Abstract:
An optical switching node and method for operating same is disclosed. If a signal received over an optical fiber is destined for the switching node at which the signal is received, the signal is processed conventionally. If there is a signal on the fiber that is destined for a different optical switching node, then a node controller will determine whether that signal requires equalization and/or regeneration. A network manager instructs the node controller whether the signal requires wavelength conversion. In accordance with the invention, if the optical signal on the fiber is destined for a different optical switching node and requires none of equalization, regeneration and wavelength conversion, the optical signal remains in the optical domain and is switched directly to an appropriate output fiber. If the node controller and network manager determine that the optical signal that is destined for a different optical switching node requires any combination of equalization, regeneration, and/or wavelength conversion, then the signal is further processed in the optical switching node to provide those functions.

Description:
TECHNICAL FIELD 
     The invention relates generally to optical communication systems, and, more particularly, to an optical switching node and method for operating same. 
     BACKGROUND OF THE INVENTION 
     Optical communications systems are used throughout the world for carrying large amounts of data and voice transmissions. Optical communication systems generally employ fiber optic cables that carry optical signals from one location to another. Typically, many optical signals are carried on a single optical fiber by using wavelength-division multiplexing (WDM). Switching nodes are located along many optical communication paths and are typically connection points for a plurality of optical fibers. The switching nodes include optical and electrical switching equipment. If a particular optical signal on an optical fiber is destined for a particular switching node, the wavelength-multiplexed optical signals on the fiber are demultiplexed at that node into individual optical signals, the individual optical signals are converted by an optical receiver at the node to respective electrical signals, the electrical signal derived from the desired optical signal is switched electronically to its final destination, and a new optical signal is derived from the electrical signal. 
     In most optical communication networks, many of the optical signals entering a switching node merely pass through the node on the way to their final destination. These optical signals that are destined for a different switching node may require regeneration, equalization and/or wavelength conversion, or may be of the appropriate wavelength, have sufficient optical power and have sufficient signal quality to be communicated directly to the next switching node. 
     Regeneration is the process of determining whether each bit of the digital signal with which an optical signal is modulated is a binary “one” or a binary “zero,” and using this information to create a new, noiseless, undistorted version of the modulated optical signal. Regeneration typically also includes retiring the modulation to reduce timing jitter. In existing optical communication systems, an optical signal is regenerated by converting the optical signal to an electrical signal, processing the electrical signal, and converting the processed electrical signal back to a noiseless, undistorted, jitter-free optical signal for retransmission. 
     Wavelength conversion involves changing the optical carrier wavelength of an optical signal, without altering the information modulated on the optical signal. Wavelength conversion typically also includes the above-mentioned regeneration process, which will be assumed for the remainder of this document. 
     Equalization is the process of adjusting the power of an optical signal so that all signals in a system are maintained at the same power level. Equalization is necessary in systems in which optical signals experience different gain or loss as they travel through the system, and is often necessary in WDM systems because of wavelength-dependent loss or gain. Equalization can be performed using a variable optical attenuator. 
     In existing optical communication systems, all optical signals received at a switching node are wavelength-demultiplexed, if necessary, converted from an optical signal to an electrical signal, and regenerated. Some advantages of such existing switching nodes are that wavelength conversion (which eases network management) and equalization are straightforward; and that electronic logic can be used to monitor the quality of the incoming signals so that upstream faults can be rapidly and precisely identified and compensated. 
     However, such existing switching nodes receive, demultiplex, regenerate and retransmit some signals that are destined for a different switching node and that do not require regeneration, equalization or wavelength conversion. Therefore, because existing switching nodes must contain sufficient resources to operate on all optical signals present at the switching node, such existing switching nodes contain more expensive resources (wavelength demultiplexers and multiplexers, receivers, transmitters, regeneration and monitoring logic) than might be necessary. 
     Therefore, it would be desirable to have a switching node that minimizes the amount of signal processing performed at the node such that only signals that require processing are processed by the node. Such a switching node would allow an optical signal that is destined for a different node and that requires no regeneration, equalization, wavelength conversion or interchange with other signals having the same wavelength to remain as an optical signal as it passes directly through the switching node. The cost of such a switching node could be less than the cost of an existing node having similar switching capacity because it could contain fewer wavelength demultiplexers and multiplexers, receivers, transmitters, regeneration and monitoring logic, etc. 
     SUMMARY OF THE INVENTION 
     The invention provides an optical switching node and method for operating same. In architecture, the invention may be considered an optical switching node comprising a fiber cross-connect that receives optical signals and a node controller in communication with the fiber cross-connect. 
