Patent Publication Number: US-2010128715-A1

Title: Protocol Conversion System in Media Communication between a Packet-Switching Network and Circuit-Switiching Network

Description:
TECHNICAL FIELD 
     The present invention relates to a technique for, when carrying out media communication of speech and images between a terminal connected to a circuit-switching network and a terminal connected to a packet-switching network, converting transmission protocols to relay media in a device that is provided between the two networks. 
     BACKGROUND ART 
     Recent years have seen the rapid popularization of bidirectional communication systems or conference systems referred to as VoIP (Voice over IP) or TVoIP (TV over IP) for communicating encoded data of speech or images by packets by way of networks. Terminals that are coming into wide use include not only this type of communication system connected to a packet-switching network, but also, among third-generation portable terminals (3G terminals), terminals capable of inter-terminal TV telephone on circuit-switching networks. These terminals are of specifications in which the encoding methods for exchanging speech and images, the transmission protocol for transmitting and receiving encoded data, and call connection protocol for mutually call connection are matched to the respective networks. For example, if the encoding methods are the same but the transmission protocols are different, the terminals are not able to communicate with each other, and as a result, realizing communication between terminals that are connected to different networks requires a device connected between the two networks for relaying the exchange of encoded data. 
     In this case, for example, it is assumed that the transmission protocol that is supported by terminals (SIP terminals) connected to a packet-switching network is RTP (Real-Time Transport Protocol)/UDP (User Datagram Protocol)/IP (Internet Protocol) and the call connection protocol for carrying out the capacity exchange of terminals is SIP (Session Initiation Protocol)/SDP (Session Description Protocol). A terminal connected to a circuit-switching network supports ITU-T H.324 recommendations (3G-324 and Q. 931 for Third-Generation portable terminals) and follows ITU-T H.223 recommendations (hereinbelow referred to as H.223) as the transmission protocol and ITU-T H.245 recommendations (hereinbelow referred to as H. 245) as the capacity exchange protocol. In this case, mutual conversion between RTP/UDP/IP and H.223 and mutual conversion between SIP and H. 245 are required in a device interposed between a packet-switching network and circuit-switching network. These issues are described below in the following four documents: 
     Document 1: Handley, M., Schulzrinne, H., Schooler, E., Rosenberg, J., “SIP: Session Initiation Protocol,” RFC 2543, March 1999. 
     Document 2: Handley, M., Jacobson, V., “SDP: Session Description Protocol,” RFC 2327, April 1998. 
     Document 3: Schulzrinne, H., Casner, S., Frederick, R., Jacobson, V., “RTP: A Transport Protocol for Real-Time Applications,” RFC 3550, July 2003. 
     Document 4: Sjoberg, J., Westerlund, M., Lakaniemi, A., Xie, Q., “Real-Time Transport Protocol (RTP) Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and the Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs,” RFC 3237, June 2002. 
     A connection system (gateway) has been proposed that is applied only to one type of encoding bit rate for realizing this communication between an SIP terminal connected to a packet-switching network and a 3G terminal. According to this connection device, a terminal connected to a packet-switching network and a terminal connected to a circuit-switching network can communicate without awareness of each other&#39;s transmission protocol and call connection protocol. In addition, a multimedia communication system has also been proposed that adopts a method of changing multiplex tables to correspond to changes in bit rate (JP-A-2003-198638 and JP-A-2002-111730) 
     DISCLOSURE OF THE INVENTION 
     However, in the transmission protocol conversion device of the above-described connection system, the specifications of the encoded data and the specifications of the transmission protocol are independent from each other, and this complicates protocol conversion adapted to the specifications of the encoded data. For example, even when specifications permitted changes in the bit rate of encoded data of speech or images during communication, problems were encountered in changing the conversion of transmission process in conjunction with fluctuation in bit rate. As a result, during communication, the relay of data was carried out with a fixed encoding bit rate. When it was necessary to change the speech encoding rate during communication in the multimedia communication system described in JP-A-2002-111730, an exchange regarding changes of multiplex tables relating to the rate change was carried out each time between terminals. After completion of the exchange of multiplex table changes, data relay was implemented with the encoding bit rate basically fixed. 
     In addition, when the bit rate of the encoded data that is supported by an SIP terminal connected to a packet-switching network differs from the bit rate of encoded data that is supported by a 3G terminal connected to a circuit-switching network, bit rate conversion was required in the connection system. In this case, however, in order to enable coping with the above-described transmission protocol conversion process, it was necessary for each terminal to adopt a respective fixed bit rate and then implement bit rate conversion between the terminals. As a result, conversion of bit rate during communication was problematic. 
     It is an object of the present invention to provide, in a system in which the transmission protocols of a packet-switching network and circuit-switching network are mutually converted, a system that can flexibly cope with a plurality of encoding bit rates. 
     To achieve the above-described object, the present invention is a protocol conversion device for, in media communication by way of a packet-switching network and circuit-switching network, converting protocols between the packet-switching network and the circuit-switching network, the protocol conversion device including a call connection unit a protocol converter. 
     The call connection unit carries out call connection processes of media communication between a terminal on the packet-switching network side and a terminal on the circuit-switching network side. The protocol converter analyzes speech packets received from the packet-switching network and specifies the encoding bit rate of the speech data in these speech packets. The protocol converter then specifies the multiplex table used in multiplexing of frames on the circuit-switching network from this encoding bit rate. The protocol converter further uses the specified multiplex table to multiplex data in the payload of packets received from the packet-switching network and thus generates frames and transmits to the circuit-switching network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing configuration of a communication system according to the first to fourth embodiments of the present invention; 
         FIG. 2  is a view for explaining the multiplex table in the first to fifth embodiments; 
         FIG. 3  shows the flow of processing when transmitting media from a circuit-switching network to a packet-switching network in the first to fifth embodiments; 
         FIG. 4  is a view for explaining the flow for generating speech RTP packets from H.223 multiplexed frames in the first and second embodiments; 
         FIG. 5  is a view for explaining an example of using a speech transcoder when generating speech RTP packets from H.223 multiplexed frames in the first and second embodiments; 
         FIG. 6  shows the flow of processing when transmitting media from a packet-switching network to a circuit-switching network in the first to fourth embodiments; 
         FIG. 7  is a view for explaining the flow for generating H.223 multiplexed frames from RTP (speech) packets in the first and second embodiments; 
         FIG. 8  is a view for explaining an example of using a speech transcoder when generating H.223 multiplexed frames from RTP (speech) packets in the first and second embodiments; 
         FIG. 9  is a view for explaining the flow when generating speech RTP packets from H.223 multiplexed frames in the third and fourth embodiments; 
         FIG. 10  is a view for explaining an example of using a speech transcoder when generating speech RTP packets from H.223 multiplexed frames in the third and fourth embodiments; 
         FIG. 11  is a view for explaining the flow when generating H.223 multiplexed frames from RTP (speech) packets in the third and fourth embodiments; 
         FIG. 12  is a view for explaining an example of using a speech transcoder when generating H.223 multiplexed frames from RTP (speech) packets in the third and fourth embodiments; 
         FIG. 13  is a block diagram showing the configuration of a connection gateway realized by the fifth embodiment; 
         FIG. 14  is a view for explaining the processing flow in the fifth embodiment; and 
         FIG. 15  is a view for explaining another processing flow in the fifth embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Explanation next regards embodiments of the present invention with reference to the accompanying figures. 
