Redundant changeover apparatus

A redundant changeover apparatus causing no frame synchronization loss even upon occurrence of changeover between working system and protection system. In case of an in-apparatus synchronization system, when two input signals which are mutually asynchronous in phase are changed over by a changeover switch, a changeover switch, a clock extractor, a PLL circuit, and a clock changing portion transmit signals with clocks before the changeover being gradually changed to clocks after the changeover. Alternatively, in case of an in-apparatus synchronize system, data are separated from clocks so that the data are once changed to data with reference clocks, and then for the data, clocks before the changeover are gradually changed to clocks after the changeover.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a redundant changeover apparatus, and in particular to a redundant changeover apparatus which makes a changeover from a working system to a protection (standby) system when a line failure or a device failure arises, or when a manual changeover is performed in node devices forming a network where transmission lines (optical fibers) are connected in a ring shape.

2. Description of the Related Art

Heretofore, with progresses of optical transmission technology, a wavelength division multiplexing (WDM) technology has been widely used, enabling a plurality of signals by using lights whose wavelength are different from each other to be multiplexed to realize a network capable of a higher transmission and extensively utilized for a transmission line of ITU Recommendation G.707 SONET/SDH device.

An optical ring network using such a WDM technology is shown inFIG. 15. This optical ring network RNW is composed of, in this example, node devices10-1to10-8, among which only the node device10-6forms a relaying or repeating node device and the other node devices10-1to10-5,10-7, and10-8respectively form a multiplexing node device.

The multiplexing node devices10-1to10-3form a network “A” together with transmission devices11-1to11-3which are also node devices connected thereto respectively. The multiplexing node devices10-4and10-5form a network “B” together with transmission devices (node devices)11-4and11-5connected thereto respectively.

Furthermore, the multiplexing node devices10-7and10-8form a SONET/SDH ring network “C” together with SONET/SDH transmission devices (node devices)11-6and11-7connected thereto respectively.

It is to be noted that the multiplexing node devices and the transmission devices are occasionally represented by general reference numerals “10” and “11”, respectively.

The node devices10-1to10-8are mutually connected through transmission lines “W” of a working system and transmission lines “P” of a protection system which are respectively duplicated, in which each of the multiplexing node devices10-1to10-5,10-7, and10-8is composed of a transmission line monitor20, a WDM device30performing optical coupling/branching per wavelength, and a redundant changeover apparatus40performing a redundant changeover at a network single wavelength channel line, as exemplified shown only in the multiplexing node device10-5, to perform a wavelength division multiplexing of a subordinate network transmission signal at the WDM device30and to perform a failure monitoring of the transmission lines W and P at the transmission line monitor20. The WDM device30is connected to the transmission device11-5through the redundant changeover apparatus40whereby upon a failure (fault) of the working transmission line W, the redundant changeover apparatus40performs the redundant changeover to the protection transmission line P to secure a connection with the transmission device11-5for achieving the protection of the transmission lines failed.

Also in the relaying node device10-6, although not shown, the redundant changeover apparatus40similarly performs a changeover from the working transmission line W to the protection transmission line P for the relaying operation.

FIG. 16shows a prior art example of the redundant changeover apparatus40in such a node device.

The redundant changeover apparatus40is composed of a frame terminal portion1, a frequency generator2, a changeover portion3, and a frame generator6.

The frame terminal portion1comprises a frame terminal unit1-1and a frame synchronizer1-2for terminating an overhead and an error correction code of a frame from the signal of the working transmission line W to extract transmission signals from the payload, so that from the frame synchronizer1-2a detection result1-3as to the synchronization is outputted.

This frame terminal portion1is similarly composed of a frame terminal unit1-4and a frame synchronizer1-5for the protection transmission line P.

