Abstract:
A method for switching a plurality of central devices including N working central devices and one protection central device of a N+1 redundancy structure provided on a LAN center, to the CATV transmission path side of which terminal devices are connected each through a cable modem, having router functions forming segments respectively at the sides of the LAN center and the CATV transmission path, and a working central device transmitting routing information of the segment on the CATV transmission path side to inform the LAN center segment of an existence of the segment on the transmission path side, so that the terminal devices are connected to appropriate networks via the LAN center, the method comprising the steps of maintaining control information of the plurality of working central devices and the protection central device to be monitored, detecting failure of a working central device by regularly polling the N working central devices, switching the detected, failed working central device to the protection central device to connect the plurality of central devices to the CATV transmission path, transferring the maintained control information of the failed central device to the protection central device, and setting the protection central device to an active status according to the transferred control information.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for switching central devices when a failure occurs in a cable modem system, which forms a local area network (LAN) by using a CATV transmission path, and a cable modem system using the method. 
     2. Description of the Related Art 
     In recent years, cable modem systems have been put in practical use, where a CATV transmission path is taken as an access network, and users can connect from their homes to Internet through a LAN interface. 
     These systems target for services to general public. Therefore, there is a need for obtaining a license as the first-class communication service. To get the license, high reliability is required as a system. 
     The conventional LAN system will be now considered. In the conventional LAN system, although a main section for inter-routers connection can be formed having redundancy, it is impossible to form sub sections directly accommodating terminals to have redundancy. The reason is because it is possible to detect a failure on a router to be connected and automatically set a bypass route by transmitting and receiving a routing protocol, such as a RIP, between routers. 
     However, it is general that such the routing protocol is not supported for a terminal such as a personal computer. Therefore, LAN connections are not set according to gateway definition of which settings can be changed manually. 
     Therefore, when a failure occurs on a router for directly storing terminals, it is impossible to perform the LAN connections by using the bypass route without changing settings of the terminal. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method for switching central devices to maintain a LAN connection by using a redundancy central device without changing settings of terminals, when a failure occurs on a central device having a router function, which directly accommodates the terminals and a cable modem system using the method. 
     The above-described object is achieved by a cable modem center device having a plurality of central devices including N working central devices and one protection central device of a N+ 1  redundancy structure provided on a LAN center, to the CATV transmission path side of which terminal devices are connected each through a cable modem, and having router functions forming segments respectively at the sides of the LAN center and the CATV transmission path, and a working central device transmitting routing information of the segment on the CATV transmission path side to inform the LAN center segment of an existence of the segment on said transmission path side, so that the terminal devices are connected to appropriate networks via the LAN center, the cable modem center device comprising a switch device provided between said plurality of central device and said CATV transmission path for switching a failed one of the plurality of central devices to said protection central device and connecting said plurality of center devices and said CATV transmission path. 
     The cable modem center devices further comprises a monitor device for monitoring said plurality of central devices, maintaining control information of the plurality of working central devices and the protection central device to be monitored, detecting failure of a working central device by regularly polling the N working central devices, switching the detected, failed working central device to the protection central device to connect the plurality of central devices to the CATV transmission path, transferring the maintained control information of the failed central device to the protection central device, and setting the protection central device to an active status according to the transferred control information. 
     Further, in one preferred mode, it is another feature that the control information of the plural working central devices and the protection device to be monitor device includes a network address of a segment on the CATV transmission path and control information of a cable modem stored under the plural working central devices and the protection devices. 
     Alternatively, in another mode, the monitor device includes a physical address (MAC address) of the segment on the transmission path of the working central device as the maintained information and the information transferred to the protection central device, and said protection central device additionally uses said physical address to activate. 
     Furthermore, in one mode, the failed central device stops sending routing information after a failure is informed. 
     Alternatively, it is a further feature that distance (HOP number) or cost data of the routing information the switched protection central device sends is made to be smaller than that of the information the working central device sends. 
     Further, in one embodiment, it is informed to the data sender that the failed central device is switched to the protection central device according to an internet control message protocol (ICMP) redirect message when receiving the data to the segment on the CATV transmission path after a failure is informed. 
     It is other feature that a resumption confirmation is performed by connecting the terminal and the failed central device, the control information of the protection central device is transferred from the monitor device to the failed central device, and the failed central device is made to be active according to the transferred information after the failure on the failed central device is restored. 
