Wavelength-division multiplex communication system and apparatus

In the WDM communication system, in the absence of data to be transmitted on an optical channel, a pilot signal transmitting means transmits the channel-unique pilot signal data on the channel. A WDM communication apparatus on a receiving end detects the pilot signal data in the received WDM signals, and on the basis of the detection result, it evaluates whether each channel is used or not. It is thus easy to recognize which channel is unused (idle) of the WDM signals, so that channel resources can be used effectively according to the traffic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength-division multiplex communication system and wavelength-division multiplex communication apparatus for use in the system.

2. Description of the Related Art

FIG. 27shows an example of an existing wavelength-division multiplex (WDM) communication system. Referring toFIG. 27, a WDM communication system100has terminal equipment,200and400, coupled to a communication network, such as an ATM (Asynchronous Transfer Mode) network, a SONET (Synchronous Optical Network), an SDH (Synchronous Digital Hierarchy) network, or an Ethernet (a registered trademark of Xerox Corporation), and an optical repeater300which is disposed between the terminal equipment,200and400, and is coupled to them through an optical transmission path (an optical fiber such as a SMF (Single Mode Fiber)),500and600. The optical repeater300amplifies and relays WDM signals bi-directionally transmitted over the optical transmission path500and600. The number of the optical repeaters (ILA)300required depends on a distance over which the WDM signals have to travel.

The terminal equipment (hereinafter simply called “terminal”)200(400) has, for example, a transponder (transceiver)201(401), a wavelength-division multiplexer (WDM coupler)202(402), and an optical amplifier,203(403) and204(404), such as an EDFA (Erbium Doped Fiber Amplifier). The optical repeater (hereinafter also called “repeater station”)300has a relay light amplifier,301and302, such as an EDFA.

In the WDM communication system100, the transponder201(401) of the terminal200(400) receives transmission signals through a required network (an ATM network, a SONET, an SDH network, or an Ethernet) and converts the received signals into light signals at predetermined wavelengths (channels). The WDM coupler202(402) multiplexes the resulting channels into WDM signals. The optical amplifier203(403) then simultaneously amplifies the WDM signals up to predetermined signal levels (power), and the amplified WDM signals are sent out on the optical transmission path500(600).

The WDM signals thus sent out on the optical transmission path500(600) are input to a repeater station300. The optical amplifier301(302) simultaneously amplifies the weakened WDM signals up to desired signal levels, thereby compensating for losses caused during transmission, and the amplified WDM signals are received by an opposing terminal equipment400(200) on a receiving end. In the receiver terminal400(200), an optical amplifier404(204) simultaneously amplifies the WDM signals, which have been received through the optical transmission path500(600), once again, thereby compensating for losses caused during transmission. A demultiplexer405(205) separates the amplified WDM signals into channels at wavelengths, and each of the thus separated channels is converted into an electric signal by the transponder401(201). The resulting electric signals are then sent out as transmission signals on a desired network, such as an ATM network, a SONET, an SDH network, or an Ethernet.

In such a WDM communication system100, it is better known that transmission loss varies with the wavelength transmitted, across the transmission band of the optical transmission path500(600). As a countermeasure, the spectrum of output or input WDM signals is monitored to separately adjust (pre-emphasis control) the transmission levels of light signals at different wavelengths (channels), so that a tilt caused in the WDM signals can be compensated for.

For this purpose, as shown inFIG. 28, for example, in the terminal200(400), the optical coupler208(408) and209(409) splits off part of the output of optical amplifiers203(403) and204(404), respectively, into a spectrum analyzer (SAU: Spectrum Analyzer Unit)210(410), where the monitoring of the spectrum of the output or input WDM signals is performed. Likewise, in the repeater station300, as shown inFIG. 29, the optical couplers304and305split off part of outputs of the optical amplifiers301and302, respectively, into an SAU306, where the monitoring of the spectrum of the output or input WDM signals is performed.

In accordance with the monitoring result, a CPU706of the SAU,210(410) and306, (described later) generates required monitoring control information (pre-emphasis setting information, or the like), and this monitoring control information is superimposed on an optical channel that is previously assigned as an OSC, by an OSC section219(419) in the terminal200(400) and by an OSC section307(or310) in the repeater station300. An optical coupler,220(420) and308, then inserts the optical channel as part (an OSC channel signal) of the WDM signals to be transmitted on an optical transmission path,500and600.

In the terminal200(400) ofFIG. 28, the reference number “206(406)” designates an optical variable attenuator for controlling the transmission level of a transmission light signal input from the transponder201(401). The reference number “207(407)” designates a photodiode (PD) for receiving the light signal, which has been controlled in transmission level by the optical variable attenuator206, and converting the light signal into an electric signal, and then inputting the resulting electric signal both into an optical circuit212(412) (described later) and into an OSC219(419). The reference number “221(421)” designates an optical coupler for splitting off part of the WDM signals, which is received through the optical transmission path600, into the OSC section219(419).

Further, in the repeater station300ofFIG. 29, the reference number “303(309)” designates an optical coupler for receiving the WDM signals input through the optical transmission path500(600) and splitting off part of the WDM signal into an OSC section (309) for the purpose of receiving an OSC channel signal. The reference number “311” designates an optical coupler for inserting an OSC channel signal into an output WDM signal to be sent out on an optical transmission path600.

Concretely, each of the SAU210(410) of the terminal200(400) inFIG. 28and the SAU306of the repeater station300inFIG. 29, has a switch701, an optical circuit employing a PD array703, an analog amplifier704, an AD (Analog to Digital) converter705, a CPU706, a bias circuit707, and a DA (Digital to Analog) converter708.

With this construction, in the SAU,210(410) and306, in response to an instruction given by the CPU706, the switch701selects one of the two inputs, the WDM signals to be sent out on an optical transmission path500and the WDM signals to be sent out on an optical transmission path600, to output to an optical circuit702. The input WDM signals at separate wavelengths are then converted by the PD array703into electric signals. At that time, the CPU706adjusts, if necessary, a bias applied to the PD array703, via the DA converter708.

The analog amplifier704amplifies the thus obtained electric signals up to desired levels, and the AD converter705then converts the electric signals into digital form to input to the CPU706. The CPU706analyzes the spectrum of the input WDM signal on each channel, or each WDM signal to be sent out on an optical transmission path500or600, to generate required monitoring control information.

The above-described SAU210(410), however, employs an highly expensive optical circuit702(PD array703), thereby significantly increasing costs for manufacturing the terminal200(400) and the repeater station300.

Further, since the WDM communication apparatus (terminal200(400) or repeater station300) combines optical channels at separate wavelengths into WDM signals, increase in line traffic on a specific channel causes access congestion in the channel, thus impairing the transmission efficiency. Hence, even in a communication apparatus, such as a WDM communication apparatus, with a large amount of transmission capacity, if the traffic is increased, it will affect better use of the transmission capacity.

Therefore, in a case where an apparatus is coupled to the WDM communication apparatus through a WDM line, the access should be balanced among the channels. As is evident fromFIGS. 28 and 29, however, since the WDM communication apparatus itself never converts light signals into electric form, it is impossible for the apparatus to recognize how much traffic the individual channels bear.

SUMMARY OF THE INVENTION

With the foregoing problems in view, it is an object of the present invention to provide a wavelength-division multiplex (WDM) communication system and a WDM communication apparatus in which the monitoring of the spectrum of WDM signals is realized without using any expensive optical circuits, and also in which unused (idle) channels are easily identified, so that effective use of channel resources according to traffic can be realized.

In order to accomplish the above object, according to the present invention, the present WDM communication system comprises a plurality of wavelength-division multiplex (WDM) communication apparatus, interconnected one another across an optical network, for transmitting a plurality of WDM optical channels at separate wavelengths, and of the plurality of WDM communication apparatus,

a first WDM communication apparatus which sends out WDM optical channels, includes: (1) means for evaluating whether or not data to be transmitted on an object one of the plurality of WDM optical channels exists; and (2) means for transmitting, if the evaluating means judges that data to be transmitted on the object WDM optical channel is absent, pilot signal data which is unique to the object WDM optical channel, and

a second WDM communication apparatus which receives the WDM optical channels from the first WDM communication apparatus, includes: (3) means for detecting the pilot signal data in the received WDM optical channels; and (4) means for evaluating whether or not each of the plurality of WDM optical channels is idle, based on detection results obtained by the pilot signal detecting means (3).

As a generic feature, the preset WDM apparatus comprises: (1) means for evaluating whether or not data to be transmitted on an object one of the plurality of WDM optical channels exists; and (2) means for transmitting, if the evaluating means judges that data to be transmitted on the object WDM optical channel is absent, pilot signal data which is unique to the object WDM optical channel.

As a preferred feature, the pilot signal transmitting means includes a framing section for framing a frame signal containing an overhead and a payload, which framing section stores the transmission data in the payload of the frame signal. The framing section includes a pilot signal adding section for storing the pilot signal data in either one or both of the overhead and the payload, if the evaluating means judges that data to be transmitted on the object WDM optical channel is absent.

As another generic feature, the present WDM apparatus comprises: (1) means for detecting a channel-unique pilot signal data, which is transmitted on an idle WDM optical channel with no data being carried thereon, in received WDM optical channels; and (2) means for evaluating whether or not each of the plurality of WDM optical channels is idle, according to detection results obtained by the pilot signal detecting means.

