Abstract:
A transmission line rerouting method includes the steps of selecting a predetermined trunk transmission line to be rerouted, from trunk transmission lines, coupling a first subscriber line accommodating apparatus which is coupled to the predetermined trunk transmission line and a second subscriber line accommodating apparatus which is coupled to a trunk transmission line other than the predetermined trunk transmission line, via a rerouting transmission line, and transmitting and receiving a data which is to be exchanged between the first subscriber line accommodating apparatus and the trunk transmission line accommodating apparatus, via the second subscriber line accommodating apparatus and the trunk transmission line other than the predetermined trunk transmission line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims the benefit of a Japanese Patent Application No. 2001-097131 filed Mar. 29, 2001, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to transmission line rerouting methods, subscriber line accommodating apparatuses and trunk transmission line accommodating apparatuses, and more particularly to a transmission line rerouting methods, a subscriber line accommodating apparatus and a trunk transmission line accommodating apparatus which are used in a digital transmission system.  
           [0004]    2. Description of the Related Art  
           [0005]    Recently, much attention is drawn to the x digital subscriber line (xDSL) system which is a digital transmission system using an existing telephone copper-wire cable (subscriber line) for telephones. The xDSL system provides a high-speed access line to the subscriber by utilizing bands up to a high frequency band on the order of several MHz.  
           [0006]    According to the xDSL system, the distance of the telephone copper-wire cable and the frequency band have an inversely proportional relationship. Hence, in order to provide a high-speed xDSL, a network structure having a short telephone copper-wire cable must be used. For example, the telephone copper-wire cable can be made short as shown in FIG.  1 , by providing a remote xDSL subscriber line accommodating apparatus near the subscriber&#39;s home.  
           [0007]    [0007]FIG. 1 is a diagram showing an example of the network structure including the remote xDSL subscriber line accommodating apparatus. For example, a remote xDSL subscriber line accommodating apparatus  100  shown in FIG. 1 accommodates one or more xDSL modems which are set up at the subscriber&#39;s home by xDSLs. In addition, an accommodating station  110  connects and accommodates one or more remote xDSL subscriber line accommodating apparatuses  100  by a trunk transmission line  120 . In order to cope with a situation where a failure or contention occurs in the trunk transmission line  120 , the remote xDSL subscriber line accommodating apparatus  100  and the accommodating apparatus  110  are in many cases connected via a rerouting transmission line  130  which is other than the trunk transmission line  120  and has a transmission band equivalent to that of the trunk transmission line  120 .  
           [0008]    For example, it is conceivable for the rerouting transmission line  130  to utilize a line switching network such as INS proposed in a Japanese Laid-Open Patent Application No. 61-262338 or a Japanese Laid-Open Patent Application No. 5-75736. In addition, it is conceivable to regard an INS line used in these Japanese Laid-Open Patent Applications No. 61-262338 and No. 5-75736 as the xDSL, so that the xDSL is used as the rerouting transmission line  130 . Furthermore, it is conceivable to use a wireless line proposed in a Japanese Laid-Open Patent Application No. 2000-124910 or the like as the rerouting transmission line  130 .  
           [0009]    However, when using the line switching network such as the INS proposed in the Japanese Laid-Open Patent Applications No. 61-262338 and No. 5-75736 for the rerouting transmission line  130 , the maximum transmission rate becomes 64 kbps, and there is a problem in that it is impossible to secure the transmission rate for the xDSL service. In order to secure the transmission rate for the xDSL service using the INS line, it is necessary to terminate a large number of INS lines to form a multi-link, but there is a problem in that such a multi-link structure is expensive. In addition, if the kind of transmission line is different between the trunk transmission line  120  and the rerouting transmission line  130 , there is a problem in that a gateway apparatus becomes necessary in the remote xDSL subscriber line accommodating apparatus  100  and the accommodating apparatus  110 .  
           [0010]    When the INS line of the Japanese Laid-Open Patent Applications No. 61-262338 and No. 5-75736 are regarded as the xDSL line, and the xDSL is used as the rerouting transmission line  130 , the distance of the rerouting transmission line  130  becomes long compared to the distance of the xDSL (subscriber line). For this reason, the transmission rate of the rerouting transmission line  130  becomes low compared to the transmission rate of the xDSL, and there is a problem in that the transmission rate of the xDSL service cannot be secured. When securing the transmission rate of the xDSL service by forming a multi-link of a plurality of xDSLs, an extremely complex control becomes necessary, and there is a problem in that the apparatuses become expensive.  
