JP6331274B2 - Communication system and communication method - Google Patents

Description

The present invention relates to a communication system and a communication how, for example, it is capable of applying to a videoconference system.

  An example of a system that requires real-time communication is a multipoint video conference system. In the multipoint video conference system, for example, as shown in FIG. 11, the video conference terminals 1-1 to 1 -N at each point transfer image data at the point to the MCU (Multipoint Control Unit) 2. By transmitting the combined image data in which the image data from each point is juxtaposed to the video conference terminals 1-1 to 1-N at each point, the MCU 2 visually observes the state of the multi-point conference room at any point. Trying to get. Here, in order to make a video conference as close as possible to a natural conference, the communication of image data from each video conference terminal 1-1 to 1-N to MCU 2 is also performed from MCU 2 to each video conference terminal 1-1 to 1-N. It is desirable that the communication of the composite image data to be real-time communication.

  However, depending on network conditions, packet loss may occur even when real-time communication is required.

  For video streams of video conferencing, when packet loss occurs and video cannot be reconstructed on the receiving side, there is a mechanism for requesting the video transmitting terminal to retransmit a complete video frame (intra frame). . H.323 related to the H.323 standard. In the H.245 standard, “videoFastUpdatePicture”, which is one of video fast update commands, is defined (see Non-Patent Document 1).

H. H.245 standard: Control protocol for multimedia communication, 8th edition, established on May 30, 2002, Information and Communication Technology Committee, pages 168-169

  In general, due to the relationship with the transmission bit rate, when an intra frame is received, a blurred video frame is often reproduced, and a method in which a fine video is gradually reproduced by a subsequent inter frame is often employed. Therefore, when viewed from the user, the replayed video feels blurred each time a new intra frame is received and played back. Also, during the time interval from when an intra frame retransmission request is made due to packet loss until the arrival of an intra frame, the video that has been corrupted continues to be played, or is played back until a new intra frame arrives. In many cases, one of the methods of continuing to display the video frame (in this case, the video seems to be temporarily stopped) is taken, but in either method, the video quality is deteriorated.

Therefore, it can be recovered in a shorter time the influence of packet loss, the communication system excluding the packet loss rate of that recovered can below a desired packet loss rate and communication how is desired.

A first aspect of the present invention is a communication system for transmitting and receiving stream data from a transmitting communication device to a receiving communication device by a data packet. (1) The receiving communication device is any one of (1-1) A packet loss detecting means for detecting a packet loss of a data packet; and (1-2) a retransmission for holding the number of retransmission requests, which is the number of times a retransmission request packet for requesting retransmission of a data packet having a packet loss is repeatedly transmitted continuously. Request number holding means, and (1-3) Retransmission request for continuously transmitting retransmission request packets by the number of retransmission requests held in the retransmission request number holding means when a data packet with packet loss occurs. (2) the communication device on the transmission side receives (2-1) a retransmission request packet for receiving the retransmission request packet. Means, (2-2) retransmission number holding means for holding the number of retransmissions, which is the number of times the data packet requested to be retransmitted is repeatedly transmitted, and (2-3) initial reception of the retransmission request packet. And a retransmission packet transmitting means for repeatedly transmitting data packets requested to be retransmitted by the number of retransmissions held in the retransmission number holding means, and (3-1) the retransmission. The communication system further includes a number of times determination means for determining the number of retransmission requests held by the request number holding means and the number of retransmissions held by the retransmission number holding means. (3-2) The number of times determination means includes a data packet and the first packet loss rate in the network in the transfer direction of the retransmission packet, the second packet loss rate in the network in the transfer direction of the retransmission request packet, about the packet loss Target value of Kettorosu rate, first the packet length of the data packet, and receives an input of the second packet length of the retransmission request packet, from among the first retransmission request number of the first predetermined value as the upper limit value, And the first request count, the first request count, the first packet loss rate, and the second packet loss rate from the first request count with the second predetermined value as an upper limit. that packet loss rate becomes equal to or less than the target value of the upper Kipa Kettorosu rate, further, the first number of retransmission requests, the first number of requests, the first packet length, the second packet loss rate, and the second packet The combination of the first retransmission request count and the first request count that minimizes the increase in network traffic calculated using the length is searched, and the first retransmission request count in the combination that can be searched. The number N1 (N1 is a positive integer) and the first request number N2 (N2 is a positive integer) are used as the number of retransmission requests held by the retransmission request number holding unit and the number of retransmissions held by the retransmission number holding unit. It is characterized by determining.

A second aspect of the present invention is a communication method in a communication system in which stream data is exchanged by a data packet from a transmission side communication device to a reception side communication device. (1-1) Packet loss of the reception side communication device The detecting means detects a packet loss of one of the data packets. (1-2) The retransmission request count holding means of the receiving side communication device repeats the retransmission request packet for requesting retransmission of the data packet of the packet loss. (1-3) the retransmission request packet transmission means of the communication device on the receiving side, when the data packet of the packet loss occurs, The retransmission request packet is repeatedly transmitted continuously as many times as the number of retransmission requests held in the holding means, and (2-1) the retransmission request packet of the communication device on the transmission side. (2-2) The retransmission number holding means of the transmission side communication device is the number of times the data packet requested to be retransmitted is repeatedly transmitted continuously. (2-3) The retransmission packet transmission means of the transmission side communication apparatus retransmits the retransmission number of retransmissions held in the retransmission number holding means in response to the first reception of the retransmission request packet. (3-1) The number-of-times determination means in the communication system holds the number of retransmission requests held by the retransmission request number holding means and the number of retransmission times holding means holds. (3-2) Further, the number of times determination means includes a first packet loss rate and a retransmission request packet in the network in the transfer direction of the data packet and the retransmission packet. Second packet loss rate in the network in the transfer direction, the target value of Rupa Kettorosu rate relates to a packet loss, the first packet length of the data packet, as well as the input of the second packet length of the retransmission request packet received in The first retransmission request count and the first request count from the first retransmission request count having the first predetermined value as the upper limit and the first request count having the second predetermined value as the upper limit. the first packet loss rate, and packet loss rate is calculated by using the second packet loss rate becomes equal to or less than the target value of the upper Kipa Kettorosu rate, further, the first number of retransmission requests, the first request The first retransmission request count and the first requirement that minimize the increase in network traffic calculated using the number of times, the first packet length, the second packet loss rate, and the second packet length. A search request combination is searched, and the first retransmission request count N1 (N1 is a positive integer) and the first request count N2 (N2 is a positive integer) in the combination that can be searched, the retransmission request count holding means And the number of retransmission requests held by the retransmission number holding means.