As another preferred feature, the present WDM apparatus further comprises means for measuring a spectrum of each of the WDM optical channels, and the foregoing pilot signal detecting means includes: an opto-electric converter for receiving the plurality of WDM optical channels at separate wavelengths and outputting an electric signal corresponding to optical power of the channels; and a pilot-signal-detecting filter section for transmitting, of the electric signal received from the opto-electric converter, a pilot signal data component unique to an idle WDM optical channel, which currently carries no data. The spectrum measuring means measures the spectrum of each of the WDM optical channels according to amplitude information of the pilot signal data component which passes through the pilot-signal-detecting filter section.

As a still another generic feature, the present WDM communication apparatus comprises:

a transmitter, including: means for evaluating whether or not data to be transmitted on an object one of the plurality of WDM optical channels exists; and means for transmitting, if the evaluating means judges that data to be transmitted on the object WDM optical channel is absent, pilot signal data, which is unique to the object WDM optical channel, to a second WDM communication apparatus of the wavelength-division multiplexed communication system;

a receiver, including: means for detecting pilot signal data in WDM optical channels which are received from the second WDM communication apparatus; and means for evaluating whether or not each of the plurality of WDM optical channels is idle, based on detection results obtained by the pilot signal detecting means; and

means for stopping, upon detection of the pilot signal data by the pilot signal detecting means of the receiver, the pilot signal transmitting means from transmitting the pilot signal data on an WDM optical channel which corresponds to the detected pilot signal.

As a further generic feature, the present WDM communication apparatus comprises: means for detecting pilot signal data, which indicates absence of data being transmitted on a WDM optical channel, in the plurality of WDM optical channels, including: an opto-electric converter for receiving the plurality of WDM optical channels at separate wavelengths and outputting an electric signal corresponding to optical power of the channels; and a pilot-signal-detecting filter section for transmitting, of the electric signal received from the opto-electric converter, a pilot signal data component unique to an idle WDM optical channel, which currently transmits no data; and means for measuring a spectrum of each of the WDM optical channels according to amplitude information of the pilot signal data component which passes through the pilot-signal-detecting filter section.

As still another preferred feature, the pilot-signal-detecting filter section includes: a lowpass filter for blocking, of an electric signal received from the opto-electric converter, a frequency component higher than the pilot signal data component; and a plurality of bandpass filters, one for each group of WDM optical channels. Each of the bandpass filters has variable passbands and transmitting pilot signal data components of output of the lowpass filter, which components correspond to the passbands of the individual bandpass filters.

As a further preferred feature, the spectrum measuring means includes: an error-correction-factor calculating section for calculating an error correction factor such that, if inputs of a same channel enter the plurality of variable bandpass filters at initialization of the apparatus, a same pilot signal data component is output from each of the plurality of variable bandpass filters; and an error correcting section for correcting errors in output of the plurality of variable bandpass filters based on the error correction factor, which has been calculated by the error-correction-factor calculating section.

As a still further preferred feature, the spectrum measuring section includes: a holder circuit for holding such amplitude information of the individual WDM optical channels separately; and an optical signal quality calculating section for calculating quality of the individual WDM optical channels based on amplitude information of the individual WDM optical channels which is obtained while the channels are idle, which amplitude information is stored in the holder circuit.

The WDM communication system and apparatus of the present invention guarantee the following advantageous results.

(1) A wavelength-division multiplex (WDM) communication apparatus on a transmitting end uses an idle (with no transmission data superimposed thereon) one of the optical channels of wavelength-division multiplex (WDM) signals to transmit pilot signal data unique to the idle channel, and the pilot signal data is detected by another wavelength-division multiplex (WDM) communication apparatus on a receiving end. It is thus possible to recognize which channels of the WDM signals are busy/idle, so that active communication control according to the channel busy/idle state can be realized. For example, since an OSC of a WDM communication apparatus notifies another WDM communication apparatus of the channel busy/idle state, it becomes possible for the latter apparatus to use an idle channel to newly establish communication.

(2) The input WDM signals are converted into electric signals corresponding to optical power of the WDM signals, and of the resulting electric signals, only a channel-unique pilot signal data component passes through a pilot signal detecting filter section (lowpass filter and bandpass filter). According to amplitude information of the component which passed through the filter section, the spectrum of each WDM channel is measured, so that spectrum measurement of the WDM signals can be realized without using any expensive optical circuit.

(3) On spectrum measurement, where a pilot signal data is transmitted on an idle channel with no data superimposed thereon, a holder circuit pre-stores the foregoing amplitude information for each wavelength separately, and on the basis of each item of the amplitude information that is obtained while each channel is idle, signal quality of the WDM optical signals is calculated. Hence, even if pilot signals cannot be detected in real time on all of the channels concurrently, it is still possible to measure the spectrum of the input WDM signals in a normal way.

(4) With the elapse of a predetermined maximum available period that has been previously set for a channel, data transmission on that channel is halted, thus making the channel idle, and pilot signal data is then transmitted on the channel for a specific period. With such a construction, if pilot signal data is not detected on a receiving end in the predetermined time period, the channel corresponding to that pilot signal data is regarded as being in a state of input interruption (loss-of-signal). As a result, there is no need for a dedicated loss-of-signal state detecting function, and an equivalent function will be realized utilizing the above-described pilot signal data.

(5) The WDM signals on multiple channels are divided into several channel groups, and variable bandpass filters are provided, one for each channel group, to detect pilot signal data corresponding to each channel group. At initialization of the apparatus, an error correction factor is calculated such that, if inputs of one and the same channel enter the variable bandpass filters, the same pilot signal data component is output from each of the variable bandpass filters. While the apparatus is in operation afterward, errors appearing in output of the variable bandpass filters can be corrected based on the thus obtained error correction factor. As a result, in a case where the variable bandpass filters are arranged, one for each channel, in parallel, it is possible to correct detection errors among the channels, thereby realizing spectrum measurement with improved accuracy.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

One preferred embodiment of the present invention will now be described with reference to the accompanying relevant drawings.

FIG. 1shows a construction of terminal equipment for use in a wavelength-division multiplex (WDM) communication system according to a first embodiment of the present invention, andFIG. 2shows a construction of a repeater station for use in the WDM communication system. Terminal equipment (hereinafter also simply called “terminal”)1is equivalent to terminal equipment200and400ofFIG. 27, and a repeater station3is equivalent to a repeater station300ofFIG. 27. As in the prior art, the terminal equipment1and the repeater station3are interconnected to one another across an optical transmission path500,600, in the present embodiment.

Referring now toFIG. 1, terminal1of the present embodiment has a transponder (transceiver section)11; variable optical attenuators12, one for each WDM channel; a wavelength-division multiplexer (beam combining coupler)13; optical amplifiers14and20; monitor beam splitting couplers15and21; an OSC beam inserting coupler16; a spectrum analyzer unit (SAU)17; an OSC unit18; OSC beam splitting coupler19; and a wavelength-division demultiplexer (beam splitting coupler)22.

Here, the transponder11receives a signal through some network such as Gigabit Ethernet (a registered trademark of Xerox Corporation), SONET/SDH, and ATM, and converts the received signal into a light signal at a predetermined wavelength, and then sends the converted light signal to a WDM communication system. In the mean time, the transponder11also receives WDM signals at separate wavelengths (hereinafter called “WDM optical channels”, or simply “WDM channels”) from a WDM communication system, and then converts the received channels into signals suitable for use in communication over the above-mentioned networks. In the present embodiment, a transmitter of the transponder11has, for example, a terminator section111, a data-framing and pilot-signal-inserting section112, a light source (LD: Laser Diode)113, an optical switch114, and an optical modulating and port-switching section115.

The terminator section111temporarily terminates a signal frame which is transmitted over the above network, and the data-framing and pilot-signal-inserting section112stores signal data—(user) data to be transmitted on a wavelength-division multiplexed channel-input from the terminator section111into a predetermined transmission signal frame (a frame construction will be detailed later). In the present embodiment, if such valid user data to be transmitted is absent, fixed-pattern data (pilot signal data) is stored in the frame.

With this construction, in a case where the WDM communication system communicates with a network, such as Gigabit Ethernet or an ATM network, in which burst transmission is performed, an idle optical channel, with no user data superimposed thereon, carries such pilot signal data. Accordingly, upon detection of the pilot signal data on a WDM channel at a receiving end of the WDM communication system, the channel is recognized to be idle. Additionally, since each WDM optical channel is given channel-unique pilot signal data, it is also possible for the receiving end to evaluate whether each WDM channel is busy or idle (a channel busy/idle state).

The light source113generates a light beam at a predetermined wavelength which is to be wavelength-division multiplexed into WDM signals, and the optical switch114can stop the light source113from emitting the light beam. The modulating and port-switching section115serves functions of: (1) a transmitter for modulating the light beam, emitted from the light source113, with output of the data-framing and pilot-signal-inserting section112, thereby generating an output light signal with the foregoing signal frame; (2) a receiver for converting and terminating optical channels which have been separated by the beam-splitting coupler22, into electric form for performing desired signal receiving processing thereon; and (3) switching the transmitting/receiving signals among multiple output destinations (ports). Such port-switching is performed in accordance with instructions (the channel busy/idle state at a downstream side) given by an OSC unit18(described later).

The variable optical attenuator12controls light signals at separate wavelengths output from the modulating and port-switching section115in optical transmission level, by separately adjusting the degree of attenuating for each of the light signals. The beam-combining coupler13combines (wavelength-division multiplexes) the optical-transmission-level-controlled light signals, which have been output from the variable optical attenuator12, thereby generating WDM signals for transmission.

