JP3931594B2 - Retransmission method for multipoint broadcast networks - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a data re-transmission method, and more particularly to a communication network in a case where an image or audio encoded signal, which is a real-time signal, is transmitted to at least two receivers via a communication network. The present invention relates to a retransmission method for a multipoint broadcast network that efficiently retransmits a transmission error occurring in the network.
[0002]
[Prior art]
In data signal communication, the data signal is divided for each packet, and the transmission side and the reception side perform communication while confirming whether the packet is normally received every time the packet is transmitted, and retransmit as necessary. Thereby, data can be communicated with high reliability. As a compensation, it takes time to retransmit, and it is known that a delay occurs. In IP (Internet Protocol) communication, whose transmission delay has been rapidly spreading in recent years, a communication protocol called TCP (Transport Protocol) / IP is used for the data signal, and the above retransmission processing is performed to enable highly reliable communication. It is known that
[0003]
Unlike a data signal, an encoded image or audio signal that requires real-time communication often employs a communication protocol that does not perform error retransmission. As a communication protocol that does not perform error retransmission in IP communication, it is known to adopt a method called UDP (User Datagram Protocol) / IP that allows a user to establish a communication protocol independently. There are two main reasons for not performing error retransmission. The first reason is that, if retransmission is performed, the delay time becomes long, so that the human psychological burden increases. The second reason is that, unlike a data signal, even if a slight transmission error occurs, the image and sound are only disturbed for a short time, which is acceptable.
[0004]
However, when an image or audio signal is transmitted over a line with a high transmission error rate, the influence of the transmission error becomes too great and the psychological burden increases. Therefore, a packet in which a transmission error is detected is retransmitted a finite number of times (usually once) as long as the retransmitted packet is in time for the decoding process on the receiving side, and the delay is extremely long due to many transmissions. It is known that the effects of transmission errors can be reduced (Nunome, Tasaka, Ishibashi “Performance measurement of continuous media retransmission control method RVTR on the Internet”, IEICE technical report, CA99-56, Dec. 1999). Since the communication protocol in this case is in a unique mode, it is desirable to adopt the above UDP / IP and implement a unique protocol called finite number of retransmissions within that frame.
[0005]
The above limited retransmission method is an excellent method because it is possible to achieve both suppression of delay time increase and countermeasures against transmission errors. We will consider applying this method when transmitting image signals and audio signals to multiple receivers in broadcast mode.
[0006]
In general, transmission errors in the case of broadcast communication include cases where a single transmission error affects multiple receivers in common and a single transmission error affects only a single receiver. The case of giving is considered. In the former, the LAN is connected to the LAN by a relatively thin transmission path, and when the transmission is performed from the transmission device in one LAN to a plurality of reception devices connected to the other LAN, the above-mentioned When packet loss occurs in a narrow transmission path, a plurality of receiving apparatuses may be commonly affected. As another example of the former, when transmitting wirelessly, it can be assumed that a plurality of receiving devices in a certain area are commonly affected by noise such as lightning.
[0007]
Examples of the latter include an overload in a network placed in the immediate vicinity of a specific receiving device, packet loss, or only a specific receiving device hidden by an obstacle when transmitting wirelessly, Can be assumed to be defective.
[0008]
Here, measures against transmission errors in the former case are considered. When a retransmission request is made when a plurality of receiving apparatuses are commonly affected by a transmission error in broadcast communication, there are the following two problems. The first problem is that retransmission is performed even when a transmission error occurs in only a part of a plurality of receiving apparatuses. Therefore, the frequency of occurrence of retransmission increases and the transmission traffic increases more than necessary. The second problem is a retransmission request. The transmission traffic itself will increase. In either case, an increase in traffic causes new transmission errors due to loss of image signals and audio signals in the communication network.
[0009]
[Problems to be solved by the invention]
The present invention has been made in order to solve the above-mentioned problems, and its purpose is not to make retransmission requests more than necessary, and traffic increases due to retransmission requests and new data is discarded. An object of the present invention is to provide a retransmission method that never happens.
