Abstract:
A network system includes a service providing apparatus; first and second backup apparatuses, each of which is configured to back up at least a part of a function of the service providing apparatus, and is connected with the service providing apparatus through a WAN; and a terminal connected with the service providing apparatus and the first and second backup apparatuses, and configured to receive a service from the service providing apparatus in a normal operation. The service providing apparatus has IP addresses of itself, the first and second backup apparatuses and the terminal, and notifies the IP addresses of the service providing apparatus and the first backup apparatus to the terminal in response to registration of the IP address of the terminal. The terminal holds the IP addresses of the service providing apparatus and the first backup apparatus, and registers the IP address of the terminal on the first backup apparatus based on the IP address of the first backup apparatus to receive a service from the first backup apparatus, when knowing a communication failure with the service providing apparatus.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to address notification, and more particularly relates to an address notifying technique in a network system and a method of notifying an address of a backup apparatus in an IP (Internet Protocol) telephony. 
   2. Description of the Related Art 
   As one of features in an IP telephony system, there is a point that a service can be provided from one point to a plurality of sites which are far away. Since this configuration is employed, it is possible to attain integrated management of data. However, simultaneously with this, the service cannot be provided even when any of network apparatuses other than a server and a terminal is failed. For this reason, a backup apparatus is installed in a necessary location in order to continue to provide the service even when the network apparatus is failed. 
   A method of using this backup apparatus will be described below. At a time of a normal operation, a VoIP (Voice over Internet Protocol) service providing apparatus belonging to a LAN (Local Area Network) provides the service for all terminals containing terminals belonging to the above LAN and terminals belonging to other LANs connected through WAN (Wide Area Network) to the above LAN. Here, the terminal implies an apparatus such as an IP telephone, all the kinds of terminal apparatuses used in the IP telephony, and a Gateway apparatus for conversion between IP channel and a conventional channel. 
   Now, it is supposed that a failure has occurred in the WAN so that a communication between a VoIP service providing apparatus and a terminal belonging to a LAN cannot be carried out. At this time, if the terminal belonging to the LAN previously obtains an IP address of a VoIP service backup unit or apparatus installed in the other LAN from the VoIP service providing apparatus, the terminal can continue to receive the service from the VoIP service backup apparatus by performing re-registration to the VoIP service backup apparatus. 
   In the normal operation, it is necessary that the LANs at the respective sites are normal when the WAN is failed. However, a network trouble is not simple, and there is a possibility that the trouble has a spreading compositely. For example, a case could be considered that a failure has occurred in another LAN so that a communication between the VoIP service backup apparatus and a terminal belonging the other LAN is impossible. In such a case, the terminal cannot receive the service even if the VoIP service providing apparatus is provided. At this time, the terminal cannot receive the service, even if there is the VoIP service backup apparatus belonging to another LAN. 
   In conjunction with the above description, Japanese Laid Open Patent Application (JP-P 2004-186766A) discloses a backup controller. In this conventional example, the backup controller automatically backs up in a remote station, a controller which centrally controls a voice exchange between a plurality of IP telephones in at least one remote station through an IP network. This backup controller has a failure monitoring section, a first automatic switching section and a second automatic switching section. In accordance with a reception state of a periodical transmission signal from a controller or a reception state of an allowable response for the periodical transmission signal to the controller, if those reception states reach a predetermined failure reference, the failure monitoring section determines that a failure has occurred. Moreover, after the failure is determined to have occurred, if those reception states reach a predetermined recovery reference, it is determined that the failure is recovered. If the failure monitoring section determines that the failure is has occurred, the first automatic switching section disconnects the IP telephone in the remote station from the controller, and registers it as a self-extension terminal to set it to be usable. Then, the IP telephone can carry out transmission/reception to/from an extension or a station line. If the failure monitoring section determines that the failure has been recovered, the second automatic switching section disconnects the IP telephone that is registered as the self-extension terminal and made usable, and then re-connects to the controller. 
   Also, Japanese Laid Open Patent Application (JP-P2004-328604A) discloses an IP telephony service system. In this conventional example, the IP telephony service system in which an IP telephony service providing apparatus provided in a main network is used to attain an IP telephony service for an IP terminal, The main network has an IP telephony service providing apparatus for providing an IF telephony service to an IP terminal; and a first database that is linked with the IP telephony service providing apparatus and stores a service data provided to the IP terminal. Inside a plurality of sub-networks, there are an IP telephony service backup apparatus for providing an IP telephony service, when a communication with the IP telephony service providing apparatus in the main network through the IP terminal becomes impossible; and a second database that is linked with the IP telephony service backup apparatus and stores service data provided to the IP terminal by the IP telephony service backup apparatus. 
