Abstract:
An alarm detection apparatus includes a plurality of alarm detectors detecting and/or cancelling alarms for identical and different error rates. The plurality of alarm detectors are grouped into a major detector unit made up of alarm detectors which detect major error rates, and a minor detector unit made up of alarm detectors which detect minor error rates. The major detector unit and the minor detector unit output detection outputs corresponding to specified detection rates thereof. A predetermined alarm detector corresponding to a part of the minor detector unit has a specified detection rate overlapping a specified detection rate of the major detector unit being controlled, so that a detection function or a detection output of the predetermined alarm detector is disabled.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to alarm detection apparatuses, and more particularly to an alarm detection apparatus which detects and/or cancels an alarm depending on an error rate of a data communication line. 
     In systems such as a Synchronous Optical Network (SONET) or a Synchronous Digital Hierarchy (SDH), a quality of a digital line is monitored by use of a B 2  byte (BIP: Bit Interleaved Parity-8) or a B 2  byte (BIP-8×N(in the case of the SONET)/BIP-N×24 (in the case of the SDH) of a frame format. The present invention is suited for application to such systems. 
     2. Description of the Related Art 
     FIG. 1 is a diagram showing a frame format of a SONET Synchronous Transport Signal-N (STS-N) for a case where N=192. At a transmitting end, a BIP operation result indicated by a hatched portion of an nth frame before scrambling is inserted into the B 2  byte of a (n+1)th frame before the scrambling. On the other hand, at a receiving end, a BIP operation result with respect to the nth frame after descrambling thereof and a B 2  byte of the (n+1)th frame after descrambling are compared, so as to detect a BIP error. According to STS- 192 , 1 B 2  byte is multiplexed 192 times, and BIP-8× 192  (8×192=1536 bits) BIP operations are carried out in total. Although not shown in FIG. 1, BIP operation and monitoring are similarly carried out with respect to the B 1  byte, although an operation range differs from that for the B 2  byte. 
     For the sake of convenience, the error rate will be described with respect to the BIP- 8  of the STS- 1  in order to simplify the description. The error rate for a case where only 1 bit within 1 frame is in error and no error exists in the other bits can be described by 1/(801×8)=1/6408≠1.5×10 −4 . Accordingly, it is possible to monitor whether or not the error rate is 1×10 −9  or greater, for example, by monitoring whether or not a BIP- 8  error of 1 bit exists in 1.5×10 5  frames or, a BIP- 8  error of 10 or more bits exist in 1.5×10 6  frames. Timewise, 1 frame period of the STS-N (STM-N) is 125 μsec, and thus, it requires at least approximately 19 sec in order to monitor the above error rate of 1×10 −9 . Actually, there are cases where a check is made to determine whether or not such a single bit error occurs 100 or more times so as to improve the monitoring accuracy. In such cases, the monitoring unit becomes 100 times the above period, that is, approximately 32 minutes. 
     FIG. 2 is a system block diagram showing a conventional alarm detector. FIG. 2 shows a typical construction which is used in common for alarm detectors  10   1  through  10   10  which will be described later. In FIG. 2, an error counter (ERCT)  11  counts an error bit B 2 E of the BIP (B 2  byte), and a protection counter (PRCT)  12  counts a number of protection times of the alarm detection and/or cancellation. A frame counter (FRCT)  13  counts a frame pulse B 2 FP which has a period of 125 μsec and is generated in synchronism with the B 2  byte of the STS-N frame. A timing decoder (TDC 1 )  15 - 1  decodes a counter output Q of the frame counter  13 , and generates an alarm detection timing signal (for example,  1 T) which is used for alarm detection. A hysteresis counter (HYCT)  14  counts a pulse signal which is generated with a period of the alarm detection timing signal  1 T. A timing decoder (TDC 2 )  15 - 2  decides a counter output Q of the hysteresis counter  14 , and generates an alarm cancel detection timing signal (for example,  10 T which is 10 times the period) which is used for alarm cancel detection. Comparators (CM 1 , CM 2 )  17 - 1  and  17 - 2 , AND gate circuits (A 1  through A 5 )  18 - 1  through  18 - 2 , a selector (SL 1 )  16 , edge detection circuits (EG 1 , EG 2  and EG 4  through EG 6 )  19 - 1 ,  19 - 2  and  19 - 4  through  19 - 6  for detecting rising and/or falling edges of input signals and generating edge pulse signals EP 1 , EP 2  and the like, a flip-flop (FF 1 )  1  for holding an alarm detection and/or cancel state, and OR gate circuits (ORI, OR 2 )  2 - 1  and  2 - 2  are connected as shown in FIG.  2 . 
     Although not shown in FIG. 2, a system clock signal CK 19  is input to a clock input terminal CK or the like of each of the counters  11  through  14  and the like. In addition, a system reset signal CL is input to a reset terminal R of each of the counters  11  through  14  and the flip-flop  1 . 
     Next, a description will be given of the operation of the alarm detector  101 . The alarm detector  10   1  is in an alarm detection mode when the flip-flop  1  is reset. In this case, the AND gate circuits  18 - 1  and  18 - 3  are closed, and the selector  16  selectively outputs the alarm detection timing signal  1 T. As a result, the error counter  11  counts the error bit signal B 2 E of the B 2  byte generated during an interval of the timing (gate) signal  1 T. In this state, the comparator  17 - 1  compares the counter output Q of the error counter  11  and a predetermined threshold value which is 980, for example. When the timing signal  1 T thereafter falls, the edge pulse signal EP 1  is generated in synchronism with this fall of the timing signal  1 T, and if the counter output Q of the error counter  11  is greater than or equal to 980 at this timing, the AND gate circuit  18 - 1  is opened and the protection counter  12  is incremented by +1. If the counter output Q of the error counter  11  is consecutively greater than or equal to 980 with respect to each detection period  1 T, the protection counter  12  is incremented by +1 each time. However, if the counter output Q of the error counter  12  becomes less than 980 at least once during the above time, the protection counter  12  is reset and the count is restarted from the beginning. In this state, the comparator  17 - 2  compares the counter output Q of the protection counter  12  and a predetermined threshold value which is 58, for example. Hence, if the counter output Q of the protection counter  12  is greater than or equal to 58 at the timing of each edge pulse signal EP 2  following the edge pulse signal EP 1 , the AND gate circuit  18 - 3  is opened and the flip-flop  1  is set to thereby output an alarm detection signal ALD 1 =1. In addition, when the flip-flop  1  is set, the protection counter  12  is reset, and the alarm detector  10   1  then assumes an alarm cancel detection mode. 
