Abstract:
A uni-direction protection switched ring node which switches from a working path to a protection path or vice versa includes a failure-information detect circuit which detects a failure occurring on the working path and a failure occurring on the protection path independently of each other as failure information. A failure-information-path identifying circuit determines whether a working path with a failure occurring thereon is an active or standby path or determines whether a protection path with a failure occurring thereon is an active or standby path. A select circuit selects either the working or protection path as an active path in accordance with results of detection and determination by the failure-information detect circuit and the failure-information-path identifying circuit respectively.

Description:
This is a continuation of U.S. patent application Ser. No. 08/929,650, filed on Sep. 15, 1997, now U.S. Pat. No. 6,038,678. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     In general, the present invention relates to a compound ring network typically comprising two networks such as a UPSR (Uni-directional Protection Switch Ring) and a BLSR (Bi-directional Line Switch Ring) connected to each other. In particular, the present invention relates to a path switching method and a path switching apparatus having a function for switching from a working path to a protection path starting from two adjacent input nodes (and ending at a common path terminating node) as a normal line (an active path) by using a path switch in such a compound ring network. 
     2. Description of the Related Art 
     First of all, as conventional technologies, the BLSR and the UPSR are explained. As described in Bellcore&#39;s GENERIC REQUIREMENTS GR-1230-CORE, Issue 1, the BLSR is a ring-network for connecting a plurality of nodes to each other by using transmission lines to form a ring-like shape wherein each two nodes thereof are connected by one path and, in the event of a failure occurring in a transmission line accommodating the path, a path route is changed-to heal the path. 
     FIGS. 1 and 2 are diagrams showing configurations of path routes in a normal operation and in the event of a failure respectively in an example of a two-fiber BLSR. In particular, FIG. 1 is a diagram showing five nodes, Node-A to Node-E denoted by reference numerals  801  to  805  respectively, connected by transmission lines  806  and  807  wherein a path  808  is established between the node  801  and the node  803 . In this state, assume that a failure  809  occurs on the transmission line  806  between the nodes  802  and  803  as shown in FIG. 2, causing the following switching to be carried out to heal the failing path. To be more specific, the failure  809  occurs on the transmission line  806  between the nodes  802  and  803  as shown in FIG.  2 . In the event of such a failure, all paths are returned at the node  802 , a node located immediately before the failure  809  occurring on the transmission line  806 , establishing new path routes till the path terminating node  803  (Node-C) of the returned path through the transmission line  807 . To be more specific, the path  808  shown in FIG. 2 is returned at the node  802 , being continued by a newly established path  810  that ends at the node  803 . In this way, the path is switched at the node  802  from the transmission line  806  to the transmission line  807 , healing the original failing path  808 . 
     As described in Chapter 3 of Bellcore&#39;s GENERIC REQUIREMENTS GR-1400-CORE, Issue 1, on the other hand, the UPSR is a network for connecting a plurality of nodes by using transmission lines to form a ring wherein each two nodes thereof are connected by two paths: a working path and a protection path. 
     FIG. 3 is a diagram showing the configuration of an example of the UPSR. In this example, the counterclockwise and the clockwise paths are working and protection paths respectively. To begin with, signal transmission from a node  701  to a node  703  is explained. Normally, both a working path  709  and a protection path  710  are established. At the node  703  at the end of the established paths, path-alarm detect units  711  and  712  are provided for the working and protection paths  709  and  710  respectively. In addition, the node  703  is also provided with a path select unit  713 . At the present time, the path select unit  713  selects the working path  709 . 
     Assume that a failure occurs on the working path  709  between the nodes  701  and  702  shown in FIG.  3 . In this case, the failure is detected by the path-alarm detect unit  711  at the node  703 . Detecting the failure, the path-alarm detect unit  711  controls the path select unit  713  to select the protection path  710  in order to recover the path from the failure. 
