Abstract:
An optical fiber protection switch that performs automatic protection switching. The optical fiber protection switch performs span and ring switching at the optical layer, thereby negating the need for SONET ADMs to perform the switching at the SONET layer. The optical fiber protection switch includes span switches and ring switches arranged to provide span and ring switching, respectively. The span switches and ring switches are implemented using 2×2 optical switches, for a total of eight 2×2 optical switches in the optical fiber protection switch. The system described herein may utilize SONET terminals and/or ATM switches in a WDM environment to support capacity increases while providing the ring and span switching functionality. A network switching element providing transparent, self-healing optical 4-fiber BLSR (OBLSR/4) is realized by using the optical fiber protection switch described herein.

Description:
RELATED APPLICATIONS 
     This application is a continuation of prior application Ser. No. 10/295,183, filed Nov. 15, 2002, now U.S. Pat. No. 6,771,849, entitled “OPTICAL FIBER PROTECTION SWITCH,” which is a continuation of prior application Ser. No. 09/207,064, filed Dec. 7, 1998, now U.S. Pat. No. 6,504,963, entitled “OPTICAL FIBER PROTECTION SWITCH,” and which are incorporated by reference into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to optical communication networking devices. More particularly, the invention relates to an optical fiber protection switch to protect optical networks, and specifically, wavelength division multiplexed optical 4-fiber Bi-Directional Line Switched Ring (BLSR) Networks. 
     B. Description of the Related Art 
     1. Optical Networking 
     Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized optical digital transmission systems that are used, respectively, in North America and internationally. SONET networks typically use synchronous add/drop multiplexers (ADM) to add and/or drop asynchronous DS-n signals onto the links. The ADM devices also re-route signals to avoid faulty communication links. This is referred to as span and ring switching. 
     2. Span Switching and Ring Switching 
     Span switching and ring switching are mechanisms to re-route traffic over optical networks. A ring is a network configuration that allows signal path redundancy between nodes on a network by interconnecting the nodes in a loop, or ring. In a four-fiber ring, the nodes are connected with a pair of working fibers creating a bi-directional communication path, and a pair of protection fibers creating a second bi-directional communication path to be used as protection for the working ring. The working and protection fibers connect each node to the two adjacent nodes in the ring topology. 
     Span switching is performed between two nodes to re-route working traffic over the protection fiber in the event of a fiber failure on the working ring. The failure may be due to a fiber cut or signal degradation due to other equipment failures. The working traffic is placed on the protection fiber by the transmitting stations, then re-routed to the working fiber/ring at the receiver, thereby bypassing the failed fiber/equipment. The working traffic from the failed fiber span is thus re-routed to the protection fiber span. 
     Bi-directional line switched ring (BLSR) is a bi-directional ring that protects against fiber failures that are more severe—such as fiber bundle cuts—where the failure occurs in both working fibers and both protection fibers between two nodes on the ring. In the event of a fiber bundle cut between two adjacent nodes, the working ring traffic is re-routed in the reverse direction along the protection fiber ring by the nodes on either side of the fiber bundle cut. Automatic protection switching (APS) may be used to perform the ring switch automatically upon signal loss. 
     Current SONET BLSR uses a SONET add-drop multiplexer (ADM) as the line termination equipment, and APS performs the span or ring switch when transmission failure occurs within the ring. In the SONET BLSR architecture, the SONET ADM performs span and ring switching by bridging the working line with the protection line, thereby protecting the working traffic. 
     3. Wavelength Division Multiplexing 
     The demand for bandwidth in the transport network has been increasing at an exponential rate. This aggressive demand has fueled the rapid deployment of wavelength division multiplexing (WDM) in the network to alleviate fiber constraints. WDM may be viewed as a parallel set of optical channels, each using a slightly different light wavelength, but all sharing a single transmission medium. WDM systems have the advantage of increasing the capacity of existing networks without the need for expensive re-cabling. 
     In the current deployment scenario, a single, multi-wavelength WDM system transports multiple, concentric, single-wavelength SONET rings. SONET ring protection is typically accomplished by ADMs, providing both span and ring protection switching. Capacity increases on the WDM ring are typically accomplished by building a new SONET ring, which includes SONET ADMs to support the protection requirements. 
     SUMMARY OF THE INVENTION 
     An optical fiber protection switch that performs automatic protection switching is provided. The optical fiber protection switch performs span and ring switching at the optical layer, thereby negating the need for SONET ADMs to perform the switching at the SONET layer. The system described herein may therefore utilize SONET terminals and/or ATM switches in a WDM environment to support capacity increases while providing the ring and span switching functionality. 
