Abstract:
The invention provides a server wherein a plurality of unit servers are arranged in parallel to each other to allow an increase in the number of clients and continuous data that can be transferred without interruption to clients using a comparatively small number of buffers. A video server includes unit servers including a file apparatus having video data stored discretely therein. When a preparation sign reporting section of a unit server reports a sign, a file read-out section of the unit server reads out the video data and stores them in a buffer, and in response to a sign of a distribution start sign reporting section, a distribution section starts distribution of the video data. A predetermined number of unit servers repeat reading out and distribution of discretely stored video data cyclically at timings set for them in this manner. Consequently, the buffers can be utilized effectively to effect distribution without interruption.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a server which transfers, for example, continuous data such as video data in response to a request from a client, and more particularly to a server effectively used with a communication system wherein a plurality of servers are coupled to each other by a network and continuous data of an image file or the like are stored discretely in the servers. 
     2. Description of the Related Art 
     Continuous data represented by video data are required to be transferred without interruption to a client. Japanese Patent Laid-Open Application No. Heisei 3-185948 or No. Heisei 6-236330 discloses a system which transfers video data via a network. Further, a server for use with a system of the type just mentioned is conventionally known and disclosed, for example, in Nikkei Electronics, No. 645, pp.133-141. The server disclosed in the document just mentioned will be hereinafter referred to as first prior art. 
     FIG. 8 shows a communication system in which the server of the first prior art Is employed. Referring to FIG. 8, the communication system shown includes a video server  101  for distributing video data, a plurality of clients  102   1  to  102   N , and a network  103  which interconnects the video server  101  and the clients  102   1  to  102   N . The video server  101  includes a file apparatus  111  for storing video data. The video data stored in the file apparatus  111  are read out by a file read-out section  112  and temporarily stored into a buffer  113 , and then sent out to the network  103  by a distribution section  114 . 
     If a request for video data is received from one of the clients  102   1  to  102   N , then the video server  101  reads out relevant data from the file apparatus  111  and temporarily stores the data into the buffer  113 . It is assumed now that a request for video data is developed from the client  102   1 . The video data in the buffer  113  are successively sent to the distribution section  114  and then sent via the network  103  to and reproduced by the client  102   1 . 
     In the communication system employing the server of the first prior art, the buffer is used so that video data as continuous data may be distributed without interruption from the video server  101  to the clients  102   1  to  102   N  side, and the buffer capacity of the buffer  113  on the video server  101  side is set to a rather high capacity in order to reduce the buffer capacity on the clients  102   1  to  102   N  side. Accordingly, the video server  101  is constructed such that it need not access the file apparatus  111  every time a request is received from any of the clients  102   1  to  102   N . 
     The communication system employing the server of the first prior art described above is subject to restriction in number of clients ( 102   1  to  102   N ) to which video data are to be supplied from a relationship to an upper limit to the capacity of a work station which constructs the video server. In order to solve this problem, two countermeasures are available: one is to raise the upper limit to the capacity of a work station, and the other is to use a plurality of video servers which operate in parallel to each other. The countermeasure to raise the upper limit to the capacity of a work station requires development of a new server having a higher capacity. On the other hand, the countermeasure to use a plurality of video servers which operate in parallel to each other is advantageous in that existing comparatively inexpensive work stations can be used for the video servers. 
     FIG. 9 shows a system wherein a plurality of video servers operate in parallel to each other. The system will be hereinafter referred to as second prior art. In FIG. 9, like elements are denoted by like reference symbols to those of FIG. 8, and overlapping description of those elements is omitted herein to avoid redundancy. Referring to FIG. 9, the system shown includes a video server  121  which in turn includes totaling M first to Mth unit servers  122   1  to  122   M . The unit servers  122   1  to  122   M  include, similarly as in the video server  101  of FIG. 8, file apparatus  111   1  to  111   M , file read-out sections  112   1  to  112   M , buffers  113   1  to  113   M , and distribution sections  114   1  to  114   M , respectively. Video data are stored discretely in units of a block in the file apparatus  111   1  to  111   M  of the unit servers  122   1  to  122   M . 
