Abstract:
Disclosed is a communication system that transmits data through a transmission path between a transmission side apparatus and a reception side apparatus, wherein the transmission side apparatus comprises a coding apparatus that creates redundantly-coded data from original data; a transmitting apparatus that sends the coded data coded by the coding unit to the transmission path; and a coding rate determining apparatus that sets and controls a coding rate in the coding unit, wherein the reception side apparatus comprises a receiving apparatus that receives the coded data sent through the transmission path; a decoding apparatus that decodes the original data from the coded data received; and a loss rate estimating apparatus that measures the loss rate on the path of the coded data sent, and wherein the coding rate determining apparatus of the transmission side apparatus varies and controls the coding rate in the coding apparatus based on the loss rate obtained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-74263, filed on Mar. 17, 2006, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates especially to a data transferring method for data transferring between data processing apparatuses connected mutually through a network, and to a communication system and a program that are applied with the method. 
     2. Description of the Related Art 
     In a network such as the Internet or an intranet, etc., when data are transmitted at a processing speed exceeding the processing speed of a relaying/processing apparatus, the data that can not be processed are lost. Such a state is referred to as “congestion state”. When congestion has occurred, the congestion will be made worse if additional data are further transmitted. 
     According to TCP (Transmission Control Protocol) that is a protocol used generally in data communication, the amount of data that can be transmitted together at one time by the transmitting side is provided corresponding to the size of a window. 
     The transmitting side does not transmit the succeeding data until the transmitting side receives an acknowledgment of reception of the data from the receiving side. Therefore, the communication performance is degraded. 
     According to TCP, as a measure against congestion, A method is taken that the amount of data to be transmitted together at one time is made small at the start of communication by setting the size of a window to be small and, when a response has arrived from the receiving side, the amount of the data to be transmitted is increased by enlarging the size of the window gradually. This method is referred to as “slow start”. 
     When RTT (Round Trip Time) that corresponds to the distance between networks is small, communication is recovered soon even when the size of the window is made small. However, when RTT is large, a problem has arisen that the transfer speed is remained to be slow because the size of the window can not be recovered so soon. 
     For the above technical problem, various suggestions have been presented that improve efficiency from the viewpoint of coding. 
     As a first conventional technique, an invention described in Japanese Patent Application Laid-Open Publication No. 1995-123043 can be listed. This invention selects and varies an appropriate coding scheme or a coding rate without degrading the efficiency even when the state of a transmission path is changed often. 
     As a second conventional technique, an invention described in Japanese Patent Application Laid-Open Publication No. 1996-265304 can be listed. This invention is a method that prevents degradation of the transmission efficiency and prevents increase of the communication time by switching the error control scheme in response to the change of the line state of a wireless line. However, according to this method, data are transmitted according to two (2) or more coding schemes, error detection is executed to each of the results of decoding, and the decoding result from which no error has been detected is accepted as the correct information. Therefore, a problem has arisen that the amount of transmission becomes large. 
     To solve the problems caused based on RTT in the above TCP communication, the inventors, etc., of the present invention have suggested to transfer data at a high speed by transmitting a packet in UDP (User Datagram Protocol) that uses no response (ACK) from the receiving side and repairing packet loss by FEC (Forward Error Correction), in a previous Japanese Patent Application Laid-Open Publication No. 2004-539446; “Carrying of a Computer Work Environment and Sharing of Data by DirectShare”, Ubiquitous Computing System Workshop, Information Processing Society of Japan, Reference for Presentation in November 2003; “P2P-type Information Sharing System-DirectShare-”, 32nd MBL/7th Joint Presentation of Studies, Reference for Presentation in March 2005; and “DirectShare: Information Sharing system using P2P”, HCI International, 2005, 22-27, Jul. 2005, Las Vegas. 
     The above suggestions by the inventors, etc., employ FEC that can code and decode at a high speed with simple calculation and small amount of memory. 
     The overview of the FEC in the above suggestions will be described referring to  FIG. 1 . Data  1 A to be transmitted from a transmission side apparatus  1  through a communication path  3  to a reception side apparatus  2  are divided into small blocks (s 1 ). Appropriate combinations are created from the divided blocks (s 2 ). Among the combinations, X or (exclusive OR) is calculated (s 3 ). The calculated data are transmitted together with information on the combinations (s 4 ). 