     The optical fiber protection switch includes a span switch and a ring switch arranged to provide span and ring switching, respectively. The span switch and ring switch are each implemented using four 2×2 optical switches, for a total of eight 2×2 optical switches in the optical fiber protection switch. 
     A network switching element providing transparent, self-healing optical 4-fiber BLSR (OBLSR/4) is realized by using the optical fiber protection switch described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the present invention will be more readily appreciated upon reference to the following disclosure when considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a preferred embodiment of the optical fiber protection switch (OPSW); 
         FIG. 2  depicts a preferred embodiment of the OPSW in a four-fiber BLSR in a normal switching condition; 
         FIG. 3  depicts a preferred embodiment of the OPSW in a four-fiber BLSR in a span switching condition; 
         FIG. 4  depicts a preferred embodiment of the OPSW in a four-fiber BLSR in a ring switching condition; 
         FIG. 5  depicts the internal switching states of two interconnected OPSWs under normal switching conditions; 
         FIGS. 6 and 7  depict the internal switching states of two interconnected OPSWs under span switching conditions; 
         FIG. 8  depicts the internal switching states of two interconnected OPSWs under ring switching conditions; 
         FIG. 9  depicts a preferred embodiment of the optical line termination equipment (OLTE); 
         FIG. 10  depicts an optical add-drop multiplexer available from JDS Fitel; 
         FIG. 11  depicts a block diagram of the OLTE, including an optical channel manager (OCM); 
         FIG. 12  depicts a preferred network configuration of the OLTE in a wavelength division multiplexed (WDM) BLSR; 
         FIGS. 13 and 14  depict alternative preferred embodiments of the OCM; and, 
         FIG. 15  demonstrates the compatibility of the OPSW/OLTE with current SONET networks. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The optical fiber protection switch  100  is designed to perform 4-fiber BLSR protection switching to bridge traffic between working fibers and protection fibers. The structure and interconnection of the optical fiber protection switch (OPSW)  100  is shown in  FIG. 1 , where eight 2×2 optical switches  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122 , and  124 , are used. The optical switch  100  includes a west ring switch  102 , an east ring switch  104 , a west span switch  106  and an east span switch  108 . As indicated in  FIG. 1 , the interface between the ring switches  102 ,  104  and the span switches  106 ,  108 , is shown with the aid of the numerical references “1” through “8” (e.g., point “1” on the west ring switch  102  connects to point “1” on the west span switch  106 , etc.). 
     The west ring switch  102  includes a first optical switch  110  and a second optical switch  112 . The east ring switch  104  includes a third optical switch  114  and a fourth optical switch  116 . The west span switch  106  includes a fifth optical switch  118  and a sixth optical switch  120 . The east span switch  108  includes a seventh optical switch  122  and an eighth optical switch  124 . The optical switches  110 – 124  are 2×2 optical switches. In a first switching state of the 2×2 optical switches the two inputs are connected to the two outputs. For example, in  FIG. 1 , the input from the W 1  fiber is connected to output “ 1 ” and the input from the P′ 2  fiber is connected to output “ 3 ”. In a second switching state of the 2×2 optical switches, the inputs are redirected to the other output, resulting in a cross-connection. For example, the input from the W 1  fiber is connected to output “ 3 ” and the input from the P′ 2  fiber is connected to output “ 1 ”. 
     Switches  110 – 124  are preferably mechanically operated optical switches. While mechanical switches tend to be somewhat slower than some electronic switches and have a shorter mean-time-to-failure, they are generally less expensive and yet provide effective channel cross-talk isolation. Mechanical fiber optic switches typically utilize solenoid-activated moving prisms or moving mirrors to direct the optical signals. Switching speed is on the order of 20 ms or faster, and insertion loss and PDL are low. The switches preferably include a feedback output by which the state of the switch may be determined. Suitable optical switches are the MFSW switch available from E-Tek, Inc. of 1865 Lundy Avenue, San Jose, Calif. 95131 or the SW, SL, or SR series switches from JDS FITEL, Inc. 570 West Hunt Club Road, Nepean, Ontario, K2G 5W8 Canada. 