According to the present invention, it can be recovered in a shorter time the influence of packet loss, it is possible to provide a communication system and a communication how that can be below a desired packet loss rate and excluded packet loss rate that was recovered.

1 is a block diagram showing a configuration of a communication system according to a first embodiment. It is a block diagram which shows the internal structure of the communication apparatus which concerns on 1st Embodiment. It is a sequence diagram in the case of transmitting transmission data as an RTP stream from one communication apparatus according to the first embodiment to the other communication apparatus. It is explanatory drawing of the qualitative meaning of the calculation formula of the packet loss rate seen from the application of the receiving side in the communication system which concerns on 1st Embodiment. It is explanatory drawing which shows the relationship between the packet loss rate of a bidirectional | two-way network and the packet loss rate seen from the application of the receiving side in the communication system which concerns on 1st Embodiment. It is explanatory drawing which shows the relationship of two parameters for evaluating communication at the time of applying the communication system which concerns on 1st Embodiment to communication of a video frame. It is a block diagram which shows the internal structure of the communication apparatus which concerns on 2nd Embodiment. 10 is a flowchart illustrating a first search method for an optimum number of retransmission requests and the number of retransmissions in the communication apparatus according to the second embodiment. 10 is a flowchart illustrating a second search method for the optimum number of retransmission requests and the number of retransmissions in the communication apparatus according to the second embodiment. 10 is a flowchart showing a third search method for the optimum number of retransmission requests and the number of retransmissions in the communication apparatus according to the second embodiment. It is a block diagram which shows the general structure of a multipoint video conference system.

(A) First Embodiment Hereinafter, a first embodiment of a communication system and a communication how according to the present invention will be described with reference to the drawings.

(A-1) Configuration of First Embodiment FIG. 1 is a block diagram showing a configuration of a communication system according to the first embodiment.

  In FIG. 1, the communication system 10 according to the first embodiment includes two communication devices 11A and 11B that perform real-time communication. Each of the communication devices 11A and 11B transmits and receives media data such as video and audio as RTP (Real-time Transport Protocol) packets. Each of the communication devices 11A and 11B transmits / receives accompanying information as an RTCP (RTP Control Protocol) packet.

  The specific device types of the communication devices 11A and 11B are not limited. However, as an example, one of the communication devices 11A and 11B is mounted on one of the video conference terminals 1-n (n is 1 to N) in the multipoint video conference system shown in FIG. A mode in which the other of the communication devices 11A and 11B is mounted on the MCU 2 can be mentioned.

  FIG. 2 is a block diagram showing an internal configuration of the communication device 11 (11A or 11B). The configuration shown in FIG. 2 may be constructed by hardware, or may be constructed so that the portion excluding the configuration of the physical layer in various transmission / reception units is realized as software (communication program) executed by the CPU. Although any construction method is adopted, it can be functionally represented in FIG.

  In FIG. 2, the communication device 11 includes an RTP transmission / reception unit 20, an RTCP transmission / reception unit 21, a data transmission / reception unit 22, a control unit 23, a transmission data storage unit 24, a transmission data extraction unit 25, a packet loss detection unit 26, a retransmission request count / A retransmission number storage unit 27 and a data communication bus 28 are included.

  The RTP transmission / reception unit 20 is in charge of transmission / reception of RTP packets transmitted / received to / from opposite communication apparatuses via a communication network. Under the control of the control unit 23, the RTP transmission / reception unit 20 transmits only the number of retransmissions stored in the retransmission request number / retransmission number storage unit 27 for the RTP packet to be retransmitted when a retransmission is requested from the opposite communication device. Send repeatedly and continuously. That is, in a situation where the number of retransmissions has not been reached, the same immediately after the previous transmission of the RTP packet ends without determining whether or not the previously transmitted (retransmitted) RTP packet has reached the opposite communication device. The RTP packet is transmitted to the opposite communication device.

  The RTCP transmission / reception unit 21 is in charge of transmission / reception of an RTCP packet transmitted / received to / from an opposite communication device via a communication network. When the RTCP transmission / reception unit 21 transmits an RTCP packet (for example, an RTCP Generic-NACK packet defined in RFC4585) requesting retransmission of the RTP packet to the opposite communication device under the control of the control unit 23, The retransmission request count / retransmission count storage unit 27 repeatedly transmits the retransmission request count continuously. That is, in a situation where the number of retransmission requests has not been reached, immediately after the previous transmission of the RTCP packet ends without determining whether or not the previously transmitted (retransmitted) RTCP packet has reached the opposite communication device, The same RTCP packet is transmitted to the opposite communication device.

  The data transmission / reception unit 22 is in charge of data transmission / reception with an application program (hereinafter, referred to as a higher-level application or simply referred to as an application) in an information processing apparatus externally or built in the communication apparatus 11.

  Each unit shown in FIG. 2 can exchange data via the data communication bus 28, and the control unit 23 controls each unit in the communication device 11 and data exchange between the units. It is.

  The transmission data storage unit 24 has a function of storing RTP data (data inserted into RTP packets) transmitted in the past by a predetermined number of packets.

  The transmission data extraction unit 25 has a function of extracting designated data from the data stored in the transmission data storage unit 24.

  The packet loss detection unit 26 has a function of detecting a packet loss that has occurred for an RTP packet destined for the communication device 11.

  The retransmission request number / retransmission number storage unit 27 stores the number of retransmission requests that is the number of repeated transmissions of RTCP packets that request retransmission, and the number of retransmissions that is the number of repeated transmissions of RTP packets that are requested to be retransmitted. is there. The number of retransmission requests and the number of retransmissions may be the same or different. The retransmission request number / retransmission number storage unit 27 stores the number of retransmission requests and the number of retransmissions in a rewritable manner. Here, the operator may arbitrarily designate the number of retransmission requests and the number of retransmissions to be stored in the retransmission request number / retransmission number storage unit 27. For example, when the communication device 11 is a video conference terminal or MCU, when each component of the multipoint video conference system is installed, the number of retransmission requests is determined by a determination method described in the second embodiment to be described later. Alternatively, the number of retransmissions may be determined, and the operator may write the determined number of retransmission requests and the number of retransmissions in the retransmission request number / retransmission number storage unit 27. Also, the number of retransmission requests and the number of retransmissions given from an external device (for example, a network management device) may be written in the retransmission request number / retransmission number storage unit 27.

