Abstract:
An optical communications system employs a plurality of optical nodes interconnected in a ring configuration by at least two optical transmission media, for example, optical fiber. The at least two optical transmission media, in this example, provide optical service transmission capacity and optical protection transmission capacity. Efficient restoration of optical communications between optical nodes in the ring, after an optical transmission media failure, is realized by employing a relatively simple and efficient optical switch matrix having a first number of possible switching states and, then, by utilizing only a second number of the switching states fewer than the first number to switch optically from the optical service transmission capacity of the failed or faulted optical transmission media to the optical protection transmission capacity of another optical transmission media. Optical switching states of the optical switch matrix are blocked that are not actively used for switching from the active optical service capacity of the faulted optical transmission media to the standby optical protection capacity of the other optical transmission media. Use of this relatively simple optical switch matrix allows for the bulk switching of the optical wavelengths as contrasted with the one-to-one switching of the optical wavelengths used in prior arrangements. In a preferred embodiment of the invention, each of the at least two optical transmission media provides both bi-directional service transmission capacity and bi-directional protection transmission capacity. In a specific embodiment of the invention, each optical transmission channel (wavelength) includes 50 percent bi-directional optical service transmission capacity and 50 percent bi-directional optical protection transmission capacity. In another embodiment of the invention, one of the at least two optical transmission media provides active optical service transmission capacity and another of the optical transmission media provides standby optical protection transmission capacity. In still another embodiment of the invention, at least four optical transmission media (optical fiber) are utilized to provide transmit and receive active optical service transmission capacity and transmit and receive standby optical protection transmission capacity. Specifically, one pair of the optical transmission media is used to provide bi-directional transmit and receive active optical service transmission capacity and another pair of the optical transmission media is used to provide the bi-directional transmit and receive standby optical protection transmission capacity.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to restoration of transmission systems and, more particularly, to restoration of optical transmission systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical transmission systems and, especially, those employing Dense Wavelength Division Multiplexing (DWDM) are desirable because they provide extremely wide bandwidths for communications channels. Each communications channel in the DWDM transmission system carries a plurality, for example, 16, 40 or even 80, optical channels (wavelengths) on a single optical fiber and single optical repeater. However, there is a trade off between providing wider bandwidth communications channels, with their corresponding lower cost of transport, and their vulnerability to large-scale disruption of communications services because of transmission medium failure. Therefore, the ability of an optical transmission system, for example, those employing DWDM, to restore itself after a transmission medium failure is very important because of its wider impact on communications services. The DWDM optical transmission systems are of particular interest because of their restoration capabilities.  
           [0003]    Prior attempts at providing adequate restoration in optical transmission systems have focused on so-called 1+1 optical protection switching and on optical cross connect systems. The 1+1 optical protection switching is limited in its application and does not efficiently use optical fiber. Known optical cross connect systems, require the use of a relatively large optical switching fabric to accommodate the capacity of the optical transmission system. Unfortunately, current technology may not support providing such a large switching fabric having an acceptable optical performance level. Moreover, use of such a large switching fabric in the optical cross connect comes with a relatively high cost. Furthermore, the optical cross connect system will be slower in terms of restoration speed than provided by prior known SONET/SDH ring transmission systems. In order to protect all wavelengths used in the optical transmission system the prior arrangements had to switch one wavelength at a time. Such switching is very inefficient.  
         SUMMARY OF THE INVENTION  
         [0004]    These problems and other limitations of prior known optical restoration systems are overcome in an optical communications system that employs a plurality of optical nodes interconnected in an optical ring transmission configuration by at least two optical transmission media, for example, optical fiber. The at least two optical transmission media, in this example, provide optical service transmission capacity and optical protection transmission capacity. Efficient restoration of optical communications between optical nodes in the ring, after an optical transmission media failure, is realized by employing a relatively simple and efficient optical switch matrix having a first number of possible switching states and, then, by utilizing only a second number of the switching states fewer than the first number to switch optically from the optical service transmission capacity of the failed or faulted optical transmission media to the optical protection transmission capacity of another optical transmission media. Optical switching states of the optical switch matrix are blocked that are not actively used for switching from the active optical service capacity of the faulted optical transmission media to the standby optical protection capacity of the other optical transmission media. Use of this relatively simple optical switch matrix allows for the bulk switching of the optical wavelengths as contrasted with the one-to-one switching of the optical wavelengths used in prior arrangements.  