The optical amplifier14simultaneously amplifies the WDM signals input from the beam-combining coupler13to compensate for losses which will be caused by light splitting by the monitor beam splitting coupler15and the OSC beam inserting coupler16, disposed subsequently to the optical amplifier14. For example, an EDFA (Erbium-doped Fiber Amplifier) is a typical example of the optical amplifier14. The monitor beam splitting coupler15splits the input from the optical amplifier14into two outputs, and then inputs one of the two outputs to the SAU17for monitoring its spectrum. The OSC beam inserting coupler16receives from the OSC unit18a light signal at a wavelength (supervisory control information) which is previously determined as an OSC channel, and combines the received light signal with the remaining one of the above outputs of the optical amplifier14.

The OSC beam splitting coupler19splits WDM signals received from a downstream side into two outputs and directs one of them into the OSC unit18. The OSC unit18extracts OSC supervisory control information (including the aforementioned channel busy/idle state) from the received WDM signals, so that the above-mentioned instructions to the modulating and port-switching section115can be issued and that use of channels at a downstream side can be coordinated according to the supervisory control information (channel busy/idle information) and the spectrum monitoring (measurement) result obtained by the SAU17.

The optical amplifier20simultaneously amplifies the WDM signals received from the OSC beam splitting coupler19to compensate for losses caused by light splitting by the OSC beam splitting coupler19and the monitor beam splitting coupler21. The monitor beam splitting coupler21splits input from the optical amplifier20into two outputs, and then inputs one of these to the SAU17to monitor its spectrum. The beam-splitting coupler22takes the other one of the outputs (WDM signals) of the monitor beam splitting coupler21, and separates the WDM signals into channels at separate wavelengths. Here, the separated optical channels are then input to the modulator and port switching section115.

The SAU17monitors the spectrum of transmitting/receiving WDM signals which have been split and directed by the foregoing monitor beam splitting couplers15and21. In the present embodiment, it is possible to realize like functions to those of the SAU17, even with no use of an expensive optical circuit (PD array). Specifically, as shown inFIG. 1, the SAU17of the present embodiment has, as its essential part, a switch51, a PD52, an amplifier53, level peak (peak/bottom) holder circuit55, an AD converter56, a CPU57, and a DA converter58.

Here, the switch51takes two inputs, one from the monitor beam splitting coupler15; the other, from the monitor beam splitting coupler21, and it selectively outputs either of them, following an instruction given by the CPU57. The PD (opto-electric converter section)52receives the WDM signals from the switch51, and then outputs an electric signal corresponding to the optical power of the received WDM signals. The amplifier53amplifies the output of the PD52to reach a predetermined level.

A filter54is a lowpass filter which allows a pilot signal component (chiefly, an AC component) to pass through it, blocking a frequency component (chiefly, a DC component due to amplified spontaneous emission (ASE)), of the output of the amplifier53, higher than the above-mentioned pilot signal data (hereinafter simply called “pilot signal”). The peak/bottom holder circuit55holds a peak and bottom values of the output of the filter54. The AD converter56converts the peak and bottom values (amplitude information) of the pilot signal component, which values are held in the peak/bottom holder circuit55, into digital form.

That is, the PD52, the filter54, and the peak/bottom holder circuit55serve as a pilot signal detecting means for detecting a pilot signal in received WDM signals.

The CPU57measures the spectrum (OSNR) of input WDM signals in accordance with the thus obtained amplitude information of the pilot signal component (a combination of multiple pilot signal components at separate wavelengths). The CPU57can evaluate whether channels are busy or idle, depending upon the detection of the pilot signal component. Moreover, such an evaluation result is notified to the OSC unit18, thereby making it possible to notify a downstream side of the above-mentioned channel busy/idle information over an OSC, so it is no longer required to monitor pilot signals at every node in the WDM communication system.

That is, the CPU57serves both as (1) an evaluating means for evaluating whether each of the plurality of WDM optical channels is busy or idle, based on detection results obtained by the above pilot signal detecting means and as (2) a notifying means (supervisory control information transmitting section) for notifying another WDM communication apparatus in the WDM communication system of the detection results, which have been obtained by the evaluating means, over an OSC which is previously assigned for communicating supervisory control information thereon.

Referring now toFIG. 2, a repeater station3of the present embodiment is symmetrically formed for realizing bi-directional communication. For example, assuming that the direction toward the right onFIG. 2is designated as a transmitting (downstream) direction, while the direction toward the left onFIG. 2is designated as a receiving (upstream) direction, the repeater station3has (1) a transmitter function including an OSC beam-splitting coupler31; an optical amplifier32; a monitor beam splitting coupler33; and an OSC beam-inserting coupler34, and (2) a receiver function including an OSC beam-splitting coupler37; an optical amplifier38; a monitor beam splitting coupler39; and an OSC beam-inserting coupler40. Additionally, the repeater station3also has (3) a supervisory controller function including an SAU35(35a,35b), and OSC sections36aand36b.

Regarding the above transmitter (receiver) function, the OSC beam-splitting coupler31(37) splits WDM signals received through an optical transmission path500(600) into two outputs, and then inputs one of the outputs to the OSC section36b(36a), and the other, to the optical amplifier32(38). The optical amplifier32(38) simultaneously amplifies the WDM signals input from the OSC beam-splitting coupler31(37) to compensate for losses which will be caused by light splitting by the monitor beam splitting coupler33(39) and the OSC beam-inserting coupler34(40), both of which are disposed subsequently to the optical amplifier32(38). As in the case of the above description, an EDFA (Erbium-doped Fiber Amplifier), for example, is applicable, as the optical amplifier32(38).

The monitor beam splitting coupler33(39) splits the input from the optical amplifier32(38) into two outputs, and then inputs one of them to the SAU35a(35b), and the other, to the OSC beam-inserting coupler34(40). The OSC beam-inserting coupler34(40) inserts an OSC light beam into the output of the monitor beam splitting coupler33(39).

The SAUs35aand35beach have a construction similar to that of an SAU17of the terminal1, so that they can monitor the channel busy/idle state of the WDM channels and their spectrum (OSNR) based on the above-mentioned pilot signal component contained in the WDM signals. Note that, since the SAUs35aand35bare dedicated to downstream and upstream directions, respectively, there is not provided any equivalent to the switch51of the SAU17of the terminal1.

In the foregoing WDM communication system of the present embodiment, the terminal1uses an idle WDM channel to transmit pilot signal data (a fixed pattern) unique to the channel, and the pilot signal data is detected by an SAU17(35) equipped to a repeater station3or to a node, such as an OADM (not shown), located en route. It is thus possible to recognize whether the channels are busy or idle (the channel busy/idle state) of the WDM signals, so that active communication control according to the channel busy/idle state is available.

More specifically, since the OSC notifies another node (OAMD, or the like) of the channel busy/idle state, it becomes possible for the node to use an idle channel to newly establish communication thereover. For example, an OADM node receives notification of the channel busy/idle state, and judges that a channel is idle, based on the notification. Onto that optical channel an optical transmitting means (an add/drop function) of the OADM node superimposes another item of transmission data, to transmit to another WDM communication apparatus.

As a result, effective use of wavelength resources in the WDM communication system is realized, thereby making it possible to reduce congestion due to a momentary upsurge in traffic.

Further, it is also possible to measure the spectrum (OSNR) of WDM signals according to the pilot signal data, thus eliminating the necessity for expensive optical circuitry.

A transponder11of a terminal1will be described hereinbelow, which transponder11copes with pilot signal data.

(A1) First Mode of Transponder11:

FIG. 3(A)andFIG. 4each depict a transponder11in a first mode.FIG. 3(A)shows a construction of a transmitter of the transponder11;FIG. 4shows a construction of a receiver of the transponder11.

As shown inFIG. 3(A), the transponder11in the first mode has a transmitter11S. The transmitter11S has a framing section11A (equivalent to the foregoing data-framing and pilot-signal-inserting section112) for generating a data frame which includes a frame pattern5, an overhead6, and a payload7, as shown inFIG. 3(B), and an E/O section11B for superimposing the data frame, which has been generated by the framing section11A, on a light signal on a predetermined WDM channel to be wavelength-division multiplexed. The framing section11A has a parallel/serial (P/S) section11A-1, an overhead inserting section11A-2, a scrambler11A-3, a pilot signal adding section11A-4, a frame pattern inserting section11A-5, and a pulse generator (PG) section11A-6. The E/O section11B has an optical modulator section11B-1and an LD (light source)11B-2.

Here, the P/S section11A-1converts data (parallel data) input through a communication network, such as an ATM network, SONET, a SDH network, and Ethernet™, into serial data. The overhead inserting section11A-2adds required overhead information (overhead6) to the output (payload data) of the P/S section11A-1. The scrambler11A-3simultaneously scrambles the output (overhead6plus payload7) of the overhead inserting section11A-2.

The pilot signal adding section11A-4inserts pilot signal data into a signal (an area composed of overhead6and payload7) which bypasses the scrambler11A-3, if data to be stored in the payload7is absent.

For this purpose, the present pilot signal adding section11A-4has, as shown inFIG. 5, a payload ALL 0s detecting section41, a protection timer42, an OR circuit43, a N-scale counter44, a T/N period timer45, a pulse generating section46, and a selector (SEL)47.

