 . Four fiber cables  464  are coupled between center stage switch  377  and output stage switches  436 . Four fiber cables  471  are coupled between input stage switches  416  and center stage switch  425 . Four fiber cables  473  are coupled between center stage switch  425  and output stage switches  436 . Four fiber cables  472  are coupled between input stage switches  416  and center stage switch  427 . Four fiber cables  474  are coupled between center stage switch  427  and output stage switches  436 . Switch traffic is routed through the fiber cables  391 - 398 ,  461 - 464 , and  471 - 474 . 
   The configuration of switch fabric subsystems into a 4,000 port optical switch fabric  500  is shown in  FIG. 14 . The upgrade method starts with 1,000 port optical switch fabric  350  that has been discussed above in connection with  FIGS. 10 through 13 . 
   In  FIG. 14 , additional switch fabric subsystems  520 ,  521 , and  522  (i.e., respective subsystems numbers  2 ,  3 , and  4 ) are added to subsystem  350  (subsystem number  1 ) to form switch fabric  500 . Switch fabric  500  has 4,000 optical inputs  504  and  4 , 000  optical outputs  514 . Each of the switch fabrics subsystems  350 ,  520 ,  521 , and  522  has 1,000 optical inputs and 1,000 optical outputs. For the sake of simplicity, subsystems  521 - 522  are shown in block diagram form as residing within block  501 . 
     FIG. 15  shows the configuration of switch fabric subsystems into an 8,000 port switch fabric  600 . The starting point is switch fabric  350 , discussed above in connection with  FIGS. 10-14 . Switch fabric subsystems  620  through  626  (i.e., respective subsystems numbers  2  through  8 ) are added to switch fabric subsystem  350  (subsystem number one) to form switch fabric  600 , which has 8,000 optical inputs  604  and 8,000 optical outputs  614 . 
   Switch fabric subsystems  620  through  626  are shown in block diagram form as part of block  601 . 
   For the sake of simplicity, ports for test and monitoring functions are not shown in  FIGS. 14 and 15  with respect to switch fabrics  500  and  600 . 
   The methods for upgrading to the higher capacity switch fabrics  500  and  600  of  FIGS. 14 and 15  by reconfiguring the fiber backplane are analogous to the method for upgrading from a 1,000 port optical switch to a 2,000 port optical switch shown in  FIGS. 10-13 . For switch fabrics  500  and  600 , one of the center stage switches is redundant. The fiber cables to this redundant switch can be disconnected and rerouted to additional subsystems. 
   Traffic can be switched from one of the active center stage switches into these additional subsystems, allowing fiber cables going to the previously active switch to be rerouted into additional switch fabric subsystems. This method is repeated until the active switch fabric subsystems have been completed reconfigured. Afterward, new fiber cables are added to the new switch fabric subsystems. 
   The fully configured 4,000 port switch fabric  500  of  FIG. 14  has four switch fabric subsystems  350 ,  520 ,  521 , and  522 . The 8,000 port fully configured switch fabric  600  of  FIG. 15  has eight switch fabric subsystems  350 ,  620 - 626 . Each of the subsystems has 1,000 optical input ports and 1,000 optical output ports. 
   As shown in  FIG. 14 , for switch fabric  500 , two fiber cables  551  are coupled between input stage switches  366  and center stage switch  375 . Two fiber cables  561  are coupled between center stage switch  375  and output stage switches  386 . Two fiber cables  553  are coupled between input stage switches  366  and center stage switch  377 . Two fiber cables  563  are coupled between center stage switch  377  and output stage switches  386 . 
   Twelve fiber cables  555  are coupled between input stage switches  366  of switch subsystem  350  and the center stage switches of switch fabric subsystems  520  through  522 . 
   Six fiber cables  552  are coupled between the input stage switches of subsystems  520 - 522  and the center stage switch  375  of subsystem  350 . Six fiber cables  554  are coupled between the input stage switches of switch fabric subsystems  520 - 522  and center stage switch  377 . 
   Six fiber cables  562  are coupled between center stage switch  375  and the output stage switches of switch fabric subsystems  520 - 522 . Six fiber cables  564  are coupled between center stage switch  377  and the output stage switches of subsystems of  520 - 522 . 
   Twelve fiber cables  565  are coupled between the center stage switches of subsystems  520 - 522  and the output stage switches  386 . 
   Each of the fiber cables  551 - 555  and  561 - 565  contains 125 fibers. For an embodiment with Clos I/O modules, the combined fiber pairs  551 / 561 ,  552 / 562 ,  553 / 563 ,  554 / 564 , and  555 / 565  have 250 fibers. 
   For the switch fabric  600  of  FIG. 15 , fiber cable  701  is coupled between input stage switches  366  and center stage switch  375 . Fiber cable  703  is coupled between input stage switches  366  and center stage switch  377 . Fiber cable  711  is coupled between center stage switch  375  and output stage switches  386 . Fiber cable  713  is coupled between center stage switch  377  and output stage switches  386 . 
   Fourteen fiber cables  704  are coupled between input stage switches  366  of switch fabric  350  and the center stage switches of subsystem  620  and the center stage switches of switch fabric subsystems  620  through  626 . Seven fiber cables  702  are coupled between the input stage switches of subsystems  620 - 626  and center stage switch  375 . Seven fiber cables  705  are coupled between the input stage switches of subsystems  620  through  626  and center stage switch  377 . 
   Seven fiber cables  712  are coupled between center stage switch  375  and the output stage switches of subsystems  620  through  626 . Seven fiber cables  715  are coupled between center stage switch  377  and the output stage switches of subsystems  620  through  626 . 
