Abstract:
A method for an intermediate node to control a level of a signal included in a wavelength-multiplexed signal and transmitted from a source node to a destination node via the intermediate node, includes: detecting a level of the signal; identifying a position of the intermediate node with respect to the source node; determining a control time based on the position; controlling, when the control time has elapsed from the detecting, a level of the signal based on the level detected at the detecting.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-249833, filed on Aug. 30, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a technology for controlling the level of each signal included in a wavelength-multiplexed signal respectively and appropriately.  
         [0004]     2. Description of the Related Art  
         [0005]     In wavelength division multiplexing (WDM) optical transmission systems, power and optical signal-to-noise ratio (SNR) of each of wavelengths being multiplexed largely affect transmission characteristics, and this requires control so that the power of each of the wavelengths is equalized. As a method for this, the following method is employed. In this method, each input of wavelengths is controlled by inputting it to a variable optical attenuator (VAT) and attenuating the power thereof.  
         [0006]      FIG. 6  is a diagram of a WDM optical transmission system in which conventional wavelength multiplexing apparatuses are connected in multiple stages. In a wavelength multiplexing apparatus  1   a  of a node  1 , a wavelength demultiplexer (DMUX)  11  demultiplexes an incident light from an input-side WDM transmission line  2   a , into lights of respective wavelengths, a variable optical attenuator of a VAT controller  12  makes the level of the light fixed for each wavelength, and a wavelength multiplexer (MUX)  13  multiplexes again the lights of the wavelengths and outputs the light multiplexed to an output-side WDM transmission line  2   b.    
         [0007]     An optical supervisory channel (OSC) controller  14  in the wavelength multiplexing apparatus  1   a  receives information for the wavelength from a wavelength multiplexing apparatus (not shown) of an immediately preceding node, and transmits the information for the wavelength to a wavelength multiplexing apparatus  1   b  of a next node. The same processing is performed in the wavelength multiplexing apparatus  1   b  of a node  2  and in wavelength multiplexing apparatuses  1   c  and  1   d  of a node  3  and a node  4 , respectively. In  FIG. 6 , reference numerals  2   c ,  2   d , and  2   e  represent a WDM transmission line.  
         [0008]     Normally, the VAT controller  12  detects the level of light output from the variable optical attenuator, for each wavelength, and controls so as to perform feedback of the level detected to the variable optical attenuator, thereby controlling the output level of the variable optical attenuator to a fixed value. The feedback control requires a certain control time from when the output level of light from the variable optical attenuator is detected until the output level is actually controlled.  
         [0009]     In the conventional WDM optical transmission system, the control time of the VAT controllers  12  is the same as one another in all the wavelength multiplexing apparatuses connected in the multiple stages. Therefore, as shown in  FIG. 7 , fluctuation in the level of a fine light from a node in an initial stage is being accumulated toward a node in a further subsequent stage, and the amplitude of the fluctuation increases. In  FIG. 7 , reference numeral  31  represents a waveform of the incident light to the node  1 ,  32  a waveform of the output light from the node  1  (incident light to the node  2 ),  33  a waveform of the output light from the node  2  (incident light to the node  3 ),  34  a waveform of the output light from the node  3  (incident light to the node  4 ), and  35  a waveform of the output light from the node  4 .  
         [0010]     The accumulation of the fluctuations in the light levels occurs in the following manner. Inputs to the VAT controller  12  of each node are light waves of different frequencies with various factors. As shown in  FIG. 8 , when the incident light  31  of a wave with a cycle twice as long as the control time is input to the node  1 , to suppress the fluctuation at time A, the VAT controller  12  controls so as to suppress its amplitude at time A in the direction of an arrow  36  and by the length of the arrow  36 .  
         [0011]     However, there is a delay, such as the control time, in the feedback control of the VAT controller  12 . Therefore, in the actual case, the control, which is indicated by an arrow  37  having the same length and the same direction as these of the arrow  36 , works at time A′ that is delayed from the time A by the control time. Thus, the output light  32  of the node  1  becomes a wave obtained by changing the incident light  31  to the node  1  by the length of the arrow  37 . In other words, the amplitude of the output light  32  increases with respect to that of the incident light  31 .  