The frequency generator2comprises a transmission frequency clock extractor2-1and a PLL (Phase Locked Loop) circuit2-2for the signals of the working transmission line W, and comprises a transmission frequency clock extractor2-3and a PLL circuit2-4for the signals of the protection transmission line P, so that based on the output signals of the PLL circuits2-2and2-4, the frame synchronizers1-2and1-5respectively detect the presence/absence of the frame synchronization.

The changeover portion3comprises a changeover switches3-11and3-12, in which a working system contact Sw of the changeover switch3-11is connected to the frame terminal unit1-1and the transmission frequency clock extractor2-1, and a protection system contact Sp of same is connected to the frame terminal unit1-4and the transmission frequency clock extractor2-3.

Also, a working system contact Sw of the changeover switch3-12is connected to the transmission frequency clock extractor2-1and a protection system contact Sp of same is connected to the transmission frequency clock extractor2-3.

The frame generator6comprises a frame adder6-1which inputs the output signal of the changeover switch3-11to be inserted with additional bits such as a frame synchronizing signal, and a frame generating unit6-2which inputs the output signal of the changeover switch3-12for the generation of a frame to be provided to the frame adder6-1.

A WDM transmission signal outputted from the redundant changeover apparatus40is subject to a frame synchronization by the frame terminal portion51comprising a frame terminal unit52and a frame synchronizer53in a receiver50.

In such a conventional ring network, a synchronization network is mainstream having a master clock source, from which clocks are provided to the node devices, thereby avoiding synchronization loss upon the redundant changeover.

On the other hand, recently, an asynchronous network is going mainstream so that master clocks may not spread over the node devices, whereas since a transparent transmission for each working system/protection system between node devices would be conducted in an asynchronous mode, changeover operations between the working system and the protection system in a failure such as a transmission failure or a device failure will be followed by instantaneous clock changeovers.

Therefore, it was disadvantageous that the receiver (seeFIG. 16) causes therein a frame synchronization loss in frequency and phase. This will be described hereinbelow referring toFIG. 17.

It is now supposed that the phases of a working system clock shown inFIG. 17Aand a working system frame shown inFIG. 17Bare outside phases of a protection system clock shown inFIG. 17Dand a protection system frame shown inFIG. 17C.

Namely, as shown inFIG. 15, the node devices10-1to10-3and the transmission devices11-1to11-3forming the network “A” are synchronized with each other, and the node devices10-4as well as10-5and the transmission devices11-4as well as11-5forming the network “B” are also synchronized with each other.

Also, the multiplexing node devices10-7and10-8as well as the transmission devices11-6and11-7forming the SONET/SDH ring network “C” are mutually in the synchronized state with each other.

However, in many cases, the networks “A”, “B”, and the SONET/SDH ring network “C” are asynchronous with each other, so that a phase shift or deviation as shown inFIGS. 17A-17Dwill arise.

Accordingly, when the phase of the working system frame shown inFIG. 17Bis instantaneously changed over to that of the protection system frame shown inFIG. 17Cby the changeover switches3-11and3-12, the phase of the overhead OH in the WDM transmission signal shown inFIG. 17Eoutputted from the frame generator6is deviated and provided to the receiver50, in which since there exists no overhead OH at the phase of a receiver clock RCK expected by the frame synchronizer53, a frame synchronization loss will arise.

Resultantly, this synchronization loss will extend to synchronizations losses {circle around (1)}-{circle around (9)} counterclockwise from the node device10-5where a changeover has arisen, as shown inFIG. 18, and eventually to the network RNW in its entirety.

Additionally, the re-synchronization of frame requires time for frame synchronization-protecting stages, and the sequence starting from the node device10-5.

The time for establishing the re-synchronization of all network nodes within the ring is an accumulation of the protection time, requiring a changeover time corresponding to the number of node devices×the protection time, resulting in a practically difficult operation.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide a redundant changeover apparatus, which can prevent a frame synchronization loss from arising even upon changeover between a working system and a protection system.

