Patent Publication Number: US-2010129075-A1

Title: Transmission system, transmission apparatus, and control method of transmission system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-300702, filed on Nov. 26, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     A certain aspect of the embodiments discussed herein is related to a transmission system, a transmission apparatus, and a control method of a transmission system. 
     BACKGROUND 
     In recent years, a protection function such as UPSR (Uni-directional path switched ring) and SNCP (Subnetwork Connection Protection) is used in a multiplexing transmission system such as SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical Network). 
     In UPSR/SNCP, when data is transmitted from a sending apparatus to a receiving apparatus by using a ring shaped transmission line of SDH/SONET, a path to one direction (e.g. right-handed direction) and a path to another direction (e.g. left-handed direction) are established in the ring shaped transmission line. The ring shaped transmission line is able to establish multiple paths. Some of the multiple paths are established as the paths to one direction, and others are established as the paths to another direction. For example, if failures occur or maintenances are done in the middle of the paths to the one direction, some of the paths mentioned above are switched to the paths to another direction. This enhances the reliability of the transmission line. 
     If there is a phase difference between two paths when some of the paths mentioned above are switched from the paths to one direction to the paths to another direction, an instantaneous interruption-free switching of the paths is impossible. Thus, installing a phase adjustment unit that adjusts the phase difference for an instantaneous interruption-free switching of the paths is known, as disclosed in Japanese Laid-open Patent Publication Nos. 08-65282 and No. 2003-348061. 
     SUMMARY 
     According to an aspect of the invention, a transmission system includes: a sending apparatus; a receiving apparatus; and an intermediate apparatus, the sending apparatus, the receiving apparatus and the intermediate apparatus being connected in a ring shape via a first transmission line used to transmit a first channel signal and a second channel signal in one direction and a second transmission line used to transmit the first channel signal and the second channel signal in another direction opposite to the one direction, the receiving apparatus including a first phase adjustment unit that adjusts a phase difference between the first channel signal of the first transmission line and the first channel signal of the second transmission line, and the intermediate apparatus including a second phase adjustment unit that adjusts a phase difference between the second channel signal of the first transmission line and the second channel signal of the second transmission line. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a transmission system in accordance with the first embodiment; 
         FIG. 2  is a block diagram of a receiving apparatus and an intermediate apparatus; 
         FIG. 3A  is a block diagram of a phase adjustment unit, and  FIG. 3B  is a time chart of each signal; 
         FIG. 4  is a diagram illustrating an overview of a phase adjustment with a phase adjustment unit in an intermediate apparatus; 
         FIG. 5  is a diagram illustrating a path of a second channel at normal times; 
         FIGS. 6A through 6C  are diagrams illustrating a connection of a cross connect unit and a selection unit; 
         FIG. 7  is a diagram illustrating an overview of when the path at normal times is a route  6 ; 
         FIGS. 8A through 8D  are diagrams illustrating a connection of a cross connect unit and a selection unit; 
         FIG. 9  is a diagram illustrating an overview of when a path during maintenance is a route  3 ; 
         FIGS. 10A through 10D  are diagrams illustrating a connection of a cross connect unit and a selection unit; 
         FIG. 11  is a diagram illustrating an overview of a system in accordance with the second embodiment; 
         FIG. 12  is a diagram illustrating an overview of a system in accordance with the third embodiment; 
         FIG. 13  is a block diagram of a phase adjustment unit in accordance with the third embodiment; 
         FIG. 14  is a diagram illustrating an overview of a system in accordance with the fourth embodiment; and 