     First Embodiment 
     A connection gateway for implementing protocol conversion is arranged between a circuit-switching network and a packet-switching network. The connection gateway, upon receiving speech encoded data and image encoded data from the packet-switching network, determines the encoding bit rate from the payload length of speech packets and converts the bit rate of the speech encoded data if necessary. The connection gateway is provided with a plurality of multiplex tables in advance, selects a multiplex table in accordance with the speech bit rate following conversion, and uses this multiplex table to multiplex the speech encoded data and image encoded data to transmit to the circuit-switching network. 
     In addition, upon receiving multiplexed data from the circuit-switching network, the connection gateway separates the multiplexed data into speech encoded data and image encoded data in accordance with the multiplex table information. Regarding the speech encoded data, the connection gateway then, if determined to be necessary from the encoded data length, converts the bit rate of the speech encoded data. The connection gateway then packetizes each of the speech encoded data and image encoded data and transmits to the packet-switching network. 
     A detailed explanation follows below with reference to the drawings. 
       FIG. 1  is a block diagram showing the configuration of the communication system according to the first embodiment. As shown in  FIG. 1 , the communication system of the present embodiment is made up from connection gateway  100 , circuit-switching terminal  101 , packet-switching terminal  102 , circuit-switching network  103 , packet-switching network  104 , and SIP server  108 . Connection gateway  100  is made up from call connection unit  105 , transmission protocol converter  106 , and speech transcoder  107 . The components that make up connection gateway  100  may be independent devices, or may exist within the same device. 
     In the present embodiment, circuit-switching terminal  101  is connected to circuit-switching network  103 . Circuit-switching terminal  101  uses H.245 in call connection processing according to 3G-324M (image compression encoding) and uses H.223 as the transmission protocol. Packet-switching terminal  102  is connected to packet-switching network  104  by IP (Internet Protocol). Packet-switching terminal  102  uses SIP as the call connection processing and uses UDP/RTP as the transmission protocol. 
     Connection gateway  100  is further connected to both circuit-switching network  103  and packet-switching network  104 . Connection gateway  100  terminates 3G-324M on the circuit-switching network  103  side, terminates SIP and UDP/RTP on the packet-switching network  104  side, and carries out relay of media and call connection processes between the two terminals. 
     In the present embodiment, circuit-switching terminal  101  is a Third-Generation (3G) portable TV telephone terminal, supports AMR (Adaptive Multi-Rate) as the speech encoding method (speech compression encoding method), and supports MPEG-4 as the image encoding method (image compression encoding method). Packet-switching terminal  102  is realized by the execution of software by a personal computer or PDA (Personal Digital Assistant). As the media encoding methods, packet-switching terminal  102  supports the same AMR and MPEG-4 as circuit-switching terminal  101  and as the transmission protocol, supports the 3GPP (3 rd  Generation Partnership Project) standards that are the standards of a 3G mobile communication system. However, the present invention is obviously not limited to these encoding methods. Although explanation regards transmission protocol and call connection processing by these protocols for the sake of simplification in the present embodiment, it will be obvious that using other protocols having the same capabilities presents no problems. 
     The following detailed explanation regards an example in which both H.223 multiplexed frames and speech RTP packets include one AMR encoded frame in the present embodiment. 
     Circuit-switching terminal  101 , when wishing to connect to packet-switching terminal  102 , first establishes a connection with connection gateway  100 . Circuit-switching terminal  101 , in order to multiplex AMR speech encoded data and MPEG-4 image encoded data by means of H.223 and transmit, reports H. 223 multiplex table information to call connection unit  105  of connection gateway  100  in a H.245 negotiation when establishing the connection. This multiplex table is necessary for making preparations for each AMR mode that is supported by circuit-switching terminal  101 . As a result, the information of a plurality of multiplex tables is here reported. 
     Connection gateway  100  acquires the capacity (AMR support mode information) of packet-switching terminal  102  from the SIP/SDP description from SIP server  108  that is connected to packet-switching network  104  and sets the plurality of multiplex tables in accordance with this information. Connection Gateway  100  then reports the multiplex table information to circuit-switching terminal  101  in an H.245 negotiation. If connection gateway  100  already knows the IP address of packet-switching terminal  102  at this time, the capacity may be acquired directly from packet-switching terminal  102  by SIP instead of from SIP server  108 . 
     Here, if connection gateway  100  ascertains the AMR mode supported by circuit-switching terminal  101  from the speech data length of one or more types indicated in the multiplex table and describes only the supported modes by the SDP notation “mode−set=” (the description can be omitted if all modes are supported) when exchanging capacity by the SIP of packet-switching terminal  102  to report to packet-switching terminal  102 , communication from packet-switching terminal  102  to circuit-switching terminal  101  can be realized by only protocol conversion and there is no need for conversion of bit rate. In the SDP notation, for example, the description “mode−set=5, 7;” as the description of modes that are supported among AMR means that modes  5  and  7  are supported and reception is possible. Alternatively, the description “mode−set=1, 2, 3, 4, 5, 6, 7;” or the lack of the description “mode−set=” means that all modes are supported. 
     If bit rate conversion is to be carried out, connection gateway  100  should transmit to packet-switching terminal  102  a SIP/SDP response indicating that all modes are supported. Connection gateway  100  should then carry out bit rate conversion such that the AMR data received from packet-switching terminal  102  becomes the AMR mode supported by circuit-switching terminal  101  that was ascertained in the H.245 negotiation and transmit to circuit-switching terminal  101 . 
     Similarly, if the AMR mode supported by packet-switching terminal  102  indicated in SIP/SDP is identical to the AMR mode supported by circuit-switching terminal  101  ascertained in the H.245 negotiation, communication is possible in the direction from circuit-switching terminal  101  to packet-switching terminal  102  by protocol conversion alone without carrying out bit rate conversion. If the supported modes are different, connection gateway  100  should perform bit rate conversion of the AMR data from circuit-switching terminal  101  to the AMR mode supported by packet-switching terminal  102  and indicated by SIP/SDP and transmit to packet-switching terminal  102 . Connection gateway  100 , upon receiving multiplexed data of speech and images from circuit-switching terminal  101  by transmission protocol converter  106 , separates each of the media of the multiplexed data based on the multiplex table information of circuit-switching terminal  101  obtained by the H. 245 negotiation. 