           [0011]    On the other hand, when using the wireless line of the Japanese Laid-Open Patent Application No. 2000-124910 as the rerouting transmission line  130 , two kinds of interfaces must be provided in the remote xDSL subscriber line accommodating apparatus  100  and the accommodating apparatus  110 , and there is a problem in that the apparatuses become expensive.  
           [0012]    Therefore, the rerouting transmission line  130  must be provided with respect to the remote xDSL subscriber line accommodating apparatus  100  and the accommodating apparatus  110  in order to continue the service even when the failure or contention occurs in the trunk transmission line  120 , but there is a problem in that the operation cost is increased thereby.  
           [0013]    On the other hand, with respect to the remote xDSL subscriber line accommodating apparatus  100 , the telephone copper-wire cable must be made short in order to provide a high-speed xDSL. Consequently, the number of subscribers which can be accommodated by one remote xDSL subscriber line accommodating apparatus  100  becomes small. In other words, the cost per subscriber accommodated by one remote xDSL subscriber line accommodating apparatus  100  increase.  
           [0014]    Furthermore, the operation cost is increased considerably by providing the rerouting transmission line  130  for each remote xDSL subscriber line accommodating apparatus  100 , and it is difficult to provide services at costs comparable to the costs of the services provided by other high-speed digital transmission systems.  
           [0015]    Moreover, the subscribers which may be accommodated by one remote xDSL subscriber line accommodating apparatus  100  are limited to the subscribers within the same area, such as within the same building or apartment. As a result, the number of subscribers which can be accommodated varies depending on each area.  
           [0016]    Accordingly, there is a problem in that the usable band greatly differs between the remote xDSL subscriber line accommodating apparatus  100  which accommodates a large number of subscribers and the xDSL subscriber line accommodating apparatus  10  which accommodates a small number of subscribers. SUMMARY OF THE INVENTION  
           [0017]    Accordingly, it is a general object of the present invention to provide a novel and useful transmission line rerouting methods, subscriber line accommodating apparatus and trunk transmission line accommodating apparatus, in which the problems described above are eliminated.  
           [0018]    Another and more specific object of the present invention to provide a transmission line rerouting methods, a subscriber line accommodating apparatus and a trunk transmission line accommodating apparatus, which can form an inexpensive rerouting transmission line without providing a backup trunk transmission line and suppress differences in the usable bands among the subscribers.  
           [0019]    Still another object of the present invention is to provide a transmission line rerouting method for rerouting a predetermined trunk transmission line of a plurality of trunk transmission lines of a system in which a plurality of subscriber line accommodating apparatuses respectively accommodating one or more subscriber lines and a trunk transmission line accommodating apparatus accommodating the plurality of subscriber line accommodating apparatuses are coupled via the plurality of trunk transmission lines, comprising the steps of (a) selecting the predetermined trunk transmission line to be rerouted, from the plurality of trunk transmission lines; (b) coupling a first subscriber line accommodating apparatus which is coupled to the predetermined trunk transmission line and a second subscriber line accommodating apparatus which is coupled to a trunk transmission line other than the predetermined trunk transmission line, via a rerouting transmission line; and (c) transmitting and receiving a first data which is to be exchanged between the first subscriber line accommodating apparatus and the trunk transmission line accommodating apparatus, via the second subscriber line accommodating apparatus and the trunk transmission line other than the predetermined trunk transmission line. According to the transmission line rerouting method of the present invention, when the predetermined trunk transmission line to be rerouted is selected, the first subscriber line accommodating apparatus connected to the predetermined trunk transmission line and the second subscriber line accommodating apparatus connected to the trunk transmission line other than the predetermined trunk transmission line via the rerouting transmission line. In addition, by transmitting and receiving the first data which is to be transmitted and received between the first subscriber line accommodating apparatus and the trunk transmission line accommodating apparatus via the second subscriber line accommodating apparatus and the trunk transmission line other than the predetermined trunk transmission line, it is possible to prevent the trunk transmission line from becoming suspended for a long time. Moreover, the contention states of the trunk transmission lines can be made uniform, and it is possible to suppress a difference from being introduced in the bands available to each of the subscribers. Furthermore, it is possible to reduce the operating cost of the system and to provide inexpensive services, because the rerouting transmission line can be formed without the need to provide a backup trunk transmission line. On the other hand, since it is possible to form a rerouting transmission line having a transmission band equivalent to that of the predetermined trunk transmission line, it is possible to minimize the deterioration of the services even when the rerouting is carried out, and the reliability of the services can be secured.  
