Patent Publication Number: US-2005135337-A1

Title: Network system using common channel signalling

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a network system using a common channel-signalling, and more particularly to a network system for transferring control signals of common channel signalling using signalling transfer points.  
      2. Description of the Related Art  
      Recently, with promotion of IP networks, a system for connecting an existing telephone exchange network to an IP network or a system for providing an IP network service through an existing telephone exchange network has been put into practical use. As such a system, configurations disclosed in Patent documents 1 and 2 are proposed.  
       FIG. 1  shows the configuration of a gateway system disclosed in the Patent document 2. An exchange  101  is installed on an existing line switching network and accommodates a subscriber terminal  102 . The exchange  101  is also connected to a SS7 (signalling system No. 7) network  110 . The SS7 network  110  transfers control signals of the No. 7 common channel signalling in the existing line switching network.  
      An IP network  120  is a network for providing an IP network service. The IP network  120  comprises a media gateway (MG)  121 , a signalling gateway (SG)  122  and a media gateway controller (MGC)  123 . The media gateway  121  is connected to the exchange  101  and a router, which is not shown in  FIG. 1 , establishing the IP network  120  by a communication line or a data line, and transmits/receives signals between the existing line switching network and the IP network  120  under the control of the media gateway controller  123  (including the format conversion of signals). The signalling gateway  122  transmits/receives the No. 7 signals to/from the exchange  101  through the SS7 network, and also transmits/receives a control signal to/from the media gateway controller  123  through the IP network  120 . Furthermore, the media gateway controller  123  controls the media gateway  121 , based on the No. 7 signals or control signals generated in response to the No. 7 signals.  
      In the above-described gateway system, when the subscriber terminal  102  originates a call requesting for an IP network service, an ISUP message, which is an MTP3 (message transfer part level-3) user message, is transmitted from the exchange  101  to the signalling gateway  122  through the SS7 network  110 . The signalling gateway  122  transmits a control signal to a corresponding media gateway controller  123  according to information stored in this message. Then, the media gateway controller  123  controls a corresponding media gateway  121  according to the received control signal. Furthermore, necessary information is returned to the exchange  101  from the media gateway controller  123  through the signalling gateway  122  and SS7 network  110 . As a result, a communication path that connects the existing line switching network and the IP network  120  is established.  
      Each node (the media gateway  121 , signalling gateway  122 , media gateway controller  123  and the like) installed in the IP network  120  is far smaller and inexpensive compared with the exchange  101  installed in the existing line switching network. By installing many such small nodes, a network configuration is made flexible. Therefore, it is anticipated that there will be a large number of nodes in the IP network.  
      In this case, if as shown in  FIG. 2A , all nodes are connected to each other, the number of links to be managed by each node, such as an SCTP (Stream Control Transmission Protocol) link increases, and accordingly a lot of resources in each node are used.  
      In order to solve this problem, the configuration as shown in  FIG. 2B , in which signalling transfer points (STP) are installed between nodes can be considered. Here, the STP is a node or device for transferring a signalling message. If this configuration is introduced into the IP network  120  shown in  FIG. 1 , each STP must be provided with a function to transfer an IP packet. Therefore, such an STP is often called “IP-STP”. A network system for attempting to increase the amount of traffic using such an STP is disclosed in Patent document 3. 
      The Patent document 1: Japanese Patent Laid-open Publication No. 2002-84363 (FIG. 1, paragraph 0022-0025)     The Patent document 2: Japanese Patent Laid-open Publication No. 2002-290551 (FIG. 1, paragraph 0031-0034)     The Patent document 3: Japanese Patent Laid-open Publication No. 2001-156922 (FIG. 1, abstract)    

      The IP network has realized a flexible system by connecting a lot of small and inexpensive communication devices unlike the existing line switching network. The configuration of the IP-STP is far smaller and much more inexpensive than that of an STP installed in the existing line switching network. Therefore, the process capability of the IP-STP is generally inferior to that of an STP installed in the existing line switching network. In other words, in the IP network, a lot of IP-STPs must be installed.  
      However, in the common channel signalling system, each node (including an IP-STP) is identified by a node identifier called “signalling point code (SPC)”. In this case, the number of bits of the SPC is fixedly determined. Therefore, as described above, when the number of IP-STPs increases, there are shortages of SPCs, which is a problem.  