     Further, objects and features of the present invention become clear by the description for explaining embodiments according to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a structural block diagram of the present invention. 
     FIG. 2 is a block diagram of a first embodiment of the present invention. 
     FIG. 3 is a structural block diagram of a central device in the first embodiment shown in FIG.  2 . 
     FIG. 4 is a structural block diagram of a switch of the first embodiment shown in FIG.  2 . 
     FIGS. 5A and 5B show control information of a cable modem in the first embodiment of FIG.  2 . 
     FIG. 6 illustrates a sequence flow chart of a LAN communication in a normal condition. 
     FIG. 7 illustrates one example of a format in a routing protocol packet (RIP). 
     FIG. 8 illustrates one example of a routing table. 
     FIG. 9 shows an ARP table in a terminal. 
     FIG. 10 illustrates one example of an ARP packet format. 
     FIG. 11 illustrates a format of a transmission frame of data (info) transmitted from a central device (#0)  10  to a host on an internet  4 . 
     FIG. 12 illustrates data (info) and a multi-frame transmitted from a host on Internet  4  to a central device (#0)  10 . 
     FIG. 13 is an example of a format of an ICMP redirect message. 
     FIG. 14 illustrates a sequence of a LAN communication when a failure occurs on a central device (#0)  10 . 
     FIG. 15 illustrates a sequence of a LAN communication when a failure occurs on a central device (#0)  10  while a terminal (TRMOn)  80  is facilitating LAN communications with a host on internet  4 . 
     FIG. 16 illustrates a sequence when a monitor device  7  maintains a MAC address on a transmission path side of a central device (#0)  10  and transmits the information to a protection device (#protection)  12  as data corresponding to the sequence shown in FIG.  14 . 
     FIG. 17 illustrates a sequence when a central device (#0) continuously transmits RIP information even after the failure occurs. 
     FIG. 18 shows an example in which a HOP number of the RIP information a central device (#protection)  12  transmits is smaller than that of the RIP information a central device (#0)  10  transmits corresponding to the sequence shown in FIG.  17 . 
     FIG. 19 shows an example in which the failed central device (#0)  10  which receives data from a router RT  3  transmits the data to a central device (#protection)  12  and concurrently transmits an ICMP redirect message to a router RT  3 . 
     FIG. 20 shows an example of an operational sequence for confirming the resumption and resuming the system after restoring a failure on a central device (#0)  10  switched to a central device (#protection)  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments according to the present invention will be now explained in accompanying with the attached drawings. Throughout the following descriptions, the same reference numerals and symbols are used to denote and identify corresponding or identical components. 
     FIG. 1 is a block diagram explaining the principle of the present invention. In FIG. 1, central devices (#0 to #m)  10  to 1 m and a central device (#protection) in are connected to an Internet  4  via a center LAN  2  and a router (RT)  3 . 
     IP addresses and media access control (MAC) addresses [(IP E0 , MAC #0E ) to (IP Er , MAC #rE )] are set for each of the above-described devices, belonging to a segment E of the center LAN  2 . 
     In this example, the protection central device  1   n  includes function for transmitting a routing protocol, such as a routing information protocol (RIP). A HOP number the protection central device  1   n  transmits, i.e., a number of routers, passed trough until reaching a target or distance information, should be smaller than that of a working central device. Or the HOP number may be expressed by the corresponding cost information. 
     The working central devices (#1˜#m)  10  and the protection central device  1   n  are connected through a physical switch device  5  to test cable modems CM and terminals TRM provided at the forward section or central devices. 
     For example, IP and MAC addresses (IP R01 , MAC #OR ) and (IP R0n , MAC R0n ) of a segment R 0  on the transmission path are respectively set to the device  10  and a test terminal (TRM 0 n)  80 , which is connected through a subordinate cable modem  70  to the device  10 . 
     IP and MAC addresses (IP Rr1 , MAC #rR ) and (IP Rrn , MAC Rrn ) of a segment Rn on the transmission path side are respectively set to the protection device  1   n  and a test terminal (TRMrn)  8   n , which is connected through a subordinate cable modem  7   n  to the device  12 . 
     A case where a failure occurs on the ith central device (#i)  1   i  will be now considered. Then, the IP and MAC addresses of the protection device  1   n  are set to addresses (IP Ri1 , MAC #iR ), which are the same as those of the central device  1   i.    