The payload ALL 0s detecting section (verifying means)41evaluates whether or not all the payload data bits are 0s (hereinafter called a “payload ALL 0s state”) before they undergo scrambling by the scrambler11A-3, to recognize the absence or existence of data to be transmitted on a WDM optical channel. If the evaluation result obtained by the payload ALL 0s detecting section41is positive, a pilot signal, in place of post-scramble data (overhead6and payload7) having been scrambled by the scrambler11A-3, is selectively output by the selector47to the frame pattern inserting section11A-5. The pilot signal is generated by the N-scale counter44, the T/N period timer45(for use in increment of pulse duty of the pilot signal), and the pulse generating section46.

Note that N is the maximum number (bit) of 0s allowed to appear consecutively (e.g. N>20); T is a pilot signal period. As shown inFIG. 6, during an idle period in which no valid data is transmitted, a pilot signal with a pattern in which logic inversion occurs every N bits in a pilot signal period T, is inserted in overhead6and payload7.

Further, by varying the pilot signal periods T with the wavelength, it is possible to generate pilot signals which are unique to the individual channels (it is possible to label each channel with a channel-unique pilot signal).

The protection timer42is provided not to output a detection signal until a certain number of data 0s are successively detected after the detection of a payload data ALL 0s state by the payload ALL 0s detecting section41.

The OR circuit43ORs output of the protection timer42with an external switching signal (an input light interrupt detection signal received from a receiver function of the transponder11or an out-of-frame-synchronization signal). With this construction, the selector47can selectively output the signal which has been received from the pulse generating section46, not only when the payload ALL 0s state is detected, but also when the external switching signal is input.

Referring now toFIGS. 3(A) and 3(B), the frame pattern inserting section11A-5adds a predetermined frame pattern5, for use in synchronization, to a signal in which a pilot signal (an area composed of overhead6and payload7)is inserted, and thereby a transmission data frame ofFIG. 3(B)is consequently generated. Note that the PG section11A-6supplies each of the elements11A-1through11A-5with a required operation clock, based on an intra-apparatus reference clock.

In the meantime, the optical modulator section11B-1of the E/O section11B modulates light of a predetermined wavelength, which is supplied from the LD11B-2, with a transmission data frame generated by the framing section11A according to an operation clock supplied from the PG section11A-6, thereby generating a transmission light signal.

That is, the aforementioned pilot signal adding section11A-4and the E/O section11B serve as a pilot signal transmitting means. If the payload ALL 0s detecting section41detects a payload ALL 0s state on a channel, whereupon it is recognized that data (payload) to be transmitted on the channel is absent, the channel-unique (labeled for each wavelength) pilot signal is transmitted over the optical channel.

Referring now toFIG. 4, a receiver function11R of a transponder11has an O/E section11C, which converts received light signals from an optical transmission path600(more specifically, from the beam-splitting coupler22ofFIG. 1) into electric form to extract a data frame therefrom, and a terminal LSI11D for performing required terminating processing, such as descrambling and S/P (serial/parallel) conversion, on the data frame thus obtained by the O/E section11C.

Thus, the O/E section11C has a photodiode (PD)11C-1, a bias circuit11C-2, a preamplifier11C-3, an equalizing filter11C-4, a timing extracting section11C-5, and a data extracting section11C-6. The terminal LSI11D has a frame timing extracting section11D-1, a pilot pattern detecting section11D-2, a descrambler11D-3, an S/P section11D-4, and a memory (buffer) section11D-5.

The PD11C-1of the O/E section11C receives light signals on predetermined optical channels through an optical transmission path600, and converts the received light signals into electronic form. The bias circuit11C-2controls a bias applied to the PD11C-1, and the preamplifier11C-3amplifies an electric signal from the PD11C-1up to a predetermined signal level.

The equalizing filter11C-4equalizes output of the preamplifier11C-3, and the timing extracting section11C-5extracts a reference transmission clock from output of the preamplifier11C-3. The Data extracting section11C-6extracts a data frame from the thus equalized signal output from the equalizing filter11C-4.

In the meantime, the frame timing extracting section11D-1of the terminal LSI11D extracts a frame timing (the foregoing frame pattern5) of a received data frame, in accordance with the reference clock extracted by the timing extracting section11C-5. The pilot pattern detecting section11D-2extracts a pilot signal from the data frame, which has been extracted by the Data extracting section11C-6, in accordance with an operation clock supplied by the frame timing extracting section11D-1. Note that the pilot pattern detecting section11D-2is notified in advance of the maximum number N of 0s allowed to appear consecutively (e.g. N>20) and a pilot signal period T.

The descrambler11D-3descrambles the received data frame obtained by the Data extracting section11C-6, thereby restoring the original data frame. The S/P section11D-4performs S/P conversion on output of the descrambler11D-3, and the memory section11D-5temporarily holds output of the S/P section11D-4. Here, at a time of detection of a pilot signal by the pilot pattern detecting section11D-2, data is defined to have a fixed pattern of “ALL 0s” or “ALL 1s”.

With this construction, the transponder11of the present embodiment is allowed to transmit downstream a channel-unique pilot signal over an idle WDM transmission channel, so that it is possible to separate a pilot signal from a received WDM signal input from the downstream side to send out only required data over an ATM network, SONET, an SDH network, or Ethernet™.

In the above-described transponder11, a pilot signal is inserted, at a time an optical channel is idle, into both of an overhead6and a payload7. Alternatively, the pilot signal may be inserted into either one of those, or otherwise, different pilot signals may be inserted into these two parts, separately.

Moreover, if a pilot signal is inserted into an overhead6, the pilot signal may be inserted into a part {a transient (undefined) overhead; unused bytes other than those (bytes A1-Z) which are significant to monitor errors} of the overhead6, not the whole part of it. Various modes of the insertion of such pilot signals will be described hereinbelow.

(A2) Second Mode of Transponder11—Pilot Signal is Inserted into an Overhead Alone:

FIG. 7(A)andFIG. 8show a transponder11in a second mode, where a pilot signal is inserted into an overhead alone.FIG. 7(A)depicts a construction of a transmitter11S of the transponder11;FIG. 8depicts a construction of a receiver11R of the transponder11ofFIG. 8.

Referring now toFIG. 7(A), the transmitter11S of the second mode has a payload scrambler11A-31anterior to an overhead inserting section11A-2and an overhead scrambler11A-32posterior to the overhead inserting section11A-2, in order to insert a pilot signal into only an overhead6. With this construction, it is possible to scramble input data separately for the payload7(hereinafter also called payload data7) and the overhead6(hereinafter also called overhead information6).

More precisely, when transmission data is present, the payload scrambler11A-31scrambles transmission payload data7. After that, the overhead inserting section11A-2adds required overhead information6to the scrambled payload data7. The overhead scrambler11A-32then scrambles the overhead information6, and the pilot signal adding section11A-4lets the resulting data pass therethrough. Finally, the frame pattern inserting section11A-5adds a frame pattern5to the output, thereby generating a transmission data frame.

Otherwise, if transmission data is absent(idle), the transmission payload data7(no data) is input to the overhead inserting section11A-2via the payload scrambler11A-3, and then passes through the overhead inserting section11A-2. The data bypasses the overhead scrambler11A-32into the pilot signal adding section11A-4, whereupon a pilot signal, instead of overhead information6, is inserted into the payload data7(seeFIG. 7(B)). Other operations than the above-described are similar to those of the transmitter11S, as was already described with reference toFIG. 3(A).

Referring now toFIG. 8, in adaptation to the insertion of a pilot signal into the overhead6alone, the terminal LSI11D of the receiver11R is equipped with an overhead extracting section11D-6. In addition, in order to make it possible to perform descrambling, S/P conversion, and various buffer processing for the payload data7and the overhead information6separately, the terminal LSI11D also has the following: a descrambler11D-31dedicated to the payload data7; an S/P section11D-41; a memory (buffer) section11D-51; a descrambler11D-32dedicated to the overhead6; an S/P section11D-42; and a memory (buffer) section11D-52. Here, the other part of the construction than the above is similar to or the same as that shown inFIG. 3(A).

In the receiver11R, the overhead extracting section11D-6extracts overhead information6from a data frame obtained by the O/E section11C, and the descrambler11D-32descrambles the extracted overhead information. The S/P section11D-42performs P/S conversion on the converted overhead information6, and then stores the overhead information6in the memory section11D-52temporarily. Meanwhile, the descrambler11D-31descrambles the remaining payload data7, and the S/P section11D-41performs P/S conversion on the converted payload data7, and then stores the payload data7in the memory section11D-51temporarily.

If the pilot pattern detecting section11D-2detects a pilot signal in the overhead6, which has been extracted by the overhead extracting section11D-6, the memories11D-51and11D-52each store a bit pattern of ALL 0s or ALL 1s.

In such a manner, in a case where a pilot signal is inserted into only an overhead6, descrambling, S/P conversion, and various kinds of buffer processing are carried out separately on an overhead6and a payload7, so that normal signal receiving processing is available.

(A3) Third Mode of Transponder11—Pilot Signal is Inserted into a Payload Alone:

FIG. 9(A)andFIG. 10show a transponder11in a third mode, where a pilot signal is inserted into only a payload7(seeFIG. 9(B)).FIG. 9(A)depicts a construction of a transmitter11S of the transponder11;FIG. 10depicts a construction of a receiver11R of the transponder11.

Referring now toFIG. 9(A), the transmitter11S of the third mode has a payload scrambler11A-31anterior to a pilot signal adding section11A-4and an overhead scrambler11A-32posterior to an overhead inserting section11A-2, in order to insert a pilot signal into only a payload7. With this construction, it is possible to scramble input data separately for payload data7and overhead information6).