     For example, in order for the first client  102   1  to reproduce video data, the communication system having the construction described above operates in the following manner. It is assumed that the first client  102   1  sends a request for video data for one second to the first unit server  122   1 . In response to the request, the file read-out section  112   1  delivers a file read-out command to the first file apparatus  111   1 . The first file apparatus  111   1  reads out relevant video data in response to the file read-out command and stores the video data once into the first buffer  113   1 . Then, the first distribution section  114   1  transmits the thus stored video data to the first client  102   1  via the network  103 . 
     The first client  102   1  reproduces the video data sent thereto while it sends a next read-out request to the next unit server  122   2 . In this manner, each of the clients  102   1  to  102   N  successively sends a request to the unit servers  122   1  to  122   M  to reproduce the video data which are continuous as a whole. The communication system which employs the server of the second prior art is advantageous in that the number of clients ( 102   1  to  102   N ) can be increased using inexpensive file apparatus ( 111   1  to  111   M ) or work stations. 
     By the way, the amount of video data is generally very large. Therefore, a hard disk is usually used for file storage apparatus. However, the hard disk employs mechanical operating members in order to read out data. Therefore, after a file read-out command is received, a waiting time such as a time for a seeking operation of a head and/or a latency time is required and object information cannot be read out immediately. 
     According to the communication system employing the server of the first prior art described hereinabove with reference to FIG. 8, in order to cover the waiting time, a buffer having a comparatively large capacity is used for the buffer  113  in the video server  101 . In particular, a comparatively large amount of video data including relevant video data is read out by a one time access to the file apparatus  111  such as a hard disk and is stored at a time into the buffer  113  having a comparatively large capacity. Then, when a request for video data following the relevant video data is received from the client  102 , the file apparatus  111  is not accessed, but the requested video data are transferred immediately from within the stored contents of the buffer  113 . Consequently, otherwise possible interruption of images on the client  102  side is prevented. 
     However, if the server of the second prior art described hereinabove with reference to FIG. 9 is used in order to increase the number of clients, a considerably long time elapses before an access is received from the same client. It is assumed that the video server  121  includes, for example, totaling  100  first to one hundredth unit servers  122   1  to  122   100 . If it is assumed that each of the unit servers  122   1  to  122   100  transfers data for one second, then 100 seconds elapse before a certain one of the unit servers  122   1  to  122   100  is accessed for the next time from the same one of the clients  102   1  to  102   N . Besides, if it is assumed that each one of the unit servers  122   1  to  122   100  supports, for example, 40 clients in average, then it requires a buffer having a capacity for 40 times 1,000 seconds. In this manner, the system which transfers video data without interruption to a desired client using the server of the second prior art is disadvantageous in that it requires a high cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a server wherein a plurality of unit servers are arranged in parallel to each other to allow an increase in number of clients and continuous data can be transferred without interruption to clients using a comparatively small number of buffers. 
     In order to attain the object described above, according to an aspect of the present invention, there is provided a server comprising a plurality of unit servers connected to a network together with a plurality of clients and having a file, in which continuous data are accommodated, stored discretely in units of a predetermined data amount therein, each of the unit servers including a file apparatus in which the discrete continuous data are stored, file read-out means for reading out the continuous data successively by the predetermined data amount from the file apparatus, a buffer for temporarily storing the continuous data read out by the file read-out means at the shortest until transfer of the stored continuous data comes to an end, transfer means for transferring the continuous data stored in the buffer to one of the clients, from which a request for the file has been received, via the network, transfer start sign reporting means for reporting a timing, at which the continuous data stored in the file apparatus are to be transferred, to the transfer means based on an order in which the continuous data stored discretely in the unit servers are to be transferred, and preparation sign reporting means for reporting a timing, at which the continuous data are to be read out from the file apparatus, to the file read-out means so that, prior to starting of transfer of the continuous data by the unit server, the continuous data may be stored by the predetermined data mount in the buffer. 
     In the server, the plurality of unit servers are provided to discretely and cyclically store continuous data individually by the predetermined data amount, and each of the unit servers reads out the continuous data successively by the predetermined data amount from the file apparatus, temporarily stores them and transfers them to one of the clients, from which a request for the file has been received. Here, upon reading out from the file apparatus, the transfer start sign reporting means reports the reading out timing, and upon transfer of the continuous data stored in the buffer, the preparation sign reporting means reports the transfer timing. The unit servers can read out the discretely stored continuous data, store them into the buffers and transfer them in a predetermined order by the predetermined amount, and consequently, the continuous data can be transferred without interruption to the client. Besides, since the buffer of each of the unit servers can process the continuous data successively by the predetermined data amount, those data which are not necessary any more because transfer of them is completed can be erased from the buffer and following continuous data of the predetermine data amount can be stored into the buffer. Accordingly, a device having a comparatively small storage capacity can be used for the buffer. 
