Patent Publication Number: US-2013231103-A1

Title: Core network and communication system

Description:
TECHNICAL FIELD 
     The present invention relates to core networks and communication systems for establishing voice communication between communication apparatuses. 
     BACKGROUND ART 
     In voice communication services using telephones, various codecs have been used. For example, the adaptive multi-rate wideband (AMR-WB) codec has been proposed as a codec for high-quality voice communication services. The AMR-WB, which is an improved version of the AMR codec currently used for telephones around the world, were standardized in 3GPP and are known as G.722.2 in ITU-T (see Non-Patent Documents 1 to 4). In the following description, the conventional AMR codec is called the adaptive multi-rate narrowband (AMR-NB) codec in order to differentiate it from the AMR-WB. 
     The transmission rate is 4.75 to 12.2 kbits/s in the AMR-NB, whereas the transmission rate in the AMR-WB is one of 12.65, 14.25, 15.85, 18.25, 19.85, 23.05, 23.85 kbits/s etc. The AMR-WB consumes a wider bandwidth than the AMR-NB. 
     Currently, the AMR-WB has been becoming dominant, but the codecs that can be used in networks depend on networks. For example, a network might use the AMR-WB and the AMR-NB, whereas another network might use only the AMR-NB. 
     When a mobile communication apparatus, such as a cell phone, performs voice communication with a communication apparatus (such as a fixed-line phone or a cell phone) connected to a communication network other than the mobile communication network to which the mobile communication apparatus subscribes, a core network that connects the mobile communication network to the communication network establishes voice communication between the communication apparatuses. As such a core network, the IP multimedia subsystem (IMS) for establishing voice over Internet protocol (VoIP) communication between communication apparatuses has been proposed (see Non-Patent Document 5). The IMS is also known as a multimedia domain (MMD). 
     PRIOR ART DOCUMENTS 
     Non-Patent Documents 
     
         
         Non-Patent Document 1: ITU-T Recommendation G.722.2; “Wideband coding of speech at around 16 kbits/s using Adaptive Multi-Rate Wideband (AMR-WB)”, International Telecommunication Union, July 2003 
         Non-Patent Document 2: 3GPP TS 26.190 V5.1.0 (2001-12), 3rd 
       
    
     Generation Partnership Project; Technical Specification Group Services and System Aspects; Speech Codec speech processing functions; “AMR Wideband speech codec; Transcoding functions”, December 2001
     Non-Patent Document 3: 3GPP TS 26.194 V5.0.0 (2001-03), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Speech Codec speech processing functions; “AMR Wideband speech codec; Voice Activity Detector (VAD)”, March 2001   Non-Patent Document 4: 3GPP TS 26.171 V5.0.0 (2001-03), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Speech Codec speech processing functions; “AMR Wideband speech codec; General Description”, March 2001   Non-Patent Document 5: 3GPP TS 23.228 V10.2.0 (2010-09), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; “IP Multimedia Subsystem (IMS)”; Stage 2 (Release 10), September 2010   

     SUMMARY OF INVENTION 
     Technical Problems 
     For use of the IMS, a mobile communication apparatus utilizes a common codec that belongs to the intersection of codecs that can be used by the mobile communication apparatus and codecs that can be supported by the media gateway control function (MGCF) of the IMS. When a codec to be used by the mobile communication apparatus is selected, codecs that can be used in the correspondent communication apparatus that is connected to another network are not taken into account. Therefore, when a mobile communication apparatus that can use both the AMR-WB and the AMR-NB originates a call in a network that can use the AMR-WB, the mobile communication apparatus uses the AMR-WB. 
     In that case, when the correspondent communication apparatus that is connected to the other network can only use the AMR-NB, the AMR-WB is used from the originating mobile communication apparatus to the media gateway (MGW) in the IMS core network, whereas the other network uses the AMR-NB. Even though the mobile communication network side uses the AMR-WB, which has a high transmission rate, since the other network side uses the AMR-NB, which has a low transmission rate, voice eventually has low voice quality as a result of the AMR-NB. Thus, it is wasteful that the mobile communication network side consumes a wide bandwidth for the AMR-WB in light of the low voice quality which is finally obtained. Therefore, it is desirable that the wide bandwidth be reduced so that radio and core-network resources can be made available for communication performed by other users. 
     Accordingly, the present invention provides a core network and a communication system capable of using the bandwidth efficiently by matching, as closely as possible, the codec used at the terminating side with the codec used at the originating side. 