For the achievement of the above object, a redundant changeover apparatus according to the present invention is characterized by comprising: a changeover unit to change over two input signals which are mutually asynchronous in phase, an extracting unit to extract clocks from output signals of the changeover unit, a PLL circuit for inputting the extracted clocks, a clock changing unit to provide the output signals with clocks changed to output clocks of the PLL circuit, and a framing unit to frame output signals of the clock changing unit with the output clocks.

Namely according to in the present invention, when two input signals which are asynchronous with each other are changed over by a changeover unit such as an optical switch, a clock extracting unit extracts clocks from output signals of the changeover unit both before and after the changeover.

Then, a clock changing unit changes clocks before the changeover to clocks after the changeover for the signal transmission with the signal frequency being gradually followed by a PLL circuit.

Then, the signal after the clock change is framed by a framing unit with the output clocks of the PLL circuit.

Thus, no rapid change arises in the phase of the output signals upon the changeover from a working system to a protection system, so that such a problem as causing a frame synchronization loss is solved.

Alternatively, a redundant changeover apparatus according to the present invention may comprise: an extracting unit to extract data and clocks respectively of two input signals which are mutually asynchronous in phase, a first and a second reference clock changing unit to change the data with reference clocks inputted externally, a first changeover unit to change over data respectively outputted from the first and the second reference clock changing unit, a second changeover unit to change over both of the extracted clocks, and clock changing unit to gradually change output data of the first changeover unit from clocks before the changeover to clocks after the changeover by the second changeover unit.

Namely, while the present invention as described above performs a redundant changeover of an in-apparatus asynchronous system, this invention adopts an in-apparatus asynchronous system such that data of two input signals which are asynchronous with each other are changed to data with reference clocks inputted externally, by a first and a second reference clock changing unit.

When the data changed with the reference clocks are changed over by a first changeover unit, a clock changing unit provides the output data of the first changeover unit with the clocks before the changeover by a second changeover unit being gradually changed to clocks after the changeover by the second changeover unit which changes over clocks extracted from the above two input signals.

Thus, even upon a redundant changeover of an in-apparatus synchronous system, a frame synchronization loss can be similarly prevented.

For the above reference clocks, in-house clocks or free-running clocks may be employed.

Also, the above clock extracting unit may extract clocks from a wavelength division multiplexing device. It is unnecessary to extract clocks from a transmission device such as a SONET/SDH transmission device.

The above input signals may comprise working input signals and protection input signals from a wavelength division multiplexing device forming a ring network, or working input signals and protection input signals from an arbitrary transmission device such as for SONET/SDH.

Furthermore, a node device comprising the above redundant changeover apparatus provided in duplicate for same transmission lines of a working system and a protection system to generate outputs of the clock changing unit in the working system and the protection system.

In this case, each changeover unit may be commonly provided to each redundant changeover apparatus.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1shows an embodiment (1) of a redundant changeover apparatus according to the present invention.

Particularly, this redundant changeover apparatus is one of an in-apparatus asynchronous system, which is used for a relaying node function shown as the relaying node device10-6inFIG. 15.

In this embodiment (1), a frame terminal portion1and a frequency generator2are the same as the prior art ones shown inFIG. 16, but a changeover portion3comprises only a single changeover switch3-1having a working system contact Sw connected to a frame terminal unit1-1and a transmission frequency clock extractor2-1, and a protection system contact Sp connected to a frame terminal unit1-4and a transmission frequency clock extractor2-3.

The output signal of the changeover switch3-1passes through a frequency generator4and a frequency regenerator5before being sent to a frame generator6.

Namely, the frequency generator4comprises a transmission frequency clock extractor4-1and a PLL circuit4-2, where a transmission frequency clock in the output signal of the changeover switch3-1is extracted by the clock extractor4-1, and the PLL circuit4-2provides as an output a clock such that a clock before the changeover performed by the changeover switch3-1is changed to a clock after the changeover by the same.