         FIG. 15  is a block diagram of a phase adjustment unit in accordance with the fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As described previously, it is known to install a phase adjustment unit that adjusts the phase difference for an instantaneous interruption-free switching of the paths. However, an apparatus will get bigger to achieve the instantaneous interruption-free switching in all paths because the size of the phase adjustment unit will get bigger. 
     A description will now be given of embodiments of the present invention with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a diagram illustrating an overview of a transmission system in accordance with the first embodiment. Referring to  FIG. 1 , a transmission line  90  such as SDH and SONET is formed into a ring shape. Multiple transmission apparatuses  10  through  19  are coupled via a transmission line  90  in a ring shape. Hereinafter, one of the multiple transmission apparatuses  10  through  19  will be described as a sending apparatus  10 , and another one will be described as a receiving apparatus  15 . Other transmission apparatuses can behave as a sending apparatus or a receiving apparatus as well. The transmission line  90  includes a first transmission line  91  used to transmit data in one direction (clockwise direction), and a second transmission line  92  used to transmit a signal in another direction opposite to the one direction (counterclockwise direction). Each of the first transmission line  91  and the second transmission line  92  has a capability of transmitting signals of multiple channels. For example, when each of the first transmission line  91  and the second transmission line  92  is a 10 Gbps transmission line, 52 Mbps VC32 can establish 192 channels. 
     The sending apparatus  10  includes a cross connect unit  80  and a selection unit  82 . The selection unit  82  selects a line from the first transmission line  91  and the second transmission line  92  for sending a transmitting signal, with respect to each channel. The cross connect unit  80  can pass and block signals of the first transmission line  91  and the second transmission line  92  with respect to each channel, and output the transmitting signal to the first transmission line  91  and second transmission line  92 . 
     The receiving apparatus  15  includes a cross connect unit  20  (a first cross connect unit), a phase adjustment unit  30  (a first phase adjustment unit), and a selection unit  40  (a first selection unit). The cross connect unit  20  can pass and block signals of the first transmission line  91  and the second transmission line  92  with respect to each channel, and can connect the first transmission line  91  and the second transmission line  92  to the phase adjustment unit  30 . The phase adjustment unit  30  can adjust a phase difference between two signals. The selection unit  40  can select a signal with respect to each channel and output the signal as a receiving signal. 
     In the above transmission system, the path (a route  1 ), which goes through the sending apparatus  10 , the transmission apparatuses  19 ,  18 ,  17 ,  16 , and the receiving apparatus  15 , and adjusts the phase difference in the receiving apparatus  15  by using the first transmission line  91  as a path to the sending apparatus  10  or the receiving apparatus  15 , and the path (a route  2 ), which goes through the sending apparatus  10 , the transmission apparatuses  11 ,  12 ,  13 ,  14 , and the receiving apparatus  15  and adjusts the phase difference in the receiving apparatus  15  by using the second transmission line  92 , are established. Some of the transmitting signals are transmitted with the route  1  by using some channels of the first transmission line  91 . Others are transmitted with the route  2  by using some channels of the second transmission line  92 . A time difference of signals occurs because the path of the route  1  is different from the path of the route  2 . The phase adjustment unit  30  adjusts the phase difference between the signal of the route  1  and the signal of the route  2 . The selection units  82  and  40  select the route  1  or the route  2  with respect to each channel. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 ROUTE 1 
                 10→19→18→17→16→15(PHASE ADJUSTMENT) 
               