      Since a different SPC is allocated to each of many IP-STPs, the work of setting information in order to manage IP-STPs in each signalling end point (SEP) becomes complex and troublesome.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to resolve the shortages of signalling point codes in the common channel signalling system. It is another object of the present invention to simplify the management of a network which uses the common channel signalling system.  
      The network system of the present invention includes a plurality of signalling end points and a plurality of signalling transfer points, and transfers a control signal of common channel signalling, based on a network identifier and a node identifier. The network system of the present invention comprises a setting unit setting a first network identifier to a logical network including the plurality of signalling end points and the plurality of signalling transfer points and also setting a second network identifier to a logical network including only the plurality of signal transfer points; and an allocation unit allocating node identifiers, that belong to the first group, to the plurality of signalling end points and the plurality of signalling transfer points in correspondence with the first network identifier and also allocating node identifiers, that belong to the second group, to the plurality of signalling transfer points in correspondence with the second network identifier. In this case, the number of node identifiers, that belong to the first group, allocated to the plurality of signalling transfer points is smaller than the number of the plurality of signalling transfer points.  
      In this network system, when a control signal is transferred between signalling transfer points, the first network identifier and a node identifier, that belong to the first group, allocated to a destination signalling endpoint is set in the control signal. When a control signal is transferred between signalling transfer points, the second network identifier and a node identifier, that belong to the second group, allocated to a destination signalling transfer point is set in the control signal. Thus, control signals can be transmitted/received between arbitrary signalling points. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an example of the configuration of the conventional gateway system;  
       FIGS. 2A and 2B  show connections between nodes;  
       FIGS. 3A and 3B  show the basic concept of the present invention;  
       FIG. 4  shows the configuration of the network system;  
       FIG. 5  shows the format of a control signal used in the common channel signalling system;  
       FIG. 6  shows an example of the allocation of a signalling point code;  
       FIG. 7  shows the configuration of the IP-STP;  
       FIGS. 8A through 8D  show an example of a management table installed in each signalling point;  
       FIGS. 9A and 9B  show an example of a routing table installed in each signalling point;  
       FIG. 10  shows the sequence of the first embodiment;  
       FIG. 11  shows the sequence of the second embodiment;  
       FIG. 12  shows the sequence of the third embodiment;  
       FIG. 13  shows the hardware configuration of the signalling end point and signalling transfer point; and  
       FIG. 14  shows the allocation of signalling point code in another embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The network system of the present invention includes a plurality of signalling end points and a plurality of signalling transfer points to connect the plurality of signalling end points, and transfers a control signal of common channel signalling, based on a network identifier and a node identifier.  
      The signalling end point (hereinafter, sometimes referred to “SEP”) is a communication node with a function to process or handle a control signal of the common channel signalling. Although the SEP is not especially limited, for example, as shown in  FIG. 1 , the SEP comprises the media gateway (MG)  121 , the signalling gateway (SG)  122  and the media gateway controller (MGC)  123 . In the following description, the signalling end point SEP is often simply called “node”. On the other hand, the signalling transfer point (hereinafter, sometimes referred to “STP”) is a communication node with a function to transfer a control signal of the common channel signalling, and in the following embodiment, it is assumed to be an “IP-STP” installed in an IP network. Each of the signalling end points and signalling transfer points is identified by a node identifier called “SPC (Signalling Point Code)”. Although the common channel signalling system is not especially limited, for example, the No. 7 signalling system is used.  
       FIGS. 3A and 3B  show the basic concept of the present invention. Here, as shown in  FIG. 3A , it is assumed that in a network, a plurality of nodes  11  are connected to each other through four IP-STPs  12   a - 12   d . Node  11  is the above-described signalling end point.  
      In this network, a different signalling point code is allocated to each of the plurality of nodes  11  and IP-STPs  12   a - 12   d . Specifically, in the example of  FIG. 3A , four signalling point codes in total are allocated to respective IP-STPS  12   a - 12   d.    
      In the network system of the present invention, as shown in  FIG. 3B , the number of signalling point codes allocated to IP-STPs is smaller than the number of IP-STPs. In other words, in the network system of the present invention, one signalling point code is allocated to a plurality of IP-STPs. For example, one signalling point code can be allocated to IP-STPs  12   a - 12   d . In this case, node  11  recognizes that only one IP-STP exists. Alternatively, one signalling point code can be allocated to two of the IP-STPs  12   a - 12   d , and another signalling point code can be allocated to the other two IP-STPs. In this case, node  11  recognizes that only IP-STPs exist.  
