Abstract:
A communication system capable of avoiding congestion in transmission of moving image data, includes (1) at least one receiving terminal, (2) a moving image delivery device for delivering moving image data to the at least one receiving terminal, (3) a moving image conversion device which has at least one moving image conversion unit for converting, in accordance with conversion parameters, the moving image data sent from the moving image delivery device, a conversion parameter setting unit for determining the conversion parameters, and a monitored result receiving unit, and (4) at least one packet switching node which has at least one data storage unit for preliminarily storing the moving image data from the moving image conversion device to be sent to the at least one receiving terminal, a data amount monitor unit for monitoring an amount of the moving image data stored in the at least one data storage unit to judge that the monitored data amount reaches a first threshold, and a monitored result sending unit for sending a congestion preview information to the moving image conversion device when the data amount monitor unit judges that the monitored data amount reaches the first threshold. The monitored result receiving unit receives the congestion preview information from the monitored result sending unit, and the conversion parameter setting unit determines the conversion parameters so that the moving image conversion unit converts the moving image data sent from the moving image delivery device into a moving image data with a smaller coding bit rate.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a communication system and method capable of avoiding congestion in transmission of moving image data. More particularly, the present invention relates to a congestion control method of controlling a data transfer amount of a moving image by changing a data compression rate at a sender or an intermediate node between a sender and a receiver depending upon characteristics of the communication line or channel used by a user when the user is browsing the moving image delivered from the server via a network such as Internet or Intranet.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    Japanese Patent Publication No. P2000-83029A discloses an image data transfer system capable of congestion avoidance by changing a data compression rate at the server and by controlling a transfer amount of the image data. This system is provided with a storage unit for storing a plurality of video data compressed in different data compression rates to each other about the same video data, and a unit for dynamically selecting either one of the plurality of video data in accordance with the load situation of a network and for transferring the selected video data to a destination through the network.  
           [0003]    However, since this system requires previous storing of a plurality of video data compressed with different data compression rates to each other, procedure of the server of the video data will become very complicate. Also, this system does not support the real time delivery of moving image data taken by a video camera for example, through a communication network.  
           [0004]    Japanese Patent Publication No. P2001-313937A published on Nov. 9, 2001 discloses a system provided with a quality control unit at a gateway or an receiving end terminal, for changing a data compression rate of the moving image in accordance with coding parameters of image conversion at a server or a gateway. The coding parameters are calculated base upon communication quality information obtained at the quality control unit.  
           [0005]    However, according to this system, since the quality control unit is located at an end node of the network, the data compression rate of the moving image is changed in response to detection of lowering of throughput, increasing of delay and occurring of packet loss due to congestion in the network. Thus, it is very difficult to completely avoid occurrence of congestion in the network.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide a communication system and method capable of avoiding congestion in transmission of moving image data, whereby congestion can be completely prevented from occurring at packet switching nodes.  
           [0007]    Another object of the present invention is to provide a communication system and method capable of avoiding congestion in transmission of moving image data, whereby congestion can be prevented from occurring without previously storing a plurality of video data compressed with different data compression rates to each other.  
           [0008]    According to the present invention, a communication system capable of avoiding congestion in transmission of moving image data, includes (1) at least one receiving terminal, (2) a moving image delivery device for delivering moving image data to the at least one receiving terminal, (3) a moving image conversion device which has at least one moving image conversion unit for converting, in accordance with conversion parameters, the moving image data sent from the moving image delivery device, a conversion parameter setting unit for determining the conversion parameters, and a monitored result receiving unit, and (4) at least one packet switching node which has at least one data storage unit for preliminarily storing the moving image data from the moving image conversion device to be sent to the at least one receiving terminal, a data amount monitor unit for monitoring an amount of the moving image data stored in the at least one data storage unit to judge that the monitored data amount reaches a first threshold, and a monitored result sending unit for sending a congestion preview information to the moving image conversion device when the data amount monitor unit judges that the monitored data amount reaches the first threshold. The monitored result receiving unit receives the congestion preview information from the monitored result sending unit, and the conversion parameter setting unit determines the conversion parameters so that the moving image conversion unit converts the moving image data sent from the moving image delivery device into a moving image data with a smaller coding bit rate.  
