Abstract:
A method of setting a path in a network using an Internet protocol to facilitate voice communications between two devices includes determining whether a first path having an adequate bandwidth for transferring a voice over Intent protocol (VoIP) packet between two label switch routers exists. The method also includes setting a new path having a bandwidth that is at least two times the necessary bandwidth for transferring a VoIP packet, when it is determined that the first path does not exist.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a voice data communication system (i.e., Internet telephone system) using an Internet protocol (IP) network, and more particularly, to an Internet telephone system capable of ensuring the quality of voice data communication (communication quality or QoS (Quality of Service)) and a path setting method. 
   2. Description of the Related Art 
   In recent years, there have been experiments to implement interactive communication through a telephone by using a network adopting an Internet protocol (in particular, Internet), and an ITU-T recommendation on a VoIP (Voice over Internet Protocol) has been issued. Also, RFCs (Request for Comments) have also been issued from an IETF (Internet Engineering Task Force). For example, the IP, voice converting method (RTP), communication system (MGCP: Media Gateway Control Protocol) are described in the IETF RFC760, IETF RFC1889, and IETF RFC2705, respectively. 
   However, the IP allows the delay in transfer of a packet and the disposition of the packet which are caused by congestion and, therefore, the IP is not suitable to real-time interactive communication. The ITU-T recommendation prescribes a method for solving a problem of the conversion between a telephone number and an IP address and a method for converting (mapping) voice data into an IP packet and, however, it prescribes no method for ensuring communication quality. The IETF proposes only a schematic standard for Internet telephone system. 
   Accordingly, predetermined communication quality must be ensured by preventing the disposition of a voice packet on the IP network and by reducing the delay in transfer of the packet to realize the Internet telephone system to satisfy user&#39;s desire. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is one object of the present invention to provide an Internet telephone system to which communication quality is ensured. 
   It is also another object of the present invention to provide an Internet telephone system capable of real-time communication without the delay in transfer. 
   It is further another object of the present invention to provide a path setting method capable of reducing the load of a router when a path is set on an IP network. 
   Other objects of the present invention will be understood apparently in the following description. 
   According to a first aspect of the present invention, an Internet telephone system implements voice communication between a telephone subscribing to a first voice network and a telephone subscribing to a second voice network via a network using an Internet protocol. The internet telephone system constitutes the network, and includes a plurality of label switch routers each of which uses a label switching technique. A first media gateway is connected across a first specific label switch router among the plurality of label switch routers and a first signaling transfer point connected to the first voice network, and assembles/resolves a VoIP packet. A second media gateway is connected across a second specific label switch router among the plurality of label switch routers and a second signaling transfer point connected to the second voice network, and assembles/resolves a VoIP packet. A path control unit checks whether or not there is a path having a residual band larger than a band necessary for transferring the VoIP packet between the first specific label switch router and the second specific label switch router. When it is determined that there is not the path, the path control unit sets a new path having a band that is equal to or more than a double band of the necessary band. A packet control unit is connected to the path control unit, and instructs the first media gateway and the second media gateway to transfer the VoIP packet via the path that is checked or set by the path control unit. 
   According to a second aspect of the present invention, in a path setting method, in order to execute voice communication between a telephone subscribing to a first voice network and a telephone subscribing to a second voice network, a path to which a band is ensured is set on a network using an Internet protocol connected between the first voice network and the second voice network. A path control unit determines whether or not there is a path having a residual band larger than a band necessary for transferring a VoIP packet between two edge label switch routers. Also, the path control unit sets a new path having band that is equal to or more than a double band of the necessary band between the two edge label switches by the path control unit, when it is determined that there is not the path. 
