Patent Publication Number: US-2003235152-A1

Title: Network system incorporating protection paths in the transmission bandwidth of a virtual concatenation signal

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an SDH ring network, and more particularly to an improvement of the reliability and efficiency of use of SDH network transmission paths.  
       [0003] 2. Description of the Related Art  
       [0004] SDH (Synchronous Digital Hierarchy) is prescribed principally for the purpose of transmitting signals for speech services. In recent years, however, the amount of traffic for data services in addition to speech services has greatly increased, and the demand is therefore increasing for the more efficient transmission of signals for data service in an SDH network.  
       [0005] Data services, of which Ethernet is representative, are available at a variety of data rates. SDH of the prior art was not capable of efficiently accommodating the signals of these data services, and various attempts have therefore been made to efficiently accommodate the signals of data services in SDH. In ITU-T G. 707 and other standards, virtual concatenation technology has been proposed for efficiently accommodating in SDH signals of data service signals.  
       [0006] According to virtual concatenation, concatenating N (where N is any number) SDH paths enables the realization of a virtual concatenation signal having a transmission bandwidth that is any number of times (a. multiple of N) the transmission bandwidth of the SDH paths, and data services of various data rates can therefore be accommodated within an SDH network. Optical ring networks achieved by using SDH are already in use in many places due to their ease of maintenance and operation. Optical ring networks that are realized by means of SDH take various forms, but a network according to MS-SPRING (Multiplex Section Shared Protection Ring) recommended by ITU-T G. 841 can be offered as a representative example. In a MS-SPRING network, a plurality of nodes are connected by ring configuration transmission paths in which signals flow in different directions. The bandwidth of the transmission paths is evenly divided between a Working and a Protection band. In an MS-SPRING network, Working and Protection paths are provided between transmission side nodes and receiving side nodes. The working paths are set in the working bandwidth, and the protection paths are set in the protection bandwidth. When a fault occurs on a particular transmission path, data that were being transmitted on the working path that was set in this transmission path is switched to the protection path that corresponds to this working path, a function that is referred to as “protection.” By means of this protection, an MS-SP RING network achieves high reliability against faults. In order to guarantee this high reliability, however, half of the transmission bandwidth that is physically possessed by this MS-SP RING network is provided only for the event of a fault, and is thus reserved.  
       [0007] As represented by an MS-SPRING, half of the transmission bandwidth in an SDH ring system is typically maintained as protection for dealing with faults. Thus, a typical SDH ring system can use only half of the transmission bandwidth and makes inefficient use of the transmission bandwidth. In contrast, an SDH ring system can be considered that, by using the protection transmission bandwidth for service only when in a normal state that is free of faults, allows a more efficient use of the transmission bandwidth. In such a system, however, the occurrence of some type of fault on the transmission path and the consequent switching to the protection bandwidth would disconnect the service that was using this protection bandwidth.  
       [0008] As an example, it is possible to simply use the protection bandwidth together with the working bandwidth as the constituent elements of a virtual concatenation signal. A virtual concatenation signal is a channel in which the timings of a plurality of SDH paths are synchronized and in which the channels of these paths are logically linked. As a result, there is the problem that the occurrence of a fault on even one of the SDH paths that serve as the constituent elements in virtual concatenation will result in a fault of the entire virtual concatenation signal that contains the faulty SDH path. In this type of system, the execution of protection to continue providing service results in the interruption all services that were accommodated in the virtual concatenation signal that employed SDH paths in the protection bandwidth as constituent elements. Despite the difficulties encountered in simultaneously achieving both efficient use of the transmission bandwidth and reliability of the transmission paths as described in the foregoing explanation, there is a strong demand from network service providers for a new method that can achieve both goals.  
       SUMMARY OF THE INVENTION  
       [0009] It is an object of the present invention to provide a network for accommodating data service and for realizing both efficient use of transmission bandwidth and reliability of a transmission path, and further, to provide a transmission device from which such a network is constructed.  
       [0010] To achieve the above-described object, a transmission device according to the present invention forms part of a network in which are applied: a virtual concatenation process for concatenating a plurality of paths to construct a single transmission bandwidth; and protection control for selecting paths that are used in accordance with the state of each of the paths in a redundancy configuration that is realized using a plurality of paths.  