      A routing table used to transfer control signals in this network is divided and managed for each logical network. In the example of  FIG. 3B , a logical network A is composed of a plurality of nodes  11  and IP-STPs  12   a - 12   d . On the other hand, a logical network B is composed of IP-STPs  12   a - 12   d . In a routing table provided for each IP-STP, information for signal transfer between nodes and signal transfer between a node and an IP-STP, and information for signal transfer between IP-STPs are separately managed.  
      As described above, in the network system of the present invention, since the number of signalling point codes to be allocated to IP-STPs is small, the shortage or exhaustion of a signalling point code is mitigated. In addition, since the number of signalling point codes to be allocated to IP-STPs is small, in each node  11 , information about IP-STPs can be easily set and managed.  
       FIG. 4  shows the configuration of a network system for the following embodiments. In this case, the network system includes two signalling end points (nodes  11 X and  11 Y), four signalling transfer points (IP-STPs  12   a  through  12   d ) and ten links for connecting these signalling points (including SEPs and STPs). These links are called “associates” in an SCTP (Stream Control Transmission Protocol).  
      In this network system, a logical network A identified by “network identifier NI=0” and a logical network B identified by “network identifier NI=15” exist. In this case, in the logical network A, nodes  11 X and  11 Y and IP-STPs  12   a - 12   d  exist, and in the logical network B, IP-STPs  12   a - 12   d  exist.  
       FIG. 5  shows the format of a control signal used in the common channel signalling system.  FIG. 5  shows the format of an MTP3 (Mmessage Transfer Part level-3). The MTP is a protocol for transferring a message of the common channel signalling system.  
      “SIO” is composed of four bits of SI area and four bits of SSF area. “Network identifier NI” is set using two bits of this SSF bits. However, in the current domestic communication of Japan, “0” is fixedly used as the network identifier NI.  
      “DPC” is an area used to set the signalling point code of a destination signalling point, and “OPC” is an area used to set the signalling point code of an originating signalling point (that is, a source signalling point). In this example, 16 bits are allocated to each of “DPC” and “OPC”. “CIC” is a circuit identification code, and includes an “A/B bit”. The “A/B bit” designates which plane is used for a communication route, a plane A or a plane B, when a network is redundantly configured.  
      The above-described control signal is generated by an arbitrary signalling point. (including nodes  11 X and  11 Y and IP-STPs  12   a - 12   d ) shown in  FIG. 4 , and is transmitted to the network. Each signalling point refers to the “network identifier NI” and “destination signalling point code DPC” of the received signal, and transfers the signal to the destination signal point.  
       FIG. 6  shows the allocation of signalling point codes. The signalling point code is allocated, for example, by the manager of a network.  
      In the system of the embodiment, the signalling point code is allocated for each network identifier. Specifically, in the logical network A identified by “network identifier NI=0”, “NI=01-01-010” and “01-01-020” are allocated to nodes  11 X and  11 Y, respectively. In addition, “01-01-000” is allocated to each of the IP-STPs  12   a  and  12   c , and “01-01-001” is allocated to each of the IP-STPs  12   b  and  12   d.    
      As described above, in this logical network, different signalling point codes are allocated to corresponding signalling endpoints (nodes  11 X and  11 Y). However, a signalling point code whose number is smaller than the number of the signalling transfer points is allocated to each of the signalling transfer points (IP-STPs  12   a - 12   d ). More specifically, two signalling point codes are allocated to four signalling transfer points. Therefore, in this case, nodes  11 X and  11 Y can see only two signalling transfer points.  
      In differently view, a route via an IP-STP identified by “01-01-000” (node  11 X, IP-STP  12   a , IP-STP  12   c  and node  11 Y) and a route via an IP-STP identified by “01-01-001” (node  11 X, IP-STP  12   b , IP-STP  12   d  and node  11 Y) exist between nodes  11 X and  11 Y. In other words, a redundant configuration is realized. The former one and the latter one are often called “plane A” and “plane B”, respectively.  
      In the logical network B identified by “network identifier NI=15”, “02-02-001”, “02-02-002”, “02-02-003” and “02-02-004” are allocated to IP-STPs  12   a ,  12   b ,  12   c  and  12   d , respectively. In other words, in this logical network, a different SPC is allocated to each signalling transfer point.  
      In the existing network where the above-described MTP3 signals are transferred, the network identifier NI is expressed using two bits in the SSF area shown in  FIG. 5 . However, in the system of the embodiment, the network identifier NI is expressed using four bits of the entire SSF area.  