           [0005]    In a preferred embodiment of the invention, each of the at least two optical transmission media provides both bi-directional optical service transmission capacity and bi-directional optical protection transmission capacity. In a specific embodiment of the invention, each optical transmission channel (wavelength) includes 50 percent bi-directional optical service transmission capacity and 50 percent bi-directional optical protection transmission capacity.  
           [0006]    In another embodiment of the invention, one of the at least two optical transmission media provides active optical service transmission capacity and another of the optical transmission media provides standby optical protection transmission capacity.  
           [0007]    In still another embodiment of the invention, at least four optical transmission media (optical fiber) are utilized to provide transmit and receive active optical service transmission capacity and transmit and receive standby optical protection transmission capacity. Specifically, one pair of the optical transmission media is used to provide bi-directional transmit and receive active optical service transmission capacity and another pair of the optical transmission media is used to provide the bi-directional transmit and receive standby optical protection transmission capacity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    [0008]FIG. 1 illustrates, in simplified block form, details of an optical ring transmission system;  
         [0009]    [0009]FIG. 2 illustrates, in simplified block diagram form, details of an optical node used in the system of FIG. 1 and including an embodiment of the invention;  
         [0010]    [0010]FIG. 3 illustrates, in simplified block diagram form, details of another version of an optical node that can be used in the system of FIG. 1 and including an embodiment of the invention;  
         [0011]    [0011]FIG. 4 illustrates, in simplified block diagram form, an optical node especially suited for a four optical fiber system and including an embodiment of the invention;  
         [0012]    [0012]FIG. 5 illustrates, in simplified block diagram form, details of applicants&#39; unique optical switch matrix employed in an embodiment of their invention;  
         [0013]    [0013]FIG. 6 is a state diagram showing the allowable optical switching states of the optical switching matrix of FIG. 5 for both terminal optical nodes and intermediate pass through optical nodes;  
         [0014]    [0014]FIG. 7 is a flow chart showing the operation of an optical node in response to a detected optical transmission media failure;  
         [0015]    [0015]FIG. 8 illustrates, in simplified block diagram form, details of an optical node effecting an optical protection switch in response to a transmission media failure on the east side of the optical node;  
         [0016]    [0016]FIG. 9 is a state diagram showing the optical switch states for effecting the optical protection switch in the optical node of FIG. 8;  
         [0017]    [0017]FIG. 10 illustrates, in simplified block diagram form, details of an optical node effecting an optical protection switch in response to a transmission media failure on the west side of the optical node;  
         [0018]    [0018]FIG. 11 is a state diagram showing the optical switch states for effecting the optical protection switch in the optical node of FIG. 10;  
         [0019]    [0019]FIG. 12 illustrates, in simplified block diagram form, details of an optical node effecting a pass through optical protection switch in response to a transmission media failure;  
         [0020]    [0020]FIG. 13 is a state diagram showing the optical switch states for effecting the pass through optical protection switch in the optical node of FIG. 12;  
         [0021]    [0021]FIG. 14 shows, in simplified block form, a plurality of optical nodes connected in a ring configuration and the optical switch matrix connections in each of the optical nodes for normal operation;  
         [0022]    [0022]FIG. 15 shows, in simplified block form, a plurality of optical nodes connected in a ring configuration and the optical switch matrix connections in each of the optical nodes for effecting an optical protection switch in response to a transmission media failure; and  