       FIG. 2  is a view for explaining a plurality of multiplex tables set by connection gateway  100 . Explanation next regards multiplex tables with reference to  FIG. 2 . 
     Multiplexed frames  1 - 4   201 - 204  are multiplexed frames that correspond to each of the modes of AMR. H.223 multiplexed frame length information and multiplex table identity information are contained in each of the H.223 headers. The multiplex table that was used in multiplexing of the H.223 multiplexed frames can be uniquely specified from among the table information acquired in a H.245 negotiation by means of the multiplex table identity information. The length of each data portion contained in a multiplexed frame can be ascertained from this table information. In other words, the length of the speech data portions (speech data lengths a 1 -a 4 ) of multiplexed frames  1 - 4   201 - 204  can be ascertained from the multiplex table identity information. 
     Explanation next regards the details of the flow of media from circuit-switching terminal  101  to packet-switching terminal  102  with reference to the figures.  FIG. 3  is a detailed view of the processing flow when transmitting media from a circuit-switching network to a packet-switching network. 
     In reception process  302 , transmission protocol converter  106  receives H.223 multiplexed data from circuit-switching terminal  101  by way of circuit-switching network  103 . In H.223 multiplexed data, multiplex table identity information is appended to each individual multiplexed frame. H.223 separation process  303  separates the encoded data of speech and images based on the multiplex table information obtained by this multiplex table identity information and the H.245 negotiation. 
     In this case, the encoded frame length in AMR has a one-to-one correspondence with the mode of the encoding bit rate, and the AMR encoding mode is therefore clear from the length of speech encoded data  304  that have been separated. In other words, if the length of the speech data portion of H. 223 multiplex table is ascertained from the multiplex table identity information obtained in the H.223 frames, then the AMR encoding mode is known. Accordingly, even if the encoding mode of speech encoded data contained in H.223 multiplexed data from circuit-switching terminal  101  should change during communication, the change of the speech encoding bit rate can be handled in a minimum of each AMR encoded frame (20 msec). 
     If this speech encoded data  304  is a speech encoding mode that is not supported in the capacity information obtained from the packet-switching terminal, the bit rate of speech encoded data  304  is converted in speech transcoder  305 . If the packet-switching terminal supports the encoding mode used in the encoding of speech encoded data  304 , processing in speech transcoder  305  is unnecessary. 
     In the present invention, the transcoder for converting the encoded bit rate may be a tandem transcoder made up from a decoder/encoder pair or may be a non-tandem transcoder that operates by, for example, parameter mapping. Speech encoded data in which the encoding bit rate has been converted according to necessity are formatted to agree with the format of RTP payload by speech payload formatting process  306 . In the present embodiment, speech payload formatting process  306  is necessary because RTP is used as the transmission protocol, but this process is unnecessary if the transmission protocol is not RTP. 
       FIG. 4  is a view for explaining the flow for generating speech RTP packets from H.223 multiplexed frames.  FIG. 5  is a view for explaining an example in which a speech transcoder is used when generating speech RTP packets from H.223 multiplexed frames. 
     Explanation next regards the flow for generating speech RTP packets from H. 223 multiplexed frames with reference to  FIG. 4  and  FIG. 5 . 
     First, in the example of  FIG. 4 , the AMR encoding mode is uniquely specified from the length of the speech data portion (speech data length a 3 ) contained in H.223 multiplexed frame  401 . The encoded data are then subjected to payload formatting process  402  to packetize as RTP packets and thus generate RTP packet  403 . 
     In the example shown in  FIG. 5 , the AMR encoding mode is specified from the length of the speech data portion (speech data length a 3 ) contained in H.223 multiplexed frame  501 . If the specified encoding mode is a mode not supported by the packet-switching terminal, conversion is implemented to an encoding mode (bit rate) supported by the packet-switching terminal in speech transcoder  502 , and the encoded data following conversion are subjected to payload formatting process  503  to packetize as RTP packets, whereby RTP packets  504  are generated. 
     If it is determined in the capacity exchange with packet-switching terminal that the packet-switching terminal is capable of receiving multiframe RTP packets, a plurality of AMR encoded frames can be contained in the same RTP packet in payload formatting processes  402  and  503 . 
     If speech data are not contained in multiplexed frames that are applied as input to H.223 separation process  303 , encoded data of silence (in the case of AMR, NO DATA frames) are packetized as RTP packets as speech encoded data  304  and transmitted to packet-switching network  104 , whereby sound quality deterioration in packet-switching terminal  102  can be suppressed to a low level. Data that have undergone RTP payload formatting are packetized as RTP packets in speech RTP packetizing process  307 . Speech RTP packets are transmitted to packet-switching terminal  102  by way of packet-switching network  104  by speech transmission process  308 . 
     Explanation next regards images. Image encoded data  309  that have been separated in H.223 separation process  303  are subjected to a process for transmitting image encoded data by RTP in image payload formatting process  310 . The image encoded data following this process are packetized as RTP packets by image RTP packetizing process  311 . Image RTP packets are transmitted to packet-switching terminal  102  by way of packet-switching network  104  by image transmission process  312 . If the transmission protocol is not RTP, image payload formatting process  310  is unnecessary, similar to speech. 
     Explanation next regards the details of the flow of media from packet-switching terminal  102  to circuit-switching terminal  101  with reference to the accompanying drawings. 
       FIG. 6  shows the details of the flow of processes when transmitting media from packet-switching network  100  to circuit-switching network  103 . 
     Connection gateway  100  receives RTP packets of speech from packet-switching network  104  in speech reception process  602 , and receives RTP packets of images in image reception process  607 . Although their sequencing has been shifted by packet-switching network  104 , these RTP packets are put in their original sequencing by speech RTP process  603  or image RTP process  608 . Further, speech encoded data  605  are extracted in speech payload formatting process  604 , and image encoded data  610  are extracted in image payload formatting process  609 . 
     The encoding mode (bit rate) of speech encoded data  605  is uniquely specified based on the RTP payload length obtained by speech RTP process  603 . If speech encoded data  605  are not of an encoding mode supported by circuit-switching terminal  101  obtained in the H.245 negotiation, the encoding bit rate of speech encoded data  605  is converted by speech transcoder  606 . If circuit-switching terminal  101  supports the encoding mode used in the encoding of speech encoded data  605 , the process in speech transcoder  606  is unnecessary. 
     Speech encoded data in which the encoding bit rate has been converted according to necessity are multiplexed together with image encoded data  610  by H.223 multiplexing process  611  and transmitted to circuit-switching terminal  101  by way of circuit-switching network  103  by transmission process  612 . In H. 223 multiplexing process  611 , the speech encoded data length is uniquely ascertained from encoding bit rate (mode) information of speech encoded data that are to be multiplexed, whereby the appropriate table is selected from among the multiplex tables reported to circuit-switching terminal  101  in H.245 and identity information that indicates this multiplex table is stored in the H.223 header. 
