Abstract:
In a ring network system, a first node device includes means for sending out a control signal for controlling one of a plurality of second node devices so as to allow signals transmitted over a protection band to pass through the second node device; each of the second node devices includes means for receiving the control signal sent out from the first node device or another second node device, means for controlling the signals transmitted over the protection band to pass through the second node device during the reception of the control signal, and means for transferring the control signal to the adjacent first node device or to the adjacent second node device during the reception of the control signal; and the first node device further includes means for receiving the control signal transferred from the adjacent second node device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a ring network system in which a plurality of node devices are connected in a ring configuration and in which a working band and a protection band combine to form a redundant transmission band pair, to a node device, and to a protection band testing method for the ring network system. 
         [0003]    2. Description of the Related Art 
         [0004]    A synchronous optical network (SONET) ring is a ring network in which a plurality of SONET/synchronous digital hierarchy transmission (SDH) devices (or SONET devices) with add-drop multiplexer (ADM) capabilities act as node devices. The SONET ring is used in networks, ranging in size from small citywide networks to large nationwide networks. Typical redundancy schemes for the SONET ring include a bi-directional line switched ring (BLSR), which comes in two varieties: a two-fiber BLSR (2F-BLSR) and a four-fiber BLSR (4F-BLSR). In the 2F-BLSR, a transmission band of a single optical fiber for each of different directions (EAST direction and WEST direction) is divided into a current band (working band) and a backup band (protection band). The current band is used for transmitting signals under normal line conditions, while the backup band is used for diverting the signals in the case of a line failure. In the 4F-BLSR, both current and backup optical fibers are provided for each of EAST and WEST directions. Hereinafter, a 2F-BLSR SONET ring will be described in detail. 
         [0005]      FIG. 8  is a diagram for illustrating the 2F-BLSR. Node devices  61 ,  62 ,  63 ,  64 , and  65  are SONET devices with ADM capabilities. An EAST line  101  is an optical fiber for EAST direction, and a WEST line  201  is an optical fiber for WEST direction. A working band  102  and a protection band  103  are provided for the EAST line  101 , and a working band  202  and a protection band  203  are provided for the WEST line  201 . An ADD line  301  is added to the working band  102  of the EAST line  101  at the node device  61 . A DROP line  302  is dropped from the working band  102  of the EAST line  101  at the node device  63 . An ADD line  401  is added to the working band  202  of the WEST line  201  at the node device  63 . A DROP line  402  is dropped from the working band  202  of the WEST line  201  at the node device  61 . 
         [0006]    The node devices  61  to  65  are connected in a ring configuration via the EAST line  101  and the WEST line  201 . The transmission band of the EAST line  101  is divided into two sub-bands, i.e., the working band  102  for current use and the protection band  103  for backup. Likewise, the transmission band of the WEST line  201  is divided into two sub-bands, i.e., the working band  202  for current use and the protection band  203  for backup. In general, the widths of a working band and a protection band are the same. For example, when an optical carrier 48 (OC48) line with a capacity of 2.4 Gbps is used, 48 channels of a synchronous transport signal- 1  (STS- 1 ) for each of the EAST line  101  and the WEST line  201  are assigned equally to the working band and protection band, i.e., channels  1  to  24  are assigned to the working band and channels  25  to  48  are assigned to the protection band. 
         [0007]    The following describes an example in which concatenated synchronous transport signal- 3   c  (STS- 3   c ) signals are transmitted between the node device  61  and the node device  63  over channels  1  to  3 . The node device  61  adds signals on the ADD line  301  to channels  1  to  3  of the EAST line  101 . The node device  62  allows the signals on channels  1  to  3  of the EAST line  101  to pass through (THRU). The node device  63  drops the signals on channels  1  to  3  of the EAST line  101  to the DROP line  302 . Likewise, the node device  63  adds signals on the ADD line  401  to channels  1  to  3  of the WEST line  201 . The node device  62  allows the signals on channels  1  to  3  of the WEST line  201  to pass through. The node device  61  drops the signals on channels  1  to  3  of the WEST line  201  to the DROP line  402 . For the node devices  61 ,  62 , and  63 , a network management system (NMS, not shown) manages ADD control, DROP control, THRU control, and the selection of appropriate channels by using ADM control signals for controlling ADM switching functions and channels to be switched. 