[0010]
[Means for Solving the Problems]
In order to solve the problem in the case where the former transmission error affects a plurality of receiving apparatuses in common, the present invention retransmits only a predetermined receiving apparatus among a plurality of receiving apparatuses. It is characterized in that a function capable of making a request is added, and other receiving apparatuses have a function of normally receiving the retransmitted signal even when the retransmission request is not made by itself.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, the configuration of a broadcast communication system to which the present invention is applied will be described.
FIG. 1 is a diagram showing a configuration of an entire real-time transmission system for video and audio using a communication network.
FIG. 1 shows a configuration in which three terminals are connected on a communication network as an example. In the terminal on the transmission side, the TV signal imaged by the camera 1 and the audio signal collected by the microphone 2 are input to the MPEG-2 encoding device 3, and the encoded signal is transmitted to the communication network 4. . In the communication network 4, for example, in IP communication, the same encoded signal is duplicated and transmitted to the MPEG-2 decoding devices 5 and 8 of a plurality of terminals according to a communication protocol called multicast.
The decoding devices 5 and 8 decode the received encoded signals, restore the original TV signals and audio signals, and output them to the image monitors 6 and 9 and the speakers 7 and 10.
[0012]
Next, the configuration of the MPEG-2 encoding device 3 will be described with reference to FIG.
The TV signal taken by the camera 1 and the audio signal collected by the microphone 2 are input to the MPEG-2 encoding device 3. Inside the encoding device 3, the TV signal is separated into a luminance signal and a color difference signal by a luminance / color difference separation circuit 21, digitized by an A / D (analog / digital) conversion circuit 22, and then MPEG-2 image code. Is input to the conversion circuit 23. The MPEG-2 image encoding circuit 23 performs high-efficiency encoding on the image signal according to a predetermined algorithm. For example, the typical sampling frequency of the original image signal is 13.5 MHz, and the typical quantization accuracy is 8 bits / sample. Therefore, the transmission speed when the original image signal is transmitted without encoding is the above two. It corresponds to the product of 108Mbit / s. In MPEG-2 image encoding, this is compressed to 6 Mbit / s, for example. The high-efficiency encoded image signal is input to the image encoding buffer 24.
[0013]
The audio signal is converted into an audio encoded signal by the MPEG-2 audio encoding circuit 29 and input to the audio encoding buffer 30. Since the typical sampling frequency of speech is 32 kHz and the typical quantization accuracy is 16 bits / sample, the information transmission rate when not encoded is equivalent to 512 kbit / s. This is compressed to, for example, 256 kbit / s by MPEG-2 audio encoding.
[0014]
The image encoded signal and the audio encoded signal stored in the image encoding buffer 24 and the audio encoding buffer 30 are multimedia-multiplexed by the microprocessor 25. The multimedia multiplexed signal is divided into predetermined length units to form a packet. Further, a serial number called an RTP sequence is assigned to each packet and written to the transmission RTP buffer 26.
[0015]
The microprocessor 27 sends the signal written in the transmission RTP buffer 26 to the communication network 4 via the communication interface 28 in accordance with a communication protocol such as UDP / IP.
[0016]
Next, the configuration of the MPEG-2 decoding device 6 will be described with reference to FIG.
A signal input via the communication network 4 is input to the microprocessor 42 via the communication interface 41, and is received and processed according to a communication protocol such as UDP / IP. Further, the signal subjected to reception processing is written in the reception RTP buffer 43.
[0017]
The microprocessor 44 reads out the RTP sequence number and the packet signal from the reception RTP buffer 43, separates and extracts the image encoded signal and the audio encoded signal from the packet signal therein, and the image decoding buffer 45 and the audio decoding respectively. Write to buffer 46.
[0018]
The MPEG-2 image decoding circuit 47 restores the image encoded signal read from the image decoding buffer 45 to the original image signal, and passes through the D / A (digital / analog) converter 48 and the luminance / color difference synthesis circuit 49. Then, it is restored to an analog TV signal and output to the image monitor 6 for display. The MPEG-2 audio decoding circuit 50 restores the audio encoded signal read from the audio decoding buffer 46 to the original audio signal, and outputs the audio to the speaker 7.