The frequency regenerator5comprises a transmission frequency clock changing unit5-1such as a memory latch to write therein the output data of the changeover switch3-1, which are then read out with the output clock from the PLL circuit4-2for performing the clock change, and then sent to the frame generator6.

The output clock from the PLL circuit4-2is similarly sent to the frame generator6, which frames the input data based on the inputted clock.

It is to be noted that the clock extractor4-1, the PLL circuit4-2, and the clock changing unit5-1form a clock changing unit.

The operation of this embodiment (1) will now be described referring to a time chart showing inFIG. 2.

It is hereby supposed that when the phases of the working system clock and the working system frame are in the states shown inFIGS. 2A and 2B, the phases of the protection system clock and the protection system frame are respectively deviated from those of the working system as shown inFIGS. 2E and 2D.

The phase of the working system clock is extracted by the transmission frequency clock extractor4-1to be provided to the PLL circuit4-2. When the changeover switch3-1is changed over from the working system contact Sw from the protection system contact Sp by the detection results1-3and1-6or the transmission monitor (seeFIG. 15), the transmission frequency clock extractor4-1extracts the protection system clock changed to be provided to the PLL circuit4-2, which does not immediately provide the phase depending on the protection system clock but gradually sifts the phase as shown.

Accordingly, the transmission frequency clock changing unit5-1in the frequency regenerator5sends the data from the changeover switch3-1with the working system clock being gradually changed to the protection system clock to the frame generator6, so that in the receiver50shown inFIG. 16, the frequency of the WDM transmission signal does not vary abruptly, thereby causing no frame synchronization loss.

Hereinbelow, the operation principle how such a clock change is gradually done by the PLL circuit4-2will be described.

FIG. 3shows a general arrangement of the PLL circuit4-2shown inFIG. 1, which is composed of a phase comparator4-21for inputting a reference frequency “fref” and a comparing frequency “fcomp” (fvco) as shown, a loop filter4-22for inputting the output signal of the phase comparator4-21, and a voltage controlled oscillator (VCO)4-23for providing the above noted comparing frequency “fcomp” by receiving the output signal of the loop filter4-22as a control voltage.

Namely, the phase comparator4-21performs a phase comparison between the reference frequency “fref” and the comparing frequency “fcomp” generated by the free running oscillation of the voltage controlled oscillator4-23, and outputs the phase difference component as a phase difference signal in the form of pulse, the AC component of which is intercepted through the loop filter4-22composed of an integration circuit and a low pass filter to obtain only the DC component.

By applying the above DC signal to the control terminal of the voltage controlled oscillator4-23having a predetermined free running frequency as a control voltage, the comparing frequency is made consistent with the reference frequency.

Because of the loop filter4-22having an integration function as mentioned above, even if the reference frequency “fref” provided to the phase comparator4-21abruptly varies, the control voltage to the voltage controlled oscillator4-23outputted from the loop filter4-22gradually varies so that the comparing frequency “fcomp” gradually follows the reference frequency “fref”. Therefore, it becomes possible to transmit the output data with the clock before the changeover performed by the changeover switch3-1being gradually changed to the clock after the changeover.

FIG. 4shows an embodiment (2) of a redundant changeover apparatus according to the present invention. This embodiment (2) is different from the embodiment (1) in that the frequency generator4substitute a SONET/SDH clock extractor4-10for the transmission frequency clock extractor4-1, substitute a SONET/SDH clock changing unit5-10for the transmission frequency clock changing unit5-1in the frequency regenerator5, and eliminates the frame generator6.

Namely, this redundant changeover apparatus40bis used for dropping in the multiplexing node devices other than the relaying node device10-6in the optical ring network shown inFIG. 15.

Therefore, the clock extractor4-10extracts the SONET/SDH clock, not the transmission frequency clock, so that the clock changing unit5-10outputs the data with the SONET/SDH clock changed from the transmission frequency clock. Since the output data of the clock changing unit5-10are not required to be framed, they are to be directly sent to any transmission device or SONET/SDH transmission device.