     At this time, a case where data are lost (sx) on the communication path  3  can be considered. 
     By applying the Gaussian elimination to the information on the combination (s 5 ), decoded data  1 B corresponding to the original data can be obtained from the data received at the data reception side apparatus  2 . 
     A coding system is characterized in that, by transmitting excessively the data to be transmitted by several percents in using FEC, even when any of packets is lost on the communication path  3  (s 4 ), the original data can be recovered or corrected from the packets that can be received. Such a coding system is referred to as RPS (Random Parity System). 
     The line state of the network may be degraded suddenly and may not be constant due to the time of day and other factors. In the congestion control like the “slow start” of TCP, a problem has arisen that lines can not be utilized effectively. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a data transferring method that enables lost data to be corrected and transmission to be continued until the original data can be recovered with the amount of data remained constant even when the line state is degraded, and a communication system and a program that are applied with the method. 
     Another object of the present invention is to provide a data transferring method that enables the above RPS to be corresponded dynamically with the line state and the decoding success rate to be improved, and a communication system and a program that are applied with the method. 
     In order to achieve the above objects, according to a first aspect of the present invention there is provided a communication system that transmits data through a transmission path between a transmission side apparatus and a reception side apparatus, wherein the transmission side apparatus comprises a coding apparatus that creates redundantly-coded data from original data; a transmitting apparatus that sends the coded data coded by the coding unit to the transmission path; and a coding rate determining apparatus that sets and controls a coding rate in the coding unit, wherein the reception side apparatus comprises a receiving apparatus that receives the coded data sent through the transmission path; a decoding apparatus that decodes the original data from the coded data received; and a loss rate estimating apparatus that measures the loss rate on the path of the coded data sent, and wherein the coding rate determining apparatus of the transmission side apparatus varies and controls the coding rate in the coding apparatus based on the loss rate obtained. 
     In the first aspect, the coding rate determining apparatus of the transmission side apparatus obtains the solutions by substituting the original data together with predetermined constant data in simultaneous equations of the Boolean algebra, determines the combination of the constant data and the solutions to be coded data, and the decoding apparatus of the reception side apparatus decodes the original data by substituting the constant data and the solutions obtained from the received coded data in the simultaneous linear equations. 
     In the first aspect, the coding rate determining apparatus of the transmission side apparatus varies and controls the coding rate of the coded data coded by the coding apparatus by varying the length of a bit string created from the number of the coded data or the original data and used for the coding. 
     As to the coding rate determining apparatus of the transmission side apparatus, when the loss rate measured by the loss rate estimating apparatus is equal to or smaller than a predetermined value, the coding rate determining apparatus varies and controls the number of the coded data in a manner the number is increased. 
     A second and a third aspects of the present invention that achieve the above objects are a data transferring method and a program that respectively causes the transmission side apparatus to execute a step of obtaining the solutions by substituting the original data together with the predetermined constant data in the simultaneous equations of the Boolean algebra and packet-transmitting the combination of the constant data and the solutions as the coding data to the reception side apparatus, and causes the reception side apparatus to execute a step of decoding the original data by substituting the constant data and the solutions obtained from the received coded data in the simultaneous linear equations; that further execute a step of causing the reception side apparatus to obtain the loss rate of the received coded data on the transmission path and a step of causing the transmission side apparatus to vary and control the coding rate of the coded data based on the obtained loss rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an schematic explanatory view of FEC in a previous suggestion; 
         FIG. 2  schematically shows the configuration of a communication system that is applied with the data transferring method according to the present invention; 
         FIG. 3  is an explanatory view for the case where binary data “0101” are transmitted using the coding principle in the previous suggestion as original data  1 A; 
         FIG. 4  is a graph showing the relation between the number of coded data and the decoding success rate; 
         FIG. 5  shows the relation between the code length and decoding success rate; and 
         FIG. 6  is a flow of an example of a method according to the present invention for obtaining the loss rate. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings, an exemplary embodiment of the present invention will be described below. The example of the embodiment is described for understanding of the present invention and the technical scope of the present invention is not limited to the example. 
       FIG. 2  schematically shows the configuration of a communication system that is applied with the data transferring method according to the present invention. 