         [0023]    [0023]FIG. 16 shows, in simplified block diagram form, details of optical monitor  206  of FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0024]    [0024]FIG. 1 shows, in simplified form, bi-directional optical transmission system  100 , which connected in a ring configuration. For brevity and clarity of exposition optical transmission system  100  is shown as including only optical nodes  101  through  104 , each incorporating an embodiment of the invention. Optical nodes  101  through  104  are interconnected by bi-directional optical transmission media  110 , which for brevity and clarity of exposition, in this example, transport active service transmission capacity and by bi-directional optical transmission media  120 , which also for brevity and clarity of exposition, in this example transports standby protection transmission capacity. In this example, optical transmission medium  110  and  120  are comprised of optical fibers and each could be comprised of a single optical fiber or two (2) optical fibers. That is, bi-directional optical transmission system  100  could be either a two (2) optical fiber or a four (4) optical fiber system. In a preferred embodiment of the invention, two (2) optical fibers are employed, each of the optical fibers ideally including 50% service bandwidth and 50% protection bandwidth. In an alternative embodiment of the invention, one of the two (2) optical fibers can carry active service transmission capacity and the other optical fiber can carry standby protection transmission capacity. In a four (4) optical fiber system, separate optical fibers are employed to transport active service transmission capacity in both directions of transmission, and separate optical fiber are employed to transport standby protection transmission capacity in both directions of transmission. The optical transmission system  100  could transport, for example, 8, 16, 32, 40, 80, etc. communications channels, i.e., wavelengths. It should be noted that in either the two (2) optical fiber arrangement or the four (4) optical fiber arrangement a separate so-called telemetry channel is employed as a maintenance channel, in addition to the communications channels. Thus, in an eight (8) channel system, nine (9) channels are transported, in a 16 channel system, 17 channels are transported and so on. The maintenance channel transports, among other things, the switching information for configuring optical nodes  101  through  104  in optical transmission system  100 . Use of the maintenance channel in transporting protection switching information in order to restore transmission in optical transmission system  100  in response to a transmission media failure or the like is described below. Two (2) and four (4) optical fiber transmission systems are known.  
         [0025]    [0025]FIG. 2 shows, in simplified block diagram form, details of optical nodes  101 - 104 , including an embodiment of the invention and operating in a normal transport mode. That is, there is no optical transmission media failure or other disruption of transmission service. Again, for brevity and clarity of exposition, the bi-directional active service transmission capacity is shown as being transported on optical transmission media  110  and the standby protection transmission capacity is shown as being transported on optical transmission media  120 . As indicated above, in a preferred embodiment of the invention, each of at least two optical fibers transports both active service transmission capacity and standby protection transmission capacity. Shown is optical receive service capacity from the west (RSCW) being supplied to input H of applicants&#39; unique optical switch matrix  201 , optical transmit service capacity to the west (TSCW) being supplied from output N of optical switch matrix  201 , optical transmit protection capacity to the west (TPCW) being supplied from output M of optical switch matrix  201 , and optical receive protection capacity from the west (RPCW) being supplied to input G of optical switch matrix  201 . Similarly, on the east side of optical node  101 , optical receive service capacity (RSCE), is supplied to input E, optical transmit service capacity (TSCE) is supplied from output K, optical receive protection capacity (RPCE) is supplied to input F and optical transmit protection capacity (TPCE) is supplied from output L, all to/from optical switch matrix  201 . RSCW supplied to input H is supplied via optical switch matrix  201  to output I and, thereafter, to optical add/drop multiplexer  210 . Similarly, RSCE supplied to input E is supplied via optical switch matrix  201  to output J and, thereafter, to optical add/drop multiplexer  211 . TSCW from optical add/drop multiplexer  211  is supplied to optical splitter S 1 , which forms two versions of it. One version of TSCW is supplied to input A and, thereafter, to output N of optical switch matrix  201 , while the other version of TSCW is supplied to input B for use if a protection switch is required. Similarly, TSCE from optical add/drop multiplexer  210  is supplied to optical splitter S 2 , which forms two versions of it. One version of TSCE is supplied to input D and, thereafter, to output K of optical switch matrix  201 , while the other version of TSCW is supplied to input C for use if a protection switch is required. Control signals (SC) for controlling operation of optical switch matrix  201  are supplied from sub controller  207 . Details of optical switch matrix  201  and its operation are described below.  