  Here, if the communication device 11 is mounted on the video conference terminal of the multipoint video conference system, the upper application encodes the image data from the imaging camera according to a predetermined encoding method. The data is input to the communication device 11 and the composite image data obtained by decoding the data from the communication device 11 is displayed on the display device.

  Further, if the communication device 11 is compatible with each point installed in the MCU of the multipoint video conference system, the host application can obtain an image of a certain point obtained by decoding the data from the communication device 11. After the data and the image data of other points obtained in the same manner are combined, the combined image data is encoded according to a predetermined encoding method, and the encoded data is input to the communication device 11. .

(A-2) Operation of the First Embodiment Next, the operation of the communication system 10 according to the first embodiment will be described with reference to FIG.

  FIG. 3 is a sequence diagram in the case where transmission data (for example, media data including image data and audio data) is transmitted as an RTP stream from the communication device 11A to the communication device 11B. In the following description, the constituent elements of the communication device 11A are described by adding “A” to the end of the reference numerals in FIG. 2, and the constituent elements of the communication apparatus 11B are further indicated by reference numerals “ B ”is added and described. FIG. 3 shows a case where the retransmission request number n1 stored in the retransmission request number / retransmission number storage unit 27B is three and the retransmission number n2 stored in the retransmission request number / retransmission number storage unit 27A is two. Is shown.

  Under the control of the control unit 23A, the RTP transmission / reception unit 20A of the communication device 11A sorts and inserts the transmission data given from the application into RTP packets, and sequentially transmits the RTP packets RTP # 101, # 102,. The control unit 23A is an RTP packet that has already been transmitted to the built-in transmission data storage unit 24A and has been inserted into a predetermined number of RTP packets immediately after transmission so as to respond to a retransmission request from the opposite communication device 11B. The transmission data is stored together with information specifying the RTP packet.

  In the communication device 11B from which the RTP packet from the communication device 11A arrives, the RTP transmission / reception unit 20B receives and processes the incoming RTP packet, but the packet loss detection unit 26B receives the header information (for example, sequence) of the received RTP packet. The occurrence of packet loss is monitored by a known detection method based on the number SN). When the packet loss detection unit 26B detects a loss (loss) of any RTP packet from the communication device 11A, the RTCP transmission / reception unit 21B immediately detects the RTP packet in which the packet loss is detected under the control of the control unit 23B. RTCP packet requesting retransmission, in which information (for example, sequence number SN) is included is repeatedly transmitted for the number of retransmission requests n1 (= 3) stored in retransmission request number / retransmission number storage unit 27B. (RTCP # 201-1 to # 201-n1).

  For example, as shown in FIG. 3, when the packet loss detection unit 26B detects a packet loss of the RTP packet RTP # 102 having the sequence number SN of 2, the RTCP transmission / reception unit 21B immediately receives the sequence number SN. The RTCP packet RTCP # 201 for requesting retransmission of the second RTP packet is repeatedly transmitted three times as the number of retransmission requests n1 stored in the retransmission request number / retransmission number storage unit 27B (RTCP # 201-1 to # 201). -3).

  When the RTCP transmission / reception unit 21A of the communication device 11A receives an RTCP packet requesting retransmission from the opposite communication device 11B, and this reception is the first reception for the packet of the sequence number requested for retransmission, immediately, Under the control of the control unit 23A, the RTP transmission / reception unit 20A causes the transmission data extraction unit 25 to extract transmission data related to the RTP packet having the sequence number requested for retransmission from the transmission data storage unit 24, and assembles the RTP packet # 301. The retransmission request number / retransmission number storage unit 27A repeatedly transmits the retransmission number n2 (= 2 times) (RTP # 301-1 to # 301-n2).

  In the example of FIG. 3, among the three RTCP packets RTCP # 201-1 to # 201-3 that are sequentially transmitted by the communication device 11B for a retransmission request, the first RTCP packet RTCP # 201-1 is sent to the communication device 11A. Although not reached, the case where the communication device 11A is reached in the second and third RTCP packets RTCP # 201-2 and # 201-3 is shown. Since the arrival of the RTCP packet RTCP # 201-2 is the first arrival in the RTCP packet that requests retransmission of the same RTP packet, the communication device 11A immediately requests the requested RTP packet # 301. Are retransmitted twice, which is the number of retransmissions n2 stored in the retransmission request number / retransmission number storage unit 27A (RTP # 301-1 and # 301-2). When the RTCP packet RTCP # 201-3 arrives, the communication device 11A has already started the retransmission operation because the same RTCP packet # 201-2 has arrived (or has already performed retransmission). The arrival of packet RTCP # 201-3 is ignored.

  The RTP packet retransmitted n2 times from the communication device 11A may or may not reach the opposite communication device 11B. Even if a plurality of retransmitted RTP packets arrive, the RTP transmitter / receiver 20B of the communication device 11B processes the retransmitted RTP packet that has arrived first as a valid packet, and the arrival of the retransmitted RTP packet that has arrived thereafter Is ignored.

  In the example of FIG. 3, the first RTP packet RTP # 301-1 reaches the communication device 11B among the two RTP packets RTP # 301-1 and 301-2 sequentially retransmitted by the communication device 11A. A case is shown in which the communication device 11B is not reached in the th RTP packet RTP # 301-2. The communication device 11B compensates for the loss of the RTP packet RTP # 102 that originally detected the packet loss when the RTP packet RTP # 301-1 arrives. By such compensation, since the data inserted into the RTP packet RTP # 102 is also given from the upper application of the communication device 11B, it can be considered that there is no loss of the RTP packet RTP # 102.

  As described above, when the RTCP packet requesting retransmission is repeatedly transmitted by the number n1 of retransmission requests, the possibility that at least one RTCP packet reaches the communication device on the other side is more than the case where only one RTCP packet is transmitted. If the retransmission RTP packet is repeatedly transmitted by the number of retransmissions n2, the possibility that at least one RTP packet reaches the communication device on the other side is considerably larger than that when only one RTP packet is transmitted. Become.

(A-3) Effect of First Embodiment According to the first embodiment, the RTCP packet that requests retransmission is repeatedly transmitted for the number of retransmission requests n1, and the retransmission RTP packet is repeatedly transmitted for the number of retransmissions n2. Since it did in this way, the packet loss rate seen from the application can be made smaller than before. Such an effect becomes greater as the number of retransmission requests n1 and the number of retransmissions n2 increase. Hereinafter, such an effect will be specifically described.