     With the buffer, since buffers having a capacity equal to an amount of data to be transferred by one transferring operation are prepared in individual unit servers for successively transferring continuous data to a client and are successively used to repeat writing of the continuous data and reading out of the continuous data for transfer, even if the amount of the continuous data is large, the continuous data can be transferred without interruption to a client using the minimized number of buffers. Consequently, the buffer can be constructed economically. Further, although an increase in number of unit servers generally gives rise to a lack in logical network resources, since the server of the present invention can use network resources when this becomes necessary, the server is advantageous also in that minimized network resources are required. 
     According to another aspect of the present invention, there is provided a server comprising a plurality of unit servers connected to a network together with a plurality of clients and having a file, in which video data are accommodated, stored discretely and cyclically in units of a predetermined data amount therein, each of the unit servers including a file apparatus in which the discrete video data are stored, file read-out means for reading out the video data successively by the predetermined data amount from the file apparatus, a buffer for temporarily storing the video data read out by the file read-out means at the shortest until distribution of the stored video data comes to an end, distribution means for distributing the video data stored in the buffer to one of the clients, from which a request for the file has been received, via the network, distribution start sign reporting means for reporting a timing, at which the video data stored in the file apparatus are to be distributed, to the distribution means based on an order in which the video data stored discretely in the unit servers are to be distributed, and preparation sign reporting means for reporting a timing, at which the video data are to be read out from the file apparatus, to the file read-out means so that, prior to starting of distribution of the video data by the unit server, the video data may be stored by the predetermined data mount in the buffer. 
     In the server, the plurality of unit servers are provided to discretely and cyclically store video data individually by the predetermined data amount or to control such storage of video data based on a table in which an order is described, and each of the unit servers reads out the video data successively by the predetermined data amount from the file apparatus, temporarily stores them and distributes them to one of the clients, from which a request for the file has been received. Here, upon reading out from the file apparatus, the distribution start sign reporting means reports the reading out timing, and upon distribution of the video data stored in the buffer, the preparation sign reporting means reports the distribution timing. The unit servers can read out the discretely stored video data, store them into the buffers and distribute them in a predetermined order by the predetermined amount, and consequently, the video data can be distributed without interruption to the client. Besides, since the buffer of each of the unit servers can process the video data successively by the predetermined data amount, those data which are not necessary any more since distribution of them is completed can be erased from the buffer and following video data of the predetermine data amount can be stored into the buffer. Accordingly, a device having a comparatively small storage capacity can be used for the buffer. 
     With the buffer, since buffers having a capacity equal to an amount of data to be distributed by one distribution operation are prepared in individual unit servers for successively distributing video data as a kind of continuous data to a client and are successively used to repeat writing of the video data and reading out of the video data for distribution, even if the amount of the video data is large, the video data can be distributed without interruption to a client using the minimized number of buffers. Consequently, the buffer can be constructed economically. Besides, a required number of buffers may be allocated immediately prior to distribution of the video data. In particular, with a conventional server, if it is assumed that it includes 100 unit servers, then buffers for 100 seconds must be prepared for each stream. However, with the buffer of the present aspect of the invention, only buffers for 2 seconds may be allocated for preparation for reading out and distribution. Consequently, the server of the discrete storage type can be constructed with a very small memory capacity. Further, although an increase in number of unit servers generally gives rise to a lack in logical network resources, since the server of the present invention can use network resources when this becomes necessary, the server is advantageous also in that minimized network resources are required. 
     The server may be constructed such that each of the unit servers includes time synchronization means for synchronizing times of the unit servers with each other, and the distribution start sign reporting means and the preparation sign reporting means output the respective reports based on the time synchronized by the time synchronization means. 
     Since the unit servers are individually connected to the network, in order that the unit servers may successively distribute data by the predetermined amount at good timings to a client, the unit servers must use the same clock among them. Thus, each of the unit servers includes time synchronization means for synchronizing times of the unit servers with each other, and the distribution start sign reporting means and the preparation sign reporting means individually perform reporting at accurate timings in the unit server. Consequently, even if the network is very large or the positions of the individual unit servers on the network are spaced by a large distance from each other, such reports as mentioned above can be delivered at accurate timings in the individual unit servers. 