FIG. 5shows an embodiment (3) of a redundant changeover apparatus according to the present invention. This embodiment (3) is different from the above-mentioned embodiments (1) and (2) in that a reference clock changing unit7and a reference clock selector8are further provided, and the changeover portion3comprises two changeover switches3-11and3-12.

Namely, while the above mentioned embodiments (1) and (2) adopt “in-apparatus asynchronous system” which makes no synchronization by particularly using the reference clock in the redundant changeover apparatus40, the embodiments (3) adopts “in-apparatus synchronous system” in which the working system data have the reference clock changed by the reference clock changing unit7-1, and the protection system data as the reference clock changed by the reference clock changing unit7-2.

The reference clock provided to the reference clock changing units7-1and7-2are furnished from the reference clock selector8which selects an in-house clock CLK1or a free-running clock CLK2based on a select signal SEL.

The changeover switch3-11has a connection so as to mutually change over the output data of the reference clock changing units7-1and7-2. Also, the changeover switch3-12has a connection so as to mutually change over the transmission frequency clocks of the working system and the protection system respectively extracted by the transmission frequency clock extractors2-1and2-3.

The data changed to have the reference clock outputted from the changeover switch3-11are provided to the transmission frequency clock changing unit5-1in the frequency regenerator.

Also, the transmission frequency clock outputted from the changeover switch3-12is provided to the PLL circuit5-2.

To the transmission frequency clock changing unit5-1the reference clock is provided from the reference clock selector8, so that the reference clock enables the data outputted from the changeover switch3-11to be written and the outputted clock from the PLL circuit5-2enables the same to be read out. Therefore, the data outputted from the transmission frequency clock changing unit5-1are to be sent to the frame generator6, with the transmission frequency clock of the working system being gradually changed to that of the protection system as in the case of the embodiments (1) and (2), thereby preventing the frame synchronization loss.

FIG. 6shows an embodiment (4) of a redundant changeover apparatus according to the present invention. This embodiment (4) is different from the above-mentioned embodiment (3) in that the frequency generator2is provided with a SONET/SDH clock extractor2-5for the data of the working system and with a SONET/SDH clock extractor2-6for the transmission signal of the protection system, and that clocks extracted by the clock extractors2-5and2-6are respectively provided to the working system contact Sw and protection system contact Sp of the changeover switch3-12.

Also, in the frequency regenerator5, a SONET/SDH clock changing unit5-10is substituted for the transmission frequency clock changing unit5-1.

Namely, while the above-mentioned embodiment (3) is applied to the relaying node device10-6shown inFIG. 15, this embodiment (4) has a dropping function in the multiplexing node devices other than the relaying node device10-6, and achieves redundant changeover to any transmission device or SONET/SDH transmission device that is a subordinate node device.

FIG. 7shows an application example of the embodiment (1) of the present invention shown inFIG. 1, in which there are provided in-apparatus asynchronous system-redundant changeover apparatuses40a-1and40a-2respectively having the same arrangement as the embodiment (1) in the counterclockwise direction and clockwise direction of the ring network RNW shown inFIG. 15.

These redundant changeover apparatuses40a-1and40a-2are employed in the relaying node device10-6shown inFIG. 15and the like as mentioned above.

Therefore, both sides of the redundant changeover apparatuses40a-1and40a-2are sandwiched by WDM devices30-1and30-2in the adjoining multiplexing node devices.

Namely, the working transmission line W1and the protection transmission line P1from the left side WDM device30-1are connected to the changeover portion3through a WDM input interface IF1composed of the frame terminal portion1and the frequency generator2in the lower redundant changeover apparatus40a-1, and the output signal thereof is sent to the right side WDM device30-2through a WDM output interface IF2composed of the frequency generator4and the frequency regenerator5.