           [0009]    An amount of the stored moving image to be sent to the receiving terminal is monitored to judge as the monitored data amount reaching a first threshold and to send a congestion preview information when reached. Then, the conversion parameters are determined so as to convert the moving image data to be delivered, into a moving image data with a coding bit rate smaller than that of the current moving image data. Since a preview of congestion is detected and a data compression rate of image data now transferring is dynamically controlled, a congestion can be completely prevented from occurring. Therefore, at the receiving terminal, it is possible to reproduce streaming type moving image data without any lack of frame and without increasing a delay. The present invention also provides effective utilization of communication channels.  
           [0010]    Furthermore, according to the present invention, since the data compression rate of the moving image data can be varied depending upon the transmission state in the network, video data with different data compression rates are not necessary to previously store in the moving image delivery device. Also, the amount of the moving image stored in the data storage unit in the packet switching node is monitored to control the transfer amount of the moving image data, no overflow of data at the packet switching node which is connected with the receiving terminal via a radio channel in a mobile communication network occurs even if the transmission rate of the radio channel abruptly changes due to radio environment change.  
           [0011]    It is preferred that the data amount monitor unit monitors an amount of the moving image data stored in the at least one data storage unit to judge as the monitored data amount reaching a second threshold which is smaller than the first threshold, that the monitored result sending unit sends a congestion avoidance information to the moving image conversion device when the data amount monitor unit judges as the monitored data amount reaching the second threshold, that the monitored result receiving unit receives the congestion avoidance information from the monitored result sending unit, and that the conversion parameter setting unit determines the conversion parameters so that the moving image conversion unit converts the moving image data sent from the moving image delivery device into a moving image data with a coding bit rate larger than that of the sent moving image data.  
           [0012]    It is also preferred that the data amount monitor unit judges as being the congestion preview state until the monitored data amount reaches the second threshold after the monitored data amount reaches the first threshold, and as being the congestion avoidance state until the monitored data amount reaches the first threshold after the monitored data amount reaches the second threshold.  
           [0013]    It is preferred that the monitored result sending unit sends the congestion preview information or the congestion avoidance information at a constant interval.  
           [0014]    It is preferred also that the data amount monitor unit calculates a new coding bit rate when the data amount monitor unit judges that the monitored data amount reaches the first threshold, that the monitored result sending unit sends the calculated new coding bit rate in addition to the congestion preview information, that the monitored result receiving unit receives the calculated new coding bit rate in addition to the congestion preview information, and that the conversion parameter setting unit determines the conversion parameters so that the moving image conversion unit converts the moving image data sent from the moving image delivery device depending upon the calculated new coding bit rate.  
           [0015]    It is further preferred that the data amount monitor unit calculates a new coding bit rate enabling to avoid congestion state after a lapse of a predetermined time, depending upon a change in the stored data amount during a predetermined period of time.  
           [0016]    It this case, preferably the data amount monitor unit calculates a low coding bit rate so as to decrease a difference between an estimate data amount after a lapse of the predetermined time and a second threshold which is smaller than the first threshold, when the change in the stored data amount during a predetermined period of time is positive change. Also, preferably, the data amount monitor unit calculates a high coding bit rate so as to increase a difference between an estimate data amount after a lapse of the predetermined time and an amount of data now stored, when the change in the stored data amount during a predetermined period of time is negative change.  
           [0017]    It is preferred that the data amount monitor unit monitors, instead of the amount of data stored in the at least one data storage unit, a data amount which is smoothed in accordance with the amount of data stored in the at least one data storage unit. Thus, even if the amount of change in the monitored data amount increases, the detection of the congestion preview state or the congestion avoidance state will not frequently occur.  
           [0018]    It is also preferred that there are a plurality of receiving terminals, and the conversion parameter setting unit includes a plurality of conversion parameter setting unit for the respective receiving terminals, that the at least one data storage unit includes a plurality of data storage unit for preliminarily storing the moving image data to be sent to the respective receiving terminals, the data amount monitor unit monitors the stored data for the respective receiving terminals, and the monitored result sending unit sends the congestion preview information or the congestion avoidance information for the respective receiving terminals, and that the monitored result receiving unit receives the congestion preview information or the congestion avoidance information for the respective receiving terminals, and the conversion parameter setting unit determines the conversion parameters for the respective receiving terminals.  