         [0008]      FIG. 9  is a diagram ( 1 ) illustrating the redundancy capabilities of a 2F-BLSR network, which has the same system configuration as that of the 2F-BLSR network illustrated in  FIG. 8 . The following describes switching operation performed for the protection of line signals when transmission over the EAST line  101  between the node device  61  and the node device  62  is interrupted due to a fiber failure or the like. 
         [0009]    First, the node device  61  adds signals on the ADD line  301  to channels  25  to  27  corresponding to the protection band  203  of the WEST line  201 . The node devices  65 ,  64 , and  63  allows the signals on channels  25  to  27  of the WEST line  201  to pass through. The node device  62  directs the signals on channels  25  to  27  to channels  1  to  3  corresponding to the working band  102  of the EAST line  101 . The node device  63  drops the signals on channels  1  to  3  of the EAST line  101  to the DROP line  302 . Then, the node device  62  directs signals on channels  1  to  3  of the WEST line  201  to channels  25  to  27  corresponding to the protection band  103  of the EAST line  101 . The node devices  63 ,  64 , and  65  allows the signals on channels  25  to  27  of the EAST line  101  to pass through. The node device  61  drops the signals on channels  25  to  27  of the EAST line  101  to the DROP line  402 . Thus, the signals transmitted over the failed line can be protected by the above-described switching operation in the ring network system. 
         [0010]    In the switching operation of the ring network system described above, auto protection switching (APS) bytes constituting K1 and K2 bytes of the section overhead of a SONET/SDH frame are used. To protect signals on the failed line, a loopback circuit is created by the exchange of messages between nodes (e.g., the node devices  61  and  62  in  FIG. 9 ) at both sides of the point of failure. In addition, the other nodes (e.g., the node devices  63 ,  64 , and  65  in  FIG. 9 ) need to allow the APS bytes containing the destination address to pass through. Japanese Unexamined Patent Application Publication No. 7-95225 discusses a method in which the address of a working node device is compared with addresses contained in APS bytes so as to increase the speed of pass-through processing for allowing the APS bytes to pass through. 
         [0011]      FIG. 10  is a diagram ( 2 ) illustrating the redundancy capabilities of a 2F-BLSR network.  FIG. 10  illustrates the same switching operation as that illustrated in  FIG. 9 . 
         [0012]    Generally, in a system with redundancy capabilities, line conditions of a protection band are not monitored. Therefore, when a channel failure in the protection band occurs, for example, on the WEST line  201  and between the node device  64  and the node device  63  and affects channels  25  to  27 , it can only be found after the completion of the switching operation illustrated in  FIG. 9  that the signals on channels  1  to  3  cannot be protected by the switching operation. 
         [0013]      FIG. 11  illustrates a line test performed on a 2F-BLSR network, which has the same system configuration as that of the 2F-BLSR network illustrated in  FIG. 8 , except that each of the node devices  61  to  65  in  FIG. 11  includes a test controller  70 . The following describes a line test performed on the working band  102  from the node device  61  to the node device  63 , and on the working band  202  from the node device  63  to the node device  61 . 
         [0014]    The node devices  61  to  65  are connected to the network management system (NMS, not shown). For a line test, the NMS sends control signals  601  to  605  to the node devices  61  to  65 , respectively, and the node devices  61  to  65  send the corresponding response signals  601  to  605  back to the NMS. To perform a test for evaluating the communication quality of a path, for example, from the node device  61  to the node device  63  under the control of the NMS, a test signal generation unit of the test controller  70  for the node device  61  sends out a test signal to the working band  102  of the EAST line  101 . Then, a test signal measurement unit of the test controller  70  for the node device  63  performs a measurement on the transmitted test signal to evaluate the communication quality. Likewise, a test signal generation unit of the test controller  70  for the node device  63  sends out a test signal to the working band  202  of the WEST line  201 . Then, a test signal measurement unit of the test controller  70  for the node device  61  performs a measurement on the transmitted test signal to evaluate the communication quality. The minimum unit of measurement for the evaluation of communication quality is STS- 1 , which is the channel unit. For the evaluation of communication quality, the number of occurrences of error signals and the cumulative number of occurrences of error signals per day or per 15 minutes are counted. 
         [0015]    In general, measurement for the evaluation of communication quality, such as that described above, is performed only on the working band of the ring network system and not on the protection band. 