   According to a third aspect of the present invention, a call control apparatus sets a path to which a band is ensured on a network using an Internet protocol connected between a first voice network and a second voice network to execute voice communication between a telephone subscribing to the first voice network and a telephone subscribing to the second voice network. The call control apparatus includes a path control unit which determines whether or not there is a path having a residual band larger than a band necessary for transferring a VoIP packet between two edge label switch routers. When it is determined that there is not the path, the call control apparatus sets a new path having a band that is equal to or more than a double band of the necessary band between the two edge label switch routers. A packet control unit controls a media gateway connected to the two edge label switch routers to transfer the VoIP packet via a path having the residual band or the new-set path. 
   According to a fourth aspect of the present invention, a router is used for a network using an Internet protocol connected between a first voice network and a second voice network to implement voice communication between a telephone subscribing to the first voice network and a telephone subscribing to the second voice network. The router sets a path having a band that is equal to or more than a double band of a band necessary for transferring a VoIP packet under the control of a call control apparatus, thereby establishing a plurality of connections in the path. 
   According to a fifth aspect of the present invention, a computer program product for implementing a call control apparatus sets a path to which a band is ensured on a network using an Internet protocol connected between a first voice network and a second voice network to execute voice communication between a telephone subscribing to the first voice network and a telephone subscribing to second voice network. The computer program product includes the steps of determining whether or not there is a path having a residual band larger than a band necessary for transferring a VoIP packet between two edge label switch routers and, when it is determined that there is not the path, setting a new path having a band that is equal to or more than a double band of the necessary band between two edge label switch routers, and controlling a media gateway connected to the two edge label switch routers to transfer the VoIP packet via the path having the residual band larger than the necessary band or the new-set path. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the structure of a conventional Internet telephone system using an IP network; 
       FIG. 2  is a diagram showing a sequence for implementing communication in the Internet telephone system in  FIG. 1 ; 
       FIG. 3  is a diagram showing the structure of an Internet telephone system according to a first embodiment of the present invention; 
       FIG. 4  is a diagram for explaining the relationship between a label switching path and the connection in the Internet telephone system in  FIG. 3 ; 
       FIG. 5  is a flowchart for explaining the operation of a label switching control unit of a label switch router shown in  FIG. 3 ; 
       FIG. 6  is a diagram showing the operation of the Internet telephone system in  FIG. 3 ; and 
       FIG. 7  is a diagram showing the construction of an Internet telephone system according to a second embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First, the conventional Internet telephone system will be described in order to apparently understand the present invention. 
   Currently, three types of Internet telephone system are known. In a first Internet telephone system, voice communication is performed between personal computers in which an Internet telephone program is installed via the Internet. In a second Internet telephone system, a device, so-called gateway is located between a normal telephone and the Internet, thereby realizing voice communication via the Internet between the telephones. A third Internet telephone system is constituted by combining the first Internet telephone system and second Internet telephone system, thereby enabling the voice communication even between the normal telephone and the personal computer. 
   Herein, a description is given of the second Internet telephone system capable of using the normal telephone with reference to  FIG. 1 . This Internet telephone system uses an Media Gateway Control Protocol (MGCP). 
   As shown in  FIG. 1 , this Internet telephone system comprises a call agent (CA)  13  which is connected to an IP network (e.g., Internet)  11  and a No. 7 signal-system network (e.g., public telephone network)  12 . The call agent  13  is also called a Media Gateway Controller (MGC). Gateways  14  and  15  comprise media gateways (MG)  141  and  151  connected to the IP network  11  and call agent  13  and signaling transfer points (STP)  142  and  152  connected to the No. 7 signal-system network  12 , respectively. 
   The operation of the second Internet telephone system will be described with reference to  FIGS. 1 and 2 . Herein, assume that a sender telephone (or an originator: not shown) is connected to the gateway  14 , and an addressed telephone (or a destination: not shown) is connected to the gateway  15 . Information indicating the relationship thereof is stored in a memory of the call agent  13 . 
   First of all, the sender telephone calls the gateway  14  and notifies a telephone number of the addressed telephone. When the call from the sender telephone is received, the signaling transfer point  142  of the gateway  14  transmits a connection request signal IAM (Initial Address Message)  1  including end point information to the call agent  13  via the No. 7 signal-system network  12  (sequence a). The end point information includes an IP address of the gateway  14  and a telephone number of the addressed telephone. 
