JP2012209839A - Session management system, number-of-sessions securing method, and number-of-sessions securing program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a session management system capable of changing the number of available sessions dynamically depending on a utilization situation of network resources.SOLUTION: On receipt of a session connection request from an originating terminal 100, a session relay device 110 mediating a communication session between communication terminals inquires of a band management and acceptance device 120 managing the network resources of the network and controlling acceptance whether or not the session connection request can be accepted to acquire a utilization situation of the network resources of the network. Based on the acquired utilization situation of the network resources of the network, the session relay device 110 re-edits information related to at least one of a codec or a band included in session information on the session connection request from the originating terminal 100 into connectable one by the network, and transmits the re-edited session connection request to a destination terminal 101 designated as a partner of the connection request.

Description

  The present invention relates to a session management system, a session number securing method, and a session number securing program, and more particularly to a session management system and a session number securing method that can reliably secure the number of sessions during network congestion in a network that guarantees a communication bandwidth. The present invention relates to a method and a session number securing program.

  As an actual example of a session management system that guarantees a bandwidth used for communication between communication terminals, an architecture of NGN (Next Generation Network) as shown in Non-Patent Document 1, which is an architecture of a QoS communication infrastructure that is being internationally standardized There is.

  In this NGN architecture, session management of multimedia services is realized using SIP (Session Initiation Protocol) proposed by IETF (Internet Engineering Task Force), and QoS management functions such as bandwidth guarantee are not disclosed in Non-Patent Document 2. This is realized in cooperation with RACS (Resource and Admission Control Subsystem) shown in FIG.

"Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN); NGN Functional Architecture", ETSI ES 282 001 Ver.3.4.1 p.p.9-31 "Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN); Resource and Admission Control Sub-System (RACS): Functional Architecture", ETSI ES 282 003 Ver.2.0.0 p.p.15-79

  Generally, in circuit-switched voice communication and data communication, since a line is secured for each session, a predetermined communication quality can be secured. However, while remaining communication resources are generated for each line, the number of sessions that can be connected is limited by the number of lines, and the surplus of resources cannot be used effectively.

  On the other hand, in voice communication and data communication based on the packet switching method, although there is no upper limit on the number of sessions, communication cannot be performed in a state where quality is ensured / guaranteed due to packet transfer delay, etc. The communication quality in each session deteriorates according to the number and communication traffic.

  As a method for solving these problems, a session management system that performs bandwidth management and admission control such as NGN described above guarantees a predetermined communication quality required for each session based on a packet switching method. Yes.

  However, since the bandwidth used in each corresponding session is set only according to the negotiation between communication terminals, it does not reflect the network congestion status, so the situation when network congestion or congestion is expected Even in the lower case, bandwidth requests similar to those in the normal time are accumulated in each session request, and the number of sessions cannot be secured sufficiently.

  In addition, the session management system that performs bandwidth management / acceptance control such as NGN described above is based on the packet switching method, but the upper limit of the number of sessions is effectively determined statically according to the plan. Therefore, the number of sessions cannot be dynamically secured according to the use status of network resources.

(Object of the present invention)
The present invention has been made in view of such circumstances, and it is possible to dynamically change the upper limit of the number of sessions according to the use state of network resources by a trade-off between communication quality of each session and guaranteed bandwidth. An object of the present invention is to provide a secure session management system, a session number securing method, and a session number securing program.

  In order to solve the above-described problems, the session management system, the session number securing method, and the session number securing program according to the present invention mainly adopt the following characteristic configuration.

  (1) A session management system according to the present invention manages a communication session via a network between a calling terminal and a called terminal, and guarantees a communication band. In the session management system, the calling terminal and the called terminal are connected to the network. A session relay device that mediates a communication session with a terminal, and a bandwidth management / acceptance device that manages the use of network resources of the network and controls reception of a communication session, and the session relay device includes: When the calling terminal receives a session connection request for the called terminal, it obtains the network resource usage status of the network by inquiring of the bandwidth management / acceptance device whether or not the session connection request can be accepted. , Network resource usage status of the acquired network Based on the information related to at least one of the codec and the band included in the session information of the session connection request received from the calling terminal, the information related to at least one of the codec and the band that can be connected to the network is re-edited. A session connection request is transmitted to the destination terminal specified as the destination of the connection request.