         [0026]    A relatively small portion of optical energy (for example, less than 2%) being transported via each of RSCW, RPCE, RPCW and RPCE is coupled via optical taps  202 ,  203 ,  204  and  205 , respectively, to optical monitor  206 . Optical monitor  206  determines whether a loss of signal (LOS) has occurred on any of the optical transports supplying optical signals to optical node  101  and, therein, optical switch matrix  201 . Details of optical monitor  206  are described below in relationship to FIG. 16. Any LOS information is supplied from optical monitor  206  to sub controller  207 , which supplies switch control (SC) signals to optical switch matrix  201  for effected any required protection switch, and to main controller  208 . In turn, main controller  208  supplies switch information, among others, to maintenance channel unit  209 . Maintenance channel unit  209  supplies switch information via the maintenance channel to optical combining units  213  and  214  where it is combined with other optical channels (if any) to be added via add/drop multiplexers  210  and  211  to TSCW and TSCE to transported to others of optical nodes  102  through  104  for use in effecting appropriate protection switches at those optical nodes. Note if a protection switch is made, then, the maintenance channel is transported as appropriate via TPCW and/or TPCE. Incoming maintenance channel information is supplied from RSCW and RSCE and if a protection switch has been made from RPCW and/or RPCE as appropriate, where it is dropped via add/drop multiplexers  210  and  211  to optical splitters  212  and  215 , respectively. Optical maintenance channel information is supplied from optical splitters  212  and  215  to maintenance channel unit  209  and, thereafter, to main controller  208 . Then any protection switch information being transported on the maintenance channel is supplied to sub controller  207  where it is determined whether a protection switch is required. If a protection switch is required appropriate switch control (SC) signals are supplied from sub controller  207  to optical switch matrix  201 . Optical communications channels dropped by add/drop multiplexers  210  and  211  are also supplied to optical splitters  212  and  215 , respectively. Optical communications channel information from optical splitters  212  and  215  is supplied to optical terminal equipment  216  as desired. Terminal equipment  216  may include, for example, a synchronous optical network/synchronous digital hierarchy (SONET/SDH) terminal, or an asynchronous transfer mode (ATM) switch, or an internet protocol (IP) router, or the like. Additionally, optical communications channel information from optical splitter  215  is supplied to peizosynchronous digital hierarchy (PDH) terminal  217 . Communications channel information from terminal equipment  216  is supplied to be added for transport to optical combining units  213  and  214  and, thereafter, to add/drop multiplexers  210  and  211 . Optical communications channel information from peizosynchronous digital hierarchy (PDH) terminal  217  is supplied to optical combining unit  213  and, thereafter, to add/drop multiplexer  210  to be added to TSCE and/or TPSE. Note that the peizosynchronous digital hierarchy (PDH) communications information from terminal  217  is span related and, therefore, is only supplied, in this example, to TSCE and/or TPCE.  
         [0027]    [0027]FIG. 3 illustrates, in simplified block diagram form, details of another version of an optical node that can be used in optical nodes  1 - 1  through  104  in the system of FIG. 1 and including an embodiment of the invention. All elements of the version of optical node  101  shown in FIG. 3 that are identical to those, described above, regarding the version of optical node  101  shown in FIG. 2 have been similarly numbered and will not be described again. The differences between the versions of optical node  101  shown in FIG. 2 and FIG. 3 are that in FIG. 3 optical demultiplexer (DMUX)  301  and optical multiplexer (MUX)  302  replace add/drop multiplexer  210 , optical splitter  212  and optical combining unit  213 , and optical demultiplexer (DMUX)  303  and optical multiplexer (MUX)  304  replace add/drop multiplexer  211 , optical splitter  214  and optical combining unit  215 . Otherwise the elements and operation of the versions of optical node  101  shown in FIGS. 2 and 3 are identical. It will be apparent to those skilled in the art how DMUX  301  and MUX  302 , and DMUX  303  and MUX  304  are a direct substitute for add/drop multiplexer  210 , optical splitter  212  and optical combining unit  213 , and for add/drop multiplexer  211 , optical splitter  214  and optical combining unit  215 , respectively.  