     A switch device  5  physically connects the devices  10  to  1   m  to the CATV transmission path  6 . When a failure occurs on the device  1   i , the switch device  5  disconnects the device  1   i  and connects the protection device  1   n  to the transmission path  6 . 
     An IP address (IP R01 ) on the transmission path side of the device  10  is set as a definition of a gateway in the terminal  80 . The failed device  1   i  stops sending the routing information. When the terminal  1   i  receives data sent to a segment on the transmission path side, the terminal  1   i  sends an Internet control message protocol (ICMP) redirect message to the sender. 
     As described above, in the present invention, the device  10  sends RIP information of a segment R 0  on the transmission path side to a segment E of the center LAN  2  on a normal condition. On the other hand, the protection device  1   n  sends the RIP information of a segment Rn on the transmission path side. Therefore, the router  3  recognizes a segment R 0  exists in the forward section of the central device  10 . 
     Additionally, the switch device  5  physically connects the device  10  to the CATV transmission path  6 . Therefore, the terminal  80  uses the device  10  as a gateway and therefore, it can be connected to the Internet  4  through the center LAN  2  and the router  3 . 
     A case where a failure occurs on the device  10  will be now considered for instance. Then, the monitor device  7  detects the failure by polling. The switch device  5  disconnects the device  10  and connects the protection device  1   n  to the CATV transmission path  6 . 
     Simultaneously, IP and MAC addresses (IP R0 , MAC #0R ), that are the same as those of the device  10 , are set to the segment Rn on the transmission path side of the protection device  1   n . Then, the protection device  1   n  starts sending the RIP information of the segment R 0  on the transmission path side to the segment E of the center LAN  2 . 
     The router  3  recognizes that the segment R 0  exists in the forward section of the protection device  1   n  by stopping sending the RIP information of the failed device  10 , reducing the HOP number of the above-described RIP information smaller than that of the device  10 , or directly informing from the failed central device  10  with the ICMP redirect message. 
     The protection device in inherits the IP and MAC addresses (IP R01 , MAC #OR ) of the segment R 0  on the transmission path side of the device  10  used as a gateway from the protection device  1   n  as they are. Therefore, it is also possible that the terminal  80  can be connected to a LAN, continuously. 
     FIG. 2 illustrates a structural diagram of one embodiment of the present invention. A cable modem system is formed of two working central devices (#0, #1)  10  and  11  and a protection device (#protection)  12 . 
     In FIG. 2, the same numerals and symbols are used to denote and identify corresponding or identical components shown in FIG. 1. A structural block diagram of a central device is shown in FIG. 3, and a structural block diagram of a switch device  5  is shown in FIG.  4 . 
     Each of central devices  10  to  12  has a commonly used structure. As illustrated in FIG. 3, each of central devices  10  to  12  includes a CPU  100  for controlling an entire central device. Further, the central device includes a memory  101  for storing a system program and data. Additionally, the each device includes a LAN connector  102  for having a MAC address  120  on the center LAN  2  side and facilitating communications with the center LAN  2 . 
     The central devices  10  to  12  further include a frame assembler  103 , a modulator  104 , a demodulator  105 , and a frame disassembler  106 . The frame assembler  103  assembles an IP packet transmitted to cable modems  70  to  72  and terminals  80  to  82  as a transmission frame, and the modulator  104  modulates the transmission frame to a RF signal and transmits it to the CATV transmission path  6 . On the contrary, the demodulator  105  demodulates the RF signals received from the CATV transmission path  6  to a transmission frame, and a frame disassembler  106  disassembles the modulated transmission frame and takes an internet protocol (IP) packet out. 
     The central devices also include a switch controller  107  transmitting an instruction for switching to the switch device  5 . 
     A system program is developed in on the above-described memory  101 . The memory  101  includes an OS section  110 , a LAN connection controller  111 , and an ARP controller  112 . 
     The OS section  110  controls the entire program system. The LAN connection controller  111  sets the MAC address on the center LAN side to the LAN connector  102  and controls the address. The ARP controller  112  maintains and administrates an address resolution protocol (ARP) table on the center LAN side and resolves the MAC address. 
     The memory  101  further includes a RIP controller  113  and a routing controller  114 . The RIP controller  113  receives and sends the routing information between the router  3  connected to the center LAN  2  and other central devices, maintains and administrates the routing table according to the routing packet RIP. The routing controller  114  maintains IP addresses  121  and  123  on the center LAN  2  and CATV transmission path  6  sides, and performs routing the IP addresses according to the routing information administrated by the controller  113 . 