     In this instance, the server may be constructed such that the preparation sign reporting means and the distribution start sign reporting means commonly include the time synchronization means as well as a table in which a time at which a report is to be outputted is described, table checking means for checking the table after each predetermined interval of time based on the time synchronized by the time synchronization means, report outputting means for outputting, when the time described in the table comes, a relevant report, and description updating means for rewriting the description of the table to a time at which another report is to be outputted. Since the preparation sign reporting means and the distribution start sign reporting means are formed from the common members, the server can be constructed economically. Further, since the two means use the table commonly and check it to output individual reports at relevant times, the reports can be outputted in a timed relationship from each other. Furthermore, since the description updating means is provided, the table can be produced in a comparatively small size and hence at a comparatively low cost. 
     The server may alternatively be constructed such that each of the unit servers further includes distribution end notification means for reporting an end of distribution of the video data by the predetermined data amount by the distribution means to the other unit servers, and distribution start sign reporting means for reporting, when an end of distribution is reported from the distribution end notification means of one of the unit servers which performs distribution immediately before then, a distribution start sign to cause the self unit server to which the distribution start sign reporting means belongs to start distribution of the video data by the predetermined amount. 
     In the server, when the unit servers are to distribute video data cyclically in an order determined in advance, one of the unit servers which completes it distribution reports this to another one of the unit servers which is to perform distribution subsequently. Consequently, reporting of distribution can be performed reasonably. Besides, in this instance, if distribution of the first unit server comes to an end before a possible maximum time elapses, the second server can start its distribution earlier accordingly, and the total time required for distribution of the entire video data to a client can be reduced as much. 
     In this instance, the server may be constructed such that each of the unit servers further includes preparation sign notification means for reporting, when a report of an end of distribution is received from the distribution end reporting means of any other one of the unit servers, a timing at which the video data are to be read out from the file apparatus of the self unit server from the time of the reception of the report, and second distribution start sign notification means for delivering, when a report of an end of distribution is received from the distribution end reporting means of any other one of the unit servers, a report to start distribution of the video data of the self unit server from the time of the reception of the report. Thus, reading out of a file from the file apparatus can be started earlier similarly as described above, and the server can achieve distribution control with a generally higher efficiency. 
     The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a video server according to a preferred embodiment of the present invention and an outline of a communication system which employs the video server; 
     FIGS.  2 ( a ) to  2 ( d ) are diagrammatic views illustrating a manner in which delimited video data are accommodated cyclically in first to fourth unit servers, respectively; 
     FIG. 3 is a timing chart illustrating a manner in which video data are distributed from the first to fourth unit servers shown in FIGS.  2 ( a ) to  2 ( d ) to a first client; 
     FIG. 4 is a flow chart illustrating contents of control for distribution of the first unit server shown in the video server of FIG. 1; 
     FIG. 5 is a block diagram showing a detailed construction of a preparation sign reporting section and a distribution start sign reporting section of the server of FIG. 1; 
     FIG. 6 is a diagrammatic view showing an example of a stream table used in the server of FIG. 1; 
     FIG. 7 is a block diagram showing a modification to the video server of FIG.  1  and an outline of a communication system employing the modified video server; 
     FIG. 8 is a block diagram showing a conventional communication system; and 
     FIG. 9 is a block diagram showing another conventional communication system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there are shown a video server as a server according to a preferred embodiment of the present invention and an outline of a communication system which employs the video server. The video server of the present embodiment is generally denoted at  201  and connected to a network  203  together with first to Nth clients  202   1  to  202   N . 
     The video server  201  includes totaling M first to Mth unit servers  211   1  to  211   M . The unit servers  211   1  to  211   M  have a same construction as each other and each includes a file apparatus  221  ( 221   1 ,  221   2 , . . . ) for storing video data. The file apparatus  221  is read out by a file read-out section  222  ( 222   1 ,  222   2 , . . . ) and temporarily stored into a buffer  223  ( 223   1 ,  223   2 , . . . ). whereafter it is sent out to the network  203  by a distribution section  224  ( 224   1 ,  224   2 , . . . ). A preparation sign reporting section  226  ( 226   1 ,  226   2 , . . . ) is connected to the file read-out section  222  so that a sign of preparation for reading out of the file apparatus  221  may be reported. Further, a distribution start sign reporting section  227  ( 227   1 ,  227   2 , . . . ) is connected to the distribution section  224  so that a sign of start of distribution of data to the network  203  may be reported. 
