Abstract:
Between a first pair of termination nodes, a first current-use packet transmission path and at least one first spare packet transmission path are defined. Between a second pair of termination nodes, a second current-use packet transmission path and at least one second spare packet transmission path are defined. When no network fault occurs, packets are transmitted between the pairs of nodes in a 1:1 transfer mode in which the pairs transfer packets via both current-use packet transmission paths only. When a network fault occurs in the first current-use packet transmission path, both transmissions are switched from a 1:1 transfer mode to a 1+1 transfer mode, in which the first pair of nodes transfer packets by using the first current-use packet transmission path and the first spare packet transmission path, and the second pair transfer packets by using the second current-use packet transmission path and the second spare packet transmission path.

Description:
CROSS-REFERENCE 
     This patent application claims a priority on convention based on Japanese Patent Application No. 2010-192255 filed on Aug. 30, 2010. The disclosure thereof is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention is related to a fault recovery method in a packet transmission network. 
     BACKGROUND ARTS 
     In a packet transmission network having a current-use packet transmission path and a spare packet transmission path used when a fault has occurred, a technique is needed in which the packet transmission network is recovered from the fault, by switching the packet transmission path at high speed without imposing a load on the packet transmission network when the fault has occurred, in order to maintain the reliability of the packet transmission network. 
     As the recovery method in the packet transmission network, a linear fault recovery method is described in ITU-T G. 8032 recommendation (Non-Patent Literature 1). As the linear fault recovery method, there are a fault detection/switching and collecting method and a fault detection/switching and separating method. In the fault detection/switching and collecting method, a termination node in the packet transmission path carries out fault detection and recovery at the same time. In the fault detection/switching and separating method, a node which relays a packet on the packet transmission path detects a fault, transmits a fault notice to a termination node of the packet transmission path, and the termination node receiving the fault notice switches the packet transmission path. Generally, the fault detection/switching and separating method is used. 
     As an example of the conventional technique, the fault detection/switching and separating method will be described.  FIG. 1  is a block diagram showing a packet transmission network  100  of the fault detection/switching and separating method in the conventional technique. The packet transmission network is configured of a node  11  to a node  18 . The node  11  and the node  12  are connected through the node  17 , and a current-use packet transmission path  110  is set therebetween. Also, the node  11  and the node  12  are connected through the node  16 , and a spare packet transmission path  120  to the current-use packet transmission path  110  is set therebetween. The node  13  and the node  14  are directly connected, and a current-use packet transmission path  111  is set therebetween. Also, the node  13  and the node  14  are connected through the node  16 , and a spare packet transmission path  121  to the current-use packet transmission path  111  is set therebetween. 
     When there is not a fault in the packet transmission network  100 , the packet transmission is performed by using the current-use packet transmission path  110 , and the current-use packet transmission path  111 . Each node transmits a continuity check message (CCM) to a neighbor node regularly. Each node manages the activeness or inactiveness of the neighbor node by receiving the CCM regularly. 
     The case where the fault has occurred in the packet transmission network  100  will be described. It is assumed that a fault has occurred between the node  12  and the node  17 . Because the node  12  does not receive the CCM from the node  17 , the node  12  detects that the fault has occurred between the node  12  and the node  17 . Similarly, because the node  17  does not receive the CCM from the node  12 , the node  17  detects that the fault has occurred between the node  12  and the node  17 . The node  17  outputs a fault notice data to the node  11  as a termination node of the packet transmission path  110 , to indicate that the fault has occurred. The node  11  receives the fault notice data, and the node  12  detects the fault in the current-use packet path  110  by not receiving the CCM from the node  17 . The node  12  switches the transmission path to the spare packet transmission path  120  from the current-use packet transmission path  110 . In this way, the fault recovery is realized in the packet transmission path. 
     Next, in the packet transmission network  100 , the internal configuration of the node of the fault detection/switching and separating method in the conventional technique will be described. Because the internal configurations of the node  11  to the node  18  are identical to each other, only the node  11  will be described. 
       FIG. 2A  and  FIG. 2B  are block diagrams showing the configuration of the node  11  in the packet transmission network  100  of  FIG. 1 . The node  11  is provided with a line card  201 - 1 , a line card  201 - 2  and a packet switch  230 . Because the internal configurations of the line card  201 - 1  and the line card  201 - 2  are identical, only the internal configuration of the line card  201 - 1  will be described. 
     The line card  201 - 1  is provided with an external reception section  210 - 1 , an internal transferring section  211 - 1 , an internal reception section  212 - 1 , an external transferring sections  213 - 1 , FDB (transferring database)  214 - 1 , a current-use/spare path switching section  215 - 1  and a path unit fault notifying section  216 - 1 . 
     The external reception section  210 - 1  is provided with a packet receiving section  217 - 1  and a fault detecting section  218 - 1 . The internal transferring section  211 - 1  is provided with a multiplexing section  219 - 1  and a packet transferring section  220 - 1 . The internal reception section  212 - 1  is provided with a packet receiving section  221 - 1  and a fault notice data processing section  222 - 1 . The external transferring section  213 - 1  is provided with a packet transferring section  223 - 1 . 
