Abstract:
Multimedia applications including a video and an audio are transmitted at respective adapted transfer rates in a server connected with a networks. The server operates on an operating system which permits a multithreading by allocating time slices to thread. For each application, data on a required transfer rate indicative of a permitted lowest transfer rate for the application is prepared. Threads are generated for respective applications. An initial number of slices are allocated to each thread to let said threads transmit said respective applications. A transfer rate of each thread is measured at a time interval. A number of slices to be allocated to each thread is calculated such that the measured transfer rate of each thread (i.e., each application) becomes equal to the required transfer rate of the application transmitted by the thread.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to a server for providing multimedia information to a network and, more particularly, to techniques for efficiently transmitting a plurality of time-series multimedia applications in a time-division multiplex by controlling the assignment of a CPU (central processing unit) operation time to the transmitted applications. 
     2. Description of the Prior Art 
     Applications including video, animation and/or audio are hereinafter referred to as time-series multimedia applications. Compressed multimedia applications may be either transmitted directly as they are (direct transmission), or once stored in a storage device and then transmitted at constant rates (stored transmission). 
     In the direct transmission, the compressed applications are launched into a network regardless of the transmission band of the network, which may cause the transmission rate to exceed the transmission band variable with time, resulting in the destruction of transmitted data. For this reason, the direction transmission is possible only when the network transmission band is sufficiently higher than the transmission rate, in which there is no need of the present invention. 
     A transmission scheme of stored multimedia at a constant rate is described in “Video server software—Video Network File System,” IEICE (The Institute of Electronics, Information and Communication Engineers) trans. Vol. J-77-A, No. 9, pp. 1-8. In this system, stored applications are read out and transmitted at a constant rate. This system reads out a frame of data for each of stored applications to be transmitted for transmission. Assuming that two applications AP 1  and AP 2  are to be transmitted and that both the frame size of the two applications are 10 Kbps, if the applications AP 1  and AP 2  are to be transmitted at rates of 10 Kbps and 5 Kbps, respectively, then the system transmits one AP 1  frame per second and one AP 2  frame per two seconds. In this way, each application is transmitted at a constant rate as long as the total load on the CPU does not exceeds the processing capacity of the CPU and the total transmission rate dose not exceeds the transmission bandwidth of the network. However, These conditions are not always satisfied. 
     It is therefore an object of the invention to provide a method and a system which are used when actually measured transmission rates of a plurality of multimedia applications differ from required transmission rates for the applications and which are for allocating time slices of the CPU to the threads for transmitting the multimedia applications such that the applications are transmitted at the required transmission rate. 
     A reduction in the transmission rate of a thread causes a delay of transmission by the thread resulting in asynchronism among transmitted applications. 
     It is another object of the invention to provide a method of and a system for temporarily changing the allocation of the available time slices to the threads so as to resolve the a synchronism among transmitted applications. 
     It is another object of the invention to provide a method and a system which are used when the number of time slices available to the threads has decreased to be insufficient to transmit the applications to be transmitted and which adjust the allocation of the available time slices to the threads according to the priorities of the applications. 
     SUMMARY OF THE INVENTION 
     A method for transmitting multimedia applications including a video and an audio at respective adapted transfer rates in a server connected with a network is provided. The server operates on an operating system which permits a multithreading by allocating time slices to thread. The method comprises the steps of: 
     for each application, preparing data on a required transfer rate indicative of a permitted lowest transfer rate for the application; 
     generating threads for respective applications; 
     allocating an initial number of slices to each thread to let the threads transmit respective applications; 