More precisely, when transmission data is present, the payload scrambler11A-31scrambles transmission payload data7. The scrambled payload data7passes through the pilot signal adding section11A-4, and then enters the overhead inserting section11A-2. After that, the overhead inserting section11A-2adds required overhead information6to the scrambled payload data7, which overhead information6is then scrambled by the overhead scrambler11A-32. To the scrambled result, a frame pattern5is added by the frame pattern inserting section11A-5.

Otherwise, if transmission data is absent, output of the P/S section11A-1bypasses the payload scrambler11A-3into the pilot signal adding section11A-4, which then inserts a pilot signal, instead of payload data7, into the output. After that, the overhead inserting section11A-2adds overhead information6to the output, which overhead information6is then scrambled by the overhead scrambler11A-32. The frame pattern inserting section11A-5then adds a frame pattern5to the output, thereby generating a transmission frame containing the pilot signal in its payload7. Other operations than the above-described are basically similar to those of the transmitter11S, as was already described with reference toFIG. 3(A)andFIG. 7(A).

Referring now toFIG. 10, in adaptation to the insertion of a pilot signal into the payload7alone, the receiver11R is equipped with a payload extracting section11D-7, instead of an overhead extracting section11D-6ofFIG. 8. The payload extracting section11D-7extracts a payload7from output of the data extracting section11C-6.

In this case, if a pilot pattern detecting section11D-2detects a pilot signal in the payload7which has been extracted by the payload extracting section11D-7, only a memory section11D-51dedicated to a payload7holds a fixed bit pattern of ALL 0s or ALL 1s. Other operations than the above-described are similar to those depicted inFIG. 8. With this construction, even in a case where a pilot signal is inserted into a payload7alone, normal signal receiving processing is available.

(A4) Fourth Mode of Transponder11—Pilot Signal is Inserted into a Payload and Part (an Undefined, or Transient, Overhead) of an Overhead Information6:

FIG. 11(A)andFIG. 12show a transponder11in a fourth mode, where a pilot signal is inserted into both a transient (or undefined) overhead62(seeFIG. 11(B)) and a payload7.FIG. 11(A)depicts a construction of a transmitter11S of the transponder11;FIG. 12depicts a construction of a receiver11R of the transponder11. InFIGS. 11, (A) and (B), andFIG. 12, like reference numbers to those that have already been described designate similar parts or elements, so their detailed description is omitted here.

Referring now toFIG. 11(A), the transmitter11S of the fourth mode includes: an undefined overhead inserting section11A-21disposed between a P/S section11A-1and an undefined overhead scrambler11A-33; a defined overhead inserting section11A-22disposed between a pilot signal adding section11A-4and defined overhead scrambler11A-34, so that a pilot signal is inserted into an undefined overhead62and a payload7. With these scramblers,11A-33and11A-34, it is possible to scramble input data separately for payload data7and for information (hereinafter called “undefined overhead information 62”) to be stored in the undefined overhead62.

Note that the defined overhead information61, as was already described above, represents bytes (A1-Z) which are significant to monitor errors, and that the undefined overhead information62represents the other remaining unused bytes.

If transmission data is absent, output of the P/S section11A-1of the transmitter11S in the fourth mode passes through the undefined overhead inserting section11A-21, and then the output bypasses the undefined overhead scrambler11A-33into the pilot signal adding section11A-4, where a pilot signal is added in place of the undefined overhead information62and the payload data7.

After that, the defined overhead inserting section11A-22adds defined overhead information61to the output of the pilot signal adding section11A-4, which information61is then scrambled by the defined overhead scrambler11A-34. Finally, the frame pattern inserting section11A-5adds a frame pattern5to the output, thereby generating a transmission data frame with an undefined overhead62and a payload7both containing a pilot signal.

Otherwise, if transmission data is present, the undefined overhead inserting section11A-21adds undefined overhead information62to payload data7. The data enters, instead of bypassing, the undefined overhead scrambler11A-33, where the input data is scrambled. After that, the output of the undefined overhead scrambler11A-33passes through the pilot signal adding section11A-4, and is then input to the defined overhead inserting section11A-22.

Then the defined overhead inserting section11A-22adds defined overhead information61to the input, which is then scrambled by the defined overhead scrambler11A-34. Finally, a frame pattern inserting section11A-5adds a frame pattern5to the input, thereby generating a transmission data frame.

Here, other operations than the above-described are basically similar to those of the transmitter11S, as was already described with reference toFIG. 3(A),FIG. 7(A), andFIG. 9(A).

Referring now toFIG. 12, in adaptation to the insertion of a pilot signal into the undefined overhead62and the payload data7, the receiver11R has a construction similar to that which is depicted inFIG. 10. In this case, however, the overhead extracting section11D-6is allowed to extract a defined overhead61and undefined overhead62separately. The defined overhead information61and the undefined overhead information62are then output to a descrambler11D-32and a descrambler11D-31, respectively.

If a pilot pattern detecting section11D-2detects a pilot signal, each of the memory sections11D-51and11D-52holds a fixed bit pattern of ALL 0s or ALL 1s. Other operations than the above-described are similar to those of the receiver11R depicted inFIG. 10. With this construction, even in a case where a pilot signal is inserted into an undefined overhead information62and a payload7, normal signal receiving processing is still available.

(A5) Fifth Mode of Transponder11—Pilot Signal is Inserted into a Payload and Part (an Undefined, or Transient, Overhead) of an Overhead6:

FIG. 13(A)shows a transponder11in a fifth mode, where a pilot signal is inserted into a defined overhead61and an undefined overhead62(seeFIG. 13(B)).FIG. 13(A)depicts a construction of a transmitter11S of the transponder11. InFIG. 13(A), like reference numbers to those which have already been described designate similar parts or elements, so their detailed description is omitted here.

Referring now toFIG. 13(B), in order to insert a pilot signal into defined overhead information61and undefined overhead information62, there are arranged, between a P/S section11A-1and a frame pattern inserting section11A-5, a defined overhead inserting section11A-22, a defined overhead scrambler11A-34, an undefined overhead inserting section11A-21, an undefined overhead scrambler11A-33, and a pilot signal adding section11A-4, in this cited sequence. With no data transmitted on a channel, output of the defined overhead inserting section11A-22is bypassed into the pilot signal adding section11A-4.

If transmission data is absent, output of the P/S section11A-1of the transmitter11S passes through the defined overhead inserting section11A-22, and then the output bypasses the defined overhead scrambler11A-34into the pilot signal adding section11A-4, where a pilot signal is added in place of the undefined overhead information62. Finally, the frame pattern inserting section11A-5adds a frame pattern5to the input, thereby generating a transmission data frame with an undefined overhead62containing a pilot signal.

Otherwise, if transmission data is present, the payload data7does not take the foregoing bypass route but the following. The defined overhead inserting section11A-22adds defined overhead information61to the payload data7. The data then enters the defined overhead scrambler11A-34, where both the defined overhead information61and the payload data7are collectively scrambled. The output of the defined overhead scrambler11A-34then enters the undefined overhead inserting section11A-21, where undefined overhead information62is added thereto.

After that, the output (data composed of the undefined overhead information62, the defined overhead information61, and the payload data7) of the undefined overhead inserting section11A-21is input to the undefined overhead scrambler11A-33, where the undefined overhead information62is scrambled. The output passes through the pilot signal adding section11A-4, and is then input into the defined overhead inserting section11A-22. Finally, the frame pattern inserting section11A-5adds a frame pattern5to the input, thereby generating a transmission data frame. Other operations than the above-described are basically similar to those of the transmitter11S, as was already described with reference toFIG. 3(A),FIG. 7(A),FIG. 9(A), andFIG. 11(A).

Here, the construction and the operations of a receiver11R are similar to those in the fourth mode (seeFIG. 12), so their detailed description is omitted here.

(A6) Sixth Mode of Transponder11—Pilot Signal is Inserted into an Undefined Overhead Alone:

FIG. 14(A)andFIG. 15show a transponder11in a sixth mode, where a pilot signal is inserted into an undefined overhead62(seeFIG. 14(B)).FIG. 14(A)depicts a construction of a transmitter11S of the transponder11;FIG. 15depicts a construction of a receiver11R of the transponder11. InFIG. 14(A)andFIG. 15, like reference numbers to those which have already been described designate similar parts or elements, so their detailed description is omitted here.

Referring now toFIG. 14(B), in order to insert a pilot signal into only an undefined overhead information62of an overhead information6, there are arranged, between a P/S section11A-1and a frame pattern inserting section11A-5, a payload scrambler11A-31, an undefined overhead inserting section11A-21, an undefined overhead scrambler11A-33, a pilot signal adding section11A-4, a defined overhead inserting section11A-22, and a defined overhead scrambler11A-34, in this cited sequence. With no data transmitted on a channel, output of the P/S section11A-1is bypassed into the pilot signal adding section11A-4.

If transmission data is absent, output of the P/S section11A-1of the transmitter11S is bypassed into the pilot signal adding section11A-4, where a pilot signal, instead of undefined overhead information62, is inserted into the input. The output of the pilot signal adding section11A-4then enters the defined overhead inserting section11A-22, where defined overhead information61is added to the input.

After that, the output of the defined overhead inserting section11A-22enters the defined overhead scrambler11A-34, where the defined overhead information61is scrambled. Finally, the frame pattern inserting section11A-5adds a frame pattern5to the input, thereby generating a transmission data frame with an undefined overhead62containing a pilot signal.