     Solution to Problems 
     A core network according to the present invention is connected to a mobile communication network to which a terminating mobile communication apparatus is connected and to an originating network to which an originating communication apparatus is connected, and establishes voice communication between the communication apparatuses. The core network includes a connection-request receiver adapted for receiving, from the originating network, a connection request that includes information indicating an originating codec selected to be used in the originating network and an identifier identifying the terminating mobile communication apparatus; a termination notification transmission section adapted for transmitting, upon receiving the connection request, a termination notification to the terminating mobile communication apparatus; a termination notification-response receiver adapted for receiving a termination notification response sent from the terminating mobile communication apparatus in response to the termination notification and indicating at least one codec that can be used by the terminating mobile communication apparatus; and a codec specifying section adapted for specifying a codec that should be used by the terminating mobile communication apparatus. When the termination notification-response receiver receives the termination notification response, the codec specifying section specifies either a common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus, indicated in the termination notification response, codecs that can be used in the core network, and the originating codec, or a codec that can be used by the terminating mobile communication apparatus and the core network and that has a transmission rate closest to the transmission rate of the originating codec, as the codec that should be used by the terminating mobile communication apparatus. 
     In the present invention, in response to the connection request sent from the originating communication apparatus, the core network sends the termination notification to the terminating mobile communication apparatus and receives the termination notification response from the terminating mobile communication apparatus. The connection request indicates an originating codec selected to be used in the originating network, and the termination notification response indicates at least one codec that can be used by the terminating mobile communication apparatus. Upon receiving the termination notification response, the core network specifies either a common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus, indicated in the termination notification response, codecs that can be used in the core network, and the originating codec, or a codec that can be used by the terminating mobile communication apparatus and the core network and that has a transmission rate closest to the transmission rate of the originating codec, as a codec that should be used by the terminating mobile communication apparatus. By doing so, the transmission rate of the codec to be used at the terminating side is made close to the transmission rate of the codec to be used at the originating side, reducing the bandwidth waste. 
     For example, it is assumed that the core network and the terminating mobile communication apparatus can use both the AMR-WB and the AMR-NB, and the originating communication apparatus can use the AMR-NB. In that case, the core network specifies the AMR-NB as the codec that should be used by the terminating mobile communication apparatus. By doing so, the terminating mobile communication apparatus and the core network use the AMR-NB codec that is the same as that in the originating communication apparatus to perform voice communication. By selecting the codec to be used for voice communication in the core network in a manner adaptive to the originating communication apparatus as described above, the codec to be used at the terminating side and the codec to be used at the originating side can be matched as closely as possible. Therefore, the terminating side uses the codec having a bandwidth suited to the voice quality to be obtained, so that the mobile communication network and the core network can use the bandwidth efficiently. In other words, the bandwidth required for the codec to be used at the terminating side is reduced in conformity to the codec to be used at the originating side, so that the radio and core-network resources can be made available for communication performed by other users. 
     It is preferable in the core network according to the present invention that, when the connection-request receiver receives the connection request, the codec specifying section temporarily specify either the originating codec indicated in the connection request, or a codec that can be used in the core network and that has a transmission rate closest to the transmission rate of the originating codec, as a first codec that should be used by the terminating mobile communication apparatus; the termination notification transmission section transmit the termination notification, which indicates the first codec, to the terminating mobile communication apparatus; and when the termination notification-response receiver receives the termination notification response, the codec specifying section specify a common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus, indicated in the termination notification response, and the first codec, as a second codec that should be used by the terminating mobile communication apparatus. 
     Codecs that can be used for voice communication between the core network and the terminating mobile communication apparatus are common codecs that belong to the intersection of codecs that can be used in the core network and codecs that can be used by the terminating mobile communication apparatus. First, the codec specifying section specifies the originating codec as the first codec if the originating codec can be used in the core network; and specifies a codec that can be used in the core network and that has a transmission rate closest to the transmission rate of the originating codec as a first codec if the originating codec cannot be used in the core network. The second codec to be selected thereafter is a common codec that belongs to the intersection of at least one codec that can be used by the terminating mobile communication apparatus and the first codec. As a result, the second codec that should be used by the terminating mobile communication apparatus is a common codec that belongs to the intersection of codecs that can be used in the core network and codecs that can be used by the terminating mobile communication apparatus and that has the same transmission rate as or a transmission rate closest to the transmission rate of the originating codec. In other words, the codec that can be used for voice communication between the core network and the terminating mobile communication apparatus and that matches the originating codec as closely as possible is used for that voice communication. 