The packet loss rate z seen from the application in the communication device 11B on the receiving side where the sequence of the RTP packet from the communication device 11A arrives can be expressed by equation (1) derived by the present inventor. In the equation (1), the network packet loss rate in the communication direction (hereinafter referred to as the forward direction) from the communication device 11A to the communication device 11B is x, and the opposite communication direction (communication from the communication device 11B to the communication device 11A). Network packet loss rate in the direction (hereinafter referred to as the reverse direction) is y, the number of retransmission requests is n1, and the number of retransmissions is n2. However, the expression (1) is stored in the reception buffer of the application as compared with the time when retransmission is completed from the time when the retransmission is requested (assuming that nTT and n2 are both 1 if RTT (Round Trip Time)). Suppose the time is long.

  FIG. 4 is an explanatory diagram of the qualitative meaning of the equation (1), that is, the packet loss rate calculation equation viewed from the receiving-side application.

  When the packet loss is seen from the application, (a) the original RTP packet from the communication device 11A to the communication device 11B does not reach the communication device 11B, and (b) the communication device 11B receives the RTCP packet for the retransmission request as n1. This is a case where no retransmitted RTP packet reaches the communication device 11B even if it is repeatedly transmitted. Event (a) represents a case where retransmission is necessary, and event (b) represents a case where the retransmission mechanism did not function.

  As shown in FIG. 4, since the forward network packet loss rate is x, the probability that the initial RTP packet from the communication device 11A to the communication device 11B will reach the communication device 11B (expected value; the same applies hereinafter) is 1−x, and the probability that the initial RTP packet from the communication device 11A to the communication device 11B does not reach the communication device 11B is x. That is, the probability that the event (a) described above occurs is x.

  The probability of event (b) is equal to one obtained by subtracting from 1 the probability of event (c) reaching the communication device 11B even with one retransmitted RTP packet. The event (c) occurs when (c1) one RTCP packet for a retransmission request reaches the communication apparatus 11A, and (c2) even one retransmission RTP packet reaches the communication apparatus 11B.

Since the network packet loss rate in the reverse direction is y, the probability that no n1 sent RTCP packets will reach the communication device 11A can be expressed as y ⁿ¹ . Therefore, the probability of the event (c1) can be expressed by 1−y ⁿ¹ . Similarly, the probability of the event (c2) can be expressed as 1−x ⁿ² . Since the event (c) is one in which the events (c1) and (c2) occur together, the probability can be expressed by (1-y ⁿ¹ ) (1-x ⁿ² ). Since the probability of the event (b) is equal to the probability of the event (c) subtracted from 1 as described above, it can be represented by 1- (1-y ⁿ¹ ) (1-x ⁿ² ).

  When the packet loss is seen from the application, as described above, both the event (a) and the event (b) occur, the above formula is used as a calculation formula of the packet loss rate seen from the receiving side application. (1) is obtained.

  From the equation (1), since the network packet loss rates x and y are both greater than 0 and less than 1, the packet loss rate z viewed from the application decreases as the number of retransmission requests n1 and the number of retransmissions n2 increase. I understand. Hereinafter, this will be described with reference to FIG.

  For example, assuming that the packet loss rate of the bidirectional network is the same (x = y), the relationship between the packet loss rate of the network and the packet loss rate seen from the application on the receiving side is as shown in FIG. become. The horizontal axis in FIG. 5 represents the packet loss rate of the network, and the vertical axis in FIG. 5 represents the packet loss rate as viewed from the application. The log scale is applied to both the vertical and horizontal axes. Curves CUR1 to CUR3 represent cases where the combinations (n1, n2) of the retransmission request number n1 and the retransmission number n2 are (1, 1), (2, 2), and (3, 3), respectively.

  For example, when the packet loss rate of the network is 10% (x = y = 0.1) and the number of retransmission requests n1 and the number of retransmissions n2 are both 1, the packet loss rate seen from the application is about 1.9. When the retransmission request count n1 and the retransmission count n2 are both 2, the packet loss rate seen from the application is reduced to about 0.19%. When the number of retransmission requests n1 and the number of retransmissions n2 are both 3, the packet loss rate seen from the application is reduced to about 0.02%.

  Decreasing the packet loss rate z seen from the application can improve the function of the application.

  Hereinafter, one communication device 11A is mounted on the video conference terminal of the multipoint video conference system, and the upper application encodes the data after the image data from the imaging camera is encoded according to a predetermined encoding method. Suppose that the other communication device 11B is mounted on the MCU of the multipoint video conference system, and the higher-level application decodes the input encoded data and returns it to image data.

  Then, by using the communication system of the first embodiment, how much the occurrence frequency of the intra frame request mentioned in the section “Problems to be solved by the invention” can be lowered is quantitatively described below. The calculation results are described in the following.

  Here, in order to simplify the calculation, the following preconditions (R1) and (R2) are assumed. (R1) The number of packets corresponding to one frame of the video stream is constant regardless of the frame type (intra frame, inter frame). (R2) The video frame rate is constant.

  Under these assumptions, the average packet rate k (packets / s) can be calculated from the bandwidth of the video stream (bit rate B (bps)) and the average size L (bytes) of the video packet according to the equation (2). .

k = B / L / 8 (2)
The network packet loss rate in the transmission direction of the video stream is set as x, and the probability that the continuous time when no packet loss occurs is t is calculated. First, the probability p that the m + 1 packets are not lost without the m packets being continuously lost is expressed by Equation (3). The time width t during which m packets are transmitted can be expressed by equation (4) using the average packet rate k of equation (2). From the equations (3) and (4), the expected value MTBF of the continuous time in which no packet loss occurs can be expressed by the equation (5).

  The expected value MTBF of the continuous time expressed by the equation (5) gradually approaches the value expressed by the equation (6) under the condition of 0 <x <1.

MTBF = (1-x) / x / k (6)
For example, the expected value MTBF of continuous time is evaluated under the condition that the bit rate B of the video stream is 1000 kbps and the average size L of the video packets is 1000 bytes. Under the conditions described above, k = 125, and the relationship between MTBF and packet loss rate x in equation (6) is as shown in FIG. 6 (logarithmic scale on both vertical and horizontal axes).

  For example, when the packet loss rate of a network in which an intra frame request is generated is 1%, the expected value MTBF of continuous time is 0.792 (sec). However, the method of the first embodiment is different from the number of retransmission requests. If both n1 and the number of retransmissions n2 are applied twice, the packet loss rate in the forward direction and the packet loss rate in the reverse direction are similarly assumed to be 1%, and the packet loss rate seen from the application is 0.00019%. The expected value MTBF of continuous time is reduced to 4210 (sec) (about 1 hour 10 minutes).