[0019]
The retransmission processing according to the present embodiment is realized by software of the microprocessors 25, 27, 42, and 44 in FIGS. Hereinafter, the present embodiment will be described in detail with reference to FIGS. The retransmission function on the transmission side is as follows. That is, writing to the transmission RTP buffer 26 by the microprocessor 25 follows the following procedure as shown in FIG.
a) When the transmission data is cyclically written in the transmission RTP buffer 26, the RTP sequence number is written at the first address (for example, address 0).
b) Every time there is data in the encoding buffer (collectively referring to the image encoding buffer and the audio encoding buffer) and the writing time in a predetermined cycle is reached, the image encoding signal and the audio encoding signal are Each of the packets is divided into packets of a certain length, packetized with an identifier for identifying images and sounds, multimedia multiplexed, and written as the multimedia multiplexed signal at the subsequent addresses.
c) Thereafter, this operation is repeated, and the RTP sequence number and the multimedia multiplexed signal are written in the transmission RTP buffer 26 in order. When the data is stored in the entire transmission RTF buffer 26, the process returns to a).
[0020]
Reading from the transmission RTP buffer 26 by the microprocessor 27 follows the following procedure as shown in FIG.
a) When the transmission RTP buffer 26 is read cyclically, at the beginning of the cycle, the sequence number at the beginning of the cycle immediately before being written in the X address is written in the Y address. Next, the current RTP sequence number is written to address X as the first sequence number for this lap.
b) In normal transmission, the sequence number and the packet signal read in order from the transmission RTP buffer 26 according to the read address are sent to the communication network according to the communication protocol. When reading from the entire transmission RTP buffer 26 is completed, the process returns to a).
c) When there is a retransmission request, the packet to be retransmitted is notified by the sequence number from the receiving side. Therefore, the address corresponding to the sequence number is decoded, and a multimedia multiplexed signal for one packet is read from the address, and is retransmitted according to the communication protocol together with the sequence number to be retransmitted.
[0021]
The retransmission function on the receiving side is as follows. Writing to the reception RTP buffer 43 by the microprocessor 42 follows the following procedure.
a) The RTP sequence number and packet signal received via the communication interface 41 are written in the reception RTP buffer 43 in a cyclic manner. When writing to the reception RTP buffer 43 for the first time, the RTP sequence number is written to the first address (for example, address 0).
b) According to a predetermined algorithm, the RTP sequence number is converted to the write address of the reception RTP buffer 43, and the RTP packet is written to the corresponding address.
c) After that, this operation is repeated, and when it is stored in the entire reception RTP buffer 43, the process returns to a).
d) The received RTP sequence number is inspected, and if there is discontinuity, a retransmission request is made assuming that the RTP sequence number or packet is discarded in the communication network. As a retransmission request method, for example, an RTP sequence number estimated to be discarded may be transmitted to the transmission side by the TCP / IP protocol or the like.
e) A null packet is written in the reception RTP buffer 43 corresponding to the RTP packet discarded in the communication network, and the ready flag is rewritten to a busy flag to indicate to the microprocessor 44 that the corresponding area has been written.
f) When the retransmitted RTP sequence number and packet arrive, the RTP sequence number is converted to the write address of the reception RTP buffer 43 according to the above algorithm, and the retransmitted RTP sequence number and packet are written.
g) After writing a packet or the like, it is rewritten to a busy flag to identify that writing has been made to the microprocessor 44 on the reading side.
[0022]
The reading process from the reception RTP buffer 43 by the microprocessor 44 is as follows.
a) Confirm that the flag of the read address of the reception RTP buffer 43 is a busy flag, confirm that the RTP sequence number and the packet are stored in the reception RTP buffer 43, and secure a certain time interval. After confirming that the RTP sequence number and packet are read.
b) After reading, the busy flag is switched to the ready flag and the microprocessor 42 is notified of the reading.
c) Discards the read RTP sequence number, separates the packet into an image encoded signal and an audio encoded signal, and writes them into an image decoding buffer and an audio decoding buffer (generally called decoding buffer), respectively.
d) The read address of the reception RTP buffer is incremented, and when reading of the entire reception RTP buffer 43 is completed, the read address is initialized to zero.