     measuring a transfer rate of each thread at a time interval; 
     calculating a number of slices to be allocated to each thread such that the measured transfer rate of each thread (i.e., each application) becomes equal to the required transfer rate of the application transmitted by the thread; and 
     allocating the calculated number of slices to each thread. 
     The number of slices are calculated by multiplying a current number of slices allocated to each thread by a ratio of the required transfer rate to the measured transfer rate. 
     In response to a detection of a synchronism among the applications, a corrected number of slices are temporarily allocated to each thread such that the applications have been synchronized by the end of a subsequent time interval. 
     In response to a detection of a decrease in a total of the measured transfer rates, the system allocates less slices to applications of lower priorities and more slices to an application of a higher priority. 
     A method for transmitting data to a plurality of destinations in a server connected with a network is also provided. The method compres the steps of; 
     creating a thread to each destination; and 
     in response to a request from one of the thread, returning a unit of data to be preferably transmitted at a time of the request, wherein the step of returning a unit of data includes the step of: 
     in response to a detection that a difference between the time of the request and a time of last request from a thread different from the requesting thread is smaller than a value associated with an unoccupied period of a central processing unit of the server, returning a same unit of data as returned at the time of last request. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The features and advantages of the present invention will be apparent from the following description of an exemplary embodiment of the invention and the accompanying drawing, in which: 
     FIG. 1 is a schematic diagram showing an exemplary network  1  including a multimedia information server  10  and client terminals  30 , the server transmitting a plurality of multimedia applications in accordance with an illustrative embodiment of the invention; 
     FIG. 2 is a schematic block diagram showing an exemplary architecture of the server  10  of FIG. 1 when it is operating; 
     FIG. 3 is a diagram showing the applications M 1  through M 3 ; 
     FIG. 4 is a diagram showing a multimedia application table  270  for containing various information on the multimedia applications  260  stored in the mass storage  106 ; 
     FIG. 5 is a diagram showing how the operating system  240  initially allocates available time slices to the threads TH 1  through TH 3 ; 
     FIG. 6 is a diagram showing an exemplary operation cycle by the threads  154 , the control program  252  and the OS  240 ; 
     FIG. 7 is a diagram showing how frames of FIG. 3 has been transmitted for the first 2 T seconds; 
     FIG. 8 is a diagram showing exemplary screens  32 - 1  and  32 - 2  of the clients  30 - 1  and  30 - 2  when the applications M 1  and M 2  are addressed to the clients  30 - 1  and  30 - 2 , respectively; 
     FIG. 9 is a flow chart showing a slice allocating operation of CPU  100  under the control of the control program  252  in accordance with the principles of the invention; 
     FIG. 10 is a flow chart showing an exemplary normal slice allocation operation  75  executed by CPU  100  under the condition of the control program  252  in accordance with the principles of the invention; 
     FIG. 11 is a diagram showing an exemplary slice allocation based on the result of step  300  of FIG. 10; 
     FIG. 12 is a diagram showing multimedia frames transmitted by the threads TH 1 -TH 3  on the basis of the allocation of FIG. 11; 
     FIG. 13 is a diagram showing exemplary screens  32 - 1  and  32 - 2  of the clients  30 - 1  and  30 - 2  when multimedia frames are transmitted as shown in FIG. 12; 
     FIG. 14 is a flow chart showing an exemplary synchronization operation according to the principles of the invention; 
     FIG. 15 is a diagram showing an exemplary slice allocation based on the result of step  318  of FIG. 14; 
     FIG. 16 is a flow chart showing an exemplary preferred audio allocation operation according to the principles of the invention; 
     FIG. 17 is a diagram showing an exemplary allocation of time slices before the preferred audio allocation operation  77  is executed; 
     FIG. 18 is a diagram showing an exemplary allocation of time slices after the preferred audio allocation operation  77  is executed; 
     FIG. 19 is a diagram showing how the transfer rates of an audio and a video change with the change in the transmission bandwidth of the network  20 ; 
     FIG. 20 is a graph showing a relationship between the throughput and the transfer delay in the network  20 ; 