     Further, if only one codec can be used by the terminating mobile communication apparatus, and that codec is the same as the first codec, that codec should be selected in the core network as the second codec. The terminating mobile communication apparatus ensures resources for the first codec when receiving the first codec temporarily specified, and if the first codec is the same as the second codec, it is possible to reduce the time required for preparing for voice communication in the terminating mobile communication apparatus. 
     The core network according to the present invention may further include a storage section adapted for storing the originating codec indicated in the connection request received by the connection-request receiver. In the core network, when the connection-request receiver receives the connection request, the codec specifying section may temporarily specify a codec that can be used in the core network, as a first codec that should be used by the terminating mobile communication apparatus; the termination notification transmission section may transmit the termination notification, which indicates the first codec, to the terminating mobile communication apparatus; and when the termination notification-response receiver receives the termination notification response, the codec specifying section may specify either the common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus, indicated in the termination notification response, the codecs that can be used in the core network, and the originating codec stored in the storage section, or the codec that can be used by the terminating mobile communication apparatus and the core network and that has the transmission rate closest to the transmission rate of the originating codec, as a second codec that should be used by the terminating mobile communication apparatus. 
     Codecs that can be used for voice communication between the core network and the terminating mobile communication apparatus are common codecs that belong to the intersection of codecs that can be used in the core network and codecs that can be used by the terminating mobile communication apparatus. First, the codec specifying section temporarily specifies a codec that can be used in the core network as the first codec that should be used by the terminating mobile communication apparatus. When the first codec is specified, it is not taken into consideration whether or not the first codec is the same as the originating codec, or whether or not the first codec and the originating codec have close transmission rates. Therefore, the first codec can be signaled to the terminating mobile communication apparatus quickly. The second codec to be selected thereafter is either a common codec that belongs to the intersection of codecs that can be used by the terminating mobile communication apparatus, codecs that can be used in the core network, and the originating codec, or a codec that can be used by the terminating mobile communication apparatus and the core network and that has a transmission rate closest to the transmission rate of the originating codec. Therefore, there is a low possibility that the first codec specified temporarily is the same as the second codec specified later. However, the terminating mobile communication apparatus ensures resources for the first codec when receiving the first codec temporarily specified, and if the first codec is the same as the second codec, the resources temporarily ensured for the first codec can be used without change for voice communication between the terminating mobile communication apparatus and the core network, so that the time required for preparing for the voice communication might be reduced. In addition, since the first codec is specified irrespective of the originating codec, the time required for specifying the first codec is short, and as a result, the time for the entire terminating procedure may be reduced. 
     A communication system according to the present invention includes the above-described core network, a mobile communication network connected to the core network, and a mobile communication apparatus that connects to the mobile communication network. The mobile communication apparatus receives the termination notification from the core network and sends, to the core network in response to the termination notification, the termination notification response, indicating the at least one codec that can be used by the mobile communication apparatus. 
     By selecting the codec to be used for voice communication in the core network in a manner adapted to the originating communication apparatus, as described above, the codec used at the terminating side and the codec used at the originating side can be matched as closely as possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the overall communication system according to embodiments of the present invention. 
         FIG. 2A  is a part of a sequence diagram indicating an example information flow in a communication system according to a first embodiment of the present invention. 
         FIG. 2B  is a part of the sequence diagram, following the part in  FIG. 2A . 
         FIG. 3  is a part of a sequence diagram indicating an information flow in a communication system according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Various embodiments of the present invention will be described below with reference to the attached drawings. 
     As shown in  FIG. 1 , a communication system according to embodiments of the present invention includes a mobile communication network  10 , an evolved packet core (EPC)  30 , and a core network  40 . A large number of mobile communication apparatuses  12  are connected to the mobile communication network  10 . The mobile communication apparatuses  12  are, for example, cell phones and other communication apparatuses having a voice communication function. The mobile communication network  10  conforms, for example, to long term evolution (LTE), but the specifications thereof are not limited to LTE. The EPC  30  manages the mobility of each of the mobile communication apparatuses  12 . In the following description, the mobile communication apparatuses  12  may be called user equipments (UEs). 