  Therefore, when a video conference of about 1 hour is performed under the above-described conditions, when the first embodiment is not used, the conference person shows video disturbance or stop due to packet loss almost every second and video blur due to intra-frame retransmission. In contrast to the experience, by using the first embodiment, the number of times can be reduced to less than once in one hour.

(B) Second Embodiment Next, a second embodiment of a communication system and a communication how according to the present invention will be described with reference to the drawings.

(B-1) Configuration of the Second Embodiment As shown in FIG. 1 described above, the communication system 10 of the second embodiment is also opposed to two communication devices 11A or 11B that exchange RTP packets and RTCP packets. It is what you are doing.

  However, in the case of the second embodiment, the internal configuration of the communication device 11 (11A or 11B) is different from that of the first embodiment. Although the internal configuration is different, in the following description, the same reference numerals for the communication apparatus as those in the first embodiment are used.

  FIG. 7 is a block diagram showing an internal configuration of the communication device 11 (11A or 11B) according to the second embodiment. The same reference numerals are given to the same or corresponding parts as those in FIG. 2 according to the first embodiment. As shown.

  In FIG. 7, the communication device 11 according to the second embodiment includes an RTP transmission / reception unit 20, an RTCP transmission / reception unit 21, a data transmission / reception unit 22, a control unit 23, a transmission data storage unit 24, a transmission data extraction unit 25, and a packet loss detection. Unit 26, packet statistical information totaling unit 30, retransmission request number / retransmission number calculation unit 31, and data communication bus 28. That is, instead of the retransmission request number / retransmission number storage unit 27 in the first embodiment, a packet statistical information totaling unit 30 and a retransmission request number / retransmission number calculating unit 31 are provided. This is the same as that of the first embodiment.

  The packet statistical information totaling unit 30 is controlled in the direction from the RTP packet transmitted from the communication device or the information of the RTP packet or RTCP packet received from the opposite communication device to the communication device 11 under the control of the control unit 23. The packet loss rate of the network and the average packet length of transmitted RTP packets and received RTCP packets are calculated. Here, the average packet length of the RTP packet is used as the average packet length of the RTP packet to be retransmitted, and the average packet length obtained including the RTP packet not related to the retransmission may be applied. You may apply the average packet length calculated | required only from the RTP packet. The average packet length of the RTCP packet is used as the average packet length of the RTCP packet requested for retransmission, and the average packet length obtained including the RTCP packet not related to the retransmission request may be applied. You may apply the average packet length calculated | required only from the request | required RTCP packet.

  The retransmission request count / retransmission count calculation unit 31 includes information obtained by the packet statistical information totaling unit 30, information obtained by the packet statistical information totaling unit of the opposing communication device, and the opposing communication device designated by the control unit 23. The number of retransmission requests n1 and the number of retransmissions n2 referred to in the first embodiment are calculated from the target packet loss rate as seen from the application. The specific calculation method of the number of retransmission requests n1 and the number of retransmissions n2 will be clarified in the operation section.

(B-2) Operation | movement of 2nd Embodiment Next, operation | movement of the communication system 10 which concerns on 2nd Embodiment is demonstrated. The operation of the second embodiment is different from the first embodiment in the determination (calculation) operation of the retransmission request number n1 and the retransmission number n2 performed by the retransmission request number / retransmission number calculation unit 31, and the determination operation. Since this is an operation for collecting information to be used, these operations will be described below. In the following description, it is assumed that the communication device is the communication device 11A in FIG. 1 and the opposite communication device is the communication device 11B in FIG. Further, in the following description, the constituent elements of the communication device 11A are distinguished by adding “A” at the end of the code, and the constituent elements of the communication device 11B are assigned with “B” at the end of the code.

  The control unit 23A of the communication device 11A, for example, when a recalculation of the retransmission request number n1 or the retransmission number n2 is instructed by an operation of an operation key (not shown) by the operator, or a predetermined review timing such as once a week. Then, each unit is controlled so as to re-determine the number of retransmission requests n1 and the number of retransmissions n2.

  Here, re-determination of the retransmission request number n1 and retransmission number n2 in the communication device 11A and re-determination of the retransmission request number n1 and retransmission number n2 in the opposite communication device 11B are executed in parallel. Is preferred.

  The operation of re-determining the number of retransmission requests n1 and the number of retransmissions n2 is the order in which the determination operation is performed after performing the operation of collecting information used in the determination operation as preprocessing. It should be noted that the operation of collecting information used in the determination operation may be performed periodically regardless of the activation of the operation for re-determining the retransmission request count n1 and the retransmission count n2.

  When information collection is activated from the control unit 23A, the packet statistical information totaling unit 30A determines the communication from the RTP packet transmitted from the communication device 11A, the RTP packet information received from the opposite communication device 11B, the information of the RTCP packet, and the like. Obtained by calculating the packet loss rate y of the network in the direction toward the device 11A, the packet length (average packet length) L1 of the RTCP packet of the retransmission request, and the packet length (average packet length) L2 of the retransmitted RTP packet The parameters y, L1, and L2 are given to the retransmission request number / retransmission number calculation unit 31A.

  When the retransmission request count / retransmission count calculation unit 31A obtains the packet loss rate y of the network in the reverse direction, the control unit 23A of the communication device 11A transmits the information to the opposing communication device 11B and is opposed to it. The packet loss rate x of the forward network is taken in from the communication device 11B and given to the retransmission request number / retransmission number calculation unit 31A. Such exchange of the packet loss rates y and x can be realized by an original communication method, but may be performed by using RTCP RR (receiver report).

  In the above description, the packet statistical information totaling unit 30A calculates both the packet length L1 of the RTCP packet of the retransmission request and the packet length L2 of the retransmitted RTP packet, but the packet statistical information totaling unit of the opposing communication apparatus 11B 30B may be provided with a function for calculating one packet length (for example, L1), and the communication apparatus 11A may complete two packet lengths L1 and L2 by exchanging information.

  Further, the control unit 23A of the communication device 11A gives the target packet loss rate Z viewed from the application of the facing communication device 11B to the retransmission request count / retransmission count calculation unit 31A. The target packet loss rate Z viewed from the application of the opposing communication device 11B may be input by an operator to an upper application of the communication device 11A using an operation input unit (not shown), and the corresponding communication What is held by the application of the device 11B may be acquired by communication.

  As described above, the packet loss rate x of the forward network, the packet loss rate y of the reverse network, the packet length L1 of the RTCP packet for the retransmission request, the packet length L2 of the retransmission RTP packet, and the opposing communication When the five parameters of the target packet loss rate Z as seen from the application of the apparatus 11B are prepared, the retransmission request number / retransmission number calculation unit 31A executes specific determination operations for the retransmission request number n1 and the retransmission number n2.