   Also, Japanese Laid Open Patent Application (JP-A-Heisei, 11-308236) discloses an emulated LAN apparatus. In this conventional example, the emulated LAN apparatus is provided with an LES/BUS server and an LAN emulation client. The server is composed of a currently-used server and a spare server, and the client holds an ATM address table corresponding to each server, and when a failure of the currently-used server is detected, its connection is switched to the spare server. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a network system and an IP address notifying method, in which a backup apparatus can be automatically and suitably selected that is used at a time of a network failure in an IP telephony system. 
   Another object of the present invention is to provide a network system and an IP address notifying method, in which a backup apparatus suitable for a terminal can be specified to minimize a possibility of a service stop. 
   In an aspect of the present invention, a network system includes a service providing apparatus; first and second backup apparatuses, each of which is configured to back up at least a part of a function of the service providing apparatus, and is connected with the service providing apparatus through a WAN; and a terminal connected with the service providing apparatus and the first and second backup apparatuses, and configured to receive a service from the service providing apparatus in a normal operation. The service providing apparatus has IP addresses of itself, the first and second backup apparatuses and the terminal, and notifies the IP addresses of the service providing apparatus and the first backup apparatus to the terminal in response to registration of the IP address of the terminal. The terminal holds the IP addresses of the service providing apparatus and the first backup apparatus, and registers the IP address of the terminal on the first backup apparatus based on the IP address of the first backup apparatus to receive a service from the first backup apparatus, when knowing a communication failure with the service providing apparatus. 
   Here, the service providing apparatus may notify the IP address of the second backup apparatus as at least one substitution backup apparatus to the terminal when knowing a communication failure with the first backup apparatus. The terminal may hold the IP address of the second backup apparatus in place of the IP address of the first backup apparatus, and register the IP address of the terminal on the second backup apparatus based on the IP address of the second backup apparatus to receive a service from the second backup apparatus, when knowing the communication failure with the service providing apparatus. 
   Also, the terminal may communicate with the first backup apparatus periodically to detect the communication failure with the first backup apparatus, and notify the detection of the communication failure with the first backup apparatus to the service providing apparatus. 
   Also, the service providing apparatus may have an IP address list of the IP addresses of a plurality of backup apparatuses including the first and second backup apparatuses, and transmit the IP address list to the terminal in response to the notice of the detection of the communication failure with the first backup apparatus from the terminal. The terminal may check a response time of each of the plurality of backup apparatuses other than the first backup apparatus based on the IP address list, and notify the check result to the service providing apparatus. The service providing apparatus may select the second backup apparatus as the at least one substitution backup apparatus from among the plurality of backup apparatuses and notifies the IP address of the second backup apparatus to the terminal. 
   Also, the service providing apparatus may belong to a first LAN, the first and second backup apparatuses may belong to second and third LANs, respectively, and connected with the first LAN through the WAN. The terminal may belong to a fourth LAN connected with the first to third LAN through the WAN. 
   Also, the service providing apparatus may be a VoIP service providing apparatus in a VoIP telephony system. 
   In another aspect of the present invention, a service providing apparatus includes a communicating section configured to communicate with a terminal and a plurality of backup apparatuses which contain first and second backup apparatuses and each of which backs up at least a part of a function of the service providing apparatus; and a storage section configured to store an IP addresses list of the plurality of backup apparatuses. The service providing apparatus further includes a processing section configured to receive registration of IP addresses from the terminal and the plurality of backup apparatuses through the communicating section, to notify the IP address of the first backup apparatus to the terminal through the communicating section in response to the registration of the IP address of the terminal. 
   Here, the processing section may notify the IP address of the second backup apparatus as at least one substitution backup apparatus to the terminal through the communicating section when knowing a communication failure with the first backup apparatus. 
   Also, the processing section may transmit the IP address list to the terminal through the communicating section in response to a request from the terminal based on detection of the communication failure with the first backup apparatus, and select the second backup apparatus as the at least one substitution backup apparatus from among the plurality of backup apparatuses based on a check result of a response time of each of the plurality of backup apparatuses other than the first backup apparatus, to notify the IP address of the second backup apparatus to the terminal through the communicating section. 