           [0019]    According to the present invention, furthermore, a communication method capable of avoiding congestion in transmission of moving image data, includes a step of converting moving image data to be delivered to at least one receiving terminal, in accordance with conversion parameters, a step of preliminarily storing the converted moving image data to be sent to the at least one receiving terminal, a step of monitoring an amount of the stored moving image data to judge that the monitored data amount reaches a first threshold, a step of sending a congestion preview information when it is judge that the monitored data amount reaches the first threshold, a step of receiving the sent congestion preview information, and a step of determining the conversion parameters so that the converting step converts the moving image data into a moving image data with a smaller coding bit rate.  
           [0020]    Since a preview of congestion is detected and a data compression rate of image data now transferring is dynamically controlled, a congestion can be completely prevented from occurring. Therefore, at the receiving terminal, it is possible to reproduce streaming type moving image data without any lack of frame and without increasing a delay. The present invention also provides effective utilization of communication channels.  
           [0021]    Furthermore, according to the present invention, since the data compression rate of the moving image data can be varied depending upon the transmission state in the network, video data with different data compression rates are not necessary to previously store in the moving image delivery device. Also, the amount of the moving image stored is monitored to control the transfer amount of the moving image data, no overflow of data at a packet switching node which is connected with the receiving terminal via a radio channel occurs even if the transmission rate of the radio channel abruptly changes due to radio environment change.  
           [0022]    It is preferred that the monitoring step includes monitoring an amount of the stored moving image data to judge that the monitored data amount reaches a second threshold which is smaller than the first threshold, that the sending step include sending a congestion avoidance information when it is judged as the monitored data amount reaching the second threshold, that the receiving step includes receiving the congestion avoidance information, and that the determining step includes determining the conversion parameters so that the converting step converts the moving image data into a moving image data with a coding bit rate larger than that of the sent moving image data.  
           [0023]    It is also preferred that the monitoring step includes judging that it is the congestion preview state until the monitored data amount reaches the second threshold after the monitored data amount reaches the first threshold, and judging that it is the congestion avoidance state until the monitored data amount reaches the first threshold after the monitored data amount reaches the second threshold.  
           [0024]    It is further preferred that the sending step includes sending the congestion preview information or the congestion avoidance information at a constant interval.  
           [0025]    It is preferred that the monitoring step includes calculating a new coding bit rate when it is judged as the monitored data amount reaching the first threshold, that the sending step includes sending the calculated new coding bit rate in addition to the congestion preview information, that the receiving step include receiving the calculated new coding bit rate in addition to the congestion preview information, and that the determining step includes determining the conversion parameters so that the converting step converts the moving image data depending upon the calculated new coding bit rate.  
           [0026]    It is also preferred that the monitoring step includes calculating a new coding bit rate enabling to avoid congestion state after a lapse of a predetermined time, depending upon a change in the stored data amount during a predetermined period of time.  
           [0027]    In this case, preferably, the monitoring step includes calculating a low coding bit rate so as to decrease a difference between an estimate data amount after a lapse of the predetermined time and a second threshold which is smaller than the first threshold, when the change in the stored data amount during a predetermined period of time is positive change. Also, preferably, the monitoring step includes calculating a high coding bit rate so as to increase a difference between an estimate data amount after a lapse of the predetermined time and an amount of data now stored, when the change in the stored data amount during a predetermined period of time is negative change.  
           [0028]    It is preferred that the monitoring step includes monitoring, instead of the stored data amount, a data amount which is smoothed in accordance with the stored data amount.  
           [0029]    It is also preferred that the converting step includes converting the moving image data to be sent to a plurality of receiving terminals, the storing step includes preliminarily storing the converted moving image data for the respective receiving terminals, the monitoring step includes monitoring the stored data for the respective receiving terminals, and the sending step include sending the congestion preview information or the congestion avoidance information for the respective receiving terminals, and that the receiving step includes receiving the congestion preview information or the congestion avoidance information for the respective receiving terminals, and the determining step includes determining a plurality of conversion parameters for the respective receiving terminals. Since the congestion state is judged for every receiving terminal, data traffic control in a public communication network is enabled.  