     In the alarm cancel detection mode, the AND gate circuits  18 - 2 ,  18 - 4  and  18 - 5  are closed, and the selector  16  selectively outputs the alarm cancel detection timing signal  10 T. Hence, the error counter  11  counts the error bit signal B 2 E of the B 2  byte generated during an interval of the timing (gate) signal  10 T. In this state, the comparator  17 - 1  compares the counter output Q of the error counter  11  and the and a predetermined threshold value which is 980, for example. When the timing signal  10 T thereafter falls, the edge pulse signal EP 1  is generated in synchronism with this fall of the timing signal  10 T, and if the counter output Q of the error counter  11  is less than 980 at this timing, the AND gate circuit  18 - 2  is opened and the protection counter  12  is incremented by +1. If the counter output Q of the error counter  11  is consecutively less than 980 with respect to each detection period  10 T, the protection counter  12  is incremented by +1 each time. However, if the counter output Q of the error counter  12  becomes greater than or equal to 980 at least once during the above time, the protection counter  12  is reset and the count is restarted from the beginning. In this state, the comparator  17 - 2  compares the counter output Q of the protection counter  12  and a predetermined threshold value which is 58, for example. Hence, if the counter output Q of the protection counter  12  is greater than or equal to 58 at the timing of each edge pulse signal EP 2  following the edge pulse signal EP 1 , the AND gate circuit  18 - 4  is opened and the flip-flop  1  is reset to thereby output an alarm detection signal ALD 1 =0. In addition, when the flip-flop  1  is reset, the protection counter  12  is reset, and the alarm detector  10   1  then assumes the alarm detection mode. 
     FIG. 3 is a diagram showing various kinds of setting information for making the alarm detection and/or cancellation. In FIG. 3, in a case where the detected error rate is 10 −3 , it is a condition for that alarm detection that the number of errors within 1 frame interval ( 1 T) is greater than or equal to 980 and that this state continues for 58 times or more. The detection time for this case is 7.25 msec. On the other hand, 10 times the period ( 10 T) of the alarm detection is employed for the alarm cancel detection in this case, and a so-called hysteresis control is carried out such that the monitoring conditions are different between the alarm detection and the alarm cancel detection. That is, the condition of the alarm cancel detection is that the number of errors within 10 frames ( 10 T) becomes less than 980 consecutively for 58 or more times. Similar conditions are determined with respect to other error rates of 10 −4  through 10 −10 , and examples of the monitoring conditions with respect to each of the error rates are shown in FIG.  3 . 
     FIG. 4 is a system block diagram showing a conventional alarm detection apparatus. FIG. 4 shows the construction for realizing the monitoring conditions shown in FIG.  3 . In FIG. 4, a major detector unit  21  detects major alarms MAJALM in the system, and includes alarm detectors  10   1  through  10   3  which respectively detect a relatively large number of error rates 10 −3  through 10 −5 . A selector (SL 1 )  23  selectively outputs one of alarm signals MAAL 1  through MAAL 3  depending on a major detection selection signal MAJRT [ 1 - 3 ] which is input to the system. Hence, the system can monitor the existence of a desired one of the major error rates which is generated. A minor detector unit  22  detects minor alarms MINALM in the system, and includes alarm detectors  10   4  through  10   10  which respectively detect a relatively small number of error rates 10 −4  through 10 −10 . A selector (SL 2 )  24  selectively outputs one of alarm signals MIAL 1  through MIAL 8  depending on a minor detection selection signal MINRT [ 1 - 7 ] which is input to the system. Hence, the system can monitor the existence of a desired one of the minor error rates which is generated. An OR gate circuit (OR 1 )  25  receives a system reset signal RST and a minor reset signal MINRST which will be described later. 
     A monitoring function of the major detector unit  21  is reset by the system reset signal RST such as a power ON reset signal. On the other hand, a monitoring function of the minor detector unit  22  is reset by the system reset signal RST or the minor reset signal MINRST. The minor reset signal MINRST is generated when a signal disconnection LOS of the line, a synchronization error LOF of the STS-N frame or the like is detected by the system. When a large number of faults is generated, it is sufficient to activate the functions of the major detector unit  21 , and the functions of the minor detector unit  22  are deactivated. 
     In the major detector unit  21 , the alarm detection signal MAAL 1  from an output terminal ALD of the alarm detector  101  is input to an input terminal ALI of the alarm detector  10   2 , and the alarm detection signal MAAL 2  from an output terminal ALD of the alarm detector  10   2  is input to an input terminal ALI of the alarm detector  10   3 . The alarm detection signals MAAL 1  and MAAL 2  are input to the edge detection circuit (EG 5 )  19 - 5  shown in FIG. 2, and act so as to forcibly set the flip-flop (FF 1 )  1  in synchronism with the rising edge thereof. Hence, if an alarm signal of the error rate of 10 −4  is detected in FIG. 4, the alarm signal of the error rate of 10 −5  is forcibly set at the same time. In addition, if an alarm signal of the error rate of 10 −3  is detected, the alarm signals of the error rates of 10 −4  and 10 −5  are forcibly set at the same time. Accordingly, even if the detecting conditions (periods) differ among the alarm detectors  10   1  through  10   3 , when an alarm of a relatively high error rate is detected, all alarms of error rates lower than this relatively high error rate are also detected at the same time, so that detections reasonably adapted to the actual error generation state is realized. Similar detections are also made in the minor detector unit  22 . 
     On the other hand, in the major detector unit  21 , it is known to input the alarm detection signal MAAL 1  from the most significant alarm detector  10   1  to input terminals MAAL 1  of each of the less significant alarm detectors  10   2  and  10   3  as indicated by a dotted line in FIG.  4 . The alarm detection signal MAAL 1  from the most significant alarm detector  10   1  is input to the edge detection circuit (EG 6 )  19 - 6  which resets or initializes the hysteresis counter  14  and the protection counter  12  in response to the falling edge of the alarm detection signal MAAL 1 , that is, in response to the alarm cancel detection, as indicated by a dotted line in FIG.  2 . Hence, when the alarm detection signal MAAL 1  from the most significant alarm detector  10   1  shown in FIG. 4 is cancelled, the detection phases for the alarm cancellation in each of the less significant alarm detectors  10   2  and  10   3  are simultaneously synchronized to the cancellation timing of the alarm detection signal MAAL 1  from the most significant alarm detector  10   1 , and the detecting operations for the alarm cancellation are simultaneously started. Although not shown in FIG. 4, it is of course possible to construct the minor detector unit  22  similarly to the major detector unit  21  described above. 
     But in the conventional system described above, in a case where the error rates such as the error rates 10 −4  and 10 −5  to be detected by the major detector unit  21  and the minor detector unit  22  overlap, no problems will occur if the detection and/or cancellation timings of the major detector unit  21  and the minor detector unit  22  match, however, the detection and/or cancellation timings may not necessarily match. For example, the detection and/or cancellation timings will not match if the reset conditions of the major detector unit  21  and the minor detector unit  22  are different. When the detection and/or cancellation timings of the major detector unit  21  and the minor detector unit  22  are different, there is a problem in that the same kind of alarm signal will be detected and/or be cancelled at different timings within the system. 