         [0016]    As described above, in the SONET BLSR network, it is possible to check the communication quality of channels in the working band. However, channels in the protection band are defined as “unused” and there is no means for creating a path for checking the communication quality of the channels. Therefore, the communication quality of channels in the protection band cannot be evaluated. The communication quality of such channels can only be evaluated when the channels are defined as being in the working band after the completion of switching operation performed due to a line failure. 
       SUMMARY OF THE INVENTION 
       [0017]    Accordingly, an object of the present invention is to provide a ring network system that includes a means for setting a path for checking the communication quality of a protection band and is capable of creating, in the protection band, a ring path starting at a first node device, passing through a second node device, and ending at the first node device. 
         [0018]    According to an aspect of the present invention, a ring network system includes a first node device, a plurality of second node devices, and a transmission line for connecting the first node device and the plurality of second node devices in a ring configuration. In the ring network system, each of the first device and the second devices combines a working band used as a normal band for transmitting signals and a protection band used as an alternative band for diverting signals to form a redundant transmission band pair. The first node device includes means for sending out a protection band control signal for controlling one of the second node devices so as to allow signals transmitted over the protection band to pass through the second node device. Each of the second node devices includes means for receiving the protection band control signal sent out from the first node device or another second node device, means for controlling the signals transmitted over the protection band to pass through the second node device during the reception of the protection band control signal, and means for transferring the protection band control signal to the adjacent first node device or to the adjacent second node device during the reception of the protection band control signal. The first node device further includes means for receiving the protection band control signal transferred from the adjacent second node device. A ring path that starts at the first node device, passes through the plurality of second node devices, and ends at the first node device is created in the protection band, and is used to evaluate the communication quality of the protection band. 
         [0019]    The present invention makes it possible to provide a ring network system that includes a means for setting a path for checking the communication quality of a protection band and is capable of creating, in the protection band, a ring path starting at the first node device, passing through the second node devices, and ending at the first node device. In the ring network system, a test signal generation unit of the first node device sends out test signals to the ring path created in the protection band, and a test signal measurement unit of the first node device performs measurements on the test signals. The communication quality of the protection band can thus be evaluated. 
         [0020]    According to another aspect of the present invention, the ring network system is characterized in that the control signal is valid when a destination address and a source address in APS bytes have the same value. 
         [0021]    According to this aspect, it is possible to provide a ring network system capable of creating a path for checking the communication quality of a protection band when a destination address and a source address in APS bytes received by a node device of the ring network system are the same. 
         [0022]    The present invention makes it possible to check the communication quality of a protection band of a ring network system. Therefore, when a transmission band is switched from a working band to a protection band for line protection in the case of a line failure, the occurrence of another line failure after the line switching due to degradation in the communication quality of the protection band can be prevented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  illustrates an exemplary ring network system. 
           [0024]      FIG. 2  is a table for explaining K1 byte and K2 byte. 
           [0025]      FIG. 3  is a diagram illustrating an exemplary configuration of a node device. 
           [0026]      FIG. 4  is a diagram ( 1 ) illustrating exemplary operations of node devices. 
           [0027]      FIG. 5  is a diagram ( 2 ) illustrating exemplary operations of node devices. 
           [0028]      FIG. 6  is a flowchart illustrating an exemplary operation of a first node device. 
           [0029]      FIG. 7  is a flowchart illustrating an exemplary operation of a second node device. 
           [0030]      FIG. 8  is a diagram for illustrating a 2F-BLSR. 
           [0031]      FIG. 9  is a diagram ( 1 ) illustrating the redundancy capabilities of a 2F-BLSR network. 
           [0032]      FIG. 10  is a diagram ( 2 ) illustrating the redundancy capabilities of a 2F-BLSR network. 
           [0033]      FIG. 11  is a diagram illustrating an exemplary line test. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    The present invention will now be described in detail with reference to the drawings. Throughout the drawings, the same or like components are denoted by the same reference numerals. 