       [0011] A protection control means executes protection control over prescribed working paths and protection paths and selects paths of either the prescribed working paths or protection paths. A bandwidth managing means operates in response to receiving the states of the working paths and protection paths from the protection control means. If protection paths are in a normal standby state, the bandwidth managing means incorporates the protection paths into a virtual concatenation signal that includes working paths. If a protection path is in an unusable state or a used state, the bandwidth managing means reconstructs the virtual concatenation signal only from paths that are selected by the protection control means. A path connection means receives working paths and protection paths as input and switches the connections of paths in accordance with control from the protection control means.  
       [0012] The present invention uses protection paths as the constituent elements of virtual concatenation during normal operation, thus efficiently utilizing the transmission bandwidth; and when a fault occurs in a working path, executes protection, and further, constructs a virtual concatenation signal only from paths that can be used in virtual concatenation, thus ensuring reliability in the event of a fault. As a result, the present invention can ensure compatibility between the efficient utilization of the transmission bandwidth and the reliability of the paths. The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, which illustrate examples of the present invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013]FIG. 1 is a block diagram showing a network system of an embodiment in accordance with the present invention.  
     [0014]FIG. 2 is a block diagram showing a node of the present embodiment.  
     [0015]FIG. 3 is a sequence chart showing the operation of each part of a node when executing protection due to the occurrence of a fault in a transmission path.  
     [0016]FIG. 4 shows the operating state of a node in FIG. 2 when executing protection due to a fault in a transmission path.  
     [0017]FIG. 5 is a block diagram showing a node in another embodiment of the present invention.  
     [0018]FIG. 6 shows the operating state of a node in FIG. 5 when executing protection due to the occurrence of a fault in a transmission path.  
     [0019]FIG. 7 is a block diagram showing a network system of yet another embodiment of the present invention.  
     [0020]FIG. 8 is a block diagram showing a node shown in FIG. 7.  
     [0021]FIG. 9 shows the operating state of the node of FIG. 8 when executing protection due to the occurrence of a fault in a transmission path. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0022] The network system of the present embodiment is an SDH ring system that is provided with the MS-SPRING capability, and further, that uses virtual concatenation process to accommodate data services. FIG. 1 is a block diagram showing the network system of this embodiment. Referring to FIG. 1, network system  10  is, as one example, a two-fiber (2F) MS-SPRING system and includes four nodes  11 . All nodes  11  have the same configuration.  
     [0023]FIG. 2 is a block diagram showing a node of the present embodiment. Referring to FIG. 2, node  11  includes: SDH interfaces  21  and  22 ; path connection unit  23 ; protection control unit  24 ; concatenation processor  25 ; bandwidth management unit  26 ; and user interface  27 .  
     [0024] SDH interfaces  21  and  22  interface with other adjacent nodes and transmit SDH lines that include a plurality of SDH paths to and from other nodes. SDH paths that are received at SDH interfaces  21  and  22  are supplied to path connection unit  23 . In addition, SDH paths from path connection unit  23  are transmitted from SDH interfaces  21  and  22 .  
     [0025] SDH interfaces  21  and  22  also monitor the state of the transmission paths and information on the SDH overhead for the receiving side of transmission paths. SDH interfaces  21  and  22  further send the state of each transmission path and information in the SDH overhead that is used in the protection protocol (hereinafter referred to as “protection information”) to protection control unit  24 .  
     [0026] Upon receiving notification of execution of protection from protection control unit  24 , SDH interfaces  21  and  22  report it to other neighboring nodes as protection information.  
     [0027] In accordance with path setting information from the host operating system (not shown in the figure), path connection unit  23  connects SDH interfaces  21  and  22  and concatenation processor  25  in units of SDH paths  28 . The host operating system is a control system that, for example, monitors the network system  10  of the present embodiment for the purpose of operation and maintenance.  
     [0028] The host operating system is connected to a prescribed node and monitors and controls this node and other nodes. A DCC (Data Communication Channel) in the overhead of the SDH frames that are communicated between nodes may be used for monitoring and controlling other nodes.  