   Also, the service providing apparatus may be a VoIP service providing apparatus in a VoIP telephony system. 
   Also, each of the plurality of backup apparatuses may have a same function as the service providing apparatus. 
   In still another aspect of the present invention, an address notifying method in a network system is achieved by holding an IP address of a service providing apparatus, and registering IP addresses of a terminal and a plurality of backup apparatuses containing first and second backup apparatuses on the service providing apparatus; by notifying the IP addresses of the service providing apparatus and the first backup apparatus to the terminal in response to the registration of the IF address of the terminal such that the IP addresses of the service providing apparatus and the first backup apparatus are held by the terminal; by providing a service from the service providing apparatus to the terminal in a normal operation; and by registering the IP address of the terminal on the first backup apparatus based on the IP address of the first backup apparatus to receive a service from the first backup apparatus, when a communication failure with the service providing apparatus has occurred. 
   Here, the address notifying method may be achieved by further notifying the IP address of the second backup apparatus as at least one substitution backup apparatus from the service providing apparatus to the terminal when a communication failure with the first backup apparatus has occurred, holding the IF address of the second backup apparatus in place of the IP address of the first backup apparatus in the terminal; and registering the IP address of the terminal on the second backup apparatus based on the IP address of the second backup apparatus to receive a service from the second backup apparatus, when the communication failure with the first backup apparatus has occurred. 
   Also, the address notifying method may be achieved by further detecting the communication failure with the first backup apparatus by the terminal by communicating with the first backup apparatus periodically; and notifying the detection of the communication failure with the first backup apparatus from the terminal to the service providing apparatus. 
   Also, the address notifying method may be achieved by further transmitting an IP address list from the service providing apparatus to the terminal in response to the notice of the detection of the communication failure with the first backup apparatus, wherein the IP address list contains the IP addresses of a plurality of backup apparatuses including the first and second backup apparatuses; checking a response time of each of the plurality of backup apparatuses other than the first backup apparatus based on the IP address list; notifying the check result of the response times from the terminal to the service providing apparatus; selecting the second backup apparatus as at least one substitution backup apparatus from among the plurality of backup apparatuses in the service providing apparatus; and notifying the IP address of the second backup apparatus to the terminal. 
   Also, the service providing apparatus belongs to a first LAN, and the first and second backup apparatuses belong to second and third LANs, respectively, and connected with the first LAN through the WAN. The terminal belongs to a fourth LAN connected with the first to third LAN through the WAN. 
   Also, the service providing apparatus may be a VoIP service providing apparatus in a VoIP telephony system. 
   In still yet another aspect of the present invention, a computer-readable software product achieves an address notifying method. The method is achieved by receiving registration of IP addresses from a terminal and a plurality of backup apparatuses which contain first and second backup apparatuses and each of which backs up at least a part of a function of the service providing apparatus; and by notifying the IP addresses of a service providing apparatus and the first backup apparatus to the terminal in response to the registration of the IP address of the terminal. 
   Here, the method may be achieved by further notifying the IP address of the second backup apparatus as at least one substitution backup apparatus from the service providing apparatus to the terminal when knowing a communication failure with the first backup apparatus. 
   Also, the method may be achieved by further transmitting an IP address list from the service providing apparatus to the terminal in response to a request from the terminal based on detection of the communication failure with the first backup apparatus, the IP address list containing the IP addresses of the plurality of backup apparatuses; and selecting the second backup apparatus as the at least one substitution backup apparatus from among the plurality of backup apparatuses based on a check result of a response time of each of the plurality of backup apparatuses other than the first backup apparatus, to notify the IP address of the second backup apparatus from the service providing apparatus to the terminal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a conceptual diagram showing a configuration of a network system according to an embodiment of the present invention; 
       FIG. 2  is a flowchart showing a process flow of the network system in the present invention; and 
       FIG. 3  is a block diagram showing a configuration of a VoIP service providing apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, a network system of the present invention will be described in detail with reference to the attached drawings. 
   First Embodiment 
   As shown in  FIG. 1 , a network system of the present invention includes a WAN  1 , a LAN  10 , LANs  20  ( 20 -j, j=1, 2, . . . , m), a VoIP service providing apparatus  100 , terminals  110  ( 110 -i, i=1, 2, . . . , n), a router  120 , a VoIP service backup units or apparatuses  200 -j, terminal  210  ( 210 -j-k, k=1, 2, . . . , n), and routers  220 -j. 
