Abstract:
In a network system where a switching exchange accommodating an ISDN provides TDMoIP packet communication over an IP network, measures for packet loss prevention need to be taken during a period during which a TDM service is packet-transferred over the IP network. When a transmitting-end switching exchange in the above network system multiplexes TDM data, the present invention varies the degree of multiplicity in accordance with the delay in the IP network and the type of data.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The present application claims priority from Japanese applications JP2008-326792 filed on Dec. 24, 2008, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a technology for providing improved reliability and enhanced efficiency when TDMoIP packet loss occurs in an IP network in a situation where a network system and a switching exchange are used to handle both an ISDN and an IP network. 
         [0003]    TDM (Time Division Multiplexing) over IP (hereinafter referred to as TDMoIP) achieves PRI (Primary Rate Interface) transmission, T1 transmission, and E1 transmission over an IP-based network or Ethernet (registered trademark)-based network. 
         [0004]    A TDMoIP transmission technology mainly involves the use of two different methods. One method is to replace a TDM network and an end-user device entirely with a communication device provided with mechanisms for voice transmission and signaling. The other method is to tunnel TDM data through a packet network by using an existing PBX (Private Branch eXchange) and multiplexer as they are. Business enterprises expect existing devices to make use of an IP network with a view toward taking advantage of cost reduction by integrating their own voice and data networks without squandering the investment in a conventional PBX or TDM device. 
         [0005]    In an apparatus that can use an existing PBX and multiplexer as they are, TDM clock jitter and wander are strictly defined for a TDM-based device. Therefore, synchronism is maintained with an extremely small delay. Meanwhile, when an IP-based packet method is used, packet delay and packet loss occur due to a conflict with bandwidth and router ports. A source device transmits packets at regular intervals over a network. However, the network does not guarantee that the packets will be delivered to a destination device at the same intervals or in the same sequence. In some cases, the packets will not be delivered to the destination device. 
         [0006]    An ISDN (Integrated Services Digital Network) is no exception. It is anticipated that packet loss may also occur due to the connection of the aforementioned IP network in a situation where the IP network is integrated with an existing ISDN. When packet loss occurs, for example, the quality of communication between telephone terminals deteriorates. Measures for packet loss prevention during a packet transfer period of an IP network have been studied (as described, for instance, in JP-A-2007-60345) because they need to be taken when an ISDN, which is a TDM service, is to be integrated into an IP-based network. 
         [0007]    The technology described in JP-A-2007-60345 packetizes a data stream in a network where a data stream is transmitted and received, copies the packets, and transmits the packets in multiple sessions. If a receiving end has a copied packet (having the same sequence number as another packet), which is copied in accordance with an RTP (Real Time Protocol) sequence number in a packet, such a copied packet is discarded. If only a copied packet has arrived at the receiving end due to packet loss, such a packet is decoded into a data stream. 
       SUMMARY OF THE INVENTION 
       [0008]    The technology described in JP-A-2007-60345 makes multiple copies of packets which are data identical with each other. Thus, the copied packets are also equal in a destination address and other items of information within an IP header. Therefore, these packets are routed along the same path. The packets can be retransmitted by predefining the number of transmissions and the transmission intervals. Further, it is possible to prevent packet loss due to bursty high traffic. However, if the path becomes physically defective, packet loss may occur because the packets cannot be recovered. In addition, an IP header is attached to an RTP packet, which is small in data size. Therefore, particularly when multiple copies are made, it is anticipated that poor bandwidth efficiency may result because the header size is relatively larger than the data size. There is a trade-off between bandwidth efficiency and packet recovery technology. 
         [0009]    To solve the above-described conventional problem and recover TDMoIP packets, the degree of multiplicity is varied with the delay in an IP network and the amount of TDM data to be transmitted. This increases the transfer efficiency and the bandwidth efficiency of low-delay transfer in consideration of bandwidth conditions. Further, generation copies obtained by multiplexing TDM data are used instead of mere packet copies. Therefore, IP headers are not duplicated so that TDMoIP packets are unfailingly delivered to a receiving-end switching exchange even if a trouble occurs in the same path. Consequently, it is possible to provide a technology for recovering TDMoIP packets even when packet loss occurs in an IP network. 
         [0010]    According to one aspect of the present invention, there is provided a network system including: a transmitter for transmitting packet data, which is obtained by packetizing time-division-multiplexed data, to a network; and a receiver for receiving the packet data. The transmitter includes a first memory section for storing segment data, which is obtained by segmenting the time-division-multiplexed data; a control section for controlling the generation of duplicate data from the segment data; a first packet processing section for multiplexing the segment data and converting the multiplexed segment data to packet data; and a transmission section for transmitting the packet data to the network. The receiver includes a reception section for receiving the packet data and a second packet processing section for converting the received packet data to time-division-multiplexed data. 