  (2) A session number securing method according to the present invention is a session number securing method for managing a communication session via a network between a calling terminal and a called terminal and securing the number of sessions in a session management system that guarantees a communication bandwidth. The network management unit manages a session relay device that mediates a communication session between the calling terminal and the called terminal in the network, and a bandwidth management that manages the use of network resources of the network and controls reception of the communication session. The session relay device inquires of the bandwidth management / acceptance device whether or not the session connection request can be accepted when the calling terminal receives a session connection request to the destination terminal. To obtain the network resource usage status of the network Based on the use status of network resources of the network, information on at least one of codec and band included in the session information of the session connection request received from the calling terminal A session connection request that has been re-edited into information relating to at least one of the information is transmitted to the destination terminal that is designated as the destination of the connection request.

  (3) The session number securing program according to the present invention is characterized in that, in at least the session number securing method described in (2), the processing in the session relay device is implemented as a program executable by a computer. .

  According to the session management system, the session number securing method, and the session number securing program of the present invention, the following effects can be obtained.

  The number of connectable sessions can be increased by a trade-off between communication quality and guaranteed bandwidth according to the use status of network resources. Particularly important is the control to increase the number of sessions that can be connected in a network congestion situation due to a trade-off between communication quality and guaranteed bandwidth, and a session relay device that mediates sessions between communication terminals. Since it can be realized only by intervening in the session negotiation, it can not only realize negotiation between communication terminals reflecting the utilization status of network resources, but also control to increase the number of sessions that can be connected in the network congestion situation Can be realized by a negotiation procedure that is completely different from the normal session negotiation in which congestion does not occur as the communication terminal of the calling terminal and the called terminal.

It is a system configuration figure showing an example of a system configuration of a session management system by the present invention. It is a sequence diagram for demonstrating an example of operation | movement of the session management system shown in FIG. FIG. 2 is an explanatory diagram for explaining a registration example in which a network congestion state is set and registered in advance in the session management system shown in FIG. 1.

  Preferred embodiments of a session management system, a session number securing method, and a session number securing program according to the present invention will be described below with reference to the accompanying drawings. In the following description, the session management system and the session number securing method according to the present invention will be described. However, the session number securing method may be implemented as a session number securing program executable by a computer, or Needless to say, the program for securing the number of sessions may be recorded on a computer-readable recording medium.

(Features of the present invention)
Prior to the description of the embodiments of the present invention, an outline of the features of the present invention will be described first. The present invention relates to a session management system that manages network resources and guarantees a band used for communication between communication terminals. It is important that the number of sessions that can guarantee bandwidth can be increased and secured even when network congestion occurs, by controlling so as to reduce the bandwidth that is accepted and secured. Features.

  More specifically, it is as follows. A session management system according to the present invention includes two communication terminals that form a communication session via a network, a session relay device that mediates a network session, a bandwidth management / acceptance device that performs bandwidth management / acceptance control, packet priority control, It is configured to include at least a switch for performing transfer, that is, a packet transfer device.

  Upon receiving an instruction from a maintenance person who has predicted network congestion or an autonomous notification regarding network congestion from the bandwidth management / acceptance device, the session relay device is aware that the network is in a congested state.

  In a normal case where the network is not congested, the session relay device that accepts the session connection request from the communication terminal determines whether or not the communication bandwidth required for the network can be secured. It manages the usage status, inquires the bandwidth management / acceptance device that controls the acceptance of the communication session, reserves the communication bandwidth, and forwards the session connection request from the communication terminal to the opposite communication terminal.

  On the other hand, after receiving a notification of network congestion from the maintenance person or from the bandwidth management / acceptance device, the session relay device receives the session connection request from the communication terminal, and receives the network bandwidth usage status and congestion status. Accordingly, a communication band lower than the request from the communication terminal is reserved, and a session connection request using the communication band lower than the request is made to the opposite communication terminal. Therefore, when establishing a communication session with the opposing communication terminal, it is intended to establish a communication session using a communication band lower than the request for the originating communication terminal that is the request source of the session connection request. A response will be sent back.

  As described above, when a network congestion situation occurs, or when the occurrence of a network congestion situation is predicted, the session relay device interrupts the session negotiation between communication terminals, thereby causing congestion. Encourage the establishment of a communication session in a communication band lower than the normal time when no occurrence occurs. Thus, when a communication session is established, each communication terminal performs an exchange in a lower communication band established by session negotiation. It will be secured. In addition, since the communication bandwidth of each communication terminal required in the network during network congestion can be kept lower than in the normal case, a larger number of sessions for the same network resource than in the normal case Can be secured.