     It is to be noted that the file apparatus  221  in each of the totaling M first to Mth unit servers  211   1  to  211   N  is formed from, for example, a hard disk, and the buffer  223  is formed from, for example, a random access memory (RAM). Meanwhile, all or some of the other components such as the preparation sign reporting section  226  and the distribution start sign reporting section  227  can be formed from a central processing unit (CPU), a read only memory (ROM) in which predetermined programs are stored, and computer elements such as a RAM for use as a working memory, all not shown. 
     Operation of the video server  201  having the construction described above when it distributes video data to one of the first to Nth clients  202   1  to  202   N  will be described below. 
     First, a process of storing video data of one file into the video server  201  will be described. Video data for one file are delimited into blocks of a suitable size, for example, of a length for one second, and the blocks are successively numbered with serial numbers beginning with “0”. 
     FIGS.  2 ( a ) to  2 ( d ) illustrate a manner in which the delimited video data are cyclically stored in the first to fourth unit servers. In particular, FIG.  2 ( a ) illustrates a stored condition of moving pictures in the first unit server  211   1 ; FIG.  2 ( b ) illustrates a stored condition of moving pictures in the second unit server  211   2 ; FIG.  2 ( c ) illustrates a stored condition of moving pictures in the third unit server  211   3 ; and FIG.  2 ( d ) illustrates a stored condition of moving pictures in the fourth unit server  211   4 . The video data for the first one second are stored with the serial number of “000” in the first unit server  211   1 ; the video data for the second one second are stored with the serial number of “001” in the second unit server  211   2 ; the video data for the third one second are stored with the serial number of “002” in the third unit server  211   3 ; and the video data for the fourth one second are stored with the serial number of “003” in the fourth unit server  211   4 . The video data for the fifth one second are stored with the serial number of “004” back again in the first unit server  211   1 . The other data are successively stored in a similar manner as described above. 
     Since the video data are successively and cyclically stored into the first to fourth unit servers  211   1  to  211   4  in this manner, in which one of the first to fourth unit servers  211   1  to  211   4  the video data for the nth one second are stored can be determined by calculating the remainder i when the value n is divided by the value “4”. This is represented by the following expression (1): 
     
       
         i=(n mod 4)  (1) 
       
     
     In the expression (1) above, if i is “0”, then the video data are stored in the first unit server  211   1 ; if i is “1”, then the video data are stored in the second unit server  211   2 ; if i is “2”, then the video data are stored in the third unit server  211   3 ; and if i is “3”, then the video data are stored in the fourth unit server  211   4 . 
     The video data stored discretely in the unit servers  211   1  to  211   4  as described above are successively transferred, as the unit servers  211   1  to  211   4  successively perform reading out and distribution of the file, for example, to the first client  202   1 . 
     FIG. 3 illustrates a manner in which the video data are distributed from the first to fourth unit servers to the first client. In the manner illustrated in FIG. 3, it is assumed that one second is required in the maximum to read out the video data from each of the file apparatus  221   1  to  221   4  of the first to fourth unit servers  211   1  to  211   4  and store the read out data into a corresponding one of the buffers  223   1  to  223   4 . If it is assumed that a request for distribution is received at time t 0  from the first client  202   1 , then the preparation sign reporting section  226   1  in the first unit server  211   1  reports a sign of reading out of a file to the file apparatus  221  at time t 0  which is preceding by one second to a distribution time. Then, when time t 1  after one second elapses comes, the distribution start sign reporting section  227   1  in the first unit server  211   1  reports a sign of start of distribution to the distribution section  224   1 , and distribution is started at this time t 1 . This will be described in more detail. 
     First, a request for distribution is outputted from the first client  202   1  and accepted by the relevant first to fourth unit servers  211   1  to  211   4 . The acceptance of the request may be performed otherwise such that the request is first accepted directly by the first unit server  211   1  in which the top portion of the video data is stored and then this is reported to the other second to fourth unit servers  211   2  to  211   4 . Or else, a client request acceptance section for collectively accepting requests from the clients  202   1  to  202   N  may be provided independently on the network  203  or a particular one of the clients  202  may have the function of such client request acceptance section such that the client request acceptance section accepts requests for distribution and reports them to required ones of the unit servers  211 . 