   The WAN  1  is a wide area network to connect terminals such as computers, which are located at geographically separate points, to allow transmission and reception of data. The WAN  1  is, for example, the Internet, a telephone communication line network and a dedicated line network. Each of the LAN  10  and the LAN  20 -j is a narrow area and small scale communication network in a company or a home. 
   The VoIP service providing apparatus  100  belongs to the LAN  10 , and assigned with an IP address  101 . Also, the VoIP service providing apparatus  100  has an IP address list indicating IP addresses of the VoIP service backup apparatuses  200 -j, and IP addresses of the terminals  110 -i and  200 -j-k. The terminals  110 -i belong to the LAN  10 , and carry out communication by using the IP addresses. For example, the terminals  110 -i are such as a computer, an IP telephone, an intelligent home electrical appliance, a game machine, and a home server. However, the terminals are not limited to those examples. Each of the routers  220 -j relays communication data between the WAN  1  and the LAN  20 -j. 
   Each of the VoIP service backup apparatuses  200 -j belongs to a corresponding one of the LANs  20 -j. Also, each of the VoIP service backup apparatuses  200 -j is assigned with the IP address  201 -j. Also, the VoIP service backup apparatus  200 -j have an IP address list indicating the IP address of the VoIP service providing apparatus  100  and the IP addresses  201 -j of the VoIP service backup apparatuses  200 -j. The terminals  210 -j-k belong to the LAN  20 -j, and carry out communication by using the IP addresses. For example, the terminal  210 -j-k is such as a computer, an IP telephone, an intelligent home electrical appliance, a game machine and a home server. However, the terminal is not limited to those examples. Each of the routers  220 -j relays communication data between the WAN  1  and a corresponding one of the LANs  20 -j. 
   It should be noted that a case that an IP address is not fixedly assigned but dynamically assigned by DHCP (Dynamic Host Configuration Protocol) will be described. When the IP addresses are assigned to the VoIP service providing apparatus  100  and the VoIP service backup apparatus  200 -j, it is necessary to notify an updated or newly assigned IP addresses to all the terminals  110 -i and  210 -j-k receiving the service from them. The notice of the IP address is issued to all the terminals  110 -i and  220 -j-k by each of the VoIP service providing apparatus  100  and the VoIP service backup apparatuses  200 -j when communication is established between the apparatus and the terminal after change or new assigning of the IP address, Or, the VoIP service backup apparatuses  200 -j notify the dynamically changed IP address to the VoIP service providing apparatus  100 , and the VoIP service providing apparatus  100  collectively manages the changed IP addresses and notifies to all of the terminals  110  and  210 . 
   Next, an example of the network system of the present invention will be described below by exemplifying a case that the number of IP addresses that can be registered in each terminal is 2. 
   The IP address of the terminal  210 -j-k is registered in the VoIP service providing apparatus  100  at the time of a normal operation and the terminal  210 -j-k receives a service from the VoIP service providing apparatus  100 , When the IP address of the terminal  210 -j-k is registered in the VoIP service providing apparatus  100 , the terminal  210 -j-k receives the IP address of the VoIP service backup apparatus  200 - 1  from the VoIP service providing apparatus  100 . The VoIP service backup apparatus  200 - 1  is the most suitable for the terminal  210 -j-k, if the VoIP service providing apparatus  100  and the LAN are normal. The VoIP service backup apparatus  200 - 1  is selected as a re-registration destination at a time of a communication failure with the VoIP service providing apparatus  100 . At this time, the selected VoIP service backup apparatus  200 - 1  may not always belong to the same LAN as the terminal  210 -j-k that is registered on the VoIP service providing apparatus  100 . 
   The terminal  210 -j-k periodically communicates with the VoIP service providing apparatus  100  and the VoIP service backup apparatus  200 - 1  during the normal operation and checks to confirm the states of them. Here, it is supposed that the communication failure between the terminal  210 -j-k and the VoIP service backup apparatus  200 - 1  is detected by the terminal  210 -j-k. 
   The service to the terminal  210 -j-k is continued because the communication with the VoIP service providing apparatus  100  is normal. However, if the communication with the VoIP service providing apparatus  100  is also failed, the service to the terminal  210 -j-k is stopped. For this reason, the terminal  210 -j-k notifies the communication failure with the VoIP service backup apparatus  200 - 1  to the VoIP service providing apparatus  100  and requests an IP address list of the variable VoIP service providing apparatuses  200 -j. The VoIP service providing apparatus  100  sends the list of the IP addresses of the VoIP service backup apparatuses  200 - 2  to  200 -m other than the VoIP service backup apparatus  200 - 1  to the terminal  210 -j-k. 