     A plurality of external networks  50  are connected to the core network  40 . The external networks  50  are circuit switching networks, such as public switched telephone networks (PSTNs) or public line mobile networks (PLMNs), or other networks (for example, networks conforming to SIP-I, in which ISUP messages are encapsulated within SIP messages and sent). A large number of communication apparatuses  52 , such as cell phones or fixed-line telephones, are connected to each of the external networks  50 . Therefore, the core network  40  establishes communication (including voice communication) between a mobile communication apparatus  12  connected to the mobile communication network  10  and capable of communicating via the IMS and a communication apparatus  52  connected to an external network  50 . 
     The core network  40  is an IMS core network. The core network  40  includes various entities in addition to those shown in the figure, but includes the following main entities. 
     A proxy call session control function (P-CSCF)  41  is a session initiation protocol (SIP) router that receives SIP messages from the mobile communication apparatus  12  and sends SIP messages to the mobile communication apparatus  12 . 
     A serving call session control function (S-CSCF)  42  is an SIP router providing the following functions:
     Managing the registration information and the provided service information of users.   Managing user sessions.   Selecting an application server  43  that provides a service for each user.
 
The S-CSCF  42  also has a routing function based on the telephone numbers of terminating apparatuses.
   

     The application server (AS)  43  provides a voice application by using SIP. Invoice communication between users, the AS  43  provides, for example, additional services such as voice guidance. 
     When the mobile communication network  10  receives a call from the external network  50 , an interrogating-CSCF (I-CSCF)  44  queries a subscriber location function (SLF)  47  about the location information of the terminating mobile communication apparatus  12 , and selects an S-CSCF to be used for communication with the mobile communication apparatus  12  according to the location information of the mobile communication apparatus  12 . In other words, the I-CSCF  44  receives an SIP message and routes the SIP message to the S-CSCF. 
     The media gateway control function (MGCF)  45  is a breakout point to a circuit switching network, in the IMS core network  40 . The MGCF  45  handles C-plane control protocol conversion between the IMS core network  40  and the external network  50 . Specifically, the MGCF  45  handles conversion between the SIP used in IMS control signals, and the ISDN user part (ISUP) or the bearer independent call control (BICC) used in control signals in a circuit switching network. The MGCF  45  controls the resources of a media gateway (MGW)  46  by using the H.248 protocol or the like. 
     The media gateway (MGW)  46  has an interface function for user data, such as voice, in the user plane (U-plane) when connected to the external network  50 . Specifically, when the codec used from a mobile communication apparatus  12  to the core network  40  and the codec used in an external network  50  are not the same, the MGW performs code conversion between the codecs. 
     In  FIG. 1 , in the core network  40 , dotted lines indicate the C-plane and solid lines indicate the U-plane. The U-plane is also called a media plane in the IMS. 
     First Embodiment 
     It is assumed that the communication apparatus  52  that connects to the external network  50  originates a call in order to perform voice communication with the mobile communication apparatus  12  that is capable of communicating via the IMS and that connects to the mobile communication network  10 . 
     In that case, the originating external network  50  sends an ISUP initial address message (IAM) as a connection request. The IAM includes information indicating the originating codec selected to be used in the originating external network  50 , and an identifier identifying the terminating mobile communication apparatus  12 . The codec selected to be used in the originating external network  50  can be used in the external network  50  and can also be used in the communication apparatus  52 . 
     The IAM is received by the MGCF (connection-request receiver)  45 . Upon receiving the IAM, the MGCF (termination notification transmission section)  45  sends a termination notification (SIP_INVITE) to the terminating mobile communication apparatus  12 . More specifically, the MGCF  45  sends SIP_INVITE to the I-CSCF  44 , and the I-CSCF  44  queries the SLF  47  about the location information of the mobile communication apparatus  12  and selects an S-CSCF  42  suited to the location of the mobile communication apparatus  12 . The I-CSCF  44  transfers SIP_INVITE to the selected S-CSCF  42 . The S-CSCF  42  selects a P-CSCF  41  to be used for communication with the mobile communication apparatus  12  on the basis of the telephone number of the mobile communication apparatus  12 , and transfers SIP_INVITE to the P-CSCF  41 . The P-CSCF  41  transfers SIP_INVITE to the mobile communication apparatus  12 . SIP_INVITE reaches the terminating mobile communication apparatus  12  through the EPC  30  and the mobile communication network  10 . 