      Each signalling point is provided with a local signalling point code table, an associate table and a routing table. The contents of these tables are set based on the above-described allocation of signalling point codes.  
       FIG. 7  shows the configuration of the IP-STP. A NIC card  21  provides an interface function to terminate a line. An OS/IP route management unit  22  performs the signal process of lower-order layers. An SCTP control unit  23  performs the signal process with respect to the SCTP (Stream Control Transmission Protocol). A routing data management unit  24  stores and manages information about the transfer of control signals (including a routing table). A transfer control unit  25  controls message transfer (MTP3 (Message Transfer Part level-3), M3UA (Mtp3-User Adaptation layer), M2UA, M2PA, Mtp3 and the like). An O/M unit  26  receives instructions from the manager of the system and sets information needed for the IP-STP.  
      The IP-STP is realized, for example, by installing the NIC card in a computer, and installing a program that describes the functions of the OS/IP route management unit  22  through O/M unit  26 .  
       FIGS. 8A through 8D  show examples of a management table provided for each signalling point. In these examples, assuming the system shown in  FIG. 4 , a management table provided for the node  11 X and that provided for the IP-STP  12   a  are shown as an example of a management table provided for each signalling end point and that provided for each signalling transfer point, respectively.  
       FIGS. 8A and 8B  are an example of a local signalling point code table provided for node  11 X and that provided for IP-STP  12   a , respectively. The local signalling point code table registers the combination of a network identifier NI for identifying a logical network to which the local signalling point belongs and an signalling point code SPC allocated to the local signalling point. Since each IP-STP (here IP-STP  12   a ) belongs to both of the logical networks A and B, as shown in  FIG. 8B , both of the signalling point code in the logical network A and the one in the logical network B are registered.  
       FIGS. 8C and 8D  are respective examples of an associate table provided for node  11 X and IP-STP  12   a  respectively. In the associate table, the network identifier NI of a logical network to which each link belongs, and a signalling point code of a destination signalling point of the link (corresponding SPC) are registered for each link (that is an associate) connected to a local signalling point. For example, in  FIG. 8C , “associate X-a”-indicates a link connecting the node  11   x  and the IP-STP  12   a  through a logical network identified by “NI=0”, and “associate X-a” indicates a link connecting the node  11 X and the IP-STP  12   b  through a logical network identified by “NI=15”. In  FIG. 8D , “associate a-c”, “associate a-b”, “associate a-d” and “associate X-a” indicate a link connecting IP-STP  12   a  and IP-STP  12   c  through a logical network identified by “NI=15”, one connecting IP-STP  12   a  and IP-STP  12   b  through a logical network identified by “NI=15”, one connecting IP-STP  12   a  and IP-STP  12   d  through a logical network identified “NI=15” and one connecting node  11 X and IP-STP  12   a  through a logical network identified by “NI=0”, respectively.  
       FIGS. 9A and 9B  are examples of a routing table provided for each signalling point. In this example, assuming the system shown in  FIG. 4 , a routing table provided for the node  11 X and that provided for the IP-STP  12   a  are shown in  FIGS. 9A and 9B , as an example of a routing table provided for each signalling end point and an example of that provided for each signalling transfer point, respectively. These routing tables are used to transfer the control signal to the destination.  
      In  FIG. 9A , on a first through third records, output associates for each destination signalling point in the logical network A identified by “network identifier NI=0” are registered. In this case, destinations registered in the third record is a signalling point identified by “SPC=01-01-001” in the logical network A, and are IP-STPs  12   b  and  12   d . In fourth through sixth records, output associates for each destination signalling point in the local network B identified by “network identifier NI=15” are registered. If a plurality of output associates exists for one destination, priority is given to each output associate.  
      As shown in  FIG. 9B , in each signalling end point, an output associate is determined by the value of the A/B bit shown in  FIG. 5 . In this example, information defining that associate X-a is selected when “A/B bit=0”, and associate X-b is selected when “A/B bit=1” is stored in the node  11 X. A signalling end point that is going to transmit a control signal can designate the plane A or B of the redundantly configured IP-STP network, using the A/B bit. In other words, by properly setting the A/B bit in a signalling end point that is going to transmit a control signal, the load balance (load distribution) of the IP-STP network can be realized.  
      Next, examples of signal transfer in the above-described network system are described.  