         [0011]    According to another aspect of the present invention, there is provided a communication method for using a transmitter for transmitting packet data, which is obtained by packetizing time-division-multiplexed data, to a network; a receiver for receiving the packet data; and a network for connecting the transmitter and the receiver, the communication method including the steps of: segmenting the time-division-multiplexed data received by the transmitter; storing the resulting segment data in a first memory section; reading the segment data from the first memory section and generating duplicate data; causing a first packet processing section to multiplex the segment data and convert the multiplexed segment data to packet data; transmitting the packet data from the transmitter to the network; and causing a second packet processing section to convert the packet data, which is received by the receiver through the network, to time-division-multiplexed data. 
         [0012]    When the TDM data is to be converted to IP packets, a transmitting-end switching exchange may store the data in a TDMoIP packet payload section with the data size of a data queue section. Further, the size of the TDM data may be rendered variable as desired by data queue control provided by the data queue section. Furthermore, when a TDMoIP packet processing section converts the TDM data to IP packets, the TDM data may be multiplexed and stored in the TDMoIP packet payload section. Moreover, when the TDM data is to be multiplexed, the degree of multiplicity may be varied with the type of TDM data and the delay time of the IP network. 
         [0013]    A protocol identification section may be rendered capable of identifying the type of TDM data. Further, a data management table in a memory may be rendered capable of calculating the delay time of the IP network from the timestamp of a TDMoIP packet and the time of TDMoIP reception by a receiving-end switching exchange. 
         [0014]    A delay information packet generation section may transmit a delay information packet indicative of the delay time to the transmitting-end switching exchange and use the delay information packet to vary the degree of multiplicity. Further, the receiving-end switching exchange may reassemble TDM data from TDMoIP packets by causing the TDMoIP packet processing section to compensate for lost packets when packet loss occurs and discard redundant TDMoIP payload data when no packet loss occurs. Moreover, the TDMoIP packet processing section may discard redundant TDMoIP payload data when a duplicate sequence number is revealed by TDMoIP header information. 
         [0015]    When TDMoIP packet loss occurs in an IP network in a situation where a network system and a switching exchange are used to handle both an ISDN and an IP network, the present invention makes it possible to provide enhanced reliability in packet recovery by making generation copies of TDM data and increase the transfer efficiency and the bandwidth efficiency of low-delay transfer in consideration of the delay in the IP network and the type of TDM data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a diagram illustrating a communication system that represents an exemplary basic embodiment of the present invention. 
           [0017]      FIG. 2  is a block diagram illustrating a switching exchange according to the present invention. 
           [0018]      FIG. 3  is a diagram illustrating a transmitting-end data management table. 
           [0019]      FIG. 4  is a diagram illustrating a receiving-end data management table. 
           [0020]      FIG. 5  is a diagram illustrating a sequence number comparison table. 
           [0021]      FIG. 6  is a diagram illustrating a data queue. 
           [0022]      FIG. 7  is a diagram illustrating a TDMoIP packet. 
           [0023]      FIG. 8  is a diagram illustrating a delay information packet. 
           [0024]    FIGS.  9 ( 1 ) to  9 ( 5 ) are diagrams illustrating an operation performed by a transmitting-end switching exchange according to a first embodiment of the present invention. 
           [0025]      FIG. 10  is a flowchart illustrating an operation performed by the transmitting-end switching exchange according to the first embodiment and second embodiment of the present invention. 
           [0026]    FIGS.  11 ( 1 ) to  11 ( 5 ) are diagrams illustrating an operation performed by a receiving-end switching exchange according to the first embodiment of the present invention. 
           [0027]      FIG. 12  is a flowchart illustrating an operation performed by the receiving-end switching exchange according to the first embodiment of the present invention. 
           [0028]      FIG. 13  is a diagram illustrating a sequence number comparison table for use in the first embodiment of the present invention. 
           [0029]    FIGS.  14 ( 1 ) to  14 ( 4 ) are diagrams illustrating a delay information packet and an operation performed by a transmitting-end switching exchange according to a second embodiment of the present invention. 
           [0030]      FIG. 15  is a diagram illustrating a transmitting-end data management table for use in the second embodiment of the present invention. 
           [0031]      FIG. 16  is a diagram illustrating a receiving-end data management table for use in the second embodiment of the present invention. 