Since this signal flow should be similarly applied to the signal flow of the transmission lines W2and P2with respect to the left side WDM device30from the right side WDM device30-2, the upper redundant changeover apparatus40a-2is similarly composed of the WDM input interface IF1, the changeover portion3, and the WDM output interface IF2.

Thus, between the WDM devices30-1and30-2, a relaying operation can be achieved without frame synchronization loss.

FIG. 8shows a modification of the application example of the embodiment (1) shown inFIG. 7.

This modification has an arrangement (quadruplex arrangement) having two transmission lines respectively for the clockwise direction and counter clockwise direction of the working transmission line W and the protection working line P in the optical ring network RNW shown inFIG. 15.

Namely, since the achievement of the duplicated network RNW inFIG. 15can not be realized with a single output signal for the redundant changeover apparatuses40a-1and40a-2inFIG. 7and so requires two output signals, each is provided in duplicate (in parallel) so as to provide the input signals to the WDM devices30-1and30-2for both of the working system and the protection system. This is further duplicated, resulting in a totally quadruplex system. It is to be noted that either the upper half or the lower half is enough for constructing the ring network shown inFIG. 15.

As regards the quadruplex arrangement, in the counter clockwise direction from the WDM device30-1to the WDM device30-2, working transmission lines W1and W3and the protection transmission lines P1and P3are provided, while in the clockwise direction, working transmission lines W2and W4and the protection transmission lines P2and P4are provided.

As regards one of them, that is a duplicate arrangement (upper part) the transmission lines W1and P1is connected to the WDM input interface IF11, which corresponds to the input interface IF1inFIG. 7, and also connected to the WDM input interface IF12in parallel thereto.

The WDM input interfaces IF11and IF12are respectively connected to the working transmission line W1and the protection transmission line P1through the optical switch3-1provided commonly to all of the interfaces and respectively through the WDM output interfaces IF25and IF26which correspond to the WDM output interface IF2inFIG. 7.

On the other hand, in the clockwise direction from the WDM device30-2to the WDM device30-1, WDM input interfaces IF15and IF16are connected in parallel with the working transmission line W2and the protection transmission line P2, and respectively connected to the transmission lines W2and P2respectively from the WDM output interfaces IF21and IF22through the optical switch3-1.

The relationship between the transmission lines W1, W2and P1, P2is similarly applied to the other duplicated arrangement (lower half), in which working transmission lines W3, W4and the protection transmission lines P3, P4are provided to achieve the redundant changeover with the WDM input interfaces IF13, IF14, IF17, IF18and the WDM output interfaces IF23, IF24, IF27, IF28.

FIG. 9shows an application example of the embodiment (2) shown inFIG. 4.

Namely, in the working transmission line W1and the protection transmission line P2from the WDM device30to the SONET/SDH transmission device11, the arrangement of the embodiment (2) inFIG. 4is inserted as it is, in which the frame terminal portion1and the frequency generator2form the WDM input interface IF1, and the frequency generator4and the frequency regenerator5form a SONET/SDH dropping interface IF3.

On the other hand, in the working transmission line W2and the protection transmission line P2from the SONET/SDH transmission device11to the WDM device30, the frame terminal portion1and the frequency generator2in the embodiment (2) shown inFIG. 4is not required, so that the working transmission line W2and the protection transmission line P2are directly connected to the changeover switch3-1from the SONET/SDH transmission device11.

Also, for providing the output signal of the changeover switch3-1to the WDM device, in addition to the frequency generator4and frequency regenerator5, a frame generator6composed of a frame adder6-1and a frame generating unit6-2is provided, in which the frequency generator4, the frequency regenerator5, and the frame generator6form a WDM output interface IF2to the WDM device30as also shown inFIG. 7.

Thus, in the dropping route from the WDM30to the SONET/SDH transmission device11, a redundant changeover can be performed by the redundant changeover apparatus40b-1, while in the adding (inserting) route from the SONET/SDH transmission device11to the WDM device30, a redundant changeover can be performed by the redundant changeover apparatus40b-2, without frame synchronization loss.