     The node controller is configured to determine whether any of the optical signals are destined for a different optical switching node. The node controller is also configured to determine whether any of the optical signals destined for the different switching node require further processing. The optical switching node also includes a signal converter that operates in response to the node controller and converts to an electrical signal only those optical signals destined for the optical switching node and those optical signals destined for the different optical switching node that require further processing. 
     The invention can also be conceptualized as a method for operating an optical switching node. The method comprises the steps of determining whether any of the optical signals received at the optical switching node are destined for a different optical switching node and determining whether any of the optical signals destined for the different switching node require further processing. Only the optical signals determined to be destined for the optical switching node or to be destined for the different optical switching node and require further processing are converted to electrical signals. 
     The invention has numerous advantages, a few which are delineated below merely as examples. 
     An advantage of the invention is that it reduces the complexity of a switching node. 
     Another advantage of the invention is that it reduces the cost of an optical switching node. 
     Another advantage of the invention is that it allows the direct passage of an optical signal through a switching node. 
     Another advantage of the invention is that it reduces the amount of resources (wavelength demultiplexers and multiplexers, receivers, transmitters, regeneration and monitoring logic) that are necessary at an optical switching node. 
     Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production. 
     Other features and advantages of the invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. These additional features and advantages are intended to be included herein within the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, as defined in the claims, can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     FIG. 1 is a block diagram illustrating an optical switching node constructed in accordance with the invention; 
     FIG. 2 is a block diagram illustrating the monitor of FIG. 1; 
     FIG. 3 is a flow chart illustrating the operation of the node controller of FIG. 1 while controlling the fiber cross-connect of FIG. 1; and 
     FIG. 4 is a flow chart illustrating the operation of the node controller of FIG. 1 while controlling the wavelength cross-connect of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram illustrating an optical switching node  100  constructed in accordance with the invention. Optical switching node  100  includes at least one, and generally a plurality of optical fibers  101  that supply optical communication signals to fiber cross-connect  107 . Optical fiber  101  may be a single optical fiber carrying a plurality of wavelength-multiplexed optical signals, or may include a plurality of optical fibers each carrying a plurality of wavelength-multiplexed optical signals. For the following discussion, it is assumed that optical fiber  101  represents a plurality of optical fibers each carrying wavelength-multiplexed optical signals. Fiber cross-connect  107  receives inputs from fiber  101  and via connection  146  from wavelength multiplexer  148 ; and provides outputs to fiber  108  and to wavelength demultiplexer  111  via connection  109 . Fiber cross-connect  107  is typically a connection matrix that routes an input signal to an appropriate output. Optical fiber  108 , connection  109 , and connection  146 , while described as single fibers or connections may include a plurality of optical fibers or connections similar to optical fiber  101 . 
     Node controller  150  controls fiber cross-connect  107  via connection  126 . Node controller  150  receives information from network manager  137  over connection  156  regarding which, if any, fibers  101  carry optical signals destined for optical switching node  100 , and which fibers  101  carry optical signals destined for an optical switching node other than node  100 . Node controller  150  is instructed by network manager  137  as to which, if any, of the fibers carrying only optical signals destined for a switching node other than node  100  carry optical signals that require wavelength conversion. 
     The network manager  137  also determines, via monitor  152  which, if any, of the fibers carrying only optical signals destined for a switching node other than node  100  carry optical signals that require regeneration or equalization. Node controller  150  is also instructed by network manager  137  which, if any, of the fibers carrying only optical signals destined for a switching node other than node  100  carry optical signals that are to exit the fiber cross-connect  107  on a single optical fiber, and which carry optical signals that are required to be interchanged with optical signals having the same wavelength on other optical fibers. Optical signals originating from a single fiber that are bound for a switching node other than switching node  100  and that require none of wavelength conversion, regeneration, equalization and interchange with optical signals having the same wavelength on other fibers are referred to as “express optical signal groups”. Express optical signal groups are represented by arrows  131 . Optical signals that require further processing are routed over connection  109  to wavelength demultiplexer  111 . 
     The operation of monitor  152  and its interaction with node controller  150  will be described in further detail with respect to FIG.  2 . 
     In accordance with the invention, if all the optical signals on an incoming fiber  101  are destined for a node other than node  100 , and none requires regeneration, wavelength conversion, equalization, or interchange with signals on other fibers, that is, if the optical signals carried by fiber  101  constitute an express optical signal group, node controller  150  will configure fiber cross-connect  107  so that such optical signals will pass directly through fiber cross-connect  107  and out of switching node  100  over fiber  108 . This occurs without conversion to an electrical signal and without wavelength demultiplexing. In this manner, any group of wavelength-multiplexed optical signals destined for an optical switching node other than optical switching node  100 , and that do not require regeneration, wavelength conversion, equalization or interchange with signals on other fibers can be switched directly through optical switching node  100  to the destination node. 