               
                 ROUTE 2 
                 10→11→12→13→14→15(PHASE ADJUSTMENT) 
               
               
                   
               
            
           
         
       
     
     When failures occur or maintenances are done in the route  2 , the path that transmits the channel signal with the route  2  is switched to the path that transmits the channel signal with the route  1 . Because of this switching, the signal can be transmitted from the sending apparatus  10  to the receiving apparatus  15  during the failures or the maintenance. Because the phase adjustment unit  30  adjusts the phase difference, an instantaneous interruption-free switching is possible. 
     The phase adjustment unit  30  with a size of the number of channels of the transmission line  90  is needed to compensate all channels of the transmission line  90  for the instantaneous interruption-free switching. For example, if the size of the transmission line  90  is 10 Gbps (192 channels), the phase adjustment unit  30  with the size of 10 Gbps (192 channels) is needed. However, if the phase adjustment unit  30  of which the size is large is used, the cost reduction and downsizing of the transmission apparatus such as the receiving apparatus  15  are prevented. Meanwhile, if the phase adjustment unit  30  with the size of 5 Gbps (96 channels) is used, the cost reduction and downsizing of the transmission apparatus such as the receiving apparatus  15  may be possible, but the instantaneous interruption-free paths will be 96 channels. Other 96 channels will become normal paths that does not compensate for the instantaneous interruption-free switching. 
     To solve the problem above, in the first embodiment, the size of the phase adjustment unit  30  in the receiving apparatus  15  is 96 channels, and is less than the number of channels of the transmission line  90 . The lack of the size is made up for with the phase adjustment unit in another transmission apparatus (e.g. an intermediate apparatus  14 ). Hereinafter, the channel signal that adjusts the phase with the phase adjustment unit  30  in the receiving apparatus  15  is described as the first channel signal, and the channel signal that adjusts the phase with the phase adjustment unit in the intermediate apparatus  14  is described as the second channel signal. 
       FIG. 2  is a block diagram around the cross connect unit  20 . The cross connect unit  20  includes splitting units  22  and  28 , multiplexing units  24  and  26 , and a connect unit  25 . The splitting units  22  and  28  split the signal of the first transmission line  91  and the signal of the second transmission line  92  with respect to each channel, respectively. The connect unit  25  can pass and block the signal of the first transmission line  91  and the signal of the second transmission line  92  with respect to each channel. The connect unit  25  can also connect the signals of the first transmission line  91  and the second transmission line  92  to the phase adjustment unit  30 , the selection unit  40 , and the output. The multiplexing units  24  and  26  multiplex the signals of multiple channels, and output them to the first transmission line  91  and the second transmission line  92 , respectively. 
     In the connect unit  25 , an input of the first channel that is a part of the multiple channels split by the splitting unit  22  is an input In 11 . An input of the second channel that is another part of the multiple channels is an input In 12 . In the same way, an input of the first channel split by the splitting unit  28  is an input In 21 , and an input of the second channel is an input In 22 . An output of the first channel outputted to the multiplexing unit  26  is an output Out 11 , an output of the second channel is an output Out 12 , an output of the first channel outputted to the multiplexing unit  24  is an output  21 , and an output of the second channel is an output Out 22 . Two outputs to the phase adjustment unit  30  are outputs OP 1  and OP 2  respectively. Inputs corresponding to the outputs OP 1  and OP 2  from the phase adjustment unit  30  to the selection unit  40  are inputs S 1  and S 2 , respectively. The signal selected from the inputs S 1  and S 2  is inputted to the input IS of the connect unit  25 . An output of the sending apparatus  15  is an output Out. 
       FIG. 3A  is a block diagram of the phase adjustment unit  30 , and  FIG. 3B  is a time chart of data that are inputted to the phase adjustment unit  30  and outputted from the phase adjustment unit  30 . Referring to  FIG. 3A , the phase adjustment unit  30  includes a control unit  32 , and adjustment units  34  and  36 . The adjustment units  34  and  36  include a storage device such as RAM (Random Access Memory). Signals are inputted to the adjustment units  34  and  36  from the outputs OP 1  and OP 2  of the cross connect unit  20 . The adjustment units  34  and  36  output signals to the inputs P 1  and P 2  of the selection unit  40 . The control unit  32  controls the adjustment units  34  and  36  based on the information, which is stored in the storage unit  31 , about the phase difference to be adjusted so that the phase difference between the signals of outputs OP 1  and OP 2  does not exist. 
     Referring to  FIG. 3B , the time difference (the phase difference) occurs between the signals of the outputs OP 1  and OP 2  of the cross connect unit  24 , and the outputs OP 1  and OP 2  are corresponding to the first transmission line and the second transmission line respectively. The adjustment units  34  and  36  store the signal data of the outputs OP 1  and OP 2  respectively, and output the signal data to the inputs P 1  and P 2  after the predetermined time t 1  and t 2  that the control unit  32  indicates, respectively. Making the predetermined times that the adjustment units  34  and  36  have different can make the time difference between the signals of the inputs P 1  and P 2  little. Thus, even though the selection unit  40  switches the input P 1  to the input P 2 , the receiving apparatus  15  can output the signal without the phase difference. 
       FIG. 4  is a diagram illustrating an overview of the phase adjustment of the second channel, in which the phase adjustment unit  30  in the receiving apparatus  15  can not adjust the phase, with the phase adjustment unit  60  in the intermediate apparatus  14  of the transmission apparatuses. The intermediate apparatus  14  includes a cross connect unit  50  (a second cross connect unit), a phase adjustment unit  60  (a second phase adjustment unit), and a selection unit  70  (a second selection unit). The configurations of the cross connect unit  50 , the phase adjustment unit  60  and the selection unit  70  are same as the cross connect unit  20 , the phase adjustment unit  30  and the selection unit  40 , respectively. 
     As described in a Table 2, the path (a route  3 ), which goes through the sending apparatus  10 , the transmission apparatuses  19 ,  18 ,  17 ,  16 , the receiving apparatus  15 , the intermediate apparatus  14 , and the receiving apparatus  15 , and adjusts the phase in the intermediate apparatus  14  by using the first transmission line  91  as the path of the second channel from the sending apparatus  10  to the receiving apparatus  15 , and the path (a route  4 ), which goes through the sending apparatus  10 , the transmission apparatuses  11 ,  12 ,  13 , the intermediate apparatus  14 , and the receiving apparatus  15 , and adjusts the phase in the intermediate apparatus  14  by using the second transmission line  92 , are established. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 ROUTE 3 
                 10→19→18→17→16→15→14(PHASE ADJUSTMENT)→15 
               