[0023]
The following conversion methods for the read / write address to the RTP buffer for transmission and reception and the RTP sequence number can be considered.
a) Let X be the RTP sequence number when writing to the head of the RTP buffer, and Y be the RTP sequence number when writing to the head of the RTP buffer one cycle before that.
b) Let L be the sum of the RTP sequence and packet signal length.
c) The sequence number A to be written is compared with the above X. When A is X or more,
Address: Write to (A−X) * L.
d) When A is less than X, write to address: (A−Y) * L.
[0024]
In the above, the following exception handling is also required.
a) When the RTP sequence number to be written at the head of the RTP buffer is discarded: X is predicted from the above Y, and a dummy signal is written to the write address corresponding to the jumped RTP packet based on the X. Only the ready flag may be simply converted to a busy flag.
b) When the arriving RTP sequence number has changed more than the immediately preceding number (not retransmission): Assuming that the channel has been switched to a new channel, no retransmission may be requested during that time.
[0025]
With the above operation, retransmission is performed once when a transmission error occurs. Furthermore, when a transmission error occurs when broadcasting an image or audio signal to a plurality of receiving side devices, which is a feature of the present embodiment, a retransmission request is made only to a limited MPEG-2 decoding device. A method for generating the above will be described with reference to FIG. Except for the software processing surrounded by a broken line, it is the same as FIG. That is,
a) When there is a retransmission request as in the prior art, the transmission side performs retransmission.
b) On the receiving side, the received RTP sequence number is decoded to detect whether it is continuous, and after detection, it is identified whether this device, which is a feature of this embodiment, is a retransmission request handling device, Retransmission is performed only when the apparatus is a retransmission request handling apparatus. A device other than the retransmission request compatible device does not make a retransmission request even if a transmission error is detected.
c) When all the receiving apparatuses receive the retransmitted signal, they replace the retransmitted signal with the received signal and decode it.
[0026]
With the above operation, when a transmission error occurs in common for all the receiving side devices, a single error retransmission request is issued to the transmitting side, and a retransmission signal is sent from the transmitting side to all the receiving side devices. It is clear that the object of the present invention can be achieved by transmitting the signal by broadcast and replacing the transmission error signal with a correct signal.
[0027]
Note that the selection of the receiving apparatus that requests retransmission is performed as follows.
a) When a common influence is given to all the receiving apparatuses and a transmission error is always caused, and in addition to that, each apparatus is affected by an individual transmission error: In this case, there are a plurality of retransmission request receiving apparatuses. It is desired that a receiving apparatus having the lowest probability of occurrence of an individual transmission error among the receiving apparatuses, for example, an apparatus arranged at a point where reception conditions are best in the case of radio, be a retransmission request terminal. This is because when a retransmission error is detected by a retransmission request receiving apparatus, there is a high possibility that it will be affected in common by other terminals. This is because a retransmission request for a common transmission error can be made by the retransmission request.
b) A common effect is given to all receiving devices, but it is a certain probability that a transmission error is caused, and in addition, each device is affected by an individual transmission error: However, it is necessary to employ a receiving apparatus with a low probability of occurrence of individual transmission errors. In addition to this, there are various ways of thinking depending on the desired quality. For example, in order to reduce the probability of retransmission as much as possible, a retransmission request may be made based on the detection result of a transmission error in one device with a low probability of occurrence of an individual transmission error. In order to increase the probability of performing the above, it is conceivable to perform a logical sum of the results detected by a plurality of devices having a low probability of occurrence of the individual transmission errors.
c) A receiving apparatus to be retransmitted may be selected in advance, and the fact that retransmission request apparatus = Yes shown in FIG. 8 may be written in software. Alternatively, a receiving apparatus that instructs a plurality of receiving apparatuses to be retransmission request terminals from the transmitting side, analyzes retransmission request signals from the plurality of receiving apparatuses, and requests retransmission simultaneously with other terminals is common. It is assumed that the occurrence of individual transmission errors is well detected and the frequency of occurrence of individual transmission errors is low, so that it can be adopted as a retransmission request receiver, and other preselected retransmission request receivers can be replaced with other receivers. good. It may be possible to select a retransmission request receiving apparatus that can be gradually trusted by repeating this process.