         [0028]    [0028]FIG. 4 illustrates, in simplified block diagram form, an optical node especially suited for a four optical fiber system and including an embodiment of the invention. All elements of the version of optical node  101  shown in FIG. 4 that are identical to those, described above, regarding the version of optical node  101  shown in FIG. 2 have been similarly numbered and will not be described again. The differences between the versions of optical node  101  shown in FIG. 2 and FIG. 4 are that in FIG. 4 four (4) optical fibers are employed to transport the incoming and outgoing optical signals and, thereby, providing the active transmit and receive service capacity, and the standby transmit and receive protection capacity. Thus, separate optical fibers are employed to transport each of RSCW, TSCW, TPCW and RPCW. Additionally, terminal equipment  401  includes a four (4) optical fiber ring, or a SONET/DSH terminal or an ATM switch or an IP router. Circuit paths are provided from optical splitters  212  and  215  to equipment  401 , and from equipment  401  to optical combining units  213  and  214  to accommodate the four optical fiber ring, as will be apparent to those skilled in the art.  
         [0029]    [0029]FIG. 5 illustrates, in simplified block diagram form, details of applicants&#39; unique optical switch matrix, e.g., optical switch matrix  201 , employed in an embodiment of their invention. Note that in this example inputs A through H and outputs I through N of optical switch matrix  201  are optical. Shown in FIG. 5 are optical splitter S 1  dual feeding optical communications channels normally intended for west bound transmission to inputs A and B of optical switch matrix  201 , and optical splitter S 2  dual feeding optical communications channels normally intended for east bound transmission to inputs C and D of optical switch matrix  201 . In this example, it is noted that input A is connected directly to output N in the west bound direction, and that input D is connected directly to output K in the east bound direction. Of course, these “direct” connections can be made in any of a number of ways, for example, they can be made by assigning optical switch units to effect the desired connections at system setup, or dynamically in response to control signals These “direct” connections significantly simplify optical switch matrix  201  and make it significantly more efficient. Optical switch matrix  201  is further comprised of controllable optical switches  501  through  505 . Optical switches  501  through  505  are controlled via switch control (SC) signals from sub controller  207  (FIG. 2) to effect the bulk switching of optical signals including communications channels being supplied to them. This bulk optical switching is an important feature of applicants&#39; unique optical switch matrix because it more efficiently effects switching of the optical signals. Again, note that switching with prior known arrangements was on an optical channel-by-optical channel basis, which is significantly less efficient than applicants&#39; use of bulk switching. It should be further noted that although optical switch matrix  201  has eight (8) inputs and six (6) outputs, only 10 switching states are allowed of which two (2) switching states are designated by preassigning the optical input and optical output connections. This preassignment of two of the optical switch states may be realized in a number of ways, for example, permanent optical connections, optical switches always switched to those optical switch states, optical switches dynamically switched to the desired states, or the like. Thus, there are effectively only eight (8) allowable switching states of optical switch matrix  201 , which significantly reduces the complexity of the switching of the optical signals and allows the use of relatively simple switching elements that are readily available. Indeed, no large complex switching matrix is required as would be in an optical channel-by-optical channel switching arrangement or in an optical cross connect switch. Further note that two (2) of the allowable optical switching states of optical switch matrix  201  are employed only in pass through optical nodes.  