     W 1 , W 2 , and W 1 ′, W 2 ′ are working fibers on the west and east sides of the switch, respectively, and P 1 , P 2 , and P 1 ′, P 2 ′ are protection fibers on west and east sides of the node, respectively. The west and east ring interface ports  130  and  132 , respectively, connect the optical switch  100  to the optical ring working and protection fibers. The west and east client equipment interface ports  134  and  136 , respectively, are depicted on the topside of optical switch  100 , and allow connection of the optical switch to client equipment. 
     There are two functional layers in the optical protection switch  100 , ring switching and span switching. Under normal operating conditions, each signal at the west ring interface port  130  of the OPSW  100  is interconnected to the corresponding underlined port of the west client equipment interface port  134  of the OPSW  100  through the internal 2×2 switches of the west ring switch  102  and west span switch  106 . For example, W 1  of ring interface port  130  is connected to W 1  of client equipment interface port  134  by way of SW 1   110  and SW 5   118 . 
     Each span switch  106 ,  108  includes a 2×2 switch for a receive fiber pair and one for a transmit fiber pair (a “fiber pair” is one working fiber and its associated protection fiber, e.g., the pair W 1 , P 1 , which, incidentally, is also referred to as the pair W 1 ′, P 1 ′ with respect to an adjacent OPSW). For example, span switch  106  includes SW 5   118  that carries receive signals nominally received from the W 1  P 1  fiber pair to equipment connected to port  134 . Span switch  106  also includes SW 6   120  that accepts signals from the equipment connected at port  134 , which are nominally directed to the W 2  P 2  fiber pair. Of course it is recognized that the actual origin and destination of such transmit and receive signals is dependent upon the switching states of the 2×2 optical switches. 
     Each ring switch is interconnected to fibers at both ports  130  and  132 . Ring switch  102  is connected to W 1  and W 2  of port  130  and to P 1 ′ and P 2 ′ of port  132 . Ring switch  104  is connected to W 1 ′ and W 2 ′ of port  132  and P 1  and P 2  of port  130 . Note that ring switches  102  and  104  do not necessarily contain a transmission medium or otherwise connect to fibers other than those that connect to the 2×2 optical switches within the ring switches. Specifically, for example, ring switch  102  need not connect to the P 2  fiber, even though  FIG. 1  depicts the lower output of SW 3   114  of ring switch  104  passing through ring switch  102  en route to west ring port  130 . The connections depicted in the drawing are merely for convenience, and in the physical embodiment of OPSW  100 , the lower output of ring switch  104  connects directly to fiber P 2  of west port  130 , bypassing ring switch  102 . The other similar connections between the west ring switch  102  and the east protection fibers bypass the east ring switch  104 , and those between the east ring switch  104  and the west protection fibers bypass the west ring switch  102 . 
       FIGS. 2 ,  3 , and  4  illustrate a four-node dense wavelength-division multiplexed (DWDM, or WDM) 4-Fiber Optical Bi-Directional Line-Switched Ring is used in here to illustrate operation of the protection switching of OPSW  100 . Generally, the OPSW  100  accomplishes the signal re-direction of ring and span switching in a manner similar to that defined in SONET BLSR standard. Bellcore Document GR-1230-CORE sets forth the ring and span switching of SONET BLSR, and is incorporated by reference herein. A significant difference between the SONET BLSR and the optical fiber BLSR (OBLSR) implemented by OPSW  100  is that the protection bridging is accomplished in the optical domain by the OPSW  100  in the optical BLSR network proposed herein, rather than in the SONET add-drop multiplexer in the SONET BLSR standard. Similar to the SONET BLSR, both optical span switching and optical ring switching can be supported by the optical BLSR.  FIG. 2  shows the working traffic flow under normal conditions, where heavy lines indicate the working traffic. 
     The OPSW implements both types of protection switching: ring switching and span switching. Ring switching is implemented when a bundle failure occurs (e.g., all four fibers are cut), and span switching is implemented when a working fiber cut occurs. The terms “fiber cut” or “fiber failure” are used here to refer generally to a connection failure that may be caused by fiber break/cut, line fiber amplifier failure, or other type of fault resulting in a degraded or unacceptable connection. The OPSW performs automatic protection switching when loss of signal indication is observed (this can be achieved by many ways such as pilot tone, use of Optical Service Channel, etc.). 
     When a fiber failure occurs, the optical protection switch  100  will reroute the multi-wavelength working traffic to the protection fiber by changing its internal switch connections. With reference to  FIG. 3 , OPSW 4   300  re-directs working traffic, as shown by arrow  302 , from the working fiber  304  to the protection fiber  306 . OPSW 3   308  redirects the traffic, as indicated by arrow  310 , prior to reaching the terminal equipment, or traffic access equipment  312 . 