     At a transmission side apparatus  1 , data  1 A to be transmitted are coded by a variable coding apparatus  10 . In this coding, a parameter for coding determined by a coding rate determining apparatus  12  according to a method described later is used. 
     The data coded by the variable coding apparatus  10  are transmitted to a reception side apparatus  2 . In the reception side apparatus  2 , a receiving apparatus  20  receives the coded data and leads the data to a decoding apparatus  21 , and recovery of data  1 B to be received corresponding to the data  1 A to be transmitted on the transmitting side is executed. 
     The data received by the receiving apparatus  20  is sent to a loss rate measuring apparatus  22 . The number of data lost on a transmission path  3  is obtained by the loss rate measuring apparatus  22  and the result thereof is transmitted by a transmitting apparatus  23  to the transmission side apparatus  1 . 
     On the transmission side, the coding rate determining apparatus  12  determines a coding parameter value based on the loss rate received by a receiving apparatus  13 . 
     In the present invention, the data communicating method that uses RPS which has been presented in the above applications filed by the inventors, etc., and is schematically described above is used as a code for which the coding rate of the variable coding apparatus  10  is variable. Though the RPS is described in detail in the above applications, for further understanding of the present invention, the data communicating method using RPS executed at the variable coding apparatus  10  and the principle thereof will be described below. 
     Data transfer of m=4 (four) bits is taken as an example. 
     This principle is based on the solution of simultaneous equations of the Boolean algebra. Assuming that the original data  1 A to be transferred are (b 0 , b 1 , b 2 , b 3 ) that are m-bit data, the solution of the following simultaneous equations are obtained using (k n0 , k n1 , k n2 , k n3 ) as a data string K nm  having appropriate m bits and n series.
 
 b   0   k   n0   +b   1   k   n1   +b   2   k   n2   +b   3   k   n3   =p   n   (1)
 
     The solution P n  is P n =P 0 , P 1 , P 2 , P 3 , . . . P n-1  where n is an integer equal to or larger than m, that is n=0, 1, 2, . . . . 
     The combination of the data string K nm  and the solution P n  of the simultaneous equations is consecutively transmitted as packets. That is, n packets K p =(K 0 p 0 , K 1 p 1 , K 2 p 2 , . . . , K n p s ) are transmitted. 
     The reception side apparatus  2  recovers the m-bit original data (b 0 , b 1 , b 2 , b 3 ) by solving the simultaneous equations of Equation (1) by substituting m-bit data series K nm =(k n0 , k n1 , k n2 , k n3 ) and the solution Pn=(P 0 , P 1 , P 2 , P 3 ) obtained from the received packets K p =(K 0 p 0 , K 1 p 1 , k 2 p 2 , . . . , K n p s ) in Equation (1). 
     As the example shown in  FIG. 3 , the case is taken where binary data “0101” are transmitted as the original data  1 A. In this case, the following is defined as the data string K n .
         K 0 =1001   K 1 =0011   K 2 =1100   K 3 =1100   K 4 =1010       

     A coding process  100  is executed by substituting the original data  1 A and the data string K n  in the simultaneous equation of Equation (1). This coding process  100  obtains the solution P n =(P 0 , P 1 , P 2 , P 3 , P 4 ) by adding the logical products of the terms. 
     As shown for the transmission path  3 , by the coding to obtain the solution of the simultaneous equations, the combination of the data string K p  and the solution P n  obtained by the coding process  100  is made into five (5) packets K 0 p 0 , K 1 p 1 , K 2 p 2 , K 3 p 3 , K 4 p 4  and transferred toward the reception side apparatus  2 . 
     In the example shown in  FIG. 3 , for the transmission path  3 , for example, it is assumed that a first packet and a second packet are exchanged and a fourth packet is lost X p . 
     The reception side apparatus  2  executes a decoding process  200  that validates a data bit corresponding to a bit  1  in the data string K n  of the received packets K 0 p 0 , K 1 p 1 , K 2 p 2 , K 3 p 3 , K 4 p 4  and creates simultaneous equations and solves the equations using the Gaussian elimination that eliminates a data bit corresponding to a bit  0 . 
     By solving the simultaneous equation created by this decoding process  200 , the recovered data  1 B that defines that 4-bit data (b 0 , b 1 , b 2 , b 3 )=(0101) can be obtained. 