     [0029] In addition, path connection unit  23  changes connections between SDH interfaces  21  and  22  and concatenation processor  25  in accordance with instructions from protection control unit  24 . Protection control unit  24  determines whether or not to execute protection based on the protection information and the state of each transmission path that is received from SDH interfaces  21  and  22 . When protection is to be carried out, protection control unit  24  instructs path connection unit  23  to carry out protection (i.e., to change connections), and notifies SDH interfaces  21  and  22  and bandwidth management unit  26  regarding the SDH paths that are to be used in carrying out protection and the state of each SDH path. The protection that is carried out in this case is “ring protection.” Ring protection is a switching operation carried out in path units from working to protection paths between two nodes in a ring network. Switching paths between two nodes entails modifying the connection of path connections not only for the two nodes but also, according to necessity, for other nodes in network system  10 .  
     [0030] Concatenation processor  25  links the plurality of SDH paths  28  between concatenation processor  25  and path connection unit  23  in accordance with the instructions from bandwidth management unit  26  and thus constructs virtual concatenation signal  29  between concatenation processor  25  and user interface  27 .  
     [0031] Bandwidth management unit  26  instructs concatenation processor  25  to concatenate SDH paths  28  based on information that is notified from protection control unit  24 . In a state in which working paths are selected and protection paths are in a normal standby state in protection control, the protection paths are included among the constituent elements of a virtual concatenation signal that includes working paths, whereby the transmission bandwidth of the protection paths is effectively utilized. When protection paths are selected and in a state of use in protection control, the virtual concatenation signal is constructed using only the selected protection paths. The selection and use of a protection path occurs when a fault has occurred on a working path or when an operator has forcibly selected the protection path. In addition, working paths are selected in protection control, and if protection paths are in an unusable state, the virtual concatenation signal is constructed only from working paths. An unusable state of a protection path is, for example, a state in which a fault has occurred in the protection path or a state in which an operator has forcibly set the protection path to non-operation. In this way, the transmission bandwidth of virtual concatenation signal  29  increases and decreases.  
     [0032] User interface  27  transmits and receives virtual concatenation signals to and from a user device (not shown in the figure).  
     [0033] Explanation next regards the operation of node  11  of the present embodiment.  
     [0034] In FIG. 2, half of the SDH paths that are accommodated in SDH interfaces  21  and  22  are working paths, and the remaining half are protection paths. In this case, the SDH paths that are accommodated in SDH interface  21  are working paths, and the SDH paths that are accommodated in SDH interface  22  are protection paths. In the normal state of protection control, normal working SDH paths are in use, and protection SDH paths are normal but still unused and in a standby state. During this time, protection SDH paths are concatenated together with working SDH paths to produce the virtual concatenation signal.  
     [0035] In FIG. 2, the two SDH paths of SDH interface  21  and the two SDH paths of SDH interface  22  are concatenated by concatenation processor  25  and then connected to user interface  27  as virtual concatenation signal  29 . In other words, virtual concatenation signal  29  in the normal state has the transmission bandwidth of four SDH paths  28 .  
     [0036]FIG. 3 is a sequence chart showing the operation of each part of node  11  when protection is executed due to the occurrence of a fault in the transmission paths. FIG. 4 shows the state of operation of node  11  when executing protection due to a fault in a transmission path.  
     [0037] Referring to FIG. 3, it will be assumed that a fault has occurred in a transmission path that is connected to SDH interface  21 , as shown in FIG. 4. SDH interface  21  detects the fault in the transmission path and notifies protection control unit  24  of the fault (Step S 1 ).  
     [0038] Protection control unit  24 , upon receiving the notification of the transmission path fault from SDH interface  21 , instructs path connection unit  23  to connect the protection SDH path to concatenation processor  25  in place of the failed working SDH path that is set as the transmission path (Step S 2 ). When protection has been executed by path connection unit  23  in accordance with the instructions of protection control unit  24 , the path connections are as shown in FIG. 4. in this way, the protection SDH path is used in place of the working SDH path and is therefore neither in a normal or standby state.  
     [0039] Protection control unit  24  further reports the execution of protection to bandwidth management unit  26 . In actuality, protection is completed with the notification of the state following execution of protection to the partner node, but in the interest of simplifying the explanation, this step is omitted in FIG. 3.  
     [0040] Upon receiving notification of the execution of protection from protection control unit  24 , bandwidth management unit  26  instructs concatenation processor  25  to delete the SDH path that, due to the execution of protection, can no longer be used as a constituent element from virtual concatenation signal  29  (Step S 3 ). This process reduces the transmission bandwidth of virtual concatenation signal  29 .  