   When the connection request signal IAM 1  is received from the signaling transfer point  142 , the call agent  13  specifies the gateway  15  associated with the addressed telephone on the basis of the end point information in the received connection request signal. Then, the call agent  13  transmits a connection generating request signal CRCX 1  to the media gateway  141  via the IP network  11  to establish the connection to the specified gateway  15  (sequence b). 
   When the connection generating request signal CRCX 1  is received, the media gateway  141  executes a connection establishing process to the gateway  15  and also transmits an answer signal ACK 1  for notifying the end of the connection establishing process to the call agent  13  via the IP network  11  (sequence c). This answer signal ACK 1  includes session description such as a protocol version. 
   When the answer signal ACK 1  is received from the media gateway  141 , the call agent  13  transmits a connection establishing request signal CRCX 2  to the media gateway  151  (sequence d). When a predetermined connection establishing process ends, the media gateway  151  transmits an answer signal ACK 2  including the session description to the call agent  13  (sequence e). 
   As mentioned above, in the IP network  11 , a one-way path is established from the media gateway  141  to the media gateway  151 . 
   Next, the call agent  13  transmits a connection request signal IAM 2  to the signaling transfer point  152  via the No. 7 signal-system network  12  (sequence f). The signaling transfer point  152  calls the addressed telephone corresponding to the telephone number included in the connection request signal IAM 2  from the call agent  13 . Simultaneously, the signaling transfer point  152  transmits an answer signal ACM (Address Complete Message) to the call agent  13  to notify the start of call of the addressed telephone (sequence g). 
   When the answer signal ACM is received from the signaling transfer point  152 , the call agent  13  transfers it to the signaling transfer point  142  (sequence h). This results in entering a state of calling the addressed telephone from the sender telephone. 
   When the addressed telephone answers the call, the signaling transfer point  152  transmits an answer signal ANM 1  (Answer Message) to the call agent  13  (sequence i). When the answer signal ANM 1  is received, the call agent  13  transmits a connection adjusting request signal MDCX including the session description to the media gateway  141  to change connection between the media gateways  141  and  151  to be interactive (sequence j). 
   The media gateway  141  changes the state of connection based on the session description included in the connection adjusting request signal MDCX. The media gateway  141  transmits an answer signal ACK 3  to the call agent  13  after changing the state of connection (sequence k). 
   Thereafter, the call agent  13  confirms that the interactive connection is established between the media gateways  141  and  151 , and transmits the answer signal ANM 2  to the signaling transfer point  142  (sequence l). 
   As mentioned above, this Internet telephone system enables the voice communication via the IP network between the sender telephone and addressed telephone (sequence m). 
   When an on-hook signal from the addressed telephone is detected, the signaling transfer point  142  transmits a communication line connection end signal REL 1  (Release Message) to the call agent  13  (sequence n). 
   When the communication line connection end signal REL 1  is received, the call agent  13  transmits a connection release request signal DLCX 2  to the media gateway  141  (sequence o). When the connection release request signal DCLX 2  is received, the media gateway  141  performs a connection releasing process and also transmits an answer signal ACK 4  including a connection parameter to the call agent  13  (sequence p). When the release of connection is confirmed by the reception of the answer signal ACK 4  from the media gateway  141 , the call agent  13  transmits a communication line release complete signal RLC 1  (Release Complete Message) to the signaling transfer point  142  (sequence q). Thereby, the signaling transfer point  142  checks the release of connection. 
   Next, the call agent  13  transmits the connection release request signal DLCX 2  to the media gateway  151  (sequence r). Similarly to the media gateway  141 , after connection releasing process, the media gateway  151  also transmits an answer signal ACK 5  including a connection parameter to the call agent  13  (sequence s). When the release of connection is confirmed by the reception of the answer signal ACK 5  from the media gateway  151 , the call agent  13  transmits a communication line connection end signal REL 2  to the signaling transfer point  152  (sequence t). When the release of connection is confirmed by the reception of the communication line connection end signal REL 2 , the signaling transfer point  152  transmits the communication line release complete signal RLC 2  to the call agent  13  (sequence u). 