         [0012]     As shown in  FIG. 9 , the output light  32  of the node  1  becomes an incident light  32  to the node  2 . In the node  2  also, the control indicated by an arrow  38  at time B actually works as the control, at time B′ delayed by the control time, indicated by an arrow  39  having the same direction and the same length as these of the arrow  38 , in the same manner as that of the VAT controller  12  in the node  1 . Thus, the output light  33  of the node  2  becomes a wave obtained by changing the incident light  32  to the node  2  by the length of the arrow  39 . In other words, the amplitude of the output light  33  increases with respect to that of the incident light  32 . The same goes for the node  3  and thereafter. The fluctuation in the light level is accumulated in this manner.  
         [0013]     Referring to the WDM optical transmission system, the following conventional technology is known. That is, Japanese Patent Application Laid-Open No. 2004-140631 discloses a wavelength multiplexing method of multiplexing wavelengths of the incident light with a plurality of wavelengths and outputting the light multiplexed; monitoring the output light multiplexed by an optical monitor to analyze each level of the wavelengths; and adjusting each incident light of the wavelengths multiplexed according to the analyzed levels of the wavelengths, for each wavelength, in a plurality of variable optical attenuators to be made to the same level as one another. In this method, output levels of the respective variable optical attenuators are detected to control respective attenuation amounts in the variable optical attenuators according to the output levels of the variable optical attenuators and the levels analyzed.  
         [0014]     In the conventional technology, careful consideration is not given to suppressing the accumulation of fluctuations in light levels in the configuration in which the nodes are connected in multiple stages. Therefore, when the number of connections of nodes is increased more and more, fluctuation in the level of a fine light from a node in the initial stage becomes largely accumulated, which may lead to error in a main signal.  
       SUMMARY OF THE INVENTION  
       [0015]     It is an object of the present invention to at least solve the problems in the conventional technology.  
         [0016]     A method according to an aspect of the present invention is a method for an intermediate node to control a level of a signal included in a wavelength-multiplexed signal and transmitted from a source node to a destination node via the intermediate node. The method includes: demultiplexing the wavelength-multiplexed signal to extract the signal; detecting a level of the signal; identifying a position of the intermediate node with respect to the source node; determining a control time based on the position; controlling, when the control time has elapsed from the detecting, a level of the signal based on the level detected at the detecting; and multiplexing the signal into the wavelength-multiplexed signal.  
         [0017]     An apparatus according to an aspect of the present invention functions as an intermediate node and controls a level of a signal included in a wavelength-multiplexed signal and transmitted from a source node to a destination node via the apparatus. The apparatus includes: a demultiplexing unit that demultiplexes the wavelength-multiplexed signal to extract the signal; a detecting unit that detects a level of the signal; an identifying unit that identifies a position of the apparatus with respect to the source node; a determining unit that determines a control time based on the position; a control unit that controls, when the control time has elapsed from when the level of the signal is detected by the detecting unit, a level of the signal based on the level detected by the detecting unit; and a multiplexing unit that multiplexes the signal into the wavelength-multiplexed signal.  
         [0018]     The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a diagram for explaining a wavelength multiplexing apparatus according to the present invention;  
         [0020]      FIG. 2  is a diagram for explaining a relevant part of the wavelength multiplexing apparatus;  
         [0021]      FIG. 3  is a diagram for explaining a sequence of a wavelength multiplexing method according to the present invention;  
         [0022]      FIG. 4  is a diagram of how fluctuations in light levels are suppressed in the present invention;  
         [0023]      FIG. 5  is a diagram for explaining the principle of how fluctuation in a light level is suppressed in the present invention;  
         [0024]      FIG. 6  is a diagram for explaining a conventional wavelength multiplexing apparatus;  
         [0025]      FIG. 7  is a diagram of how fluctuations in light levels are accumulated in a conventional technology; and  
         [0026]      FIGS. 8 and 9  are diagrams for explaining the principle of how fluctuation in a light level is accumulated in the conventional technology. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Exemplary embodiments of a wavelength multiplexing method and an apparatus therefor according to the present invention are explained in detail below with reference to the accompanying drawings.  