           [0032]    FIGS.  17 ( 1 ) to  17 ( 3 ) are diagrams illustrating an operation performed by the transmitting-end switching exchange according to the second embodiment of the present invention. 
           [0033]      FIG. 18  is a diagram illustrating a sequence number comparison table for use in the second embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 
       First Embodiment 
       [0035]    A first embodiment of the present invention will now be described.  FIG. 1  is an overall view of a communication system in which a switching exchange  40  ( 41 ) is used. The switching exchange  40  ( 41 ) is used for a subscriber and connected to an ISDN line to which a telephone terminal  10  ( 11 ) and a PC terminal  20  ( 21 ) are connected through a terminal adapter  30  ( 31 ) (connection/signal conversion device). The terminal adapter  30  ( 31 ) is hereinafter referred to as the TA  30  ( 31 ). 
         [0036]    Various data are handled by the telephone terminal  10  and PC terminal  20 . For example, the telephone terminal  10  handles continuous voice data and the PC terminal  20  handles single-shot data for email transmission/reception. All such data are passed through the TA  30  and converted to IP packets in the transmitting-end switching exchange  40 . These IP packets are handled as TDMoIP packets  1000 , passed through an IP network  50 , converted to TDM data in the receiving-end switching exchange  41 , and forwarded to the telephone terminal  11  and PC terminal  21  through the TA  31 . These steps are performed to provide a conversation between the telephone terminal  10  and telephone terminal  11  and email transmission/reception between the PC terminal  20  and PC terminal  21 . 
         [0037]      FIG. 2  is a diagram illustrating the configuration of the switching exchange  40  ( 41 ). Data transmitted from the telephone terminal  10  ( 11 ) and PC terminal  20  ( 21 ) is forwarded to the TA  30  ( 31 ), passed through an ISDN line, and terminated by a circuit termination section  401  ( 411 ). This data is TDM data and transmitted in the form of a continuous frame. The circuit termination section  401  ( 411 ) is controlled by a CPU  403  ( 413 ) and a control section  402  ( 412 ). To determine whether the TDM data is voice data of a telephone or the like or common data such as email data, a protocol identification section  406  identifies the protocol of the TDM data. 
         [0038]    When voice data is handled, the degree of multiplicity, which will be described later, is decreased in consideration of delay. When, on the other hand, email data or other common data is handled, the degree of multiplicity is increased to enhance the transfer efficiency and the bandwidth efficiency of low-delay transfer. 
         [0039]    A TDM packet processing section  408  ( 418 ) converts the TDM data to IP packets. The TDMoIP packets  1000  are transmitted to the IP network  50  through a packet transmission configuration (a packet SW section  409  ( 419 ) and a port section  404  ( 414 )). All the above steps are common to the transmitting-end switching exchange  40  and receiving-end switching exchange  41 . 
         [0040]    The following description applies to the transmitting-end switching exchange  40  only. The data to be converted to TDMoIP packets is temporarily queued in a data queue  40 B in a memory section  40 A. Data queues can be set up as desired. The number of data queues that are set represents the number of generations of copies. A data queue control section  40 C controls the generation copies (duplicates) in the data queues. 
         [0041]    Next, the TDM data needs to be multiplexed before being converted to TDMoIP packets. The degree of multiplicity is determined in accordance with the type of data as described earlier or determined in accordance with delay in the IP network as described later. Multiplexing is provided by the TDM packet processing section  408 . The degree of multiplicity is determined on the basis of a data management table  40 D in the memory section  40 A. Multiplexed TDM data is stored in a TDMoIP payload section  1001  which will be described later with reference to  FIG. 6 . An IP header  1005 , a UDP header  1004 , a timestamp  1003 , and a sequence number  1002  are attached to the payload section to generate a TDMoIP packet  1000 . The TDMoIP packet  1000  is transmitted over the IP network  50  to the receiving-end switching exchange  41  via the packet SW section  409  and port section  404 . At the time of transmission, the data stored in the TDMoIP payload section  1001  is copied to make generation copies. The number of data queues  40 B that are set as described later with reference to  FIG. 6  represents the number of generations of copies. 