     Moreover, the memory  101  includes a cable modem controller  115 , a MAC controller  116  and an ARP controller  117 . The cable modem controller  115  maintains and administrates control information of the cable modem connected to the central device through the CATV transmission path  6  (refer to ID numbers shown in (1) of FIG.  5  and upward/downward RF signal frequencies shown in (2) of FIG. 5) and executes an authorization, distance control, received or transmitted RF signal level control, and settings of RF signal frequencies to the modulator and demodulator in the cable modem. The MAC controller  116  maintains a MAC address  126  on the CATV transmission path  6  side and controls a MAC layer. The ARP controller  117  maintains and administrates the ARP table on the CATV transmission path  6  side and executes the MAC address resolution. 
     The switch device  5 , of which detail description is illustrated in FIG. 4, mixes outputs RFout from each modulators  104  of the devices  10  and  11  and an input RFin of the demodulator  105  inside and connects to contact switches #i and i 0 . 
     In other words, a mixer  500  mixes the outputs RFout from the modulator  104  of the device  10  and the input RFin from the demodulator  105  of the device  10 . The input RFin and output RFout from the device  10  are connected to a contact switch SW 00  of the switch  510 . On the other hand, a mixer  501  mixes an output RFout from the modulator  104  of the device  11  and an input RFin from the demodulator  105  of the device  11 . The input RFin and the output RFout from the device  11  are connected to a contact switch SW 00  of a switch  511 . 
     Additionally, the working devices  10  and  11  are connected to the protection device  12  through a divider  502  and a mixer  503 . An output RFout from the modulator  104  of the protection device  12  and an input RFin from the demodulator  105  in the device  12  are respectively connected to the divider  502  and the mixer  503 . The input and output from the protection device  12  are mixed in mixers  504 ,  505  and  506  through the divider  502  and the mixer  503 . The mixed signals are respectively connected to the contact switches SW 01 , SW 11  and SW 20  of the switches  510 ,  511  and  512 . 
     The contact switches SW 02  and SW 12  of the switches  510  and  511  are connected to the CATV transmission path  6 , and the contact switch SW 22  of the switch  512  is terminated. The contact switches SW 03 , SW 13  and SW 23  of the switches  510 ,  511  and  512  are respectively connected to the test terminal after the mixer  507  mixes and divides the contact switches SW 03 , SW 13  and SW 23 , individually. 
     In the switches  510 ,  511  and  512 , the contact switches SWi 0  and SWi 1  are respectively connected to the contact switches SWi 3  and SWi 2  when a power is ON. When the power is OFF, the contact switch SWi 0  is connected to the contact switch SWi 2 , and the contact switches SWi 1  and SWi 3  are terminated. In this example, i means 0 to 2. The above-described conditions of these switches  510 ,  511  and  512  can be expresses as follows: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 SW0 
                 SW1 
                 SW2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Normal Condition 
                 off 
                 off 
                 on 
               
               
                 When a failure occurs on a central device #0 
                 on 
                 off 
                 off 
               
               
                 When a failure occurs on a central device #1 
                 off 
                 on 
                 off 
               
               
                   
               
             
          
         
       
     
     In other word, when the condition is normal, i.e., the devices  10  and  11  are operated, the switches  510  and  511  become OFF, and only the switch  512  becomes ON. Then, the devices  10  and  11  are respectively connected to the CATV transmission path  6 . 
     When a failure occurs on the device  10 , the protection device  12  receives information of the failure on the device  10  from the monitor device  7 . Then, the switch  510  becomes ON and the switches  511  and  512  become OFF. Therefore, the device  10  is connected to the test terminal, and the protection device  12  is connected to the CATV transmission path  6  instead of the device  10 . 
     Similarly, when a failure occurs on the device  11 , the device  11  is physically switched to the protection device  12 . 
     The monitor device  7  is formed of a hardware in which a LAN card is mounted on a personal computer. A software is formed of an OS where multi-task environment is provided, such as Windows NT, a monitor device program driven in a constant period, an automatic switching program driven when a failure occurs, and a resumption confirmation program for transmitting information manually. 
     The monitor program maintains a network address, i.e., an IP address, of the segment on the center LAN  2  side of a central device to be monitored including the protection device, and is driven in a predetermined period of the OS. When the central device is not replied or information of failure occurrence from the device, the automatic switching program is driven. 