     A concrete path switching method for inter-ring connection configurations is disclosed in U.S. Pat. No. 5,390,164 with a title “Ring Interworking between Bi-directional Line-Switching Ring Transmission Systems.” This document describes a switching method for maintaining communication connectivity of transmission systems in the event of a failure in a configuration comprising ring networks connected to each other by two ring nodes wherein two paths, working and protection paths, are established. Particularly, in a SONET (Synchronous Optical Network) signal, low-speed signals each known as a VT signal are multiplexed and, in addition, a multiplex frame structure for forming a high-order STS-1 signal is adopted. Thus, even if failures occur at the low-order VT-signal level, in the multiplexing process to form an STS-1 signal, pieces of information on the failures appear to be normal in controlling a switching operation. The switching method disclosed in the above reference is a switching method for reducing the probability of selecting such an abnormal lowlevel signal, being aimed at a case in which failures occur on the working and protection paths independently to the bitter end. Thus, the scope of the method does not include a case in which one failure affects both the working and protection lines at the same time in the inter-ring connection configuration. 
     A network system in which the UPSR and the BLSR described above are connected to each other is taken into consideration. According to Bellcore&#39;s GENERIC REQUIREMENTS GR-1230-CORE, in the event of path switching taking place in the BLSR due to a transmission-line failure, a time of 50 ms between the occurrence of the failure and the recovery of the line achieved through the path switching is allowed. 
     As shown in FIG. 3, transmission lines of a UPSR form a ring-like shape. To put it in detail, two paths in a UPSR, the working and protection paths, are set to form a circle starting from two adjacent interconnection nodes connected to a BLSR to a path terminating node as shown in FIG. 4. A difference in transmission route between the working and protection paths makes the number of nodes included in the working path different from the number of nodes included in the protection path. Thus, there is a difference in arrival time of alarm information at the path terminating node between a failure occurring on the working path and a failure occurring on the protection path. This difference in arrival time of alarm information at the path terminating node is attributed to the difference in path length caused by the difference in code count between the working and protection paths. As a result, even if a path failure with a maximum recovery time limit of 50 ms described above passes through the interconnection node at the same time, it may arrive at the path terminating node at different times. 
     Path switching is carried out to always select either the working and protection paths as a normal (active) path. Thus, in a case with a small difference in arrival time of failure alarm information described above, there are observed erroneous switching operations in which a selector once selects a path with a longer propagation time which appears to be the normal path before switching back later on to the path with a shorter propagation time. Particularly, in the case of a SONET signal, since low-speed paths each known as a VT signal are multiplexed and a multiplex frame structure for forming a high-order STS-1 signal is further adopted, it is quite within the bounds of possibility that erroneous switching operations described above are inadvertently carried out simultaneously on a plurality of low-speed paths. 
     Normally, in a transmission apparatus for handling a SONET signal, software is used for monitoring the switching state of a path. When a plurality of path erroneous switching operations described above occur at the same time, two switching operations, ‘switch-over’ and then ‘switch-back’, are carried out on each path. In this case, the monitoring software must perform very complex software processing in order to monitor the switching state of each path. Assume that erroneous switching operations occur in all VT-1.5 signals accommodated in an STS-3 signal. In this case, 168 (=84×2) wasteful switching-state monitoring operations must be carried out in a short period of time. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to provide a path switching apparatus and a path switching method that are capable of preventing an erroneous switching operation from being carried out in the event of simultaneous failures occurring on the working and protection paths at the same time mostly in the course of path switching in a BLSR of a compound ring network system including a UPSR such as a network comprising a BLSR and a UPSR connected to each other. 
     In order to achieve the object described above, the present invention provides an alarm detect unit (a path switching apparatus) for selecting an active path comprising a path-alarm detect circuit and a guard timer for a working path as well as a path-alarm detect circuit and a guard timer for a protection path wherein, when an alarm is detected on the active path, alarm information, that is, information on the generation of the alarm, is delayed by a predetermined time by the guard timer of the working or protection path that serves as the active path whereas, when an alarm is detected on a standby path, alarm information, that is, information on the recovery of the alarm, is delayed by a predetermined time by the guard timer of the working or protection path that serves as the standby path and, finally, either the working or protection path is then selected as the active path in accordance with pieces of alarm information output by the two guard timers which indicate the line-failure-occurrence states of the active and standby paths. 