     FIG. 21 is a diagram showing a network data table  280  for the network  20 ; 
     FIG. 22 is a flow chart showing an exemplary transfer rate optimizing operation according to the principles of the invention; 
     FIG. 23 is a diagram showing exemplary timing of data requests from the threads TH 1  and TH 3 ; 
     FIG. 24 is an example of a sharable interval table according to the invention; 
     FIG. 25 is a flow chart showing an exemplary data read operation executed by CPU  100  under the control of a data read subroutine included in the control program  252  in response to a data request from a packet transmitter thread THi in a single media distribution in accordance with the principles of the invention; 
     FIG. 26 is a diagram showing an exemplary allocation of slices to the threads TH 1  and TH 2 ; 
     FIG. 27 is a diagram showing how frames are returned to the threads in response to the requests issued as shown in FIG. 23 in an allocation state of FIG. 26; 
     FIG. 28 is a diagram showing an exemplary allocation of slices to the threads TH 1  and TH 2 ; 
     FIG. 29 is a diagram showing how frames are returned to the threads in response to the requests issued as shown in FIG. 23 in an allocation state of FIG. 28; 
     FIG. 30 is a diagram showing an example of a frame stream transmitted from the server  1  to a client  30 ; 
     FIG. 31 is a diagram showing an exemplary received time table stored in a client  30  according to the invention; 
     FIG. 32 is a flow chart showing an exemplary receiving operation executed by a CPU of a client  30  when the client receives a frame addressed thereto through the IF  31 ; and 
     FIG. 33 is a flow chart showing a part of a display operation of the client  30 . 
     Throughout the drawing, the same elements when shown in more than one figure are designated by the same reference numerals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram showing an exemplary network  1  including a multimedia information server  10  and client terminals  30 - 1 ,  30 - 2 , . . . , the server transmitting a plurality of multimedia applications in accordance with an illustrative embodiment of the invention. The server  10  and the clients  30  are interconnected with a network  20 . The network  20  may be any suitable network which provide a time-division multiplexed multimedia application transfer with a limited transfer bandwidth. 
     The multimedia information server  10  may be any suitable computer comprising: a CPU  100 ; a read only memory (ROM)  102 ; a random access memory (RAM)  104 ; a mass storage device  106 ; a man-machine interfaces (IFs)  120 ; an output devices  122  such as a display device; input devices  124  such as a keyboard, a mouse, etc.; and a communication IF through which multimedia applications are launched into the network  20 . In order to take in multimedia application, the server  10  further comprises a optical disc device  134  which plays an optical disc such as a DVD (digital video disc), a CD (compact disc), etc. and a multimedia data input IF  132  for receiving multimedia application data from the external. 
     FIG. 2 is a schematic block diagram showing an exemplary architecture of the server  10  of FIG. 1 when it is operating. The mass storage device  106  stores an operating system  240 , a multimedia server program  250  for transmitting multimedia applications in accordance with the principles of the invention, and multimedia applications  260 . The multimedia server program  250  includes a control program  252  which controls the transmission operations and a transmitter program  254  which is threaded during operation to actually transmit an application  260  specified by the control program  252 . The operating system  240  allocates available time slices of the CPU  100  to packet transmitter threads  154  (i.e., to the application to be transmitted). The control program  215  determines the number of time slices to be allocated to each thread  154  based on required transfer rates stored in the mass storage device  106  and actual transfer rates measured by monitoring the transmitted data streams as detailed below. 
     It is noted that the way of the mass storage  106  storing a multimedia application is either such that the whole application is stored continuously or such that a fraction of the application is sequentially stored and read out for transmission just as is done in a buffer. 
     In the following description of the operation, it is assumed that three multimedia applications, say, a first motion picture M 1 , a second motion picture M 2  and an audio M 3  are to be transmitted. FIG. 3 is a diagram showing the applications M 1  through M 3 . In FIG. 3, each of the application, Mi (i=1, 2, 3), comprises frames of data, Fi- 1 , Fi- 2 , Fi- 3 , . . . It is assumed that each frame Fi-j (j=1, 2, . . . .) should be presented (i.e., transmitted) every T second. In other words, the frame frequency of the materials M 1  through M 3  is 1/T Hz. 