     In addition, according to the conventional system described above which synchronizes the alarm cancel detection timings of each of the less significant alarm detectors  10   2  and  10   3  to the cancellation timing of the alarm detection signal MAAL 1  from the most significant alarm detector  10   1 , the following problems occur. 
     FIGS. 5 and 6 are timing charts for explaining the operation of the conventional alarm detection apparatus. In FIGS. 5 and 6, it is assumed for the sake of convenience that the detection period is  1 T,  3 T and  5 T in the most significant order, and that no hysteresis control is carried out for the alarm cancel detection. 
     FIG. 5 shows a case where an error B 2 E generated at a high density disappears quickly. The most significant alarm signal ALD 1  is quickly set by the generation of the high density burst error, and the less significant alarm signals ALD 2  and ALD 3  are simultaneously set forcibly in response to the setting of the most significant alarm signal ALD 1 . Next, when the most significant alarm signal ALD 1  is quickly reset (cancelled) due to a rapid decrease of the error generation density, the phases of the alarm cancel detection timing signals  3 T and  5 T for the less significant alarms are synchronized to the cancellation timing of the alarm signal ALD 1 . Hence, in this particular case, the alarm signal ALD 2  is cancelled at a timing  3 T after the cancellation of the most significant alarm signal ALD 1 , and the alarm signal ALD 3  is cancelled at a timing  5 T after the cancellation of the most significant alarm signal ALD 1 . Consequently, it is possible to quickly carry out the alarm detection and/or cancellation operation which is adapted to the actual generation and/or disappearance of the error B 2 E. 
     On the other hand, FIG. 6 shows a case where the error B 2 E generated at a high density gradually reduces its generation rate and disappears. The most significant alarm signal ALD 1  is quickly set by the generation of the high density burst error, and is reset quickly as the density thereafter decreases. With respect to the alarm signal ALD 2 , the alarm cancel detection is started in synchronism with the cancellation timing of the most significant alarm signal ALD 1 . However, the error density is greater than or equal to a first predetermined value in the first  3 T interval and the alarm signal ALD 2  is not reset in this first  3 T interval, and is finally reset in the second  3 T interval. With respect to the alarm signal ALD 3 , the alarm cancel detection is started in synchronism with the cancellation timing of the most significant alarm signal ALD 1 . However, the error density is greater than or equal to a second predetermined value which is lower than the first predetermined value in the first and second  5 T intervals and the alarm signal ALD 3  is not reset in these first and second  5 T intervals, and is finally reset in the third  5 T interval. Accordingly, there is a problem in that the less significant alarm signal ALD 3  is not reset for a considerably long time. This problem is caused by the overlap of the alarm cancel detection timings between the less significant alarm detectors  10   2  and  10   3 , and because the error bit B 2 E in the intervals are taken into account for the evaluation by both of the alarm detectors  10   2  and  10   3 . In this particular case, the periods of the alarm detection cancellation are  1 T,  3 T and  5 T, but the periods of the actual alarm detection cancellation are generally much larger and are  10 T,  100 T and  1000 T, for example. As a result, the differences among the periods of the actual alarm detection cancellation is extremely large, thereby making the delay of the alarm cancel detection no longer negligible. 
     Furthermore, as shown in FIG. 2, when each alarm detector  10  is constructed to include the frame counter  13  and the hysteresis counter  14 , there is a problem in that the circuit scale of the frame counter  13  and the hysteresis counter  14  becomes larger for the alarm detectors provided for the lower error rates as compared to the alarm detectors provided for the higher error rates. In other words, the most significant alarm detector  10   1  shown in FIG. 2 simply generates the alarm detection timing signal  10 T and the alarm cancel detection timing signal  10 T based on the frame pulse signal B 2 FP, but the least significant alarm detector  10   10  shown in FIG. 4 must generate the alarm detection and alarm cancel detection timing signals  4000000 T based on the frame pulse signal B 2 FP. For this reason, a large scale counter circuit is required in the alarm detection apparatus as a whole. Further, since the scale of the counter differs for each alarm detector  10 , there is another problem in that a common circuit construction cannot be used for each of the alarm detectors  10 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful alarm detection apparatus in which the problems described above are eliminated. 
     Another and more specific object of the present invention is to provide an alarm detection apparatus which can appropriately detect and/or cancel the alarm by use of a simple construction. 
     Still another object of the present invention is to provide an alarm detection apparatus comprising a plurality of alarm detectors detecting and/or cancelling alarms for identical and different error rates, where the plurality of alarm detectors are grouped into a major detector unit made up of alarm detectors which detect major error rates and a minor detector unit made up of alarm detectors which detect minor error rates, the major detector unit and the minor detector unit output detection outputs corresponding to specified detection rates thereof, and a predetermined alarm detector corresponding to a part of the minor detector unit has a specified detection rate overlapping a specified detection rate of the major detector unit being controlled, so that a detection function or a detection output of the predetermined alarm detector is disabled. According to the alarm detection apparatus of the present invention, it is possible to effectively avoid an undesirable situation where the alarm detection and/or cancel signals are output from the major and minor detector units at different timings for the overlapping detection rates. In addition, it is also possible to effectively avoid a contradictory situation where the alarm detection and/or cancel signal output from the minor detector unit is for a detection rate higher than that for the major detector unit. 
     A further object of the present invention is to provide an alarm detection apparatus comprising a plurality of alarm detectors detecting and/or cancelling alarms for different error rates, and means, responsive to an alarm detection in an arbitrary alarm detector of the plurality of alarm detectors, for forcibly setting an alarm detection output of each of the plurality of alarm detectors which detect error rates smaller than that detected by the arbitrary alarm detector, where each of the alarm detectors starts a detection period for an alarm cancel detection thereof in synchronism with a detection of an alarm cancellation in an alarm detector which detects an error rate one level higher than an error rate detected thereby. According to the alarm detection apparatus of the present invention, no overlap of the alarm cancel detection periods occur between the alarm detectors. Hence, it is possible to effectively avoid an undesirable situation where the same error signal is counted by both two alarm detectors within the respective alarm cancel detection periods. As a result, it is possible to effectively avoid a situation where the alarm cancel detection is unnecessarily extended as was the case of the conventional alarm detection apparatus. 
     Another object of the present invention is to provide an alarm detection apparatus comprising a plurality of alarm detectors detecting alarms for different error rates, where each of the plurality of alarm detectors detects an alarm of its own detection rate depending on an input timing signal and outputting a timing signal having a period which is n times a period of the input timing signal, and the plurality of alarm detectors are successively coupled in a cascade connection so that a timing signal output from one alarm detector is input to another alarm detector provided in a next stage. According to the alarm detection apparatus of the present invention, it is possible to greatly reduce the circuit scale of a timing generating circuit within each alarm detector, and the circuit scale of the alarm detection apparatus as a whole is greatly reduced. In addition, since the same circuit construction can basically be used for the alarm detectors, the construction of the alarm detection apparatus becomes simple and easy to design, thereby making it possible to reduce the cost of the alarm detection apparatus. 