   The above-mentioned processes result in releasing the connection between the message gateways  141  and  151 . 
   As will be apparently understood in the above description, an MGCP is used for controlling the VoIP gateways  14  and  15  from the outside. That is, the MGCP defines an interface between the call agent  13  and the media gateways  141  and  151 . By using the MGCP, the call agent  13  can control the media gateways  141  and  151  and also can notify the call agent  13  of an event from the media gateways  141  and  151 . 
   A description is given of an Internet telephone system according to a first embodiment of the present invention with reference to  FIGS. 3 to 7 . 
   As shown in  FIG. 3 , this Internet telephone system has a call agent (CA) or call control apparatus  31 . A control terminal  32  is connected to the call agent  31 . An Internet protocol (IP) network  33  includes a plurality of label switch routers (LSR)  34 ,  35 ,  36 ,  37 , and  38 . A plurality of voice networks  39  and  40  includes at least telephones (or voice terminals)  41  and  42 , respectively. The voice networks  39  and  40  are directly connected to the signaling transfer points  43  and  44 , respectively. The signaling transfer points  43  and  44  are connected to a public telephone network and are directly connected to media gateways (MG)  45  and  46 , respectively. The media gateways  45  and  46  are directly connected to label switch routers  34  and  36 , respectively. 
   The call agent  31  has an LSP control unit  311  and a VoIP packet control unit  312 . The call agent  31  controls a call of the IP network and, for example, is arranged in an exchange station of the public telephone network. The LSP control unit  311  stores a maximum band settable between adjacent label switch routers in an internal memory (not shown). The LSP control unit  311  stores the connection relationship between the signaling transfer points  43  and  44  and the media gateways  45  and  46  in the internal memory. The LSP control unit  311  stores the connection relationship between the telephones included in the voice networks  39  and  40  and the media gateways  45  and  46  in an internal memory (telephone number table) with correlation to telephone numbers of the telephones. Further, the LSP control unit  311  stores the connection relationship between the media gateways  45  and  46  and the label switch routers  34  and  36  in the internal memory. Information indicative of the connection relationships is supplied, for example, through a control terminal  32  by a maintenance person. 
   When connection request signals (off-hook signal and dial signal) are received from the signaling transfer points  43  and  44 , the LSP control unit  311  searches the internal memory and specifies the media gateway  45  or  46  connected to the signal transfer point  43  or  44  and the label switch router  34  or  36  connected thereto. The specified media gateway and label switch router are called a sender media gateway and a sender edge label switch router, respectively. The LSP control unit  311  searches the telephone number table, and specifies the media gateway  45  or  46  connected to the voice network  39  or  40  including the telephone corresponding to the addressed telephone number included in the connection request signal and the label switch router  34  or  36  connected thereto, respectively. The specified media gateway and label switch router are called an addressed media gateway and the addressed edge label switch router, respectively. The label switch routers  35 ,  37 , and  38  located between the sender edge label switch router and the addressed edge label switch router are called core label switch routers. The LSP control unit  311  notifies the VoIP packet control unit  312  of addresses (IP addresses) assigned to the sender and addressed edge media gateways  45  and  46 . 
   The LSP control unit  311  manages a label switching path (LSP) on the IP network. For example, the LSP control unit  311  controls the sender and addressed edge label switch routers  34  and  36  and confirms the label switching path which exists therebetween. The LSP control unit  311  provides a new label switching path between the sender and addressed label switch routers  34  and  36  if the necessity may arise. Each of the label switching paths to be set on the IP network has a wide band capable of accommodating a plurality of connections (e.g., approximately 100 to 1000 connections are preset). In other words, the label switching path has a band which is equal to or more than a plurality of times of a band necessary for transferring the VoIP packet. One label switching path can accommodate a plurality of connections and, therefore, the LSP control unit  311  needs no setting of label switching path each time a connection is established. As a consequence, the load of the overall IP network is reduced. 