     In response to SIP_INVITE, the terminating mobile communication apparatus  12  sends a termination notification response (SIP — 183 message (session progress)) indicating at least one codec that can be used by the terminating mobile communication apparatus  12 . The P-CSCF  41  (termination notification-response receiver) receives the SIP — 183 message. The SIP — 183 message reaches the MGCF  45  through the S-CSCF  42  and the I-CSCF  44 . 
     The MGCF  45  (codec Specifying Section) has a function of specifying a codec that should be used by the terminating mobile communication apparatus  12 . Specifying the codec includes two stages. 
     First, upon receiving a connection request (IAM) from the external network  50 , the MGCF  45  temporarily specifies either the originating codec indicated by the IAM or a codec that can be used in the IMS core network  40  and that has a transmission rate closest to the transmission rate of the originating codec, as a first codec that should be used by the terminating mobile communication apparatus  12 . More specifically, if the originating codec can be used in the core network  40 , that originating codec is specified as the first codec; and, if the originating codec cannot be used in the core network  40 , a codec that can be used in the core network  40  and that has a transmission rate closest to the transmission rate of the originating codec is specified as the first codec. The MGCF  45  (termination notification transmission section) sends a termination notification (SIP_INVITE) indicating the first codec to the terminating mobile communication apparatus  12 . 
     Thereafter, upon receiving the termination notification response (SIP — 183 message) indicating at least one codec that can be used by the terminating mobile communication apparatus  12 , the MGCF  45  specifies a common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus  12 , indicated in the termination notification response, and the first codec, as a second codec that should be used by the terminating mobile communication apparatus  12 . 
     The MGCF  45  functions as a second codec notification transmission section, and sends a second codec notification (SIP_PRACK) to the terminating mobile communication apparatus  12 . SIP_PRACK reaches the terminating mobile communication apparatus  12  through the I-CSCF  44 , the S-CSCF  42 , and the P-CSCF  41 , and then through the EPC  30  and the mobile communication network  10 . 
     The second codec, selected in the latter stage, is a common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus  12  and the first codec. As a result, the second codec, which should be used by the terminating mobile communication apparatus  12 , is a common codec that belongs to the intersection of codecs that can be used in the core network  40  and codecs that can be used by the terminating mobile communication apparatus  12  and that has the same transmission rate as or a transmission rate closest to the transmission rate of the originating codec. In other words, the second codec is either a common codec that belongs to the intersection of at least one codec that can be used by the terminating mobile communication apparatus  12 , codecs that can be used in the core network  40 , and the originating codec, or a codec that has a transmission rate closest to the transmission rate of the originating codec and that can be used by the terminating mobile communication apparatus  12  and the core network  40 . In summary, a codec that can be used for voice communication between the core network  40  and the terminating mobile communication apparatus  12  and that has a transmission rate as close as possible to the transmission rate of the originating codec is used for that voice communication. By doing so, the transmission rate of the codec to be used at the terminating side is made close to the transmission rate of the codec to be used at the originating side, reducing the bandwidth waste. 
     For example, it is assumed that the core network  40  and the terminating mobile communication apparatus  12  can use both the AMR-WB and the AMR-NB, and that the originating communication apparatus  52  can use the AMR-NB. In that case, the core network  40  specifies the AMR-NB as the second codec, which should be used by the terminating mobile communication apparatus  12 . By doing so, the terminating mobile communication apparatus  12  and the core network  40  use the AMR-NB that is the same as that in the originating communication apparatus  52  to perform voice communication. By selecting the codec to be used for voice communication in the core network  40  in a manner adaptive to the originating communication apparatus  52  as described above, the codec to be used at the terminating side and the codec to be used at the originating side can be matched as closely as possible. Therefore, the terminating side uses the codec having a bandwidth suited to the voice quality to be obtained, so that the mobile communication network and the core network can use the bandwidth efficiently. In other words, the bandwidth required for the codec to be used at the terminating side is reduced in conformity to the codec to be used at the originating side, so that the radio and core-network resources can be made available for communication performed by other users. 
     If only one codec can be used by the terminating mobile communication apparatus  12 , and that codec is the same as the first codec, that codec is specified as the second codec by the MGCF  45 . If the terminating mobile communication apparatus ensures resources for the first codec when receiving the first codec temporarily specified, the terminating mobile communication apparatus  12 , and if the first codec is the same as the second codec, it is possible to reduce the time required for preparing for voice communication in the terminating mobile communication apparatus  12 . 