[0028]
Next, in order to explain a specific operation of the retransmission function, an example of an operation sequence is shown. The microprocessor 42 in the decoding apparatus determines whether a retransmission request is necessary or not based on the continuity of sequence numbers in the RTP packet. The transmission of a retransmission request command from the microprocessor 42 in the decoding apparatus is preferably notified to the transmission side encoding apparatus by a highly reliable TCP / IP protocol. Here, as a method of specifying the transmitting side encoding device,
a) A specific address or the like may be assigned to the encoding device in advance, and the address may be set in the decoding device that makes a retransmission request.
b) The transmission encoding device may be specified from the data received by the decoding device.
[0029]
When the retransmission request command is received by the microprocessor 27 in the encoding apparatus, it is determined whether or not the data can be retransmitted from the sequence number in the retransmission request command, and retransmission is possible. That is, the corresponding data in the transmission RTP buffer 26 has not been updated. If so, the corresponding data is transmitted via the communication interface 28. The retransmitted data is judged by the reception side microprocessor 42 as to whether or not the data can be received from the sequence number in the RTP packet, and can be received, that is, even if the microprocessor 44 has not read the corresponding data from the reception RTP buffer 43. For example, the data may be stored in the corresponding reception RTP buffer 43, or the retransmitted data may be stored unconditionally. However, in the latter case, since the corresponding data may be written after being read by the processor 44, the above busy FLAG remains standing. Therefore, it is necessary to write even if busy FLAG when writing after rounding the RTP buffer.
[0030]
FIG. 9 shows a retransmission sequence when retransmission of a decoding apparatus having a retransmission function is successful. When data B is received, the microprocessor 42 in the decoding apparatus detects a sequence skip and sends a retransmission request command C using, for example, the TCP / IP protocol. The macro processor 27 in the encoding apparatus acquires the sequence number of the retransmission data in the retransmission request command. When it is determined that retransmission is possible as described above, retransmission data D to E are transmitted using the UDP / IP protocol. The decoding device writes the received data in the RTP buffer at the address calculated from the sequence number in the received RTP packet.
[0031]
When the arrived RTP sequence number changes by a predetermined number or more than the immediately preceding number, it can be considered that the retransmission request C is not transmitted because it is assumed that the transmission apparatus has been switched as shown in FIG.
[0032]
The microprocessor 42 in the decoding apparatus detects a sequence skip in the received data B and sends a retransmission request command C using the TCP / IP protocol. The microprocessor 27 in the encoding apparatus sends a retransmission request command C in the retransmission request command. If the requested retransmission data D has already disappeared as a result of checking the sequence number in the retransmission data after obtaining the sequence number and calculating the index on the transmission RTP buffer, the retransmission data D may not be transmitted as shown in FIG. desirable.
[0033]
The microprocessor 42 in the decoding apparatus detects a sequence skip and sends a retransmission request command B using the TCP protocol. The microprocessor 27 in the encoding apparatus acquires the sequence number in the retransmission request command. The microprocessor 27 inside the encoding device sends retransmission data D using the UDP / IP protocol. When the received data is written in the received RTP buffer at the address calculated from the sequence number in the received RTP packet, the microprocessor 42 in the decoding device writes the dummy bit in the received RTP buffer in advance when sending the retransmission request command. If there is no match as a result of comparison with the sequence number to be received, the received data may be discarded as shown in FIG.