     The FDB  214 - 1  is a table which retains destination data necessary to transfer a packet when transferring the packet, such as a MAC address of a next destination node, and the number of a port connected to a network to which the destination node of the packet is connected. 
     The current-use/the spare path switching section  215 - 1  refers to the FDB  214 - 1  to transfer the reception packet to the next destination node. The packet received from which of the current-use packet transmission path and the spare packet transmission path should be transferred is determined by the fault notice data processing section  222 - 1  notifying to the current-use/spare path switching section  215 - 1 . 
     The path unit fault notifying section  216 - 1  specifies the packet transmission path passing through a faulty section by referring to the FDB  214 - 1 , and generates the fault notice data in units of the packet transmission paths. The path unit fault notifying section  216 - 1  transfers the fault notice data to the multiplexing section  219 - 1 . 
     The packet receiving section  217 - 1  of the node  11  receives the packet transmitted to a port of the line card  201 - 1  connected to the neighbor node of the node  11   
     The fault detecting section  218 - 1  confirms whether a CCM is received from the node connected to the port of the line card  201 - 1 . When the reception of the CCM within a constant time cannot be confirmed, the fault detecting section  218 - 1  detects that the fault has occurred, and transmits a fault detection notice to the path unit fault notifying section  216 - 1 . The fault detecting section  218 - 1  transfers a packet except for the CCM to the current-use/spare path switching section  215 - 1 . 
     The multiplexing section  219 - 1  multiplexes the packets transferred from the current-use/spare path switching section  215 - 1  and the path unit fault notifying section  216 - 1 . 
     The packet transferring section  220 - 1  transfers the packet which is received from the line card  201 - 1 , toward the line card with a port connected with the next destination the node. 
     The packet receiving section  221 - 1  receives the packet from a port of any of the line cards of the nodes  11  through packet switch  230 . 
     The fault notice data processing section  222 - 1  refers to the received fault notice data and the FDB  214 - 1  to determine the packet transmission path to be switched, and instructs the current-use/spare path switching section  215 - 1  to switch the path from the current-use packet transmission path to the spare packet transmission path. 
     The packet transferring section  223 - 1  transfers the packet from the port connected with the next destination node which neighbors the node  11 . 
     The packet switch  230  in the node  11  transfers the packet which is received from the port of any of the line cards to the port of the line card which is connected with the destination node. 
     In the fault detection/switching and separating method of the conventional technique, when a fault has occurred among the nodes for the packet transmission path, a termination node of the packet transmission path generates the fault notice data which contains data for specifying the packet transmission path in which the fault has occurred. The fault notice data unicasted for the termination node of the packet transmission path. The termination node receiving the fault notice data switches the packet transmission path from the current-use packet transmission path to the spare packet transmission path. At that time, there sometimes is a site where the fault has occurred between the nodes of the spare packet transmission path after the switching. In this case, once again, the fault notice data is transmitted to the termination node of the packet transmission path after the switching, and the packet transmission path is switched once again. In this way, in the fault detection/switching and separating method of the conventional technique, when the fault has occurred simultaneously at a plurality of sites, it is assumed to successively switch the packet transmission path, and there is possibility that switching time has been extended. 
     CITATION LIST 
     
         
         [Non-Patent Literature 1]: ITU-t (international telecommunication union telecommunication standardization sector) G.8032 The Ethernet ring protection method 
       
    
     SUMMARY OF THE INVENTION 
     The present invention provides recovery from a fault in a packet transmission path at high speed even in the condition that the fault has occurred at a same time in a plurality of the packet transmission paths. 
     In an aspect of the present invention, a packet transmission system including a plurality of nodes connected by a packet transmission path. A current-use packet transmission path is used as the packet transmission path when a fault does not occur, and a spare packet transmission path is used as the packet transmission path when the fault has occurred. Each of the plurality of nodes includes: a transfer mode switching section configured to switch a transfer mode when transferring a received packet to a destination node to a 1:1 transfer mode or a 1+1 transfer mode, wherein the packet is transferred by the current-use packet transmission path in the 1:1 transfer mode, but the spare packet transmission path is not used, and the packet is transferred by the current-use packet transmission path and the spare packet transmission path in the 1+1 transfer mode; a path controller configured to instruct the transfer mode switching section to switch the transfer mode; a packet transferring section configured to transfer the received packet in the 1:1 transfer mode when the fault does not occur in the packet transmission network; and a reception path selecting section configured to select one of the packets received from the current-use packet transmission path and the spare packet transmission path in the 1+1 transfer mode. 