       First Embodiment 
       [0035]      FIG. 1  illustrates an exemplary ring network system (ring network system  1 ) of the present invention. An EAST line  101 , which is typically an optical fiber, is a transmission line for connecting node devices  10 ,  20 ,  30 ,  40 , and  50  in EAST direction in a ring configuration. A WEST line  201 , which is typically an optical fiber, is a transmission line for connecting the node devices  10 ,  20 ,  30 ,  40 , and  50  in WEST direction in a ring configuration. A working band  102  and a protection band  103  are provided for signals transmitted over the EAST line  101 . The working band  102  is provided for current use and the protection band  103  is provided for backup. Likewise, a working band  202  and a protection band  203  are provided for signals transmitted over the WEST line  201 . The working band  202  is provided for current use and the protection band  203  is provided for backup. An ADD line  301  is added at the node device  10 . A DROP line  302  is dropped at the node device  30 . Likewise, an ADD line  401  is added at the node device  30 . A DROP line  402  is dropped at the node device  10 . A control/response signal  500  is a signal for interfacing with a network management system (NMS, not shown). A protection band control signal  501  is used to establish a path in a protection band for checking the communication quality of the protection band. The protection band control signal  501  is a control signal transmitted from each of the node devices  10 ,  20 ,  30 ,  40 , and  50  to the adjacent node device in WEST direction. Although not illustrated in  FIG. 1 , there is also the protection band control signal  501  transmitted from each node device to the adjacent node device in EAST direction. 
         [0036]    The ring network system  1  in  FIG. 1  has a SONET ring configuration, which is the same as that described with reference to  FIG. 8 . Specifically, the ring network system  1  is a 2F-BLSR network in which a transmission band of a single optical fiber for each of different directions (EAST direction and WEST direction) is divided into a current band (working band) and a backup band (protection band). The node devices  10 ,  20 ,  30 ,  40 , and  50  are SONET devices with ADM capabilities and are connected via the EAST line  101  and the WEST line  201 . The transmission band of the EAST line  101  is divided into two sub-bands, i.e., the working band  102  for current use and the protection band  103  for backup. Likewise, the transmission band of the WEST line  201  is divided into two sub-bands, i.e., the working band  202  for current use and the protection band  203  for backup. In general, the widths of a working band and a protection band are the same. For example, when an optical carrier 48 (OC48) line with a capacity of 2.4 Gbps is used, 48 channels of a synchronous transport signal- 1  (STS- 1 ) for each of the EAST line  101  and the WEST line  201  are assigned equally to the working band and protection band, i.e., channels  1  to  24  are assigned to the working band and channels  25  to  48  are assigned to the protection band. 
         [0037]    Both the EAST line  101  and the WEST line  201  have a SONET/SDH frame format (not shown). For example, the line overhead (LOH) of a SONET OC48 frame and the multiplex section overhead (MSOH) of an SDH synchronous transport module- 16  (STM- 16 ) frame has an APS field containing K1 and K2 bytes, which are used for the control of line switching operations and for the transfer of line statuses. 
         [0038]      FIG. 2  is a table for explaining K1 and K2 bytes. The first to fourth bits of K1 byte represent switch request signals (or switch requests) for the control of line switching operations. For example, FS-S (FS: forced switch) corresponding to switch request  1110  and FS-R corresponding to switch request  1101  represent control signals for directing transmitted signals from a working line toward a protection line in response to certain commands, when the protection line is in a normal state. Specifically, FS-S represents a control signal acting as a forced span switch and FS-R represents a control signal acting as a forced ring switch. Additionally, SD-S (SD: signal degrade) corresponding to switch request  1001  and SD-R corresponding to switch request  1000  represent control signals for directing transmitted signals from the working line toward the protection line, when the protection line is in a normal state, upon detection of signal degradation on the working line. Specifically, SD-S represents a control signal indicating that there is span signal degradation on the working line, while SD-R represents a control signal indicating that there is ring signal degradation on the working line. SD-P corresponding to switch request  1010  represents a signal used in a 4F-BLSR network and indicating that there is signal degradation on a protection line. 
         [0039]    The fifth to eighth bits of K1 byte represent destination node IDs. The first to fourth bits of K2 byte represent source node IDs. 
         [0040]    The fifth bit of K2 byte represents line types. The sixth to eighth bits of K2 byte represent signals for transferring line statuses and line switching statuses. 
         [0041]    A source node device (represented by a source node ID) from which the node device is to be switched transmits a switch request signal to a destination node device (represented by a destination node ID) to which the node device is to be switched. 
         [0042]    The following describes an example in which the node device  10  in  FIG. 1  checks the line conditions of a protection band. 