           [0020]    A further object of the present invention is to provide a subscriber line accommodating apparatus for accommodating one or a plurality of subscriber lines, comprising trunk transmission line coupling means for coupling trunk transmission lines to a trunk transmission line accommodating apparatus which accommodates one or a plurality of subscriber line accommodating apparatuses; and transfer means for monitoring a state of the trunk transmission lines and transferring a first transmitting data which is to be transmitted to the trunk transmission line accommodating apparatus to an other subscriber line accommodating apparatus depending on a monitored result, so that a first receiving data which is to be received from the trunk transmission line accommodating apparatus is transferred from the other subscriber line accommodating apparatus. According to the subscriber line accommodating apparatus of the present invention, when the predetermined trunk transmission line to be rerouted is selected, the first subscriber line accommodating apparatus connected to the predetermined trunk transmission line and the second subscriber line accommodating apparatus connected to the trunk transmission line other than the predetermined trunk transmission line via the rerouting transmission line. In addition, by transmitting and receiving the first data which is to be transmitted and received between the first subscriber line accommodating apparatus and the trunk transmission line accommodating apparatus via the second subscriber line accommodating apparatus and the trunk transmission line other than the predetermined trunk transmission line, it is possible to prevent the trunk transmission line from becoming suspended for a long time. Moreover, the contention states of the trunk transmission lines can be made uniform, and it is possible to suppress a difference from being introduced in the bands available to each of the subscribers. Furthermore, it is possible to reduce the operating cost of the system and to provide inexpensive services, because the rerouting transmission line can be formed without the need to provide a backup trunk transmission line. On the other hand, since it is possible to form a rerouting transmission line having a transmission band equivalent to that of the predetermined trunk transmission line, it is possible to minimize the deterioration of the services even when the rerouting is carried out, and the reliability of the services can be secured.  
           [0021]    Another object of the present invention is to provide a trunk transmission line accommodating apparatus for accommodating one or a plurality of subscriber line accommodating apparatuses, comprising trunk transmission line coupling means for coupling to the one or plurality of subscriber line accommodating apparatuses respectively accommodating one or more subscriber lines via one or more trunk transmission lines; and transmitting and receiving means for monitoring a state of the one or more trunk transmission lines and transferring a transmitting data which is to be transmitted to a first subscriber line accommodating apparatus to a second subscriber line accommodating apparatus depending on a monitored result, so that a receiving data which is to be received from the first subscriber line accommodating apparatus is received from the second subscriber line accommodating apparatus. According to the trunk transmission line accommodating apparatus of the present invention, when the predetermined trunk transmission line to be rerouted is selected, the first subscriber line accommodating apparatus connected to the predetermined trunk transmission line and the second subscriber line accommodating apparatus connected to the trunk transmission line other than the predetermined trunk transmission line via the rerouting transmission line. In addition, by transmitting and receiving the first data which is to be transmitted and received between the first subscriber line accommodating apparatus and the trunk transmission line accommodating apparatus via the second subscriber line accommodating apparatus and the trunk transmission line other than the predetermined trunk transmission line, it is possible to prevent the trunk transmission line from becoming suspended for a long time. Moreover, the contention states of the trunk transmission lines can be made uniform, and it is possible to suppress a difference from being introduced in the bands available to each of the subscribers. Furthermore, it is possible to reduce the operating cost of the system and to provide inexpensive services, because the rerouting transmission line can be formed without the need to provide a backup trunk transmission line. On the other hand, since it is possible to form a rerouting transmission line having a transmission band equivalent to that of the predetermined trunk transmission line, it is possible to minimize the deterioration of the services even when the rerouting is carried out, and the reliability of the services can be secured.  