     The optical protection switch  100  can support optical ring switching as shown in  FIG. 4 . In the event that the fiber bundle  404  connecting OPSW 1   400  and OPSW 2   402  fails, OPSW 1   400  and POSW 2   402  redirect the signals as indicated. Traffic received by OPSW 1   400  on the west W 2  fiber would normally be sent out on the east W 2 ′ fiber, but is instead redirected to the west-bound P 1  fiber as shown by arrow  406 . The traffic similarly redirected by OPSW 2   402  (shown by arrow  408 ) is received by OPSW 1   400  on the west P 2  fiber. OPSW 1   400  redirects this traffic to the east side of the traffic access equipment  312  as shown by arrow  410  so that it may be processed as if it had been received directly from OPSW 2   402  over the W 1 ′ fiber. Thus it may be seen that the protection fibers are used to re-route the working traffic and effectively replace the failed working fibers in bundle  404 . 
     In  FIGS. 5–8 , the components of OPSWs  100 W (west) and  100 E (east) will be referred to by the same numbering system as in  FIG. 1 , with a W or E appended when necessary to differentiate between the west and east OPSW switches  100 W and  100 E.  FIG. 5  depicts the states of the internal switches of two interconnected OPSWs  100 W and  100 E. The span switches  106 ,  108  and ring switches  102 ,  104  are shown in a first switching state. That is, the signal paths from the respective input/output ports  130 ,  132 ,  134  and  136 , to the interface between the ring and span switches are not redirected by the span and ring switches. The states of the internal 2×2 switches for each OPSW  100 W,  100 E which correspond to the first switching state of the ring and span switches may also be seen in  FIG. 5 . Each 2×2 optical switch is preferably in an un-switched state allowing signals to pass through without redirection. The remainder of the ring network is not shown in  FIGS. 5–8  because only the two switches adjacent to the failed link will switch, while the other nodes remain in the first switching state. 
     A second switching state of the span switches  106 ,  108  is shown in  FIGS. 6 and 7 . The second switching state redirects the signal from the working fiber to the protection fiber, or vice-versa. This occurs when there has been a single working fiber failure. In  FIG. 6 , the east W 1  (W 1 ′) fiber is inoperative, hence span switch  136 W of OPSW  100 W redirects the working traffic received on W 1 ′ of port  136 W to ring switch  102 W. Ring switch  102 W then directs the traffic to the protection fiber P 1 ′ through port  132 W. In OPSW  100 E, on the east side of the fiber failure, the working traffic received on the protection fiber P 1  of port  130 E passes through ring switch  104 E to span switch  106 E. Span switch  106 E redirects the traffic to the W 1  fiber of port  134 E so that traffic access equipment  312  (not shown) connected to ports  134 E can process it as normal working traffic. 
     With reference to  FIG. 7 , the working fiber W 2  is in a failure condition, and span switches  136 W and  134 E redirect the working traffic around the failed fiber, using the protection fiber P 2 . The signal to be transmitted from  100 E to  100 W containing the working traffic is received from traffic access equipment  312  (not shown) on port  134 E on the W 2  fiber. As shown in  FIG. 7 , span switch  106 E redirects the working traffic to the protection fiber P 2  at port  130 E by redirecting the signal to ring switch  104 E instead of ring switch  102 E. 
     In the ring switching operation depicted in  FIG. 8 , the working traffic between two isolated nodes is redirected in the opposite direction over the protection fibers. Ring switch  104 W receives working traffic intended for OPSW  100 E from span switch  108 W on the line indicated by ‘2’ at the top left of ring switch  104 W. Ring switch  104 W redirects this traffic out to P 2  of port  130 W. The signal passes around the ring over the P 2  protection fiber and is received by OPSW  100 E at the P 2 ′ fiber of port  132 E. The signal is directed to the W 1  fiber of port  134 E by ring switch  102 E (via span switch  106 E). In this manner, the traffic access equipment  132  connected to port  134 E may process the working traffic in the normal fashion. The working traffic headed from OPSW  100 E to OPSW  100 W is similarly redirected by ring switch  102 E over protection fiber P 1  to ring switch  104 W, which then redirects the working traffic to the W 2 ′ fiber of port  108 W for processing. 