         [0028]      FIG. 1  is a diagram of an example of a WDM optical transmission system in which wavelength multiplexing apparatuses according to the present invention are connected in multiple stages. In a wavelength multiplexing apparatus  4   a  of a node  1 , a wavelength demultiplexer (DMUX)  41  demultiplexes an incident light from an input-side WDM transmission line  5   a  into lights of respective wavelengths. And a variable optical attenuator  46  of a VAT controller  42  controls levels of the lights so as to be fixed for each wavelength, and then, a wavelength multiplexer (MUX)  43  multiplexes again the lights of the wavelengths and outputs the light multiplexed to an output-side WDM transmission line  5   b.    
         [0029]     The VAT controller  42  braches part of the light output from the variable optical attenuator (VAT)  46 , and detects the level of the light branched, by a photodetector (PD)  47  such as a photodiode. A controller  48  of the VAT controller  42  performs feedback control on a light attenuation amount in the variable optical attenuator  46 , based on the level detected by the photodetector  47 . An optical supervisory channel (OSC) controller  44  of the wavelength multiplexing apparatus  4   a  receives, via the WDM transmission line  5   a , information for wavelengths and information to identify a position of a node from a wavelength multiplexing apparatus (not shown) of an immediately preceding node. A node position identifying unit  49  of the VAT controller  42  receives information to identify a position of its own node from the OSC controller  44 .  
         [0030]     The controller  48  of the VAT controller  42  controls a control time required for feedback control of the light attenuation amount in the variable optical attenuator  46  according to the position of the own node identified by the node position identifying unit  49 . More specifically, the controller  48  controls the time from when an output level of the variable optical attenuator  46  is detected by the photodetector  47  until the control according to the output level actually works. A relationship between the position of a node and a control time is previously stored in a memory  45  that is formed with a nonvolatile memory such as an Electrically Erasable Programmable Read-Only Memory (EEPROM). The OSC controller  44  transmits, via the WDM transmission line  5   b , information for wavelengths and information to identify the position of the node, to a wavelength multiplexing apparatus  4   b  of a next node.  
         [0031]     The wavelength multiplexing apparatus  4   b  of a node  2 , and wavelength multiplexing apparatuses  4   c  and  4   d  of respective node  3  and node  4  are the same as that of the wavelength multiplexing apparatus  4   a . In each of the nodes, the VAT controller  42  is provided for each wavelength, but, for simplicity of the figure,  FIG. 1  shows only one VAT controller  42  in each node. In  FIG. 1 , reference numerals  5   c ,  5   d , and  5   e  represent WDM transmission lines, respectively. Furthermore,  FIG. 1  shows the four stages, out of multiple stages, in which the nodes are connected to each other, but the number of connections of nodes is not limited. Therefore, nodes may be connected in five stages or more, or even two stages or three stages. These nodes connected in multiple stages construct a ring network or an open-type ring network.  
         [0032]      FIG. 2  is a diagram of a main portion of the wavelength multiplexing apparatus according to the present invention. The VAT controller  42  includes the variable optical attenuator  46 , the photodetector  47 , the controller  48 , and the node position identifying unit  49 . The node position identifying unit  49  includes a node position information receiver  50  and a control time receiver  51 . The node position information receiver  50  receives information to identify the position of its own node from the OSC controller  44 , and transmits a signal for selecting a control time corresponding to the information to identify the position of the own node.  
         [0033]     The memory  45  includes a control-time table storage unit  56 . For example, as shown in  FIG. 10 , the control-time table storage unit  56  stores a control-time table for defining control times corresponding to parameters for node positions, respectively. As an example, in the table shown in  FIG. 10 , the control times Ta, Tb, Tc, and Td are getting shorter in this order with a node position located in a more subsequent stage, in other words, following the ascending order of values (1, 2, 3, 4) to identify node positions.  
         [0034]     The control-time table storage unit  56  selects a control time selected by the node position information receiver  50 , from the control-time table, and transmits the control time selected to the control time receiver  51 . The control time receiver  51  sets the time required for controlling a light level by the variable optical attenuator  46 , in the control time received from the control-time table storage unit  56 . The controller  48  controls the variable optical attenuator  46  using the control time set by the control time receiver  51 .  