(Configuration example of embodiment)
Next, an example of the system configuration of the session management system according to the present invention will be described with reference to FIG. FIG. 1 is a system configuration diagram showing an example of a system configuration of a session management system according to the present invention.

  The session management system shown in FIG. 1 includes at least a calling terminal 100, a called terminal 101, a session relay device 110, a bandwidth management / acceptance device 120, and packet transfer devices 130 and 131.

  The calling terminal 100 is a communication terminal that transmits a session request for packet communication, and the called terminal 101 is a communication terminal that receives a session request from the calling terminal 100, and between the calling terminal 100 and the called terminal 101, Packet communication is performed using a communication session in which bandwidth is guaranteed as a network.

  The session relay device 110 is a server having a function of mediating a session between communication terminals in communication between the calling terminal 100 and the called terminal 101, and the bandwidth management / accepting device 120 checks the network congestion state. A server for monitoring and managing network bandwidth and performing communication session admission control. The packet transfer apparatuses 130 and 131 perform bandwidth management / acceptance in communication between the calling terminal 100 and the called terminal 101. This is a switch for performing priority control / transfer of packets in a state where a communication band is secured based on control from the device 120.

  For example, the session management system shown in FIG. 1 performs the following operation. The two terminals, the calling terminal 100 and the called terminal 101, are mediated by the session relay device 110 when starting packet communication, and perform a procedure for establishing a session. At this time, the two terminals, that is, the calling terminal 100 and the called terminal 101 negotiate at least one of the codec used in packet communication or the communication band, and the session relay apparatus 110 uses the codec and request that are going to be used for packet communication. For the bandwidth, by making an inquiry to the bandwidth management / acceptance device 120, accept at least one of the codec or the communication bandwidth that is going to be used for packet communication between the two terminals of the originating terminal 100 and the terminating terminal 101, It is determined whether a communication session can be established.

  If the bandwidth management / acceptance device 120 determines that the communication session can be established by accepting the codec or communication bandwidth to be used for packet communication between the originating terminal 100 and the terminating terminal 101, the bandwidth The management / acceptance device 120 counts information related to the codec and communication band to be used for packet communication between the two terminals of the calling terminal 100 and the called terminal 101 in bandwidth management, Control is performed to allocate a communication band related to the communication session to the packet transfer apparatuses 130 and 131 that perform the packet transfer operation between the two terminals.

  That is, the bandwidth management / acceptance device 120 manages the use status of network resources, and in the exchange with the session relay device 110, that is, for packet communication between the two communication terminals of the calling terminal 100 and the called terminal 101. When receiving an inquiry from the session relay device 110 regarding the codec or communication band to be used, the session of packet communication between the two terminals, the source terminal 100 and the destination terminal 101, reflecting the use status of the network resource A response relating to the possibility of establishment is sent to the session relay device 110.

(Description of operation example of embodiment)
Next, an example of the operation of the session management system shown in FIG. 1 will be described in more detail with reference to FIG. FIG. 2 is a sequence diagram for explaining an example of the operation of the session management system shown in FIG. 1, and performs a negotiation operation for establishing a communication session between two communication terminals, that is, the calling terminal 100 and the called terminal 101. In this case, an example of the flow of control signals transmitted / received among the calling terminal 100, the session relay device 110, the bandwidth management / acceptance device 120, and the destination terminal 101 is shown.

  First, the calling terminal 100 transmits a session connection request for the called terminal 101 to the session relay apparatus 110 (sequence S101). This session connection request includes at least session information including a codec, a communication band, and the like that can be used for communication by the calling terminal 100 as well as destination information for designating the destination terminal 101 that is a communication partner.

  The session relay device 110 that has received the session connection request from the calling terminal 100 determines whether to accept the bandwidth management / acceptance device 120 based on the session information including the codec and communication band included in the session connection request. A request / bandwidth reservation request is created and transmitted to the bandwidth management / acceptance device 120 (sequence S102).

  In the bandwidth management / acceptance device 120 that has received the acceptance determination request / bandwidth reservation request from the session relay device 110, it is included in the reception determination request / bandwidth reservation request according to the network bandwidth usage status and congestion status. It accepts session information consisting of a codec, a communication band, etc. as it is, determines whether or not to allow establishment of a communication session, and if so, The bandwidth is reserved and registered as a network resource in the packet transfer apparatuses 130 and 131 that perform the transfer, and an acceptance determination response / bandwidth reservation response indicating that is returned to the session relay apparatus 110 that has made the inquiry (sequence S103). .