     [0041] Although the transmission bandwidth of the virtual concatenation signal is consequently reduced, virtual concatenation is realized by concatenating only SDH paths that are available for service, and service can be maintained.  
     [0042] In FIGS. 3 and 4, a case was shown in which working SDH path and protection SDH path could not be concatenated in a virtual concatenation signal due to the execution of protection that was brought about by a fault in a working SDH path. The occurrence of a fault in a protection SDH path also prevents the use of the protection SDH path as a constituent element of virtual concatenation. In this case, the instruction from protection control unit  24  to path connection unit  23  maintains the connection of the SDH path from SDH interface  21  to concatenation processor  25  and cuts off only the SDH path from SDH interface  22 . The instruction from bandwidth management unit  26  to concatenation processor  25  is to delete the SDH paths that can no longer be used from virtual concatenation signal  29 .  
     [0043] The bandwidth of the virtual concatenation signal is also reduced as described hereinabove when an operator forcibly switches an SDH path or places SDH interface  22  or a protection SDH path in a state of halted operation. In other words, a protection SDH path can be incorporated in virtual concatenation signal  29  when it is in a normal state and free of faults, and moreover, in a standby state and not being used in protection control. In the end, the use of a protection SDH path in place of a working SDH path in protection control makes the protection SDH path a constituent element of a virtual concatenation signal. In this case, the working SDH path is dropped as a constituent element of the virtual concatenation signal.  
     [0044] In addition, when recovery from a transmission path fault is detected by SDH interface  21  and reported to protection control unit  24 , protection control unit  24  returns the path connections to the original state (refer to FIG. 2). The protection SDH path thus returns to a state of possible use as a constituent element of virtual concatenation. When the protection SDH path enters the state of possible use, bandwidth management unit  26  instructs concatenation processor  25  to take the protection SDH path as a constituent element of virtual concatenation (refer to FIG. 2), whereby virtual concatenation signal  29  again attains the original expanded transmission bandwidth.  
     [0045] As described in the foregoing explanation, node  11  of the present embodiment enables the efficient use of the transmission bandwidth by using protection SDH paths as constituent elements of virtual concatenation during normal operation, and when a fault occurs in a working SDH path, both uses protection SDH paths in place of working SDH paths in protection control and dynamically reconstitutes the virtual concatenation signal using only SDH paths that can normally be used to maintain service. Node  11  of the present embodiment therefore enables both efficient use of the transmission bandwidth and reliability of the transmission paths.  
     [0046] Explanation next regards another embodiment of the present invention.  
     [0047] In this embodiment of the present invention, the network system is again assumed to be an MS-SPRING SDH ring system. FIG. 5 is a block diagram showing a node of this embodiment of the present invention. Referring to FIG. 5, node  30  includes: SDH interfaces  31  and  32 , path connection unit  33 , protection control unit  34 , concatenation processor  35 , bandwidth management unit  36 , user interface  37 , and path monitor unit  40 . SDH interfaces  31  and  32 , concatenation processor  35 , and user interface  37  each have the same configuration as SDH interfaces  21  and  22 , concatenation processor  25 , and user interface  27 , respectively, that were shown in FIG. 2.  
     [0048] Path connection unit  33 , although similar to path connection unit  23  shown in FIG. 2, differs in that it further includes AIS insertion unit  41 . AIS insertion unit  41  sends an AIS (Alarm Indication Signal) in place of SDH paths from SDH interfaces  31  and  32  that can no longer be connected to path monitor unit  40  when connections are altered in accordance with instructions from protection control unit  34 . Protection control unit  34  determines whether or not protection is to be executed, as with protection control unit  24  of FIG. 2, but does not need to report the execution of protection to bandwidth management unit  36 .  
     [0049] Bandwidth management unit  36  instructs concatenation processor  35  to concatenate SDH paths  38 , as with bandwidth management unit  26  in FIG. 2. However, bandwidth management unit  36  is notified of the data line on which an AIS is received from path monitor unit  40  without receiving the notification of protection execution from protection control unit  34 , and instructs concatenation processor  35  to delete the SDH paths that were connected to the data line on which the AIS is received from the virtual concatenation signal.  