         [0043]    Upon receipt of the control signal  500  from the NMS, the node device  10  sends out to the adjacent node device  50  the protection band control signal  501  for establishing a path for checking the communication quality of a protection band. Upon receipt of the protection band control signal  501 , the node device  50  puts the protection band  203  into a “pass-through” state in which signals transmitted from the node device  10  over the protection band  203  (e.g., channels  25  to  48 ) are directly transmitted to the node device  40 . Then, the node device  50  transfers the protection band control signal  501  to the adjacent node device  40 . Likewise, upon receipt of the protection band control signal  501 , the node device  40  puts the protection band  203  into a “pass-through” state and transfers the protection band control signal  501  to the adjacent node device  30 . Upon receipt of the protection band control signal  501 , the node device  30  puts the protection band  203  into a “pass-through” state and transfers the protection band control signal  501  to the adjacent node device  20 . Upon receipt of the protection band control signal  501 , the node device  20  puts the protection band  203  into a “pass-through” state and transfers the protection band control signal  501  to the adjacent node device  10 . Then, upon receipt of the protection band control signal  501 , the node device  10  sends out the response signal  500  to the NMS. 
         [0044]    With the above-described operation of the node devices  10 ,  20 ,  30 ,  40 , and  50  in the ring network, a path starting and ending at the node device  10  is created in the protection band  203  of the ring network system  1 . 
         [0045]      FIG. 3  is a diagram illustrating an exemplary configuration of a node device. The node device includes line interfaces  11  and  12 , an intra-office line interface  13 , an ADM unit  14 , and a test controller  15  including a test signal generation unit  151  and a test signal measurement unit  152 . A control/response signal  601  is a signal for interfacing the node device with an NMS. An ADD line  701  and a DROP line  802  are lines for interfacing the node device with intra-office devices. 
         [0046]    Signals transmitted over the EAST line  101  are received at the line interface  12 . In the ADM unit  14 , signals on a predetermined “drop” channel are transmitted through the intra-office line interface  13  to the DROP line  802 . Signals from the ADD line  701  are transmitted through the intra-office line interface  13 , added to a predetermined “add” channel, and transmitted through the line interface  11  to the EAST line  101 . 
         [0047]    Likewise, signals transmitted over the WEST line  201  are received at the line interface  11 . In the ADM unit  14 , signals on a predetermined “drop” channel are transmitted through the intra-office line interface  13  to the DROP line  802 . Signals from the ADD line  701  are transmitted through the intra-office line interface  13 , added to a predetermined “add” channel, and transmitted through the line interface  12  to the WEST line  201 . 
         [0048]    The control signal  601  from the NMS is detected at the test controller  15 , which controls add-drop operations in the ADM unit  14 , controls the protection band control signal  501  for establishing a path for checking the communication quality of a protection band, and performs control for setting signals for K1 and K2 bytes. 
         [0049]    The ADM unit  14  is capable of performing add/drop operations on channel signals on the EAST line  101  and WEST line  201 , allowing the channel signals to pass through, returning and diverting the channel signals from the working band to appropriate channels in the protection band for protecting line signals, directing test signals generated by the test signal generation unit  151  to specific channels in the protection band, and directing the test signals in the protection band to the test signal measurement unit  152 . The ADM unit  14  is also capable of directing signals on the ADD line  701  to the protection band and directing signals in the protection band to the DROP line  802 . 
         [0050]    According to the present embodiment described above, it is possible to create, in a protection band of a ring network, a ring path for transmitting test signals to the protection band, and thus possible to test the state of the protection band. 
       Second Embodiment 
       [0051]      FIG. 4  is a diagram ( 1 ) illustrating exemplary operation of node devices. Of all the node devices  10  to  50  in the ring network system  1  illustrated in  FIG. 1 , the node devices  20  and  40  are omitted in  FIG. 4 . Intra-office devices  61 ,  63 , and  65  are connected to the node devices  10 ,  30 , and  50 , respectively, within the corresponding offices (stations). An NMS  71  controls the node devices  10 ,  30 , and  50  and the like. Each of the node devices  10 ,  30 , and  50  includes line interfaces  11  and  12 , an intra-office line interface  13 , an ADM unit  14 , and a test controller  15 . A control/response signal  601  interfaces between the NMS  71  and the node device  10 . Although  FIG. 4  illustrates the connection between the NMS  71  and the node device  10  only, the NMS  71  is connected to either of the node devices  30  and  50  in the same manner as to the node device  10 . 