         [0042]    The following description applies to the receiving-end switching exchange  41  only. TDMoIP packets  1000  received through a reception configuration (the port section  414  and the packet SW section  419  in a protocol processing section  415 ) are disassembled in the TDM packet processing section  418  and reassembled into TDM data. Packet disassembly is achieved by removing the IP header  1005 , UDP header  1004 , timestamp  1003 , and sequence number  1002 . When the packets are disassembled, a sequence number comparison table  41 E is searched for previously received TDMoIP packets  1000 . When any existing sequence number coincides with a sequence number  41 E- 1  in the table, duplicate data in copied TDMoIP payload  10001  is discarded. The duplicate data is memorized in the field of payload internal data  41 E- 2 , which will be described later with reference to  FIG. 5 , in accordance with a data count  1000 - 1 - 3 , which will be described later with reference to  FIG. 7 , and length data  1000 - 1 - 2  attached to each piece of data  1000 - 1 - 1 . If no existing sequence number coincides with a sequence number  41 E- 1  in the table, it is possible that packet loss has occurred in the IP network  50 . In this instance, therefore, already delivered TDMoIP packets  1000  are reassembled into TDM data. The TDM data are multiplexed. Therefore, after all the multiplexed TDM data are obtained upon reassembly, they are converted to voice or other data. The resulting data is then transmitted to the telephone terminal  11  and PC terminal  21  through the circuit termination section  411  and TA  31 . 
         [0043]    Delay information for determining the degree of multiplicity will now be described. A delay information packet processing section  417  transmits a delay information packet  2000  from the receiving-end switching exchange  41  to the transmitting-end switching exchange  40 . The delay information packet  2000  contains delay information indicative of delay time between the transmitting-end switching exchange  40  and receiving-end switching exchange  41 , which is derived from a data management table  41 D in the receiving-end switching exchange  41 . 
         [0044]    The transmitting-end switching exchange  40  receives the delay information packet  2000 , causes a delay information packet processing section  407  to disassemble the packet and acquire the delay information  2000 - 1 , and writes the acquired delay information  2000 - 1  into the data management table  40 D. The delay information is then used to calculate the degree of multiplicity, which varies with the delay in the IP network. 
         [0045]      FIG. 3  is a diagram illustrating the data management table  41 D for the receiving-end switching exchange  41 . When a TDMoIP packet  1000  is received, a timestamp  1000 - 2 , which will be described later with reference to  FIG. 7 , is stored in the field of transmitting-end switching exchange transmission time  41 D- 1 , and the time of TDMoIP packet reception is stored in the field of receiving-end switching exchange reception time  41 D- 2 . The difference between the above two stored values is then calculated as the delay time of the IP network and stored in the field of delay time  41 D- 3 . The delay information packet  2000  containing the above calculation result is transmitted to the transmitting-end switching exchange  40 . The delay information packet  2000  will be described later with reference to  FIG. 8 . 
         [0046]      FIG. 4  is a diagram illustrating the data management table  40 D for the transmitting-end switching exchange  40 . When the delay information packet  2000  is received, the calculation result contained in the packet is stored in the field of delay time  40 D- 2 . The degree of multiplicity  40 D- 3  is then calculated from the delay time. The degree of multiplicity of TDM data is determined from the result of calculation. In the field of protocol identification  40 D- 1 , the aforementioned identified data type is entered. The result of protocol identification, which is based on the volume of data, may be voice data, common data such as email data or Web data, or other data such as fax data. 
         [0047]      FIG. 5  is a diagram illustrating the sequence number comparison table  41 E in the receiving-end switching exchange  41 . When there is any duplicate sequence number in the field of sequence number  41 E- 1 , duplicate data corresponding to the duplicate sequence number is discarded from the field of payload internal data  41 E- 2 . 
         [0048]      FIG. 6  is a diagram illustrating a data queue  40 B ( 41 B), which is in the transmitting-end switching exchange  40  or receiving-end switching exchange  41 . The aforementioned data queue control section  40 C ( 41 C) can be used to preselect the number (N) of generations of data queue copies. In the example shown in  FIG. 6 , N=3. It means that up to three generations of copies can be made. 
         [0049]      FIG. 7  is a diagram illustrating a TDMoIP packet  1000 . The TDMoIP packet contains TDMoIP payload  1000 - 1 , a timestamp  1000 - 2 , a sequence number  1000 - 3 , a UDP header  1000 - 4 , and an IP header  1000 - 5 . The TDMoIP payload  1000 - 1  is composed of the sum total of multiplexed TDM data (segment data)  1000 - 1 - 1 , length values of the multiplexed TDM data, and a data count  1000 - 1 - 3 . The data count  1000 - 1 - 3  indicates the size of the data queue (N or less). In the example in  FIG. 6 , which assumes that the data queue size N=3, the data count of the TDMoIP payload  1000 - 1  is 3 or less. As regards generation-copied data, two pieces of data A are multiplexed into data ( 1 )″; two pieces of data B are multiplexed into data ( 2 )′; and two pieces of data C are multiplexed into data ( 3 ). The timestamp  1000 - 2  represents the time of transmission from the transmitting-end switching exchange  40 . The sequence number  1000 - 3  is a sequence number for packetization and assigned at the time of transmission. The UDP header  1000 - 4  and IP header  1000 - 5  are attached to the above-described elements to form a TDMoIP packet  1000 . The resulting TDMoIP packet  1000  is transmitted and received over the IP network  40 . 