     After the request for distribution is accepted, the first to fourth unit servers  211   1  to  211   4  individually set distribution times. Here, since it is assumed that one second is required in the maximum for preparation of reading out of a unit of video data stored discretely, if the request is accepted at time to from the first client  202   1 , then the start time t 1  for distribution corresponding to the accepted request is the point of time elapsed by one second from time t 0 . Thus, when the request for distribution is accepted at time t 0 , the first unit server  211   1  starts distribution at time t 1  elapsed by one section from time t 0 . 
     FIG. 4 illustrates a flow of such control of the first unit server as described above. Referring to FIG. 4, if the request for distribution is accepted at time t 0  (step S 101 ; Y (Yes)), then the first unit server  221   1  waits that the time at which video data are to be read out from the file apparatus  221   1  of itself (step S 102 ). Since distribution of video data is first performed by the first unit server  211   1 , the read time comes simultaneously with the acceptance of the request (step S 102 ; Y). Thus, the preparation sign reporting section  226   1  reports a read-out sign to the file read-out section  222   1  (step S 103 ). In response to the read-out sign, the file read-out section  222   1  secures a memory area for video data for one second in the buffer  223   1 , reads out video data from the self file apparatus  221   1  and stores the video data into the secured memory area. 
     The first unit server  211   1  supervises that the time t 1  elapsed by one second after read-out time t 0  in step S 102  comes (step S 104 ), and when time t 1  comes (step S 104 ; Y), the distribution start sign reporting section  227   1  reports a sign of start of distribution to the distribution section  224   1  (step S 105 ). In response to the sign of start of distribution, the distribution section  224   1  successively reads out the video data stored in the buffer  223   1  from time t 1  and distributes the video data to the first client  202   1  via the network  203 . At a point of time when the distribution comes to an end (step S 106 ; Y), the first unit server  211   1  releases the memory area in the buffer  223   1  used for the distribution (step S 107 ). This allows storage of following video data. Then, it is checked whether or not the distribution of a series of video data to the first client  202   1  is completed (step S 108 ), and if the distribution is not completed as yet (step S 108 ; N), then the control sequence returns to step S 102 , in which it is supervised that time t 4  illustrated in FIG. 3 comes. Thereafter, control for distribution is performed in a similar manner. Then, if the distribution for the series of video data is completed (step S 103 ; Y), then the control is ended (End). 
     On the other hand, the second unit server  211   2  develops a sign of reading out of a file at time t 1  as seen in FIG. 3, and then develops a sign of start of distribution at time t 2  elapsed by one second after time t 1 . Further, the third unit server  211   3  signs reading out of a file at time t 2  and signs start of distribution at time t 3  elapsed by one second after time t 2 . The fourth unit server  211   4  signs reading out of a file at time t 3  and signs start of distribution at time t 4  elapsed by one second after time t 3 . Thereafter, control for distribution of video data is cyclically repeated by the first to fourth unit servers  211   1  to  211   4  in a similar manner. Contents of the control of the second unit servers  211   2  to  211   4  are similar to those of the control of the first unit server  211   1  illustrated in FIG. 4 except that the time at which video data are to be read out is different individually. 
     FIG. 5 shows the preparation sign notification means and the distribution start sign notification means where they are formed from hardware components. Referring to FIG. 5, the preparation sign reporting section  226  and the distribution start sign reporting section  227  are formed from common components. In particular, the preparation sign reporting section  226  and the distribution start sign reporting section  227  are formed from a synchronization section  231  for synchronizing the times of the unit servers  211   1  to  211   M  with each other, a timer  232  which is started at a time synchronized by the synchronization section  231 , a stream table checking and rewriting section  233  which is interrupted each time the timer  232  counts up a predetermined time, and a stream table  234  connected to the stream table checking and rewriting section  233 . 
     FIG. 6 shows an example of the stream table. The stream table  234  represents a relationship among the client number, the image number, the image block number, the read-out time and the distribution start time. Here, the client number is a number of a client which is a destination of distribution, and the image number is a number of an image to be distributed. The video block number is a number representing a distributed portion in an image. The read-out time is a time at which a sign of start of reading out is to be developed, and the distribution start time is a time at which a sign of start of distribution is to be developed. 