           [0030]    Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 shows a block diagram illustrating a configuration of a communication system as a preferred embodiment according to the present invention;  
         [0032]    [0032]FIG. 2 shows a flow chart illustrating a congestion monitoring program provided in a data amount monitor unit in the communication system shown in FIG. 1;  
         [0033]    [0033]FIG. 3 illustrates a method of calculating a new coding bit rate based upon the monitored results when a congestion preview state is detected;  
         [0034]    [0034]FIG. 4 illustrates a method of calculating a new coding bit rate based upon the monitored results when a congestion avoidance state is detected;  
         [0035]    [0035]FIG. 5 shows a graph of an actual change in the stored data amount and a smoothed change in the stored data amount with respect to a time;  
         [0036]    [0036]FIG. 6 shows a frame structure of information representing a review of congestion and an avoidance of congestion, formed in a monitored result sending unit; and  
         [0037]    [0037]FIG. 7 shows a block diagram illustrating configuration of a communication system adaptable to a plurality of users as another embodiment according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    As shown in FIG. 1, a communication system illustrated as a preferred embodiment according to the present invention includes a moving image delivery device  10 , a moving image conversion device  11 , a packet switching device or node  12 , and first and second receiving terminals  13   1 , and  13   2  for first and second users respectively.  
         [0039]    The moving image delivery device  10  is provided with a moving image delivery unit  100  for sending moving image data to the moving image conversion device  11  via a first communication line or channel  14 . The moving image data sent may be preliminarily stored in the moving image delivery device  10  or may not be stored therein.  
         [0040]    The moving image conversion device  11  is provided with a moving image conversion unit  110 , a monitored result receiving unit  111  and a conversion parameter setting unit  112 .  
         [0041]    The moving image conversion unit  110  obtains conversion parameters from the setting unit  112 , converts, depending upon the conversion parameters, the moving image data received via the channel  14 , and then sends the converted moving image data to the packet switching device  12  the via a second communication line or channel  15 .  
         [0042]    The monitored result receiving unit  111  receives a congestion preview information or a congestion avoidance information, containing destination information for specifying a receiving terminal from the packet switching device  12  via a fourth communication line or channel  17 , and transfers the received information to the setting unit  112 .  
         [0043]    The setting unit  112  determines the moving image conversion parameters to decrease an amount of the transferred moving image data by a predetermined amount when a congestion preview information is transferred. If a new coding bit rate calculated in accordance with the monitored result is attached to the congestion preview information received at the receiving unit  111 , the setting unit  112  changes the moving image conversion parameters depending upon the attached new coding bit rate.  
         [0044]    Also, the setting unit  112  determines the moving image conversion parameters to increase an amount of the transferred moving image data by a predetermined amount when a congestion avoidance information is transferred. If a new coding bit rate calculated in accordance with the monitored result is attached to the congestion avoidance information received at the receiving unit  111 , the setting unit  112  changes the moving image conversion parameters depending upon the attached new coding bit rate.  
         [0045]    The packet switching node  12  is provided with a data storage unit  120 , a data amount monitor unit  121  and a monitored result sending unit  122 .  
         [0046]    The data storage unit  120  temporarily stores the moving image data received via the second channel  15 , and then sends the moving image data to a third communication line or channel  16  which is defined by the destination information for specifying a receiving terminal, contained in the data. The amount of data stored in the storage unit  120  will be gradually increased when the amount of data capable to transfer through the third channel  16  is smaller than the stored amount of data to be transferred.  
         [0047]    The data amount monitor unit  121  monitors a stored amount of data in the storage unit  120 , and activates the monitor result sending unit  122  when recognizing that the stored data amount reaches a first threshold.  
         [0048]    The monitor result sending unit  122  produces a congestion preview information containing a destination information for specifying a receiving terminal when triggered by the monitor unit  121 , and sends the produced information to the moving image conversion device  11  via the fourth channel  17 . The sending unit  122  may send the congestion preview information with attaching a new coding bit rate calculated in accordance with the monitored result.  
         [0049]    The data amount monitor unit  121  also monitors a stored amount of data in the storage unit  120 , and activates the monitor result sending unit  122  when recognizing that the stored data amount decreases lower than a second threshold after it reaches the first threshold.  