     FIG. 4 is a diagram showing a multimedia application table  270  for containing various information on the multimedia applications  260  stored in the mass storage  106 . Each record of the table  270  at least comprises the fields of application ID MIDi and required transfer rate RTRi of the application. According to the table  270 , the required transfer rates of the applications M 1 , M 2  and M 3  are 10 Kbps, 5 Kbps and 10 Kbps, respectively. 
     For each application to be transmitted, Mi, the control program  252  creates a packet transmitter thread THi. The operating system (OS)  240  responsively allocates some time slices to each thread THi in a predetermined manner. It is assumed that the OS  240  allocates the same number of slices to each thread THi as shown in FIG.  5 . In FIG. 6, the time slice labeled “VACAUNT” is a time period for which the CPU  100  has nothing to do. The time slice labeled “OS” is a time period for which the CPU  100  is occupied by the OS  240  or other application program(s). 
     Once slices are allocated to the packet transmitter threads  154 , the threads start respective transmission operations. FIG. 6 is a diagramn showing exemplary operation cycle by the threads  154 , the control program  252  and the OS  240  during transmission operation. While the threads  154  are transmitting the applications M 1  through M 3  (block  50 ), the control program  252  measures the transfer rates MTR 1  through MTR 3  of the threads TH 1  through TH 3  (block  60 ). The control program  252  preferably periodically determines how to allocate time slices to the threads  154  on the basis of the measure transfer rates MTR 1 - 3  to inform the OS  240  of the determined numbers of slices to be allocated (block  70 ). Then, the OS  240  allocates the determined numbers of time slices to the threads  154  (block  80 ). The cycle comprisine blocks  50  through  80  continues till the applications M 1  through M 3  are exhausted. 
     In block  50 , each thread THi  154  reads out the first frame Fi- 1  of the specified application Mi, adds a destination address to the read frame Fi- 1  data to form a packet, and sends out the packet to the network  20 . It should be noted that in reading out a frame of data, a thread THi may read out the frame data either as it is or in a thinned-out manner. It is further noted that a thread THi may compress and encode the read frame data before assembling the read frame data into a packet. 
     The process from reading out a frame of a multimedia application Mi to sending out the assembled packet has to be done for the slice time allocated to the application Mi or the thread THi. However, a time period necessary for the process varies depending on the type and the format of the application Therefore the amounts of transmitted data from the threads  154  may vary even if the same time is allocated to the threads  154 . It is assumed that frames of the applications M 1  through M 3  has been transmitted for the first 2T seconds as shown in FIG.  7 . 
     On the other hand, a client terminal, say,  30 - 1  receives a packet that is addressed to the terminal  30 - 1  and supplies the multimedia data contained in the packet to a suitable output device  32 - 1 . FIG. 8 shows how the transmitted frames of FIG. 7 are presented in the output devices  32 - 1  and  32 - 2  of the clients  30 - 1   30 - 2  when the application M 1  is addressed to the client  30 - 1  and the applications M 2  and M 3  are addressed to the client  30 - 2 . However, if data is transmitted at a rate exceeding the processing capacity of the client, say,  30 - 2 , then this results in defective pictures in case of image data and noisy sounds in case of audio data as shown in the output device  32 - 2  of FIG.  8 . 
     Turning back to FIG. 6, the control program  252  periodically measures transfer rates of the threads TH 1  through TH 3 , MTR 1  through MTR 3 , respectively (in block  60 ). Assuming that the data amounts of the applications M 1  through M 3  for T seconds (i.e., a frame&#39;s worth of each application Mi) are 10 Kbps, then the average transfer rates for the first 2T seconds, MTR 1  through MTR 3 , are 5 Kbps, 10 Kbps and 20 Kbps, respectively as seen from FIG.  7 . 
     In block  70 , the control program  252  determines how to allocate time slices to each thread THi on the basis of the measured transfer rates MTR 1  through MTR 3 . FIG. 9 is a flow chart showing a slice allocating operation  70  of CPU  100  under the control of the control program  252  in accordance with the principles of the invention. 
     After step  60  of FIG. 6, CPU  100  makes a test to see if the applications M 1  though M 3  can be transmitted at the required transfer rates RTR 1  through RTR 3  respectively in step  71 . Specifically, CPU  100  makes the following test: 
     