         [0030]    [0030]FIG. 6 is a state diagram showing the allowable switching states of the optical switch matrix  201  of FIG. 5 for both terminal optical nodes and intermediate pass through optical nodes. Note that the allowable switching states are indicated by a “dot” in the middle of a square representative of an allowable switching state, and a switched state or preassigned, e.g. a permanently connected, switched state is indicated by a “X” in the square representative of the switched state or preassigned state. Thus, as seen in FIG. 6, input A is preassigned, i.e., permanently connected, to output N, input B can be controllably connected to output L, input C can be controllably connected to output M, input D is preassigned, i.e., permanently connected, to output K, input E can be controllably connected to output J, input F can be controllably connected to output I or output M, input G can be controllably connected to output J or output L and input H can be controllably connected to output I, all of optical switch matrix  201  shown in FIG. 5.  
         [0031]    Returning to FIG. 5, the above controllable switching states of simplified optical switch matrix  201 , are realized by employing controllable optical switching units  501  through  505 . To this end, inputs H and F are supplied to individual inputs of optical switching unit  501 . Inputs E and G are supplied to individual inputs of optical switching unit  502 . Inputs B and C are supplied to individual inputs of optical switching unit  503 . One output of optical switching unit  501  is supplied to optical output I. Consequently, an optical signal supplied via either input H or input F can be controllably supplied to output I via optical switching unit  501 , in response to control signals SC. One output of optical switching unit  502  is supplied to optical output J. Consequently, an optical signal supplied via either input E or input G can be controllably supplied to output J via optical switching unit  502 , in response to control signals SC. Another output from optical switching unit  501  is supplied to one input of optical switching unit  505 , and an output from optical switching unit  503  is supplied to another input of optical switching unit  505 . Consequently, an optical signal supplied via either input C or input F can be controllably supplied to output M via optical switching units  505 ,  501  and  503 , in response to control signals SC. Another output from optical switching unit  502  is supplied to one input of optical switching unit  504 , and another output from optical switching unit  503  is supplied to another input of optical switching unit  504 . Consequently, an optical signal supplied via either input B or input G can be controllably supplied to output L via optical switching units  504 ,  502  and  503 , in response to control signals SC.  
         [0032]    [0032]FIG. 7 is a flow chart showing the operation of main controller  208  of an optical node, in response to a detected optical transmission media failure. The process is started in step  701  in response to a failure indication from sub controller  207 . Step  702  indicates that optical monitor  206  has indicated a failure. Then, step  703  starts a so-called millisecond (msec) counter clock. Step  704  tests to determine if a predetermined threshold time interval in step  703 . If the test result in step  704  is NO, step  705  resets the counter of step  703 , and control is returned to step  703 . Thereafter, steps  703 ,  704  and  705  are iterated unit step  704  yields a YES result and control is transferred to step  706 . Step  706  causes instructions to be sent via the maintenance channel to other optical nodes in the optical ring communications system including appropriate optical switch states. Then, step  707  starts a millisecond (msec) counter clock. Step  708  tests to determine if confirmation is received via the maintenance channel that the switching of the switch states sent in step  706  have been completed within a predetermined time out interval, T, as indicated by the counter in step  707 . If the test result in step  708  is NO, step  709  stops the switching process because the attempt at restoration has failed. If the test result in step  708  is YES, step  710  indicates that the optical transmission system restoration has been completed.  
         [0033]    [0033]FIG. 8 illustrates, in simplified block diagram form, details of an optical node, e.g.,  101 , effecting an optical protection switch in response to a transmission media failure on the east side of the optical node. Upon optical detector  206  detecting the failure, sub controller  207  sends optical switch control signals SC to optical switch matrix  201  and to main controller  208 . Optical switch matrix  201  effects the optical switch indicated in dashed outline. That is, input C is controllably connected to output M and input G is controllably connected to output J. Note that input H remains connected to output I, and the preassigned, i.e., permanent, optical connections of input A to output N and input D to output K remain intact. FIG. 9 is a state diagram showing the optical switch states, indicated by “X”, for effecting the optical protection switch in the optical node of FIG. 8. Main controller  208  transmits instructions, via the maintenance channel, including appropriate optical switch states to the other optical nodes in the optical ring transmission system.  