     In another aspect of the present invention, a fault recovery method is provided in a packet transmission network which includes a current-use packet transmission path used when a fault has not occurred, and a plurality of spare packet transmission paths used when the fault has occurred. The fault recovery method includes: transferring a reception packet in a 1:1 transfer mode by a packet transferring section of a termination node of the current-use packet transmission path, when the fault has not occurred, wherein in the 1:1 transfer mode, the termination node transfers the reception packet by using only the current-use packet transmission path; detecting the fault between nodes of the current-use packet transmission path by a fault detecting section of the termination node; broadcasting a packet containing a fault notice data to the nodes by the packet transferring section of the termination node, wherein the fault notice data contains a data indicating that the fault has been detected in the current-use packet transmission path; transferring the fault notice data to a neighbor node by the packet transferring section of one node which receives the fault notice data; transmitting a 1+1 transfer mode switching packet by the packet transferring section of the termination node of any packet transmission path which receives the fault notice data; switching a transfer mode from the 1:1 transfer mode to the 1+1 transfer mode by a transfer mode switching section of the termination node which has transmitted the 1+1 transfer mode switching packet, wherein the node transfers the reception packet by using the current-use packet transmission path and the plurality of spare packet transmission paths in the 1+1 transfer mode; selecting the reception packet from one of the current-use packet transmission path and the plurality of spare packet transmission paths by a reception path selecting section of the termination node which receives the reception packet in the 1+1 transfer mode; and transferring the reception packet to a next destination node by the packet transferring section of the termination node which receives the reception packet in the 1+1 transfer mode. 
     In still another aspect of the present invention, a computer-readable non-transitory storage medium in which a program code is stored is provided to realize a fault recovery method in a packet transmission network which includes a current-use packet transmission path used when a fault has not occurred, and a plurality of spare packet transmission paths used when the fault has occurred. The fault recovery method includes: transferring a reception packet in a 1:1 transfer mode by a packet transferring section of a termination node of the current-use packet transmission path, when the fault has not occurred, wherein in the 1:1 transfer mode, the termination node transfers the reception packet by using only the current-use packet transmission path; detecting the fault between nodes of the current-use packet transmission path by a fault detecting section of the termination node; broadcasting a packet containing a fault notice data to the nodes by the packet transferring section of the termination node, wherein the fault notice data contains a data indicating that the fault has been detected in the current-use packet transmission path; transferring the fault notice data to a neighbor node by the packet transferring section of one node which receives the fault notice data; transmitting a 1+1 transfer mode switching packet by the packet transferring section of the termination node of any packet transmission path which receives the fault notice data; switching a transfer mode from the 1:1 transfer mode to the 1+1 transfer mode by a transfer mode switching section of the termination node which has transmitted the 1+1 transfer mode switching packet, wherein the node transfers the reception packet by using the current-use packet transmission path and the plurality of spare packet transmission paths in the 1+1 transfer mode; selecting the reception packet from one of the current-use packet transmission path and the plurality of spare packet transmission paths by a reception path selecting section of the termination node which receives the reception packet in the 1+1 transfer mode; and transferring the reception packet to a next destination node by the packet transferring section of the termination node which receives the reception packet in the 1+1 transfer mode. 
     According to the present invention, even in the state that a fault has occurred to at a same time in a plurality of packet transmission paths, the fault in the packet transmission path at high speed can be recovered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain exemplary embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram showing a packet transmission network  100  to explain fault detection/switching and separation method in conventional art; 
         FIG. 2A  is a block diagram of one of nodes which configure the packet transmission network of  FIG. 1 ; 
         FIG. 2B  is a block diagram of one of the nodes which configure the packet transmission network  100  of  FIG. 1 ; 
         FIG. 3  is a diagram showing a packet transmission network in which the fault does not occur to in the present exemplary embodiment of the present invention does; 
         FIG. 4  is a diagram showing the packet transmission network in case of the fault occurrence in the present exemplary embodiment of the present invention; 
         FIG. 5  is the packet transmission network  300  at the time of the 1+1 transfer mode in the present exemplary embodiment of the present invention; 
         FIG. 6  is a diagram showing the configuration of the packet transmission network in 1+1 transfer mode in the packet transmission network according to the present exemplary embodiment of the present invention; 
         FIG. 7A  is a block diagram showing the configuration of each of the nodes which configures the packet transmission network of  FIG. 3 ; 
         FIG. 7B  is a block diagram of each of the nodes which configures the packet transmission network  300  of  FIG. 3 ; 
         FIG. 7C  are a block diagram of the nodes which configures the packet transmission network of  FIG. 3 ; and 
         FIGS. 8A ,  8 B and  8 C are diagram showing the switching of the packet transfer mode in the present exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, a fault recovery of the present invention will be described with reference to the attached drawings. 
     (Configuration) 
     First, the internal configuration of each of nodes in an exemplary embodiment of the present invention will be described.  FIG. 7A ,  FIG. 7B , and  FIG. 7C  are block diagrams showing the configuration of each of the nodes which configures a packet transmission network  300  of  FIG. 3 . 
     The node is configured of a plurality of line cards  501  ( 501 - 1 ,  501 - 2 ,  501 - 3 ) and a packet switch  530 . The node shown in  FIG. 7A ,  FIG. 7B , and  FIG. 7C  is configured from three of the line card  501 - 1 , the line card  501 - 2  and the line card  501 - 3 . The number of line cards to be installed is changed appropriately based on a scale, environment and so on of the packet transmission network to be controlled. 
     The line card  501 - 1  is provided with an external reception section  510 - 1 , an internal transferring section  511 - 1 , an internal reception section  512 - 1 , an external transferring section  513 - 1 , an FDB  514 - 1 , a transfer mode switching section  515 - 1 , a broadcast fault notifying section  516 - 1  and a path controller  517 - 1 . 