           [0022]    Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a diagram showing an example of a network structure including a remote xDSL subscriber line accommodating apparatus;  
         [0024]    [0024]FIG. 2 is a diagram for explaining the operating principle of a transmission line rerouting method according to the present invention;  
         [0025]    [0025]FIG. 3 is a system block diagram showing an embodiment of a remote xDSL subscriber line accommodating apparatus according to the present invention;  
         [0026]    [0026]FIG. 4 is a system block diagram showing an embodiment of a system including the xDSL subscriber line accommodating apparatuses according to the present invention; and  
         [0027]    [0027]FIG. 5 is a sequence diagram for explaining an embodiment of a transmission line rerouting procedure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    A description will be given of various embodiments of a transmission line rerouting method according to the present invention, a subscriber line accommodating apparatus according to the present invention, and a trunk transmission line accommodating apparatus according to the present invention, by referring to FIGS. 2 through 5. In the embodiments described hereunder, the xDSL is utilized as an example of the digital transmission system, but the present invention is of course applicable to any kind of digital transmission system.  
         [0029]    First, a description will be given of the operating principle of the present invention, by referring to FIG. 2. FIG. 2 is a diagram for explaining the operating principle of the transmission line rerouting method according to the present invention. In FIG. 2, each of remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  accommodates one or more xDSL modems set up at the subscriber homes, for example, by connecting to a corresponding one of xDSLs  14   a  and  14   b . In addition, an accommodating station  11  connects to and accommodates the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  via respective trunk transmission lines  12   a  and  12   b.    
         [0030]    The remote xDSL subscriber line accommodating apparatus  10   a  and the remote xDSL subscriber line accommodating apparatus  10   b  are connected by a rerouting transmission line  13  using one or more xDSLs. The remote xDSL subscriber line accommodating apparatus  10   a  and the remote xDSL subscriber line accommodating apparatus  10   b  are set up within a separating distance of several km, for example, so that a required band may be secured for the rerouting transmission line  13 .  
         [0031]    When a failure or a contention occurs in one of the trunk transmission line  12   a  and the trunk transmission line  12   b , the rerouting transmission line  13  is used to connect the remote xDSL subscriber line accommodating apparatus  10   a  or the remote xDSL subscriber line accommodating apparatus  10   b  to the accommodating station  11  via the other of the trunk transmission lines  12   a  and the trunk transmission line  12   b  in which no failure or contention is generated. For example, when the failure or contention (hereinafter simply referred to as an abnormality) occurs in the trunk transmission line  12   b , for example, the remote xDSL subscriber line accommodating apparatus  10   b  is connected to the accommodating station  11  via the rerouting transmission line  13 , the remote xDSL subscriber line accommodating apparatus  10   a  and the trunk transmission line  12   a.    
         [0032]    When using the xDSL as the rerouting transmission line  13 , an interface required for the rerouting transmission line  13  can be realized by a slight modification to the hardware or the signal processing firmware, because the circuit of such an interface is similar to that of an interface which accommodates the subscriber line. Accordingly, the remote xDSL subscriber line accommodating apparatus  10   a  and the remote xDSL subscriber line accommodating apparatus  10   b  can utilize the xDSLs as the rerouting transmission line  13  by making the slight modification to the hardware or the signal processing firmware.  
         [0033]    By employing as the xDSL a xDSL in which the up-transmission rate and the down-transmission rate are symmetrical, it becomes unnecessary to modify the setting on the center side or the remote side of the rerouting transmission line  13  when an abnormality is generated in the trunk transmission lines  12   a  and  12   b , thereby making it possible to reduce a switching time which would otherwise be longer if the setting were to be modified.  
         [0034]    When the trunk transmission lines  12   a  and  12   b  are in a normal state, the rerouting transmission line  13  is used to transfer control information between the remote xDSL subscriber line accommodating apparatus  10   a  and the remote xDSL subscriber line accommodating apparatus  10   b . Hence, the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  exchange with each other, through notification, control information which includes abnormality information of the trunk transmission lines  12   a  and  12   b , switching request and the like, so that each of the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  can independently carry out a transmission line rerouting process when an abnormality occurs in the trunk transmission line  12   a  or  12   b.    
         [0035]    In addition, control paths using the trunk transmission lines  12   a  and  12   b , for example, may be set between the accommodating station  11  which accommodates the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  and the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  which are mutually connected via the rerouting transmission line  13 . In this case, it is possible to independently carry out the transmission line rerouting process in the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  and the accommodating station  11  when the abnormality is generated in the trunk transmission line  12   a  or  12   b.    