     An example information flow in a communication system according to the first embodiment will be described below with reference to  FIG. 2A  and  FIG. 2B . For convenience, the mobile communication network  10  and the EPC  30  are omitted in the figures. In the following description, it is assumed that a communication apparatus  52  connected to an external network  50  originates a call in order to perform voice communication with a UE. In this example, the originating network is a PSTN, but it maybe another circuit switching network, such as a PLMN, or another type of network. 
     The telephone number of the terminating mobile communication apparatus  12  is input to the communication apparatus  52 . The originating PSTN sends an IAM to the MGCF  45  (connection-request receiver) as a connection request that indicates the originating codec and an identifier (such as the telephone number) identifying the terminating mobile communication apparatus  12 . 
     Upon receiving the IAM, the MGCF selects an MGW to be used for this voice communication, and activates the MGW by using the H.248 protocol. The MGCF  45  (codec specifying section) also specifies either the originating codec signaled from the originating network, or a codec that has a transmission rate close to the transmission rate of the originating codec and that can be used in the IMS core network  40 , as the first codec. The MGCF  45  (termination notification transmission section) generates SIP_INVITE that includes Initial SDP Offer describing the first codec, according to the session description protocol (SDP), and sends it to the I-CSCF  44 . The I-CSCF  44  queries of the SLF  47  about the location information of the mobile communication apparatus  12 , and transfers SIP_INVITE to an S-CSCF  42  suited to the location of the mobile communication apparatus  12 . 
     Upon receiving SIP_INVITE, the S-CSCF  42  requests the AS to perform service control, if necessary, and also selects a P-CSCF  41  to be used for communication with the mobile communication apparatus  12  on the basis of the telephone number of the terminating mobile communication apparatus  12  and transfers SIP_INVITE to the P-CSCF  41 . The P-CSCF  41  transfers SIP_INVITE to the mobile communication apparatus  12 . 
     Upon receiving SIP_INVITE, the UE generates an SIP — 183 message (session progress) that includes SDP Answer describing the codec capacity of the UE (codecs that can be used by the UE) and sends it. The P-CSCF  41  (termination notification-response receiver) receives the SIP — 183 message. The SIP — 183 message reaches the MGCF  45  through the S-CSCF  42  and the I-CSCF  44 . 
     On the other hand, the MGCF  45  uses the H.248 protocol to signal the codec already selected by the MGCF (that is, the first codec) to the MGW, and the MGW ensures resources required for the codec. 
     Upon receiving the termination notification response (SIP — 183 message) indicating at least one codec that can be used by the UE, the MGCF  45  specifies a common codec that belongs to the intersection of the codec that can be used by the UE, indicated in the SIP — 183 message, and the codec already selected by the MGCF (that is, the first codec) as the second codec that should be used by the UE. 
     The MGCF  45  (second codec notification transmission section) sends a second codec notification (SIP_PRACK) that includes 2nd SDP Offer indicating the codec newly selected by the MGCF (that is, the second codec) to the UE. SIP_PRACK reaches the UE through the I-CSCF  44 , the S-CSCF  42 , and the P-CSCF  41 . 
     Referring now to  FIG. 2B , the UE sends back SIP — 200 OK to the core network  40 . SIP — 200 OK is received by the P-CSCF and reaches the MGCF through the S-CSCF and the I-CSCF. The MGCF uses the H.248 protocol to signal the codec newly selected by the MGCF (that is, the second codec) to the MGW, and the MGW ensures resources required for the codec. 
     Upon receiving SIP — 200 OK, the MGCF confirms whether or not voice-media resources have been ensured inside the MGCF (precondition control). After the confirmation, the MGCF sends SIP_Update signaling that the MGCF has ensured the resources to the UE. 
     Upon receiving SIP_Update, the UE sends SIP — 200 OK for signaling that the voice-media resources have been ensured inside the UE to the MGCF. When the voice-media resources have been ensured inside the UE (when the precondition is satisfied), the UE starts USER ALERT and sends SIP — 180 Ringing to the MGCF. This message indicates that the terminating apparatus is being called. Upon receiving SIP — 180 Ringing, the MGCF sends an ISUP address complete message (ACM) to the external originating network  50 . 
     Further, when the user of the UE responds to the calling and the UE becomes off-hook, the UE sends SIP — 200 OK. Upon receiving SIP — 200 OK, the MGCF sends an answer message (ANM) to the external originating network  50 . Then, the MGCF uses the H.248 protocol to signal to the MGW that the communication apparatus  52  has become off-hook and voice-media communication has started. Then, the MGCF sends SIP_ACK to the UE. 