         [0050]      FIG. 8  is a diagram illustrating the delay information packet  2000 . The delay information packet  2000  contains payload (delay information)  2000 - 1 , a UDP header  2000 - 2 , and an IP header  2000 - 3 . As the packet is to be transmitted and received over the IP network  50 , the IP header  2000 - 3  and UDP header  2000 - 2  are attached to the payload (delay information)  2000 - 1 . The payload (delay information)  2000 - 1  is to be stored in the field of delay time  41 D- 3 , which is described earlier with reference to  FIG. 3 . 
         [0051]    An operation of the present embodiment will now be described in detail.  FIGS. 9-1  to  9 - 5  illustrate an operation performed by the transmitting-end switching exchange  40  in the communication system that represents an exemplary basic embodiment shown in  FIG. 1 .  FIG. 10  is a flowchart illustrating an operation performed by the transmitting-end switching exchange  40  in the communication system that represents an exemplary basic embodiment shown in  FIG. 1 . 
         [0052]    Referring to FIG.  9 ( 1 ), it is assumed that the data continuously transmitted from the telephone terminal  10  and PC terminal  20  through the TA  30  is A+B+C, and that the size of the data queue  40 B is set to 3 (step  10001  of  FIG. 10 ), and further that the degree of multiplicity is 2. 
         [0053]    Next, the protocol identification section  406  identifies the protocol type (step  10002  of  FIG. 10 ) to determine whether voice data or common data is to be transmitted. It is temporarily assumed here that voice data is to be transmitted. If a delay information packet is received (step  10003  of  FIG. 10 ), the delay time indicated by the delay information packet is compared against the delay time stored in the data management table (step  10004  of  FIG. 10 ). When the delay time indicated by the delay information packet is the same as the delay time stored in the data management table, TDM data is received (step  10007  of  FIG. 10 ) with the stored value. When, on the other hand, the delay time indicated by the delay information packet is not the same as the delay time stored in the data management table, the degree of multiplicity is newly calculated (step  10005  of  FIG. 10 ) and determined (step  10006  of  FIG. 10 ) before TDM data reception (step  10007  of  FIG. 10 ). 
         [0054]    Referring to FIG.  9 ( 2 ), TDMoIP payload is formed. As the size of the data queue  40 B is 3, the queue size is 3. Therefore, up to three generations of TDM data are stored in the payload  1000 - 1  within a TDMoIP packet as detailed later with reference to FIG.  9 ( 5 ). Two pieces of data A multiplexed into data ( 1 ), two pieces of data B multiplexed into data ( 2 ), and two pieces of data C multiplexed into data ( 3 ) are stored as individual payload data  1000 - 1 - 1  (step  10008  of  FIG. 10 ). Further, data length values  1000 - 1 - 2  and a data count  1000 - 1 - 3  are attached to each piece of payload data. As it is assumed that the data queue size is 3, the value 3 is stored as the data count. 
         [0055]    Referring to FIG.  9 ( 3 ), TDMoIP packetization is performed. In addition, a copy is made at the time of data transmission. The data queue  20 A has a First-In-First-Out (FIFO) structure, stores data ( 2 ) in the first queue position, and sequentially stores data ( 3 ) and data ( 4 ). 
         [0056]    A data length value  1000 - 1 - 2 , a data count  1000 - 1 - 3 , a sequence number (step  10009  of  FIG. 10 ), an IP header, a UDP header, and a timestamp (step  10010  of  FIG. 10 ) are attached to data ( 1 ). Data ( 1 ) is then converted to a TDMoIP packet. When the TDMoIP packet is to be transmitted (step  10012  of  FIG. 10 ), data ( 1 ) is copied (step  10011  of  FIG. 10 ). Data ( 1 )′, which is a copy of data ( 1 ), is stored in the first queue position (data ( 1 )′ and data ( 2 )). 