  The basic concept of the method for determining the number of retransmission requests n1 and the number of retransmissions n2 is as follows.

First, when the number of retransmission requests n1 and the number of retransmissions n2 are determined, the packet loss rate z as seen from the application of the opposing communication device 11B calculated according to the above-described equation (1) is the target packet loss rate Z The following is required. Therefore, it is necessary to satisfy the inequality (7).

  Secondly, the increase e of network traffic due to the retransmission mechanism when retransmission functions is suppressed as much as possible.

The traffic increase amount e can be expressed by equation (8). The first term on the right side of the equation (8) is an increase in traffic due to the retransmission request RTCP packet being transmitted n1 times. Since the retransmission RTP packet is transmitted n2 times when at least one RTCP packet of the retransmission request reaches the communication apparatus 11A, the probability that the retransmission RTP packet is transmitted n2 times is 1-y ⁿ¹ It is. Therefore, the amount of increase in traffic due to the retransmission RTP packet being transmitted n2 times is expressed by the second term on the right side of equation (8).

  The retransmission request count / retransmission count calculation unit 31A searches for a combination in which the increase e of network traffic shown in the formula (8) is minimized among the combinations of the retransmission request count n1 and the retransmission count n2 that satisfy the formula (7). Then, the number of retransmission requests n1 and the number of retransmissions n2 of the combinations that can be searched are determined as the number of retransmission requests n1 and the number of retransmissions n2 to be applied. Note that the retransmission request count n1 and the retransmission count n2 are naturally positive integers.

  A specific search method (a method for determining the number of retransmission requests n1 and the number of retransmissions n2) is not limited as long as the optimum number of retransmission requests n1 and the number of retransmissions n2 can be searched. Hereinafter, three specific search methods (first to third search methods) will be described in order. In the first to third search methods, an upper limit value n1max of the retransmission request number n1 and an upper limit value n2max of the retransmission number n2 are introduced. In addition, when the target packet loss rate Z viewed from the application is equal to or higher than the packet loss rate x of the forward network, the target packet loss rate Z can be achieved without applying the technical idea of the second embodiment. In the following description and flowchart, the description of handling in this case is omitted.

  FIG. 8 is a flowchart showing a first search method for the optimum number of retransmission requests and the number of retransmissions.

  First, the retransmission request count / retransmission count calculation unit 31A performs the forward network packet loss rate x, the reverse network packet loss rate y, the retransmission request RTCP packet packet length L1, and the retransmission RTP packet packet length. Five parameters of L2 and the target packet loss rate Z viewed from the application of the opposing communication apparatus 11B are set (step S100), and the retransmission request number candidate n1 is set to an initial value 0 (step S101).

Thereafter, after incrementing the retransmission request number candidate n1 by 1 (step S102), the smallest value n2 satisfying the expression (9) is set as the retransmission number candidate n2 combined with the current retransmission request number candidate n1 (however, When the small value n2 is larger than the upper limit value n2max, it is handled that there is no value n2 that satisfies the equation (9)), and the combination obtained this time is held (step S103). Here, equation (9) is an inequality obtained by modifying the inequality shown in equation (7) above so that n2 is on the left side (since x and y are larger than 0 and smaller than 1, The direction of the inequality sign changes when converted to a logarithm). Satisfying equation (9) is equivalent to satisfying equation (7).

  Thereafter, it is determined whether or not the retransmission request number candidate n1 at that time is the upper limit value n1max (step S104). If the retransmission request number candidate n1 is not the upper limit value n1max, the process proceeds to step S102 described above.

  On the other hand, if retransmission request number candidate n1 at that time is upper limit n1max, retransmission request number / retransmission number calculation unit 31A has the same number of retransmission number candidates n2 in the held n1max combinations. The values are grouped, and only the combination with the smallest number of retransmission request candidates n1 is left out of each group, and the combination that is not grouped is left as it is because there is no retransmission number candidate n2 with the same value, and is a candidate. The combinations are narrowed down (step S105).

  Then, for each remaining combination candidate of retransmission request number n1 and retransmission number candidate n2, the traffic increase amount e is calculated according to the equation (8), and the retransmission request number candidate n1 of the combination candidate having the smallest traffic increase amount e. The retransmission number candidate n2 is determined as the retransmission request number n1 and the retransmission number n2 to be applied (step S106).

  FIG. 9 is a flowchart showing a second search method for the optimum number of retransmission requests and the number of retransmissions.

  First, the retransmission request count / retransmission count calculation unit 31A performs the forward network packet loss rate x, the reverse network packet loss rate y, the retransmission request RTCP packet packet length L1, and the retransmission RTP packet packet length. Five parameters of L2 and the target packet loss rate Z as viewed from the application of the opposing communication apparatus 11B are set (step S200), and the retransmission request number candidate n1 and the retransmission number candidate n2 are set to initial values 0 ( Step S201).

  Thereafter, after incrementing the retransmission number candidate n2 and the retransmission request number candidate n1 by 1 (steps S202 and S203), the traffic increase amount e according to the equation (8) for the combination of the current retransmission request number candidate n1 and the retransmission number candidate n2. Is calculated and held (step S204).

  Next, it is determined whether or not the retransmission request number candidate n1 at that time is the upper limit value n1max (step S205). If the retransmission request number candidate n1 is not the upper limit value n1max, the process proceeds to step S203 described above.

  On the other hand, if retransmission request number candidate n1 at that time is upper limit n1max, retransmission request number / retransmission number calculation unit 31A sets retransmission request number candidate n1 to initial value 0 (step S206). Then, it is determined whether or not the retransmission number candidate n2 at that time is the upper limit value n2max (step S207). If the retransmission number candidate n2 is not the upper limit value n2max, the process proceeds to step S202 described above.

  By repeating the loop processing between steps S202 to S207 until the loop processing is exited, the traffic increase amount e is calculated and held for all combinations of the retransmission request number candidate n1 and the retransmission number candidate n2.

  At this time, the retransmission request number / retransmission number calculation unit 31A sorts all combinations of the retransmission request number candidate n1 and the retransmission number candidate n2 in ascending order of the traffic increase amount e (step S208). Then, the first combination that satisfies the inequality of the above-described equation (7) is found from the sorted head side, and the retransmission request number candidate n1 and retransmission number candidate n2 of the found combination are applied to the retransmission request number n1 and the retransmission. The number of times n2 is determined (step S209).