     Still another object of the present invention is to provide an alarm detection apparatus comprising a timing generator generating a plurality of kinds of timing signals required to detect and/or cancel alarms of different error rates based on a pulse signal having a basic period, a plurality of alarm detectors detecting and/or cancelling alarms for different alarms, each of the plurality of alarm detectors detecting and/or cancelling an alarm depending on a detection rate thereof based on an input timing signal, and a timing selector, interposed between the timing generator and the plurality of alarm detectors, distributing the plurality of kinds of timing signals from the timing generator to the plurality of alarm detectors, where the timing selector supplies an alarm detection timing signal with respect to a corresponding alarm detector when no alarm is detected by the corresponding alarm detector, and supplies an alarm cancel detection timing signal with respect to the corresponding alarm detector when an alarm is detected by the corresponding alarm detector. According to the alarm detection apparatus of the present invention, it is possible to simplify the circuit construction because the circuit parts are separated depending on the primary functions. Further, the timing generator can generate the timing signals with periods of freely variable multiplication factors relative to the period of the input timing signal, and select the desired alarm detection timing signal and alarm cancel detection timing signal from these timing signals and supply these signals to the alarm detector. Therefore, it is possible to easily construct a flexible alarm detection apparatus in terms of the various alarm detection and/or cancellation conditions to be satisfied. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a frame format of a SONET Synchronous Transport Signal-N (STS-N) for a case where N=192; 
     FIG. 2 is a system block diagram showing a conventional alarm detector; 
     FIG. 3 is a diagram showing various kinds of setting information for making the alarm detection and/or cancellation; 
     FIG. 4 is a system block diagram showing a conventional alarm detection apparatus; 
     FIG. 5 is a timing chart for explaining the operation of the conventional alarm detection apparatus for a case where an error B 2 E generated at a high density disappears quickly; 
     FIG. 6 is a timing chart for explaining the operation of the conventional alarm detection apparatus for a case where the error B 2 E generated at a high density gradually reduces its generation rate and disappears; 
     FIG. 7 is a system block diagram for explaining the operating principle of the present invention; 
     FIG. 8 is a system block diagram showing a first embodiment of an alarm detection apparatus according to the present invention; 
     FIG. 9 is a system block diagram showing an alarm detector of the first embodiment of the alarm detection apparatus; 
     FIG. 10 is a timing chart for explaining the operation of the first embodiment of the alarm detection apparatus for a case where an error B 2 E generated at a high density disappears quickly; 
     FIG. 11 is a timing chart for explaining the operation of the first embodiment of the alarm detection apparatus for a case where the error B 2 E generated at a high density gradually reduces its generation rate and disappears; 
     FIG. 12 is a system block diagram showing a second embodiment of the alarm detection apparatus according to the present invention; 
     FIG. 13 is a system block diagram showing an alarm detector of the second embodiment of the alarm detection apparatus; 
     FIG. 14 is a timing chart for explaining the operation of the second embodiment of the alarm detection apparatus; 
     FIG. 15 is a system block diagram showing a third embodiment of the alarm detection apparatus according to the present invention; 
     FIG. 16 is a system block diagram showing a timing generator of the third embodiment of the alarm detection apparatus; 
     FIG. 17 is a system block diagram showing a part of a timing selector of the third embodiment of the alarm detection apparatus; 
     FIG. 18 is a system block diagram showing another part of the timing selector of the third embodiment of the alarm detection apparatus; 
     FIG. 19 is a system block diagram showing an alarm detection module of the third embodiment of the alarm detection apparatus; and 
     FIG. 20 is a system block diagram showing an alarm detector of the third embodiment of the alarm detection apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First, a description will be given of the operating principle of the present invention, by referring to FIG.  7 . 
     An alarm detection apparatus shown in FIG. 7 includes a plurality of alarm detectors  30 . For the sake of convenience, only three alarm detectors  30   1  through  30   3  are shown in FIG.  7 . Each of the alarm detectors  30   1  through  30   3  makes an alarm detection with its own detection rate depending on an input timing signal TI, and generates and outputs a timing signal TO having a period which is n times that of the input timing signal TI based on the input timing signal TI, where n is an arbitrary integer. The alarm detectors  30   1  through  30   3  are successively connected in a cascade connection, so that the output timing signal TO of one alarm detector  30  is input to another alarm detector  30  in a next stage as the input timing signal TI. Hence, each of the alarm detectors  30   1  through  30   3  makes the alarm detection with respect to a different error rate. 
     In this case, each of the alarm detectors  30   1  through  30   3  only needs to generate an output timing signal TO having a period which is at a maximum only about 10 times that of an input timing signal TI input thereto. For example, with respect to the input timing signal TI having the period  1 T, the periods of the output timing signals TO of the alarm detectors  30   1  through  30   3  respectively are  4 T,  40 T and  400 T. 
     For this reason, the scale of the timing generation circuit or counter circuit within each of the alarm detectors  30   1  through  30   3  is greatly reduced, thereby making it possible to greatly reduce the circuit scale of the alarm detection apparatus as a whole. In addition, since the same circuit construction can be used for each of the alarm detectors  30   1  through  30   3 , the construction of the alarm detection apparatus becomes simple and easy to design, and as a result, the cost of the alarm detection apparatus can be reduced. 
     Next, a description will be given of a first embodiment of the alarm detection apparatus according to the present invention. FIG. 8 is a system block diagram showing the first embodiment of the alarm detection apparatus. In FIG. 8, those parts which are the same as those corresponding parts in FIG. 4 are designated by the same reference numerals, and a description thereof will be omitted. 
     In FIG. 8, a major detector unit  21  includes alarm detectors  20   1  through  20   3  and a selector (SL 1 )  23  which are connected as shown, and a minor detector unit  22  includes alarm detectors  20   4  through  20   10 , a selector (SL 2 )  24 , and OR gate circuits (O 1 , O 2  and O 3 )  25 ,  26  and  27  which are connected as shown. Each of the alarm detectors  20   1  through  20   10  has a construction which is basically the same as that of the alarm detector  10  shown in FIG. 2 described above. 
     This embodiment is characterized in that, when overlapping error rates such as 10 −4  and 10 −5  are detected between the major detector unit  21  and the minor detector unit  22 , for example, the detection made in the major detector unit  21  is given priority over the detection made in the minor detector unit  22 , and the detection in the minor detector unit  22  is deactivated. As a result, it is possible to effectively avoid an undesirable situation where the alarm detection and/or cancellation signals MAJALM and MINALM are output at different timings from the major detector unit  21  and the minor detector unit  22  with respect to the same error rate. 