         [0057]    Referring to FIG.  9 ( 4 ), a copy is made at the time of transmission in the same manner as indicated in FIG.  9 ( 3 ). The data queue  40 B stores data ( 3 ) in the first queue position, and sequentially stores data ( 4 ) and data ( 5 ). Data ( 1 )′ and data ( 2 ) are handled as single payload data. A data length value  1000 - 1 - 2 , a data count  1000 - 1 - 3 , a sequence number (step  10009  of  FIG. 10 ), an IP header, a UDP header, and a timestamp (step  10010  of  FIG. 10 ) are attached to the single payload data. The single payload data is then converted to a TDMoIP packet. When the TDMoIP packet is to be transmitted (step  10011  of  FIG. 10 ), data ( 1 )′ and data ( 2 ) are copied (step  10012  of  FIG. 10 ). Data ( 1 )″ and data ( 2 )′, which are copies of data ( 1 )′ and data ( 2 ), respectively, are stored in the first queue position (data ( 1 )″, data ( 2 )′, and data ( 3 )). 
         [0058]    Referring to FIG.  9 ( 5 ), N ( 3  in FIG.  9 ( 5 )) copies are made of queued data. The data queue  40 B stores data ( 4 ) in the first queue position, and sequentially stores data ( 5 ) and data ( 6 ). Data ( 1 )″, data ( 2 )′, and data ( 3 ) are handled as single payload data. A data length value  1000 - 1 - 2 , a data count  1000 - 1 - 3 , a sequence number (step  10009  of  FIG. 10 ), an IP header, a UDP header, and a timestamp (step  10010  of  FIG. 10 ) are attached to the single payload data. The single payload data is then converted to a TDMoIP packet. When the TDMoIP packet is to be transmitted (step  10012  of  FIG. 10 ), data ( 2 )′ and data ( 3 ) are copied (step  10011  of  FIG. 10 ). Data ( 2 )″ and data ( 3 )′, which are copies of data ( 2 )′ and data ( 3 ), respectively, are stored in the first queue position (data ( 2 )″, data ( 3 )′, and data ( 4 )). Here, as it is assumed that the size of the data queue  40 B is 3 (N=3), it is possible to copy up to three generations, namely, up to three successive data sequentially obtained upon TDMoIP payload generation. As is the case with identification based on data type and the degree of multiplicity based on delay, an object is to provide increased network efficiency by increasing the packet size (payload size). Sequence number assignment is performed in such a manner that the same sequence number is attached to all generations of the same data. 
         [0059]      FIGS. 11-1  to  11 - 5  illustrate an operation performed by the receiving-end switching exchange  41  in the communication system that represents an exemplary basic embodiment shown in  FIG. 1 .  FIG. 12  is a flowchart illustrating an operation performed by the receiving-end switching exchange  41  in the communication system that represents an exemplary basic embodiment shown in  FIG. 1 . 
         [0060]    Referring to FIG.  11 ( 1 ), the receiving-end switching exchange  41  receives a TDMoIP packet  1000  (step  11001  of  FIG. 12 ), stores the reception time of the received TDMoIP packet  1000  in the data management table ( 41 D- 2  in  FIG. 3 ), and calculates the delay time (step  11002  of  FIG. 12 ). The data queue  41 B stores data ( 1 ) (step  11003  of  FIG. 12 ), which is obtained by disassembling the TDMoIP packet  1000 . Packet disassembly is achieved by removing the IP header  1005 , UDP header  1004 , timestamp  1003 , and sequence number  1002 . At the time of packet disassembly, the sequence number comparison table  41 E is searched for previously received TDMoIP packets  1000 . If any existing sequence number coincides with a sequence number  41 E- 1  in the table, duplicate data in copied TDMoIP payload  10001  is discarded. The duplicate data is memorized in the field of payload internal data  41 E- 2 , which will be described later with reference to  FIG. 5 , in accordance with a data count  1000 - 1 - 3 , which is shown in  FIG. 7 , and length data  1000 - 1 - 2  attached to each piece of data  1000 - 1 - 1 . If no existing sequence number coincides with a sequence number  41 E- 1  in the table, it is possible that packet loss has occurred in the IP network  50 . In this instance, therefore, already delivered TDMoIP packets  1000  are reassembled into TDM data. In the resulting state, only data ( 1 ) is placed in the first queue position after TDMoIP packet disassembly. 
         [0061]    Referring to FIG.  11 ( 2 ), the order in which TDMoIP packets  1000  arrive at the receiving-end switching exchange is not guaranteed because the IP network  50  lies between the switching exchanges  40 ,  41 . It is therefore assumed that the third TDMoIP packet  1000  transmitted from the transmitting-end switching exchange  40  is queued so as to store data ( 1 )″, data ( 2 )′, and data ( 3 ) in the second queue position (step  11003  of  FIG. 12 ), as is the case described with reference to FIG.  11 ( 1 ). 