       
           |MTRa−RTRa|&lt;b,   (0) 
       
     
     where the suffix “a” indicates that the application is an audio application, and b is a positive value. The value b may a constant or vary with time. If the value b is variable, the value b may be automatically changed by measuring the change of the transfer rate MTRa of the audio application(s) such that the equality (0) is not affected by the change in the transfer rate MTRa. Since M 3  is an audio application and MTR 3  and RTR 3  are 20 Kbps and 10 Kbps as described above, a test is made to see if |MTR 3 −RTR 3 |=10&lt;b.                    (     1   +   α     )     ·       ∑   i        RTRi       &lt;       ∑   i        MTRi       ,           (   1   )                                
     where α is a positive constant. 
     If the inequality (0) and/or (1) is true, CPU  100  stores 0 in a location M or sets a flag M to 0 in step  72  (the flag M is used in a preferred audio allocation operation detailed later). In step  73 , CPU  100 l makes another test in step  73  to see if the total transfer rate is smaller than the maximum transfer rate of the network  20 . Alternatively, the following test may be made.                    ∑   i        MTRi     &lt;       (     1   +   β     )     ·       ∑   i        RTRi         ,           (   2   )                                
     where β is a constant larger α. In this case, it is preferable to set the right side of the inequality (2) to a value near the maximum transfer rate of the network  20 . If the test result is YES in step  73 , which means that the overall transmission rate is in an acceptable range, then CPU  100  performs a normal slice allocation operation (detailed later) in step  75  and ends the slice allocating operation  70 . 
     If the test result is NO in decision step  71 , which means that one or more application can not be transmitted at a rate not lower than a required transfer rate, then CPU  100  performs a preferred audio allocation operation (detailed later) in step  77 . If the test result is NO in decision step  73 , which means that the measured transfer rates are so high that the delay time in the network  20  can not be neglected, then CPU  100  performs a transfer rate optimizing operation (detailed later) in step  79 . After step  77  or  79 , CPU  100  ends the slice allocating operation. 
     Assuming that the constants α and β are 0 and 0.5, respectively, we discuss the above-mentioned example. As described above, the sum of the required transfer rates is 25 Kbps (=5+10+5 in FIG.  4 ), and the sum of the measured transfer rates is 35 Kbps (=5+10+20). It is clear that the inquality (1) is satisfied. Further, since 35&lt;1.5·25(=37.5), the inequality (2) is also satisfied. Thus, the normal transfer rate subroutine is executed in this case. 
     Normal Slice Allocation Operation 
     FIG. 10 is a flow chart showing an exemplary normal slice allocation operation  75  executed by CPU  100  under the condition of the control program  252  in accordance with the principles of the invention. In step  300 , CPU  100  calculates the number of slices to be allocated to the threads TH 1 -TH 3  based on the measured transfer rates MTR 1 -MTR 3  such that the transfer rates become equal to the required transfer rates RTR 1 -RTR 3 . 
     One of allocation schemes adaptable in this case is to calculating the number of slices to be allocated to the thread THi simply by using the ratio of RTRi/MTRi. Specifically, assuming that the number of slices having been allocated to a thread THi is SNi and the number of slices to be allocated to a thread THi is SNi′, the number SNi′ is given by the equation: 
     