  On the other hand, if it is determined that session establishment using session information including the codec and communication band included in the reception determination request / bandwidth reservation request is impossible, the calling terminal 100 and the receiving terminal 101 As network resources in the packet transfer devices 130 and 131 that transfer packets between two terminals, based on the bandwidth usage status and congestion status of the network, among the session information included in the acceptance determination request / bandwidth reservation request, Exclude the band that cannot be permitted, perform reservation registration for the lower band that can be permitted (that is, the band where the communication quality deteriorates), and send an acceptance determination response / bandwidth reservation response indicating that to the inquiry source To the session relay device 110 (sequence S103).

  In the session relay device 110 that has received the acceptance determination response / bandwidth reservation response from the bandwidth management / acceptance device 120, an appropriate reflection of the bandwidth usage status and congestion status of the network is performed based on the reception determination response / bandwidth reservation response. The session connection request is re-edited into the form extracted from the session information of the session connection request accepted from the calling terminal 100, and transmitted to the destination terminal 101 (sequence). S104). That is, the session relay device 110 includes a lower rate codec and a lower bandwidth according to the network bandwidth usage status and the congestion status in the session information included in the session connection request received from the calling terminal 100. Select only one and re-edit it to a session connection request that includes the selected low-rate codec and session information focused on low bandwidth.

  In the receiving terminal 101 that has received the session connection request transferred from the session relay device 110, the packet with the calling terminal 100 is the same as in the normal session connection request reception process, regardless of whether the network is in a congested state. Negotiation operation related to a communication session for communication is performed. That is, the destination terminal 101 determines whether or not packet communication with the calling terminal 100 is possible based on the session information included in the received session connection request, that is, whether or not the session connection request can be accepted. When the session connection request can be accepted, a session connection response including session information including at least one of the codec and the band selected for use in communication with the calling terminal 100 is created. When the reception is impossible, a session connection response indicating that the session connection request is rejected is generated and transmitted to the session relay device 110 (sequence S105).

  When the session relay apparatus 110 receives a session connection response from the destination terminal 101, the session relay apparatus 110 determines whether or not the destination terminal 101 has accepted the session connection request. If the session relay apparatus 110 accepts the session connection response, it is included in the session connection response. Based on the session information, the codec and communication band to be used for packet communication between the two terminals of the calling terminal 100 and the called terminal 101 are determined, and it is required to secure and guarantee the determined communication band as a network A bandwidth reservation request to be transmitted is transmitted to the bandwidth management / acceptance device 120 (sequence S106).

  In the bandwidth management / acceptance device 120 that has received the bandwidth securing request from the session relay device 110, the bandwidth is transferred in the packet transfer devices 130 and 131 that transfer packets between the two terminals, that is, the originating terminal 100 and the terminating terminal 101. The bandwidth requested by the securing request is secured, and the bandwidth reserved previously is released. Thereafter, the bandwidth management / acceptance device 120 sends a bandwidth reservation response indicating that the network resource used for packet communication between the two terminals of the calling terminal 100 and the called terminal 101 is reserved to the session relay device 110 that is the request source. In response, the message is returned (sequence S107).

  In the session relay device 110 that has received the bandwidth securing response from the bandwidth management / accepting device 120, based on the bandwidth securing response, as a response to the session connection request from the originating terminal 100, the originating terminal 100 and the terminating terminal 101 A session connection response indicating that the network resource used for packet communication between the two terminals has been secured is created and transmitted to the calling terminal 100 (sequence S108).

  The originating terminal 100 that has received the session connection response from the session relay device 110 performs packet communication with the destination terminal 101 using the session information such as codec and bandwidth included in the session connection response as the negotiation result. Recognizing that the establishment of the communication session for performing is completed, packet communication is performed using the established communication session.

  As described above, when the session relay apparatus 110 interrupts the session negotiation between the two terminals of the calling terminal 100 and the called terminal 101 and the network congestion occurs, the communication is lower than the normal case where there is no congestion. It will automatically prompt the establishment of a communication session in the band. Therefore, when a communication session is established, each terminal of the calling terminal 100 and the called terminal 101 performs packet communication using a lower communication band that is established in the session negotiation and whose bandwidth is guaranteed. Therefore, the communication quality conscious of each terminal of the calling terminal 100 and the called terminal 101 is secured. In addition, when the network is congested, the bandwidth required for the communication between the calling terminal 100 and the called terminal 101 in the network can be reduced as compared with the normal time when no congestion occurs. On the other hand, it is possible to secure a larger number of sessions than in the normal case.