         [0062]    Referring to FIG.  11 ( 3 ), any duplicate received packet is discarded in accordance with a sequence number. As is the case described with reference to FIG.  11 ( 1 ), the data queue  41 B stores data ( 1 ) in the first queue position, data ( 1 )″, data ( 2 )′, and data ( 3 ) in the second queue position, and data ( 1 )′ and data ( 2 ) in the third queue position (step  11003  of  FIG. 12 ). Next, the sequence numbers are compared with each other (step  11004  of  FIG. 12 ). Copies of data ( 1 ) (data ( 1 )″ and data ( 1 )′), which are placed in the second and third queue positions as indicated in the field of payload internal data  41 E- 2 , are discarded from the sequence number comparison table  41 E shown in  FIG. 13  because they are duplicated and assigned the same sequence number ( 1001 ). 
         [0063]    Referring to FIG.  11 ( 4 ), the data queue  41 B stores data ( 2 )′ in the first queue position because data ( 1 ) is deleted, stores data ( 2 ) in the second queue position, and stores data ( 2 )″, data ( 3 )′, and data ( 4 ) in the third queue position. 
         [0064]    Referring to FIG.  11 ( 5 ), it is assumed that TDMoIP packets  1000  are sequentially received, and that data ( 2 ) to ( 3 ) have the same sequence number. In such an instance, the data queue  41 B stores data ( 4 )″, data ( 5 )′, and data ( 6 ) in the first queue position, stores data ( 5 )″, data ( 6 )′, and data ( 7 ) in the second queue position, and stores data ( 6 )″, data ( 7 )′, and data ( 8 ) in the third queue position. As all the data required for data ( 1 ) to ( 3 ) are readied, they are reassembled into data (A+B+C) for one frame of TDM data (step  11006  of  FIG. 12 ). After completion of TDM data reassembly, the delay information packet is transmitted to the receiving-end switching exchange (step  11007  of  FIG. 12 ). 
       Second Embodiment 
       [0065]    A second embodiment of the present invention will now be described.  FIGS. 14-1  to  14 - 4  illustrate a delay information packet and an operation performed by the transmitting-end switching exchange. The second embodiment will also be described with reference to  FIG. 10  which is a flowchart illustrating an operation performed by the transmitting-end switching exchange. For illustration purposes, it is assumed that TDMoIP packet processing is performed with the data queue size set to 3 (step  10001  of  FIG. 10 ) and the degree of TDM data multiplicity set to 3. 
         [0066]    Referring to FIG.  14 ( 1 ), the delay time of the IP network  50 , which is calculated from the data management table  41 D shown in  FIG. 15 , is 10 ms. The receiving-end switching exchange  41  transmits the delay time to the transmitting-end switching exchange  40  by using a delay information packet  2000 . Upon receipt of the delay information packet  2000  (step  10003  of  FIG. 10 ), the transmitting-end switching exchange  40  memorizes the delay time (10 ms), compares it against the delay time stored in the data management table (step  10004  of  FIG. 10 ), calculates the degree of multiplicity  40 D- 3  from the delay time (step  10004  of  FIG. 12 ), and memorizes the value 3 instead of the value 2 (step  10005  of  FIG. 12 ), as the degree of multiplicity ( FIG. 16 ). 
         [0067]    Referring to FIG.  14 ( 2 ), the degree of multiplicity is set to 3. As the degree of multiplicity is 3 and the size of the data queue  40 B is 3 (N=3), up to three generations of packets are stored in the TDMoIP packet internal payload  1000 - 1 . Three pieces of data A multiplexed into data ( 1 ), three pieces of data B multiplexed into data ( 2 ), and three pieces of data C multiplexed into data ( 3 ) are stored as individual payload data (step  10008  of  FIG. 10 ). Further, data length values  1000 - 1 - 2  and a data count  1000 - 1 - 3  are attached to each piece of payload data. As it is assumed that the data queue size is 3, the value 3 is stored as the data count. 
         [0068]    Referring to FIG.  14 ( 3 ), TDMoIP packetization is performed. In addition, a copy is made at the time of data transmission. The data queue  40 B stores data ( 2 ) in the first queue position, and sequentially stores data ( 3 ) and data ( 4 ). A data length value  1000 - 1 - 2 , a data count  1000 - 1 - 3 , a sequence number (step  10009  of  FIG. 10 ), an IP header, a UDP header, and a timestamp (step  10010  of  FIG. 10 ) are attached to data ( 1 ). Data ( 1 ) is then converted to a TDMoIP packet. When the TDMoIP packet is to be transmitted (step  10012  of  FIG. 10 ), data ( 1 ) is copied (step  10011  of  FIG. 10 ). Data ( 1 )′, which is a copy of data ( 1 ), is stored in the first queue position (data ( 1 )′ and data ( 2 )). Subsequently, data ( 1 ), data ( 2 ), and data ( 3 ) are sequentially copied (in the same manner as indicated in FIG.  9 ( 4 )). 