         [0036]    When using the xDSL in which the up-transmission rate and the down-transmission rate are asymmetrical, it becomes necessary to modify the setting at the center side or the remote side of the rerouting transmission line  13  when the abnormality is generated in the trunk transmission lines  12   a  and  12   b . In this case, the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b  exchange with each other, through notification, control information which includes abnormality information of the trunk transmission lines  12   a  and  12   b , switching request and the like, and modify the setting at the center side or the remote side of the rerouting transmission line  13  by disconnecting the rerouting transmission line  13 . In other words, the setting can be modified at the center side or the remote side of the rerouting transmission line  13  depending on the direction in which the broad band is required, thereby making it possible to secure the band required for the rerouting transmission line  13 .  
         [0037]    In a case where the setting at the center side or the remote side of the rerouting transmission line  13  cannot be modified during operation, two xDSLs may be used as the rerouting transmission line  13 . In this case, one xDSL may be set in advance as the center side and the other xDSL may be set in advance as the remote side, so as to realize a pseudo-state in which the same band is used for the two directions.  
         [0038]    Next, a description will be given of the structures of the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b , by referring to FIG. 3. FIG. 3 is a system block diagram showing an embodiment of the remote xDSL subscriber line accommodating apparatus which may be used as the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b , that is, an embodiment of the subscriber line accommodating apparatus according to the present invention. A remote xDSL subscriber line accommodating apparatus  10  shown in FIG. 3 includes a user line termination section  20 , a multiplexing and demultiplexing section  22 , a switch control section  23 , a xDSL termination section  24 , and a trunk transmission line termination section  25 .  
         [0039]    The user line termination section  20  includes one or more xDSL termination sections  21   a  through  21   n  respectively terminating the xDSLs which are used as the user lines. In addition, the xDSL termination sections  21   a  through  21   n  are connected to the multiplexing and demultiplexing section  22 .  
         [0040]    The xDSL termination section  24  is connected to the xDSL termination section  24  of the adjacent remote xDSL subscriber line accommodating apparatus  10  via the xDSL  26 , and terminates this xDSL  26 . This xDSL  26  is used as the rerouting transmission line  13 . The xDSL termination section  24  is connected to the multiplexing and demultiplexing section  22 .  
         [0041]    The trunk transmission line termination section  25  is connected to an accessing accommodating apparatus set up in the accommodating station  11  via a trunk transmission line  27 , and terminates this trunk transmission line  27 . The trunk transmission line termination section  25  is connected to the multiplexing and demultiplexing section  22 .  
         [0042]    The switch-control section  23  receives various switching requests from within or outside the remote xDSL subscriber line accommodating apparatus  10 , and instructs switching of the multiplexing and demultiplexing section  22  depending on the switching request. The multiplexing and demultiplexing section  22  carries out a routing process among the user line termination section  20 , the xDSL termination section  24  and the trunk transmission line termination section  25 , depending on the switching instruction from the switching control section  23 .  
         [0043]    For example, when an abnormality is generated in the trunk transmission line  27  or when transferring a specific user line to another remote xDSL subscriber line accommodating apparatus  10 , the switching request is generated inside or outside the remote xDSL subscriber line accommodating apparatus  10  and supplied to the switching control section  23 . The switching request may be generated, for example, by an operation system provided outside the remote xDSL subscriber line accommodating apparatus  10  or, by a control unit provided inside the remote xDSL subscriber line accommodating apparatus  10 . The control unit provided inside the remote xDSL subscriber line accommodating apparatus  10  generates the switching request by utilizing failure information and contention information of the trunk transmission line termination section  25 , for example.  
         [0044]    The multiplexing and demultiplexing section  22  changes a transfer destination of the data supplied from the user line from the trunk transmission line termination section  25  to the xDSL termination section  24 , and changes a transfer destination of the data supplied from the xDSL  26  to the user line termination section  20 , depending on the switching request received from the switching control section  23 . Accordingly, when an abnormality is generated in the trunk transmission line  27 , the remote xDSL subscriber line accommodating apparatus  10  can utilize the xDSL  26  as the rerouting transmission line.  