  More specifically, by using the procedure described with reference to FIG. 2, it is possible to reflect the network bandwidth usage status and the congestion status in the session negotiation between the two communication terminals of the calling terminal 100 and the called terminal 101. Even during network congestion, a larger number of sessions can be secured by a trade-off between communication quality such as codec and bandwidth and guaranteed bandwidth. Further, in this procedure as shown in FIG. 2, the session relay apparatus 110 is merely involved in the negotiation between the communication terminals of the calling terminal 100 and the called terminal 101. Essentially, the calling terminal 100, the called terminal, A communication session that is similar to the establishment of a communication session based on a negotiation result between terminals of the terminal 101, and that uses at least one of a codec or a band permitted by the band management / acceptance device 120 according to the use state of network resources. As a result, the communication quality agreed between the terminals of the calling terminal 100 and the called terminal 101 is finally guaranteed.

  In the above description, as described in the sequences S102 and S103, the bandwidth management / accepting device 120 autonomously determines the network congestion status based on the management state of the network resource. However, the present invention is not limited to such a case. For example, as shown in FIG. 3, the present invention may be set in advance using information indicating that a network congestion state has been predicted. FIG. 3 is an explanatory diagram for explaining a registration example in which the network congestion state is set and registered in advance in the session management system shown in FIG.

  In FIG. 3A, the network administrator predicts the congestion state of the network in advance, and transmits it as a congestion management request to the bandwidth management / acceptance device 120 (sequence S201). The accepting device 120 shows a case where the network congestion status is set and registered in advance according to the information specified in the congestion management request.

  In FIG. 3B, the network administrator predicts the network congestion state in advance and transmits it as a congestion management request to the session relay device 110 instead of the bandwidth management / acceptance device 120. (Sequence S202) shows a case where the session relay apparatus 110 sets and registers the network congestion status in advance according to the information specified in the congestion management request. Therefore, the session relay device 110 is included in the session connection request from the calling terminal 100 when creating the acceptance determination request / bandwidth reservation request to be transmitted to the bandwidth management / acceptance device 120 in the sequence S102 of FIG. Session information consisting of the current codec and communication band, etc., is appropriately edited to session information consisting of codec and communication band that can be used according to network congestion conditions, and created as an acceptance determination request / bandwidth reservation request It is also possible to make it.

  Further, in FIG. 3C, when the bandwidth management / acceptance device 120 predicts the congestion status of the network based on the management state of the network resource, the session management relay 120 sends a session relay as a congestion management request. By transmitting to the apparatus 110 (sequence S203), the session relay apparatus 110 also shows a case where the network congestion state is set and registered in advance according to the information specified in the congestion management request. Even in such a case, as in the case of FIG. 3B, the session relay device 110 creates an acceptance determination request / bandwidth reservation request to be transmitted to the bandwidth management / acceptance device 120 in the sequence S102 of FIG. In addition, the session information including the codec and the communication band included in the session connection request from the calling terminal 100 is appropriately edited to the session information including the codec and the communication band that can be used according to the network congestion state. Thus, it can be created as an acceptance determination request / bandwidth reservation request.

  In the system configuration example of FIG. 1, there are one session relay device 110 and one bandwidth management / acceptance device 120 interposed between the calling terminal 100 and the called terminal 101, but a plurality of session relay devices, A configuration in which a bandwidth management / acceptance device is interposed may be employed. In such a case, for example, when the session relay device accepts a session connection request transferred from another session relay device, the session relay device is handled in the same manner as when a session connection request from the calling terminal 100 is accepted. The procedure similar to the procedure described above in FIG. 5 is performed, and as a session relay device that receives a session connection request from the calling terminal 100, an operation of selecting a session relay device that is closest to the calling terminal 100 is added. An operation of selecting a bandwidth management / acceptance device that manages the packet transfer devices 130 and 131 that transfer packets between the originating terminal 100 and the destination terminal 101 from among a plurality of bandwidth management / acceptance devices is added. In this way, it can be operated in exactly the same manner as in the above-described embodiment.

(Explanation of effect of embodiment)
As described in detail above, according to the present embodiment, the following effects can be obtained.