     Referring back to FIG. 5, the synchronization section  231  synchronizes the times of the unit servers  211   1  to  211   M  with each other at a common time so that the first to fourth unit servers  211   1  to  211   M  may distribute video data in correct order. Such synchronization can be realized by means of a hardware element for synchronization among the unit servers  211   1  to  211   M  so that a same clock signal generated by a clock generation circuit not shown is supplied commonly to the unit servers  211   1  to  211   M . Or, a software program for a network time protocol (ntp) or the like may be used to synchronize the unit servers  211   1  to  211   M  via the network  203 . 
     The timer  232  generates an interruption to the stream table checking and rewriting section  233  after each predetermined interval of time beginning with a time at which synchronism is established by the synchronization section  231 . The stream table checking and rewriting section  233  every time refers to the stream table  234  to check whether or not the present point of time has come to or passed a time described in the stream table  234 . If it is assumed that the time of interruption processing is 18:10.21, then this is the same as the read-out time for the client number “381” in the stream table  234  shown in FIG.  6 . Therefore, the preparation sign reporting section  226  reports a readout sign  241  to the file read-out section  222  so that video data of the read-out block “000” of the image number “001” may be read out. 
     Thereafter, the relevant unit server  211  ( 211   1  to  211   M ) rewrites the image block number of “000” to “004” and rewrites the read-out time described as “18:10.21” to “18:10.25” which is later by 4 seconds so that reading out of video data of the next image block number may be performed. 
     Further, the stream table checking and rewriting section  233  of the relevant unit server  211  performs checking of the stream table  234  each time an interruption is generated from the timer  232 , and if the checked time comes to 18:10.22, then the stream table checking and rewriting section  233  discriminates that the distribution start time for the image block “000” of the client number “381” comes. Then, at this point of time, a distribute start sign  242  is sent out from the distribution start sign reporting section  227  to the distribution section  224 . Also in this instance, the relevant unit server  211  rewrites the distribution start time from “18:10.22” to “18:10:26” which is later by 4 seconds so that control for starting distribution of video data of the next image block number may be performed similarly. 
     Modification 
     FIG. 7 shows a video server as a modification to the embodiment of the present invention described above and an outline of a communication system which employs the video server. Description of common elements of the modified video server and the communication system to those described hereinabove with reference to FIG. 1 is omitted herein to avoid redundancy. A video server  301  connected to a first network  203   1  together with first to Nth clients  202   1  to  202   N  includes first to Mth unit servers  302   1  to  302   M . The first to Mth unit servers  302   1  to  302   M  have a same construction as each other and each includes a file apparatus  221  ( 221 ,  221   2 , . . . ) for storing video data. The file apparatus  221  is read out by a file read-out section  222  ( 222   1 ,  222   2 , . . . ) and temporarily stored into a buffer  223  ( 223   1 ,  223   2 , . . . ), whereafter it is sent out to the first network  203   1  by a distribution section  305  ( 305   1 ,  305   2 , . . . ). A preparation sign reporting section  306  ( 306   1 ,  306   2 , . . . ) is connected to the file read-out section  222  so that a sign for preparation of read-out of the file apparatus  221  may be reported. Further, a distribution start sign reporting section  307  ( 307   1 ,  307   2 , . . . ) is connected to the distribution section  224  so that a sign of start of distribution of data to the first network  203   1  may be reported. Furthermore, a distribution end reporting section  308  ( 308   1 ,  308   2 , . . . ) is connected to the distribution section  224  so that completion of distribution of the relevant unit server  302  ( 302   1  to  302   M ) may be reported. 
     While the present modified video server employs the first network  203   1  which is a substantially same network as the network  203  in the embodiment described above, it further includes a second network  203   2  connected to the preparation sign reporting sections  306 , distribution start sign reporting sections  307  and distribution end reporting sections  308  of the unit servers  302   1  to  302   M . In particular, in the modified video server  301 , the preparation sign reporting sections  306  and the distribution start sign reporting sections  307  receive a common clock signal via the second network  203   2  to establish synchronism between them, and also a timing at which distribution is completed is sent out to the second network  203   2 . 