               
                   
                 ROUTE 4 
                 10→11→12→13→14(PHASE ADJUSTMENT)→15 
               
               
                   
                   
               
            
           
         
       
     
     When the route  1  and the route  2  illustrated in  FIG. 1  are established, the phase difference between the route  1  and the route  2  is measured. The storage unit  31  in the phase adjustment unit  30  stores the information about the phase difference. The control unit  32  indicates the time t 1  and the time t 2  illustrated in  FIG. 3B  to the adjustment units  34  and  36  based on the phase difference measured. In the same way, when the route  3  and the route  4  illustrated in  FIG. 4  are established, the phase difference between the route  3  and the route  4  is measured, and the storage unit  31  in the phase adjustment unit  60  stores the information about the phase difference. 
       FIG. 5  illustrates the path of the second channel at normal times. As described in Table 3, the path (a route  5 ), which goes through the sending apparatus  10 , the transmission apparatuses  19 ,  18 ,  17 ,  16 , and the receiving apparatus  15  by using the first transmission line  91  as the path of the second channel from the sending apparatus  10  to the receiving apparatus  15 , and the path (route  6 ), which goes through the sending apparatus  10 , the transmission apparatuses  11 ,  12 ,  13 , the intermediate apparatus  14 , and the receiving apparatus  15  and adjusts the phase in the intermediate apparatus  14  by using the second transmission line  92 , are established. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 ROUTE 5 
                 10→19→18→17→16→15 
               
               
                   
                 ROUTE 6 
                 10→11→12→13→14→15 
               
               
                   
                   
               
            
           
         
       