       FIG. 7  is a view for explaining the flow of generating H.223 multiplexed frames from RTP (speech) packets.  FIG. 8  is a view for explaining an example of using a speech transcoder when generating H.223 multiplexed frames from RTP (speech) packets. 
     Explanation next regards the flow for generating H.223 multiplexed frames from RTP (speech) packets with reference to  FIG. 7  and  FIG. 8 . 
     In the example of  FIG. 7 , the AMR encoding mode (bit rate) contained in RTP packet  701  is uniquely specified from the RTP payload length of speech RTP packet  701 . The RTP payload is then subjected to payload formatting process  702  to extract the speech encoded data. The image encoded data contained in the image RTP packets received from packet-switching network  104  are similarly extracted. 
     The multiplex table appropriate to the AMR encoding mode is selected from among the H.223 multiplex tables that were reported to circuit-switching terminal  101  in the H.245 negotiation, and this multiplex table is used to multiplex the speech encoded data and image encoded data and generate H. 223 multiplexed frames  703 . 
     In the example of  FIG. 8 , the AMR encoding mode (bit rate) contained in RTP packet  801  is uniquely specified from the RTP payload length of speech RTP packet  801 . If this mode is not supported by circuit-switching terminal  101 , conversion is implemented by speech transcoder  803  to an encoding mode (bit rate) supported by circuit-switching terminal  101 . The speech encoded data following conversion and the image encoded data extracted from the image RTP packets received from packet-switching network  104  are then multiplexed using the multiplex table appropriate to the AMR encoding mode after bit rate conversion that was selected from among the H.223 multiplex tables reported to the circuit-switching terminal in the H.245 negotiation to generate H.223 multiplexed frames  804 . 
     If there are no speech data at the timing of multiplexing of speech encoded data and image encoded data by H.223 multiplexing process  611  (if speech data have not been received from the packet-switching network), encoded data of silence (a NO DATA frame in the case of AMR) should be multiplexed in place of the speech data and transmitted to circuit-switching network  103 , whereby deterioration in sound quality in circuit-switching terminal  101  can be suppressed to a low level. 
     Further, in circuit switching, an upper limit generally applies to the transmission bit rate of H.223 multiplexed data and transmitting multiplexed data of a rate that exceeds this limit causes delays. However, to suppress the bit rate of the multiplexed data, a portion of the audible speech encoded data may also be replaced with encoded data of silence (NO DATA frames in the case of AMR) that has a smaller amount of data (low bit rate) to carry out the H.223 multiplexing process and transmission to circuit-switching network  103  then implemented. 
     Second Embodiment 
     A connection gateway for implementing protocol conversion is arranged between a circuit-switching network and a packet-switching network. The connection gateway, upon receiving speech encoded data and image encoded data from the packet-switching network, determines the encoding bit rate from bit rate information contained in the speech encoded data and then converts the speed bit rate if necessary. The connection gateway is provided with a plurality of multiplex tables in advance, selects a multiplex table according to the speech bit rate after conversion, and uses this multiplex table to multiplex the speech encoded data and image encoded data and transmit to the circuit-switching network. 
     In addition, the connection gateway, upon receiving multiplexed data from the circuit-switching network, separates the multiplexed data into speech encoded data and image encoded data in accordance with multiplex table information. The connection gateway then determines the encoding bit rate from the bit rate information contained in the speech encoded data for the speech encoded data and converts the speech bit rate if necessary. The connection gateway then packetizes the speech encoded data and image encoded data and transmits to the packet-switching network. 
     A detailed explanation follows below with reference to the figures. 
     The second embodiment is similar to the first embodiment but differs in the method of determining the bit rate (mode) of speech encoded data. Explanation here chiefly regards the differing portions. 
     The configuration of the communication system according to the second embodiment is similar to that of the first embodiment shown in  FIG. 1 . In addition, the call connection protocol, transmission protocol, and encoding method of the second embodiment are also similar to the first embodiment but are obviously not limited to the protocols and method that are shown by way of example. 
     In addition, the second embodiment is also similar to the first embodiment regarding the call connection processes between circuit-switching terminal  101  and packet-switching terminal  102 , such as the process of H.245 negotiation and processes relating to multiplex tables ( FIG. 2 ). 
     Explanation next regards the flow of media from circuit-switching terminal  101  to packet-switching terminal  102  with reference to  FIG. 3 . 
     In reception process  302 , transmission protocol converter  106  receives H.223 multiplexed data from circuit-switching terminal  101  by way of circuit-switching network  103 . In the H.223 multiplexed data, multiplex table identity information is appended to each of the multiplexed frames. In H.223 separation process  303 , the encoded data of speech and images are separated based on this multiplex table identity information and multiplex table information received by H.245. 
     Regarding AMR, mode information of the encoding bit rate (encoding bit rate information) is contained at the head of encoded data, and the AMR encoding mode is therefore determined from separated speech encoded data  304 . In this way, changes in the speech encoded data bit rate can be dealt with at each AMR encoded frame (20 msec) at a minimum despite changes during communication in the encoding mode of speech encoded data contained in H. 223 multiplexed data from circuit-switching terminal  101 . 
     If these speech encoded data  304  are not supported by a speech encoding mode included in the capacity information obtained from the packet-switching terminal, the encoding bit rate of speech encoded data  304  is converted by speech transcoder  305 . If the packet-switching terminal supports the encoding mode used in the encoding of speech encoded data  304 , the process in speech transcoder  305  is not necessary. 
     Speech encoded data in which the encoding bit rate has been converted as necessary are formatted to match the format of the RTP payload by speech payload formatting process  306 . In the present embodiment, speech payload formatting process  306  exists because RTP is used as the transmission protocol, but this process becomes unnecessary if the transmission protocol is not RTP. 
     Explanation next regards the flow of media from packet-switching terminal  102  to circuit-switching terminal  101  with reference to  FIG. 6 . 
     Connection gateway  100  receives speech RTP packets from packet-switching network  104  in speech reception process  602 , and receives image RTP packets in image reception process  607 . Even though the sequencing of these RTP packets has been shifted by packet-switching network  104 , the original sequencing is arranged by speech RTP process  603  and image RTP process  608 . Speech encoded data  605  are further extracted in speech payload formatting process  604 , and image encoded data  610  are extracted in image payload formatting process  609 . 