         [0050]    In this case, the monitor result sending unit  122  produces a congestion avoidance information containing a destination information for specifying a receiving terminal when triggered by the monitor unit  121 , and sends the produced information to the moving image conversion device  11  via the fourth channel  17 . The sending unit  122  may send the congestion avoidance information with attaching a new coding bit rate calculated in accordance with the monitored result.  
         [0051]    The receiving terminals  13   1  and  13   2  are provided with moving image display units  130   1  and  130   2  for displaying moving images based upon the data received through the third channel  16 , respectively.  
         [0052]    In modification, the aforementioned moving image conversion unit and data amount monitor unit may be located in the same node.  
         [0053]    [0053]FIG. 2 illustrates a congestion monitoring program provided in the data amount monitor unit  121 .  
         [0054]    Assuming that q_now [kilo bytes] indicate an amount of data stored now or at this time, q_prev [kilo bytes] indicates an amount of data stored previously or at previous time, q_max [kilo bytes] indicates the first threshold, q_min [kilo bytes] the second threshold, and m indicates a counter threshold, respectively.  
         [0055]    First, at step S 11 , an amount of now stored data q_now is compared with the first and second thresholds q_max and q_min.  
         [0056]    If it is judged at step S 11  that the amount of the now stored data q_now is equal to or smaller than the second threshold q_min, the amount of the previous stored data q_prev is compared with the second threshold q_min at step S 12 .  
         [0057]    If it is judged at step S 11  that the amount of the now stored data q_now is equal to or larger than the first threshold q_max, the amount of the previous stored data q_prev is compared with the first threshold q_max at step S 13 .  
         [0058]    If it is judged at step S 13  that the amount of the previous stored data q_prev is equal to or larger than the first threshold q_max, a counter is incremented by one at step S 14 .  
         [0059]    Then, contents in the counter is compared with the counter threshold m at step S 15 .  
         [0060]    If it is judged that the contents in the counter is equal to or larger than the counter threshold m at step S 15  or that the amount of the previous stored data q_prev is smaller than the first threshold q_max at step S 13 , the counter is reset and a congestion preview flag is set at step S 16 . Thus, it is judged that a congestion preview state is detected and a necessary procedure such as calculation of a new coding bit rate based upon the monitored results will be executed at step S 17 .  
         [0061]    If it is judged that the contents in the counter is smaller than the counter threshold m at step S 15 , the program will end.  
         [0062]    If it is judged at step S 11  that the amount of the now stored data q_now is smaller than the first threshold q_max and larger than the second threshold q_min, the counter is reset at step S 18 .  
         [0063]    If it is judged at step S 12  that the amount of the previous stored data q_prev is larger than the second threshold q_min, the congestion preview flag is checked at step S 19 . If it is set, the program will proceed to step S 20 , but if it is not set, the program will end. Also at step S 12 , if it is judged that the amount of the previous stored data q_prev is equal to or smaller than the second threshold q_min, the program will end.  
         [0064]    At step S 20 , the counter is reset and also the congestion preview flag is reset. In this case, it is judged that an avoidance of congestion state is detected and a necessary procedure such as calculation of a new coding bit rate based upon the monitored results will be executed at step S 21 .  
         [0065]    [0065]FIG. 3 illustrates a method of calculating a new coding bit rate based upon the monitored results when a congestion preview state is detected.  
         [0066]    The first and the second thresholds q_max and q_min are preliminarily determined to values smaller than the maximum amount of stored data q_limit [kilo bytes]. The data amount monitor unit  121  will check the amount of data stored in the storage unit  120  at a time interval of T [seconds].  
         [0067]    When the stored data amount exceeds the first threshold q_max for the first time, it is judged as a congestion preview state. Then, a new coding bit rate for the moving image data now transferred is determined so that the stored data amount decreases to the second threshold q_min after a lapse of m×T [seconds]. The determination of the new coding bit rate is performed as follows.  