         [0034]    [0034]FIG. 10 illustrates, in simplified block diagram form, details of an optical node, e.g.,  101 , effecting an optical protection switch in response to a transmission media failure on the west side of the optical node. Upon optical detector  206  detecting the failure, sub controller  207  sends optical switch control signals SC to optical switch matrix  201  and to main controller  208 . Optical switch matrix  201  effects the optical switch indicated in dashed outline. That is, input B is controllably connected to output L and input F is controllably connected to output I. Note that input H remains connected to output I and the preassigned, i.e., permanent, optical connections of input A to output N and input D to output K remain intact. FIG. 11 is a state diagram showing the optical switch states, indicated by “X”, for effecting the optical protection switch in the optical node of FIG. 8. Main controller  208  transmits instructions, via the maintenance channel, including appropriate optical switch states to the other optical nodes in the optical ring transmission system.  
         [0035]    [0035]FIG. 12 illustrates, in simplified block diagram form, details of an optical node, e.g.,  102 , effecting a pass through optical protection switch in response to a transmission media failure and instructions received via the maintenance channel. As shown in dashed outline, input F is connected to output M and input G is connected to output L. FIG. 13 is a state diagram showing the optical switch states, indicated by “X”, for effecting the optical protection switch in the optical node of FIG. 12. Main controller  208  transmits instructions, via the maintenance channel, including appropriate optical switch states to the other optical nodes in the optical ring transmission system.  
         [0036]    [0036]FIG. 14 shows, in simplified block form, a plurality of optical nodes, namely,  1401  through  1404 , connected in a ring configuration and the optical switch matrix connections in each of the optical nodes for normal operation. The optical connects are the same as those shown in optical switch matrix  201  of FIG. 2 and are not explained again here.  
         [0037]    [0037]FIG. 15 shows, in simplified block form, a plurality of optical nodes, namely,  1501  through  1504 , connected in a ring configuration and the optical switch matrix connections in each of the optical nodes for effecting an optical protection switch in response to a transmission media failure. As shown, the optical transmission media failure is to the east of optical node  1501  and to the west of optical node  1504 . Thus optical node  1501  responds to a detected east side optical media failure, and optical node  1504  responds to a detected west side optical failure. The optical switch connections effected in optical node  1501  are identical to those shown in optical switch matrix  201  of FIG. 8 in response to an east side optical media failure, as described above in relationship to FIG. 8. The optical switch connections effected in optical node  1504  are identical to those shown in optical switch matrix  201  of FIG. 10, as described above in relationship to FIG. 10.  
         [0038]    [0038]FIG. 16 shows, in simplified block diagram form, details of optical monitor  206  of FIG. 2. In this example, optical monitor  206  is comprised of four (4) LOS detector units, namely,  1600 - 1  through  1600 - 4 . Incoming optical signal RSCW is supplied to LOS detector RSCW  1600 - 1  from optical tap  202  (FIG. 2), incoming optical signal RPCW is supplied to LOS detector RPCW  1600 - 2  from optical tap  204 , incoming optical signal RSCE is supplied to LOS detector RSCE  1600 - 3  from optical tap  203  and incoming optical signal RPCE is supplied to LOS detector RPCE  1600 - 4  from optical tap  205 . Optical monitors  1600 - 1  through  1600 - 4  are all identical and, therefore, only optical monitor  1600 - 1  will be explained in detail. Thus, optical monitor  1600 - 1  includes optical filter  1601 , which, in this example, is a conventional optical wavelength multiplexer that is utilized to remove the maintenance channel from incoming optical signal RSCW. The remaining optical signal of RSCW includes the communications channels, i.e., wavelengths, and is supplied to optical interference filter  1602 , which, in this example, is a known optical band-pass filter. Specifically, interference filter  1602  separates a supplied optical signal into a so-called in-band optical signal and