Abstract:
An optical transmission apparatus includes a demultiplexer for separating wavelength-division multiplexing light received from a first optical transmission line into signals of different wavelengths to transmit the signals to an outside and a multiplexer for multiplexing signals of different wavelengths received from the outside to transmit multiplexed signals to a second optical transmission line. An input check unit is provided for monitoring a power level of a signal separated by the demultiplexer and for providing an output indicative thereof. An output adjuster is provided for intercepting a signal from the outside so as to inhibit receipt of the signal from the outside by the multiplexer depending on the output of the input check unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to optical transmission equipment, and more particularly, to optical transmission equipment that prevents malfunction derived from communication of fault information between transceivers.  
         [0003]     2. Description of Related Art  
         [0004]     In the past, optical transmission systems have been designed on the assumption that audio signals are transmitted over a trunk line including multiple telephone lines, and requested to provide super-reliable, very long-distance, and high-definition performance. On the other hand, there is a demand for low-cost data transmission targeted on base-to-base communication in a firm or interconnection between local area networks (LANs). An optical transmission system designed for the low-cost data transmission has been demanded and actively introduced. The data transmission techniques are based on the Ethernet technology. The specifications for optical transceivers concerning the products and characteristics of the optical transceivers are made public so that products of a plurality of optical transceiver manufacturers will be compatible with one another. Moreover, when a plurality of vendors provide devices, modules, and pieces of equipment, a low-cost system can be realized. Some optical transceiver manufacturers apply unique specifications to their optical transceivers.  
         [0005]     The related art of the present invention will be described in conjunction with  FIG. 1  to  FIG. 5 .  FIG. 1  is a block diagram explanatory of the configuration of a conventional bidirectional optical transmission system.  FIG. 2  is an explanatory diagram concerning actions to be performed in the conventional bidirectional optical transmission system in case a fault takes place.  FIG. 3  is a state transition diagram explanatory of a fault notification facility to be included in a conventional optical transceiver.  FIG. 4  is a sequence diagram explanatory of the fault notification facility of the conventional optical transceiver.  FIG. 5  is a state transition diagram explanatory of another fault notification facility to be included in the conventional optical transceiver.  
         [0006]      FIG. 1  shows the configuration of a bidirectional optical transmission system employing two-conductor optical fibers. An optical transceiver  110  comprises an optical transmitter  111  and an optical receiver  112 , and an optical transceiver  120  comprises an optical transmitter  121  and an optical receiver  122 . The optical transceiver  110  and optical transceiver  120  are linked by two-conductor optical fibers  131  and  132 . Thus, optical transmission between two points is realized.  
         [0007]     Now, a case where a fault takes place on one of the communication links included in the bidirectional optical transmission system shown in  FIG. 1  will be discussed below. If the optical fiber  131  is broken or if the optical fiber  131  is incorrectly coupled to the optical receiver  122 , the optical receiver  122  cannot receive any optical signal. However, since the communication link of the optical fiber  132  is held intact, no problem occurs in reception by the optical receiver  112 . Therefore, although the fault has occurred, the optical transceiver  110  is unaware of the occurrence of the fault.  
         [0008]     In general, optical transceivers are designed so that if a fault occurs, a special signal will be transmitted in addition to data that should be conveyed. Referring to  FIG. 2 , transfer of signals in case of a fault will be described. In  FIG. 2 ( a ), for example, if a fault occurs on the optical fiber  131 , the optical receiver  122  detects the fault. In  FIG. 2 ( b ), the optical transmitter  121  initiates transmission of a first fault notification signal (hereinafter called a fault detection signal), which signifies that a fault has been detected, to the optical receiver  112 . When the optical receiver  112  detects the fault detection signal, the optical transceiver  110  recognizes occurrence of the fault. Furthermore, the optical receiver  120  having detected interception of a signal recognizes that both the remote transmitter  111  and local receiver  122  have detected the fault. Moreover, the optical receiver  110  having detected the fault detection signal recognizes that both the local transmitter  111  and remote receiver  122  have detected the fault. Thus, the optical transceivers  110  and  120  can locate a faulty component.  
         [0009]     Moreover, the optical transceiver  110  having received the fault detection signal must suspend data transfer because the fault has occurred downstream the local optical transmitter  111 . On the other hand, the optical transmitter  111  must continuously transmit a certain signal so that immediately after the faulty component linking the optical transmitter  111  and optical receiver  122  is recovered to enable communication, the fact that the faulty component is recovered can be recognized. Therefore, a signal other than the fault detection signal, which signifies that data transfer is suspended because the fault detection signal has been detected and a standby state is under way (hereinafter called a standby signal), is adopted as a second fault notification signal. This method is widely adopted. After the optical receiver  112  detects the fault detection signal as shown in  FIG. 2 ( b ), the optical transmitter  111  suspends data transfer and transmits the standby signal instead.  