Otherwise, if transmission data is present, the payload data7does not take the foregoing bypass route but the following. After the payload scrambler11A-31scrambles the payload data7, the undefined overhead inserting section11A-21adds undefined overhead information62to the scrambled data. The undefined overhead scrambler11A-33scrambles the undefined overhead information62.

After being scrambled, the data passes through the pilot signal adding section11A-4, and then enters the defined overhead inserting section11A-22, where the defined overhead information61is added to the input data. After that, the data is input to the defined overhead scrambler11A-34, where the added defined overhead information61is scrambled. Finally, the frame pattern inserting section11A-5adds a frame pattern5to the data, thereby generating a transmission data frame. Other operations than the above-described are basically similar to those of the transmitter11S, as was already described with reference toFIG. 3(A),FIG. 7(A),FIG. 9(A),FIG. 11(A), andFIG. 13(A).

Referring now toFIG. 14(B), in adaptation to the insertion of a pilot signal into the undefined overhead62of an overhead6, a terminal LSI11D of the receiver11R includes a payload extracting section11D-7for extracting payload data7. Further, in order to perform descrambling, S/P conversion, and various kinds of buffer processing for defined overhead information61, undefined overhead information62, and payload data7, separately, the terminal LSI11D has a descrambler11D-31, an S/P section11D-41, and a memory (buffer) section11D-51, each of which is dedicated to payload data7. Further, the terminal LSI11D also has a descrambler11D-32, an S/P section11D-42, and a memory (buffer) section11D-52, each of which is dedicated to defined overhead information61. Still further, the terminal LSI11D has a descrambler11D-33, an S/P section11D-43, and a memory (buffer) section11D-53, each of which is dedicated to undefined overhead information62. Construction other than the above-described is similar to or the same as that shown inFIG. 3(A).

With this construction, in the receiver11R of the sixth mode, the payload extracting section11D-7extracts payload data7from a data frame obtained by an O/E section11C, and the extracted payload data7is then descrambled by the descrambler11D-31. The descrambled output undergoes S/P conversion carried out by the S/P section11D-41, and is then temporarily stored in the memory section11D-51.

In the meantime, the remaining defined overhead information61and undefined overhead information62are input to the descramblers11D-32and11D-33, respectively, where the information,61and62, is then descrambled. After that, the outputs of the descramblers61and62are S/P converted by the S/P sections11D-42and11D-43, respectively, and are then temporarily stored in the memory sections11D-52and11D-53, respectively.

After that, if the pilot pattern detecting section11D-2detects a pilot signal in the undefined overhead information62, each of the memories holds a fixed bit pattern of ALL “0s” or ALL “1s”.

With such a construction, even in a case where a pilot signal is inserted into the undefined overhead62, it is possible to carry out descrambling, S/P conversion, and various kinds of buffer processing for the defined overhead61, the undefined overhead information62, and the payload7, separately, so that normal signal receiving processing is available.

(A7) Seventh Mode of Transponder11—Separate Pilot Signals are Inserted into an Undefined Overhead and a Payload:

FIG. 16(A)andFIG. 17show a transponder11in a seventh mode.FIG. 16(A)depicts a construction of a transmitter11S of the transponder11;FIG. 17depicts a construction of a receiver11R of the transponder11. InFIG. 16(A)andFIG. 17, like reference numbers to those which have already been described designate similar parts or elements, so their detailed description is omitted here.

Referring now toFIG. 16(B), in order to insert separate pilot signals, that is, pilot signal1and pilot signal2, into an undefined overhead62and a payload7, there are arranged, between a P/S section11A-1and a frame pattern inserting section11A-5, a payload scrambler11A-31, a pilot signal adding section11A-41for adding pilot signal2, an undefined overhead inserting section11A-21, an undefined overhead scrambler11A-33, a pilot signal adding section11A-42for adding pilot signal1, a defined overhead inserting section11A-22, and a defined overhead scrambler11A-34, in this cited sequence. With no data to be transmitted on a channel, output of the P/S section11A-1is bypassed into the pilot signal adding section11A-41, and output of the undefined overhead inserting section11A-21is bypassed into the pilot signal adding section11A-42.

In the transmitter11S of the seventh mode, if transmission data is absent, output of the P/S section11A-1is bypassed into the pilot signal adding section11A-41, where pilot signal2is inserted in place of payload data7. The output of the pilot signal adding section11A-41passes through the undefined overhead inserting section11A-21, and then bypasses the undefined overhead scrambler11A-33into the pilot signal adding section11A-42, where pilot signal1is inserted in place of undefined overhead information62.

The output of the pilot signal adding section11A-42is then input to the defined overhead inserting section11A-22, where defined overhead information61is added to the input. The defined overhead information61is scrambled by the defined overhead scrambler11A-34, and finally, the frame pattern inserting section11A-5adds a frame pattern5to the input, thereby generating a data frame with an undefined overhead62and a payload7which contain pilot signal1and pilot signal2, respectively.

Since separate pilot signals1and2are inserted in the undefined overhead information62and the payload data7, respectively, it is permissible to use pilot signal2(the pilot signal period is T2; the maximum number of 0s allowed to appear consecutively is N2) to indicate that a channel is idle, and it is also permissible to use pilot signal1(the pilot signal period is T1; the maximum number of 0s allowed to appear consecutively is N1) to represent (notify) power level control which is carried out by a SAU17(35) according to results of spectrum monitoring.

Otherwise, if transmission data is present, the transmission payload data7does not take the foregoing bypass route but the following. After the payload scrambler11A-31scrambles the payload data7, the data passes through the pilot signal adding section11A-41into the undefined overhead inserting section11A-21, where undefined overhead information62is added to the input. The undefined overhead information62is scrambled by the undefined overhead scrambler11A-33.

The thus scrambled undefined overhead information62then passes through a pilot signal adding section11A-42into the defined overhead inserting section11A-22, where defined overhead information61is added. The defined overhead scrambler11A-34scrambles the defined overhead information61. Finally, the frame pattern inserting section11A-5adds a frame pattern5to its input, thereby generating a data frame. Other operations than the above-described are basically similar to those of the foregoing transmitter11S.

Referring now toFIG. 17, in adaptation to the insertion of separate pilot signals, pilot signal1and pilot signal2, into the undefined overhead62and the payload data7, a terminal LSI11D of the receiver11R includes a payload extracting section11D-7, a terminal LSI11D-8for extracting undefined overhead information62, a pilot pattern detecting section11D-21for detecting pilot signal2which is inserted into the payload7, a pilot pattern detecting section11D-22for detecting pilot signal1which is inserted into the undefined overhead62.

In order to perform descrambling, S/P conversion, and various kinds of buffer processing for the defined overhead information61, the undefined overhead information62, and the payload data7, separately, the terminal LSI11D has a descrambler11D-31, an S/P section11D-41, and a memory (buffer) section11D-51, each of which is dedicated to payload data7. Further, the terminal LSI11D also has a descrambler11D-32, an S/P section11D-42, and a memory (buffer) section11D-52, each of which is dedicated to defined overhead information61. Still further, the terminal LSI11D has a descrambler11D-33, an S/P section11D-43, and a memory (buffer) section11D-53, each of which is dedicated to undefined overhead information62. Construction other than the above-described is similar to or the same as that already described.

With this construction, in the receiver11R of the seventh mode, the payload extracting section11D-7extracts payload data7from a data frame obtained by an O/E section11C, and the extracted payload data7is then descrambled by the descrambler11D-31. The descrambled output undergoes S/P conversion carried out by the S/P section11D-41, and is then temporarily stored in the memory section11D-51.

If the pilot pattern detecting section11D-21detects a pilot signal in the payload7which has been extracted by the payload extracting section11D-7, the memory section11D-51stores a pattern of ALL 0s or ALL 1s.

In parallel with this, an undefined overhead extracting section11D-8extracts an undefined overhead62from a data frame which is obtained by the O/E section11C. The extracted undefined overhead is then descrambled by the descrambler11D-31, and the descrambled output undergoes S/P conversion carried out by the S/P section11D-43, and is then temporarily stored in the memory section11D-53.

If the pilot pattern detecting section11D-22detects a pilot signal in the undefined overhead62which has been extracted by the undefined overhead extracting section11D-8, the memory section11D-53stores a pattern of ALL 0s or ALL 1s.

The remaining defined overhead61is input to the corresponding descrambler11D-32, which then descrambles the defined overhead61. After that, the defined overhead information61undergoes S/P conversion carried out by the S/P section11D-42, and is then temporarily stored in the memory section11D-52.

With such a construction, even in a case where separate pilot signals, pilot signal1and pilot signal2, are inserted into the undefined overhead62and the payload7, it is possible to carry out descrambling, S/P conversion, and various kinds of buffer processing for the defined overhead61, the undefined overhead information62, and the payload7, separately, so that normal signal receiving processing is available.

(B) Details of SAU,17and35:

Details of the SAU,17and35, as has been described with reference toFIGS. 1 and 2, will be explained hereinbelow. In the following description, in cases where no distinction needs to be made between an SAU17of the terminal1and an SAU35of the repeater station3, both will be simply designated as the “SAU”, with no reference character added thereto.