   Fourteen fiber cables  714  are coupled between the center stage switches of subsystems  620  through  626  and output stage switches  386 . 
   Each of the fiber cables  701 - 705  and  711 - 715  contains 125 fibers. For an embodiment with Clos I/O modules, each combined fiber cable pair  701 / 711 ,  702 / 712 ,  703 / 713 ,  704 / 714 , and  705 / 715  has 250 fibers. 
   The 8,000 port Clos switch fabric  600  of  FIG. 15  can be upgraded to the 16,000 port switch fabric  800  of  FIG. 16 . Switch fabric  800  has 16,000 optical input ports  804  and 16,000 optical output ports  814 . Switch fabric  800  has sixteen switch fabric subsystems. The starting point is switch fabric subsystem  810 , which is subsystem number one. Subsystem  810  has an input stage  970 , a center stage  980 , and an output stage  990 . Switch fabric subsystem  810  includes 125 eight port by sixteen port input stage switches  976  and  125  sixteen port by eight port output stage switches  996 . An additional fifteen switch fabric subsystems  820  through  835  (i.e., subsystems numbers  2 - 16 ) are coupled to subsystem  810 . The switch fabric subsystems  820  through  835  are shown in block diagram form as part of block  801 . 
   For the sake of simplicity, the ports for test and monitoring functions are not shown as part of switch fabric  800  in  FIG. 16 . 
   The 8,000 port switch fabric  600  shown in  FIG. 15  is upgraded to 16,000 port switch fabric  800  by replacing each pair of 1,000 port center stage switches of switch fabric  600  with a single 2,000 port center stage switch. Thus, subsystem  810  of  FIG. 16  includes a 2,000 port by 2,000 port center stage switch  986 . Each of the additional subsystems  820  through  835  also contains a 2,000 port by 2,000 port center stage switch. Each of the 2,000 port by 2,000 port center stage switches (including center stage switch  986 ) has a total of sixteen fiber input connectors and sixteen fiber output connectors, each of the fiber connectors having 250 fibers. 
   For fully configured switch fabric  800 , fiber cable  802  couples input stage switches  976  to center stage switch  986 . Fiber cable  808  couples center stage switch  986  with output stage switches  996 . 
   Fifteen fiber cables  805  couple input stage switches  976  to the center stage switches of switch fabric subsystems of  820  through  835 . Fifteen fiber cables  806  couple the center stage switches of switch fabric subsystems  820  through  835  to center stage switch  986 . 
   Fifteen fiber cables  812  couple center stage switch  986  to the center stage switches of switch fabric subsystems  820  through  835 . The fifteen fiber cables  815  couple the center stage switches of switch fabric subsystems  820  through  835  to the output stage switches  996 . 
   Fiber cables  802  and  808  have 125 fibers. Each of fiber cables  805 ,  806 ,  812 , and  815  consist of 15 cables each with 125 fibers. Each of the fiber cables  805 ,  806 ,  812 , and  815  consist of 15 cables each with 125 fibers. For an embodiment with Clos input/output modules, input cable  802  and output cable  808  can be combined into one cable with 250 fibers. Similarly, input and output cables  805  and  815  or input and output cables  806  and  812  can be combined into 15 groups each with 250 fibers. 
   The upgrade methods described above in connection with  FIGS. 9-15  do not interrupt working optical signals. For alternative embodiments, the switch capacity upgrades could still be employed even if working optical signals were interrupted or switched off. 
   For alternative embodiments, the fiber backplane upgrade approaches of  FIGS. 9-16  could be used for smaller or larger switch fabrics. 
   For yet other alternative embodiments, the fiber backplane approaches of  FIGS. 9-16  could operate in reverse in order to downgrade switch fabric capacity. For example, with respect to  FIGS. 9-13 , a fiber backplane downgrade would entail rerouting traffic to fiber cables  391 - 397 , removing fiber cables  461 - 464  and  471 - 474 , rerouting traffic away from fiber cables  392  and  396 , moving fiber cables  392  and  396  from center stage switch  427  to center stage switch  375 , rerouting traffic away from fiber cables  394  and  398 , moving fiber cables  394  and  398  from center stage switch  425  to center stage switch  377 , and rerouting traffic through fiber cables  391 - 398 . The switch fabric would thereby be downgraded from a redundant Clos 2,000 port switch fabric with two switch subsystems to a 1,000 port switch fabric with one switch subsystem. Similar approaches could be used to downgrade the 4,000 port, 8,000 port, and 16,000 port switch fabrics shown in respective  FIGS. 14-16 . 
   Although embodiments of the invention have been described that specify, for example, the number of optical inputs, optical outputs, the number of fiber cables and fibers, the number of switch stages, and the number of switch subsystems, it is to be appreciated that other embodiments are contemplated that include different numbers of inputs, outputs, fiber cables, fibers, subsystems, and stages, etc. Although for some embodiments, ports for testing and monitoring functions were not shown for the sake of simplicity, it is to be appreciated that for various embodiments the ports for the testing and monitoring functions can be included and can have various numbers of lasers, detectors, and fiber inputs and outputs. Furthermore, although particular upgrade methods have been described with respect to specific number of ports, input switches, output switches, center stage switches, and subsystems, other upgrade methods are contemplated that involve different numbers of input switches, center stage switches, output stage switches, input ports, output ports, and subsystems. Although particular Clos input/output modules have been discussed, Clos input/output modules of different sizes with different numbers of inputs and outputs, different numbers of mirror arrays, and different number of fibers, detectors, taps, and lasers are contemplated. 
   In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.