         [0010]      FIG. 2 (C) shows a state established immediately after the faulty component is recovered. Since the faulty component is recovered, the optical receiver  122  detects the standby signal. When the standby signal is detected, the optical transmitter  121  resumes data transmission. In  FIG. 2 ( d ), when the optical receiver  112  receives data instead of the fault detection signal, the optical transmitter  111  resumes data transmission.  
         [0011]     As mentioned above, the optical transceiver  110  and optical transceiver  120  that are opposed to each other check occurrence of a fault and locate a faulty component. When recognizing that the faulty component has recovered, the optical transceivers resume bidirectional data communication.  
         [0012]     When the foregoing change in the state of an optical transceiver is summarized, it is plotted like the state transition diagram of  FIG. 3 . The normal state is state  0  in which data is transmitted. In this state, if an optical transceiver detects a fault, the optical transceiver changes the state thereof into state  1  and transmits the fault detection signal. In state  1  or state  0 , if the optical transceiver receives the fault detection signal, it changes the state thereof into state  2  and transmits the standby signal. In state  1  or state  0 , if the optical transceiver receives data or the standby signal, it returns to state  0  and resumes data transmission.  
         [0013]     Referring to the state transmission diagram, a procedure to be followed by optical transceivers in case a fault takes place and a procedure to be followed thereby after a faulty component is recovered will be described in conjunction with the sequence diagram of  FIG. 4 . In  FIG. 4 ( a ), if a fault occurs, the optical transceiver  120  detects the fault, changes states from state  0  to state  1 , and transmits the fault detection signal. Thereafter, the optical transceiver  110  detects the fault detection signal, changes states from state  0  to state  2 , and transmits the standby signal.  
         [0014]     In  FIG. 4 ( b ), after the faulty component is recovered, the optical transceiver  120  detects the standby signal. The optical transceiver  120  then changes states from state  1  to state  0  and resumes data transmission. The optical transceiver  110  then detects data, changes states from state  2  to state  0 , and resumes data transmission.  
         [0015]     An example of a facility for detecting a fault and recovering a faulty component, there is, for example, a fault notification facility to be adapted to the Ethernet having a throughput of ten gigabits per second. The Institute of Electrical and Electronic Engineers of the U.S. has stipulated as a standard IEEE802.3ae the specifications for the fault notification facility for the 10 Gbps Ethernet. This document reads “detection of a local fault” in place of “DETECTION OF FAULT” described in  FIG. 3 , reads “transmission of a remote fault signal” in place of “TRANSMISSION OF FAULT DETECTION SIGNAL” described in  FIG. 3 , reads “reception of the remote fault signal” in place of “RECEPTION OF FAULT DETECTION SIGNAL” described in  FIG. 3 , reads “transmission of an idle signal” in place of “TRANSMISSION OF STANDBY SIGNAL” described in  FIG. 3 , reads “reception of data or the idle signal” in place of “RECEPTION OF DATA OR STANDBY SIGNAL” described in  FIG. 3 , and describes that a faulty component is located and recovered according to the same mechanism.  
         [0016]     The fault detection facility that uses two signals of the fault detection signal and standby signal has been described so far. Improvement of safety using the fault detection facility has been discussed in many aspects. Referring to  FIG. 2 ( b ), the optical transmitter  111  continues transmission of a standby signal until a faulty component is recovered. Conceivable as the cause of the fault is the failure of the optical transmitter  111  or optical receiver  122 , of the breakage or incorrect coupling of the optical fiber  131 . Except the case where the optical transmitter  111  has failed, the standby signal may be released as an optical signal to a space outside equipment during a period during which a fault takes place or work of recovering a faulty component is in progress. As a means for minimizing the adverse effect of the release of the optical signal to the space outside equipment, a technique of suppressing the optical power of the standby signal has been proposed.  
         [0017]     For example, a document, “Evaluating Open Fiber Control” (Ken Herrity, [online], September, 2000, IEEE802.3ae 10 Gb/s Task Force Plenary Meeting, [retrieved on June, describes a technique for the 10 Gbps Ethernet for suppressing a means optical power by intermittently transmitting a standby signal.  FIG. 5  is a state transition diagram concerning the technique. When a fault detection signal is received, an optical transceiver changes the state thereof into state  2 . The standby signal is then transmitted. If data or the standby signal is not received for a certain period of time (T 1 ), a faulty component is recognized not to have been recovered. The optical transceiver then changes the state thereof into state  3 . In state  3 , transmission of the standby signal is suspended because there is a possibility that light is released to the space outside equipment over a downstream optical fiber (optical output is intercepted). However, as long as state  3  persists, when the faulty component is recovered, an opposite transceiver cannot receive the standby signal. Consequently, communication cannot be resumed. Therefore, the optical transceiver returns to state  2  again after elapse of a certain period of time (T 2 ) and transmits the standby signal. As long as the faulty component is not recovered, the state of the optical transceiver continuously changes between state  2  and state  3 . Optical powers are evened between an on period (T 1 ) during which light is propagated and an off period (T 2 ) during which light is intercepted. For example, if the T 1  and T 2  values are equal to each other, a mean optical power is a half of an original optical power. If the T 2  value is nine times larger than the T 1  value, the mean optical power is suppressed to be a one-tenth of the original optical power.  