     The external reception section  510 - 1  is provided with a packet receiving section  518 - 1  and a fault detecting section  519 - 1 . The internal transferring section  511 - 1  is provided with a multiplexing section  520 - 1  and a packet transferring section  521 - 1 . The internal reception section  512 - 1  is provided with a packet receiving section  522 - 1  and a path control data extracting section  523 - 1 . The external transferring section  513 - 1  is provided with a reception path selecting section  524 - 1  and a packet transferring section  525 - 1 . 
     Because the line card  501 - 2  and the line card  501 - 3  have the same configuration as that of the line card  501 - 1 , each processing section of the line cards will be described by using the line card  501 - 1  as an example. 
     The FDB  514 - 1  is a table which has a destination data which is necessary to transfer a received packet, such as a MAC address of the next destination node, and a number of a port connected to a network as a destination of the packet, upon transfer of the received packet. 
     The transfer mode switching section  515 - 1  switches a transfer mode of the packet received by the line card  501 - 1  to a 1:1 transfer mode or a 1+1 transfer mode. 
     In the 1:1 transfer mode, the packets are transferred by using the current-use packet transmission path  310  and the current-use packet transmission path  311 , and a spare packet transmission path  320  and a spare packet transmission path  321  are not used. 
     In the 1+1 transfer mode, the packets are transferred by using the spare packet transmission path  320  and the spare packet transmission path  321 , in addition to the current-use packet transmission path  310  and the current-use packet transmission path  311 . In the 1+1 transfer mode, an identical packet is transferred on the current-use packet transmission path  310  and a corresponding spare packet transmission path  320 . In the same way, an identical packet is transferred to the current-use packet transmission path  311  and a corresponding spare packet transmission path  321 . 
     The transfer mode switching section  515 - 1  refers to the FDB  514 - 1  to acquire the data necessary to transfer a packet such as the MAC address of the next destination node and the port number of the node  31  to which the next destination node is connected, in order to transfer the packet received by the line card  501 - 1  to the next destination node. The transfer mode switching section  515 - 1  transfers the received packet by using one of the current-use packet transmission path and the spare packet transmission path in the 1:1 transfer mode. The transfer mode switching section  515 - 1  transfers the received packet by using the current-use packet transmission path and the spare packet transmission path in the 1+1 transfer mode. There is a case that a plurality of spare packet transmission paths are provided for the single current-use packet transmission path, depending on the configuration of the packet transmission network. 
     The transfer mode switching section  515 - 1  refers to the FDB  514 - 1  in the 1:1 transfer mode, to acquire data of a next destination node when using a the current-use packet transmission path, or data of a next destination node when using the spare packet transmission path. 
     The transfer mode switching section  515 - 1  refers to the FDB  514 - 1  in the 1+1 transfer mode, to acquire data of the next destination node when using the current-use packet transmission path, and data of the next destination node when using the spare packet transmission path. 
     The transfer mode switching section  515 - 1  transmits a 1+1 transfer mode switching packet to the current-use packet transmission path and the spare packet transmission path in case to switch from the 1:1 transfer mode to the 1+1 transfer mode. Oppositely, in case to switch from the 1+1 transfer mode to the 1:1 transfer mode, the transfer mode switching section  515 - 1  transfers a 1+1 transfer mode cancelling packet to the current-use packet transmission path and the spare packet transmission path. An instruction control between the 1+1 transfer mode the 1:1 transfer mode is performed to the transfer mode switching section  515 - 1  by the path controller  517 - 1 . 
     When receiving a fault occurrence notice from the fault detecting section  519 - 1 , a broadcast fault notifying section  516 - 1  generates a fault notice data to be broadcasted. The fault notice data includes data showing that a fault has been detected in the current-use packet transmission path, but the data for specifying the current-use packet transmission path in which the fault has occurred is not essential. The broadcast fault notifying section  516 - 1  transfers the generated broadcast fault notice data to the multiplexing section  520 - 1 . As the transmission of the fault notice data, there are a method of transmitting regularly or periodically until the path is recovered, and a method of transmitting several times immediately after the fault has occurred. 
     The path controller  517 - 1  instructs the transfer mode switching section  515 - 1  to switch to the 1+1 transfer mode. When receiving a transmission path selection data from the path control data extracting section  523 - 1 , the path controller  517 - 1  determines a packet transmission path by a predetermined method, and transfers packet transmission path selection agreement data which contains data for identifying the determined packet transmission path, to the multiplexing section  520 - 1 . In the packet transmission path selection data, the data for selecting the packet transmission path to be used is contained. For example, the data of a packet reception condition such as a bit rate, a packet loss rate and so on of the packet transmission path are contained. 
     Also, a quality monitoring result of the packet transmission path performed in the reception path selecting section  524 - 1  is transferred to the path controller  517 - 1 . 
     The packet receiving section  518 - 1  receives a packet transmitted from the neighbor node to a port of the line card  501 - 1 . 
     The fault detecting section  519 - 1  monitors whether or not a CCM (continuity check message) has been received from the neighbor node connected with the port of the line card  501 - 1 , and detects that a fault has occurred, when the CCM does not arrive within a predetermined time. When detecting the occurrence of the fault, the fault detecting section  519 - 1  notifies the occurrence of the fault to the broadcast fault notifying section  516 - 1 . 