  The number of connectable sessions can be increased by a trade-off between communication quality and guaranteed bandwidth according to the use status of network resources. In particular, the important point is that the control for increasing the number of sessions that can be connected in a network congestion situation by the trade-off between the communication quality and the guaranteed bandwidth is performed between the communication terminals by the session relay device 110 that mediates the session between the communication terminals. It can be realized simply by intervening in the session negotiation of the network, so it is possible not only to realize negotiation between communication terminals reflecting the utilization status of network resources, but also to increase the number of sessions that can be connected in the network congestion situation Control can be realized by a negotiation procedure that is completely different from the normal session negotiation in which congestion does not occur for the communication terminals of the calling terminal 100 and the called terminal 101.

  The configuration of the preferred embodiment of the present invention has been described above. However, it should be noted that such embodiments are merely examples of the present invention and do not limit the present invention in any way. Those skilled in the art will readily understand that various modifications and changes can be made according to a specific application without departing from the gist of the present invention.

100 source terminal 101 destination terminal 110 session relay device 120 bandwidth management / acceptance device 130 packet transfer device 131 packet transfer device

Claims (9)

  In a session management system that manages a communication session between a calling terminal and a called terminal via a network and guarantees a communication band, a session that mediates a communication session between the calling terminal and the called terminal to the network A relay device and a bandwidth management / acceptance device that manages network resource usage status of the network and controls reception of a communication session, wherein the session relay device sends a session connection request to the destination terminal by the calling terminal. Upon receipt, the bandwidth management / acceptance device is inquired whether the session connection request can be accepted, thereby obtaining the network resource usage status of the network, and the obtained network resource usage status of the network. Based on the set received from the calling terminal. The connection request partner as a session connection request that has been re-edited into information related to at least one of the codec and / or bandwidth that can be connected to the network. A session management system which transmits to the destination terminal specified as a destination.   When the session relay device recognizes that the network is in a congested state as the network resource usage status of the network acquired from the bandwidth management / acceptance device, the session connection request session received from the calling terminal The session management according to claim 1, wherein information on at least one of a codec and a band included in the information is re-edited to a lower-rate codec and a lower-rate band that can be connected to the network. system.   When the bandwidth management / accepting device or the session relay device receives, as a congestion management request, information predicting the congestion state of the network from a network administrator, the bandwidth management / accepting device or the session relay device indicates in advance the congestion status of the network specified by the congestion management request. 3. The session management system according to claim 1, wherein setting registration is performed.   When the session relay apparatus receives information predicting the congestion state of the network from the bandwidth management / acceptance apparatus as a congestion management request, the session relay apparatus sets and registers in advance the congestion state of the network specified by the congestion management request. The session management system according to any one of claims 1 to 3.   A session number securing method for managing a communication session via a network between a calling terminal and a called terminal and securing a number of sessions in a session management system that guarantees a communication band, the network including the calling terminal and the A session relay device that mediates a communication session with a destination terminal, and a bandwidth management / acceptance device that manages the use of network resources of the network and controls reception of a communication session, the session relay device, When the calling terminal receives a session connection request for the called terminal, it obtains the network resource usage status of the network by inquiring of the bandwidth management / acceptance device whether or not the session connection request can be accepted. The network resources of the acquired network Based on the situation, the information regarding at least one of the codec and the band included in the session information of the session connection request received from the calling terminal is edited into the information regarding at least one of the codec and the band that can be connected to the network. A method for securing the number of sessions, characterized in that the corrected session connection request is transmitted to the destination terminal specified as the destination of the connection request.   When the session relay device recognizes that the network is in a congested state as the network resource usage status of the network acquired from the bandwidth management / acceptance device, the session connection request session received from the calling terminal 6. The number of sessions according to claim 5, wherein information on at least one of a codec and a band included in the information is re-edited to a lower-rate codec and a lower-rate band connectable with the network. How to secure.   When the bandwidth management / accepting device or the session relay device receives, as a congestion management request, information predicting the congestion state of the network from a network administrator, the bandwidth management / accepting device or the session relay device indicates in advance the congestion status of the network specified by the congestion management request. 7. The method for securing the number of sessions according to claim 5, wherein setting registration is performed.   When the session relay apparatus receives information predicting the congestion state of the network from the bandwidth management / acceptance apparatus as a congestion management request, the session relay apparatus sets and registers in advance the congestion state of the network specified by the congestion management request. The method for securing the number of sessions according to claim 5, wherein:   9. The session number securing method according to claim 5, wherein the process in the session relay device is implemented as a program executable by a computer.

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