         [0018]     In  FIG. 5 , even if a data signal or the standby signal is received in state  3 , state  3  is not changed to state  0 . This is because after the standby signal is transmitted in state  2 , since no response is returned within the certain period of time (T 1 ), negotiation or handshaking is thought to be reset at the same time when a transition is made to state  3 .  
         [0019]     Moreover, Japanese Unexamined Patent Application Publication No. 2001-217778 describes a method adopting as a standby signal a signal whose duty factory is small (short pulse train) and a technique for suppressing the power of the standby signal itself by employing a signal whose level or power itself is low. This method or technique refers to a case where a special signal whose power itself is different from that of a data signal or a fault detection signal is adopted as the standby signal to be transmitted in state  2  shown in the state transition diagram of  FIG. 3 .  
         [0020]     Japanese Unexamined Patent Application Publication No. 05-206945 describes an optical transceiver effective in extending the service life of a light-emitting device by disabling the light-emitting device from working when the absence of a main signal in two directions is found by monitoring the level of a received signal.  
         [0021]     Japanese Unexamined Patent Application Publication No. 2004-015084 describes wavelength-division multiplexing transmission equipment that prevents a deadlock from occurring between transponders.  
         [0022]     Japanese Unexamined Patent Application Publication No. 2003-110585 describes an Ethernet terminal that detects occurrence of a fault on a transmission line between terminals interconnected over the Ethernet and that even when disconnecting a link with an opposite terminal, does not notify the opposite terminal of the fact.  
         [0023]     Japanese Unexamined Patent Application Publication No. 2002-057635 describes optical signal monitoring equipment that when receiving a fault notification signal contained in an optical signal sent from upstream equipment, intercepts optical output to associated downstream equipment.  
         [0024]     Problems the present invention attempts to solve will be described in conjunction with  FIG. 6  to  FIG. 8 .  FIG. 6  is a block diagram of a wavelength-division multiplexing system having optical transceivers and pieces of wavelength-division multiplexing transmission equipment interconnected.  FIG. 7  and  FIG. 8  are sequence diagrams explanatory of a fault notification facility of each optical transceiver.  
         [0025]     In order to realize transmission of a larger throughput using an Ethernet optical transceiver, the use of the optical transceiver in combination with wavelength-division multiplexing (WDM) transmission equipment would prove effective. The WDM is a method of combining a plurality of optical signals having different wavelengths, and transmitting the optical signals over a single optical fiber. In the WDM, as the number of wavelengths to be multiplexed gets larger, a total transmission throughput increases proportionally. This permits an optical fiber to exhibit a large data-carrying capacity.  
         [0026]     When wavelength-division multiplexing transmission equipment and an optical transceiver are interconnected, the wavelengths of light to be transmitted by the optical transceiver are limited as described below. First, the bandwidth of light to be transmitted by the wavelength-division multiplexing transmission equipment is limited depending on the bandwidth of light to be transmitted over an optical fiber or the bandwidth of light to be amplified by an optical amplifier for long-distance transmission. Moreover, when the number of wavelengths to be multiplexed is increased, the difference between adjacent wavelengths gets smaller. This brings about a crosstalk between signals. Therefore, the wavelength of each signal must be strictly managed in the order of nanometers. As for the wavelength of each signal, any of specific wavelengths set in the form of, generally a “grid” is adopted. On the other hand, the wavelengths of signals to be transmitted by an optical transceiver that does not support wavelength-division multiplexing, such as, a general Ethernet optical transceiver are defined in the specifications for the optical transceiver to encompass an error of several tens of nanometers or more. Consequently, when the Ethernet transceiver is directly connected to the wavelength-division multiplexing transmission equipment, the crosstalk is intensified and the band use efficiency is deteriorated. At the worst, even reception may be hard to do.  
         [0027]     When an optical transceiver that does not support wavelength-division multiplexing transmission (that does not manage wavelengths in the order of nanometers) must be connected to wavelength-division multiplexing transmission equipment, a device called a transponder is connected between the optical transceiver and wavelength-division multiplexing transmission equipment in order to realize a configuration like the one shown in  FIG. 6 . Wavelength-division multiplexing transmission equipment  141  comprises a multiplexer  142  that multiplexes a plurality of wavelengths and a demultiplexer  143  that separates a signal, which has wavelengths multiplexed, into signals of different wavelengths. The wavelength-division multiplexing transmission equipment  141  is opposed to wavelength-division multiplexing transmission equipment  151 , which has the same components as the wavelength-division multiplexing transmission equipment  141 , by way of optical fibers  131  and  132 . A transponder  113  is interposed between an optical transceiver  110  and the wavelength-division multiplexing transmission equipment  141 . The transponder  113  comprises a transmission transponder  114  that converts a signal received from an optical transmitter  111  into a signal to be subjected to wavelength-division multiplexing, and a reception transponder  115  that converts a signal received from the wavelength-division multiplexing transmission equipment  141  to a signal that can be received by the optical transceiver.  