     In the description of the modified video server  301 , several steps when a distribution request is accepted from any of the first to Nth clients  202   1  to  202   N  will be hereinafter referred to as initial steps, and the following steps after a steady state is thereafter entered will be hereinafter referred to as steady steps. Further, in the following description, an example wherein the first client  202   1  develops a request for distribution similarly as in the embodiment described hereinabove and then the unit servers  302   1  to  303   4  cyclically distribute video data will be described. 
     If the first client  202   1  develops a request for distribution, then the request is accepted by a relevant one of the first to fourth unit servers  302   1  to  302   4 . In the initial steps, the preparation sign reporting section or sections  306  of a unit server or servers  302  set in advance develop a read-out preparation sign to the corresponding file read-out section or sections  222 . In the present example, the preparation sign reporting section  226   1  of the unit server  302   1  and the preparation sign reporting section  226   2  of the unit server  302   2  output a read-out preparation sign to the corresponding file read-out sections  222   1  and  222   2 , respectively. 
     In the unit server  302   1 , the file read-out section  222   1  secures a memory area for video data for one second in the buffer  223   1 . Then, the file read-out section  222   1  reads out the video data for one second from the file apparatus  221   1  and stores the video data into the thus secured memory area of the buffer  223   1 . 
     When one second elapses after the request for distribution is accepted from the first client  202   1 , storage of the video data into the buffer  223   1  comes to an end. Thus, in the initial steps, when one second elapses in this manner, the distribution start sign reporting section  307   1  delivers a distribution start sign to the distribution section  305   1 . The operations up to this are the initial steps. 
     Then, a steady state is entered. In the steady state, the distribution section  3051  reads out, in response to the distribution start sign received from the distribution start sign reporting section  307   1 , the video data from the relevant memory area of the buffer  223   1  and distributes the video data to the first client  202   1  via the first network  203   1 . When the distribution of the video data of the image block number comes to an end, the first unit server  302   1  releases the relevant memory area of the buffer  223   1 . Then, the unit server  302   1  outputs a distribution end sign to the distribution end reporting section  308   1 . Upon reception of the distribution end sign, the distribution end reporting section  308   1  reports the distribution end sign to the distribution start sign reporting section  307   2  of the second unit server  302   2  via the second network  203   2 . Simultaneously, the distribution end reporting section  308  reports the distribution end sign also to the preparation sign reporting section  306   3  of the third unit server  302   3 . 
     Upon reception of the distribution end sign, the distribution start sign reporting section  307   2  of the unit server  302   2  distributes video data prepared already in the buffer  223   2  to the first client  202   1 . Further, the preparation sign reporting section  306   3  of the third unit server  302   3  having received the distribution end sign reports, in response to the sign, a read-out preparation start sign to the file read-out means  222   3  to start reading out of the relevant file of the file apparatus  221   3 . In this manner, in the modification described above, a distribution end sign is sent to one of the unit servers  302 , which is to start distribution subsequently, using the second network  203   2  so that distribution may be started. Consequently, successive image blocks of video data can be distributed successively without interruption from the first to fourth unit servers  302   1  to  302   4  to the first client  202   1 . 
     While the communication system which employs the modified server employs the first network  203   1  for distribution of video data and employs the second network  203   2  for reporting of a distribution end sign, the employment of the first network  203   1  and the second network  203   2  is intended to merely facilitate description, and countermeasures for distribution of video data and for reporting of a distribution end sign are not limited to the specific networks. For example, a single network which integrates the first and second networks  203   1  and  203   2  may be employed instead. 
     Further, the communication systems in which the servers of embodiment and the modification are employed presume that a logical network is provided in advance from the distribution section  224  or  305  of a unit server  211  or  302  to a client  202 . However, if the number of unit servers  211  or  302  increases very much, then it may possibly become impossible to provide logical networks from one client  202  to all of the unit servers  211   1  to  211   M  or  302   1  to  302   M  in advance. In order to solve this, a logical network should be provided between the distribution section  224  or  305  and the client  202  when this becomes necessary. In addition, a logical network can be provided between the distribution section  224  or  305  and the client  202  when this becomes necessary if the preparation sign reporting section  226  or  306  reports a read-out preparation sign to the file read-out section  222  and also reports a distribution preparation sign to the distribution section  224  or  305 . 
     Further, while the communication systems which employ the servers of the embodiment and the modification described above presume video data, naturally the present invention can be applied similarly also to transfer of any other continuous data. 
     Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention as set forth herein.