     Second Embodiment 
     A second embodiment is basically similar to the first embodiment, and the MGCF (codec specifying section)  45  specifies a codec that should be used by the terminating mobile communication apparatus  12  in the two stages. In the second embodiment, however, the first codec and the second codec that should be used by the terminating mobile communication apparatus  12  are different from those used in the first embodiment. Differences from the first embodiment will be mainly described below. 
     First, upon receiving a connection request (IAM) from the external network  50 , the MGCF (storage section)  45  stores the originating codec indicated in the IAM, and also temporarily specifies one of codecs that can be used in the IMS core network  40  as the first codec that should be used by the terminating mobile communication apparatus  12  irrespective of the originating codec indicated in the IAM. When the first codec is specified, it is not taken into consideration whether or not the first codec is the same as the originating codec, or whether or not the first codec and the originating codec have close transmission rates. Therefore, the first codec can be signaled to the terminating mobile communication apparatus quickly. The MGCF (termination notification transmission section) sends a termination notification (SIP_INVITE) indicating the first codec to the terminating mobile communication apparatus  12 . 
     Thereafter, upon receiving the termination notification response (SIP — 183 message) indicating at least one codec that can be used by the terminating mobile communication apparatus  12 , the MGCF  45  specifies a common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus  12 , indicated in the termination notification response, codecs that can be used in the core network  40 , and the originating codec stored in the storage section, as the second codec that should be used by the terminating mobile communication apparatus  12 . If there is no common codec, a codec that can be used by the terminating mobile communication apparatus  12  and the core network  40 , and that has a transmission rate closest to the transmission rate of the originating codec is specified as the second codec that should be used by the terminating mobile communication apparatus  12 . 
     The MGCF  45  functions as a second codec notification transmission section and sends a second codec notification (SIP_PRACK) to the terminating mobile communication apparatus  12 . SIP_PRACK reaches the terminating mobile communication apparatus  12  through the I-CSCF  44 , the S-CSCF  42 , and the P-CSCF  41 , and then through the EPC  30  and the mobile communication network  10 . 
     Since the first codec, which is specified temporarily and quickly, is specified irrespective of the originating codec, there is a low possibility that the first codec is the same as the second codec and is specified as the second codec. However, the terminating mobile communication apparatus ensures resources for the first codec when receiving the first codec temporarily specified, and if the first codec is the same as the second codec, the resources temporarily ensured for the first codec can be used without change for voice communication between the terminating mobile communication apparatus  12  and the core network  40 , so that the time required for preparing for the voice communication might be reduced. In addition, since the first codec is specified irrespective of the originating codec, the time required for specifying the first codec is short, and as a result, the time for the entire terminating procedure may be reduced. 
     The second codec, selected in the latter stage, is either a common codec that belongs to the intersection of the at least one codec that can be used by the terminating mobile communication apparatus  12 , codecs that can be used in the core network  40 , and the originating codec, or a codec that can be used by the terminating mobile communication apparatus  12  and the core network  40  and that has a transmission rate closest to the transmission rate of the originating codec. In other words, the codec that can be used for voice communication between the core network  40  and the terminating mobile communication apparatus  12  and that has a transmission rate as close as possible to the transmission rate of the originating codec is used for that voice communication. By doing so, the transmission rate of the codec to be used at the terminating side is made close to the transmission rate of the codec to be used at the originating side, reducing the bandwidth waste. 
     For example, it is assumed that the core network  40  and the terminating mobile communication apparatus  12  can use both the AMR-WB and the AMR-NB, and the originating communication apparatus  52  can use the AMR-NB. In that case, the core network  40  specifies the AMR-NB as the second codec, which should be used by the terminating mobile communication apparatus  12 . By doing so, the terminating mobile communication apparatus  12  and the core network  40  use the AMR-NB codec that is the same as that in the originating communication apparatus  52  to perform voice communication. By selecting the codec to be used for voice communication in the core network  40  in a manner adaptive to the originating communication apparatus  52  as described above, the codec to be used at the terminating side and the codec to be used at the originating side can be matched as closely as possible. Therefore, the terminating side uses the codec having a bandwidth suited to the voice quality to be obtained, so that the mobile communication network and the core network can use the bandwidth efficiently. In other words, the bandwidth required for the codec to be used at the terminating side is reduced in conformity to the codec to be used at the originating side, so that the radio and core-network resources can be used for communication performed by others. 