   The label switching path may be provided in advance between any desired two label switch routers in the LSP control unit  311 , irrespective of the connection request signal. In this case, if no congestion is caused, the label switching path needs no control or no adjustment between the label switch routers. As mentioned above, because each label switching path can accommodate a large number of connections, e.g., 100 to 1000 connections. The label switching path to be preset may be provided across three or more label switch routers. The band of the label switching path may be changed in accordance with day of the week or time. 
   The VoIP packet control unit  312  notifies the addressed media gateway of a service port number assigned to the sender media gateway. The VoIP packet control unit  312  notifies the sender media gateway of a service port number assigned to the addressed media gateway. Further, the VoIP packet control unit  312  notifies the sender media gateway and addressed media gateway of a fact that the connection between the sender media gateway and the addressed media gateway is established. 
     FIG. 4  shows the relationship between the label switching path and VoIP connection. As shown in  FIG. 4 , when the VoIP connections  47  and  48  are established between the media gateways  45  and  46 , label switching paths  49  and  50  are sequentially established between the edge label switch routers  34  and  36 . The label switching paths  49  and  50  are established similarly to, for example, the sequences shown in  FIG. 2 . In each label switching path, one or a plurality of VoIP connections  47  and  48  are sequentially formed in the one-way direction, respectively. Incidentally, in the label switching paths  49  and  50 , the edge label switch routers  34  and  35  are connected for the purpose of the convenience of description. Actually, a label switching path is established between two adjacent routers. Therefore, a new label switching path may be established between the edge label switch routers  34  and  35 , and an existing label switching path may be used between the core label switch router  35  and edge label switch router  36 . 
   Referring back to  FIG. 3 , the sender media gateway and addressed media gateway answer the notification from the VoIP packet control unit  312 , and establishes a connection through the label switching path between the sender label switch router and the addressed label switch router. The sender and addressed media gateways convert analog/digital data and assemble/resolve a packet (or VoIP data) between the voice networks  39  and  40  and the IP network  33 . 
   Each label switch router has a route control unit (QoS routing protocol) and a label switching control unit (MPLS). When the route control unit calculates a route, information on communication quality is considered. In the IETF draft-querin-qos-routing-ospf-05.txt, the aforementioned route control unit is described. The label switching control unit uses an MPLS (Multi-Protocol Label Switching) technique to realize the assurance of band. In the IETF Draft-ietf-mpls arch-06.txt, the MPLS is described. 
   A band reservation system (RSVP: ReSerVation Protocol) and a service differentiating system (Diff-Serv: Differentiated Service) are known as techniques for ensuring the communication quality. The former technique is described in the IETF RFC2205, and the latter technique is described in the IETF RFC2475. However, the RSVP periodically optimizes a communication route and, therefore, the load is applied to the IP network. The Diff-Serv throws out the packet when the congestion is caused. Then, this Internet telephone system reduces the load of the IP network, and uses the MPLS so as to ensure the communication quality. 
   The sender edge label switch router calculates the shortest route to the addressed edge label switch router under the control operation of the LSP control unit  311 . In this case, as the shortest route, a route is selected to ensure the band equal to “band requested by information on the communication quality×predetermined number” (=band of label switching path). If there has already been label switching path in the calculated shortest route, the sender edge label switch router determines whether or not a connection can be established. In other words, the sender edge router determines whether or not a band necessary for transferring the VoIP packet remains in the existing label switching path. If the connection can be established, the sender edge label switch router does not establish a new label switching path. If there is not a label switching path on the shortest route and if there is no residual band for establishing the connection in existing the label switching path, the sender edge label switch router establishes a new label switching path on the shortest route. 
   The addressed edge label switch router operates in the same manner as that of the sender edge label switch router, and establishes the label switching path to obtain the shortest route to the sender edge label switch router if the necessity may arise. 