  FIG. 10 is a flowchart showing a third search method for the optimum number of retransmission requests and the number of retransmissions.

  First, the retransmission request count / retransmission count calculation unit 31A performs the forward network packet loss rate x, the reverse network packet loss rate y, the retransmission request RTCP packet packet length L1, and the retransmission RTP packet packet length. Five parameters of L2 and the target packet loss rate Z viewed from the application of the opposing communication apparatus 11B are set (step S300), and the retransmission request number candidate n1 and the retransmission number candidate n2 are set to initial values 0 ( Step 301).

  Thereafter, the retransmission number candidate n2 and the retransmission request number candidate n1 are incremented by 1 (steps S302 and S303), and the condition of the inequality of the expression (7) is satisfied for the combination of the current retransmission request number candidate n1 and the retransmission number candidate n2. If it is satisfied and information is satisfied, the information of the current combination is held (step S304).

  Next, it is determined whether or not the retransmission request number candidate n1 at that time is the upper limit value n1max (step S305). If the retransmission request number candidate n1 is not the upper limit value n1max, the process proceeds to step S303 described above.

  In contrast, if retransmission request number candidate n1 at that time is upper limit n1max, retransmission request number / retransmission number calculation unit 31A sets retransmission request number candidate n1 to initial value 0 (step S306). Then, it is determined whether or not the retransmission number candidate n2 at that time is the upper limit value n2max (step S307). If the retransmission number candidate n2 is not the upper limit value n2max, the process proceeds to step S302 described above.

  By repeating the loop processing between steps S302 to S307 until the loop processing is exited, information on combinations satisfying the condition of the inequality (7) among all combinations of the retransmission request number candidate n1 and the retransmission number candidate n2 Only retained.

  At this time, the retransmission request number / retransmission number calculation unit 31A calculates the traffic increase amount e according to the equation (8) for each combination satisfying the condition of the inequality of the equation (7) (step S308). A combination having the smallest traffic increase amount e is found, and the retransmission request number candidate n1 and retransmission number candidate n2 of the found combination are determined as the retransmission request number n1 and retransmission number n2 to be applied (step S309).

  Although not shown in FIGS. 8 to 10, the retransmission request count / retransmission count calculation unit 31 </ b> A determines the retransmission count n <b> 2 to retransmit only the retransmission count n <b> 2 when the RTP packet needs to be retransmitted. Is stored internally. On the other hand, since it is the opposite communication apparatus 11B that transmits the RTCP packet requesting retransmission by the determined retransmission request count n1, the determined retransmission request count n1 from the retransmission request count / retransmission count calculator 31A. Under the control of the control unit 23A that has received the notification, the determined retransmission request count n1 is transferred to the opposing communication device 11B.

  In the above, the determination operation of the retransmission request number n1 and the retransmission number n2 related to the transfer of the RTP packet from the communication device 11A to the communication device 11B has been described. However, regarding the transfer of the RTP packet from the communication device 11B to the communication device 11A, The retransmission request count / retransmission count calculator 31B of the device 11B determines the retransmission request count n1 and the retransmission count n2, and the determined retransmission request count n1 is notified to the communication device 11A. In the communication device 11A, for example, the retransmission request number / retransmission number calculation unit 31A holds the notified retransmission request number n1, and when requesting retransmission to the opposite communication device 11B, notifies the retransmission request RTCP packet. Is transmitted only n1 times.

(B-3) Effects of the Second Embodiment According to the second embodiment, the retransmission request count n1 and the retransmission count n2 are determined and applied, so that the application load can be minimized while minimizing an increase in network load. Can achieve the packet loss rate required.

  Here, by periodically determining the number of retransmission requests n1 and the number of retransmissions n2, and reflecting the determination results in the retransmission mechanism, it is possible to dynamically cope with changes in the network packet loss rate over time. Become.

  Incidentally, in the case of the first embodiment, by increasing the number of retransmission requests and the number of retransmissions, the packet loss rate seen from the application can be significantly reduced. However, transmitting an RTCP packet for retransmission request and an RTP packet for retransmission more than necessary will waste the network bandwidth, which is a limited resource. In the second embodiment, the number of retransmission requests n1 and the number of retransmissions n2 do not increase unnecessarily, and the increase in the load on the network is minimized while keeping the packet loss rate viewed from the application below the required rate. be able to.

(C) Other Embodiments In the description of each of the above embodiments, various modified embodiments have been referred to, but further modified embodiments as exemplified below can be given.

  In the second embodiment, the packet loss rate of the forward network and the packet loss rate of the reverse network are shown separately, but the packet loss rates of the forward and reverse networks are equal. The number of retransmission requests and the number of retransmissions may be determined.

  In the second embodiment, the packet length of the retransmission request RTCP packet is also measured. However, the fixed length prepared in advance by the system is applied as the packet length of the retransmission request RTCP packet. May be. In addition, even when the RTP packet payload is fixed, such as when the encoding method in the upper application uses a fixed-length encoding method, the system prepares in advance as the packet length of the retransmitted RTP packet. The fixed length may be applied.

  In the second embodiment, the upper limit of the number of retransmission requests and the upper limit of the number of retransmissions are set to search for the optimum number of retransmission requests and the number of retransmissions. However, at least one of the number of retransmission requests and the number of retransmissions is An intentional lower limit (for example, twice) may be provided to search for the optimum number of retransmission requests and the number of retransmissions.

  In the second embodiment, in the transmission of the RTP packet from the communication device 11A to the communication device 11B, the communication device 11A determines the optimum number of retransmission requests and the number of retransmissions, and the optimum number of retransmission requests is determined. Although what is notified to the communication device 11B is shown, the determination subject is not limited to the communication device 11A.

  For example, in the transmission of the RTP packet from the communication device 11A to the communication device 11B, the communication device 11B collects necessary information and receives the information to determine the optimum number of retransmission requests and the number of retransmissions. The optimal number of retransmissions may be notified to the communication device 11A. Further, for example, in transmission of an RTP packet from the communication device 11A to the communication device 11B, the communication device 11A determines and stores an optimal number of retransmissions, and the communication device 11B determines and stores an optimal number of retransmission requests. There may be. Further, for example, the network management device receives the necessary information from the communication device 11A and the communication device 11B, determines the optimum number of retransmission requests and the number of retransmissions, and notifies the communication device 11B of the determined optimum number of retransmission requests. Alternatively, the determined optimum number of retransmissions may be notified to the communication device 11A. In this case, the network management device is also a component of the communication system.

  In each of the above-described embodiments, the communication device is shown as having a higher application program (software). However, the higher device may have a hardware configuration.