         [0028]     An optical signal sent from the optical transmitter  111  included in the optical transceiver  110  is transferred to the transmission transponder  114  included in the transponder  113 , and converted into a signal that has any of wavelengths managed in the form of a grid (managed in the order of nanometers) and that is supported by the wavelength-division multiplexing transmission equipment. The optical signal having the wavelength thereof converted falls on the multiplexer  142  included in the wavelength-division multiplexing transmission equipment  141 . The optical signal is then combined with other optical signal, whereby a wavelength-multiplexed signal is produced. The wavelength-multiplexed signal propagates along the optical fiber  131 , and then reaches a demultiplexer  153  included in the wavelength-division multiplexing transmission equipment  151 . The wavelength-multiplexed signal is then separated into signals of different wavelengths. The separated optical signals are transferred to the reception transponder  125  included in the transponder  123 , converted into signals supported by an optical transceiver, and then received by the optical receiver  122 .  
         [0029]     Even on the opposite side of the system, an optical signal sent from the optical transmitter  121  is received by the optical receiver  112  via the transponder  124 , multiplexer  152 , optical fiber  132 , demultiplexer  143 , and transponder  115 . Thus, when a transponder in which wavelengths are managed for the purpose of wavelength-division multiplexing is interposed between an optical transceiver in which wavelengths are not managed, such as, an Ethernet transceiver and wavelength-division multiplexing transmission equipment, transmission of a large throughput (Ethernet-based wavelength-division multiplexing transmission) can be realized inexpensively.  
         [0030]     Moreover, some transponders have a loading facility for performing encoding that is intended for error detection or error correction, signal addition that is adapted to a control signal to be transferred between transponders, or reshaping or reproduction of a wave. When this kind of transponder is employed, a certain delay time is produced between a received signal and a transmitted signal.  
         [0031]     In the system having the configuration shown in  FIG. 6 , when the optical transceiver  110  and optical transceiver  120  perform fault notification according to different state transition diagrams, that is, when the optical transceiver  120  performs fault notification according to the state transition diagram of  FIG. 3  and the optical transceiver  110  performs fault notification according to the state transition diagram of  FIG. 5 , malfunction may occur at the time of starting up the optical transceiver  110 . This phenomenon will be described below.  
         [0032]      FIG. 7  shows a recovery sequence to be followed when one of the optical transceivers that are included in the configuration shown in  FIG. 6  and connected opposite to each other, that is, the optical transceiver  110  is rebooted (restarted).  FIG. 7  also shows the state of the transmission transponder  114  connected to the optical transmitter  111 . For brevity&#39;s sake, the description of the actions of the reception transponder  125 , opposite transmission transponder  124 , and opposite reception transponder  115  will be omitted.  
         [0033]     When the optical transceiver  110  is rebooted, the opposite optical transceiver  120  detects a fault, changes states from state  0  to state  1 , and transmits a fault detection signal to the optical transceiver  110 . When the rebooting of the optical transceiver  110  is completed, the fault detection signal transferred from the opposite transmitter is received. The optical transceiver  110  changes the state thereof into state  2 , and transmits a standby signal to the optical transceiver  120 .  
         [0034]     Assume that a delay occurs in the transmission transponder  114  after reception of the standby signal until transmission thereof. If a delay in transmission of the standby signal occurs in the transmission transponder  114 , the optical receiver  120  delays by the delay time in detecting the standby signal and returning to state  0 . Consequently, the optical transceiver  110  delays in receiving a data signal. At this time, before the data signal reaches the optical transceiver  110 , if a certain period of time T 1  described in conjunction with the state transition diagram of  FIG. 5  elapses after the optical transceiver  110  enters state  2 , the optical transceiver  110  changes the state thereof into state  3 . Consequently, transmission of the standby signal is suspended and recovery work itself is suspended. The optical transceiver  110  suspends transmission during a certain period of time T 2 . Thereafter, the optical transceiver  110  returns to state  2  and transmits the standby signal. However, since the delay has occurred in the transmission transponder  14 , if the optical transceiver  110  cannot receive the data signal during the period of time T 1 , the optical transceiver  110  reenters state  3 . Transmission of the standby signal is suspended. The optical transceiver  110  repeats the same actions and falls into a loop state in which state  2  and state  3  are repeatedly alternated. Eventually, it becomes impossible to recover the optical transceiver  110  after rebooting.  