     The multiplexing section  520 - 1  multiplexes packets from the broadcast fault notifying section  516 - 1 , the transfer mode changing receptacle  515 - 1 , and the path controller  517 - 1 . 
     The packet transferring section  521 - 1  transfers the packet received by the line card  501 - 1  to a line card with a port connected with a next destination node. 
     The packet receiving section  522 - 1  receives the packet through the port of either of the line cards of the node through the packet switch  530 . 
     The path control data extracting section  523 - 1  transfers to the path controller  515 - 7 , the fault notice data and the packet transmission path selection data of the packet transmission path for its own interface to be switched, of the packets transferred from the packet switch  530 . Also, the path control data extracting section  523 - 1  generates and transfers a CCM to the neighbor node. 
     The reception path selecting section  524 - 1  selects either of the current-use packet transmission path or the spare packet transmission path by monitoring the quality of the packet transmission path and receives the packet of the selected path from the packets received in the 1+1 transfer mode. The reception path selecting section  524 - 1  transfers the packet received from the line card  501 - 1  to the next destination node. The reception path selecting section  524 - 1  transfers the received packet to the next destination node without selecting the received packet in the 1:1 transfer mode. The selecting process of the received packet will be described below. 
     The packet transferring section  525 - 1  transfers a packet to the outside from the port which the next destination node is connected. 
     When receiving the packet from the line card, the packet switch  530  transfers the packet to the line card with the port to which the destination node is connected. 
     (Operation) 
     Next, the recovery method of the packet transmission network  300  in the present exemplary embodiment of the present invention will be described. 
     First, a communication condition when any fault does not occur in the packet transmission network  300  will be described.  FIG. 3  is the packet transmission network  300  according to the present exemplary embodiment of the present invention in which any fault does not occur. The packet transmission network  300  is configured from nodes  31  to  38 . The nodes  31  and  32  are connected through the node  37 , and the current-use packet transmission path  310  is set. Also, the nodes  31  and  32  are connected through the node  36 , and the spare packet transmission path  320  is set. The nodes  33  and  34  are directly connected with each other, and the current-use packet transmission path  311  is set. Also, the nodes  33  and  34  are connected through the node  36 , and the spare packet transmission path  321  is set. When there is not a fault in the packet transmission network  300 , it is supposed that the packet transmission is performed by using the current-use packet transmission path  310  and the current-use packet transmission path  311 . 
     Each of the nodes  31  to  38  receives the CCM transmitted periodically from the neighbor node and monitors the validity of the packet transmission path between the neighbor nodes. 
     When the fault does not occur in the packet transmission network  300 , the packet is transferred by using the current-use packet transmission path  310  and the current-use packet transmission path  311 , and the spare packet transmission path  320  and the spare packet transmission path  321  are not used (the 1:1 transfer mode). 
     First, with reference to  FIG. 3 ,  FIG. 7A ,  FIG. 7B , and  FIG. 7C , the operation in the 1:1 transfer mode will be described by using the node  31  as an example. The transfer of the packet received by the port of the line card  501 - 1  as an example will be described. The node  37  as the next destination node is connected to the port of the line card  501 - 2 , when transferring the packet by using the current-use packet transmission path  310 . The node  36  as the next destination node is connected to the port of the line card  501 - 3 , when transferring the packet by using the spare packet transmission path  320 . 
     The packet received by the packet receiving section  518 - 1  of the external reception section  510 - 1  is transferred to the transfer mode switching section  515 - 1  through the fault detecting section  519 - 1 . Because the 1:1 transfer mode is set, the transfer mode switching section  515 - 1  transfers the packet only to the node  37  as the next destination node when using the current-use packet transmission path  310 . The received packet is transferred in order of the multiplexing section  520 - 1 , the packet transferring section  521 - 1 , the packet switch  530 , the packet receiving section  522 - 2 , the path control data extracting section  523 - 2 , the reception path selecting section  524 - 2 , the packet transferring section  525 - 2  in the node  31 , and is transferred from the node  31  to the node  37 . 
     Next, the case where the fault has occurred in the packet transmission network  300  will be described. When the fault has occurred in the packet transmission network  300 , the switching between the current-use packet transmission path  310  and the spare packet transmission path  320  is performed by the node  31  and the node  32  as termination nodes. In the same way, the nodes  33  and  34  as termination nodes perform the switching between the current-use packet transmission path  311  and the spare packet transmission path  321 . 
       FIG. 4  is the packet transmission network  300  in case of the occurrence of the fault in the present exemplary embodiment of the present invention. In  FIG. 4 , it is assumed that the fault has occurred between the node  32  and the node  37 . Because the CCM from the node  37  cannot be received in the predetermined time, the node  32  detects that the fault has occurred between the node  32  and the node  37 . In the same way, because the CCM from the node  32  cannot be received in the predetermined time, the node  37  detects that the fault has occurred between the node  32  and the node  37 . 
     Because detecting the faults, the node  32  and the node  37  generate and broadcast the fault notice data to the neighbor nodes, respectively. The nodes  31  to  38  receive the fault notice data and transfer the fault notice data to the neighbor nodes. When the node already received the fault notice data, the fault notice data is not transferred to the neighbor node. 