     If speech encoded data  605  are not an encoding mode supported by circuit-switching terminal  101  obtained in the H.245 negotiation, the bit rate of speech encoded data  605  is converted by speech transcoder  606 . If circuit-switching terminal  101  supports the encoding mode used in the encoding of speech encoded data  605 , the process in the speech transcoder  606  is unnecessary. Speech encoded data in which the encoding bit rate has been converted as necessary are multiplexed together with image encoded data  610  by H.223 multiplexing process  611  and transmitted to circuit-switching terminal  101  by way of circuit-switching network  103  by transmission process  612 . In H.223 multiplexing process  611 , the speech encoded data length is uniquely ascertained from the encoding bit rate (mode) information of the speech encoded data that are multiplexed, the appropriate table is selected from among the multiplex tables reported to circuit-switching terminal  101  in H.245, and the identity information that indicates the multiplex table is stored in the H. 223 header. 
     The second embodiment is similar to the first embodiment regarding processing other than the process of identifying the bit rate of AMR encoded frames from the encoding mode information contained in the above-described AMR speech encoded data. 
     Third Embodiment 
     A connection gateway for implementing protocol conversion is arranged between a circuit-switching network and a packet-switching network. The connection gateway, upon receiving speech encoded data and image encoded data from the packet-switching network, determines the encoded frame length from the payload length of the speech packets, separates the received data into encoded frames, and converts the speech bit rate if necessary. The connection gateway is provided with a plurality of multiplex tables in advance, selects a multiplex table according to the speech bit rate after conversion, and uses the multiplex table to multiplex the speech encoded data and image encoded data and then transmit to circuit-switching network. 
     The connection gateway, upon receiving multiplexed data from the circuit-switching network, separates the multiplexed data into speech encoded data and image encoded data according to multiplex table information. The connection gateway then converts the speech bit rate if determined to be necessary based on the encoded data length. The connection gateway next packetizes the speech encoded data and image encoded data and transmits to the packet-switching network. 
     When separating the payload of packets to encoded frames, the speech encoding bit rate obtained from the call connection process may be used. A detailed explanation follows below with reference to the figures. 
     The third embodiment is similar to the first embodiment but differs in that the speech encoded frames contained in H.223 multiplexed frames and speech encoded frames contained in speech RTP packets are both pluralities. Explanation now chiefly regards the portions that differ. 
     The configuration of the communication system according to the third embodiment is similar to that of the first embodiment shown in  FIG. 1 . In addition, the call connection protocol, transmission protocol, and encoding method of the third embodiment are similar to the first embodiment, but the present invention is obviously not limited to the protocol and encoding method shown by way of example. 
     In addition, the third embodiment is similar to the first embodiment with regard to call connection processes between circuit-switching terminal  101  and packet-switching terminal  102  such as the H.245 negotiation and processes relating to multiplex tables ( FIG. 2 ). 
     Explanation next regards the flow of media from circuit-switching terminal  101  to packet-switching terminal  102  with reference to  FIG. 3 . 
     Transmission protocol converter  106  receives H.223 multiplexed data from a terminal by way of circuit-switching network  103  in reception process  302 . In H. 223 multiplexed data, multiplex table identity information is appended to each multiplexed frame. H.223 separation process  303  separates encoded data of speech and images based on this multiplex table identity information and the multiplex table information obtained by H.245. 
     Here, if the length of the speech encoded data is equal to an integer multiple of the length of one AMR encoded frame, the AMR encoding mode (bit rate) is clear from one AMR encoded frame length obtained by dividing the speech encoded data into an integer number of divisions. 
     If the speech encoding mode determined from speech encoded data  304  is not supported by a speech encoding mode contained in the capacity information obtained from the packet-switching terminal, the encoding bit rate of speech encoded data  304  is converted by speech transcoder  305 . If the packet-switching terminal supports the encoding mode used in encoding speech encoded data  304 , the process in speech transcoder  305  is not necessary. Speech encoded data in which the encoding bit rate has been converted as necessary are formatted to match the format of the RTP payload by speech payload formatting process  306 . In the present embodiment, speech payload formatting process  306  exists because RTP is used as the transmission protocol, but this process is unnecessary if the transmission protocol is not RTP. 
       FIG. 9  is a view for explaining the flow in generating speech RTP packets from H.223 multiplexed frames.  FIG. 10  is a view for explaining an example of using a speech transcoder when generating speech RTP packets from H.223 multiplexed frames. 
     Explanation next regards the flow when generating RTP packets of speech from H.223 multiplexed frames with reference to  FIG. 9  and  FIG. 10 . 
     In the example of  FIG. 9 , assuming that the length of the speech data portion (speech data length a 4 ) contained in H.223 multiplexed frame  901  is an integer multiple of AMR encoded frame length f, the AMR encoding mode and frame number can be uniquely specified, and each of AMR encoded frames  902 - 904  can be separated. AMR encoded frame  902  is subjected to payload formatting process  905  to convert to RTP packets and thus generate RTP packet  906 . 
     In the example of  FIG. 10 , if the length of the speech data portion (speech data length a 4 ) contained in H.223 multiplexed frame  1001  is an integer multiple of AMR encoded frame length f, the AMR encoding mode and frame number can be uniquely specified based on this relation. If the AMR encoding mode is a mode not supported by the packet-switching terminal, conversion is implemented to an encoding mode (bit rate) that is supported by the packet-switching terminal by means of speech transcoder  1005 . The encoded data following conversion are then subjected to payload formatting process  1006  to packetize as RTP packets and thus generate RTP packets  1007 . 
     If it is clear from the capacity exchange with the packet-switching terminal that the packet-switching terminal has the capability to receive RTP packets of multiframes, a plurality of AMR encoded frames can be contained in the same RTP packet in payload formatting processes  905  and  1006 . If the packet-switching terminal lacks the capability to receive RTP packets of multiframes, an RTP packet payload formatting process and RTP packetizing are carried out for each speech encoded frame. 
     Explanation next regards the flow of media from packet-switching terminal  102  to circuit-switching terminal  101  with reference to  FIG. 6 . 
     Connection gateway  100  first receives speech and image RTP packets from packet-switching network  104  in speech reception process  602  and image reception process  607 , respectively. The sequencing of these RTP packets that has been shifted by packet-switching network  104  is rearranged by means of speech RTP process  603  or image RTP process  608 . Speech encoded data  605  are extracted in speech payload formatting process  604 , and image encoded data  610  are extracted in image payload formatting process  609 . 
     In speech payload formatting process  604 , if a plurality of AMR encoded frames is contained in one speech RTP packet, a plurality of speech encoded data items are obtained by separating into each individual AMR encoded frame. If RTP or RTP payload formatting is not used as the transmission protocol, the length of the payload portion is an integer multiple of one speech encoded frame, and each individual can therefore be separated. In this process, the frame length can be estimated from the encoding mode that is supported by packet-switching terminal  102  and obtained in the call connection process. 