     [0050] The function of path monitor unit  40  is not present in FIG. 2, this function being to monitor the SDH paths from path connection unit  33  and detect AIS. If a data line exists on which an AIS is received from path connection unit  33 , path monitor unit  40  notifies bandwidth management unit  36  of this data line. Explanation next regards the operation of the node of FIG. 5. FIG. 6 shows the operating state of node  30  when protection is executed due to the occurrence of a fault on a transmission path.  
     [0051] It is first assumed that a fault occurs on a transmission path that is connected to SDH interface  31  as shown in FIG. 6. SDH interface  31  detects the fault in the transmission path and notifies protection control unit  34  of the fault.  
     [0052] Protection control unit  34 , upon receiving the notification of the transmission path fault from SDH interface  31 , instructs path connection unit  33  to connect a protection SDH path to path monitor unit  40  in place of the SDH path that has failed. When protection is executed at path connection unit  33  in accordance with the instruction from protection control unit  34 , the path connections are as shown in FIG. 6. The protection SDH path is thus no longer available as a constituent element of virtual concatenation signal  39 . In addition, an AIS is transmitted to path monitor unit  40  on a data line that, until this point, had been connected to SDH interface  32 . This process is well known as a squelch process in ring systems.  
     [0053] Path monitor unit  40  reports the data line on which the AIS was detected to bandwidth management unit  36 . Based on the notification from path monitor unit  40 , bandwidth management unit  36  instructs concatenation processor  35  to delete SDH paths on which the AIS was detected from virtual concatenation signal  39 , whereby the transmission bandwidth of virtual concatenation signal  39  is reduced.  
     [0054] As a result, virtual concatenation is realized by concatenating only those SDH paths that are available for service, and service is maintained.  
     [0055] Further, when the recovery from the transmission path fault is detected by SDH interface  31  and reported to protection control unit  34 , protection control unit  34  returns the path connections to the original state (refer to FIG. 5), whereby the protection SDH paths become available for use as constituent elements of virtual concatenation. The AIS ends when the protection SDH paths become available for use, and the end of AIS is reported from path monitor unit  40  to bandwidth management unit  36 . Bandwidth management unit  36  instructs concatenation processor  35  to take the protection SDH paths as constituent elements of virtual concatenation (see FIG. 5), whereby virtual concatenation signal  39  again attains the original expanded transmission bandwidth.  
     [0056] A case has been described in which a protection SDH path is used in place of a working SDH path due to the execution of protection arising from the occurrence of a fault in the working transmission path, and the concatenation of the protection SDH path with working SDH paths to broaden the bandwidth of a virtual concatenation signal is therefore prevented. In addition to this case, a protection SDH path becomes unavailable for use as a constituent element of virtual concatenation in a case in which a fault occurs on a protection transmission path. The bandwidth of a virtual concatenation signal is also reduced in cases in which an operator forcibly switches the SDH path or a protection SDH path placed in a non-operating state. In other words, a protection SDH path can be incorporated in virtual concatenation signal  39  when it is in a normal state that is free of faults, and moreover, in a standby state and not in use due to protection switching. AIS insertion unit  41  transmits AIS in place of SDH paths from SDH interfaces  31  and  32 , which cannot connect to path monitor unit  40  in any of these cases.  
     [0057] Explanation next regards another embodiment of the present invention. The network system of this embodiment of the present invention is a linear system and not a ring system. FIG. 7 is a block diagram showing the network system of this embodiment of the present invention. Referring to FIG. 7, network system  50  includes two nodes  51 , both nodes  51  being of the same configuration.  
     [0058]FIG. 8 is a block diagram showing the configuration of node  51  that is shown in FIG. 7. Referring to FIG. 8, node  51  includes: SDH interface  61  and  62 , line selection unit  63 , protection control unit  64 , concatenation processor  65 , bandwidth management unit  66 , and user interface  67 .  
     [0059] SDH interfaces  61  and  62  interface with other neighboring nodes and transmit on SDH lines having a plurality of SDH paths with other nodes. The SDH lines that are received at SDH interfaces  61  and  62  are supplied to line selection unit  63 . SDH lines from line selection unit  63  are transmitted from SDH interfaces  61  and  62 .  
     [0060] In addition, SDH interfaces  61  and  62  monitor information of the SDH overhead and the state of the transmission paths for the receiving side of the transmission paths. SDH interfaces  61  and  62  then send the state of each of the transmission paths and information of the SDH overhead that is used in protection protocol (hereinbelow referred to as “protection information”) to protection control unit  64 .  