FIG. 10shows a modification of the application example of the embodiment (2) shown inFIG. 9. Also in this modification, as in the modification inFIG. 8, a quadruplex transmission line is set up, that is four working transmission lines W1-W4and four protection transmission lines P1-P4connect the WDM device30-1to the SONET/SDH transmission device11, and the WDM input interfaces IF11-IF14and WDM output interfaces IF21-IF24are provided between the WDM device30-1and the optical switch3-1as in the modification shown inFIG. 8.

Also, between the optical switch3-1and the SONET/SDH transmission device11, only the SONET/SDH dropping interface IF3in the redundant changeover apparatus40b-1shown inFIG. 9is related, so that the working transmission line W1, the protection transmission line P1, the working transmission line W3, and the protection transmission line P3are respectively provided with a dropping interfaces IF31-IF34, which is different from the modification inFIG. 8.

FIG. 11shows an application example of the embodiment (3) of the in-apparatus synchronous system-redundant changeover apparatus shown inFIG. 5, which is connected between the WDM devices30-1and30-2to serve the relaying node function as in the application example of the embodiment (1) inFIG. 7.

Namely, in the counter clockwise direction from the WDM device30-1to the WDM device30-2, an in-apparatus synchronous system-redundant changeover apparatus40c-1is inserted, while in the clockwise direction from the WDM device30-2to the WDM device30-1, an in-apparatus synchronous system-redundant changeover apparatus40c-2is inserted.

These redundant changeover apparatuses40c-1and40c-2have the same arrangement as the embodiment (3) shown inFIG. 5.

In this case, the frame terminal portion1, the frequency generator2, and the reference clock changing portion7form a WDM input interface IF4, and the frequency regenerator5and the frame generator6form a WDM output interface IF5.

Thus, even in the case where the redundant changeover apparatus adopts the in-apparatus synchronous system, the redundant changeover between the WDM devices can be performed without frame synchronization loss.

FIG. 12shows a modification of the application example of the embodiment (3) shown inFIG. 11. Also in this modification, the embodiment (3) shown inFIG. 5is inserted in the working transmission lines W1-W4and the protection transmission lines P1-P4connected between the WDM device30-1and WDM device30-2in the same manner as the application example inFIG. 11.

In addition, an optical switch and an electric switch are provided as the changeover switches3-11and3-12commonly to the interfaces.

It is to be noted that to the interfaces the reference clock is furnished from the reference clock selector8.

For this purpose, WDM input interfaces IF41-IF44having the same arrangement as the WDM input interface IF4shown inFIG. 11and WDM output interfaces IF51-IF54having the same arrangement as the WDM output interface IF5are inserted.

Namely, between the WDM device30-1and the optical switch3-11or the electric switch3-12, two WDM input interfaces IF41and IF42are connected in parallel to the transmission line W1, and two WDM input interfaces IF43and IF44are connected in parallel to the transmission lines W3and P3.

Also, the WDM output interfaces IF51and IF52are respectively inserted in the transmission lines W2and P2, and the WDM output interfaces IF53and IF54are inserted in the transmission lines W4and P4.

Between the optical switch3-11or the electric switch3-12and the WDM device30-2, WDM output interfaces IF55and IF56are inserted in the transmission lines W1and P1, and WDM input interfaces IF45and46are inserted in the transmission lines W2and P2.

Similarly in the transmission lines W3and P3, WDM output interfaces IF57and IF58are inserted, and in the transmission lines W4and P4, WDM input interfaces IF47and IF48are inserted.

FIG. 13shows an application example of the embodiment (4) shown inFIG. 6

In this application example, for the embodiment (4), redundant changeover apparatuses40d-1and40d-2of an in-apparatus synchronous system are inserted between the WDM device30and the SONET/SDH transmission device11.