     The detection in the minor detector unit  22  can be deactivated by various methods. According to one method, the function of the alarm detector  20   4  is reset by the major selection signal MAJRT[ 2 ]=1, and the functions of the alarm detectors  20   4  and  20   5  are reset by the major selection signal MAJRT[ 3 ]=1, as shown in FIG.  8 . 
     On the other hand, according to another method, the alarm detection signal MIAL 2  from the alarm detector  20   4  is turned OFF at the input side of the selector (SL 2 )  24  by the major selection signal MAJRT[ 2 ]=1, and the alarm detection signals MIAL 2  and MIAL 3  from the alarm detectors  20   4  and  20   5  are turned OFF at the input side of the selector (SL 2 )  24  by the major selection signal MAJRT[ 3 ]=1, although not shown in FIG.  8 . According to this latter method, the alarm detectors  20   4  and  20   5  can operate normally, so that the minor detector unit  22  as a whole can operate normally. 
     Of course, the application of the above described features is not limited to this embodiment, and similar applications can be made with respect to each of the following embodiments and also to the conventional alarm detection apparatus shown in FIG.  2 . 
     Furthermore, this embodiment is also characterized in that, the alarm detector  20  provided at each stage starts the detection period for its own alarm cancellation in phase synchronism with the alarm cancel detection in an alarm detector  20  provided at a preceding stage, that is, in a level which is one level more significant alarm detector  20 . 
     A more detailed description will be given of this latter characteristic of this embodiment, by referring to FIG.  9 . FIG. 9 is a system block diagram showing the alarm detector  20  of this embodiment. In FIG. 9, those parts which are the same as those corresponding parts in FIG. 2 are designated by the same reference numerals, and a description thereof will be omitted. 
     The basic construction of the alarm detector  20  shown in FIG. 9 is similar to that of the conventional alarm detector  10  shown in FIG.  2 . However, in FIG. 9, the edge detection circuit (EG 6 )  19 - 6  does not receive the alarm detection signal MAAL 1  or MIAL 2  of the most significant error rate as is the case of the conventional alarm detector  10 , but instead receives an alarm detection signal PALD detected and/or cancelled by an alarm detector  20  which is provided at a preceding stage, that is, in an alarm detector  20  which is provided in a level which is one level more significant. Hence, the edge detection circuit (EG 6 )  19 - 6  of each alarm detector  20  outputs a pulse signal by detecting a falling edge (alarm cancellation) of the alarm detection signal PALD of an immediately preceding stage (that is, from an alarm detector  20  in a level which is one level more significant), and resets (initializes) the detection phase of its own alarm cancel detection. Hence, the hysteresis counter  14  and the protection counter  12  of the alarm detector  20  are reset. 
     FIGS. 10 and 11 are timing charts for explaining the operation of this embodiment of the alarm detection apparatus. In FIGS. 10 and 11, it is assumed for the sake of convenience that the detection period is  1 T,  3 T and  5 T in the most significant order, and that no hysteresis control is carried out for the alarm cancel detection, as in the case of the timing charts shown in FIGS. 5 and 6. 
     FIG. 10 shows a case where an error B 2 E generated at a high density disappears quickly. The most significant alarm signal ALD 1  is quickly set by the generation of the high density burst error, and the less significant alarm signals ALD 2  and ALD 3  are simultaneously set forcibly in response to the setting of the most significant alarm signal ALD 1 . Next, when the most significant alarm signal ALD 1  is reset (cancelled) due to a rapid decrease of the error generation density, the phase of the alarm cancel detection timing signal  3 T in a level which is one level less significant than the alarm signal ALD 1  is synchronized to the cancellation timing of the alarm signal ALD 1 . In addition, the alarm signal ALD 2  is cancelled at a timing  3 T after the cancellation of the alarm signal ALD 1 , and the phase of the alarm cancel detection timing signal  5 T in a level which is one level less significant than the alarm signal ALD 2  is synchronized to the cancellation timing of the alarm signal ALD 2 . Furthermore, the alarm signal ALD 3  is cancelled at a timing  5 T after the cancellation of the alarm signal ALD 2 . Hence, in this particular case, the alarm cancel detection of the alarm signals ALD 1 , ALD 2  and ALD 3  occurs with a regularity of  1 T,  3 T thereafter, and  5 T thereafter, reflecting the instantaneous disappearance of the error signal B 2 E. 
     On the other hand, FIG. 11 shows a case where the error B 2 E generated at a high density gradually reduces its generation rate and disappears. The most significant alarm signal ALD 1  is quickly set by the generation of the high density burst error, and is reset as the density thereafter decreases. With respect to the alarm signal ALD 2 , the alarm cancel detection thereof is started in phase synchronism with the cancellation timing of the most significant alarm signal ALD 1 . However, the error density is greater than or equal to a first predetermined value in the first  3 T interval and the alarm signal ALD 2  is not reset in this first  3 T interval, and is finally reset in the second  3 T interval. With respect to the alarm signal ALD 3 , the alarm cancel detection thereof is started in phase synchronism with the cancellation timing of the alarm signal ALD 2 , and the alarm signal ALD 3  is reset in a first  5 T interval. Accordingly, in this particular case, the alarm cancel detection of the alarm signals ALD 1 , ALD 2  and ALD 3  occurs with a regularity of  1 T,  2 × 3 T thereafter, and  5 T thereafter, reflecting the gradual disappearance of the error signal B 2 E. 
     When the time charts shown in FIGS. 10 and 11 for this embodiment are compared with the time charts shown in FIGS. 5 and 6 for the conventional case, it may be seen that this embodiment can reduce the cancellation timing of the least significant alarm signal ALD 3  by  4 T compared to the conventional case. This reduction in the cancellation timing of the alarm signal ALD 3  is realized because the alarm cancel detection timing signals  3 T and  5 T do not overlap, and double counting of the same error signal B 2 E by the two is avoided. Therefore, it is possible to effectively avoid a situation where the alarm cancellation times of the less significant alarm signals unstably become longer or shorter depending on the disappearing state of the error signal B 2 E as is the case of the conventional alarm detection apparatus, and this embodiment can realize an alarm detection apparatus having an improved response. 
     FIG. 12 is a system block diagram showing a second embodiment of the alarm detection apparatus according to the present invention. In FIG. 12, those parts which are the same as those corresponding parts in FIG. 4 are designated by the same reference numerals, and a description thereof will be omitted. 