   Each label switch router located on the label switching path transfers the VoIP data as if it flowed on the label switching path by using information which is called a label. Similarly to a CSR (Cell Switch Router) used for an asynchronous transfer mode (ATM) network, the label switch router can transfer the IP packet (i.e., fast transfer), without performing the IP process. 
   A description is given of the operation of an Internet telephone system in  FIG. 3  hereinlater with reference to  FIG. 5 . Herein, assume that the telephone  41  is a sender telephone and the telephone  42  is an addressed telephone. 
   To start with, the sender telephone  41  calls the call agent  31  and notifies the call agent  31  of the telephone number of the addressed telephone  42 . This notification is transmitted to the LSP control unit  311  of the call agent  31  as the connection request signal IAM by the signaling transfer point  43  connected to the network  39  including the sender telephone  41 . The connection request signal IAM includes a telephone number of the sender telephone  41 , telephone number of the addressed telephone  42 , and information that the transmission (and band assurance) of the VoIP packet is desired. 
   When the connection request signal IAM is received, the LSP control unit  311  specifies the sender signaling transfer point  43  which transmits the connection request signal IAM. The LSP control unit  311  searches the internal memory, and specifies the sender media gateway  45  connected to the sender signaling transfer point  43  and the sender edge label switch router  34  connected to the sender media gateway  45 . Also, the LSP control unit  311  specifies the addressed signaling transfer point  44  connected to the voice network  40  including the addressed telephone  42 , the addressed media gateway  46  connected to the addressed signaling transfer point  44 , and the addressed edge label switch router  36  connected to the addressed media gateway  46 , based on the addressed telephone number (step A 1 ). 
   The LSP control unit  311  determines whether or not the sender media gateway  45  and addressed media gateway  46  are specified in step A 1  (step A 2 ). If at least one of the sender media gateway  45  and addressed media gateway  46  is not specified, the LSP control unit  311  cancels processes after that. If both the sender media gateway  45  and addressed media gateway  46  are specified, IP addresses of the specified sender media gateway  45  and addressed media gateway  46  are notified to the VoIP packet control unit  312 . 
   When the notification of the IP address is received from the LSP control unit  311 , the VoIP packet control unit  312  requests to issue a call to the sender media gateway  45  (step A 3 ). When the request to issue a call is received from the VoIP packet control unit  312 , the sender media gateway  45  notifies the VoIP packet control unit  312  of a service port number of the sender media gateway  45 . 
   When the notification of the service port number of the sender media gateway  45  is received from the sender media gateway  45 , the VoIP packet control unit  312  notifies the addressed media gateway  46  of the IP address and service port number of the sender media gateway  45  and requests to issue a call (steps A 4  and A 5 ). When the notification of the service port number of the sender media gateway  45  is not received from the sender media gateway  45 , the VoIP packet control unit  312  performs a cancel process in step A 16  and cancels processes after that. 
   The addressed media gateway  46  notifies the VoIP packet control unit  312  of the service port number of the addressed media gateway  46  in accordance with the request to issue a call from the VoIP packet control unit  312 . When the notification of the service port number is not received from the addressed media gateway  46 , the VoIP packet control unit  312  performs a cancel process in step A 16  and cancels processes after that (step A 6 ). 
   Next, the LSP control unit  311  checks whether or not the label switching path has already been established between the edge label switch routers  34  and  36  (step A 7 ). If the label switching path has not been established yet between the edge label switch routers  34  and  36 , the LSP control unit  311  requests the addressed edge label switch router  36  to establish the label switching path to the sender edge label switch router  34  (steps A 8  and A 9 ). 
   The addressed edge switching router  36  tries to establish the label switching path to the sender edge label switch router  34  and notifies the LSP control unit  311  of the result (step A 10 ). If the addressed edge label switch router  36  establishes the label switching path to the sender edge label switch router  34 , the LSP control unit  311  requests the sender edge label switch router  34  to establish the label switching path to the addressed edge label switch router  36  (step A 11 ). If the addressed edge label switch router  36  does not establish the label switching path to the sender edge label switch router  34 , the LSP control unit  311  performs a cancel process in step A 16  and cancels processes after that. 