In this case, the arrangement of the redundant changeover apparatus40d-1is the same as that of the embodiment (4) inFIG. 6, whereas the redundant changeover apparatus40d-2requires no frame terminal portion to the working signal and the protection signal from the SONET/SDH transmission device11so that those signals are directly inputted to the frequency generator2which forms a SONET/SDH input interface IF8in combination with the reference clock changing portion7.

It is to be noted that the transmission frequency clock extractor2-1and the PLL circuit2-2used in the embodiment (4) are not required also in the frequency generator2.

In this way, an in-apparatus synchronous system-redundant changeover is achieved between the WDM device30and the SONET/SDH transmission device.

FIG. 14shows a modification of the application example of the embodiment (4) shown inFIG. 13.

Also in this modification, the WDM device30and SONET/SDH transmission devices11are connected through the working transmission lines W1-W4and the protection transmission lines P1-P4.

Additionally, between the WDM device30and the switches3-11,3-12, the WDM input interfaces IF61and IF62are connected in parallel to the transmission lines W1and P1, and the WDM input interfaces IF63and IF64are connected in parallel to the transmission lines W3and P3.

In the transmission lines W2and P2, the WDM output interfaces IF51and IF52are respectively inserted, and in the transmission lines W4and P4, the WDM output interfaces IF53and IF54are inserted.

Furthermore, between the optical switch3-11and the SONET/SDH transmission device11, the SONET/SDH dropping interfaces IF71and IF72are inserted in the transmission lines W1and P1, and the SONET/SD H dropping interfaces IF73and IF74are inserted in the transmission lines W3and P3.

In the transmission lines W2and P2, SONET/SDH input interfaces IF81and IF82are inserted in parallel, and in the transmission lines W4and P4, SONET/SDH input interfaces IF83and IF84are inserted in parallel.

Also in this modification, the optical switch3-11and the electric switch3-12form the changeover portion3as in the case of the modification shown inFIG. 12.

As described above, a redundant changeover apparatus according to the present invention is arranged such that in case of as an in-apparatus asynchronous system, when two input signals which are mutually asynchronous in phase are changed over, a PLL circuit and a clock changing unit transmit signals with clocks before the changeover being gradually changed to clocks after the changeover, or in case of an in-apparatus synchronous system, data are separated from clocks so that the data are once changed to data with the reference clock and then for the data, the clocks before the changeover is gradually changed to clocks after the changeover. Therefore, frame phases are not rapidly varied, so that frame synchronized state can be maintained in the latter receiver.

Accordingly, the present invention can achieve a transparent transmission of any signals because of an asynchronous network, in which a redundant changeover can be performed without synchronization loss spreading over the network in its entirety.

Because of the transparency, it becomes possible to connect a redundant changeover apparatus to an asynchronous multiplexing transmission line such as WDM, and directly connect, to a network, the transmission signal which was required to be format-converted into e.g. SONET/SDH in the past because the frame is generated from arbitrary data.

In case of a SONET/SDH transmission device, it has been required to perform pointer processing for absorbing the difference between device frequency and input frequency to change an input signal to device clocks, whereas in the present invention no pointer processing is required because the input signal is linearly followed by device clocks, enabling a transparent transmission including clocks.

Furthermore, it is possible to construct a network (ring network) like a synchronous network by means of overhead having an equal function to SONET/SDH.

Also, upon occurrence of line failures or the like in the WDM device, which lacks a redundant changeover function, it becomes unnecessary to make redundant changeover in SONET/SDH devices forming a subordinate network in the presence of the redundant changeover apparatus. Accordingly, for network signals to be multiplexed, it becomes unnecessary to frequently change the frame format or transmission speed for example at the time of redundant changeover within SONET/SDH devices.

Furthermore, without being multiplexed with e.g. SONET/SDH signals, direct WDM of arbitrary transmission signals will cause no line disconnection due to the redundant changeover.