         [0068]    First, an amount of data to be stored if no amount control is executed is estimated on the analogy of the history of change in the stored data amount. The most simple way for performing this is to extrapolate a line with a gradient of ΔQ/T [kilo bytes/second] where ΔQ [kilo bytes] is the amount of change between the previous stored data amount q_prev [kilo bytes] and the now stored data amount q_now [kilo bytes], namely ΔQ=q_now−q_prev. Therefore, the estimated increasing amount for m×T [seconds] is m×ΔQ [kilo bytes]. Then, the data amount to be decreased during a period of time m×T [seconds] becomes m×ΔQ+(q−q_min) [kilo bytes] 
         [0069]    Thus, the new coding bit rate should be determined by decreasing {m×ΔQ+(q−q_min)}×8/(m×T) [kilo bps] from the current coding bit rate.  
         [0070]    [0070]FIG. 4 illustrates a method of calculating a new coding bit rate based upon the monitored results when a congestion avoidance state is detected.  
         [0071]    When the stored data amount falls below the second threshold q_min after the congestion preview state was detected, it is judged as a congestion avoidance state. In this case, if no amount control is executed, the stored data amount will continue decreasing and as a result a capacity of the third channel will not be effectively utilized. Then, a new coding bit rate for the moving image data now transferred is determined within a range no congestion preview occurs. The determination of the new coding bit rate is performed as follows.  
         [0072]    First, an amount of data to be stored if no amount control is executed is estimated on the analogy of the history of change in the stored data amount. The most simple way for performing this is to extrapolate a line with a gradient of ΔQ/T [kilo bytes/second] where ΔQ [kilo bytes] is the amount of change between the previous stored data amount q_prev [kilo bytes] and the now stored data amount q_now [kilo bytes], namely ΔQ=q_now−q_prev. Therefore, the estimated decreasing amount for m×T [seconds] is m×ΔQ [kilo bytes]. Then, the data amount to be increased during a period of time m×T [seconds] becomes m×ΔQ [kilo bytes] 
         [0073]    Thus, the new coding bit rate should be determined by decreasing (m×ΔQ)×8/(m×T)=ΔQ×8/T [kilo bps] from the current coding bit rate.  
         [0074]    [0074]FIG. 5 illustrates characteristics of an actual change in the stored data amount and a smoothed change in the stored data amount with respect to a time.  
         [0075]    In case that a time period T for monitoring the stored data amount is short, the amount of change in the monitored data amount will increase and therefore the detection of the congestion preview state or the congestion avoidance state may frequently occur. In order to prevent frequent detections of the congestion preview state or the congestion avoidance state, it is desired to smooth a value of the stored data amount used in the judgment of the congestion. For example, the amount of data stored now q_now may be smoothed every time packets were stored by the following equation;  
           q   —   now =(1 −W )× q   —   prev+W×q   —   actual    
         [0076]    where q_actual indicates an actual amount of data now stored and q_rev indicates an amount of stored data calculated before packets were stored. The smoothed change in the stored data amount shown in FIG. 5 corresponds to the calculated result using this equation and assuming as W=0.5.  
         [0077]    [0077]FIG. 6 illustrates a frame structure of information representing a review of congestion and an avoidance of congestion, formed in the monitored result sending unit  122 .  
         [0078]    As will be noted from this figure, the frame includes a control header field  60  indicating whether this information represents the congestion preview state or the congestion avoidance state, a field  61  representing the number n of information to be sent, an identification information  62   1  to  62   n  of first to n-th receiving terminals and new coding bit rate information  63   1  to  63   n  for the respective receiving terminals.  
         [0079]    As shown in FIG. 7, a communication system adaptable to a plurality of users illustrated as another embodiment according to the present invention includes a moving image delivery device  70 , a moving image conversion device  71 , a packet switching device or node  72 , and receiving terminals  73  consisting of first to nth receiving terminals  73   1  to  73   n  for first to nth users respectively.  
         [0080]    The moving image delivery device  70  is provided with moving image delivery units  700   a  and  700   b  for the respective contents of moving images. The delivery unit  700   a  sends moving image data to first and second moving image conversion units  710   1  and  710   2  in the moving image conversion device  71  via first communication lines or channels  74   1  and  74   2 , respectively. The delivery unit  700   b  sends moving image data to a nth moving image conversion unit  710   n  in the moving image conversion device  71  via a first communication line or channel  74   n . The moving image data sent may be preliminarily stored in the moving image delivery device  70  or may not be stored therein.  
         [0081]    The moving image conversion device  71  is provided with moving image conversion units  710   1  to  710   n , a monitored result receiving unit  711  and a conversion parameter setting unit  712 .  