a so-called out-of-band optical signal. The in-band optical signal is comprised of, for example, optical signals having wavelengths within a predetermined range, one example being 1548 nm (nano-meters) to 1562 nm (i.e., λi) and the out-of-band optical signal is comprised of optical signals out side of the predetermined in-band range of wavelengths. Filter  1602  supplies the in-band optical signal to photodetector  1603  and the out-of-band optical signal to photodetector  1604 . Photodetectors  1603  and  1604  convert the optical signals supplied thereto into electrical signals (e.g., current) in well known fashion. The current from photodetector  1603  is supplied to current-to-voltage converter  1605 , which converts it to a voltage signal. One such current-to-voltage converter, which may be employed for converter  1605 , in this example, is a conventional 2V/mA converter. The voltage signal is supplied from converter  1605  to amplifier  1606 , which is essentially a buffer amplifier, i.e., a 1:1 amplifier. The output from amplifier  1606  is supplied to a negative input of comparator (C)  1607 . The current from photodetector  1604  is supplied to current-to-voltage converter  1608 , which converts it to a voltage signal. One such current-to-voltage converter, which may be employed for converter  1608 , in this example, is also a conventional 2V/mA converter. The voltage signal is supplied from converter  1608  to amplifier  1609 , which in this example is a 50:1 amplifier. This amplification factor of 50:1 is used to equalize the in-band signal with the out-of-band signal. This 50:1 amplification factor is employed as a threshold for determining whether a LOS has occurred. Indeed, we have recognized that when a LOS has not occurred, the in-band signal level is approximately 60 times larger than the out-of-band signal level. When a LOS has occurred the in-band signal level is approximately 40 times as large as the out-of-band signal level. The output from amplifier  1609  is supplied to a positive input of comparator (C)  1607 . Comparator  1607  yields a low state, i.e., logical zero (0), output when the output from amplifier  1606  is greater than the output from amplifier  1609 , i.e., the in-band signal level is greater than the amplified out-of-band signal level. Otherwise, comparator  1607  yields a high state, i.e., logical one (1), output. The output from comparator  1607  is supplied to pulse width detector  1610 , which detects the duration that the output from comparator remains in a high state after a low-to-high state transition. If the duration of the high state output from pulse width detector  1610  persists for a predetermined interval, for example, for between zero (0) and 3.2 seconds, it is concluded that a LOS has occurred. When a LOS has occurred pulse width detector  1610  supplies as outputs LOS_W and {overscore (LOS_W)}, which are supplied to sub controller  207  (FIG. 2).  
         [0039]    LOS detector RPCW  1600 - 2 , LOS detector RSCE  1600 - 3  and LOS detector RPCE  1600 - 4  are essentially identical to LOS detector RSCW  1600 - 1  in both structure and operation. LOS detector  1600 - 2  supplies as outputs LOS_X and {overscore (LOS_X)}, LOS detector  1600 - 3  supplies as outputs LOS_Y and {overscore (LOS_Y)} and LOS detector  1600 - 4  supplies as outputs LOS_Z and {overscore (LOS_Z)}, all of which are supplied to sub controller  207 .  
         [0040]    Sub controller  207  effects the following logic:  
         If LOS_W and {overscore (LOS_Z)} then PROT_SWITCH_WEST=HIGH  (1)  
         If LOS_Z and {overscore (LOS_W)} then {overscore (PROT_SWITCH_WEST)}=HIGH  (2)  
         If LOS_W and LOS_Z then WEST remains in current state  (3)  
         If {overscore (LOS_W)} and {overscore (LOS_Z)} then WEST remains in current state  (4)  
         If LOS_Y and {overscore (LOS_X)} then PROT_SWITCH_EAST=HIGH  (5)  
         If LOS_X and {overscore (LOS_Y)} then {overscore (PROT _SWITCH_EAST)}=HIGH  (6)  
         If LOS_Y and LOS_X then EAST remains in current state  (7)  
         If {overscore (LOS_Y)} and {overscore (LOS_X)} then EAST remains in current state  (8).  
         [0041]    It will be apparent to those skilled in the art that appropriate interface apparatus is required to interface an optical node and the optical switch matrix therein to the optical transmission media. The interface apparatus will necessarily be different to some extent depending on whether two or four optical transmission media, e.g., optical fibers, are employed.