       
           SNi′=SNi·(RTRi/MTRi).   (3) 
       
     
     Since SN 1 =SN 2 =SN 3 =2 according to FIG. 5, we have: 
     
       
           SN   1 ′=2·(10/5)=4 
       
     
     
       
           SN   2 ′=2·(5/10)=1 
       
     
     and 
     
       
           SN   3 ′=2·(10/20)=1. 
       
     
     Accordingly, four slices are allocated to the thread TH 1  and one slice is allocated to each of the threads TH 2  and TH 3  for every T sec. as shown in FIG.  11 . The changing of slice allocation results in data transmission for the next 2T sec. as shown in FIG.  12 . In this case, the block  60  of FIG. 6 again measures the transfer rates of the threads TH 1  through TH 3  to obtain: 
     
       
           MTR   1 =10 Kbps, 
       
     
     
       
           MTR   2 =5 Kbps, 
       
     
     and 
     
       
           MTR   3 =10 Kbps. 
       
     
     In this case, the frames of FIG. 12 are presented on relevant output devices  32 - 1  and  32 - 2  of the clients  30 - 1  and  30 - 2  as shown in FIG.  13 . 
     Though the ratios RTRi/MTRi are simply used for the calculation of the number of time slices to be allocated to the threads TH 1 -TH 3 , the allocated slice numbers may be adaptively changed at each execution of step  300 . 
     Thus, even if CPU  100  has changed the slice allocation for its own processing or other application programs, the multimedia information server  10  can automatically recover the appropriate slice allocation. 
     Further, CPU  100  make s a test in step  302  to see if the multimedia applications need synchroniation. If not, then CPU  100  simply ends the normal slice allocation operation  75 . If not, CPU  100  synchronize the applications with each other in step  304 . The synchronization scheme is to transmit data of applications other than the application that has been transmitted most for the subsequent measurement interval Tm (Tm is assumed to be 2T sec. in this specific example) such that the applications will have been synchronized at the end of the subsequent measurement interval. 
     FIG. 14 is a flow chart showing an exemplary synchronization operation according to the principles of the invention. In step  310 , CPU  100  calculates, for each application, the difference Δi of the transmitted data amount minus the data amount that should have been transmitted for the last measurement interval (Tm), ie., 
       Δi= ( MTRi−RTRi ) ·Tm.   
     In this example, assuming Tm to be 2T, we obtain: 
     
       
         Δ1=−10 T K bits  
       
     
     
       
         Δ2=+10 T K bits  
       
     
     
       
         Δ3=+20  T K bits  (4) 
       
     
     In step  312 , CPU  100  finds the time period Te that it takes to transmit the excessively transmitted data of the most-sent application:              Te   =     Δ                   i   /   RTRi                   =       (     MTRi   -   RTRi     )     ·     Tm   /     RTRi   .                                      
     Since the most-sent application is M 3  in this specific example, we obtain Te=2T(=20T/10). 
     In step  314 , CPU  100  calculates, for each application, the amount of data to be transmitted for the time period Te, i.e., RTRi·Te. In this case, the amounts of data to be transmitted for the time period 2T are: 
     
       
         20 T K bits for  M   1   
       
     
     
       
         10 T K bits for  M   2   (5) 
       
     
     0 for M 3  (20 Kbits of data has been excessively transmitted). 
     In step  316 , CPU  100  calculates, for each application Mi, the correction value Ci defined by: 
     
       
           Ci=RTRi·Te−Δi ( =RTRi·Te− ( MTRi−RTRi)·Tm).   
       