         [0035]     A delay occurring in a transponder is attributable partly to a startup time required by the transponder. Although no signal input has been detected in the transponder so far, if production of a signal input is initiated, the internal circuit of the transponder must be started in order to provide a signal output. This causes a delay. Moreover, when the transponder is recovered from the no-signal state, if human manipulations are required, a delay time is naturally produced until a worker autonomously performs recovery work. If a slow-start facility that does not abruptly transmit a large-power signal but increases power little by little is included, a delay occurs for a period of time required until the power is increased to the level permitting a receiver to recognize the signal.  
         [0036]     Conventionally, optical transceivers, transponders, pieces of wavelength-division multiplexing transmission equipment, and opposed stations included in a WDM system are manufactured by the same manufacturer. However, as far as Ethernet-based wavelength-division multiplexing transmission is concerned, if the transponders and pieces of wavelength-division multiplexing transmission equipment are manufactured by the same manufacturer, the wavelength-division multiplexing transmission equipment manufacturer is requested to provide a product to which diverse optical transceivers manufactured by numerous manufacturers can be connected.  
         [0037]     As described in the Japanese Unexamined Patent Application Publication No. 2001-217778, whichever of a method employing as a standby signal a signal (short pulse train) whose duty factor is small and a method employing a signal whose level or power itself is low is adopted, unless transponders support the method, recovery from a fault is impossible.  FIG. 8  shows a sequence to be followed when an optical transceiver is rebooted. When the optical transceiver  110  is rebooted, the optical transceiver  120  opposite to optical transceiver  110  detects a fault, changes states from state  0  to state  1 , and transmits a fault detection signal to the optical transceiver  110 . When the rebooting of the optical transceiver  110  is completed, the optical transceiver  110  receives the fault detection signal transferred from the opposite transmitter, enters state  2 , and transmits a standby signal to the optical transceiver  120 . Although the transmission transponder  114  receives the standby signal, if the transmission transponder  114  does not transmit the standby signal but intercepts transfer of the standby signal, a data signal is not returned to the optical transceiver  110 . Even in this case, after a period of time T 1  elapses, the optical transceiver  110  changes the state thereof into state  3 , and suspends transmission of the standby signal. After a period of time T 2  elapses, the optical transceiver returns to state  2  and resumes transmission of the standby signal. If the transmission transponder  114  intercepts transfer of the standby signal, the optical transceiver  110  reenters state  3 . Likewise, the optical transceiver  110  falls into a loop state in which state  3  and state  2  are repeatedly alternated, and is not recovered from a fault.  
         [0038]     The phenomenon that the transmission transponder  113  intercepts transfer of a standby signal takes place in a case where although a signal (short pulse train) whose duty factor is small or a signal whose level or power itself is low is adopted as the standby signal to be transmitted from the optical transceiver  110 , the transmission transponder  114  does not support the special standby signal and does not therefore recognize a received signal as an effective signal. In particular, if a special signal unique to a manufacturer of an optical transceiver is adopted as the standby signal, the transponder cannot deal with the signal.  
         [0039]     In order to solve the foregoing problem attributable to the interaction between the fault notification facility included in a transceiver and signal processing performed in a transponder, the fault notification facility of the transceiver must be improved and the signal delay occurring in the transponder must be overcome. Otherwise, the problem is solved by temporarily invalidating the fault notification facility itself. However, if the optical transceiver has already been incorporated in a router or optical transmission equipment, upgrading of the optical transceiver or modification of settings is often hard to do. Moreover, in an equipment installation site or the like, there is difficulty in modifying settings for lack of a satisfactory equipment setting environment or equipment setting data. These cases cannot be coped with by updating the optical transceiver or transponder.  
       SUMMARY OF INVENTION  
       [0040]     An object of the present invention is to provide optical transmission equipment permitting a transceiver, which has a fault notification facility, to recover from a fault even when the transceiver is connected to the optical transmission equipment.  
         [0041]     In bidirectional transmission, a photoreceiver that monitors the intensity of an optical signal sent over a first transmission line, and an output adjuster disposed on a path of the optical signal sent from an optical transceiver over a second transmission line are used to decrease the power level of the optical signal to transmitted over the second transmission line when the intensity of the received optical signal decreases to become lower than a predetermined intensity. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWING  
       [0042]      FIG. 1  shows a block diagram illustrating a conventional bidirectional optical transmission system.  
         [0043]      FIG. 2  shows an explanatory diagram concerning actions to be performed in case a fault occurs in the conventional bidirectional optical transmission system.  
         [0044]      FIG. 3  shows a state transition diagram explanatory of a fault notification facility of a conventional optical transceiver.  
         [0045]      FIG. 4  shows a sequence diagram explanatory of the fault notification facility of the conventional optical transceiver.  
         [0046]      FIG. 5  shows a state transition diagram explanatory of other fault notification facility of the conventional optical transceiver.  
         [0047]      FIG. 6  shows a block diagram of a wavelength-division multiplexing system explanatory of an object of the present invention.  
         [0048]      FIG. 7  shows a sequence diagram of a fault notification facility of an optical transceiver explanatory of the object.  
         [0049]      FIG. 8  shows a sequence diagram showing a sequence to be followed by the fault notification facility of the optical transceiver and being explanatory of the object.  