   The terminal  210 -j-k checks a response time of each of the VoIP service backup apparatuses  200 - 2  to  200 -m with regard to the connection to each of the VoIP service backup apparatuses based on the IP address list. Then, the terminal  210 -j-k sends the check result to the VoIP service providing apparatuses  100 . The VoIP service providing apparatus  100  considers the number of terminals assigned to each of the VoIP service backup apparatuses in the list, and a band of the line used for the connection between the sites, in accordance with the check result from the terminal  210 -j-k, and re-selects an optimal one of the VoIP service backup apparatuses, for example, the VoIP service backup apparatus  200 - 2 , and then notifies the IP address  201 - 2  of the VoIP service backup apparatus  200 - 2  to the terminal  210 -j-k. 
   The terminal  210 -j-k receives the notice and stores this IP address  201 - 2  therein as a backup registration destination when the communication with the VoIP service providing apparatus  100  is failed. Thus, a continuous operation is carried out. 
   The process flow of the network system according to the present invention will be described below with reference to  FIG. 2 .
     (1) Step S 101     

   The IP address of the terminal  210 -j-k is registered on the VoIP service providing apparatus  100  in order for the terminal  210 -j-k to receive a service at the time of the normal operation.
     (2) Step S 102     

   When being registered on the VoIP service providing apparatus  100 , the terminal  210 -j-k receives the IP address of the VoIP service backup apparatus  200 - 1  from the VoIP service providing apparatus  100 . The VoIP service backup apparatus  200 - 1  is selected as an optimal apparatus by the VoIP service providing apparatus  100 .
     (3) Step S 103     

   The terminal  210 -j-k registers the VoIP service backup apparatus  200 - 1  herein as a re-registration destination of the IP address of the terminal  210 -j-k when the communication with the VoIP service providing apparatus  100  is failed,
     (4) Step S 104     

   The terminal  210 -j-k periodically communicates with the VoIP service backup apparatus  200 - 1  in the normal operation and checks whether or not the VoIP service backup apparatus  200 - 1  is normally operated.
     (5) Step S 105     

   If the VoIP service backup apparatus  200 - 1  is in the normal state, the communication with the VoIP service backup apparatus  200 - 1  is normal. Thus, the service to the terminal  210 - 1  is continued.
     (6) Step S 106     

   However, if the communication failure between the terminal  210 -j-k and the VoIP service backup apparatus  200 - 1  is detected by the terminal  210 -j-k and the communication with the VoIP service providing apparatus  100  in this state is also failed, the service to the terminal  210 -j-k is stopped. For this reason, when detecting the communication failure with the VoIP service backup apparatus  200 - 1 , the terminal  210 -j-k notifies the communication failure with the VoIP service backup apparatus  200 - 1  to the VoIP service providing apparatus  100  and requests the IP address list of the variable VoIP service backup apparatuses.
     (7) Step  5107     

   The VoIP service providing apparatus  100  sends the IP address list of the VoIP service backup apparatuses  200 - 2  to  200 -m other than the variable VoIP service backup apparatus  200 - 1  to the terminal  210 - 1 .
     (8) Step S 108     

   The terminal  210 -j-k checks a response time of each of the VoIP service backup apparatuses  200 - 2  to  200 -m with regard to the connection to terminal  210 -j-k and notifies the check result to the VoIP service providing apparatus  100 . That is, the communication speed with each of the VoIP service backup apparatuses  200 - 2  to  200 -m is checked, and the check result is notified to the VoIP service providing apparatus  100 . It should be noted that as for the once-checked VoIP service backup apparatus  200 , the check result may be stored and notified to the VoIP service providing apparatus  100 . Consequently, the check may not be performed thereon from a next time.
     (9) Step S 109     

   The VoIP service providing apparatus  100  considers the number of the terminals assigned to each of the VoIP service backup apparatuses  200 - 2  to  200 -m and the band of the line used for the connection between the sites, in accordance with the notified check result from the terminal  210 -j-k. The VoIP service providing apparatus  100  re-selects an optimal one of the VoIP service backup apparatuses  200 - 2  to  200 -m based on the consideration result. In this example, the optimal apparatus is the VoIP service backup apparatus  200 - 2 . The VoIP service providing apparatus  100  notifies the IP address  201  of the VoIP service backup apparatus  200 - 2  to the terminal  210 -j-k.