     
     In this example, the correction values for the applications M 1  and M 2  are 30T Kbits and 0 bit, respectively. That is, in order to synchronize the applications, it is only necessary to transmit the application M 1  at a rate of 15 Kbps (=Ci/Tm) for subsequent measurement interval 2T (=Tm). CPU  100  allocates time the slices to relevant threads accordingly in step  318 . In step  320 , CPU  100  actually transmit applications at a rate of 15 Kbps for subsequent measurement interval 2T (=Tm). In this case, slices are allocated to the thread TH 1  as shown in FIG.  15 . In step  322 , CPU  100  restores the slice allocation as determined in step  300  and ends the synchronization operation  304 . 
     According to the invention, the synchronization of the transmitted media is automatically maintained. 
     Preferred Audio Allocation Operation 
     The throughput or the amount of transferred data of the network  20  may lower or the available slices of CPU  100  may decrease to the extent that the applications to be transmitted can no longer transmitted at their required transfer rates. In such a case, a preferred audio allocation operation is executed. 
     FIG. 16 is a flow chart showing an exemplary preferred audio allocation operation according to the principles of the intention. In this operation, referring to the priorities stored in the multimedia application table  270  of FIG. 4, more slices are allocated as possible to the applications of higher priority by reducing the number of slices allocated to lower-priority applications. 
     In step  330 ,CPU  100  adjusts the subsequent transfer rates TRv of image applications (which is denoted as Mv) as follows: 
     