     An example information flow in the communication system according to the second embodiment will be described below with reference to  FIG. 3 . In the same way as in  FIGS. 2A and 2B , for convenience, the mobile communication network  10  and the EPC  30  are omitted in the figures. In the following description, it is assumed that a communication apparatus  52  connected to an external network  50  originates a call in order to perform voice communication with an UE. In this example, the originating network is the PSTN, but it may be another circuit switching network, such as a PLMN, or another type of network. 
     The telephone number of the terminating mobile communication apparatus  12  is input to the communication apparatus  52 . The originating PSTN sends an IAM to the MGCF  45  (connection-request receiver) as a connection request that indicates the originating codec and an identifier identifying the terminating mobile communication apparatus  12  (such as the telephone number). 
     Upon receiving the IAM, the MGCF selects an MGW to be used for this voice communication, and activates the MGW by using the H.248 protocol. The MGCF  45  (storage section) also holds, that is, stores, the originating codec signaled from the originating network. The MGCF  45  (codec specifying section) also specifies one of codecs that can be used in the IMS core network  40  (codec owned by the MGCF), as a first codec, irrespective of the originating codec or the transmission rate thereof. The MGCF  45  (termination notification transmission section) generates SIP_INVITE that includes Initial SDP Offer describing the first codec, according to the session description protocol (SDP), and sends it to the I-CSCF  44 . The I-CSCF  44  queries the SLF  47  about the location information of the mobile communication apparatus  12 , and transfers SIP_INVITE to an S-CSCF  42  suited to the location of the mobile communication apparatus  12 . 
     Upon receiving SIP_INVITE, the S-CSCF  42  requests the AS to perform service control, if necessary, and also selects a P-CSCF  41  to be used for communication with the mobile communication apparatus  12  on the basis of the telephone number of the terminating mobile communication apparatus  12  and transfers SIP_INVITE to the P-CSCF  41 . The P-CSCF  41  transfers SIP_INVITE to the mobile communication apparatus  12 . 
     Upon receiving SIP_INVITE, the UE generates an SIP — 183 message (session progress) that includes an SDP answer describing the codec capacity of the UE (codecs that can be used by the UE) and sends it. The P-CSCF  41  (termination notification-response receiver) receives the SIP — 183 message. The SIP — 183 message reaches the MGCF  45  through the S-CSCF  42  and the I-CSCF  44 . 
     The MGCF  45  uses the H.248 protocol to signal the codec already selected by the MGCF (that is, the first codec) to the MGW, and the MGW ensures resources required for the codec. 
     Upon receiving the termination notification response (SIP — 183 message) indicating at least one codec that can be used by the UE, the MGCF  45  specifies the second codec. The MGCF  45  specifies a common codec that belongs to the intersection of the at least one codec that can be used by the UE, indicated in the SIP — 183 message, codecs that can be used in the core network  40 , and the originating codec stored in the storage section, as the second codec that should be used by the terminating mobile communication apparatus  12 . If there is no common codec, however, a codec that can be used by the UE and the core network  40  and that has a transmission rate closest to the transmission rate of the originating codec is specified as the second codec that should be used by the UE. 
     The MGCF  45  (second codec notification transmission section) sends a second codec notification (SIP_PRACK) that includes 2nd SDP Offer indicating the codec newly selected by the MGCF (that is, the second codec) to the UE. SIP_PRACK reaches the UE through the I-CSCF  44 , the S-CSCF  42 , and the P-CSCF  41 . 
     The subsequent steps are the same as those described in the first embodiment with reference to  FIG. 2B . 
     Other Modifications 
     In the above-described embodiments, example codecs are the AMR-WB and the AMR-NB. Other codecs may be used in the communication system. 
     In the above-described embodiments, the core network  40  is an IMS core network. A core network according to the present invention is not limited to an IMS core network. 
     In the above-described embodiments, the identifier used to identify the originating communication apparatus is the telephone number of the originating communication apparatus. 
     However, other identifiers may be used. 
     REFERENCE NUMERALS 
     
         
           10 : Mobile communication network 
           12 : Mobile communication apparatus 
           30 : EPC 
           40 : Core network 
           41 : P-CSCF (termination notification-response receiver) 
           42 : S-CSCF 
           43 : AS 
           44 : I-CSCF 
           45 : MGCF (connection-request receiver, termination notification transmission section, codec specifying section, storage section) 
           46 : MGW 
           50 : External network 
           52 : Communication apparatus