     
     Now the behavior of the transmission system will be described. The first channel will be described first.  FIG. 6A  is a diagram illustrating a connection relation of the cross connect unit  20  of the first channel, which is corresponding to the inputs In 11  and In 21 , where the receiving apparatus  15  adjusts the phase difference between the route  1  and the route  2  with the route  1  and the route  2 .  FIG. 6B  is a diagram illustrating a connection relation of the cross connect unit  50 .  FIG. 6C  is a diagram illustrating the selection of the selection unit  40 . 
     Referring to  FIG. 1  and  FIG. 6A , the cross connect unit  20  of the receiving apparatus  15  connects the input In 11  and the input In 21  to the output OP 1  and the output OP 2 , respectively. Referring to  FIG. 6B , the cross connect unit  50  of the intermediate apparatus  14  connects the input In 11  and the input In 21  to the output Out 11  and the output Out 21 , respectively. Thus, the cross connect units  20  and  50  pass the first channel signals of the first transmission line  91  and the second transmission line  92 . Referring to  FIG. 6C , the selection unit  40  selects the input P 2  at normal times. This means that the path of the first channel signal from the sending apparatus  10  to the receiving apparatus  15  is the route  2 . When the transmission apparatuses  11  through  14  or the transmission line  90  are maintained (or failures occur), the selection unit  40  selects the input P 1 . As illustrated in  FIG. 6A , the cross connect unit  20  outputs the input IP from the selection unit  40  to the output Out. Thus, the path of the first channel from the sending apparatus  10  to the receiving apparatus  15  is the route  1 . When the path of the first channel signal is switched to the route  1  from the route  2 , the instantaneous interruption-free switching of the route is possible, because the phase adjustment unit  30  adjusts the phase difference. 
     Now the behavior of the second channel at normal times will be described.  FIG. 7  is a diagram illustrating an overview of when the path at normal times is the route  6 .  FIGS. 8A ,  8 B,  8 C, and  8 D are illustrating the selection relation of the cross connect unit  20 , the selection unit  40 , the cross connect unit  50  and the selection unit  70 , respectively. Note that  FIG. 7  schematically illustrates that the output of the selection unit  70  is not inputted into the cross connect unit  50  for simplification. 
     As illustrated in  FIG. 7  and  FIG. 8A , the inputs In 12  and In 21  of the second channel of the cross connect unit  20  in the receiving apparatus  15  are connected to the output OS 1  and the output OS 2  respectively. As illustrated in  FIG. 8B , the selection unit  40  selects the input S 2 . As illustrated in  FIG. 8A , the input IS from the selection unit  40  is connected to the output Out. Accordingly, the second channel signal of the second transmission line  92  is outputted from the receiving apparatus  15 . As illustrated in  FIG. 7  and  FIG. 8C , the inputs In 12  and In 22  of the cross connect unit  50  in the intermediate apparatus  14  are connected to the outputs Out 12  and OP 2  respectively. As illustrated in  FIG. 8D , the selection unit  70  selects the input P 2 . As illustrated in  FIG. 8C , the input IP from the selection unit  70  is connected to the output Out 22 . Accordingly, in the intermediate apparatus  14 , the second channel signal of the second transmission line  92  is outputted to the second transmission line  92  via the phase adjustment unit  60 . 
     Now the behavior of the second channel during the maintenance will be described. When the maintenance work is needed in the transmission apparatuses  11  through  13  or the transmission line  90  between the sending apparatus  10  and the intermediate apparatus  14 , the cross connect units  20  and  50  establish the route  3  and route  4  illustrated in  FIG. 4 .  FIG. 9  is a diagram illustrating the overview of when the path during the maintenance is the route  3 .  FIGS. 10A ,  10 B,  10 C, and  10 D illustrate the selection relation of the cross connect unit  20 , the selection unit  40 , the cross connect unit  50 , and the selection unit  70  respectively. 
     As illustrated in  FIG. 9  and  FIG. 10A , the inputs In 21  and In 22  of the second channel of the cross connect unit  20  in the receiving apparatus  15  are connected to the outputs Out 12  and OS 2  respectively. Thus, the second channel signal of the first transmission line  91  passes through the cross connect unit  20 . As illustrated in  FIG. 10B , the selection unit  40  selects the input S 2 . As illustrated in  FIG. 10A , the input IS from the selection unit  40  is connected to the output Out. Accordingly, the second channel signal of the second transmission line  92  is outputted from the receiving apparatus  15 . As illustrated in  FIG. 9  and  FIG. 10C , the inputs In 12  and In 22  of the cross connect unit  50  in the intermediate apparatus  14  are connected to the outputs OP 1  and OP 2  respectively. As illustrated in  FIG. 10D , the selection unit  70  selects the input P 1 . As illustrated in  FIG. 10C , the input IP from the selection unit  70  is connected to the output Out 22 . Accordingly, in the intermediate apparatus  14  the second channel signal of the first transmission line  91  is outputted to the second transmission line  92  via the phase adjustment unit  60 . 