     Wavelength cross-connect  114  receives input from wavelength demultiplexer  111  via connection  112 , from transmitter  157  via connection  162 , and from wavelength processor  159  via connection  161 . Wavelength cross-connect  114  includes outputs to wavelength multiplexer  148  via connection  144 , to receiver  158  via connection  163 , and to wavelength processor  159  via connection  160 . 
     Wavelength cross-connect  114  is typically an optical connection matrix that routes the input optical signals to appropriate outputs. It is assumed herein that all switching performed by wavelength cross-connect  114  is performed on optical signals. Alternatively, there may be applications in which wavelength cross-connect  114  performs wavelength switching electrically. For example, wavelength cross-connect  114  may be implemented having receivers similar to receiver  158  (to be described below) on its input ports (connection  112 ), the receivers used to convert incoming optical signals to electronic signals, an electronic crossbar switching matrix (not shown) to interconnect electronic input ports to electronic output ports, and transmitters similar to transmitter  157  (to be described below) on its output ports (connection  144 ) to convert the switched electronic signals to optical signals. Connections  112 ,  144 ,  160 ,  161 ,  162  and  163 , while described as single connections, may include a plurality of connections. 
     Node controller  150  controls wavelength cross-connect  114  via connection  128 . Node controller  150  receives from network manager  137  over connection  156  information regarding which, if any, optical signals on connection  112  are destined for an optical switching node other than node  100  and require interchange with optical signals having the same wavelength on other fibers, but which do not require wavelength conversion or equalization. These optical signals are described as “express optical signals” and are indicated by arrows  132 . 
     Optical signals on connection  112  that do not require interchange with optical signals having the same wavelength on other fibers are directed to either wavelength processor  159  via connection  160 , or to receiver  158  via connection  163  and will be described below. Information regarding whether the optical signals require regeneration or equalization is provided to wavelength cross-connect  114  from monitor  152 , through node controller  150 . In accordance with the invention, node controller  150  will configure wavelength cross-connect  114  so that the optical signals that require none of wavelength conversion, equalization, and regeneration will pass directly through wavelength cross-connect  114  as express optical signals  132  and out to wavelength multiplexer  148  via connection  144  without being converted to electrical signals. In this manner, any optical signal destined for an optical switching node other than optical switching node  100 , and that does not require regeneration, wavelength conversion, or equalization (i.e., an express optical signal) can be directly interchanged with an optical signal on another fiber and switched through optical switching node  100  to the destination node. 
     In accordance with the invention, if any optical signal on fiber  101  will terminate at optical switching node  100 , then all of the optical signals on that fiber are sent over connection  109  from fiber cross-connect  107  to wavelength demultiplexer  111 . Wavelength demultiplexer  111  demultiplexes all of the wavelength-multiplexed optical signals on that fiber and sends each demultiplexed optical signal over connection  112  to wavelength cross-connect  114 . The wavelength cross-connect  114  selects the optical signals that will terminate at optical switching node  100  and routes those optical signals over connection  163  to receiver  158 . The optical signals that terminate at switching node  100  are sometimes referred to as “dropped signals.” 
     While shown as a single block, receiver  158  is typically a plurality of receivers. Receiver  158  receives the dropped optical signal over connection  163  from wavelength cross-connect  114  and converts the optical signal to an electrical signal. The electrical signal is then analyzed to determine whether each bit represents a binary 1 or a binary 0 value. Receiver  158  then creates a new, noiseless, undistorted, jitter-free version of that electrical signal for transmission over connection  124  to local entities that may include synchronous optical network/synchronous digital hierarchy (SONET/SDH) digital cross-connects, asynchronous transfer mode (ATM) switches, Internet protocol (IP) routers, etc. 
     In accordance with the invention, if any optical signal on fiber  101  requires regeneration, wavelength conversion or equalization, then all of the optical signals on that fiber are sent over connection  109  from fiber cross-connect  107  to wavelength demultiplexer  111 . Wavelength demultiplexer  111  demultiplexes all of the wavelength-multiplexed optical signals on the fiber and sends each demultiplexed optical signal over connection  112  to wavelength cross-connect  114 . The wavelength cross-connect  114  selects the optical signals that require regeneration, wavelength conversion or equalization and routes said optical signals over connection  160  to wavelength processor  159 . While shown as a single block, wavelength processor  159  is typically a plurality of devices, including wavelength converter(s)  147 , regenerator(s)  142  and variable optical attenuator(s)  141 . 