         [0052]    As in the case of the ring network system illustrated in  FIG. 1  or  FIG. 8 , paths for the intra-office device  61  and the node device  63  are created between the node device  10  and the node device  30 . In the node device  10 , signals transmitted from the intra-office device  61  through the ADD line  301  are received at the intra-office line interface  13  and added to the working band  102  of the EAST line  101  in the ADM unit  14 , on the basis of ADM information defined according to information from the NMS  71 . Then, in the node device  30 , the signals are dropped from the working band  102  in the ADM unit  14 , on the basis of ADM information defined according to information from the NMS  71 , and output from the intra-office line interface  13  through the DROP line  302  to the node device  63 . Likewise, in the node device  30 , signals transmitted from the intra-office device  63  through the ADD line  401  are received at the intra-office line interface  13  and added to the working band  202  of the WEST line  201  in the ADM unit  14 , on the basis of ADM information defined according to information from the NMS  71 . Then, in the node device  10 , the signals are dropped from the working band  202  in the ADM unit  14 , on the basis of ADM information defined according to information from the NMS  71 , and output from the intra-office line interface  13  through the DROP line  402  to the node device  61 . 
         [0053]    The node device  10  receives, from the NMS  71 , the control signal  601  for establishing a path for checking the communication quality of the protection band  203 . On the basis of the received control signal  601 , the node device  10  creates an “add” path in the ADM unit  14  such that test signals generated by the test signal generation unit  151  of the test controller  15  are transmitted over the protection band  203 . Then, the node device  10  assigns an ID value to the adjacent node device  50  such that the source node ID in the first to fourth bits of K2 byte in the WEST line  201  is the same as the destination node ID in the fifth to eighth bits of K1 byte (e.g., the destination node ID=0000 and the source node ID=0000). 
         [0054]    The node device  50  terminates the WEST line  201  at the line interface  11  and detects K1 and K2 bytes. When the destination node ID and the source node ID are the same, the ADM unit  14  creates a through path that allows signals transmitted over the protection band  203  to pass through. Then, the node device  50  assigns an ID value to the adjacent node device  30  such that the source node ID in the first to fourth bits of K2 byte in the WEST line  201  is the same as the destination node ID in the fifth to eighth bits of K1 byte. Examples of possible combinations of node IDs are: the destination node ID=0000 and source node ID=0000 that are transmitted from the node device  10 ; destination node ID=0100 and source node ID=0100; and destination node ID=0010 and source node ID=0010. 
         [0055]    The node device  30  operates in the same manner as the node device  50 . The node device  30  terminates the WEST line  201  at the line interface  11  and detects K1 and K2 bytes. When the destination node ID and the source node ID are the same, the ADM unit  14  creates a through path that allows signals transmitted over the protection band  203  to pass through. Then, the node device  50  assigns an ID value to the adjacent node device  10  such that the source node ID in the first to fourth bits of K2 byte in the WEST line  201  is the same as the destination node ID in the fifth to eighth bits of K1 byte. For example, a destination node ID and a source node ID that are to be used may be those transmitted from the node device  30 , or those newly created. 
         [0056]    The node device  50  terminates the WEST line  201  at the line interface  11  and detects K1 and K2 bytes. If the destination node ID and the source node ID are the same, the ADM unit  14  creates a “drop” path so that the evaluation of communication quality can be performed, in the test signal measurement unit  152  of the test controller  15 , on the signals transmitted over the protection band  203 . 
         [0057]    The above-described operations of the node devices  10 ,  30 , and  50  in the ring network create a path  300  that starts and ends at the node device  10  in the protection band  203  of the ring network system, and allow test signals generated by the test signal generation unit  151  to be transmitted over the path  300  and measured by the test signal measurement unit  152 . The communication quality of the protection band  203  can thus be evaluated. 
         [0058]    Although only the protection band  203  of the WEST line  201  has been described above, the protection band  103  of the EAST line  101  can also be evaluated in the same manner as that described above. 
         [0059]    If the above-described condition “destination node ID=source node ID” is cancelled, the through path in the ADM unit  14  in each of the node devices  30  and  50  and the “drop” path in the ADM unit  14  of the node device  10  are also cancelled. 
         [0060]    According to the present embodiment described above, it is possible to create, in node devices of a ring network, a path for transmitting test signals over a protection band of the ring network. 
       Third Embodiment 
       [0061]    In the present embodiment based on the configuration illustrated in  FIG. 4 , a destination node ID and a source node ID that satisfy “destination node ID=source node ID” in K1 and K2 bytes are transmitted, as a protection band control signal, from a node device to an adjacent node device so that a path for checking the communication quality of a protection band can be established. It is also possible to use one of 101, 100, and 011 in the sixth to eighth bits of K2 byte in  FIG. 2  as the protection band control signal. 