     When the node receiving the fault notice data is a termination node in either of the packet transmission paths, the packet transmission to the spare packet transmission path is started in addition to the current-use packet transmission path. Through this operation, the identical packet is transferred onto the current-use packet transmission path and the spare packet transmission path (the 1+1 transfer mode).  FIG. 5  is the packet transmission network  300  in the 1+1 transfer mode in the present exemplary embodiment of the present invention. In  FIG. 5 , the nodes  31  and  32  are the termination nodes in the current-use packet transmission path  310 , and the nodes  33  and  34  are the termination nodes in the current-use packet transmission path  311 . When receiving the fault notice data, the nodes  31  and  32  start the packet transmission onto the spare packet transmission path  320  in addition to the current-use packet transmission path  310 . In the same way, when receiving the fault notice data, the nodes  33  and  34  start the packet transmission onto the spare packet transmission path  321  in addition to the current-use packet transmission path  311 . The fault does not occur on the current-use packet transmission path  311 , but when receiving the fault notice data, the nodes  33  and  34  perform the switching from the 1:1 transfer mode to the 1+1 transfer mode between the node  33  and the node  34 . 
     After switched to the 1+1 transfer mode, the nodes  31  and  32  monitor the quality of the current-use packet transmission path  310  and the spare packet transmission path  320 . The quality monitoring is performed by calculating a bit rate of the packet received from the packet transmission path. Because the fault has occurred in the current-use packet transmission path  310  so that the packet cannot be received, the packet transmission path with high quality is the spare packet transmission path  320 . The nodes  31  and  32  transfer the received packet to the next destination node by the spare packet transmission path  320  of high quality. 
     In the same way, after switched to the 1+1 transfer mode, the nodes  33  and  34  monitor the quality of the current-use packet transmission path  311  and the spare packet transmission path  321 . Because the fault does not occur in the current-use packet transmission path  311 , the transmission and reception of the packet is possible. When it is possible to use both of the current-use packet transmission path and the spare packet transmission path in similar quality, the nodes  31  and  32  select the current-use packet transmission path  311 , because it is not necessary to switch to the spare packet transmission path. 
       FIG. 6  is the packet transmission line  300  in cancellation of the 1+1 transfer mode in the present exemplary embodiment of the present invention. After determining the packet transmission path to be used for the transfer of the received packet as the spare packet transmission path  320 , the nodes  31  and  32  stop the transferring of the packet by using the current-use packet transmission path  310 , and switches from the 1+1 transfer mode to the 1:1 transfer mode in which the spare packet transmission path is used. After determining the packet transmission path to be used for the transfer of the received packet as the current-use packet transmission path  311 , the nodes  33  and  34  stops the transfer of the packet by using the spare packet transmission path  321 , and switches from the 1+1 transfer mode to the 1:1 transfer mode in which the current-use packet transmission path  311  is used. 
     As mentioned above, in the present exemplary embodiment of the present invention, the node detects the fault occurrence, and generates the fault notice data to broadcast to the other nodes. When the node which receives the fault notice data is a termination node in any of the packet transmission paths, the transfer mode is switched to the 1+1 transfer mode uniformly by regarding as the fault occurrence, regardless of the existence or non-existence of the fault in the packet transmission path. After that, one of the packet transmission paths is selected, and the transfer mode is switched from the 1+1 transfer mode to the 1:1 transfer mode. Because the fault notice data is not generated for each of the packet transmission paths, it is not necessary to perform processing on data for specifying the packet transmission path. Because a processing quantity can be more reduced, as compared with a conventional case, there are not a problem of straining a bandwidth of the packet transmission path, a problem of the delay of generation of the fault notice data and so on. Also, when the fault has occurred, especially, when the faults have occurred in the plurality of packet transmission paths at the same time, a delay does not occur in the recovery of the packet transmission paths in the fault recovery method in the present exemplary embodiment in which operations of all the packet transmission paths are carried out in an integrated fashion. 
     Next, the details of the switching operation of the packet transfer mode will be described.  FIGS. 8A ,  8 B and  8 C are diagrams showing the switching of the packet transfer mode in the present exemplary embodiment of the present invention.  FIGS. 8   a ,  8 B and  8 C show packet transfer states to the current-use packet transmission path  311  and the spare packet transmission path  321  between the nodes  33  and  34 . The switching operation of the packet transfer mode will be described by using the transfer of a packet in one direction from the node  33  to the node  34  as an example. 
     When receiving the fault notice data from the neighbor node, the node  33  transmits the 1+1 transfer mode switching packet to the current-use packet transmission path  311  and the spare packet transmission path  321  at a same time. After that, the node  33  transfers the received packet to the current-use packet transmission path  311  and the spare packet transmission path  321  at the same time, and the transfer mode shifts to the 1+1 transfer mode. 
       FIG. 8A  is a diagram showing before and after transmitting the 1+1 transfer mode switching packet  410  from the node  33  to the node  34 . In  FIG. 8A , the packet transfer mode between the nodes  33  and  34  is switched from the 1:1 transfer mode in which the current-use packet transmission path  311  is used, to the 1+1 transfer mode in which the current-use packet transmission path  311  and the spare packet transmission path  321  are used, after transmission of the 1+1 transfer mode switching packet. 