     In order to solve the problems described above, the present invention provides the following means. 
     According to one aspect of the present invention, there is provided a method for switching from a working path to a protection path or vice versa in a UPSR of a compound network connecting the UPSR to another ring network through interconnection nodes wherein: 
     a path is set from the other ring network to a path terminating node in the UPSR; 
     the working path and the protection path starting from the interconnection nodes and ending at the path terminating node form a circle; and 
     either the working path or the protection path is selected as an active path, 
     the method comprising the steps of: 
     detecting a failure occurring on the working path and a failure occurring on the protection path independently of each other as failure information; 
     detecting a failure information of an active path; 
     detecting a failure information of a standby path; and 
     selecting either the working or protection path as the active path. 
     According to another aspect of the present invention, there is provided a method for switching from a working path to a protection path or vice versa in a UPSR of a compound network connecting the UPSR to another ring network through an interconnection node wherein: 
     a path is set from the other ring network to a path terminating node in the UPSR; 
     the working path and the protection path starting from the interconnection nodes and ending at the path terminating node form a circle; and 
     either the working path or the protection path is selected as an active path, 
     the method comprising the steps of: 
     detecting a failure occurring on the working path and a failure occurring on the protection path independently of each other as failure information; 
     detecting a failure information of an active path; 
     detecting a failure information of a standby path; 
     delaying failure detection time for the failure information of the active path by a first predetermined time; 
     delaying failure recovery time for the failure information of the standby path by a second predetermined time; and 
     selecting either the working or protection path as the active path by comparing the failure information of the active path delayed by a first predetermined time with the failure information of the standby path delayed by the second predetermined time. 
     According to still another aspect of the present invention, there is provided a path switching apparatus for switching from a working path to a protection path or vice versa in a UPSR of a compound network connecting the UPSR to another ring network through an interconnection node wherein: 
     a path is set from the other ring network to a path terminating node in the UPSR; 
     the working path and the protection path starting from the interconnection nodes and ending at the path terminating node form a circle; and 
     either the working path or the protection path is selected as an active path, 
     the apparatus comprising: 
     nodes in the UPSR; 
     a failure-information detect circuit for detecting a failure occurring on the working path and a failure occurring on the protection path independently of each other as failure information; 
     a failure-information-path identifying circuit for determining whether a working path with a failure occurring thereon is an active or standby path or for determining whether a protection path with a failure occurring thereon is an active or standby path; and 
     a select circuit for selecting either the working or protection path as the active path in accordance with results of detection and determination by the failure-information detect circuit and the failure-information-path identifying circuit respectively. 
     According to a still further aspect of the present invention, there is provided a path switching apparatus for switching from a working path to a protection path or vice versa in a UPSR of a compound network connecting the UPSR to another ring network through an interconnection node wherein: 
     a path is set from the other ring network to a path terminating node in the UPSR; 
     the working path and the protection path starting from the interconnection nodes and ending at the path terminating node form a circle; and 
     either the working path or the protection path is selected as an active path, 
     the apparatus comprising: 
     nodes in the UPSR; 
     a first failure-information detect circuit for detecting a failure occurring on the working path; 
     a second failure-information detect circuit for detecting a failure occurring on the protection path; 
     a first guard circuit for delaying failure detection time for failure information of the working or protection path selected as an active path by a first predetermined time; 
     a second guard circuit for delaying failure recovery time for failure information of the working or protection path selected as a standby path by a second predetermined time; and 
     a select circuit for selecting either the working or protection path as the active path in accordance with delayed failure information output by the first guard circuit and delayed failure information output by the second guard circuit. 
     It is advantageous to implement the methods and apparatuses for solving the problems described above by software. 