         [0045]    Next, a description will be given of a system including the remote xDSL subscriber line accommodating apparatuses  10  described above, by referring to FIG. 4. FIG. 4 is a system block diagram showing an embodiment of the system including the xDSL subscriber line accommodating apparatuses according to the present invention.  
         [0046]    In FIG. 4, the remote xDSL subscriber line accommodating apparatus  10   a  includes xDSL termination sections  31   a  through  31   n  and  32 , a path selecting section  33 , a control section  34 , and a transmission line interface (IF) section  35 . Similarly, the remote xDSL subscriber line accommodating apparatus  10   b  includes xDSL termination sections  41   a  through  41   n  and  42 , a path selecting section  43 , a control section  44 , and a transmission line interface (IF) section  45 . The accessing accommodating apparatus  11  includes transmission line interface (IF) sections  37   a  and  37   b , a path selecting section  38 , a control section  39 , and a backbone trunk interface (IF) section  50 .  
         [0047]    In FIG. 4, the path selecting sections  33  and  43  correspond to the multiplexing and demultiplexing section  22  shown in FIG. 3, and the control sections  34  and  44  correspond to the switching control section  23  shown in FIG. 3. In addition, the transmission line interface sections  35  and  45  shown in FIG. 4 correspond to the trunk transmission line termination section  25  shown in FIG. 3.  
         [0048]    Two kinds of modes, namely, a master mode and a slave mode, may be set with respect to the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b . In this embodiment, the remote xDSL subscriber line accommodating apparatus  10   a  is set to the master mode, and the remote xDSL subscriber line accommodating apparatus  10   b  is set to the slave mode. The xDSL termination section  32  of the remote xDSL subscriber line accommodating apparatus  10   a  which is set to the master mode operates in an ADSL transceiver unit at the central office (ATU-C) mode. In addition, the xDSL termination section  42  of the remote xDSL subscriber line accommodating apparatus  10   b  which is set to the slave mode operates in an ATU at the remote end (ATU-R) mode.  
         [0049]    The xDSL termination section  32  and the xDSL termination section  42  are connected via the xDSL, and this xDSL is used as the rerouting transmission line  13 . In FIG. 4, each xDSL termination section which operates in the ATU-C mode is indicated by a designation “ATU-C”. In addition, each xDSL termination section which operates in the ATU-R mode is indicated by a designation “ATU-R”.  
         [0050]    The remote xDSL subscriber line accommodating apparatus  10   a  includes one or more xDSL termination sections  31   a  through  31   n , and the xDSL termination sections  31   a  through  31   n  respectively accommodate xDSL termination sections  30   a  through  30   n  which are set up within the subscriber homes. The one or more xDSL termination sections  31   a  through  31   n  are connected to the path selecting section  33 .  
         [0051]    The demote xDSL subscriber line accommodating apparatus  10   b  includes one or more xDSL termination sections  41   a  through  41   n , and the xDSL termination sections  41   a  through  41   n  respectively accommodate xDSL termination sections  40   a  through  40   n  which are set up within the subscriber homes. The one or more xDSL termination sections  41   a  through  41   n  are connected to the path selecting section  43 .  
         [0052]    The transmission interface section  35  of the remote xDSL subscriber line accommodating apparatus  10   a  is connected to a transmission line interface section  37   a  of the accessing accommodating apparatus  11  via a trunk transmission line  36 . The transmission line interface section  35  is connected to the path selecting section  33 . In addition, the transmission line interface section  45  of the remote xDSL subscriber line accommodating apparatus  10   b  is connected to a transmission line interface section  37   b  of the accessing accommodating apparatus  11  via a trunk transmission line  46 . The transmission line interface section  45  is connected to the path selecting section  43 .  
         [0053]    The control section  34  of the remote xDSL subscriber line accommodating apparatus  10   a  receives various switching requests supplied from inside or outside the remote xDSL subscriber line accommodating apparatus  10   a , and instructs switching of the path selecting section  33  depending on the switching request. The path selecting section  33  carries out a routing process among the xDSL termination sections  31   a  through  31   n , the xDSL termination section  32  and the transmission line interface section  35 , depending on the switching instruction from the control section  34 .  