     The automatic switching program maintains a network address, i.e., an IP address and a sub net mask, of the segment on the transmission path side of the central devices including the protection device to be monitored and the control information, such as upward/downward RF frequencies, cable modem ID number, of the cable modem stored in the segment. 
     Then, the information of the failed device is transmitted to the protection device by driving from the monitoring program and resetting the protection device, and therefore, the protection device becomes an active condition. 
     The resumption-confirming program is manually driven from a console after resuming the failed device. The program includes functions for enabling the resumption confirmation of the failed device and transferring the original information to the failed or protection devices after confirming the resumption in order to obtain the previous condition before the failure occurs. 
     FIG. 6 shows a sequence of a LAN communication in a normal condition. Devices  10  and  11  send a routing protocol, i.e., RIP, to a router  3  at regular intervals (STEP S 1 ), for example, 30 seconds in general, and send information of a segment, i.e., a network, subordinate to the own terminals to a center LAN  2 . 
     In here, a structural example of a routing protocol (RIP) packet format is shown in FIG.  7 . In other words, the device  10  sends the information of a segment R 0 , and the device  11  sends the information of the segment R 1 . The router  3  forms a routing table illustrated in FIG. 8 inside according to the above-described information. 
     The router  3  recognizes which terminal is subordinate to which device according to the routing table to make LAN communications enable. 
     The monitor device  7  performs polling for each device  10 ,  11  and  12  at regular intervals and monitors existences of a failure according to the reply for the polling. 
     A case where a terminal  80  starts facilitating LAN communications with a host on an Internet  4  will be now considered. At first, the terminal  80  queries the MAC address for the IP address (IP R01 ) set in the gateway definition of the terminal according to an ARP request (ARP protocol) (STEP S 3 - 1 , S 3 - 2 , S 3 - 3 ). Thereby, the device  10  informs the terminal  80  that the address is MAC #OR  according to the ARP reply (STEP S 4 - 1 , S 4 - 2 , S 4 - 3 ). 
     The terminal  80  stores the information in an ARP table shown in FIG. 9 thereof. Then, the terminal  80  uses the above-described MAC address (MAC #OR ) to send the data (info) of real LAN communication (STEP S 5 - 1 , S 5 - 2 , S 5 - 3 ). 
     The device  10  which receives the data (info) executes the ARP protocol for the router  3 , similarly to the above-described case (STEP S 6 ) and repeats the above-described received data (info) to the router  3 . Finally, the data (info) is repeated to the host on the Internet  4  (STEP S 7 ). 
     One example of a request message from the device  10  to the router  3  when executing the ARP protocol for the router  3  (STEP S 6 ) and an ARP packet format of the reply message sent from the router  3  as described above are shown in FIG.  10 . 
     The reply message sent from the host on the Internet  4  is also repeated in the reversed direction and is achieved to the terminal  80 . Therefore, as shown in the step S 8  of FIG. 6, the data (info) is exchanged between the host on the Internet  4  and terminal  80 . 
     The format of transmitted frame of the data (info) sent from the device  10  to the host on the Internet  4  in this case is as shown in FIG.  11 . The format includes a terminal ID (TID) and data following a synchronous signal. The data following the synchronous signal is repeated, and an error check cyclic code CRC is attached at the end of the repetition. 
     On the other hand, the data (info) sent from the host on the internet  4  to the device  10  is transmitted with a multi-frame format as shown in FIG.  12 . MF means a multi-frame synchronous signal, and F means a frame synchronous signal. ACK and channel signals are alternatively located in a frame. 
     An ACK signal is formed of a header and an error check cyclic code, and a channel signal is formed of a header, data and an error check cyclic code. 
     In this example, when the terminal  80  sends the data to the above-described host, the ARP protocol is omitted if the above-described MAC address exists on the ARP table stored in the terminal  80 . 
     However, it is general to erase the information corresponding to the IP and MAC addresses from the ARP table if the information is not used for twenty minutes. 
     FIG. 14 illustrates a sequence of LAN communication when a failure occurs on the device  10 . The device  10  and the protection device  12  transmit routing protocols, such as a RIP, at regular intervals and respectively transmit the data of each subordinate segments, i.e., networks (STEP S 1 ). 
     In other words, the device  10  transmits the information of the segment R 0 , and the device  11  transmits the information of the segment R 1 . The router  3  forms a routing table inside according to each information. 