         [0082]    The moving image conversion units  710   1  to  710   n  obtain conversion parameters from the setting unit  712 , convert, depending upon the conversion parameters, the moving image data received via the channels  74   1  to  74   n , and then send the converted moving image data to packet switching device  72  the via second communication line or channels  75   1  to  75   n , respectively.  
         [0083]    The monitored result receiving unit  711  receives a congestion preview information or a congestion avoidance information, containing destination information for specifying a receiving terminal from the packet switching device  72  via a fourth communication line or channel  77 , and transfers the received information to the setting unit  712 .  
         [0084]    The setting unit  712  determines the moving image conversion parameters to decrease an amount of the transferred moving image data by a predetermined amount when a congestion preview information is transferred. If a new coding bit rate calculated in accordance with the monitored result is attached to the congestion preview information received at the receiving unit  711 , the setting unit  712  changes the moving image conversion parameters depending upon the attached new coding bit rate.  
         [0085]    Also, the setting unit  712  determines the moving image conversion parameters to increase an amount of the transferred moving image data by a predetermined amount when a congestion avoidance information is transferred. If a new coding bit rate calculated in accordance with the monitored result is attached to the congestion avoidance information received at the receiving unit  711 , the setting unit  712  changes the moving image conversion parameters depending upon the attached new coding bit rate.  
         [0086]    The packet switching node  72  is provided with first to nth data storage units  720   1  to  720   n , a data amount monitor unit  721  and a monitored result sending unit  722 .  
         [0087]    The data storage units  720   1  to  720   n  temporarily store the moving image data received via the second channels  75   1  to  75   n , and then send the moving image data to third communication lines or channels  76   1  to  76   n  which are defined by the destination information for specifying a receiving terminal, contained in the data. The amount of data stored in the storage units  720   1  to  720   n  will be gradually increased when the amount of data capable to transfer through the third channels  76   1  to  76   n  is smaller than the stored amount of data to be transferred.  
         [0088]    The data amount monitor unit  721  monitors a stored amount of data in each of the storage units  720   1  to  720   n , and activates the monitor result sending unit  722  when recognizing that the stored data amount reaches a first threshold.  
         [0089]    The monitor result sending unit  722  produces a congestion preview information containing a destination information for specifying a receiving terminal when triggered by the monitor unit  721 , and sends the produced information to the moving image conversion device  71  via the fourth channel  77 . The sending unit  722  may send the congestion preview information with attaching a new coding bit rate calculated in accordance with the monitored result.  
         [0090]    The data amount monitor unit  721  also monitors a stored amount of data in each of the storage units  720   1  to  720   n , and activates the monitor result sending unit  722  when recognizing that the stored data amount decreases lower than a second threshold after it reaches the first threshold.  
         [0091]    In this case, the monitor result sending unit  722  produces a congestion avoidance information containing a destination information for specifying a receiving terminal when triggered by the monitor unit  721 , and sends the produced information to the moving image conversion device  71  via the fourth channel  77 . The sending unit  722  may send the congestion avoidance information with attaching a new coding bit rate calculated in accordance with the monitored result.  
         [0092]    The receiving terminals  73   1  to  73   n  are provided with moving image display units  730   1  to  730   n  for displaying moving images based upon the data received through the third channels  76   1  to  76   n , respectively.  
         [0093]    In modification, the aforementioned moving image conversion units and data amount monitor units may be located in the same node.  
         [0094]    As will be noted, in this embodiment, the moving image delivery units  700   a  and  700   b  are provided for the respective contents of moving images. The moving image delivery unit  700   a  delivers moving image data to the first and second users upon their requests and the moving image delivery unit  700   b  delivers moving image data to the nth user upon its request. The moving image conversion units  710   1  to  710   n  and the data storage units  720   1  to  720   n  are provided for the respective users because traffic conditions and capacity of receiving terminals differ from each other. Since the congestion state is judged for every receiving terminal, data traffic control in a public communication network is enabled.  
         [0095]    Other configurations, operations and advantages of this embodiment are the same as those in the embodiment of FIG. 1.  
         [0096]    In modification, a plurality of moving image conversion devices and a plurality of packet switching devices may be provided for the respective users.  
         [0097]    Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.