     In FIG. 12, each of alarm detectors  30   1  through  30   10  is only provided with a single timing counter which corresponds to the hysteresis counter  14 , for example. The alarm detectors  30   1  through  30   3  are connected in a cascade connection within a major detector unit  31 , and the alarm detectors  30   4  through  30   10  are connected in a cascade connection within a minor detector unit  32 . The alarm detectors  30   1  through  30   10  efficiently generate timing signals  1 T,  10 T,  4 T,  40 T,  400 T,  4000 T and the like having various kinds of periods required for the alarm detection and/or cancellation. For example, the alarm detector  30   1  generates the timing signals  4 T and  10 T from the input timing signal  1 T, and uses the timing signal  1 T for the alarm detection and the timing signal  10 T for the alarm cancel detection. On the other hand, alarm detector  30   2  generates a timing signal  40 T from the input timing signal  4 T, and uses the timing signal  4 T for the alarm detection and the timing signal  40 T for the alarm cancel detection. Accordingly, the circuit scale of the counters and the alarm detection apparatus as a whole is greatly reduced in this embodiment. 
     The internal construction of each of the alarm detectors  30   1  through  30   10  of this embodiment shown in FIG. 12 will be described later. In the major detector unit  31 , the alarm detector  30   1  inputs the frame pulse B 2 FP corresponding to the timing signal  1 T and uses this timing signal  1 T for the alarm detection thereof, and also generates the timing signal  4 T having a period which is 4 times the period of the timing signal  1 T by use of an internal counter and outputs this timing signal  4 T. The alarm detector  30   1  also internally generates the timing signal  10 T having a period which is 10 times the period of the timing signal  1 T and uses this timing signal  10 T for the alarm cancel detection thereof. The alarm detector  30   2  inputs the timing signal  4 T from the alarm detector  30   1  and uses this timing signal  4 T for the alarm detection thereof. The alarm detector  30   2  also generates the timing signal  40 T having a period which is 10 times the period of the timing signal  4 T by use of an internal counter and uses this timing signal  40 T for the alarm cancel detection thereof. The alarm detector  30   3  inputs the timing signal  40 T from the alarm detector  30   2  and uses this timing signal  40 T for the alarm detection thereof, and also generates the timing signal  400 T having a period which is 10 times the period of the timing signal  40 T. The alarm detector  30   3  uses the timing signal  400 T for the alarm cancel detection thereof, and outputs this timing signal  400 T. 
     The minor detector unit  32  operates similarly to the major detector unit  31 . But in the minor detector unit  32 , the alarm detectors  30   4  through  30   6  obtain the input timing signals  4 T through  400 T from the alarm detectors  30   1  through  30   3  of the major detector unit  31 . For this reason, internal counters may be omitted in the alarm detectors  30   4  and  30   5 . In other words, the timing signals may be mutually used between the major and minor detector units  31  and  32  for parts where the detection rates overlap, thereby enabling a further reduction in the circuit scale of the counter circuit. In the minor detector unit  32 , the alarm detector  30   6  inputs the timing signal  400 T and uses this timing signal  400 T for the alarm detection thereof. In addition, the alarm detector  30   6  generates a timing signal  4000 T having a period which is 10 times the period of the timing signal  400 T by use of an internal counter, and uses this timing signal  4000 T for the alarm cancel detection thereof. The alarm detector  30   7  inputs the timing signal  4000 T from the alarm detector  30   6  and uses this timing signal  4000 T for the alarm detection thereof. The alarm detector  30   7  also generates a timing signal  40000 T having a period which is 10 times the period of the timing signal  4000 T by use of an internal counter, and uses this timing signal  40000 T for the alarm cancel detection thereof. The timing signal  40000 T from the alarm detector  30   7  is input to the next alarm detector  30   8 , and similar operations are carried out by the alarm detectors  30   8  through  30   10 . Accordingly, each internal counter used by the alarm detectors  30   4  through  30   10  only needs to make a count on the order of 10-count, and the circuit scale of the counter circuit can greatly be reduced. In addition, since the circuit construction used for the alarm detectors  30   1  through  30   3  of the major detector unit  31  can be used in common for the alarm detectors  30   4  through  30   10  of the minor detector unit  32 , it is possible to realize an alarm detection apparatus having an arbitrary number of stages at a low cost. 
     FIG. 13 is a system block diagram showing a typical construction of the alarm detector  30  of this embodiment. The construction shown in FIG. 13 may be used in common for each of the alarm detectors  30   1  through  30   10  shown in FIG.  12 . In FIG. 13, those parts which are the same as those corresponding parts in FIG. 9 are designated by the same reference numerals, and a description thereof will be omitted. More particularly, an upper half portion of the alarm detector  30  shown in FIG. 13 related to the alarm detection and/or cancel control may be the same as a corresponding part of the alarm detector  20  shown in FIG.  9 . However, in FIG. 13, one of the timing signals input to the selector (SL 1 )  16 , that is, the alarm detection timing signal, is the timing signal  1 T itself. Furthermore, in FIG. 13, the other of the timing signals input to the selector (SL 1 )  16 , that is, the alarm cancel detection timing signal, is a timing signal  10 T′ which is generated based on a counter output Q of the hysteresis counter  14 . 
     A description will be given of a lower half portion of the alarm detector  30  shown in FIG. 13 related to the timing generation. The lower half portion of the alarm detector  30  includes an edge detection circuit (EG 3 )  19 - 3 , the hysteresis counter  14 , a latch circuit (LTH 1 )  33 , a comparator (CM 3 )  17 - 3 , the timing decoders (TDC 1  and TDC 2 )  15 - 1  and  15 - 2 , and the selector (SL 1 )  16  which are connected as shown. 
     The edge detection circuit (EG 3 )  19 - 3  detects rising edges of the input timing signal T, such as the timing signals  1 T,  4 T and  40 T, and outputs an edge pulse signal. The hysteresis counter  14  counts this edge pulse signal, and cooperates with the timing decoder (TDC 2 )  15 - 2  so as to generate the timing signal  10 T having a period which is 10 times the period of the input timing signal T. Hence, in the case where the input timing signal  1 T is input to the alarm detector  30 , a constant phase relationship is always maintained between this input timing signal  1 T and the output timing signal  10 T which is generated. Similarly, the constant phase relationship is always maintained between the input timing signal  10 T (or  4 T) and the output timing signal  100 T (or  40 T) which is generated in an alarm detector  30  provided at a next stage. Accordingly, this constant phase relationship is always maintained regardless of the alarm signal detection and/or cancellation in each alarm detector  30 , and the alarm detector  30  in each stage can thus use the input and output timing signals  1 T and  10 T, for example, for the alarm detection and the alarm cancel detection thereof. 
     In the alarm detector  30 , it is desirable that the alarm cancel detection period which is 10 times the period of the alarm detection starts immediately after the alarm detection, but it may not necessarily be the case. For example, in the case of the alarm detector  30   1 , the alarm detection signal ALDI becomes ALD 1 =1 when an error of 10 −3  or greater occurs within each  1 T period 58 consecutive times. In this case, if the alarm detector  30   1  starts the count of the protection stage from the first  1 T period, the counter output Q of the hysteresis counter  14  at the time of the alarm detection is “58” and the one&#39;s digit is “8” which is not an accurate multiple of 10. Accordingly, if the alarm cancel detection period is started immediately after the alarm detection when Q=58, the first alarm cancel detection period ends when the hysteresis counter  14  counts Q=9 and Q=10. In other words, the alarm cancel detection period in this case would be  8 T shorter than the original alarm cancel detection period  10 T. But this is merely one example, and in actual practice, the burst error signal B 2 E may occur at any count phase of the hysteresis counter  14 , and the first alarm cancel detection period may vary arbitrarily in a range of  1 T to  9 T. Therefore, this second embodiment eliminates this problem of the varying first alarm cancel detection period by using the following construction. 