       FIG. 8B  is a diagram showing the operation when the node  34  receives the 1+1 transfer mode switching packet  410 . The packet transfer mode between the nodes  33  and  34  is set to the 1:1 transfer mode in which the packet transmission path  311  and the packet transmission path  321  are used. When the node  34  receives the 1+1 transfer mode switching packet, the quality monitoring of the current-use packet transmission path  311  and the spare packet transmission path  321  is started. The quality monitoring is carried out by calculating a bit rate and so on. The packet received from the packet transmission path with the higher quality is transferred to the next destination node. The node  34  retains the packet received during the quality monitoring in a buffer without transferring to the next destination node until the quality monitoring ends. 
       FIG. 8C  is a diagram showing an operation before and after the node  33  transmits the 1+1 transfer mode cancelling packet  411  to the node  34 . In  FIG. 8C , the packet transfer mode between the nodes  33  and  34  is switched from the 1+1 transfer mode in which the current-use packet transmission path  311  and the spare packet transmission path  321  are used, to the 1:1 transfer mode in which the current-use packet transmission path  311  is used, after transmission of the 1+1 transfer mode cancelling packet. 
     The packet transmission path to be use in the 1:1 transfer mode may be determined based on the result of exchange of the packet transmission path selection data by the nodes  33  and  34  and may be determined based on the result of the quality monitoring in the 1+1 transfer mode shown in  FIG. 8B . Also, as a method of selecting the packet transmission path, there are a method of selecting the packet transmission path to receive the 1+1 transfer mode switching packet earliest, a method of selecting the current-use packet transmission path with a priority, and a method of selecting the spare packet transmission path with a priority, and so on. 
     In  FIG. 8C , the packet transmission path to be used in the 1:1 transfer mode is determined as the current-use packet transmission path  311  which is the packet transmission path having used before switched in the 1+1 transfer mode. In such a case, it is assumed that the node  33  receives the fault notice data in the state that the fault does not occur in the current-use packet transmission path. When the current-use packet transmission path is determined to be a high quality by the node  33  and the node  34  for the reason that the fault does not occur in the current-use packet transmission path, it is desirable to use the current-use packet transmission path, in order to prevent an unnecessary switching operation to the spare packet transmission path. 
     In the transmission of the 1+1 transfer mode cancelling packet, there are a method of transmitting it when a constant time has elapsed after switching to the 1+1 transfer mode, or a method of transmitting it when a packet is transmitted to a transmission side node to indicate the completion of selection and is received, after the termination node on the reception side has selected the packet transmission path based on a result of the quality monitoring in the 1+1 transfer mode. 
     In the present exemplary embodiment of the present invention, the node on the transmission side switches the packet transfer mode from the 1+1 transfer mode to the 1:1 transfer mode when transmitting the 1+1 transfer mode cancelling packet. Therefore, even if the packet transmission path selected through the quality monitoring in the 1+1 transfer mode is different from the packet transmission path selected between the nodes on switching to the 1:1 transfer mode, the problems can be prevented of double transmission of the identical packet, the disorder of the packet transmission order, the packet loss and so on, by switching of the packet transmission path. 
     Next, the internal operation of the node in case of the fault occurrence in the present exemplary embodiment of the present invention will be described. First, the case where its own node detects a fault will be described. As an example, with reference to  FIG. 4 ,  FIG. 7A ,  FIG. 7B , and  FIG. 7C , the internal operation of the node  32  in case where the packet transfer mode of the packet transmission path between the nodes  31  and  32  is switched when the fault has occurred between the nodes  32  and  37  will be described. It is assumed that with respect to the state of the connection between the node  32  and another node, the port of the line card  501 - 1  of the node  32  is connected with the node  36 , the port of the line card  501 - 2  of the node  32  is connected with the node  37 , and the port of the line card  501 - 3  of the node  32  is connected with the node  38 . 
     Because the reception of the CCM from the node  37  cannot be confirmed in a predetermined time, the fault detecting section  519 - 2  of the node  32  detects that the fault has occurred. The fault detecting section  519 - 2  of the node  32  transmits a fault detection notice to the broadcast fault notifying section  516 - 2 . 
     The broadcast fault notifying section  516 - 2  generates one fault notice data. The broadcast fault notifying section  516 - 2  transfers the fault notice data to the packet switch  530  through the multiplexing section  520 - 2  and the packet transferring section  521 - 2 . 
     The packet switch  530  transfers the fault notice data to all the line cards  501 - 1  to  501 - 3  installed on the node  32 . Because the operation when receiving the fault notice data from the packet switch  530  is same even in any of the line cards, the operation will be described, by using the line card  501 - 3  as an example, hereinafter. 
     The line card  501 - 3  receives the fault notice data by the packet receiving section  522 - 3  of the internal reception section  512 - 3 . The packet receiving section  522 - 3  transfers the fault notice data to the path control data extracting section  523 - 3 . The path control data extracting section  523 - 3  transfers the fault notice data to the path controller  517 - 3  and the reception path selecting section  524 - 3 . The reception path selecting section  524 - 3  transfers the fault notice data to the packet transferring section  525 - 3 . The packet transferring section  525 - 3  transmits the fault notice data to the neighbor node. 