     The First Embodiment  
      In the first embodiment, the sequence in the case where a control signal is transferred from node  11 X to node  11 Y is described. In this example, the following information is set in a control signal generated by node  11 X in the format shown in  FIG. 5 . That is to say, “NI=0” is set in the SSF area. In this case, “NI=0” can be set using the predetermined two bits specified in advance in the SSF area as in the conventional method. Alternatively, “NI=0” can be set using four bits of the entire SSF area. A signalling point code “01-01-020” indicating node  11 Y and a signalling point code “01-01-010” indicating node  11 X are set in the DPC area and the OPC area, respectively. Furthermore, it is assumed that “0” is set in “A/B bit”. A signal information area stores information, commands or the like to be notified to node  11 Y.  
      The control signal generated as described above is transferred from node  11 X to node  11 Y in the following sequence. This sequence is described below with reference to  FIG. 10 . 
      (1) The node  11 X firstly selects the associate X-a according to “A/B bit=0” set in the control signal. Then, the control signal is transmitted through the selected associate X-a. As a result, this control signal is received by the IP-STP 12   a.       (2) The IP-STP  12   a  extracts a network identifier NI and a destination signalling point code DPC from the received control signal, and refers to the routing table using those values as retrieval keys. In this case, the routing table provided for IP-STP  12   a  is as shown in  FIG. 9A . Therefore, “a-c” is obtained as an output associate, based on “NI=0” and “SPC=01-01-020”. Here, an output associate is selected according to the priority registered in the routing table. Then, the IP-STP  12   a  outputs the control signal received from the node  11 X to associate a-c. As a result, this control signal is transferred to the IP-STP  12   c.       (3) The operation of the IP-STP  12   c  is basically the same as that of the above-described IP-STP  12   a . Specifically, the IP-STP  12   c  determines an output associate using the routing table, and outputs the control signal received from the IP-STP  12   a  to the determined associate. As a result, this control signal is transferred to the node  11 Y.     (4) When the node  11 Y detects that the destination signalling point code DPC of the received control signal coincides with a signalling point code of the node  11 Y itself, the node  11 Y performs an operation corresponding to a notice or command stored in the received control signal.    

      As described above, in the signal transfer between signalling end point, a control signal is transmitted to a desired signalling end point by setting “NI=0” as a network identifier and setting an signalling point code allocated on the logical network that is identified by that network identifier as a destination. In this case, each IP-STP transfers the control signal to the destination referring to information managed in correspondence with the network identifier in the routing table.  
     The Second Embodiment  
      In the second embodiment, the sequence in the case where a failure occurs on the associate X-b that connects the node  11 X and the IP-STP  12   b  is described. In this example, it is assumed that a failure occurs on the associate X-b while the control signal is transmitted from the node  11 X to the IP-STP  12   b . Specifically, it is assumed that before the failure has occurred, “NI=0” and “SPC=01-01-010” are set in the control signal, and the control signal is transmitted from the node  11 X to the IP-STP  12   b  through the associate X-b. It is also assumed that the node  11 X has detected the failure by some method (for example, a notice from the IP-STP  12   b ). The sequence of the second embodiment is described below with reference to  FIG. 11 . 
      (1) The node  11 X sets the same network identifier NI and destination signalling point code DPC as that before the failure occurs in a subsequent control signal or a control signal to be re-transmitted. Specifically, “NI=0” and “DPC=01-01-010” are set in this control signal. However, since there is a failure on the associate X-b, “0” which indicates the plane A configured by the IP-STPs  12   a  and  12   c  is set in the A/B bit of this control signal.     (2) The node  11 X transmits the control signal according to the A/B bit. Specifically, node  11 X outputs the control signal to the associate X-a. As a result, the control signal is received by the IP-STP  12   a.       (3) The IP-STP  12   a  extracts the network identifier NI and destination signalling point code DPC from the received control signal and refers to the routing table using these values as retrieval keys. In this case, the routing table provided for the IP-STP  12   a  is as shown in  FIG. 9A . Therefore, “a-b” is obtained as an output associate, based on “NI=0” and “DPC=01-01-001”. Then, the IP-STP  12   a  outputs the received control signal to the associate a-b. As a result, this control signal is transferred to the IP-STP  12   b.       (4) The IP-STP  12   b  performs an operation corresponding to a notice or a command stored in the received control signal.    

      As described above, even if a failure occurs in a link between a signalling end point and a signalling transfer point, the control signal is transferred to a target destination through another route by the same network identifier NI and destination signalling point code DPC as that before the failure occurs.  