     The monitor device  7  performs polling of each devices  10 ,  11  and  12  to detect the failure on the device  10  (STEP S 2 ). The monitor device  7  further transfers the IP address (IP R01 ), the sub net mask (SUB R0 ) and the control information including CM 0 n of the subordinate cable modem on the transmission path side of the failed device  10  to the protection device  12 . Then, the monitor device  7  instructs the device  12  to transit the condition to active (STEP S 9 ). In here, it is also possible that the device  10  informs the failure detection to the monitor device  7  by itself. 
     The protection device  12  instructs the switch device  5  to connect the own device instead of the device  10 . The monitor device  7  may directly perform this instruction to the switch  5 , too. 
     The active device  12  starts sending the routing protocol, such as a RIP, at regular intervals and informs the router  3  that the segment R 0  is subordinate to the device  12  (STEP S 11 ). 
     The router  3  changes the contents on the routing table when the device  10  stops sending the RIP and the protection device  12  starts sending the RIP (STEP S 12 ). 
     When the terminal  80  starts LAN communications with the host on the internet  4 , the terminal  80  queries the MAC address for the IP address (IP R01 ) set in the gateway definition thereof to the host according to the ARP protocol at first (STEP S 13 ). Then, the device  12  informs the terminal  80  that the address is MAC #rR  . The terminal  80  stores the information in the ARP table thereof (STEP S 14 ). 
     Next, the terminal  80  uses the above-described MAC address (MAC #rR ) to send the data (info) of real LAN communications (STEP S 15 - 1 , S 15 - 2 , S 15 - 3 ). 
     The device  12 , which receives the data (info), executes the ARP protocol for the router  3  as described above (STEP S 16 ). After that, the router  3  repeats the received data (info), and the data (info) is repeated to the host on the internet  4 , finally (STEP S 17 ). 
     FIG. 15 illustrates a sequence of LAN communications when a failure occurs on the device  10  while the terminal  80  facilitates LAN communications with the host on the internet  4 . 
     In FIG. 15, the terminal  80  facilitates LAN communications according to the sequence illustrated in FIG. 6, and the MAC address (MAC #0R ) is stored for the IP address (IP R01 ) in the ARP table. 
     As explained in FIG. 14, the monitor device  7  detects the failure on the device  10 , makes the protection device  12  active, switches the switch device  5  (STEP S 10 ), and changes the contents on the routing table on the router  3  (STEP S 11 ). 
     In here, since the terminal  80  continuously transmits the data (info), and the correspondence between the IP address (IP R01 ) and the MAC address (MAC #OR ) is registered on the ARP table of the terminal  80 , the ARP protocol is omitted. Then, the MAC address (MAC #OR ) stored on the transmitted data (info) does not coincide with the MAC address (MAC #rR ) on the transmission path side of the device  12 . Therefore, the device  12  can not receive the data (STEP S 18 ). 
     After erasing the correspondence between the IP address (IPR R01 ) and the MAC address (MAC #OR ) from the ARP table in the terminal  80 , i.e., after twenty minutes have been elapsed in general, the terminal  80  executes the ARP protocol again. Therefore, the device  12  informs the MAC address (MAC #rR ) to the terminal  80 . Then, the terminal  80  stores the information on the ARP table thereof again (STEP S 19 ). 
     Thereby, the terminal  80  can resume LAN communications by using the above-described MAC address (MAC #rR ) (STEP S 20 ). 
     FIG. 16 illustrates a sequence when the monitor device  7  maintains the MAC address on the transmission path side of the device  10  and transfers it to the protection device  12  as information in opposite to the sequence of FIG.  15 . 
     When the terminal  80  continuously sends the data (info) after switching because of the failure on the device  10 , the MAC address (MAC #OR ) described in the transmitted data (info) coincides with the MAC address (MAC #OR ) on the transmission path side of the device  12 . This means that the device  12  can receive the data (info), and therefore, the data (info) can be continuously transferred. 
     FIG. 17 illustrates a sequence when the RIP is continuously sent even after a failure occurred on the device  10 . 
     The device  12  becomes active (STEP S 10 ) and starts sending the RIP, and the failed device  10  continuously sends the RIP (STEP S 22 ). Then, the router  3  does not change the routing table for the segment R 0 . 
     Therefore, the router  3  repeats the data (info) from the host on the internet  4  to the device  10  in the same state. Accordingly, the data (info) does not reach the terminal  80  (STEP S 23 ). 