(B1) SAU in First Mode:

FIG. 18shows an SAU in a first mode. Referring now toFIG. 18, the SAU has a PD52, an amplifier53, a filter54, a peak/bottom holder circuit55, an AD converter56, a CPU57, and a DA converter58, as has been already described with reference toFIG. 1(FIG. 2), and the SAU also has an optical shutter50, a drive circuit50A, a split buffer section63, AD converters64and65, a DA converter66, a lowpass filter (LPF)68, and a splitter circuit69. Additionally, the SAU also includes, as functions realized by the CPU57, equalizing sections57-1,57-3,57-5, a bias control section57-2, a correction factor calculating section57-4, and a difference calculating section57-6. InFIG. 18, the foregoing filter54is realized by a capacitor which removes a DC component from an input signal, and the amplifier53is an AGC (Automatic Gain controlled) amplifier.

The optical shutter50is controlled by the drive circuit50A to block input WDM signals received from the aforementioned beam splitting coupler15,21,33, or39(seeFIGS. 1 and 2). The LPF68allows only a predetermined low-frequency component of one output (voltage level V1) of the PD52to pass therethrough. The LPF68is set, for example, to have a passband such that if the foregoing data frame has a frame period of 8 kHz, channels higher than 4 kHz are cut off.

The splitter circuit69splits input electric signals, which have been amplified by the amplifier53, into two outputs. One of the outputs enters the filter54, while the other enters the AD converter65. As has already been described, the filter54receives the one of the outputs (voltage level V2), containing a DC (mainly due to an ASE light) and AC components, from the splitter circuit69. The filter54allows the AC component alone to pass through it, blocking the DC components.

With this construction, if a pilot signal is contained in a received data frame, as has been already described, the pilot signal component can be extracted from the received data frame, and its peak and bottom values are held in a peak/bottom holder circuit55. These values are converted into digital form by the AD converter56, and are then input to the CPU57(the equalizing section57-5).

The split buffer section63splits the output of the filter54into multiple signals which correspond in number with the WDM channels. The individual split signals are temporarily held in the split buffer section63, and then input to filter sections70(detailed later) prepared one for each wavelength (channel). The AD converter64converts the output of the PD52into a digital signal, and the AD converter65converts the other one of the outputs of the splitter circuit69into digital form. A bias circuit67controls a bias current applied to the PD52, in response to a bias control signal which is obtained by the bias control section57-2of the CPU57and is then converted into digital form by the DA converter58.

In the CPU57, the equalizing section57-1performs equalizing processing on the output of the AD converter64(that is, the output of the PD52) to obtain an average of the output voltage level V1of the PD52. On the basis of the average value, the bias control section57-2generates the foregoing bias control signal. At that time, the bias control section57-2calculates the total power P0of the received WDM signal from an average of output voltage level V1of the PD52and the voltage level of the bias control signal.

The equalizing section57-3equalizes the output of the splitter circuit69(that is, the output of the amplifier53), which output has been converted into digital form by the AD converter65, to obtain an average of the amplified output of the amplifier53. The correction factor calculating section57-4executes factor-correcting processing on the average of amplified output voltage level V2, which has been obtained by the equalizing section57-3, thereby obtaining a correction value P1for the amplified output voltage level V2.

The equalizing section57-5equalizes the peak and bottom values of the pilot signal component, which values have been converted into digital form by the AD converter56, and the difference calculating section57-6calculates a difference between the thus obtained average values of the peak and bottom values, thereby obtaining amplitude information P3of the pilot signal component. Here, using the foregoing items P1and P3, P1/P3is calculated to obtain an average modulation degree, which is for use in controlling gain of the amplifier53and in bias controlling carried out by the bias control section57-2.

Referring now toFIG. 20(A), each of the filter sections70, provided one for each channel, has a bandpass filter (BPF)71, a DC converter72, an AD converter73, and an equalizing section74.

The BPF71allows only a frequency component, of the output signal of the split buffer section63, which component corresponds to channel x (x=1 to n), where n is the number of channels contained in the WDM signal (n is an integer greater than 2). The DC converter72converts the frequency component of the channel x, which has passed though the BPF71, into a DC signal.

The AD converter73converts the DC signal of the channel x into digital form. The equalizing section74then equalizes the digital signal of the channel x, which has been obtained by the AD converter73, thereby obtaining an average (CHx receiving data) of the digital signal of the channel x.

That is, the above-mentioned lowpass filter68and each filter section70(bandpass filter71) serve as a pilot-signal-detecting filter section, which receives an electric signal from the PD52and allows a pilot signal component alone, of the received electric signal, to pass through it. The pilot signal component is unique to the idle optical channel (carrying no transmission data thereon) on which the pilot signal component has been transmitted. Note that the functions of the filter section70are realized by the CPU57as part of its functions.

With this construction, an SAU of the present embodiment measures the power, OSNR, and average OSNR of the received WDM signal on each channel in the following manner.

A WDM signal input through the optical shutter50is converted by the PD52into an electric signal (voltage level V1). After the LPF68cuts off the electric signal's frequency components of 4 kHz or higher, the signal is input to the amplifier53to be amplified. At that time, the output of the PD52then enters the bias control section57-2, via the AD converter64and the equalizing section57-1, for use in bias feedback control carried out by the PD52.

Further, the output of the amplifier53is split by the splitter circuit69into two outputs, and one of them is input into the AD converter65, and the other, into the filter54. The AD converter65converts the amplified output (voltage level V2) into digital form, and then inputs the digital signal to the equalizing section57-3. The equalizing section57-3obtains an average of the output voltage level V2, and the correction factor calculating section57-4calculates a correction value P1for the average value.

In the meantime, the filter54allows an AC component (pilot signal component) alone, out of the input signal received from the splitter circuit69, to pass therethrough, and the output of the filter54is input to both of the peak/bottom holder circuit55and the split buffer section63. As a result, the peak/bottom holder circuit55holds a peak and bottom values of the pilot signal component. These values are converted into digital form by the AD converter56, and the equalizing section57-5obtains their average values. The difference calculating section57-6then calculates a difference between those average values, thereby obtaining the pilot signal's amplitude information P3.

On the other hand, the signal which is split into the split buffer section63is divided by the split buffer section63into n outputs which correspond in number to the channels of the WDM signal. The split outputs are input, one to each filter section70, where CHx receiving data is obtained by the BPF71, the DC converter72, the AD converter73, and the equalizing section74. The thus obtained CHx receiving data is for use in the CPU57's factor correction processing (seeFIG. 22(A)).

Specifically, in a case where the filter sections70are provided, one for each channel, in parallel, it is required that variation (deviation) due to difference in the circuitry conversion efficiency among the filter sections70be compensated for. Hence, the CPU57multiplies each item of CHx receiving data by a factor Kx (P0), thereby obtaining the channel power Pchx of each WDM channel. Here, taking into consideration the fact that the voltage level V1depends on bias control by the PD52, the factor Kx (P0) is expressed as a function of the total power P0obtained by the bias control section57-2. Actually, the factor Kx (P0) is expressed by the production of the total power P0and a constant.

The CPU57, as shown inFIG. 22(B), obtains OSNR of each channel, by dividing the thus obtained channel power Pchx (P31) of each channel by a value P11, where the value P11is obtained by the following expression:
P11=(total powerP0−the sum total of channel powerPchxof the multiple channels)×correction factorB1
where, B1=grid width per channel/EDF gain-bandwidth.

The CPU57, as shown inFIG. 22(C), obtains an average OSNR of a WDM signal containing multiple channels, by dividing the sum total (P3) of the channel power of the multiple channels by a value P12, where the value P12is obtained by the following expression:
P12=(total powerP0−the sum total of channel powerPchxof the multiple channels)×correction factorB2
where, B2=channel grid width×the maximum number of channels/EDF gain-bandwidth.

The foregoing filter section70may be constructed, for example, as shown inFIG. 20(B). Specifically, the filter section70ofFIG. 20(B)has a BPF71, a peak holder circuit72A, a bottom holder circuit72B, AD converters73A and73B, equalizing sections74A and74B, and a difference calculating section75. A peak and bottom values of a pilot signal component which passes through the BPF71are held in the peak holder circuit72A and the bottom holder circuit72B, respectively. The peak and bottom values each are converted into digital form by the corresponding AD converters73A and73B, respectively, and average values of those are obtained by the equalizing sections74A and74B, respectively. The difference calculating section75then calculates the difference between the average values.

This construction, in comparison with the construction depicted inFIG. 20(A), makes it possible to obtain CHx receiving data with improved precision.

Further, with use of a passband-variable BPF, instead of the passband-fixed BPF71, the necessity for the multiple filter sections70, one for each channel, in parallel is eliminated. Theoretically speaking, a single variable BPF can cover all the channels. In that case, however, the trace width of the passband of the variable BPF is increased, resulting in increased measuring time.

Therefore, the multiple channels of the WDM signal are grouped into several channel groups, and each of the groups is given a filter section70with a variable BPF which covers a band of the individual channel group. It is thus possible to obtain CHx receiving data by using filter sections70significantly reduced in number than the channels, thus restraining increase of the measuring time.

For example, assuming the channels constituting a WDM signal are divided into three channel groups, there should be prepared three filter sections70for the same number of routes. Each of the filter sections70, as shown inFIG. 21, for example, has a BPF71′, a DC converter72, an AD converter73, an equalizing section74, a memory76, and a factor multiplier section77. The CHx receiving data, which is obtained from a pilot signal of one of the channel groups and which passes through a BPF71′ prepared for the channel group, is stored in the memory76on a channel-by-channel basis.