         [0069]    Referring to FIG.  14 ( 4 ), N copies are made of queued data. The data queue  40 B stores data ( 4 ) in the first queue position, and sequentially stores data ( 5 ) and data ( 6 ). Data ( 1 )″, data ( 2 )′, and data ( 3 ) are handled as single payload data. A data length value  1000 - 1 - 2 , a data count  1000 - 1 - 3 , a sequence number (step  10009  of  FIG. 10 ), an IP header, a UDP header, and a timestamp (step  10010  of  FIG. 10 ) are attached to the single payload data. The single payload data is then converted to a TDMoIP packet. When the TDMoIP packet is to be transmitted (step  10012  of  FIG. 10 ), data ( 2 )′ and data ( 3 ) are copied (step  10011  of  FIG. 10 ). Data ( 2 )″ and data ( 3 )′, which are copies of data ( 2 )′ and data ( 3 ), respectively, are stored in the first queue position (data ( 2 )″, data ( 3 )′, and data ( 4 )). 
         [0070]      FIGS. 17-1  to  17 - 3  illustrate an operation performed by the receiving-end switching exchange  41  in the communication system that represents an exemplary basic embodiment shown in  FIG. 1 . The following example illustrates a case where a TDMoIP packet  1000  is lost in the IP network  50 . The example will also be described with reference to  FIG. 12 , which is a flowchart illustrating an operation performed by the transmitting-end switching exchange. 
         [0071]    Referring to FIG.  17 ( 1 ), it is assumed that a TDMoIP packet  1000  containing data ( 1 ) is already received by the receiving-end switching exchange  41 . The reception time of the TDMoIP packet  1000  is stored in the data management table to calculate the delay time (step  11002  of  FIG. 12 ). Here, as indicated in FIG.  11 ( 1 ), which illustrates the first embodiment, the data queue  41 B stores data ( 1 ) (step  11003  of  FIG. 12 ) after disassembly of the TDMoIP packet  1000 . It is now assumed that a TDMoIP packet  1000  containing data ( 1 )′ and data ( 2 ) and a TDMoIP packet  1000  containing data ( 1 )″, data ( 2 )′, and data ( 3 ) are lost in the IP network  50 . 
         [0072]    Referring to FIG.  17 ( 2 ), a TDMoIP packet  1000  containing data ( 2 )″, data ( 3 )′, and data ( 4 ) and a TDMoIP packet  1000  containing data ( 3 )″, data ( 4 )′, and data ( 5 ) are delivered to the data queue because the TDMoIP packet  1000  containing data ( 1 )′ and data ( 2 ) and the TDMoIP packet  1000  containing data ( 1 )″, data ( 2 )′, and data ( 3 ) are lost. In the same manner as described with reference to FIG.  17 ( 1 ), the TDMoIP packets  1000  are disassembled to compare the sequence numbers (step  11004  of  FIG. 12 ). As the sequence number comparison table  41 E indicates that the sequence number  1001 , which is in the field of sequence number  41 E- 1 , is not duplicated, a TDMoIP packet  1000  containing data ( 1 ) is reassembled into TDM data. 
         [0073]    The sequence number  1002 , on the other hand, is duplicated. Therefore, data ( 3 )″ and data ( 4 )′, which are the copies of data ( 3 ) and data ( 4 ) and placed in the third queue position of the payload internal data  41 E- 2 , are discarded (step  11005  of  FIG. 12 ). As data ( 2 ) and ( 3 ) are readied in the above-mentioned step in addition to data ( 1 ), they are reassembled into data (A+B+C) for one frame of TDM data (step  11006  of  FIG. 12 ). After completion of TDM data reassembly, the delay information packet  2000  is transmitted to the transmitting-end switching exchange  40  (step  11007  of  FIG. 12 ). 
         [0074]    The exemplary configuration according to the first or second embodiment described above makes it possible to provide enhanced reliability in packet recovery by making generation copies of TDM data and increase the transfer efficiency and the bandwidth efficiency of low-delay transfer in consideration of the delay in an IP network and the type of TDM data when TDMoIP packet loss occurs in the IP network  50  in a situation where a network system and a switching exchange are used to handle both an ISDN and IP network.