       
           TRv=MTRv+ ( MTRa−RTRa ) ·Ad,   
       
     
     where Ad is a positive variable coefficient. If the measured transfer rate MTRa of the audio application Ma (a=3 in this case) is smaller than the required transfer rate RTRa of the audio application Ma, the transfer rates TRv of image applications Mv is reduced. If the measured transfer rate MTRa is larger than the required transfer rate RTRa of the audio application Ma, the transfer rates TRv of image applications Mv is increased. In step  331 , CPU  100  allocates as many slices as possible to the audio application (M 3  in this case). In step  332 , CPU  100  makes a test to see if M=0. If so, which means that this preferred audio allocation operation has not been executed, then CPU  100  proceeds to step  336 . Otherwise, CPU  100  updates the value of Ad in step  334 . The coefficient Ad is changed in any of the following ways: 
     1. If the unequal relation between the measured and the required transfer rates is the same as the last preferred audio allocation operation, then the coefficient Ad is increased and, otherwise, decreased. 
     1. The degree of the increase and the decrease of (1) is made proportional to the difference between the measured and the required transfer rates. 
     1. The coefficient Ad is set as Ad=RTRA−MTRa. After step  334 , CPU  100  set the flag M to 1. 
     This causes the coefficient Ad to converge rapidly. 
     For example, If the allocated slices are allocated as shown in FIG. 17 in step  71 , then an execution of the preferred audio allocation operation results in an allocation of FIG.  18 . FIG. 19 shows how the transfer rates of an audio and a video change with the change in the transmission bandwidth of the network  20 . 
     Transfer Rate Optimizing Operation 
     Generally speaking, a network  20  is characterized in that the more the transfer rate increases, the longer the delay time becomes as shown in FIG.  20 . This is because a single transmission bandwidth is shared by TDM (time-division multiplexing). Almost all of the conventionally used networks have this characteristic. The transfer rate optimizing operation minimizes the delay that occurs in the network  20 . 
     The relationship between the maximum transfer rate and the delay time is preferably measured in advance with respect to the network  20  and stored in the mass storage device  106 , Alternatively, the relationship may be periodically and automatically measured. 
     FIG. 22 is a flow chart showing an exemplary transfer rate optimizing operation according to the principles of the invention. In step  340 , CPU  100  finds a transfer rate ratio (which enables a low delay transmission) associated with the total transfer rate          ∑   i        MTRi                          
     in the network data table  280 . Since the total transfer rate (MTR 1 +MTR 2 +MTR 3  in this example) is  15 , the transfer rate ratio that enables a low delay transmission is 85%. That is, the transfer rate that enables the low delay transmission is 13 Kbps (=15 Kbps×85%). In step  342 , CPU  100  allocates slices to each thread THi according to the obtained transfer rate (13 Kbps in this specific example). This allocation is such that more slices are reduced for applications with a lower priority. This allocation is preferably performed as the above-described preferred audio allocation operation of FIG.  16 . 
     Distribution of the Same Data 
     It is sometimes the case when the same application is transmitted to a plurality of clients. Though all of the above-described features can be applied to this case, further techniques is available for this case. 
     It is assumed that the packet transmitter threads TH 1  and TH 3  issues data requests at the timing as shown in FIG.  23 . It often happens that the threads TH 1  and TH 3  issue the data requests not at the same time but in vary near timing. If the same data are used or shared by the thread TH 1  and TH 2  in such a case, it is very efficient. 
     FIG. 24 is an exemplary sharable interval table  290  according to the invention. A sharable interval (SI) is a time period in which a data sharing is permitted between requests from the threads TH 1  and TH 2 . In the sharable interval table  290 , a sharable interval (SI) is associated with corresponding unoccupied (vacant) period of CPU  100 . 
     FIG. 25 is a flow chart showing an exemplary data read operation executed by CPU  100  under the control of a data read subroutine included in the control program  252  in response to a data request from a packet transmitter thread THi in a single media distribution in accordance with the principles of the invention. In step  350 , CPU  100  obtains a sharable interval (SI) associated with the vacant period of CPU  100  from a sharable interval table  290 . In step  352 , CPU  100  sets Ti to the current time (i.e., the requested time). In step  354 , CPU  100  makes a test to see if it (=T 1 ) is more than SI after the last requested time (T 2 ). i.e., T 1 &gt;T 2 +SI. If so, the CPU  100  returns a new frame of data in step  358 . If not, the CPU  100  returns the same frame of data in step  356 . After step  356  or  358 , CPU  100  sets the value of T 1  and T 2  to the current time in step  360 . 
     If, for example, the threads TH 1  and TH 2  issue requests at the timing as shown in FIG. 23 when slices are allocated as shown in FIG. 26, then since the vacant period of FIG. 26 is in the range from 101 to 300 ms, the value of SI is set to 1 sec. As a result, the data read subroutine returns frames to the requesting threads as shown in FIG.  27 . Similarly, if the threads issue requests at the timing as shown in FIG. 23 when slices are allocated as shown in FIG. 28, then SI is set to 3 sec., resulting in returning frames as shown in FIG.  29 . 
     Though in the above embodiment, the sharable interval SI has changed depending on the load of CPU  100 , i.e., the length of vacant period, the sharable interval SI may be set to a constant value, say, 1 sec. In this case, the step  350  is omitted. 
     Client Terminal 
     Each time a client terminal  30  receives a frame, the terminal  30  preferably stores the frame number and the received time or the current time so hat the terminal  30  can display the received frames in the same timing as they were received. 
     It is assumed that the multimedia server  1  transmits frames of the multimedia application M 1  in the timing shown in FIG.  30 . For the sake of simplicity, only frames of M 1  are shown in FIG.  30 . It is also assumed that the frames transmitted from the server  1  reaches a destination client  30  in the same timing as the transmission timing. 
     FIG. 32 is a flow chart showing an exemplary reception operation executed by a CPU  33  of a client  30  when the client receives a frame addressed to the client through the IF  31 . In step  370 , CPU  33  receives a frame of packets addressed to the client. In step  372 , CPU  33  makes a test to see if the received frame is one of a new application. If so, then CPU  33  stores the frame in a new name in step  374  and creates a received time table in the new name in step  376 . Otherwise, CPU  33  stores the frame in a relevant location in step  378 . After step  376  or  378 , CPU  33  stores the frame number of the received frame and the received time (or the current time) in the received time table of the received frame in step  380 . In step  382 , CPU  33  makes a test to see if the frame is the end of frame stream (i.e., the last frame of an application). If not, CPU  33  returns to step  370 . If so, then CPU  33  ends the reception operation. 
     If a user requests the client  30  to display one of multimedia applications stored in a just-described manner, the client  30  operates as shown in FIG.  33 . Specifically, in step  386  of FIG. 33, CPU  33  displays each of the frames of the requested application at the time specified by the received time data associated with the frame in the received time table of FIG.  31 . Doing this enables a correct reproduction of a time series data. 
     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.