     If speech encoded data  605  are not an encoding mode supported by circuit-switching terminal  101  and obtained in the H.245 negotiation, a speech transcoder  606  converts the bit rate of speech encoded data  605 . If the circuit-switching terminal supports the encoding mode used in encoding speech encoded data  605 , the process in speech transcoder  606  is unnecessary. Speech data in which the encoding bit rate has been converted as necessary are multiplexed together with image encoded data  610  by H.223 multiplexing process  611  and transmitted by way of circuit-switching network  103  to the circuit-switching terminal by transmission process  612 . In H.223 multiplexing process  611 , the speech encoded data length is uniquely distinguished by the encoding bit rate (mode) information of the multiplexed speech encoded data. As a result, the speech data length that is a multiple of the number of speech encoded frames contained in the same H.223 frame is ascertained, the appropriate table is selected from among the multiplex tables reported to the circuit-switching terminal in H.245, and identity information indicating this multiplex table is stored in the H.223 header. 
       FIG. 11  is a view for explaining the flow of generating H.223 multiplexed frames from RTP (speech) packets.  FIG. 12  is a view for explaining an example of using a speech transcoder when generating H.223 multiplexed frames from RTP (speech) packets. 
     Explanation next regards the flow of generating H.223 multiplexed frames from RTP (speech) packets with reference to  FIG. 11  and  FIG. 12 . 
     In the example of  FIG. 11 , speech data, which are the RTP payload of speech RTP packet  1101 , are subjected to payload formatting process  1102  to extract individual AMR encoded frames  1103 - 1105 . When RTP and RTP payload formatting are not followed as the transmission protocol and if the payload length is an integer multiple of one speech encoded frame, each individual speech encoded frame is separated based on this relation. These speech encoded frames and the image encoded data that were contained in the image RTP packets similarly received from packet-switching network  104  are multiplexed to generate H.223 multiplexed frames  1106 . In this multiplexing, of the H.223 multiplex tables that were reported to circuit-switching terminal  101  in the H.245 negotiation, the multiplex table is used that is appropriate to the AMR encoding mode according to the sum of the encoded frame lengths contained in the same H.223 multiplexed frame. In the example of  FIG. 11 , a plurality of speech encoded frames are multiplexed in the speech data portion of one H.223 multiplexed frame. However, if there are only tables corresponding to a single speech encoded frame in the speech data portion of the H.223 multiplex tables, a plurality of H.223 multiplexed frames are generated in which individual speech encoded frames are each multiplexed. In the example of  FIG. 12 , speech data that are the RTP payload of speech RTP packet  1201  are subjected to payload formatting process  1202  to extract each of individual AMR encoded frames  1203 - 1205 . The AMR encoding mode (bit rate) is uniquely specified from the length of these AMR encoded frames. If the specified AMR encoding mode is a mode not supported by circuit-switching terminal  101 , speech transcoder  1206  effects conversion to an encoding mode (bit rate) that is supported by circuit-switching terminal  101 . The obtained speech encoded frames are multiplexed with image encoded data that were contained in image RTP packets received from packet-switching network  103  to generate H.223 multiplexed frames  1207 . In this multiplexing, of the H.223 multiplex tables reported to the circuit-switching terminal in the H.245 negotiation, the multiplex table is used that is appropriate to the AMR encoding mode following bit rate conversion. In the example of  FIG. 12 , a plurality of speech encoded frames are multiplexed in the speech data portion of one H. 223 multiplexed frame. However, if there are only tables corresponding to a single speech encoded frame in the speech data portion of H.223 multiplex tables, a plurality of H.223 multiplexed frames are generated in which individual speech encoded frames are each multiplexed. 
     In  FIGS. 11 and 12 , examples were described in which the number of AMR encoded frames contained in one H.223 multiplexed frame was three, but the number of AMR encoded frames contained in a multiplexed frame is of course not limited to this number. 
     Apart from the above-described content, the third embodiment is the same as the first embodiment. 
     Fourth Embodiment 
     A connection gateway for implementing protocol conversion is arranged between a circuit-switching network and a packet-switching network. The connection gateway, upon receiving speech encoded data and image encoded data from the packet-switching network, determines one encoded frame length from the bit rate information contained in the speech encoded data to separate into encoded frames, and if necessary, converts the speech bit rate. The connection gateway is provided with a plurality of multiplex tables in advance, selects a multiplex table according to the speech bit rate after conversion, and uses the multiplex table to multiplex the speech encoded data and image encoded data and transmit to the circuit-switching network. 
     Upon receiving multiplexed data from the circuit-switching network, the connection gateway separates the multiplexed data into speech encoded data and image encoded data in accordance with the multiplex table information. The connection gateway then, regarding the speech encoded data, converts the speech bit rate if determined to be necessary from the bit rate information contained in the speech encoded data. The connection gateway then packetizes each of the speech encoded data and image encoded data and transmits to the packet-switching network. In addition, the speech encoding bit rate obtained from the call connection process may be used when separating the packet payload in encoded frame units. 
     A detailed explanation follows below with reference to the accompanying drawings. 
     The fourth embodiment is similar to the third embodiment, but differs only with respect to the method of determining the bit rate (mode) of speech encoded data. Explanation here chiefly regards the differing points. 
     The configuration of the communication system according to the fourth embodiment is similar to that of the first embodiment shown in  FIG. 1 . In addition, the call connection protocol, transmission protocol, and encoding method in the fourth embodiment are all similar to the first embodiment, but the fourth embodiment is of course not limited to the protocols and encoding method shown by way of example. 
     In addition, the fourth embodiment is also similar to the first embodiment regarding the call connection processes of the circuit-switching terminal  101  and packet-switching terminal  102  such as the process of the H.245 negotiation and the process regarding the multiplex tables ( FIG. 2 ). Explanation next regards the flow of media from circuit-switching terminal  101  to packet-switching terminal  102  with reference to  FIG. 3 . 
     In reception process  302 , transmission protocol converter  106  receives H.223 multiplexed data from circuit-switching terminal  101  by way of circuit-switching network  103 . In the H.223 multiplexed data, multiplex table identity information is appended to each of the multiplexed frames. H.223 separation process  303  separates the encoded data of speech and images based on this multiplex table identity information and multiplex table information obtained in H.245. Here, AMR contains mode information of the encoding bit rate at the head of encoded data, and the AMR encoding mode is therefore clear from the data of separated speech encoded data  304 , whereby changes in the encoding mode of speech encoded data contained in H.223 multiplexed data from circuit-switching terminal  101  during communication can be dealt with in changes of the speech encoded bit rate for each AMR encoded frame (20 msec) at a minimum. 
     If these speech encoded data  304  are not supported by a speech encoding mode contained in the capacity information obtained from the packet-switching terminal, the encoding bit rate of speech encoded data  304  is converted by speech transcoder  305 . If the packet-switching terminal supports the encoding mode used in encoding speech encoded data  304 , the process in speech transcoder  305  is unnecessary. 