     If the optical signal selected by the wavelength cross-connect  114  requires equalization, the variable optical attenuator  141  in wavelength processor  159  attenuates the optical signal to the desired value and sends the equalized optical signal back to wavelength cross-connect  114  via connection  161 . If the selected optical signal requires regeneration, the regenerator  142  in wavelength processor  159  determines whether each bit represents a binary 1 or a binary 0 value, and then creates a new, noiseless, undistorted version of the optical signal at the same wavelength. The regenerated optical signal is sent back to optical wavelength cross-connect  114  via connection  161 . 
     The regeneration process may or may not involve conversion of the optical signal to an electrical signal and back to an optical signal. If the optical signal to selected by the wavelength cross-connect  114  requires wavelength conversion, the wavelength converter  147  in wavelength processor  159  determines whether each bit represents a binary 1 or a binary 0 value, and then creates a new, noiseless, undistorted, jitter-free version of the optical signal at the desired wavelength. This wavelength-shifted, regenerated optical signal is sent back to optical wavelength cross-connect  114  via connection  161 . 
     If a selected optical signal requires equalization and wavelength conversion or regeneration, the optical signal is switched by wavelength cross-connect  114  to wavelength processor  159  where one of these operations is performed. The signal is then sent back to wavelength cross-connect  114 , and switched back to wavelength processor  159  for the second operation. The processing is performed in this manner because the implementation contemplated for the wavelength processor  159  is better suited to perform one operation each time the signal is passed through the processor. In this manner, the complexity of the wavelength processor  159  can be minimized because the need for communication between the wavelength processor and the node controller  150  regarding configuration of the wavelength processor is minimized. Alternatively, it is foreseeable that multiple operations could be performed in a single pass through the wavelength processor. 
     Equalized, wavelength-converted and/or regenerated optical signals entering wavelength cross-connect  114  via connection  161  are switched to wavelength multiplexer  148  via connection  144 . The optical signals then enter fiber cross-connect  107  via connection  146 , where they are switched to an appropriate output fiber  108 . 
     Electrical information signals originating from local entities that are to be transmitted through optical switching node  100  and onto fiber  108  are received over connection  138  and supplied to transmitter  157 . Transmitter  157  converts the electrical information signals into optical signals at the appropriate wavelength and supplies them over connection  162  to wavelength cross-connect  114 . The wavelength cross-connect  114  sends the optical signals over connection  144  to wavelength multiplexer  148 , where they are multiplexed and switched by the fiber cross-connect  107  onto an appropriate output fiber  108 . 
     Node controller  150  communicates with wavelength cross-connect  114 , wavelength processor  159  and transmitter  157  over connections  128 ,  154  and  155 , respectively. 
     FIG. 2 is a block diagram illustrating the monitor  152  of FIG.  1 . When commanded by the node controller  150  of FIG. 1, the monitor  152  analyzes the optical signal chosen by node controller  150 , and determines whether this optical signal requires regeneration or equalization. Monitor  152  is connected to input fiber  101  via tap  171  and connection  151 . Tap  171  removes a small amount of light from fiber  107  and directs this light through connection  151  to tunable optical bandpass filter  166 . Tunable optical bandpass filter  166  is tuned by a tuning control signal  164  received from node controller  150  over connection  164 . The tuning control signal  164  may be represented by an electrical, optical, or a mechanical signal. Tuning control signal  164  directs tunable optical bandpass filter  166  to pass one of the optical signals present on connection  151  to receiver  167  via connection  169  and to block all others. Tuning control signal  164  originates in node controller  150 , which determines the wavelength of the optical signal that is to be passed by filter  166 , and thus which optical signal is analyzed. The single optical signal output from filter  166  passes to receiver  167  via connection  169 , where it is converted to an electrical signal. 
     Receiver  167  is similar to receiver  158  described above. The electrical signal generated by the receiver  167  passes to signal analyzer  168  via connection  170 . Signal analyzer  168  measures the electrical signal strength and determines the optical power therefrom. Signal analyzer  168  then estimates the bit-error-ratio of the signal, and sends this information to node controller  150  via connection  153 . 
     The node controller  150  uses the bit-error ratio estimate to determine whether the selected optical signal requires regeneration. If the bit-error ratio estimate is worse than a predetermined threshold, then the optical signal requires regeneration. If the bit-error ratio estimate is better than the threshold, the optical signal is passed through the switching node without regeneration. Estimation of the bit-error ratio may occur using known techniques, such as, for example, measurement of optical signal-to-noise ratio; eye pattern estimation; or parity check. 