         [0050]      FIG. 9  shows a block diagram of a wavelength-division multiplexing system explanatory of an embodiment of the present invention;  
         [0051]      FIG. 10  shows a sequence diagram showing a sequence to be followed by the fault notification facility included in the optical transceiver and being explanatory of the embodiment of  FIG. 9 .  
         [0052]      FIG. 11  shows a block diagram of wavelength-division multiplexing equipment explanatory of the embodiment of  FIG. 9 .  
         [0053]      FIG. 12  shows a block diagram of wavelength-division multiplexing transmission equipment explanatory of another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0054]     Before beginning a detailed description of the subject invention, mention of the following is in order. When appropriate, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing figure drawings. Arrangements may be shown in block diagram form in order to avoid obscuring the invention, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the present invention is to be implemented, i.e., such specifics should be well within purview of one skilled in the art. In other instances, detailed descriptions of well-known methods and components are omitted so as not to obscure the description of the invention with unnecessary/excessive detail. Where specific details (e.g., circuits, flowcharts) are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that the invention can be practiced without, or with variation of, these specific details. Finally, it should be apparent that differing combinations of hard-wired circuitry and software instructions can be used to implement embodiments of the present invention, i.e., the present invention is not limited to any specific combination of hardware and software.  
         [0055]     Referring to drawings, a mode for implementing the present invention will be described below by presenting embodiments.  
         [0056]     An embodiment of the present invention will be described in conjunction with  FIG. 9  to  FIG. 12 .  FIG. 9  is a block diagram of a wavelength-division multiplexing system explanatory of the embodiment.  FIG. 10  is an explanatory diagram of the embodiment showing a sequence to be followed by a fault notification facility included in an optical transceiver.  FIG. 11  is a block diagram of a wavelength-division multiplexing transmission equipment explanatory of the embodiment.  FIG. 12  is a block diagram of wavelength-division multiplexing transmission equipment explanatory of a variant embodiment.  
         [0057]     Referring to  FIG. 9 , an optical signal sent from an optical transmitter  11  included in an optical transceiver  10  is temporarily transferred to a transmission transponder  14  included in a transponder  13 . The transmission transponder  14  converts the wavelength of the optical signal into any of wavelengths managed in the form of a grid (in the order of nanometers) and supported by wavelength-division multiplexing transmission equipment. The resultant optical signal is supplied to a multiplexer  42  via an output adjuster  47  included in wavelength-division multiplexing transmission equipment  41 , and is combined with an other signal to produce a wavelength-multiplexed signal. The wavelength-multiplexed signal propagates along an optical fiber  31 , and then reaches a demultiplexer  53  included in wavelength-division multiplexing transmission equipment  51 . The wavelength-multiplexed signal is separated into signals of different wavelengths, and then transferred to a reception transponder  25  included in a transponder  23 . After the resultant signals are converted into signals supported by an optical transceiver, they are received by an optical receiver  22  included in an optical transceiver  20 . Moreover, part of the signals separated by the demultiplexer  53  is branched out by a photocoupler  54  and routed to a photoreceiver  55 .  
         [0058]     On the opposite side of the system, an optical signal sent from an optical transmitter  21  is transferred to a transmission transponder  24 . The transmission transponder  24  converts the wavelength of the optical signal to any of wavelengths managed in the form of a grid, and transfers the resultant optical signal to the wavelength-division multiplexing transmission equipment  51 . Herein, the transmitted signal reaches an optical multiplexer  52  via an output adjuster  57 . The signal is then combined with an other signal to produce a wavelength-multiplexed signal. The wavelength-multiplexed signal propagates along an optical fiber  32  and reaches a demultiplexer  43 . The wavelength-multiplexed signal is then separated into signals of different wavelengths. The separated optical signals are transferred to a reception transponder  15  and converted into signals supported by an optical transceiver. The resultant signals are received by the optical receiver  12 .  
         [0059]     An electrical signal sent from the photoreceiver  55  is transferred to an input check circuit  56 . The input check circuit  56  checks the electrical signal to see if the optical power level agrees with a certain value. If the optical power level is equal to or smaller than a certain reference value, the output adjuster  57  is controlled in order to intercept an optical signal to be transmitted. The description of a photocoupler  44 , a photoreceiver  45 , and an input check circuit  46  included in the wavelength-division multiplexing transmission equipment  41  is omitted. The photocoupler  44 , photoreceiver  45 , and input check circuit  46  act in the same manner as the photocoupler  54 , photoreceiver  55 , and input check circuit  56  included in the wavelength-division multiplexing transmission equipment  51 .  
         [0060]     Both of the optical signal sent from the optical transmitter  11  and the optical signal sent from the transmission transponder  14  are continuous signal light. Therefore, some signal is transmitted even in a standby (idle) state, and signal light will never cease. The same applies to the optical signal sent from the other optical transmitter  21  and the optical signal sent from the transponder  24 .  