     The speech encoded data in which the encoding bit rate has been converted as necessary are formatted to match the format of the RTP payload by speech payload formatting process  306 . Speech payload formatting process  306  exists in the present embodiment because RTP is used as the transmission protocol, but this process is unnecessary if the transmission protocol is not RTP. Explanation next regards the flow of media from packet-switching terminal  102  to circuit-switching terminal  101  with reference to  FIG. 9  and  FIG. 10 . 
     In the example of  FIG. 9 , the frame length of speech encoded frame  902  is clear from encoding mode information contained at the head of the speech encoded data. The frame length of the next speech encoded frame  903  is clear from the encoding mode information contained in the head of speech encoded frame  903 . The frame length of the next speech encoded frame  904  is clear from the encoding mode information contained at the head of speech encoded frame  904 . In this way, each of encoded frames  902 - 904  can be separated, and the encoding mode (bit rate) of each is clear. AMR encoded frame  902  is next subjected to payload formatting process  905  and converted to RTP packets to generate RTP packet  906 . 
     On the other hand, in the example of  FIG. 10 , each of individual speech encoded frames  1002 - 1004  is separated based on the encoding mode information contained in the speech data contained in H.223 multiplexed frame  1001 , as in the case of  FIG. 9 , and the encoding mode information can be acquired. If speech encoded frames  1002 - 1004  are encoding modes not supported by packet-switching terminal  102 , the encoding modes are converted by speech transcoder  1005  to encoding modes (bit rate) supported by the packet-switching terminal. The encoded data after conversion are then subjected to payload formatting process  1006  to convert to RTP packets to generate RTP packet  1007 . 
     If it is clear from the capacity exchange with the packet-switching terminal that the packet-switching terminal is capable of receiving multiframe RTP packets, payload formatting processes  905  and  1006  can contain a plurality of AMR encoded frames in the same RTP packet. 
     Explanation next regards the transmission of media from packet-switching terminal  102  to circuit-switching terminal  101  with reference to  FIG. 6 . Connection gateway  100  receives RTP packets of speech and images from packet-switching network  104  in speech reception process  602  and image reception process  607 , respectively. The sequencing of these RTP packets that has been shifted by packet-switching network  104  is rearranged by speech RTP process  603  and image RTP process  608 . Speech encoded data  605  are extracted in speech payload formatting process  604 , and image encoded data  610  are extracted in image payload formatting process  609 . 
     In speech payload formatting process  604 , if a plurality of AMR encoded frames is contained in one speech RTP packet, each of the AMR encoded frames is separated to acquire a plurality of speech encoded data items. If RTP or RTP payload format is not used as the transmission protocol, the length of one speech encoded frame can be specified from the encoding mode information contained at the head of each speech encoded frame, whereby each individual encoded frame is separated based on this length of one speech encoded frame. 
     Regarding content other than that described above, the fourth embodiment is the same as the third embodiment. 
     Fifth Embodiment 
       FIG. 13  is a block diagram showing the configuration of a computer processing system for realizing the connection gateway in the fifth embodiment. This computer processing system is realized as a protocol conversion server and executes a protocol conversion process for enabling communication of media between a packet-switching network and a circuit-switching network similar to the first to fourth embodiments. 
     Referring to  FIG. 13 , the computer processing system is made up from data processing unit (CPU)  1301 , bit rate converter (speech transcoder)  1302 , memory unit  1303 , call connection unit  1304 , and input/output unit  1305 . Bit rate converter  1302  converts the data rate of speech encoded data. Memory unit  1303  stores various types of data such as processing programs and the information of a plurality of multiplex tables. Call connection unit  1304  carries out the call connection processes that include such processes as the H.245 negotiation. Input/output unit  1305  realizes the input to and output from data processing unit  1301  of the media that are exchanged between packet-switching network  104  and circuit-switching network  103 . 
     The processes relating to transmission protocol converter  106 , speech transcoder  107 , and call connection unit  105  in connection gateway  100  shown in  FIG. 1  are chiefly executed by data processing unit  1301  of  FIG. 13 . 
     The basic functions of the fifth embodiment are similar to those of the first embodiment. The fifth embodiment is further similar to the first embodiment with regard to the call connection protocol, transmission protocol, and encoding method. The fifth embodiment is further similar to the first embodiment regarding the call connection processes between circuit-switching terminal  101  and packet-switching terminal  102  such as the H.245 negotiation and the multiplex tables ( FIG. 2 ). The present invention is not limited to particular protocols and encoding method. 
     The processes of connection gateway  100  of the present embodiment are carried out by the reading and execution, by data processing unit  1301 , of the processing program for protocol conversion that has been stored in memory unit  1303 . In other words, the processing program that has been read controls the operations of data processing unit  1301 , whereby data processing unit  1303  carries out the processes for protocol conversion. 
     To begin bidirectional communication between terminals  101  and  102  that are connected to circuit-switching network  103  and packet-switching network  104 , respectively, requires various negotiations relating to protocol conversion by way of call connection unit  1304 . Call connection unit  1304  carries out the exchange of, for example, an H.245 negotiation regarding the change of multiplex tables when altering the bit rate in communication between terminals. In addition, call connection unit  1304  acquires exchange capacity information for comprehending SIP/SDP relating to terminal  102  of packet-switching network  104 . Necessary information such as a plurality of multiplex tables (information) is then stored in memory unit  1303  in advance by the notification of all multiplex tables (the allotment of data) by means of negotiations by the processing of call connection unit  1304  and data processing unit  1301 . 
       FIG. 14  is a view showing the basic processing functions and the flow of processes of data processing unit  1301  for media that are transmitted and received between circuit-switching network  103  and packet-switching network  104 .  FIG. 14  shows an example that corresponds to the first and second embodiments. The actual flow of processes for media conversion between circuit-switching network  103  and packet-switching network  104  are as shown in  FIGS. 3-8 , and this figure therefore shows only the basic flow of processes. Explanation next regards the processing functions and flow of media between a circuit-switching terminal and packet-switching terminal according to the protocol conversion processes that correspond to the first embodiment in the fifth embodiment with reference to  FIG. 14 . 
     Data processing unit  1301 , upon receiving packets from packet-switching network  104  ( 1402 ), refers to the speech encoding bit rate based on the payload length of these speech packets ( 1403 ). Data processing unit  1301  further, upon receiving multiplexed data from circuit-switching network  103  ( 1409 ), refers to the speech encoding bit rate based on the speech data length obtained from the multiplex table of these multiplexed data ( 1410 ). 