     When receiving the fault notice data from the path control data extracting section  523 - 3 , the path controller  517 - 3  instructs the transfer mode switching section  515 - 3  to switch the transfer mode to the 1+1 transfer mode. 
     When receiving a switching instruction from the path controller  517 - 3 , the transfer mode switching section  515 - 3  generates the 1+1 transfer mode switching packet. The transfer mode switching section  515 - 3  refers to the FDB  514 - 3 , to detect all the packet transmission paths which are connected with the ports of the line card  501 - 3  of the node  32 . The transfer mode switching section  515 - 3  transfers the 1+1 transfer mode switching packet to the multiplexing section  520 - 3  of the internal transferring section  511 - 3 , in order to transfer the 1+1 transfer mode switching packet to the detected termination node of the packet transmission path. After that, the transfer mode switching section  515 - 3  copies the packet received by the port of the line card  501 - 3 , and transfers the received packet by using the current-use packet transmission path and the spare packet transmission path. In an example of  FIG. 4 , when transmitting the packet received from the node  38  to the node  31 , the packet received by the packet receiving section  518 - 3  of the external reception section  510 - 3  is copied by the transfer mode switching section  515 - 3  in the node  32 . The transfer mode switching section  515 - 3  refers to the FDB  514 - 3 , to transmit the packet to the node  31  by using both of the packet transmission path  310  and the packet transmission path  320 . 
     Next, a case where the packet received in the 1+1 transfer mode is transferred to the next destination node will be described. The case where the node  32  transfer to the node  38 , the packet received from the current-use packet transmission path  310  and the spare packet transmission path  320  will be described, by using it as an example. After switching to the 1+1 transfer mode, the reception path selecting section  524 - 3  of the node  32  carries out the quality monitoring with respect to the packets received from the current-use packet transmission path  310  and the spare packet transmission path  320 . Because the fault has occurred in the current-use packet transmission path  310  so that the packet cannot be received, the packet transmission path with high quality becomes the spare packet transmission path  320 . The reception path selecting section  524 - 3  transfers to the next destination node, the packet received from the spare packet transmission path  320 . 
     Next, the case where the transfer mode is switched from the 1+1 transfer mode to the 1:1 transfer mode will be described. The node  32  receives the packet transmission path selection data from the node  31 . The packet transmission path selection data is transferred by using either one of the current-use packet transmission path  310  and the spare packet transmission path  320  or both. In the example of  FIG. 4 , because the fault has occurred in the current-use packet transmission path  310 , the case where the node  32  receives the packet transmission path selection data from the spare packet transmission path  320  will be described. The node  32  extracts the packet transmission path selection data by the path control data extracting section  523 - 1  of the line card  501 - 1 , and transfers to the path controller  517 - 1 . The path controller  517 - 1  refers to the packet transmission path selection data to determine the packet transmission path uniquely. The path controller  517 - 1  transfers to the multiplexing section  520 - 1 , the packet transmission path selection agreement data containing the data which distinguishes the uniquely determined packet transmission path, and transfers toward the node  31 . After that, the transfer mode switching section  515 - 1  is instructed to switch from the 1+1 transfer mode to the 1:1 transfer mode. 
     The transfer mode switching section  515 - 1  transfers the 1+1 transfer mode cancelling packet to the current-use packet transmission path  310  and the spare packet transmission path  320 . After transmission of the 1+1 transfer mode cancelling packet, the transfer mode switching section  515 - 1  transmits the packet only to the uniquely determined spare packet transmission path  320 . 
     The above is the internal operation of the node when a fault is detected in its own node. 
     Next, the operation when another node detects the fault, and the fault notice data generated by the other node is received will be described. For example, when receiving the fault notice data from the line card  501 - 2  of the node  31 , the fault notice data is transferred to the line card  501 - 1  to  501 - 3  through the packet switch  530 . The processing since then is same as the processing when detecting the fault in its own node and generating the fault notice data. 
     As mentioned above, in this exemplary embodiment, when a node of the packet transmission network  300  detects a fault, the node does not generate the fault notice data for each of the packet transmission paths. Therefore, the node never processes the fault notice data for each of the packet transmission paths. In this exemplary embodiment, the node having detected the fault generates and broadcasts single fault notice data. The termination node on the packet transmission path has received the fault notice data and switches the mode to the 1+1 transfer mode uniformly, regarding as the fault occurrence, even when the fault does not occur in the packet transmission path. After that, the node switches to the 1:1 transfer mode. In the recovery method of the present exemplary embodiment, even when the fault has occurred at a same time among the plurality of packet transmission paths, the successive switching of the packet transmission path due to the detection of the fault in the packet transmission path never occurs. Therefore, the switching time is never elongated and can recover the fault in the packet transmission path at high speed. 
     As described above, the exemplary embodiments of the present invention have been described with reference to the drawings. However, the present invention is not limited to the above embodiments and can be appropriately modified by a person in the art in a range which does not deviate from the scope of the present invention.