     That is, in this embodiment, the latch circuit (LTH 1 )  33  latches the counter output Q of the hysteresis counter  14  by a rising edge of its own alarm detection signal ALD 1 . The comparator (CM 3 )  17 - 3  compares the latched output of the latch circuit (LTH 1 )  33  and the counter output Q of the hysteresis counter  14 , and outputs a match pulse signal when the two compared outputs match. The timing decoder (TDC 1 )  15 - 1  generates the timing signal  10 T′ which turns ON from the match pulse signal to a next match pulse signal. Accordingly, if the detection of the alarm signal ALD 1 =1 is made at the timing when the counter output Q of the hysteresis counter  14  is Q=8T as described above, the latch circuit (LTH 1 )  33  holds the value 8T, and the timing decoder (TDC 1 )  15 - 1  outputs a timing signal  10 T′ which turns ON from the time when the counter output Q of the hysteresis counter  14  is Q=8T to the next time when the counter output Q becomes Q=8T. As a result, it is possible to obtain an alarm cancel detection timing signal  10 T′ which is in phase synchronism with the detection of the alarm signal ALD 1 =1 and has a period  10 T from the start. 
     FIG. 14 is a timing chart for explaining the operation of the second embodiment of the alarm detection apparatus. More particularly, FIG. 14 shows the signal timings related to the alarm detectors  30   1  and  30   2 . In the alarm detector  30   1 , the hysteresis counter (HYCTa)  14  generates the alarm cancel detection timing signal  10 T based on the input timing signal  1 T. In this particular case, the alarm detection period of the alarm detector  30   2  in the next stage is  4 T, and for this reason, although not shown in FIG. 13, another counter (HYCTb) is provided to generate the timing signal  4 T which is supplied to the alarm detector  30   2 . The provision of this other counter (HYCTb) is peculiar to the alarm detector  30   1 , and it is unnecessary to provide the additional counter (HYCTb) in the other alarm detectors  30   2  through  30   10 . 
     Furthermore, the alarm detector  30   1  monitors the input error signal B 2 E in each  1 T period, and outputs the alarm detection signal ALD 1 =1 at the 58th  1 T of the consecutive  1 Ts satisfying B 2 E≧980. The latch circuit (LTH 1 )  33  latches the counter output Q=4 of the hysteresis counter (HYCTa)  14  at this point in time, and the comparator (CM 3 )  17 - 3  compares the latched output  4  from the latch circuit (LTH 1 )  33  and the counter output Q of the threshold counter (HYCTa)  14 . The comparator (CM 3 )  17 - 3  outputs the match signal every time the counter output Q of the threshold counter (HYCTa)  14  becomes Q=4. Hence, the alarm cancel detection timing signal  10 T′ in his case is generated at the phase shown in FIG. 14, and the detection period amounting to  10 T can be secured from the start of the alarm cancel detection period. 
     FIG. 15 is a system block diagram showing a third embodiment of the alarm detection apparatus according to the present invention. In FIG. 15, those parts which are the same as those corresponding parts in FIG. 12 are designated by the same reference numerals, and a description thereof will be omitted. In this embodiment, the structure of the second embodiment of the alarm detection apparatus described above is efficiently divided depending on the primary functions and restructured. 
     In this embodiment, the alarm detection apparatus includes a timing generator  41 , a timing selector  42 , a major detector unit  43 , and a minor detector unit  44  which are connected as shown in FIG.  15 . The timing generator  41  generates various kinds of timing signals, such as  1 T,  4 T and  40 T, which are required for the alarm detection operation at each stage. The timing selector  42  distributes the various kinds of timing signals from the timing generator  41  to the major detector unit  43  and the minor detector unit  44 . 
     FIG. 16 is a system block diagram showing the timing generator  41  of this embodiment. The timing generator  41  includes a flip-flop circuit (FF)  411 , and counter units CU 2  through CU 8  which are connected as shown in FIG.  16 . The flip-flop circuit (FF)  411  generates a timing signal TIM 1  ( 1 T) based on the input frame pulse signal B 2 FP. In the counter unit CU 2 , an edge detection circuit EG 2  detects a rising edge of the timing signal  1 T, and generates an edge pulse signal. A counter CTR 2  counts this edge pulse signal from the edge detection circuit EG 2 , and cooperates with a timing decoder TDC 2  to generate a timing signal TIM 2  ( 4 T) having a period which is 4 times the period of the timing signal  1 T. 
     On the other hand, in the counter unit CU 3 , an edge detection circuit EG 3  detects a rising edge of the timing signal  4 T, and generates an edge pulse signal. A counter CTR 3  counts this edge pulse signal from the edge detection circuit EG 3 , and cooperates with a timing decoder TDC 3  to generate a timing signal TIM 3  ( 40 T) having a period which is 10 times the period of the timing signal  4 T. 
     Similarly thereafter, timing signals TIM 4  through TIM 8  ( 400 T through  4000000 T) are generated by the cascade connection of counter units CU 4  through CU 8 , so that the period of the timing signal generated from one stage is 10 times that of the timing signal generated from an immediately preceding stage of the cascade connection. In this particular case shown in FIG. 16, alarm cancel detection timing signals are generated by the timing selector  42 , and only the alarm detection timing signals are generated by the timing generator  41 . 
     FIGS. 17 and 18 are system block diagrams showing the timing selector  42  of this embodiment. FIG. 17 shows a circuit part of the timing selector  42  corresponding to the major detector unit  43 , and FIG. 18 shows a circuit part of the timing selector  42  corresponding to the minor detector unit  44 . 
     In FIG. 17, a circuit part of a major hysteresis timing generator  42   a  including an edge detection circuit EG 1 , a hysteresis counter HYC 1 , a latch circuit LTH 1 , a comparator CM 1 , and timing decoders TDC 1  and TDC 2  has the same construction as the corresponding circuit part of the alarm detector  30  shown in FIG. 13 including the edge detection circuit (EG 3 )  19 - 3 , the hysteresis counter (HYCT)  14 , the latch circuit (LTH 1 )  33 , the comparator (CM 3 )  17 - 3  and the timing decoders (TDC 1  and TDC 2 )  15 - 1  and  15 - 2 . By providing one hysteresis timing generator  42   a  with respect to the major detector unit  43 , it is possible to further reduce the circuit scale of the alarm detection apparatus. 