     The calculation of the simultaneous equations of Equation (1) that is the coding process  100  in the transmitting apparatus  1  can be solved very quickly by a computer process because only counting the number of “1s (ones)”, that is, observing the parity bits has to be executed after executing logical multiplication (AND) of each bit b n  and each data string K n  in the Boolean algebra. 
     The process  200  that solves the simultaneous equations in the reception side apparatus  2  can also be executed quickly using “exclusive OR (Xor)”. 
     Assuming that the original data  1 A are divided into a plurality of pieces, the number of pieces divided when a plurality of blocks of the data string K p  is “n”, and that the number of the coded data created by the RPS coding is “m”, the redundancy degree of the number of the coded data to the original data  1 A can be represented as m/n. 
     Measurement was conducted based on the recognition of the inventors that, in the RPS coding, the tolerance against loss of data can be varied by varying the number of coded data to be created and the length of the bit string (code length) used in the coding. 
       FIG. 4  is a graph showing the relation between the number of coded data and the decoding success rate obtained when the code length is fixed to be 32 bits while the number of coded data to be created is being increased. 
     When the data loss rate on the transmission path  3  is, for example, 0.1% (Graph a), the probability of successful decoding can be improved by increasing the number of coded data to be created (that is, increasing the redundancy degree). 
     However, it could be confirmed that the probability of successful decoding is not significantly varied even though the redundancy degree is made 1.3 time as large when the loss rate is 5% (Graph b) and 20% (Graph c). 
       FIG. 5  shows the measurement result obtained when the loss rate is fixed to be 20% and the code length and the redundancy degree are varied, that is the relation between the code length and the decoding success rate. 
     In  FIG. 5 , the axis of abscissas represents the code length and the axis of coordinate represents the probability that decoding is failed. From the measurement result shown in  FIG. 5 , it was confirmed that, as the code length becomes longer, the probability of successful decoding becomes higher even for a same redundancy degree. 
     From the above confirmation, the inventors have recognized that, in the RPS coding, efficient data transmission is enabled by coping with data loss by increasing the number of created coded data when the loss rate is small and coping with data loss by increasing the code length when loss rate has become large. 
     Therefore, according to the present invention, in the system configuration of  FIG. 2 , the received signal to be received is received in the receiving apparatus  20  in the reception side apparatus  2 , the loss rate (loss percentage) is obtained by the loss rate measuring apparatus  22 , and the obtained loss rate is transmitted to the transmission side apparatus  1 . 
     When the transmission side apparatus  13  has received the loss rate, the value is sent to the coding rate determining apparatus  12 . The coding rate determining apparatus  12  increases the number of the created coded data based on the loss rate, that is, the redundancy degree is increased and, when the loss rate has exceeded a predetermined threshold value, controls the variable coding apparatus  10  to increase the code length. 
     The important point is how to obtain the loss rate on the transmission path  3  of the data. 
       FIG. 6  is a flow of an example of a method according to the present invention for obtaining the loss rate. 
     For example, a serial number N is given to each of the data to be transmitted. The number of data used for measurement is set to be “M” in the receiving apparatus  20  in the reception side apparatus  2 . 
     When a datum has been received (step S 1 ), the serial number N of the received datum is compared with a minimum value MIN (step S 2 ). When the serial number N of the received datum is not larger than the minimum value MIN, the serial number is stored as the minimum value (step S 3 ). 
     When the serial number N of the received datum is larger than the minimum value MIN (step S 2 , YES), the value N is stored as MAX=N. 
     The above process is repeated until the number of times of reception reaches M (step S 5 , NO). When the number of times of reception reaches M (step S 5 , YES), the loss rate={(MAX−MIN)−M}/M is obtained (step S 6 ). 
     That is, the difference between, the difference between the maximum vale MAX and the minimum value MIN of the serial numbers for the number of times of reception M of the data, and the number of times of reception M of the data is the number of the lost data. Therefore, the percentage of the number of times of reception M of the data to the number of the lost data is obtained as the loss rate. 
     As described above, by applying the present invention, data can be continuously transmitted until the lost data are corrected and the original data can be recovered with the amount of data remained constant even when the line state is degraded. Therefore, amore reliable data communication system can be provided. 
     While the illustrative and presently preferred embodiments of the present invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.