     In addition, the transmission system used in the networks described above can be one conforming to the SONET prescribed by ANSI and the SDH (Synchronous Digital Hierarchy) prescribed in the ITU-T recommendations. 
     A signal transmitted through each path can be one conforming to the VT (Virtual Tributary) prescribed by ANSI and the TU (Tributary Unit) prescribed in the ITU-T recommendations. 
     As an alternative, a signal transmitted through each path can be one conforming to the STS (Synchronous Transport Signal) prescribed by ANSI and the AU (Administrative Unit) prescribed in the ITU-T recommendations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described in conjunction with the following drawings in which: 
     FIG. 1 is a configuration diagram showing a path route in a normal operation of an example of a BLSR; 
     FIG. 2 is a configuration diagram showing a path route in the event of a failure occurring in the example of a BLSR shown in FIG. 1; 
     FIG. 3 is a configuration diagram showing an example of a UPSR; 
     FIG. 4 is a configuration diagram showing path routes in a normal operation in a network system connecting a BLSR and a UPSR; 
     FIG. 5 is a configuration diagram showing path routes in the event of a failure occurring in the network system connecting a BLSR and a UPSR shown in FIG. 4; 
     FIG. 6 is a configuration diagram showing path routes after a path switching operation has been carried out in the event of the failure occurring in the network system connecting a BLSR and a UPSR as shown in FIG. 5; 
     FIG. 7 is a block configuration diagram showing a path switching apparatus (employed in each UPSR node) as provided by the present invention; 
     FIG. 8 is a table showing switching logic embraced by a switch control circuit employed in the path switching apparatus shown in FIG. 7; 
     FIG. 9 is time charts showing a difference in arrival time of failure-alarm information at a path terminating node in the network system connecting a BLSR and a UPSR shown in FIG. 4; 
     FIG. 10 is time charts showing path switching operations of an alarm management circuit employed in the path switching apparatus shown in FIG. 7; 
     FIG. 11 is time charts showing a path switching operation in the event of a failure arriving earlier at an input bus  101  of the path switching apparatus shown in FIG. 7; 
     FIG. 12 is time charts showing a path switching operation in the event of a failure arriving earlier at an input bus  102  of the path switching apparatus shown in FIG. 7; and 
     FIG. 13 is a diagram used for explaining the configuration of a typical node in a UPSR system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will become more apparent from a study of the following detailed description of some preferred embodiments with reference to the accompanying diagrams. In particular, FIGS. 4 to  6  are configuration block diagrams used for explaining propagation of a failure occurring in a path switching operation of a BLSR of a network system connecting the BLSR and a UPSR to which the present invention is applied. In FIGS. 4 to  6 , reference numerals  201  to  205  denote Node-A to Node-E respectively in the BLSR and reference numerals  206  to  211  are Node-a to Node-e respectively in the UPSR. Reference numerals  212  and  213  each denote a transmission line in the BLSR and reference numerals  214  and  215  are each a transmission line in the UPSR. Reference numeral  216  is a ring interconnect line for connecting the BLSR to the UPSR and reference numeral  217  denotes a transmission path in the BLSR. Reference numerals  218  and  219  are a working path and a protection path in the UPSR respectively. Reference numeral  220  and  221  denote alarm detect units employed at each node in the UPSR for the working and protection paths  218  and  219  respectively. Reference numeral  222  is a path select unit for selecting a path in accordance with alarm detection results output by the alarm detect units  220  and  221 . 
     FIG. 4 is a diagram showing a path established from Node-A  201  in the BLSR to Node-c  208  in the UPSR. In the BLSR, the path starts from Node-A  201  and ends at Node-C  203  by way of Node-B  202 . The path is continued from Node-C  203  in the BLSR to Node-a  206  in the UPSR and to Node-f  211  in the UPSR by way of Node-D  204  in the BLSR. In the UPSR, the continuing path starts from Node-a  214  and ends at Node-c  208  by way of Node-b  207 , (serving as a working path). On the other hand, the path passing through Node-D  204  in the BLSR and Node-f  211  in the UPSR becomes a protection path which also ends at Node-c  208  by way of Node-e  210  and Node-d  209  in the UPSR. 