In this case, also, since two or more filter sections70are provided, one for each channel group, in parallel, probable circuitry errors among the channel groups should be taken into consideration. It is the factor multiplier section (error correcting section)77that takes in charge of the correcting of such circuitry errors. More specifically, on initial operation of the present apparatus, CHx receiving data on one and the same channel is input to the filter sections70. The factor multiplier section77then stores factors (error correction factors) such that equal outputs can be obtained among the filters70, and uses the factors to correct circuitry errors which would be detected in the CHx receiving data during later operations. InFIG. 21, factor “1”, factor “G1”, and factor “G2” are stored in the upper-most factor multiplier section77, the middle factor multiplier section77, and the lower-most factor multiplier section77, respectively.

In other words, the CPU57, which serves as a spectrum measuring means, also carries a function as an error-correction-factor calculating section for calculating an error correction factor such that, if the same channel is input to the variable BPFs71′ at initialization of the apparatus, the same pilot signal data component is output from each of the variable BPFs71′. On the basis of the error correction factor obtained by the error-correction-factor calculating section, the factor multiplier section77performs error-correcting processing on the outputs of the BPFs71′, while the apparatus is in operation.

As has been described under item (A), if the pilot signal is constantly transmitted by using an overhead6(for example, an undefined overhead62), it is possible for a receiving end to detect the pilot signal constantly on every channel. Otherwise, if the pilot signal is transmitted only on an idle channel (for example, in a case where only a payload7is used to carry a pilot signal), no pilot signal is detected on a busy channel (where transmission data is stored in the payload7).

In order to guarantee the above-described spectrum measurement even in the latter case, it is required to pre-store pilot signal components (CHx receiving data) which are detected while the channels are idle. Hence, as in the example ofFIG. 23, a register (holder circuit)83is provided to store the channel power Pchx obtained by the above-described factor correction processing.

A comparator82then compares current channel power Pchx, with the threshold of the channel power Pchx, which is held in a pilot-detecting-level threshold holder section81. As a comparison result, if the current channel power Pchx is the threshold or smaller, it is judged that the pilot signal is absent, and output of the comparator82is supplied to the register83as a write-protect signal to prevent the previously measured channel power Pchx now in the register83from being overwritten.

It is thus possible to avoid conceivable effects on spectrum measurement, which effects would be caused by a removed (undetectable) pilot signal unavoidably deleted as a channel is busy. Hence, even if pilot signals cannot be detected in real time on all of the channels concurrently, it is still possible to measure the spectrum of the input WDM signal in a normal way.

In other words, the CPU57of the present embodiment serves also as an optical signal quality calculating section for calculating OSNR of a light signal on each channel, based on the channel power Pchx (amplitude information), stored in the register83, of the corresponding channel while idle.

(B2) SAU in Second Mode:

FIG. 19shows an SAU in a second mode. This SAU differs from that ofFIG. 18in that a peak/bottom holder circuit,55A and55B, an AD converter,56A and56B, are disposed both before and after the filter54. Moreover, as functions realized by the CPU57, the SAU is also provided with an equalizing section,57A-5and57B-5, a difference calculating section,57A-6,57B-6,57C-6, and an arithmetic operation section57-7.

Here, the peak/bottom holder circuit55A holds peak and bottom values of output (voltage level V2) of the amplifier53, from which output the filter54has not yet removed a DC component. The AD converter56A converts each of the peak and bottom values stored in the peak/bottom holder circuit55A into digital form.

The peak/bottom holder circuit55B and the AD converter56B are similar to the peak/bottom holder circuit55and the AD converter56ofFIG. 18, respectively. The peak/bottom holder circuit55B stores a peak and bottom values of the amplifier output (that is, a pilot signal component) after the filter54removes a DC component therefrom. The AD converter56B converts each of the peak and bottom values stored in the peak/bottom holder circuit55B into digital form.

After that, in the CPU57, the equalizing section57A-5obtains averages of the peak and bottom values of the digital signals (two pairs of peak and bottom values of an input and output signals to and from the filter54), which values have been obtained by the AD converters56A and56B. In the meantime, the equalizing section57B-5obtains averages of the peak and bottom values of the digital signals (the output signal (pilot signal component) output from the filter54), which have been obtained by the AD converter56B.

Using the two pairs of the peak and bottom average values, which have been obtained by the equalizing section57A-5, the difference calculating section57A-6calculates a difference between the average peak value of one of the two pairs and the average bottom value of the other of the pairs, thereby obtaining amplitude information P21, while the difference calculating section57B-6calculates a difference between the remaining average peak and bottom values of the two pairs, thereby obtaining amplitude information P22.

The equalizing section57B-5and the difference calculating section57C-6are similar to those that have been described with reference toFIG. 18. As in the case ofFIG. 18, the equalizing section57B-5obtains averages of the peak and bottom values of the pilot signal component, which values have been converted into digital form by the AD converter56B, and the difference calculating section57C-6calculates a difference between the thus obtained average values, thereby obtaining amplitude information P3of the pilot signal.

After that, the arithmetic operation section57-7calculates (P21+P22)/2, where P21and P22are amplitude information obtained by the foregoing difference calculating sections57A-6and57B-6, respectively, and thereby an average value P1of the amplitude information P21and P22is obtained. Using the thus obtained average value P1and the amplitude information P3, which has been obtained by the difference calculating section57C-6, the arithmetic operation section57-7further calculates P3/P1, thereby obtaining a modulation degree for use in bias controlling on the PD52carried out by the bias control section57-2.

In other words, with use of the SAU in the second mode, a bias applied to the PD52can be adjusted with improved accuracy, based on the averages of the peak and bottom values of the signal from which its DC component has not yet been removed. Accompanying this, the spectrum measurement itself is also improved in accuracy. At that time, the CPU57carries out spectrum measurement in a similar manner to that which has been described referring toFIG. 22(A)throughFIG. 22(C). Further, the filter section70may have a construction similar to that which has been described with reference toFIG. 20(A),FIG. 20(B), orFIG. 21. Still further, the channel power Pchx of channels which is obtained while the channels are idle, may be stored in the register83(seeFIG. 23), as has already been described.

If a channel falls in input interrupt (loss of signal) in the above-described embodiment, it is permissible to use the channel to superimpose data to be transmitted thereon, although the aforementioned pilot signal, which indicates that the channel is idle, is absent. Hence, as shown inFIG. 25, loss-of-signal detection couplers30B are provided, one for each channel-dedicated input port of the WDM communication apparatus. The loss-of-signal detection coupler30B splits off part of the signal light on each channel into the SAU35. If the SAU35detects a state of loss of signal, notification as such is transmitted from the OSC to an upstream device (an OADM node, or the like). With this construction, upon receipt of such loss-of-signal detection information, the node evaluates whether or not the channel can be used to transmit new data, thus coordinating use of the channels.

InFIG. 25, like reference numbers to those that have already been described designate similar parts or elements, so their detailed description is omitted here. The reference character “30A” designates an optical variable attenuator for adjusting the level of a light signal on each channel; and “30C”, an OR circuit for ORing (1) the notification information (the aforementioned loss-of-signal detection information, channel-busy information of the present apparatus detected by the SAU35, or the like) received from the SAU35and (2) busy information (indicating which channels are being used at a downstream node) from a downstream side, which information is detected by the OSC section36a.

In the construction ofFIG. 25, the OSC of the downstream node is capable of notifying an upstream node that a specific channel is being used downstream. Upon receipt of this notification, it is possible for the upstream node to avoid erroneously judging the channel is idle (it is possible for the upstream node to coordinate the use of the channels, based on the channel-busy state monitored by the SAU35and on the downstream channel busy/idle state obtained by the OSC).

Further, the above-described loss-of-signal detection can be realized by utilizing a pilot signal, without using any loss-of-signal detection coupler30B. Specifically, the maximum available period is previously set for each channel on a transmitter end. Channel are made idle when their predetermined time periods lapse, and a pilot signal is then transmitted on the channel for a specific period. An SAU of a receiving end, as shown inFIG. 24, for example, has a register84and a timer85, in place of the construction depicted inFIG. 23. If the register83is kept write-protected for the foregoing predetermined period, which is timed by the timer85, the information (channel power Pchx) stored in the register84, which information is the same as that stored in the register83, is rewritten into the loss-of-signal detection information.

With no detection of a pilot signal on a channel in the foregoing predetermined period, the SAU judges the channel is in a loss-of-signal state. The loss-of-signal detection coupler30B is thus no longer required.

Further, as shown inFIG. 26, if the receiver11R (seeFIGS. 4,8,10,12,15, and17) of the transponder11, as has been described above, detects a pilot signal, the transmitter11S (seeFIGS. 3,7,9,11,13,14, and16) halts insertion of a pilot signal into a transmission data frame, thereby notifying a downstream node of the detection of the pilot signal.

More specifically, the transponder11is equipped with a pilot signal transmission halting means. If the pilot pattern detecting section (pilot signal detecting means)11D-2(or11D-21,11D-22) of the receiver11R detects a pilot signal, the pilot signal transmission halting means stops a pilot signal adding section11A-4(or11A-41,11A-42) which is provided for a channel corresponding to the detected pilot signal, from adding a pilot signal, thereby consequently stopping the E/O section11B from sending out a pilot signal.

With this construction, even without using an OSC, it is still possible to notify a downstream node of availability of the WDM channels, so that increment of the capacity of the OSC is restrained. In this case, it is preferable that separate pilot signals are prepared to transmit in a transmission direction and in its opposite direction. One of the pilot signals is stored in an overhead6to indicate a channel is idle in the transmission direction, while the other is stored in a payload7to indicate a channel is idle in the opposite direction.

Further, the present invention should by no means be limited to the above-illustrated embodiment, but various changes or modifications may be suggested without departing from the gist of the invention.