     As described above, the route  6  is used as the path of the second channel from the sending apparatus  10  to the receiving apparatus  15  at normal times. During the maintenance the route  6  is switched to the route  3 . When the path of the second channel signal is switched to the route  3  from the route  6 , the instantaneous interruption-free switching of the route is possible because the phase adjustment unit  60  adjusts the phase difference. 
     According to the first embodiment, as described in  FIG. 1  and  FIG. 9 , in the receiving apparatus  15 , the phase adjustment unit  30  adjusts the phase difference between the first channel signal of the first transmission line  91  and the first channel signal of the second transmission line  92 . In the intermediate apparatus  14 , the phase adjustment unit  60  adjusts the phase difference between the second channel signal of the first transmission line  91  and the second channel signal of the second transmission line  92 . As described above, by dividing the phase adjustment between the receiving apparatus  15  and the intermediate apparatus  14 , the size reduction of the phase adjustment unit  30  in the receiving apparatus  15 , and the downsizing and cost reduction of the receiving apparatus  15  are possible. 
     Additionally, as illustrated in  FIG. 6A  and  FIG. 6C , the selection unit  40  in the receiving apparatus  15  selects one of the first channel signal of the first transmission line  91  adjusted and the first channel signal of the second transmission line  92  adjusted, and outputs it. As illustrated in  FIGS. 10A and 10B , the cross connect unit  20  outputs the second channel signal of the first transmission line  91 , and passes the second channel signal of the second transmission line  92 . As illustrated in  FIG. 6B , the cross connect unit  50  of the intermediate apparatus  14  passes the first channel signal of the first transmission line  91  and the first channel signal of the second transmission line  92 . As illustrated in  FIGS. 10C and 10D , the selection unit  70  selects one of the second channel signal of the first transmission line  91  adjusted and the second channel signal of the second transmission line  92  adjusted, and outputs it to the first transmission line  91 . Accordingly, the phase difference adjustment of the first channel is executed in the phase adjustment unit  30  of the receiving apparatus  15 , and the phase difference adjustment in the second channel is executed of the phase adjustment unit  60  of the intermediate apparatus  14 . 
     According to the configurations described above, even though the number of channels, which can adjust the phase differences, of each of the phase adjustment unit  30  and the phase adjustment unit  60 , is less than the sum of channels of the first channel and the second channel, the path using the first channel and the second channel can be the instantaneous interruption-free path. 
     Furthermore, the transmission apparatus that is the receiving apparatus  15  can behave as the intermediate apparatus. When the transmission apparatus behaves as the receiving apparatus, as described in  FIG. 6A  and  FIG. 6C , the selection unit  40  selects one of the first channel signal of the first transmission line  91  adjusted and the first channel signal of the second transmission line  92  adjusted, and outputs the selected signal to the external. Meanwhile, when the transmission apparatus behaves as the intermediate apparatus, as illustrated in  FIGS. 8A ,  8 D,  10 C and  10 D, the selection unit  70  selects one of the second channel signal of the first transmission line  91  adjusted and the second channel signal of the second transmission line  92  adjusted, and sends the selected signal to the second transmission line  92 . As described above, the selection units  40  and  70  can output the selected signal to both the second transmission line  92  and the external. 
     When the transmission apparatus behaves as the receiving apparatus, the cross connect unit  20  outputs the second channel signal of the second transmission line  92  to the external, and passes the second channel signal of the first transmission line  91  as illustrated in  FIGS. 10A and 10B . Meanwhile, when the transmission apparatus behaves as the intermediate apparatus, as illustrated in  FIG. 6B , the cross connect unit  50  passes each of the first channel signal of the first transmission line  91  and the first channel signal of the second transmission line  92 . As described above, the cross connect units  20  and  50  can execute all operations mentioned above. 
     Second Embodiment 