     The Third Embodiment  
      In the third embodiment, the sequence in the case where a failure occurs in the associate X-a that connects the node  11 X and the IP-STP  12   a  is described below. More specifically, the sequence in the case where the IP-STP  12   a  detects the failure of the associate X-a and notifies all adjacent IP-STPs of the fact is described. The sequence of the third embodiment is described below with reference to  FIG. 12 . 
      (1) When the IP-STP  12   a  detects the failure, the IP-STP  12   a  refers to the associate table shown in  FIG. 8D  and recognizes all signalling points adjacent to the IP-STP  12   a . Then, the IP-STP  12   a  generates control signals whose destinations are these recognized signalling points. However, since there is a failure in the associate X-a, a control signal whose destination is the node  11 X is not generated. Specifically, control signals whose destinations are the IP-STPs  12   b ,  12   c  and  12   d , respectively, are generated. In this case, these control signals are transmitted/received between signalling transfer points, “network identifier NI=15” is set in each of these control signals. As destination signalling point codes, values corresponding to “network identifier NI=15” (02-02-002, 02-02-003 and 02-02-004) are set. These control signals are transfer prohibition signals indicating that signal transfer through the associate X-a should be prohibited.     (2) The control signals generated thus are transmitted to each adjacent signalling point. In this case, a control signal with “NI=15, SPC=02-02-002”, a control signal with “NI=15, SPC=02-02-003” and a control signal with “NI=15, SPC=02-02-004” are transmitted to the IP-STP  12   b  through the associate a-b, to the IP-STP  12   c  through the associate a-c, and to the IP-STP  12   d  through the associate a-d, respectively.     (3) Each of the IP-SWTP  12   b  through  12   d  performs an operation corresponding to the received control signal (transfer prohibition signal). Specifically, for example, the routing table of each of the IP-STP is updated in such away that the associate X-a can be excluded from the transfer route of the control signal.    

      As described above, if a failure occurs in a link connected to an IP-STP, a transfer prohibition signal is transmitted from the IP-STP that has detected the failure to other IP-STPs. An IP-STP that has received the transfer prohibition signal updates the routing table in such a way as to set information for realizing a route that goes around a link in which the failure has occurred. Therefore, after that, the control signal goes around the link in which the failure has occurred and is transferred to a desired signalling point.  
       FIG. 13  shows the hardware configuration of a signalling end point and a signalling transfer point. The signalling end point and signalling transfer point of the embodiment can be realized, for example, by the computer shown in  FIG. 13 . The computer comprises a CPU  51 , a memory  52 , a storage unit  53 , a user I/F unit  54  and a communication I/F unit  55 . A program that describes the operation of the signalling end point or signalling transfer point is stored in the storage unit  53  (or the memory  52 ), and by executing the program, the above-described operations are realized. The tables shown in  FIGS. 8A through 8D ,  9 A and  9 B are stored in the memory  52  (or the storage unit  53 ).  
      The network identifier NI and signalling point code SPC are determined, for example, by a network manager and are set in a corresponding signalling point through the user I/F unit  54  or communication I/F unit  55 . In this case, “the setting of network identifiers” and “the allocation of signalling point codes (node identifiers)” include the operation of registering determined values in a table provided for each signalling point.  
      Although in the above-described embodiments, the IP-STP network is duplicated by allocating two signalling point codes to the IP-STP network, the present invention is not limited to this configuration. Specifically, as shown in  FIG. 14 , only one signalling point code may be allocated to the entire IP-STP network. In other words, an identical signalling point code may be allocated to all IP-STPs configuring the IP-STP network. Alternatively, more than three signalling point codes may be allocated to the IP-STP network. However, even in this case, the number of signalling point codes allocated to the entire IP-STP network must be smaller than the number of IP-STPs configuring the IP-STP network.  
      Although in the above-described embodiments, the “No. 7” is adopted as one form of the common channel signalling system, the present invention is not limited to this, and the present invention is applicable to a network system that transfers a signal of another common channel signalling system.  
      Furthermore, although in the above-described embodiments, a system for providing an IP network service through the existing line switching network is assumed, the present invention is not limited to this. Specifically, the signalling end point of the present invention is not limited to a media gateway, a signalling gateway and a media gateway controller. The signalling transfer point of the present invention is not limited to an IP-STP provided for an IP network.  
      According to the present invention, since the number of signalling point codes to be allocated to signalling transfer points is small, the shortage of the signalling point codes in a common channel signalling system can be resolved or mitigated. Accordingly, the management of a network using a common channel signalling system can be simplified.