     The OPSW  100  is shown in  FIG. 9  in a preferred configuration appropriate for use as an optical line termination equipment (OLTE)  910  in a WDM four-fiber BLSR network. With reference to  FIG. 9 , the working and protection ring fibers are connected to the west and east ring interface ports  130  and  132 , respectively. The protection interface ports P 1 , P 1 ′, P 2 , and P 2 ′ on the top of the switch are interconnected with each other through optical regenerators (optical amplifiers)  202  and  204 . Working traffic from W 1 , W 1 ′, W 2 , and W 2 ′ appearing at the client equipment interface ports  134 ,  136  are connected with wavelength multiplexers (MUX)  206 ,  208  and demultiplexers (DEMUX)  210 ,  212 . The multiplexers and demultiplexers  206 – 212  provide access to terminal equipment such as ATM switches, IP routers, SONET DCS, ethernet networks, frame relay (FR), etc. Each wavelength of the WDM signal, or each channel, is connected either to a client transceiver or is directly passed from DEMUX to MUX (DEMUX  212  to MUX  208 , or DEMUX  210  to MUX  206 ). In this way, all optical signals are transported, added, or dropped, through the WDM BLSR network. 
     The OCM  900  of  FIG. 9  may have dedicated add-drop functionality, or may be implemented on a configurable optical add drop multiplexer (COADM), such as that available from JDS Fitel. The COADM depicted in  FIG. 10  selectively adds and drops wavelength channels from a DWDM multi-channel stream using 2×2 optical switches  500  placed between demultiplexers  502  and multiplexers  504 . Each 2×2 switch may simultaneously drop a channel on a given wavelength and add a new channel of the same wavelength (or a different wavelength, provided there is no spectral conflict). The COADM is available in opto-mechanical and solid state switch versions. 
     The OLTE, therefore consists of the OPSW  100 , and two optical channel managers (OCM)  900 . The OCM provides traffic or channel access to the individual wavelengths in the WDM network.  FIG. 11  depicts a block diagram of the OLTE, including the OCM  900  and the OPSW  100 , while the overall network configuration is shown in  FIG. 12 . 
       FIGS. 13 and 14  depict alternative preferred embodiments of the OCM. Note that for simplicity  FIGS. 13 and 14  depict the components required for only one working fiber/protection fiber pair. It is understood that each OCM is capable of handling two working/protection fiber pairs. It is also understood that the OCM  900  may optionally include optical amplifiers  202 ,  204 , which are also not depicted in  FIGS. 13 and 14 .  FIG. 13  shows an optical channel manager OCM  900 A having demultiplexers  212 ,  206 , multiplexers  208 ,  210 , and cross connect switch  920 . The cross connect switch  920  is preferably an optical cross connect switch, but may alternatively be an electrical cross connect switch. Suitable optical cross connect switches are available from such manufacturers as NEC and Fujitsu. The cross connect switch also provides an interface  925  to terminal equipment that further processes the optical channels that are added or dropped by the switch  920 . 
     The OCM  900 A also includes regenerators  930 ,  940 . The regenerators are capable of receiving signals over a broad spectrum and regenerate it at the desired wavelength. Suitable regenerators are available from Fujitsu. The regenerators receive the optical signal of a given wavelength and adjust the spectrum to occupy a desired wavelength. One function of the regenerators  930 ,  940  is to shift the optical signal from one wavelength to another. Each regenerator  930 ,  940  provides an optical signal output at a different wavelength such that the various outputs may be combined by multiplexer  208  (or  210 ) without interfering with each other. One or more regenerators  940  may be reserved as backup regenerators to be used in the event of failure of any other regenerator  930 . The backup regenerators  940  may have an output at a predetermined wavelength, or alternatively, may include a tunable laser such that the backup regenerators  940  provide an output at any specified wavelength, preferably the same wavelength as the failed regenerator unit. 
       FIG. 14  shows an alternative embodiment of an OCM  900 B. The OCM  900 B is a simplifed version of OCM  900 A in that only the working traffic is switched by cross connect switch  950 , while the protection traffic is routed straight through the OCM  900 B. 
       FIG. 15  demonstrates the compatibility of the OPSW/OLTE with current SONET networks. The signals received on working and protection fibers pass through an optical wavelength drop switch  960  which extracts, or demultiplexes, the SONET channel to be processed independently. The SONET channel is then recombined with the OLTE traffic using wavelength add switch  970 . 
     A preferred embodiment of the present invention has been described herein. It is to be understood, of course, that changes and modifications may be made in the embodiment without departing from the true scope of the present invention, as defined by the appended claims.