         [0035]     The OSC controller  44  includes an optical-to-electrical converter  60 , an inter-OSC communication data decompressor  61 , a node position adder  62 , a node positional setting unit  63 , a selector  64 , a node position information transmitter  65 , an inter-OSC communication data collector  66 , an optical signal monitor/controller  67  for each wavelength, an “Add” information receiver  68  for each wavelength, and an electrical-to-optical converter  69 . The optical-to-electrical converter  60  receives an optical signal for inter-OSC communication information sent from the OSC controller  44  of an immediately preceding node, and converts the optical signal to an electrical signal. The inter-OSC communication information converted to the electrical signal is transmitted to the inter-OSC communication data decompressor  61 .  
         [0036]     The inter-OSC communication data decompressor  61  extracts a value to identify a node position from the inter-OSC communication information received from the optical-to-electrical converter  60 . The value to identify a node position is stored in a data frame used for inter-OSC serial communication, for each wavelength. The value to identify a node position for each wavelength extracted is transmitted to the node position adder  62 . The remaining information, of the inter-OSC communication information received from the optical-to-electrical converter  60 , after the value to identify a node position for each wavelength is excluded is transmitted from the inter-OSC communication data decompressor  61  to the optical signal monitor/controller  67  for each wavelength.  
         [0037]     The node position adder  62  adds 1 to the value to identify the node position received from the inter-OSC communication data decompressor  61 , for each wavelength, to set the value as a value to identify the position of its own node, and transmits the value to identify the position of the own node to the selector  64 . On the other hand, the node positional setting unit  63  sets 1 as the value to identify the position of the own node, and transmits the value to the selector  64 .  
         [0038]     The Add information receiver  68  for each wavelength receives information, indicating that the own node is a node in the initial stage for a light with a wavelength, from an operator  7 , and sets the value of Add information for the light with the wavelength to, for example, 1. If the own node is not the node in the initial stage, the Add information receiver  68  sets the value of Add information to, for example, 0. The node in the initial stage indicates a node on a network to which light is initially input.  
         [0039]     When the value of Add information transmitted from the Add information receiver  68  for each wavelength is 0, the selector  64  selects the value (value of a preceding node position+1) received from the node position adder  62  provided for each wavelength, as a value to identify the position of the own node for each wavelength, and selects the value “1” received from the node positional setting unit  63  when the value of Add information is 1. The value to identify the position of the own node selected by the selector  64  is transmitted to the node position information transmitter  65 .  
         [0040]     The node position information transmitter  65  transmits the value to identify the position of the own node received from the selector  64 , to the inter-OSC communication data collector  66  and to the node position information receiver  50  of the VAT controller  42 . How to control the control time in the variable optical attenuator  46  based on the value to identify the position of the own node sent to the node position information receiver  50  is as explained above. The inter-OSC communication data collector  66  adds the value to identify the position of the node received from the node position information transmitter  65 , to the information received from the optical signal monitor/controller  67  for each wavelength, and transmits the value to the electrical-to-optical converter  69 . The electrical-to-optical converter  69  converts the electrical data received from the inter-OSC communication data collector  66  to an optical signal, and transmits the optical signal to the OSC controller  44  of the next node.  
         [0041]      FIG. 3  is a diagram of an example of a sequence in the wavelength multiplexing method. When light of a client (wavelength: λX) is input to the node  1 , the OSC controller  44  of the node  1  sets 1 in a node position parameter for λX in the data frame for the inter-OSC communication information (step S 1 ). The inter-OSC communication information including the node position parameter for λX is sent to the node  2  (step S 2 ).  
         [0042]     In the node  1 , the node position parameter for λX is also sent to the node position identifying unit  49  of the node  1  (step S 3 ). The node position identifying unit  49  of the node  1  selects, for example, 64 milliseconds (ms) from the control-time table in the control-time table storage unit  56  as a control time corresponding to the value of the node position parameter that is 1 (step S 4 ). And the controller  48  of the node  1  starts controlling the light attenuation amount based on the control time in the variable optical attenuator  46  for λX that is 64 ms (step S 5 ).  