     Data processing unit  1301  then determines whether bit rate conversion is necessary from the payload length of speech packets, and if necessary, speech encoded data and image encoded data received from packet-switching network  104  undergo bit-rate conversion to the bit rate of circuit-switching network  103  in bit rate converter  1302  ( 1404 ). Data processing unit  1301  next selects from memory unit  1303  the appropriate multiplex table that corresponds to the speech bit rate following conversion ( 1405 ), multiplexes the speech encoded data based on this multiplex table ( 1406 ), and transmits the obtained multiplexed data to circuit-switching network  103  ( 1407 ). 
     Data processing unit  1301  further, in accordance with the multiplex table information ( 1410 ), separates the multiplexed data received from circuit-switching network  103  to speech encoded data and image encoded data ( 1411 ). Data processing unit  1301  then determines the bit rate from the encoded data length for the speech encoded data and converts the bit rate if necessary ( 1412 ). Data processing unit  1301  then packetizes each of speech encoded data and image encoded data ( 1413 ) and transmits to packet-switching network  103  ( 1414 ). 
     Explanation next regards the flow of media between circuit-switching terminal  101  and packet-switching terminal  102  by means of the protocol conversion process that corresponds to the second embodiment in the fifth embodiment. Data processing unit  1301 , upon receiving packets from packet-switching network  104  ( 1402 ), extracts bit rate information contained in speech encoded data obtained from these packets ( 1403 ). Data processing unit  1301  further, upon receiving multiplexed data from circuit-switching network  103  ( 1409 ), extracts bit rate information contained in the speech encoded data obtained from these multiplexed data ( 1410 ). 
     Data processing unit  1301  then determines whether bit rate conversion is necessary from the bit rate information contained in the speech encoded data, and if necessary, implements bit-rate conversion of the speech encoded data and image encoded data received from packet-switching network  104  in bit rate converter  1302  ( 1404 ). Data processing unit  1301  next selects from memory unit  1303  the appropriate multiplex table based on the speech bit rate ( 1405 ) and uses this multiplex table to multiplex ( 1406 ) and transmit to circuit-switching network  103  ( 1407 ) 
     Data processing unit  1301  further, based on the multiplex table information ( 1410 ), separates the multiplexed data received from circuit-switching network  103  into speech encoded data and image encoded data ( 1411 ). Data processing unit  1301  then, for the speech encoded data, determines whether bit rate conversion is necessary from the bit rate information contained in the speech encoded data, and if necessary, implements bit rate conversion ( 1412 ). Data processing unit  1301  then packetizes each of the speech encoded data and image encoded data ( 1413 ) and transmits to packet-switching network  104  ( 1414 ). 
       FIG. 15  shows the basic processing functions and the flow of processes of data processing unit  1301  for media that are transmitted and received between circuit-switching network  103  and packet-switching network  104 .  FIG. 15  shows an example that corresponds to the third and fourth embodiments. The actual media conversion process flow between circuit-switching network  103  and packet-switching network  104  is as shown in  FIGS. 9-12 , and this figure shows only the basic process flow. 
     Explanation next regards the flow of media between a circuit-switching terminal and a packet-switching terminal by the protocol conversion processes that correspond to the third embodiment in the fifth embodiment with reference to  FIG. 15 . The protocol conversion process shown here is an example that takes as an object speech encoded data that contain a plurality of speech encoded frames. 
     Data processing unit  1301 , upon receiving packets from packet-switching network  104  ( 1502 ), refers to the speech encoding bit rate from the payload length obtained from these packets ( 1503 ). Data processing unit  1301  further, upon receiving multiplexed data from circuit-switching network  103  ( 1510 ), refers to the bit rate from the speech data length obtained from the multiplex table of these multiplexed data ( 1511 ). 
     Data processing unit  1301  then, upon receiving speech encoded data and image encoded data from packet-switching network  104 , determines the length of one encoded frame of the plurality of speech encoded frames of the speech encoded data from the payload length of a speech packet, and separates the encoded data into encoded frames based on the length of one encoded frame ( 1504 ). Data processing unit  1301  further converts bit rate of the speech encoded data if necessary ( 1505 ). Data processing unit  1301  then selects the appropriate multiplex table from memory unit  1303  in accordance with the speech bit rate after conversion ( 1506 ), multiplexes in accordance with the multiplex table ( 1507 ), and transmits to circuit-switching network  103  ( 1508 ). Data processing unit  1301 , in accordance with the multiplex table information ( 1511 ), separates multiplexed data received from circuit-switching network  103  into speech encoded data and image encoded data ( 1512 ). Data processing unit  1301  then determines bit rate from the encoded data length for the speech encoded data, and if necessary, implements bit rate conversion ( 1513 ). Data processing unit  1301  next packetizes each of speech encoded data and image encoded data ( 1514 ) and transmits to packet-switching network  104  ( 1515 ). When separating the payload of speech packets into encoded frame units in the above-described process, a processing function may be provided for obtaining the speech encoding bit rate from the call connection process of call connection unit  1304  and the speech encoding bit rate obtained thereby may then be used. 
     Explanation is next given by  FIG. 15  of the flow of media between a circuit-switching terminal and a packet-switching terminal by the protocol conversion process that corresponds to the fourth embodiment in the fifth embodiment. The protocol conversion process also takes as an object speech encoded data composed of a plurality of speech encoded frames. 
     Data processing unit  1301 , upon receiving packets from packet-switching network  104  ( 1502 ), extracts bit rate information contained in speech encoded data obtained from these packets ( 1503 ). Data processing unit  1301  further, upon receiving multiplexed data from circuit-switching network  103  ( 1510 ), extracts bit rate information contained in speech encoded data obtained from these multiplexed data ( 1511 ). 
     Data processing unit  1301  then determines the length of one encoded frame from the bit rate information contained in the speech encoded data and separates the speech encoded data and image encoded data received from packet-switching network  104  into encoded frames based on the length of one encoded frame ( 1504 ). Data processing unit  1301  further implements bit rate conversion of the encoded frames if necessary ( 1505 ). Data processing unit  1301  next selects from memory unit  1303  the appropriate multiplex table according to the speech bit rate after conversion ( 1506 ), multiplexes by means of this multiplex table ( 1507 ), and transmits to circuit-switching network  103  ( 1508 ). 
     Data processing unit  1301  further, in accordance with the multiplex table information ( 1511 ), separates multiplexed data received from circuit-switching network  103  into speech encoded data and image encoded data ( 1512 ). Data processing unit  1301  determines from bit rate information contained in the speech encoded data whether bit rate conversion is necessary for the speech encoded data, and if necessary, implements bit rate conversion ( 1513 ). Data processing unit  1301  next packetizes each of speech encoded data and image encoded data ( 1514 ) and transmits to packet-switching network  104  ( 1515 ). 
     In the above-described processes, a processing function may be added for obtaining the speech encoding bit rate from the call connection process of call connection unit  1304  when separating the payload of speech packets into encoded frame units, and the speech encoding bit rate obtained thereby may then be used.