     Now, assume that a failure  224  occurs on a transmission line between Node-B  202  and Node-C  203  in the BLSR as-shown in FIG.  5 . At that time, the line failure  224  is detected at Node-C  203  and Node-D  204 . As a result, the path to Node-C  203  and Node-D  204  is switched to a path from Node-E  205 . A state of the network after the path switching is shown in FIG.  6 . 
     In the BLSR, a time between the occurrence of the line failure  224  and the recovery of the path back to a normal state through completion of a path switching operation is prescribed to be within  50 ms in Bellcore&#39;s specifications. Thus, a time delay of up to  50 ms caused by a path failure affects the working and protection paths  218  and  219  in the UPSR. Since the route of the working path  218  is different from the route of the protection path  219 , on the other hand, there is a difference in arrival time of failure-alarm information at Node-c  208  between the working and protection paths  218  and  219 . The difference Td in arrival time of failure-alarm information at Node-c  208  between the working and protection paths  218  and  219  in the case of the example shown in FIG. 5 can be expressed as Td=2 (Te+Tt) where notation Te denotes a delay due to propagation through each node and notation Tf is a delay due to propagation between two consecutive nodes. 
     FIG. 7 is a block configuration diagram showing a path switching apparatus (employed in each UPSR node) as provided by an embodiment of the present invention. In the figure, reference numerals  101  and  102  each denote an input path whereas reference numeral  103  is a path selector for selecting either the input path  101  or the input path  102  (as an active path). Reference numerals  104  and  105  are alarm detect circuits for the input paths  101  and  102  respectively. Reference  106  is a guard timer for delaying alarm information generated by the alarm detect circuit  104  by a predetermined time and reference  107  is a guard timer for delaying alarm information generated by the alarm detect circuit  105  by a predetermined time. 
     Reference numeral  108  is an alarm management circuit for processing the alarm information output by the alarm detect circuit  104  and delayed alarm information output by the guard timer  106 . To put in detail, when the input path  101  is serving as an active path, the alarm management circuit  108  computes the logical product of the alarm information and the delayed alarm information. When the input path  101  is serving as a standby path, on the other hand, the alarm management circuit  108  computes the logical sum of the alarm information and the delayed alarm information. By the same token, reference numeral  109  is an alarm management circuit for processing the alarm information output by the alarm detect circuit  105  and delayed alarm information output by the guard timer  107 . To put in detail, when the input path  102  is serving as an active path, the alarm management circuit  109  computes the logical product of the alarm information and the delayed alarm information. When the input path  102  is serving as a standby path, on the other hand, the alarm management circuit  109  computes the logical sum of the alarm information and the delayed alarm information. 
     Reference numeral  110  is a switch control circuit for outputting a control signal to the path selector  103  to select either the input path  101  or  102  as a normal (active) path on the basis of the pieces of alarm information of the input paths  101  and  102  received from the alarm management circuits  108  and  109  respectively. The switch control circuit  110  also notifies the alarm management circuit  108  of the selection status of the input path  101 , that is, whether the input path  101  is serving as an active or standby path. By the same token, the switch control circuit  110  also notifies the alarm management circuit  109  of the selection status of the input path  102 , that is, whether the input path  102  is serving as an active or standby path. FIG. 8 is a table showing switching logic embraced by the switch control circuit  110  employed in the path switching apparatus shown in FIG.  7 . 
     FIG. 13 is a diagram used for explaining the configuration of a typical node provided by the present invention. As shown in the figure, the node comprises two high-speed interface units  904 , an interconnecting switch  903 , a low-speed interface unit  905  and a path selection switch  100 , (that is, the path switching apparatus shown in FIG.  7 ). Sandwiched by the high-speed interface units  904 , the interconnecting switch  903  is used for switching signals exchanged between the high-speed interface units  904 , a drop signal transmitted from the high-speed interface units  904  to the low-speed interface unit  905  and an add signal transmitted from the low-speed interface signal  905  to the high-speed interface unit  904 . Inserted on the path of the drop signal, the path selection signal  100  selects one of the two input paths, carrying out path switching based on the switching logic shown in FIG.  8 . 