     In other words, the major hysteresis timing generator  42   a  is provided with a data multiplexer MUX 1  which corresponds to an AND-OR circuit. For example, when the major detection selection signal MAJRT[ 1 ]=1, the alarm signal MAAL 1  is supplied to the clock input terminal CK of the latch circuit LTH 1 , and the timing signal TIM 1  ( 1 T) is supplied to the edge detection circuit EG 1 . In this case, the major hysteresis timing generator  42   a  generates a hysteresis timing signal HT ( 10 T) which is in phase synchronism with the rising edge of the alarm signal MAAL 1 . On the other hand, a selector SL 1  is enabled by the major detection selection signal MAJRT[ 1 ]=1, and outputs the alarm detection timing signal TIM 1  ( 1 T) as a major timing signal TMJ 1  when the alarm detection signal MAAL 1 =0, and outputs the hysteresis timing signal HT ( 10 T) from the major hysteresis timing generator  42   a  as an alarm cancel detection timing signal when the alarm detection signal MAAL 1 =1. Similarly, a selector SL 2  is enabled by the major detection selection signal MAJRT[ 2 ]=1, and outputs the alarm detection timing signal TIM 2  ( 4 T) as a major timing signal TMJ 2  when the alarm detection signal MAAL 2 =0, and outputs the hysteresis timing signal HT ( 40 T) from the major hysteresis timing generator  42   a  as an alarm cancel detection timing signal when the alarm detection signal MAAL 2 =1. Further, a selector SL 3  is enabled by the major detection selection signal MAJRT[ 3 ]=1, and outputs the alarm detection timing signal TIM 3  ( 40 T) as a major timing signal TMJ 3  when the alarm detection signal MAAL 3 =0, and outputs the hysteresis timing signal HT ( 400 T) from the major hysteresis timing generator  42   a  as an alarm cancel detection timing signal when the alarm detection signal MAAL 3 =1. 
     In FIG. 18, a circuit part of a minor hysteresis timing generator  42   b  including an edge detection circuit EG 2 , a hysteresis counter HYC 2 , a latch circuit LTH 2 , a comparator CM 2 , and timing decoders TDC 3  and TDC 4  has the same construction as the corresponding circuit part of the major hysteresis timing generator  42   a  shown in FIG.  17 . In addition, selectors SL 4  through SLA of the minor hysteresis timing generator  42   b  is arranged similarly to the selectors SL 1  through SL 3  of the major hysteresis timing generator  42   a . By providing one hysteresis timing generator  42   b  with respect to the minor detector unit  44 , it is possible to further reduce the circuit scale of the alarm detection apparatus. 
     FIG. 19 is a system block diagram showing an alarm detection mode of the third embodiment of the alarm detection apparatus. The construction of alarm detectors  40   1  through  40   10  shown in FIG. 19 will be described later in conjunction with FIG.  20 . In FIG. 19, those parts which are the same as those corresponding parts in FIG. 12 are designated by the same reference numerals, and a description thereof will be omitted. 
     In the major detector unit  43 , the alarm detection signal MAAL 1  output from the alarm detector  40   1  is selectively output from the selector (SEL 1 )  23  when the major detection selection signal MAJRT[ 1 ]=1. In addition, the major timing signal TMJ 1  from the timing selector  42  is also input to the alarm detector  40   1 , where this major timing signal TMJ 1  is the timing signal  1 T when the alarm detection signal MAAL 1 =0 (detection mode) and is the timing signal  10 T which is in phase synchronism with the rising edge of the alarm detection signal MAAL 1  when the alarm detection signal MAAL 1 =1 (cancel detection mode). The alarm detectors  40   2  and  40   3  operate similarly to the alarm detector  40   1 , and the alarm detectors  40   4  through  40   10  of the minor detector unit  44  also operate similarly to the alarm detector  40   1  of the major detector unit  43 . 
     FIG. 20 is a system block diagram showing a typical construction of the alarm detector  40  which may be used in common for any of the alarm detectors  40   1  through  40   10  of this third embodiment. In FIG. 20, those parts which are the same as those corresponding parts in FIG. 13 are designated by the same reference numerals, and a description thereof will be omitted. 
     The alarm detector  40  shown in FIG. 20 has an extremely simple construction, because the circuit part related to the timing generation, such as the counters  13  and  14 , and the circuit part related to the timing selection, such as the selector (SL 1 )  16 , are separated from the alarm detector  40 . When the alarm detection signal ALD 1  output from the alarm detector  40  is ALD 1 =0 (detection mode), the alarm detection timing signal TM 1  ( 1 T) is input to the alarm detector  40 , and when the alarm detection signal ALD 1  is ALD 1 =1 (cancel detection mode), the alarm cancel detection timing signal TM 1  ( 10 T′) which is in phase synchronism with the rising edge of the alarm detection signal ALD 1  is input to the alarm detector  40 . Hence, according to this third embodiment, it is possible to efficiently monitor the error rates of desired monitoring conditions, using a simple circuit having a small circuit scale. 
     In this third embodiment, the major hysteresis timing generator  42   a  and the minor hysteresis timing generator  42   b  are provided in common for each of the detection rates in the timing selector  42 , but the present invention is not limited to this arrangement. For example, a hysteresis timing generator may be provided for each of the major detection rates and the minor detection rates. In this case, it is possible to generate various kinds of alarm detection timing signals of arbitrary multiplication factors in the timing generator  41 , and the timing selector  42  can generate alarm detection timing signals having different multiplication factors for each detection rate. Accordingly, it is possible to realize an inexpensive alarm detection apparatus which can easily cope with various alarm detection and/or cancellation conditions. 
     In addition, it is possible to eliminate the major and minor hysteresis timing generators  42   a  and  42   b  from the timing selector  42 , and instead generate various kinds of timing signals having arbitrary multiplication factors in the timing generator  41  for the alarm detection and for the alarm cancel detection if necessary. In this case, the alarm cancel detection timing signal from the timing generator  41  is input to an input terminal  1  of the selector SL 1 , so as to switch the alarm detection timing signal and the alarm cancel detection timing signal from the timing generator  41 . Accordingly, it is possible to eliminate the major and minor hysteresis timing generators  42   a  and  42   b  from the timing selector  42  shown in FIGS. 17 and 18, thereby further reducing the circuit scale of the alarm detection apparatus, and making it possible to easily cope with various alarm detection and/or cancellation conditions. 
     On the other hand, although the present invention is applied to the STS-N frame of the SONET in each of the embodiments described above, the present invention can of course be applied to the monitoring of the Synchronous Transfer Mode-N (STM-N) frame of the SDH, and various other kinds of frame signals. 
     Therefore, according to the present invention, it is possible to improve the operation reliability, responses to the alarm detection and/or cancellation, and the like of the alarm detection apparatus. In addition, it is possible to construct a high-performance alarm detection apparatus having a greatly reduced and simplified circuit construction, at a low cost. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.