     Node controller  150  uses the optical power measurement supplied by signal analyzer  168  to determine whether the signal requires equalization. If the optical power does not fall within a predetermined range, then the signal requires equalization. If the optical power is within the specified range, the signal is passed through the switching node  100  without equalization. 
     FIG. 3 is a flow chart  200  illustrating the operation of the node controller  150  while controlling the operation of the fiber cross-connect  114  of FIG.  1 . In block  201  it is determined whether one or more optical signals on fiber  101  (FIG. 1) are destined for switching node  100  of FIG.  1 . Network manager  137  of FIG. 1 supplies this information. If one or more of the optical signals on fiber  101  are destined for switching node  100 , then, in block  207 , all of the optical signals on fiber  101  are switched by the fiber cross-connect  107  to wavelength demultiplexer  111  for processing as described above with respect to FIG.  1 . 
     If, in block  201 , it is determined that no optical signals on fiber  101  are destined for switching node  100 , then in block  202  it is determined whether one or more optical signals on fiber  101  require equalization, regeneration, and/or wavelength conversion. Monitor  152  of FIG. 1 supplies information regarding equalization and regeneration. Network manager  137  of FIG. 1 supplies information regarding wavelength conversion. 
     If it is determined in block  202  that one or more optical signals on fiber  101  require equalization, regeneration, and/or wavelength conversion, then in block  207 , all of the optical signals on fiber  101  are switched to wavelength demultiplexer  111  as described above. If, in block  202 , it is determined that none of the optical signals on fiber  101  requires equalization, regeneration or wavelength conversion, then in block  204  it is determined whether any of the optical signals on fiber  101  are required to be interchanged with optical signals having the same wavelength on other fibers. Network manager  137  of FIG. 1 supplies this information. 
     If, in block  204  it is determined that none of the optical signals on fiber  101  is required to be interchanged with an optical signal having the same wavelength on other fibers (i.e., all of the optical signals on fiber  101  are bound for the same output fiber), then the optical signal group on fiber  101  is an express optical signal group and in block  206  all of the optical signals on input fiber  101  are switched to the appropriate output fiber  108  by fiber cross-connect  107  for transmission over fiber  108 . In this manner, the express optical signal group is routed directly through switching node  100  without conversion to electrical signals. 
     If, in block  204  it is determined that some or all of the optical signals on fiber  101  are to be interchanged with signals having the same wavelength on other fibers, then in block  207  all of the optical signals on that fiber  101  are switched to wavelength demultiplexer  111 . 
     FIG. 4 is a flow chart  300  illustrating the operation of the node controller  150 , while controlling the wavelength cross-connect  114  of FIG.  1 . 
     In block  301  it is determined whether any of the optical signals output by wavelength demultiplexer  111  is destined for switching node  100  of FIG.  1 . Network manager  137  of FIG. 1 supplies this information. If any of the optical signals is destined for switching node  100 , then in block  306  the wavelength cross-connect  114  will send the optical signal destined for switching node  100  over connection  163  to the receiver  158  for conversion to an electrical signal and distribution to local entities as described above. 
     If it is determined, in block  301 , that none of the optical signals output by wavelength demultiplexer  111  is destined for switching node  100 , in block  302  it is determined whether any of the optical signals output by wavelength demultiplexer  111  requires equalization, regeneration, and/or wavelength conversion. Monitor  152  of FIG. 1 supplies information regarding equalization and regeneration. Network manager  137  of FIG. 1 supplies information regarding wavelength conversion. If any optical signal requires equalization, regeneration and/or wavelength conversion then, in block  307 , that optical signal is switched to the wavelength processor  159  for further processing as described above. 
     If it is determined in block  302  that none of the optical signals output by wavelength demultiplexer  111  require equalization, regeneration and wavelength conversion (i.e., if the optical signal is an express optical signal), then in block  304 , wavelength cross-connect  114  will switch the optical signal to wavelength multiplexer  148  via connection  144 , where it is multiplexed and forwarded via connection  146  to an appropriate output fiber through fiber cross-connect  107  and onto fiber  108 . In this manner, the node controller  150  determines whether any optical signal destined for an optical switching node other than optical switching node  100  requires equalization, regeneration, and/or wavelength conversion. If no further processing is required on such optical signal, the optical signal is passed directly through optical switching node  100  without conversion to an electrical signal, thus significantly reducing the resources required at optical switching node  100 . 
     It will be apparent to those skilled in the art that many modifications and variations may be made to the preferred embodiments of the invention, as set forth above, without departing substantially from the principles of the invention. All such modifications and variations are intended to be included herein within the scope of the invention, as defined in the claims that follow.