         [0043]     In the node  2 , the node position parameter for λX in the data frame for the inter-OSC communication information is incremented by 1, to be set to 2 (step S 6 ). Inter-OSC communication information including the node position parameter for the λX is sent to the node  3  (step S 7 ). Furthermore, in the node  2 , the node position parameter for λX is also sent to the node position identifying unit  49  of the node  2  (step S 8 ).  
         [0044]     The node position identifying unit  49  selects, for example, 16 milliseconds (ms) from the control-time table in the control-time table storage unit  56 , as a control time corresponding to the value of the node position parameter that is 2 (step S 9 ). And the controller  48  of the node  2  starts controlling the light attenuation amount based on the control time in the variable optical attenuator  46  for λX that is 16 ms (step S 10 ).  
         [0045]     In the following, the same goes for the node  3  (steps S 11  to S 15 ) and the node  4  (steps S 16  to S 20 ). However, in the node  3 , the node position parameter for λX in the data frame for the inter-OSC communication information is 3 (step S 1 ), and the control time in the variable optical attenuator  46  corresponding to the value is 4 milliseconds (ms) (steps S 14 , S 15 ). Furthermore, in the node  4 , the node position parameter for λX in the data frame for the inter-OSC communication information is 4 (step S 16 ), and the control time in the variable optical attenuator  46  corresponding to the value is 1 millisecond (ms) (steps S 19 , S 20 ).  
         [0046]     In the embodiment of the present invention, the control time in the variable optical attenuator  46  in the respective nodes becomes ¼ of the control time in the variable optical attenuator  46  in the immediately preceding node. Therefore, as shown in  FIG. 4 , the fluctuation in the level of a fine light from the initial node can be suppressed from being accumulated in the node in the subsequent stage. As shown in  FIG. 4 , reference numeral  81  represents a waveform of the incident light to the node  1 ,  82  a waveform of the output light from the node  1  (incident light to the node  2 ),  83  a waveform of the output light from the node  2  (incident light to the node  3 ),  84  a waveform of the output light from the node  3  (incident light to the node  4 ), and  85  a waveform of the output light from the node  4 .  
         [0047]     The reason that the accumulation of the fluctuations in the light levels can be suppressed as shown in  FIG. 4  is explained below as compared with the conventional technology with reference to  FIG. 8  and  FIG. 9 , for simplicity. A relationship between the incident light  81  and the output light  82  in the node  1  is the same as that between the incident light  31  and the output light  32  in the conventional node  1  of  FIG. 8 . However, as shown in  FIG. 5 , in the node  2 , to suppress fluctuation of the incident light  82  to the node  2  at time C, the VAT controller  42  controls so as to suppress the amplitude of the incident light  82 , in the direction indicated by an arrow  88  and by the length of the arrow  88  at time C.  
         [0048]     In contrast to this, the control actually works as indicated by an arrow  89  in the same direction and by the same length as these of the arrow  88  at time C′ delayed by a time of ¼ of the control time in the node  1 . Therefore, the output light  83  from the node  2  becomes a wave such that the incident light  82  to the node  2  is changed by the length of the arrow  89 , and the amplitude decreases. The same goes to the node  3  and thereafter. In this manner, even if the stages of the nodes increase, the increase in the amplitude of the fluctuation can be suppressed.  
         [0049]     It should be noted that the present invention is not limited to the embodiments, and hence, various modifications are possible. For example, the control time in the variable optical attenuator  46  of the respective nodes is not limited to ¼ of the control time in the variable optical attenuator  46  of an immediately preceding node. However, if the control time in the variable optical attenuator  46  is made shorter following a node in a further subsequent stage, the effect of suppressing the amplitude of the fluctuation is higher, which is preferable. Furthermore, the present invention is also applicable to a ring network or a network in any form other than an open-type ring network.  
         [0050]     According to one aspect of the present invention, the control time required for controlling the output level of a light for each wavelength is set to a shorter time than the control time in the immediately preceding node, thereby suppressing the fluctuation in the level of light from the initial node, from being accumulated in the node in the subsequent stage. Thus, in the WDM optical transmission system in which the nodes are connected in multiple stages, the number of connections of nodes in multiple stages can be remarkably increased.  
         [0051]     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.