     Path switching operations which are carried out in the event of a line failure occurring in a network system as has been described with reference to FIGS. 4 to  6  are explained by referring to the path switching apparatus shown in FIG. 7, the switching logic shown in FIG.  8  and the node shown in FIG.  13 . 
     First of all, individual operations carried out by the path switching apparatus are explained. Assume that an alarm indicating a line-failure-occurrence state with a difference Td in arrival time of failure-alarm information between the working and protection paths  218  and  219  like one shown in FIG. 7 is input through the input paths  102  and  1032 . A failure propagated through the input path  101  is detected as an alarm by the alarm detect circuit  104  and output as alarm information to the alarm management circuit  108  and the guard timer  106 . In the guard timer  106 , the alarm information supplied thereto is delayed by a predetermined time Tt before being output to the alarm management circuit  108 . The time Tt is set at a value equal to or longer than a maximum propagation delay time Tdmax which is determined from the number of nodes and differences in propagation delay time between nodes in the UPSR system. In the alarm management circuit  108 , different kinds of processing depending on the selection status of the input path  101 , that is, depending whether the input path  101  is serving as an active or standby path are carried out on pieces of alarm information supplied from the alarm detect circuit  104  and the guard timer  106  and a result of the processing is output to the switch control circuit  110 . 
     Outputs in the path switching apparatus  108  are shown in FIG.  10 . In the figure, reference numeral  401  denotes a line-failure-occurrence state of the input path  101  and reference numeral  402  is the output of the alarm detect circuit  104 . Reference numeral  403  denotes the output of the guard timer  106  and reference numeral  404  is the output of the alarm management circuit  108  with the input path  101  serving as an active path. Reference numeral  405  is the output of the alarm management circuit  108  with the input path  101  serving as a standby path. When the input path  101  is serving as an active path, the alarm management circuit  108  computes the logical product of the alarm information  402  generated by the alarm detect circuit  104  and the delayed alarm information  403  generated by the guard timer  106 , outputting the logical product as the output  404 . When the input path  101  is serving as a standby path, on the other hand, the alarm management circuit  108  computes the logical sum of the alarm information  402  generated by the alarm detect circuit  104  and the delayed alarm information  403  generated by the guard timer  106 , outputting the logical product as the output  405 . In the case being explained, since the input path  101  is serving as an active path, the signal  404  shown in FIG. 10 is supplied to the switch control circuit  110 . 
     For the input path  102 , on the other hand, the same processing as that for the input path  101  is carried out. That is to say, a failure propagated through the input path  102  is detected as an alarm by the alarm detect circuit  105  and output as alarm information to the alarm management circuit  109  and the guard timer  107 . In the guard timer  107 , the alarm information supplied thereto is delayed by the predetermined time Tt before being output to the alarm management circuit  109 . 
     Since the input path  102  is serving as a standby path, the alarm management circuit  109  computes the logical sum of the alarm information generated by the alarm detect circuit  105  and the delayed alarm information generated by the guard timer  107 , outputting the logical product corresponding to the output  405  shown in FIG. 10 to the switch control circuit  110 . Receiving the signals output by the alarm management circuits  108  and  109 , the switch control circuit  110  outputs a switch control signal to the path selector  103  in accordance with the select logic shown in FIG. 8, that is, logic for always selecting the normal path. In addition, the selection status of the input paths is supplied to the alarm management circuits  108  and  109 . 
     The operation of the switch control circuit  110  is explained for each case as follows by referring to FIGS. 11 and 12 respectively. 