[0034]
FIG. 13 shows a case where retransmission data is transmitted to a decoding apparatus that does not make a retransmission request. That is, the decoding apparatus internal microprocessor 42 of the receiving terminal that requests retransmission transmits a sequence skip and sends a retransmission request command B to the encoding apparatus using the TCP protocol. The receiving terminal that does not request retransmission and the selected decoding apparatus internal microprocessor 42 detect the sequence skip and hold dummy bits in the reception RTP buffer as write sequence skip information. The sequence number in the retransmission request command received by the microprocessor 27 in the encoding apparatus is acquired, and the microprocessor 27 in the encoding apparatus sends retransmission data D using the UDP protocol. Regardless of whether the transmission request is made or not, the microprocessor 42 in the decoding apparatus writes the received data into the received RTP buffer calculated from the sequence number in the received data, and when the sequence skip is detected in advance, As a result of comparison with the sequence number to be received written together with the dummy bit, if it matches, writing to a predetermined address of the receiving RTP buffer may be performed, and if not matched, the received data may be discarded.
[0035]
The retransmission request command packet format is shown in FIG. FIG. 15 shows an example of a transmission command when a sequence skip of received data is actually detected.
[0036]
【The invention's effect】
As described above, according to the present invention, only a limited decoding terminal makes a retransmission request.
When retransmission is applied to broadcast communication, unnecessary retransmission is not performed, and it is possible to prevent the communication network from being congested due to an increase in the traffic of the retransmission request itself and a new packet loss from occurring in the communication network.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a real-time transmission system for video and audio using a communication network.
FIG. 2 is a block configuration diagram of an MPEG-2 encoding device.
FIG. 3 is a block configuration diagram of an MPEG-2 decoding apparatus.
FIG. 4 is a flowchart of writing to and reading from a transmission side RTP buffer.
FIG. 5 is a flowchart of writing to and reading from a transmission side RTP buffer.
FIG. 6 is a diagram of a write and read process to a receiving side RTP buffer.
FIG. 7 is a diagram showing a process of writing to and reading from a reception side RTP buffer.
FIG. 8 is a flowchart of a write process to a reception side RTP buffer in the present embodiment.
FIG. 9 is a sequence diagram for explaining the operation of the present embodiment;
FIG. 10 is a sequence diagram for explaining the operation of the present embodiment;
FIG. 11 is a sequence diagram for explaining the operation of the present embodiment;
FIG. 12 is a sequence diagram illustrating an operation of the present embodiment.
FIG. 13 is a sequence diagram for explaining the operation of the present embodiment;
FIG. 14 is an explanatory diagram showing an example of a retransmission request command employed in the present embodiment.
FIG. 15 is an explanatory diagram showing an example of a retransmission request command employed in the present embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Camera 2 Microphone 3 MPEG-2 encoding apparatus 4 Communication network 5, 8 Decoding apparatus 6, 9 Image monitor 7, 10 Speaker 21 Luminance and color difference separation circuit 22 A / D (analog-digital) conversion circuit 23 MPEG-2 image Encoding circuit 24 Image encoding buffer 25, 27, 42, 44 Microprocessor 26 Transmission RTP buffer 28, 41 Communication interface 29 MPEG-2 audio encoding circuit 30 Audio encoding buffer 43 Reception RTP buffer 45 Image decoding buffer 46 Audio Decoding buffer 47 MPEG-2 image decoding circuit 48 D / A (digital analog) converter 49 Luminance color difference synthesis circuit 50 MPEG-2 audio decoding circuit

Claims (2)

In a retransmission method of broadcast data transmitted from a transmission device to a plurality of reception devices via a wireless network,
Among the plurality of receiving devices, to the receiving device arranged at the point where the reception condition is the best , set to transmit a retransmission request to the transmitting device for the transmission error of the broadcast data,
The broadcast data is transmitted again to the plurality of reception devices when the retransmission request is received from the reception device set to transmit the retransmission request by the transmission device. Retransmission method. In the broadcast data retransmission method according to claim 1 ,
The broadcast data retransmission method, wherein the transmission apparatus sets a reception apparatus that transmits the retransmission request.

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