     (10) Step S 110     

   The terminal  210 - 1  receives the notice and stores the IP address  201 - 2  of the VoIP service backup apparatus  200 - 2  as a backup registration destination when the communication with the VoIP service providing apparatus  100  is failed. Thus, the operation is continued. 
   An example of the configuration of the VoIP service providing apparatus will be described below with reference to  FIG. 3 . The VoIP service providing apparatus  100  contains a communicating section  102 , a processing section  103  and a storage section  104 . 
   The communicating section  102  communicates with the LAN  10 , and NIC (Network Interface Card) is exemplified. The IP address  101  is assigned to this NIC. That is, if there are a plurality of NICs, there are a plurality of IP addresses  101 . The processing section  103  executes processes of the present invention such as the provision of the service and notification of the IP addresses. That is, the processing section  103  executes the programs for carrying out these processes. As the processing section  103 , CPU is exemplified. The storage section  104  stores the data required to provide the service and the programs for executing the processes of the present invention. As the storage section  104 , a memory or hard disc is exemplified. A recording medium or external storage is allowable. The data required to provide the service includes the content provided as the service and the IP address list used in the present invention. However, actually, there is a case that the content is provided by a different server. In such a case, the VoIP service providing apparatus  100  is not required to hold the content. The IP address of the server providing this content may be reported to the terminal side. 
   It should be noted that the configuration of the VoIP service backup apparatus  200  is desired to be similar to the configuration of the VoIP service providing apparatus  100 . In case of the same configuration, when the communication with the VoIP service providing apparatus  100  is failed, the VoIP service backup apparatus  200  can function instead of the VoIP service providing apparatus  100 . 
   Second Embodiment 
   Next, the network system according to the second embodiment of the present invention will be described below. The number of the servers that can be registered on the terminal side may be 3 or more. In this case, the VoIP service providing apparatus  100  can send the IP addresses of the VoIP service backup apparatuses as preferable apparatuses, to which priority orders are assigned, in accordance with the check result of the connection. That is, when the number of the IP addresses of the servers that can be registered by each terminal is 3 or more, each terminal registers the IP addresses in the order starting from the high priority order. 
   For example, in the description of the above embodiment, if the terminal  210 -j-k detects and knows the communication failure between the terminal  210 -j-k and the VoIP service backup apparatus  200 - 1 , the VoIP service providing apparatus  100  notifies the IP address list to the terminal  210 -j-k, when re-selecting the optimal one of the VoIP service backup apparatuses  200 - 2  to  200 -m, which are arranged in the order starting from the optical apparatus. The order at this time is assumed to be the priority order. The terminal  210 -j-k determines the backup registration destination when the communication with the VoIP service providing apparatus  100  is failed, in accordance with the priority order in the notified IP address list. When detecting the communication failure of the VoIP service backup apparatus  200 - 1  having the highest priority order, the terminal  210 -j-k selects the VoIP service backup apparatus  200  having the next highest priority order. Or, the backup registration destination when the communication with the VoIP service providing apparatus  100  is failed is determined in accordance with a preset priority order, in spite of whether or not it is optimal. At this time, the priority order is considered to be able to be freely set. 
   In the above description, the terminal communicates with the VoIP service backup apparatus periodically to check the state of the VoIP service backup apparatus. However, the VoIP service providing apparatus  100  may communicate with the VoIP service backup apparatus periodically, instead of the terminal. In this case, when the VoIP service providing apparatus  100  detects the communication failure with the VoIP service backup apparatus, the VoIP service providing apparatus  100  may select the optimal substitution backup apparatus based on the check result and the like and notify an IP address of the optimal substitution backup apparatus to the terminal. 
   It should be noted that in the foregoing description, the IP address is used as the data to specify the VoIP service providing apparatus  100  and the VoIP service backup apparatuses  200 . Actually, they are not limited to the IP-address. Instead of the IP address, the identification data indicating the VoIP service providing apparatus  100  and the VoIP service backup apparatus  200  may be used. 
   Also, the above has been described under the assumption that the function for checking the connection exists inside the VoIP service providing apparatus. However, as a different apparatus, it may exist outside.