     First of all, consider a case in which, with the input path  101  selected as an active path, a failure on the input path  101  arrives earlier than that on the input path  102  by the time Td. This case is explained by referring to FIG.  11 . In the path switching apparatus shown in FIG. 7, the failure  501  on the input path  101  causes alarm information detected by the alarm detect circuit  104  and alarm information delayed by the guard timer  106  by the time Tt to be supplied to the alarm management circuit  108 . Since the input path  101  is serving as an active path, the alarm management circuit  108  produces the logical product  503  of the alarm information supplied by the alarm detect circuit  104  and the delayed alarm information supplied by the guard timer  106  as shown in FIG.  11 . On the other hand, a failure  502  on the input path  102  lags behind the failure  501  on the input path  101 . Much like the input path  101 , in the path switching apparatus shown in FIG. 7, the failure  502  on the input path  102  causes alarm information detected by the alarm detect circuit  105  and alarm information delayed by the guard timer  107  by the time Tt to be supplied to the alarm management circuit  109 . Since the input path  102  is serving as a standby path, on the other hand, the alarm management circuit  109  produces the logical sum  504  of the alarm information supplied by the alarm detect circuit  105  and the delayed alarm information supplied by the guard timer  107  as shown in FIG.  11 . Receiving the logical product  503  and the logical sum  504  from the alarm management circuits  108  and  109  respectively, the switch control circuit  110  outputs a switch control signal  506  shown in FIG. 11 for controlling the path selector  103  in accordance with the select logic shown in FIG.  8 . It is obvious from FIG. 8 that, with the input path  101  serving as an active path, switching will take place only if the input path  102  does function normally and a failure does occur on the input path  101 . From the logical product  503  and the logical sum  504  shown in FIG. 11, the present case does not match a condition for path switching to take place as shown in FIG. 8, causing no switching to occur in the event of the failure. In addition, since Tt is set at a value equal to or longer than the maximum value Tdmax of Td, in the present case, no switching takes place. 
     Next, consider a case in which, with the input path  101  selected as an active path, a failure on the input path  101  arrives later than that on the input path  102  by the time Td. This case is explained by referring to FIG.  12 . In the path switching apparatus shown in FIG. 7, the failure  601  on the input path  101  causes alarm information detected by the alarm detect circuit  104  and alarm information delayed by the guard timer  106  by the time Tt to be supplied to the alarm management circuit  108 . Since the input path  101  is serving as an active path, the alarm management circuit  108  produces the logical product  603  of the alarm information supplied by the alarm detect circuit  104  and the delayed alarm information supplied by the guard timer  106  as shown in FIG.  12 . On the other hand, a failure  602  on the input path  602  leads ahead of the failure  601  on the input path  101 . Much like the input path  101 , in the path switching apparatus shown in FIG. 7, the failure  602  on the input path  102  causes alarm information detected by the alarm detect circuit  105  and alarm information delayed by the guard timer  107  by the time Tt to be supplied to the alarm management circuit  109 . Since the input path  102  is serving as a standby path, on the other hand, the alarm management circuit  109  produces the logical sum  604  of the alarm information supplied by the alarm detect circuit  105  and the delayed alarm information supplied by the guard timer  107  as shown in FIG.  12 . Receiving the logical product  603  and the logical sum  604  from the alarm management circuits  108  and  109  respectively, the switch control circuit  110  outputs a switch control signal  606  shown in FIG. 12 for controlling the path selector  103  in accordance with the select logic shown in FIG.  8 . From the logical product  603  and the logical sum  604  shown in FIG. 12, much like the case described above, the present case does not match a condition for path switching to take place as shown in FIG. 8, causing no switching to occur in the event of the failure. 
     As described above, the present invention exhibits an effect that, in a network system with a BLSR connected to a UPSR, in the event of simultaneous failures occurring at the same time on both the working and protection paths of the UPSR due to path switching in the BLSR, it is possible to prevent an erroneous switching operation from occurring in a phenomenon that would not cause path switching to take place in a stand-alone UPSR, making complex software processing unnecessary.