       Fourth Embodiment 
       [0062]      FIG. 5  is a diagram ( 2 ) illustrating exemplary operations of node devices. A ring network system illustrated in  FIG. 5  has the same system configuration as that illustrated in  FIG. 4 . In the present embodiment, signals coming from the intra-office device  61  through the ADD line  301  and transmitted from the node device  10  to the node device  30  are used as test signals. The communication quality of the protection band  203  of the WEST line  201  can thus be checked by the node device  10 , as in the case of the second embodiment. 
         [0063]    The node device  10  receives, from the NMS  71 , the control signal  601  for establishing a path for checking the communication quality of the protection band  203 . On the basis of the received control signal  601 , the ADM unit  14  of the node device  10  adds signals from the ADD line  301  to the working band  102  of the EAST line  101  and, at the same time, to the protection band  203  of the WEST line  201 . This allows a path  300  that starts and ends at the node device  10  to be created in the protection band  203  of the ring network system, as in the case of the second embodiment. 
         [0064]    Since the path  300  that starts and ends at the node device  10  is created in the protection band  203  of the ring network system, and signals on the ADD line  301  are transmitted over the path  300  and measured by the test signal measurement unit  152 , the communication quality of the protection band  203  can be evaluated. 
       Fifth Embodiment 
       [0065]      FIG. 6  is a flowchart illustrating an exemplary operation of a first node device, which is equivalent to the node device  10  described with reference to  FIGS. 1 ,  3 , and  4 . The first node device serves as the start and end points for creating a round path to be used in testing a protection band. 
         [0066]    Step S 01 : From the NMS, the first node device receives a user request, in the form of a line test control signal, for performing a line test on the protection band. 
         [0067]    Step S 02 : The first node device determines whether the protection band is not in use in the ring network. 
         [0068]    Step S 03 : The first node device sends out, to an adjacent second node device, a protection band control signal for establishing a path for checking the communication quality of the protection band. In the protection band control signal, a destination node ID in K1 byte and a source node ID in K2 byte may be the same, or the sixth to eighth bits of K2 byte may be defined as one of 101, 100, and 011. 
         [0069]    Step S 04 : The first node device determines whether a protection band control signal has been received from the direction opposite the direction in which the protection band control signal was sent out in step S 03 . 
         [0070]    Step S 05 : If it is determined in step S 04  that the protection band control signal has not been received, the first node device determines in step S 05  whether the amount of time that has elapsed from the time when the protection band control signal was sent out in step S 03  exceeds a timeout period predefined according to the characteristics of the ring network system. 
         [0071]    Step S 06 : If it is determined in step S 02  that the protection band is in use, or if it is determined in step S 05  that a timeout occurs, the first node device returns an “abnormal” response to the NMS in response to the user request received in step S 01 . 
         [0072]    Step S 07 : If the first node device has received in step S 04  the protection band control signal before a timeout occurs, in other words, if the protection band control signal sent out in step S 01  has been transmitted around the entire ring network and a round path for testing the protection band has been created, the first node device returns a “normal” response to the NMS. 
         [0073]      FIG. 7  is a flowchart illustrating an exemplary operation of a second node device, which is equivalent to either of the node devices  20 ,  30 ,  40 , and  50  described with reference to  FIGS. 1 ,  3 , and  4 . In the second node device, a protection band is placed in a pass-through state so that a round path for testing the protection band can be created. 
         [0074]    Step S 11 : From an adjacent first node device or another second node device, the second node device receives a protection band control signal for establishing a path for checking the communication quality of the protection band. In the protection band control signal, a destination node ID in K1 byte and a source node ID in K2 byte may be the same, or the sixth to eighth bits of K2 byte may be defined as one of 101, 100, and 011. 
         [0075]    Step S 12 : The second node device determines whether the protection band is not in use in the ring network. 
         [0076]    Step S 13 : If it is determined in step S 12  that the protection band is not in use, the second node device keeps the protection band in a through state. 
         [0077]    Step S 14 : The second node device sends out a protection band control signal to the adjacent first or second node devices in the same direction as the flow of the protection band control signal received in step S 11 . The protection band control signal to be sent out is a control signal defined in the same manner as in step S 11 . 
         [0078]    In the protection band of a SONET ring composed of the first node device and the plurality of second node devices, a round path starting and ending at the first node device and passing through the second node devices can thus be created and allow the communication quality of the protection band to be tested.