  In each of the above embodiments, a communication system that transmits and receives stream data in both directions has been described. However, the technical idea of the present invention can also be applied to a communication system that transmits and receives stream data only in one direction. For example, the technical idea of the present invention can be applied to communication between a video distribution device that distributes video data and a terminal that receives the distribution. In such a one-way communication system, it is only necessary to set only the optimum number of retransmissions for the transmission-side communication apparatus and only the optimum number of retransmission requests for the reception-side communication apparatus.

  DESCRIPTION OF SYMBOLS 10 ... Communication system 11, 11A, 11B ... Communication apparatus, 20 ... RTP transmission / reception part, 21 ... RTCP transmission / reception part, 22 ... Data transmission / reception part, 23 ... Control part, 24 ... Transmission data storage part, 25 ... Transmission data extraction part , 26 ... packet loss detection unit, 27 ... retransmission request number / retransmission number storage unit, 28 ... data communication bus, 30 ... packet statistical information totaling unit, 31 ... retransmission request number / retransmission number calculation unit.

Claims (6)

In a communication system that transmits and receives stream data by data packets from a communication device on the transmission side to a communication device on the reception side,
The communication device on the receiving side is
A packet loss detection means for detecting a packet loss of any data packet;
Retransmission request number holding means for holding the number of retransmission requests, which is the number of times to repeatedly transmit a retransmission request packet for requesting retransmission of data packets with packet loss, and
A retransmission request packet transmitting unit that continuously transmits a retransmission request packet by the number of retransmission requests held in the retransmission request number holding unit when a packet packet of packet loss occurs, and
The transmission side communication device is:
A retransmission request packet receiving means for receiving the retransmission request packet;
Retransmission number holding means for holding the number of retransmissions, which is the number of times to repeatedly transmit data packets for which retransmission has been requested, continuously;
In response to the first reception of the retransmission request packet, a retransmission packet transmission unit that repeatedly transmits data packets requested to be retransmitted by the number of retransmissions held in the retransmission number holding unit,
The communication system further includes a number of times determination unit for determining the number of retransmission requests held by the retransmission request number holding unit and the number of retransmissions held by the retransmission number holding unit.
The number of times determination means is
The first packet loss rate in the network in the direction of the data packet and the retransmission packet, the second packet loss rate in the network in the transfer direction of the retransmission request packet, the target value of Rupa Kettorosu rate relates to a packet loss, the data In response to the input of the first packet length of the packet and the second packet length of the retransmission request packet,
From the first number of retransmission requests with the first predetermined value as the upper limit, and from the first number of requests with the second predetermined value as the upper limit,
The first number of retransmission requests, the first number of requests, the first packet loss rate, and packet loss rate is calculated by using the second packet loss rate becomes equal to or less than the target value of the upper Kipa Kettorosu rate,
Furthermore, the amount of increase in network traffic calculated using the first number of retransmission requests, the first number of requests, the first packet length, the second packet loss rate, and the second packet length is minimized. Search for the combination of the number of times of the first retransmission request and the number of times of the first request,
The number of retransmission requests held by the retransmission request number holding means and the number of retransmission requests held by the retransmission request number holding means are N1 (N1 is a positive integer) and the first request number is N2 (N2 is a positive integer) in the combination that can be searched. A communication system characterized in that it is determined as the number of retransmissions held by the number holding means. The transmission side communication device has first packet statistical processing means for obtaining the second packet loss rate and the second packet length to be input to the number-of-times determining means by statistical processing of received packets,
The receiving-side communication apparatus has second packet statistical processing means for obtaining the first packet loss rate and the first packet length to be input to the number-of-times determining means by statistical processing of received packets. The communication system according to claim 1.   The communication system according to claim 1 or 2, wherein the number-of-times determining means is provided in the communication device on the transmission side. A communication method in a communication system for transmitting and receiving stream data by a data packet from a communication device on a transmission side to a communication device on a reception side,
The packet loss detecting means of the receiving side communication device detects the packet loss of any data packet,
The retransmission request number holding means of the communication device on the receiving side holds a retransmission request number that is the number of times the retransmission request packet for requesting retransmission of a packet loss data packet is repeatedly transmitted continuously.
The retransmission request packet transmission means of the communication device on the receiving side continuously transmits the retransmission request packet by the number of retransmission requests held in the retransmission request number holding means when a packet loss data packet occurs. And
The retransmission request packet receiving means of the transmission side communication device receives the retransmission request packet,
The retransmission number holding means of the communication device on the transmission side holds the number of retransmissions, which is the number of times the data packet requested to be retransmitted is repeatedly transmitted continuously,
In response to the initial reception of the retransmission request packet, the retransmission packet transmission means of the transmission side communication device continuously repeats the data packet for which retransmission has been requested by the number of retransmissions held in the retransmission number holding means. To
The number-of-times determining means in the communication system determines the number of retransmission requests held by the retransmission request number holding means and the number of retransmissions held by the retransmission number holding means,
Further, the number of times determining means is
The first packet loss rate in the network in the direction of the data packet and the retransmission packet, the second packet loss rate in the network in the transfer direction of the retransmission request packet, the target value of Rupa Kettorosu rate relates to a packet loss, the data In response to the input of the first packet length of the packet and the second packet length of the retransmission request packet,
From the first number of retransmission requests with the first predetermined value as the upper limit, and from the first number of requests with the second predetermined value as the upper limit,
The first number of retransmission requests, the first number of requests, the first packet loss rate, and packet loss rate is calculated by using the second packet loss rate becomes equal to or less than the target value of the upper Kipa Kettorosu rate,
Furthermore, the amount of increase in network traffic calculated using the first number of retransmission requests, the first number of requests, the first packet length, the second packet loss rate, and the second packet length is minimized. Search for the combination of the number of times of the first retransmission request and the number of times of the first request,
The number of retransmission requests held by the retransmission request number holding means and the number of retransmission requests held by the retransmission request number holding means are N1 (N1 is a positive integer) and the first request number is N2 (N2 is a positive integer) in the combination that can be searched. A communication method characterized by determining the number of retransmissions held by the number holding means. The first packet statistical processing means of the communication device on the transmission side acquires the second packet loss rate and the second packet length, which are input to the number determining means, by statistical processing of the received packet,
Second packet statistical processing means of the receiving communication device is input to the number determination unit, and the first packet loss rate, and the first packet length, it is obtained by statistical processing of the received packet The communication method according to claim 4.   6. The communication method according to claim 4, wherein the number-of-times determining means is provided in the transmission-side communication apparatus.

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