   The sender edge label switch router  34  tries to establish the label switching path to the addressed edge label switch router  36  and notifies the LSP control unit  311  of the result (step A 12 ). If the sender edge label switch router  34  does not establish the label switching path to the addressed edge label switch router  36 , the LSP control unit  311  performs the cancel process in step A 16  and cancels processes after that. 
   If the sender edge label switch router  34  establishes the label switching path to the addressed edge label switch router  36 , the LSP control unit  311  notifies the VoIP packet control unit  312  of such a fact. The VoIP packet control unit  312  controls the media gateway  46  and transmits an incoming call address and the IP address of the sender telephone to the addressed telephone  42 . When it is detected that the addressed telephone  42  is hooked off, the media gateway  46  notifies the VoIP packet control unit  312  of such a fact. Thereafter, the VoIP control unit  312  notifies the sender media gateway  45  of the IP address and service port number of the addressed media gateway  46  and requests to change the connection of call (step A 17 ). 
   Thereafter, the voice communication via the IP network becomes possible between the sender telephone  41  and addressed telephone  42 . That is, the media gateway  45  converts a voice signal from the telephone  41  into the VoIP packet, adds a header including the IP address and service port number of the media gateway  46 , and transmits the VoIP packet added with the header to the media gateway  46 . Also, the media gateway  46  converts a voice signal from the telephone  42  into the VoIP packet, adds a header including the IP address and service port number of the media gateway  45 , and transmits the VoIP packet added with the header to the media gateway  45 . When the VoIP packet to be addressed to the telephone  41  is received, the media gateway  45  converts the received VoIP packet into a voice signal and transmits it to the telephone  41 . Also, when the VoIP packet to be addressed to the telephone  42  is received, the media gateway  46  converts the received VoIP packet into a voice signal and transmits it to the telephone  42 . 
   On the other hand, if the LSP control unit  311  detects that there has already been a label switching path between the edge label switch routers  34  and  36  in step A 8 , the LSP control unit  311  checks whether or not a new connection can be established in the detected label switching path (step A 13 ). That is, the LSP control unit  311  checks whether or not there is an unused band necessary for the voice communication and whether or not the band necessary for the voice communication can be ensured in the detected label switching path. 
   When the LSP control unit  311  determines that a new connection cannot be established in the detected label switching path, the processing routine advances to step A 9  from step A 14 . When the LPS control unit  311  determines that a new connection can be established in the detected label switching path, the LPS control unit  311  requests the edge label switch routers  34  and  36  to establish a new connection in the detected label switching path. In other words, the LSP control unit  311  requests the edge label switch routers  34  and  36  to flow the VoIP data packet between the edge label switch routers  34  and  36  via the detected label switching path (step A 15 ). 
   After that, as mentioned above, the voice communication via the IP network is possible between the sender telephone  41  and the addressed telephone  42 . 
   Next, referring to  FIG. 6 , a description is given of the operation of the label switching control unit MPLS of each label switch router. 
   The edge label switch router  34  adds a label to the IP data packet to be transmitted from the media gateway  45 . Value of the label depends on a route through which the IP packet passes (label switching path). If all of the IP packets are transmitted to the media gateway  46  via the same label switching path, the same label (such as L=1) is added to the all of the IP packets (process  1 ). 
   The core label switch router  35  transfers the IP packet which is transmitted by the edge label switch router  34  in accordance with the label which is added to the transmitted to IP packet (input label). In this case, the core label switch router  35  exchanges the input signal to another label signal (output label). Because the core label switch router  35  stores the relationship between the input label and the transfer destination (and output label) in the internal memory, the IP packet can be transferred at a high speed on the basis of only the input label without referring to the header of the IP packet. Only if the label which is added to the received IP packet is not stored in the internal memory, the core label switch router  35  recognizes the transfer destination by referring to the header of the IP packet. Also, the core label switch router  35  assigns a new output label and stores the relationship between the input label and the output label. Herein, the input label (L=1) is added to the all of the IP packets and, therefore, the output label (L=2) is added in place of the input label, thereby transferring the IP packet with the added output label to the edge label switch router  36  (process  2 ). 