     FIG. 18 illustrates a sequence when the HOP number of the RIP information the protection device  12  sends is smaller than that of the RIP information the device  10  sends. Thereby, the router  3  recognizes that the segment R 0  is subordinate to the protection device  12  and changes the routing table (STEP S 24 ). 
     Accordingly, the router  3  which receives the data (info) sent from the host on the internet  4  changes the destination for repeating to the protection device  12 , and therefore, the data (info) reaches the terminal  80 . 
     FIG. 19 shows an example where the failed device  10  which receives data (info) from the router  3  transmits the data to the protection device  12  (STEP S 25 ) and concurrently transmits an ICMP redirect message to the router  3  (STEP  26 ). Thereby, the router  3  recognizes that the segment R 0  is subordinate to the protection device  12  and changes the routing table. Therefore, the router  3  which receives the data (info) sent from the host on the internet  4  changes the destination for repeating to the protection device  12 , and therefore, the data (info) reaches the terminal  80  (STEPS S 27 - 0  to S 27 - 4 ). 
     FIG. 20 shows an example of operational sequence for confirming the resumption and resuming the system after restoring the failure on the device  10  which is switched to the protection device  12  according to the sequence shown in FIG.  14 . 
     After the failure on the device  10  is restored (STEP S 28 ), the monitor device  7  transfers an IP address (IP Rr1 ), a sub net mask (SUB Rr ) and the control information including CMrn of the subordinate cable modem on the transmission path side of the protection device  12  to the device  10 . Additionally, the monitor device  7  instructs the device  10  to transit to an active status (STEP S 29 ). 
     The active device  10  starts sending the routing protocol, such as the RIP, at regular intervals and informs the router  3  that the segment Rr is subordinate to the device  10 . The router  3  changes the contents on the routing table according to the command of starting transmission of the RIP information of the device  10 . 
     When the test terminal  80  starts facilitating the LAN communications with the host on the internet  4  to confirm the resumption of the device  10 , the switch  5  queries a MAC address for the IP address (IP Rr1 ) set in the gateway definition of the terminal according to an ARP protocol at first. For this query, the device  10  informs that the address is MAC #OR  as an ARP reply (STEP S 31 ). 
     The terminal  80  stores the information on the ARP table thereof (STEP S 32 ). Next, the terminal  80  uses the above-described MAC address (MAC #OR ) to transmits the data (info) of the real LAN communications (STEP S 33 - 1 , S 33 - 2  and S 33 - 3 ). The device  10  that receives the data (info) performs the ARP protocol for the router  3 , similarly to the above-described case. After that, the device  10  repeats the received data (info) to the router  3  (STEP S 34 ). Finally, the data (info) is repeated to the host on the internet  4  (STEP S 35 ). 
     Additionally, the monitor device  7  transfers the original IP address (IP R01 ), sub net mask (SUB R0 ) and the control information including (CM 0 n) of the subordinate cable modem and instructs the device  10  to transit to an active status. After confirming the resumption of the device  10 , the protection device  12  is switched to the device  10  to return to a normal status. The monitor device  7  concurrently transfers the IP address (IP Rr1 ), sub net mask (SUB Rr ) and the control information including CMrn of the subordinate cable modem on the original transmission path of the device  10  to the protection device  12  and instructs the device  12  to transit to an active condition (STEP S 36 ). 
     The protection device  12  instructs the switch  5  to connect the device  10  instead of the own device  12 . The monitor device  7  can perform this instruction to the switch device  5  by itself. 
     The active device  10  and the protection device  12  start sending the routing protocol, such as a RIP, at regular intervals and inform the router  3  that the segment Rr is returned to be subordinate to the protection device  12 . Thereby, the router  3  changes the contents on the routing table according to the command for starting the transmission of the RIP in the terminals  10  and  12  (STEP S 37 ). 
     Although the present invention has been described with reference to embodiments, the invention is not restricted to those. LAN communications can be continued without changing setting information of a terminal when a failure occurs on a cable modem of the terminal according to the present invention. 
     Additionally, it is possible to avoid a non-active condition of the communication to ensure changes of a routing table in a router. Therefore, a reliability of a cable modem system can be improved sharply. 
     Further, it can be simplified to confirm a resumption of a failed terminal using a test terminal. 
     The present invention may be embodied in other specific forms without departing from the sprit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.