         [0054]    The control section  44  of the remote xDSL subscriber line accommodating apparatus  10   b  receives various switching requests supplied from inside or outside the remote xDSL subscriber line accommodating apparatus  10   b , and instructs switching of the path selecting section  43  depending on the switching request. The path selecting section  43  carries out a routing process among the xDSL termination sections  41   a  through  41   n , the xDSL termination section  42  and the transmission line interface section  45 , depending on the switching instruction from the control section  44 .  
         [0055]    The accessing accommodating apparatus  11  forms an embodiment of the trunk transmission line accommodating apparatus according to the present invention.  
         [0056]    The transmission line interface sections  37   a  and  37   b  of the accessing accommodating apparatus  11  are connected to a path selecting section  38 . A backbone trunk interface section  50  is connected to the path selecting section  38 . A control section  39  of the accessing accommodating apparatus  11  receives various switching requests supplied from inside or outside the accessing accommodating apparatus  11 , and instructs switching of the path selecting section  38  depending on the switching request. The path selecting section  38  carries out a routing process among the transmission line interface section  37   a , the transmission line interface section  37   b  and the backbone trunk interface section  50 , depending on the switching instruction from the control section  39 .  
         [0057]    A control path utilizing the rerouting transmission line  13  is set between the control section  34  of the remote xDSL subscriber line accommodating apparatus  10   a  and the control section  44  of the remote xDSL subscriber line accommodating apparatus  10   b . In addition, control paths utilizing the trunk transmission lines  36  and  46  are set between the control section  39  of the accessing accommodating apparatus  11  and the control sections  34  and  44  of the remote xDSL subscriber line accommodating apparatuses  10   a  and  10   b.    
         [0058]    [0058]FIG. 5 is a sequence diagram for explaining an embodiment of a transmission line rerouting procedure of the present invention. FIG. 5 shows the transmission rerouting procedure for a case where a failure is generated in the trunk transmission line  36 . In a normal state, path information A, B and C for carrying out the routing process is respectively set in the path selecting sections  33 ,  43  and  38 . For example, the data supplied from the xDSL termination section  31   a  to the path selecting section  33  is routed to the transmission line interface section  35  depending on the path information A.  
         [0059]    When a failure is generated in the trunk transmission line  36 , the transmission interface section  35  in a step S 1  detects this failure generated in the trunk transmission line  36 , and notifies transmission line failure information with respect to the control section  34 . After the step S 1 , the control section  34  in a step S 2  notifies the transmission line failure information to the control section  44  of the remote xDSL subscriber line accommodating apparatus  10   b  via the control path utilizing the rerouting transmission line  13 . More particularly, the transmission line failure information is notified from the control section  34  to the control section  44  via the path selecting section  33 , the xDSL termination section  32 , the xDSL termination section  42  and the path selecting section  43 .  
         [0060]    After the step S 2 , the control section  44  in a step S 3  notifies the transmission line failure information to the control section  39  of the accessing accommodating apparatus  11  via the control path utilizing the trunk transmission line  46 . More particularly, the transmission line failure information is notified from the control section  44  to the control section  39  via the path selecting section  43 , the transmission line interface section  45 , the transmission line interface section  37   b  and the path selecting section  38 .  
         [0061]    By the above described process, the control section  34  of the remote xDSL subscriber line accommodating apparatus  10   a , the control section  44  of the remote xDSL subscriber line accommodating apparatus  10   b , and the control section  39  of the accessing accommodating apparatus  11  recognize the failure generated in the trunk transmission line  36 .  
         [0062]    When the xDSLs used have an up-transmission rate and a down-transmission rate which are asymmetrical, it is necessary to modify the setting at the center side or the remote side of the rerouting transmission line  13 . In a step S 4 , the control section  34  notifies a direction change instruction to the xDSL termination section  32  to instruct the change in direction of the rerouting transmission line  13 . Further, in a step  5 , the control section  44  notifies a direction change instruction to the xDSL termination section  42  to instruct the change in direction of the rerouting transmission line  13 . By the processes carried out by the steps S 4  and S 5 , the xDSL termination section  32  and the xDSL termination section  42  are switched from the ATU-C mode to the ATU-R mode or, from the ATU-R mode to the ATU-C mode. In a step S 6 , the rerouting transmission line  13  is temporarily disconnected and thereafter reconnected.  