         [0061]      FIG. 10  shows a sequence to be followed for rebooting when the optical transceiver  10  is supposed to perform fault notification according to the state transition diagram of  FIG. 5  and the optical transceiver  20  is supposed to perform fault notification according to the state transition diagram of  FIG. 3 .  FIG. 10  also shows the state of the output adjuster  57  interposed between the transmission transponder  24  and multiplexer  52 . The illustration of the actions of the output adjuster  47  is omitted for brevity&#39;s sake.  
         [0062]     When the optical transceiver  10  is rebooted, the optical transceiver  20  detects a fault, changes states from state  0  to state  1 , and transmits a fault detection signal to the optical transceiver  10 . When the optical transceiver  10  is rebooted, an input to the photoreceiver  55  ceases at the same time. The input check circuit  56  detects interception of an input. This causes the output adjuster  57  to change the state thereof into an output off state. Consequently, the fault detection signal sent from the optical transceiver  20  to the optical transceiver  10  is intercepted by the optical adjuster  57 .  
         [0063]     On the other hand, although the rebooting of the optical transceiver  10  is completed, a signal sent from an opposite transmitter is not detected. The optical transceiver  10  therefore enters state  1 . Consequently, the optical transceiver  10  transmits a fault detection signal to the optical transceiver  20 . After a delay occurs in the transponder, the fault detection signal reaches the optical transceiver  20 . First, the photoreceiver  55  receives the fault detection signal, and the input check circuit  56  recognizes that input light is recovered. This causes the output adjuster  57  to change the state thereof into an output on state. When the fault detection signal reaches the optical transceiver  20 , the optical transceiver  20  receives the fault detection signal. Consequently, the optical transceiver  20  changes the state thereof into state  2  and initiates transmission of a standby signal. Since the output adjuster  57  has already entered the output on state, the standby signal passes through the output adjuster  57  and heads for the optical transceiver  10 .  
         [0064]     At this time, the optical transceiver  10  is in state  1 . Therefore, whichever of the transponders causes the fault detection signal or standby signal to delay, the optical transceiver  10  will not change the state thereof into state  3 . When the optical transceiver  10  receives the standby signal from the optical transceiver  20 , the optical transceiver  10  is reset to state  0 . The reset optical transceiver  10  resumes data transfer. When a data signal reaches the optical transceiver  20 , the optical transceiver  20  is also reset to state  0  and resumes data transfer. Thus, even when a delay occurs in a transponder, both the optical transceivers are recovered to a state in which they can transfer data.  
         [0065]     According to the present embodiment, when a fault is defected on a link with an opposite Ethernet optical transceiver, control is implemented so that the opposite Ethernet optical transceiver will transmit a fault detection signal.  
         [0066]     Incidentally, a response time constant (control time constant) required by the input check circuit  56  is determined to meet a condition that the interception performed by the output adjuster  57  should work so that the optical transceiver  10  having been rebooted immediately previously will not receive a fault detection signal. In consideration of the fact that signal light is not a burst signal but is continuous light, the interception may be a slow action that requires about several hundreds of milliseconds.  
         [0067]     Referring to  FIG. 11 , the input check circuit and output adjuster shown in  FIG. 9  will be explained in detail. The input check circuit comprises two comparators  61  and  62  and a functional mask circuit  65 . The output adjuster is realized with an optical amplifier  70 . The reason why the optical amplifier  70  is used to adjust an output is that compact optical amplifiers are disposed in association with wavelengths on the input stage of the wavelength-division multiplexing transmission equipment  51  in order to even the levels of the signals that have the wavelengths and are transferred to the demultiplexer  52  (output adjustment). The optical amplifier  70  is used for output adjustment. The optical amplifier  70  comprises an erbium doped optical fiber (EDF)  71 , a pumping source  72  for supplying excitation light with which the EDF  71  is excited, a WDM coupler  73  for combining the excitation light with signal light sent from the transponder  24 , and an optical amplifier control circuit  74  that feeds a control current to the pumping source  72 .  
         [0068]     The reason why the optical amplifier  70  can cause a signal output to go off or decay will be described. As long as sufficient power (for example, several tens of milliwatts) of excitation light is supplied to the EDF  71 , the optical amplifier  70  amplifies signal light. However, when power of excitation light is not sufficiently supplied, a loss occurring when the light passes through the EDF  71  exceeds a gain to attenuate signal light. The present embodiment makes the most of this characteristic of the optical amplifier  70 .  
         [0069]     Referring back to  FIG. 11 , a monitor voltage proportional to optical power transferred to the photoreceiver  55  is transferred to the comparators  61  and  62 . The comparator  61  compares the monitor voltage with a first reference voltage equivalent to an input of −20 dBm of the transponder  25 . If the monitor voltage falls below the first reference voltage, the comparator  61  recognizes interception of a signal, and transmits an output interception signal to the optical amplifier control circuit  74 . In response to the output interception signal, the optical amplifier control circuit  74  decreases an excitation current to be fed to the pumping source  72 . Consequently, signal light sent from the transponder  24  is attenuated to have optical power, with which the opposite equipment recognizes interception of an output, by means of the optical amplifier  70 .  