     [0061] SDH interfaces  61  and  62  further, upon receiving notification of protection execution from protection control unit  64 , report this notification as protection information to other adjacent nodes. Line selection unit  63  connects SDH interfaces  61  and  62  and concatenation processor  65  in units of SDH lines  68  in accordance with line setting information from the host operating system (not shown in the figures). Line selection unit  63  also modifies connections in accordance with instructions from protection control unit  64 .  
     [0062] Protection control unit  64  determines whether or not to execute protection based on the protection information and the state of each of the transmission paths that have been received from SDH interfaces  61  and  62 . When protection is to be executed, protection control unit  64  instructs the execution of protection (i.e., the alteration of connections) to line selection unit  63  and reports the execution of protection to SDH interfaces  61  and  62  and bandwidth management unit  66 . The protection that is executed here is, for example, linear protection. Linear protection is an operation of switching in units of SDH lines from working to protection between two partner nodes. This case is assumed to be a 1+1 redundancy configuration that one protection SDH line is prepared to one working SDH line. But, n+1 redundancy configuration is available. Furthermore, n+m(n&gt;=m) redundancy configuration is available. Concatenation processor  65  concatenates the SDH paths that are included within the plurality of SDH lines  68  between concatenation processor  65  and line selection unit  63  in accordance with instructions from bandwidth management unit  66  and constructs virtual concatenation signal  69  between concatenation processor  65  and user interface  67 .  
     [0063] Based on the information that is notified from protection control unit  64 , bandwidth management unit  66  instructs concatenation processor  65  to concatenate the SDH lines that are included in SDH lines  68 , whereby the transmission bandwidth of virtual concatenation signal  69  is increased.  
     [0064] User interface  67  transmits a virtual concatenation signal to and receives a virtual concatenation signal from user devices (not shown in the figures).  
     [0065] Explanation next regards the operation of node  51  of the present embodiment.  
     [0066] In FIG. 8, the SDH lines accommodated in SDH interface  61  are assumed to be working lines, and the SDH lines that are accommodated in SDH interface  62  are assumed to be protection lines. In the normal state of protection control, normal working SDH lines are in use and protection SDH lines are in a normal but unused standby state. At this time, the SDH paths that are included in protection SDH lines are concatenated by means of virtual concatenation together with SDH paths that are included in the working SDH lines. FIG. 9 shows the operating state of node  51  when executing protection due to the occurrence of a fault in a transmission path.  
     [0067] First, a fault is assumed to occur in a transmission path that is connected to SDH interface  61  as shown in FIG. 9. SDH interface  61  detects the fault in the transmission path and reports the detection to protection control unit  64 .  
     [0068] Protection control unit  64 , having received the notification of a transmission path fault from SDH interface  61 , instructs line selection unit  63  to connect the SDH line of the protection transmission path to concatenation processor  65  in place of the SDH line of the transmission path in which the fault occurred. When protection is executed in line selection unit  63  in accordance with the instruction of protection control unit  64 , the connections of the SDH lines are as shown in FIG. 9. The protection SDH line is thus used in place of the working SDH line and is no longer in the normal standby state, and accordingly, the concatenation of SDH paths that are included in the protection SDH line with SDH paths that are included in the working SDH line to expand the transmission bandwidth is no longer possible. Bandwidth management unit  66 , upon receiving notification of the execution of protection from protection control unit  64 , instructs concatenation processor  65  to delete the SDH paths that can no longer be used as constituent elements due to the execution of protection from virtual concatenation signal  69 , whereby the transmission bandwidth of virtual concatenation signal  69  is reduced, but service is maintained.  
     [0069] When recovery from the transmission path fault is detected in SDH interface  61  and reported to protection control unit  64 , protection control unit  64  returns the line connections to the original state (see FIG. 8), whereby SDH paths that are included in the protection SDH line enter a state of potential use as constituent elements of virtual concatenation. When the protection SDH line is in a useable state, bandwidth management unit  66  instructs concatenation processor  65  to treat the SDH paths that are included in the protection SDH line as constituent elements of virtual concatenation (refer to FIG. 8), whereby virtual concatenation signal  69  again returns to the original expanded transmission bandwidth. While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.