     The second embodiment is an example that the transmission apparatus  18  is another intermediate apparatus. As described in  FIG. 11 , the transmission apparatus  18  includes the cross connect unit  72 , the phase adjustment unit  74 , and the selection unit  76 . The cross connect unit  72 , the phase adjustment unit  74 , and the selection unit  76  behave same as the cross connect unit  50 , the phase adjustment unit  60 , and the selection unit  70  in the first embodiment. In this configuration, the path (a route  7 ), which goes through the sending apparatus  10 , the transmission apparatuses  19 ,  18 ,  17 ,  18  and the receiving apparatus, and adjusts the phase in another intermediate apparatus  16 , and the path (a route  8 ), which goes through the sending apparatus  10 , the transmission apparatuses  11 ,  12 ,  13 ,  14 , the receiving apparatus  15 , another intermediate apparatus  16 , and the receiving apparatus  15 , and adjusts the phase in another intermediate apparatus  16 , are established. Accordingly, the instantaneous interruption-free switching of the path is possible even when the maintenance is done in the transmission apparatuses  17  through  19 , the sending apparatus  10  or the transmission line  90  between the transmission apparatuses  19  and  16 . 
     The configurations of all transmission apparatuses can be same as the configuration of the intermediate apparatus  14 . Thus, the phase adjustment can be done by using the phase adjustment unit of the transmission apparatus that is other than the transmission apparatus next to the receiving apparatus  15 . Additionally, the phase adjustment can be done in more than three phase adjustment units. As described above, multiple intermediate apparatuses can be installed. 
     Third Embodiment 
     The third embodiment is an example that the phase adjustment unit adjusts the phase difference based on the signal transmitted from the sending apparatus.  FIG. 12  is a block diagram in accordance with the third embodiment. As described in  FIG. 12 , the sending apparatus  10  includes a J1 insert unit  84 . The J1 insert unit  84  inserts the J1 multi-frame into the overhead as a signal for adjusting the phase difference between the first transmission line  91  and the second transmission line  92 . Other configurations are same as the configurations illustrated in  FIG. 4 . 
       FIG. 13  is the block diagram of the phase adjustment units  30  and  60 . As described in  FIG. 13 , the phase adjustment unit  30  includes phase extraction units  33  and  35 , a phase comparison unit  37 , and a phase difference control unit  38 . The phase extraction units  33  and  35  extract the J1 multi-frame from the signals of the first transmission unit  91  and the second transmission unit  92  respectively. The phase comparison unit  37  compares the phases extracted by the phase extraction units  33  and  35 . The phase difference control unit  38  determines the time that the adjustment units  34  and  36  delay the phase based on the comparison result of the phase comparison unit  37 . The control unit  32  controls the phase difference between the signal of the first transmission line  91  and the signal of the second transmission line  92 , as described in  FIG. 3B . 
     According to the third embodiment, the sending apparatus  10  transmits the signal to adjust the phase difference between the first transmission line  91  and the second transmission line  92 , and the phase adjustment unit  30  and the phase adjustment unit  60  can adjust the phase difference based on the signal mentioned above. Note that the method disclosed in the Japanese Patent Laid Open. 2005-244806 can be used for the method of the phase adjustment with the J1 multi-frame. 
     Fourth Embodiment 
       FIG. 14  is a block diagram in accordance with the fourth embodiment. As described in  FIG. 14 , a control apparatus  100  controls the sending apparatus  10 , the receiving apparatus  15 , the intermediate apparatus  14 , and the transmission apparatuses  11  through  13  and  16  through  19 .  FIG. 15  is a block diagram of the phase adjustment units  30  and  60 . The control unit  32  is controlled by the control apparatus  100 . When the route  1  and the route  2  in  FIG. 1  are established and when the route  3  and the route  4  in  FIG. 4  are established, the phase difference between the route  1  and the route  2  and the phase difference between the route  3  and the route  4  are stored in a storage unit  101 . When the phase adjustment units  30  and  60  adjust the phase, the control apparatus  100  indicates the information about the phase difference stored in the storage unit  101  to the control unit  32 . The control unit  32  controls the adjustment units  34  and  36  based on the information about the phase difference indicated. 
     Note that as described in the fourth embodiment the control apparatus  100  that controls the transmission system can store the information about the phase difference into the storage unit  101 . 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.