         [0070]     The comparator  62  compares the monitor voltage with a second reference voltage equivalent to an input of −15 dBm of the transponder  25 . If the monitor voltage exceeds the second reference voltage, the comparator  62  recognizes recovery from signal interception, and transmits an output return signal to the optical amplifier control circuit  74 . In response to the output return signal, the optical amplifier control circuit increases an excitation current to be fed to the pumping source  72 . Thus, the optical amplifier  70  performs output adjustment so that signal light sent from the transponder  24  will be provided as a predetermined optical output (for example, 0 dBm).  
         [0071]     The functional mask circuit  65  is interposed between the comparators  61  and  62  and the optical amplifier control circuit  74 . The functional mask circuit  65  can disable transmission of the output interception signal or output return signal to the optical amplifier control circuit. This is intended to eliminate the possibility that since the occasion on which the present embodiment is set to work is limited to the time of startup of the equipment or the time of troubleshooting, malfunction of the input check circuit adversely affects normal operation.  
         [0072]     According to the present embodiment, there is provided wavelength-division multiplexing transmission equipment that permits a transceiver, which has a fault notification facility, to recover from a fault even when the transceiver is connected to the equipment.  
         [0073]     The reason why the present embodiment includes two comparators is to stabilize the action of the input check circuit by differentiating a reference voltage based on which interception of a signal is recognized from a reference voltage based on which recovery of a signal is recognized. Alternatively, one comparator may be employed and reference voltages may be switched. Moreover, a fiber amplifier is adopted as the output adjuster. Alternatively, a semiconductor amplifier or a variable attenuator will do. Moreover, the circuit blocks have been described as if they are analog circuits. Alternatively, the functions of the circuit blocks may be realized with digital circuits or software controls.  
         [0074]     Referring to  FIG. 12 , an embodiment slightly different from the foregoing embodiment will be described below. The present variant embodiment adopts an input check circuit  58 , which is incorporated in the reception transponder  25 , as a circuit that checks if there is an input from an opposite equipment.  
         [0075]     In short, a transponder has a photoelectric conversion circuit and an electro-optic conversion circuit connected in series with each other. Consequently, a voltage level resulting from photoelectric conversion is used to discriminate interception of a signal from recovery of a signal. The transponder  25  includes the input check circuit  58  composed of the comparators  61  and  62  shown in  FIG. 11  and, if necessary, the functional mask circuit  64 . The input check circuit  58  and optical amplifier  70  are electrically connected to each other in order to transfer a control signal. The wavelength-division multiplexing transmission equipment and transponder are often installed in the same room, and the combination of the wavelength-division multiplexing transmission equipment and transponder may therefore be called the wavelength-division multiplexing transmission equipment.  
         [0076]     According to the present invention, there is provided wavelength-division multiplexing transmission equipment permitting a transceiver, which has a fault notification facility, to recover from a fault even when the transceiver is connected to the equipment. The wavelength-division multiplexing transmission equipment directly monitors optical power of an input to a transponder, and can therefore check an event highly precisely. Moreover, the photocoupler and photoreceiver included in the wavelength-division multiplexing transmission equipment in accordance with the aforesaid embodiment are unnecessary.  
         [0077]     Furthermore, even when the input check circuit is incorporated in the optical receiver  22  instead of the transponder  25 , the present invention is applicable. Moreover, the input check circuit  58  may be included in the transponder  25 . Both the input check circuit  58  and output adjuster  57  may be included in the transponder  25 .  
         [0078]     Except a case where the input check circuit  58  included in the reception transponder is used to check if an optical signal is transferred from opposite equipment, the presence of the reception transponders  25  and  15  shown in  FIG. 9  is not a must. Even when the reception transponders  25  and  15  are not included, the present invention works without a problem.  
         [0079]     A case where a signal whose duty factor is small (short pulse train) or a signal whose level or power itself is low is adopted as a standby signal to be sent to the optical transceiver  10  will be discussed below. Since the transmission transponder  14  does not support the special standby signal, if the special standby signal is intercepted, it is possible to recover equipment by applying the present invention. This is because, referring to the sequence diagram of  FIG. 10 , the optical transceiver  10  does not enter state  2  and does not therefore transmit a standby signal.  
         [0080]     The aforesaid facility need not always be active. If a fault attributable to a unique fault notification facility of an optical transceiver or the input/output specifications for a transponder takes place, a faulty component must be recovered. In this case, the aforesaid facility can be validated automatically or temporarily validated through operation performed by an installation worker.  
         [0081]     According to the present invention, there is provided optical transmission equipment permitting a transceiver, which has a fault notification facility, to recover from a fault even when the transceiver is connected to the optical transmission equipment.