   If the label which is added to the received IP packet indicates the transfer to the media gateway  46 , the edge label switch router  36  removes the label from the IP packet and transfers only the IP packet to the media gateway  46  (process  3 ). 
   The transfer of packet to the media gateway  45  from the media gateway  46  is executed in the manner similar to the foregoing. 
   In this Internet telephone system, the band of connection which has been established once is ensured, and the delay in transfer of the packet is decreased by using the label switch router. Further, in the Internet telephone system, the setting of the path is not necessary every connection because the band of label switching path is set to be large. Accordingly, the number of negotiation between the routers is decreased and the setting of path is possible in a short time in the Internet telephone system. 
   Note that the program in which the above-mentioned operation is described is supplied to an electric computer, thereby realizing the call agent, MPLS server, and label switch router. 
   Next, a description is given of an Internet telephone system according to a second embodiment of the present invention with reference to  FIG. 7 . Points different from those of the first embodiment will be mainly described hereinbelow. 
   Referring to  FIG. 7 , the Internet telephone system comprises a call agent (CA) or call control apparatus  71 , an MPLS server  72  of the label switching system (MPL), an IP network  80  including a plurality of label switch routers (LSR)  73 - 79 , and media gateways  45  and  46 . The label switch routers  73  and  74  are called edge label switch routers while the label switch routers  75 - 79  are called core label switch routers. 
   The media gateway  45  is directly connected to the label switch router  73 , and the media gateway  46  is directly connected to the label switch router  74 . 
   The call agent  71  has a VoIP packet control unit  312 , and manages a connection between the media gateways  45  and  46 . If the assurance of band is necessary when the connection is established between the media gateways  45  and  46 , the call agent  71  requests the LSP control unit  311  of the MPLS server  72  to form the label switching path between the edge label switch router  73  and  74 . 
   The MPLS server comprises the LSP control unit  311  and a route control unit (QoS routing protocol)  721 . The LSP control unit  311  answers the inquiry of an address from the VoIP packet control unit  312 , and establishes and manages the label switching path. The route control unit  721  performs routing together with the edge label switch routers  73  and  74  and core label switch routers  75  to  79 . The LSP control unit  311  sets the route when the route control unit  721  operates. 
   The route control unit  721  is connected to all of the label switch routers  73 - 79 , and collectively manages the label used on the IP network. Consequently, each label switch router needs no management of the label. That is, the negotiation between the routers becomes unnecessary. Since the route control unit  721  manages the labels in a lump, it is possible to transfer the packets from the sender edge label switch router to the addressed edge label switch router by using a single label. In other words, the exchange of label is unnecessary in each label switch router and, therefore, the packet can be transferred at a higher speed. 
   In this Internet telephone system, the label switching path is established in the same manner as that of the Internet telephone system according to the first embodiment. Thereafter, similarly to the case of the Internet telephone system according to the first embodiment, the voice communication is possible between the sender telephone and the addressed telephone in this Internet telephone system. In this case, each label switch router refers to label information which is stored in the route control unit  721 , and executes the transfer of the VoIP packet. 
   Although the embodiments of the present invention are described in the foregoing, the present invention is not limited to the above embodiments and can variously be modified without departing the range of the spirit. For example, it is possible to use a personal computer in which an Internet telephone program is installed, instead of the telephone. It is also possible to use a telephone having an automatic answering function such as an answering telephone. In this case, the telephone having an automatic answering function executes operations such as recording, reproducing, and transfer of a voice signal. The IP network is not limited to the Internet, and a LAN and WAN using an Internet protocol may be employed.