         [0063]    In a step S 7 , the control section  34  changes the path information A set in the path selecting section  33  to path information A′. In a step S 8 , the control section  44  changes the path information B set in the path selecting section  43  to path information B′. Moreover, in a step S 9 , the control section  39  changes the path information C set in the path selecting section  38  to path information C′.  
         [0064]    For example, the data supplied from the xDSL termination section  31   a  to the path selecting section  33  is routed to the xDSL termination section  32  depending on the path information A′ as indicated by the step S 9 , and the data is supplied to the path selecting section  38  via the xDSL termination section  42 , the path selecting section  43 , the transmission line interface section  45  and the transmission line interface section  37   b.    
         [0065]    Next, a more detailed description will be given of a path selection switching process. For example, the path selecting section  33  changes the output port of each user data in the up-direction, input from the xDSL termination sections  31   a  through  31   n , from the transmission line interface section  35  to the xDSL termination section  32 . Further, the path selecting section  33  changes the output ports of addresses  1  through n of each of the user data in the down-direction, input from the xDSL termination section  32 , to the xDSL termination sections  31   a  through  31   n . As a result, the remote xDSL subscriber line accommodating apparatus  10   a  can transmit and receive each user data via the rerouting transmission line  13 .  
         [0066]    The path selecting section  43  sets the output port of each user data in the up-direction, input from the xDSL termination section  42 , to the transmission line interface section  45 , and changes the addresses  1  through n of each of the user data of the remote xDSL subscriber line accommodating apparatus  10   a  to addresses m+1 through m+n, so as not to overlap the addresses  1  through m of each of the user data of the remote xDSL subscriber line accommodating apparatus  10   b.    
         [0067]    In addition, the path selecting section  43  sets the output port of the user data of the addresses m+1 through m+n, input from the transmission line interface section  45 , to the xDSL termination section  42 , and changes the addresses m+1 through m+n of the user data to the addresses  1  through n. By the above described processes, the remote xDSL subscriber line accommodating apparatus  10   b  can transmit and receive each user data of the remote xDSL subscriber line accommodating apparatus  10   a  via the rerouting transmission line  13  and the trunk transmission line  46 .  
         [0068]    The path selecting section  38  changes the input port of the user data of the remote xDSL subscriber line accommodating apparatus  101  from the transmission line interface section  37   a  to the transmission line interface section  37   b , and changes the addresses  1  through n of each of the user data to the addresses m+1 through m+n. In addition, the path selecting section  38  changes the output port of the user data of the addresses  1  through n, input from the backbone trunk interface section  50 , from the transmission line interface section  37   a  to the transmission line interface section  37   b , and changes the addresses  1  through n of each of the user data to the addresses m+1 through m+n. By the above described processes, the accessing accommodating apparatus  11  can transmit and receive each user data of the remote xDSL subscriber line accommodating apparatus  10   a  via the trunk transmission line  46 .  
         [0069]    In the transmission line rerouting procedure shown in FIG. 5, the transmission line interface section  35  detects the failure generated in the trunk transmission line  36 . However, the failure generated in the trunk transmission line  36  may be detected by the transmission interface section  37   a . In this case, the control section  39  notifies the transmission line failure information to the control section of the remote xDSL subscriber line accommodating apparatus  10   b  via a control path utilizing the trunk transmission line  46 . In addition, the controller  44  notifies the transmission line failure information to the control section  34  of the remote xDSL subscriber line accommodating apparatus  10   a  via a control path utilizing the rerouting transmission line  13 .  
         [0070]    Moreover, in the transmission line rerouting procedure shown in FIG. 5, it is assumed for the sake of convenience that the failure is generated in the trunk transmission line  36 . However, a similar procedure may be carried out when a contention is generated in the trunk transmission line  36 . Moreover, it may be seen that a procedure similar to that shown in FIG. 5 may be carried out when a failure or contention is generated in the trunk transmission line  46 .  
         [0071]    In addition, when selecting a trunk transmission line which is to be rerouted, an abnormality in the trunk transmission line may be detected based on the traffic in each of the trunk transmission lines connecting the trunk transmission line accommodating apparatus (accessing accommodating apparatus) and the subscriber line accommodating apparatuses (remote xDSL subscriber line accommodating apparatuses), so that the trunk transmission line having a large or largest traffic is selected as the trunk transmission line which is to be rerouted.  
         [0072]    Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.